An Experimental Study of Convective Heat Transfer on a Body Disturbed by Local Airflow-Part 2: Stimulation Structure of Local Airflow Through Natural Convection
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4800
An Experimental Study of Convective Heat
Transfer on a Body Disturbed by Local
Airflow-Part 2: Stimu lation Structure of
Local Airflow Through Natural Convection
Listiani Nurul Huda, DrEng
Hiroshi Homma, TeknDr
Nohomi Matsu bara
Member ASHRAE
ABSTRACT
To stimulate a thermal or contact receptor on a body
surface, a local airflow must pass through the natural convection that rises around a body from its own metabolic heat. The
effect of local airflows on local convective heat transfer was
statistically examined using thermal manikin . experimental
results. The effect ofthe local airflows was evaluatedfrom the
changes in the natural convection boundary layer temperature
and the changes in the body surface heat fluxes . The effects of
nozzle surface velocities and arrival velocities after traveling
the distance between the nozzle and a body surface were
compared. Though the boundary layer temperature changes
differed irregularly between the back ofthe neck and ankle, the
surface heatflux changes were significantly larger at the ankle
than at the back ofthe neck. The results indicated that the local
airflows affected the ankle more strongly than the neck.
INTRODUCTION
When economy is sought for summer air conditioning in
rapidly developing Asian societies, a higher air velocity than
the presently applied upper limit seems to be acceptable. This
may allow an air conditioner to supply air at a higher temperature than the present practice, which may improve the performance of an air conditioner. Thermal comfort standards such
as ASHRAE Standard 55-2004 (ASHRAE 2004) and 1SO7730 (ISO 1994), suggest a limit for the allowable air velocity.
The table of optimum and acceptable ranges of operative
temperature for a sedentary occupant indicates that the mean
air speed should be less than 0.15 mls (30 fpm), and the air
speed must be no greater than 0.25 mls (50 tpm), even if a
measure is provided for the affected person to control the
velocity. These standards were developed for relatively cool
regions. For summer air conditioning, the ASHRAE standard
indicates that thermal comfort can be attained at an air temperature above the comfort zone if the heat transfer is adjusted by
increasing the air velocity, but the air velocity is limited to less
than 0.8 mls (160 fpm) to avoid blowing of papers. However,
in Southeast Asia, it is believed that not only does a higher air
velocity than this not disturb thermal comfort but also that it
causes a sense of freshness and encouragement for the room
occupants.
Provided that such an increase in an air velocity is
accepted for an occupied zone, it is impractical to distribute
the higher air velocity evenly throughout the occupied space.
On the contrary, if a body site where comfort is promoted by
local airflow is found, comfort can be improved by concentrating the local airflow to it. This may result in more economical air conditioning. In this context, if a draft-sensitive
location is found on the body surface, comfort air conditioning
may be established by avoiding a high velocity or cold air at
it. Draft has been treated as a difficult factor to control in an
indoor environment. However, once the stimulation of a local
airflow is well understood, it may be a possible to utilize it,
rather than to try to avoid it, for air-conditioned buildings or
vehicle cabins.
Metabolic heat dissipation causes natural convection
around a human body. The properties and magnitude of this
convection were well explained and were suggested to playa
considerable role in the indoor environment (Lewis et aI. 1969;
Homma et al. 1988). In a subjective assessment of thermal
comfort by office workers in Britain, Fishman (1978)
mentioned that once the draft was sufficiently high to interfere
with the natural convection over the body, it became important. Thus, there is an inconsistency in thermal comfort condi-
Listiani Nurul Huda is a lecturer in the Department of Industrial Engineering, University of Sumatera Utara, Medan, Indonesia. Hiroshi
Homma is a professor and Nohomi Matsubara is a student in the Department of Architecture and Civil Engineering, Toyohashi University
of Technology, Toyohashi, Japan.
© 2005 ASHRAE.
185
tions for local air movement. On one hand, higher intolerance
at the back ofthe neck for cold draft is emphasized compared
to the ankle (Houghten et al. 1938; Fanger and Christensen
1986), while on the other hand, the importance of the temperature in the lower part of a room is suggested (Wyon et aI.
1969; Gonzalez and Nishi 1976). This inconsistency may be
caused by regard or disregard of the natural convection in the
experiments of the two parties (Horoma et aI. 1988). The interaction ofthe natural convection and forced airflow around the
neck of a thermal manikin was investigated by Melikov and
Zhou (1996). The heat flux at the neck was found to increase
by more than 30% compared to the case of natural convection
when an airflow at a velocity of 0.3 mls was directed from the
back. An experimental study on the interaction of natural
convection and room airflow by an air conditioner was
conducted by Jia et al. (2002), using the particle image velocimetry technique.
However, another reason may account for this inconsistency. The higher tolerability of the ankle than the back ofthe
neck was explained as a result of acclimatization of the location to the room environment of the air-conditioning practices
of the 1930s, which produced a large vertical temperature
gradient (Houghten et aI. 1938). However, the back ofthe neck
may have more sensitive receptors for temperature and air
movement than the ankle for some physiological reason. The
mechanism of the effect of draft may be divided into two
stages from a standpoint of thermal comfort. The first stage is
how local airflow reaches a body surface; this is a physical part
of heat transfer. The second stage is physiological, involving
the sensitivity distribution ofthermal and contact receptors by
location. Therefore, we chose to analyze this inconsistency by
separating the physical stimulation of local airflow from the
physiological response.
To examine the physical side ofthis mechanism, an experiment was designed to study the influence of local airflow on
various body locations. In this design, the natural convection
around a body was considered as a protective, or resistance,
factor in metabolic heat transfer, while any impinging local
airflow was considered to be a penetrating factor from the
environment into the natural convection air layer. The general
characteristics of this influence have been reported in
ASHRAE Transactions (Homma 2001). In the present report,
the detail of this influence is examined statistically.
natural convection boundary layer and ofthe surface heat flux
by the local airflows were measured. These changes were used
to describe how the local airflow penetrated the natural
convection boundary layer and stimulated the surfaces at the
two locations. The thermal manikin was warmed to a normal
temperature for a human body, so it was enveloped by the natural convection.
The thermal manikin used had the posture of a seated
male. It consisted of nine segments of cast aluminum hollow
shells. The inside of each segment was regulated to have a
temperature of36±0.5°C (96.8±0.9°F) by circulating hot air in
it. The resulting surface temperatures were 31 °C and 30°C
(87.8°F and 86.0°F), respectively, at the back of the neck and
at the ankle. The unclothed manikin was placed on an office
chair, from which the back rest and its support had been
removed.
Two local airflow blowers were constructed to supply the
airflow for this test; its largest temperature difference was
10°C (l8°P) from the room temperature, and its highest velocity was 1.5 mls (300 fpm) from a nozzle with a diameter of
50 mm (2 in.).
The natural convection boundary air layer has a thickness
of 8 to 25 rom (0.3 to 1.0 in.) beside the ankle, and about
50 rom (2 in.) in front ofthe head. The thickness is supposed
to be about 0.2 m (8 in.) at its best developed upper body side
(Fiedorowicz 1975; Horoma et al. 1988). To detect the influence of local airflows of various velocities and temperatures,
the position of the temperature sensor must be as close as
possible to the object's surface, and its precise position must
be measured. For these reasons, the sensors for the boundary
layer temperature change measurement were positioned at a
distance of 5 rom (0.2 in.) from the surface of the back of the
neck and the side of the ankle. The temperature sensors were
0.2 rom (8 mils) diameter copper constantan thermocouples.
The temperatures were measured with a precision of 0.1 °c
(0.2°F).
For the heat flux measurement, heat flux sensors were
attached to the surface of the manikin with an adhesive layer.
The average sensitivity of the heat flux sensors was 0.0124
millivolts for a heat flux of 1 W /m 2 (0.32 Btu/h ft2) .
EXPERIMENTAL PROCEDURE
In the present study, only the average changes in the
boundary layer temperatures or the heat fluxes with and without local airflows on the centerline of the local airflow are
treated.
Horizontal local airflows were directed through the natural convection boundary layers at the back of the neck and the
side of the ankle of a life-sized thermal manikin. At the back
of the neck, the centerline of the local airflow nozzle was
aligned to the front-back centerline of the manikin. At the side
of the ankle, it was arranged perpendicular to the protruding
surface of the ankle. The horizontal local airflow was regulated to produce various temperature differences from the air
temperature in the experimental space, and its velocity was
also changed. The resulting changes of the temperatures in the
The laboratory used for the measurement was not to influence this change. The measurement of the changes in the
boundary layer temperature was executed by changing the
temperature differences of the local airflows from the room
temperature to - 10°C, -5°C, O°C, SoC, and 10°C (- 18°F, - 9°F,
O°F, 9°F, and 18°F) and also by changing the velocity to 0.25 ,
0.50, 0.75, 1.00, and 1.25 mls (50, 100, 150, 200, and 250
fpm) . The distance between the nozzle outlet and the object
surfaces was fixed to 0.4 m (16 in). The total number of the
measured conditions was 25 .
186
ASHRAE Transactions: Research
In the heat flux change measurement, the temperature of
the local airflow was changed to the same temperature differences as the boundary layer temperature measurement. The
velocity was changed to 0.25, 0.50, 0.75, and 1.00 mls (50,
100, 150, and 200 fpm) . Further, the distances between the
nozzles and the object surfaces were changed to 0.25 , 0.40,
and 0.60 m (0.82, 1.33, and 2.0 ft). The decayed velocities
after traveling these distances in an almost unlimited space are
called the "arrival velocity" and are shown in Table I. A total
of 60 conditions of local airflows were examined.
The details of the experimental arrangements and the
direct results of this study were reported in an earlier paper
(Homrna 2001).
RESULTS
The influence of the local airflow on the boundary layer
temperature changes and heat flux changes was examined
statistically using correlation coefficient (R) and variance ratio
(F) tests.
Table 1.
Centerline Velocity Decay
Arrival Velocity
Distance from Nozzle (m)
Nozzle Velocity
0.25
0.40
0.60
1.00
0.25
0.113
0.083
0.024
0.010
0.50
0.339
0.205
0.098
0.024
0.75
0.509
0.326
0.179
0.041
1.00
0.693
0.498
0.306
0.084
1.25
0.891
0.640
0.404
0.140
Temperature Changes in the
Natural Convection Boundary Layer
Figures la and Ib show the temperature changes at the
back of the neck and at the ankle, respectively. The means of
the temperature changes induced by the 25 local airflows were
-2.46 and -2.94, respectively, at the back ofthe neck and at the
ankle. Their standard deviations were 2.696 and 2.604, respectively. The correlation coefficients at the two locations were
0.636 and 0.707, and the variance ratios were 18 and 27,
respectively. The relation between the local airflow temperature differences and the boundary layer temperature changes
were significant at P < 0.005 . These statistics all show that the
temperature changes were larger, and the relation to the local
airflows was stronger at the ankle than at the back of the neck.
Two velocities were evaluated: the velocity at the nozzle
and the arrival velocity. Heat transfer on a surface is expressed
as proportional to the velocity or to the square root of the
velocity (ASHRAE 2001) according to the range of the velocity. The results of the tests are shown in Table 2. The relation
between the nozzle velocities and the boundary layer temperature changes showed a negative gradient. The correlation
coefficients of the back of the neck and the ankle were 0.606
and 0.418, respectively, and were significant with P < 0.005
and P < 0.025, respectively.
The relation between the temperature changes and the
nozzle velocity, arrival velocity, and their square roots had
almost the same variance ratios of 15, and they were significant with P < 0.005 . However, the nozzle velocity had a
slightly larger variance ratio than the arrival velocity. The
correlation coefficients at the ankle were -0.46, and the average variance ratio of the above four variations of velocity
terms was 6.9. The average variance ratio was larger at the
back of the neck than at the ankle. At the back ofthe neck, the
variance ratios indicated that the temperature differences had
2
2
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local airflow temperature difference [OC]
(a) back of neck
Figure 1
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local airflow temperature difference [OC]
(b) ankle
Boundary layer temperature changes by local airflows.
ASH RAE Transactions : Research
187
Forced convection heat transfer coefficients were calculated
to be 8.6 and 1O.3W/m 2 X (2.7 and 3.3 Btu/h if), respectively,
for the neck and the ankle at an air velocity of 0.5 mls (100 fpm)
from the equation for a single cylinder (ASHRAE 2001). The
temperature changes at a nozzle velocity of 0.5 mls (100 fpm),
which were calculated from the regression constants, were l.28°e and -2.lO o e (2.3°F and 3.8°F), respectively, for the back
of the neck and the ankle. Ifa temperature change was proportional to the forced convection heat transfer, the difference
between the measured temperature changes at the two locations
seemed too large. The strength of the natural convection at the
two locations seemed to influence this difference.
showed a large variance ratio at the ankle. The correlation
coefficients and variance ratios showed the same tendency.
The variance ratios between the surface heat flux changes
and the products of the temperature differences and nozzle
velocities were slightly larger at the back of the neck than the
ankle. Even when the arrival velocity was used in the product,
the change in the variance ratio was slight. The correlation
coefficients showed that the back of the neck had a stronger
relation with the product than the ankle. This difference was
shown more clearly in the variance ratios. As the arrival velocity was nearly proportional to the reciprocal of the travel
distance to the nozzle velocity, as can be seen from Table 1, a
local airflow index I, calculated as the product of the local
airflow temperature difference IlT and the nozzle velocity V,
divided by the travel distance d, as
Change in Heat Flux
Figures 2a and 2b show the surface heat flux changes at
the two locations with the temperature difference of the local
airflow on the abscissa.
1 = ~T
The intercepts, gradients, correlation coefficients, and
variance ratios with the conditions of the local airflow are
shown in Table 3. In the single regression analysis, the gradient of the heat flux changes to the local airflow temperature
differences had a slightly larger negative value at the back of
the neck than the ankle. The gradients ofthe four variations of
the velocity terms were, on average, larger at the ankle than at
the back ofthe neck. At the back of the neck, the average of the
variance ratios of the nozzle velocity and its square root was
30, and that of the arrival velocity and its square root was 42.
The corresponding values at the ankle were 34 and 52, respectively. The proportion of the variance ratio of the arrival velocity to that of the nozzle velocity was considerably larger in the
heat flux changes than in the boundary layer temperature
changes; however, the square root of the arrival velocity
60
55
50
x
d
V
'
was employed as an element of heat transfer. The correlation
coefficients and the variance ratios for this index varied
slightly when they were compared with the results of the
products of the local airflow temperature difference and the
arrival velocity at the back of the neck but showed clear
improvement at the ankle.
In the multiple regression analysis, the average variance
ratio was 55 at the back of the neck (P < 0.005) when nozzle
velocity was applied; it increased to 86 when the arrival velocity was applied. These values were 32 and 63, respectively, at
the ankle (P < 0.005). These indicate that the arrival velocity
had a stronger influence than the nozzle velocity in the heat
flux changes. The opposite relations were found in the boundary layer temperature changes.
distance [m] x
velocity [m/s]
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- •• - OAOxO.50
- ... - o.40x1 .00
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(a) neck
10
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local airflow temperature difference
Figure 2
- .• - o.40xO.75
r---....
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-5
-10
r--....
.... .~ .... -.
t-~
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.:~
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20
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-10
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local airflow temperature difference ["C]
(b) ankle
•• - • ·060x1.00
Surface heatflux changes with the temperature difference a/the local airflow an the abscissa.
AS HRAE Transactions: Research
189
Table 3.
Influence of Local Airflows on Heat Flux Changes
Back of Neck
Independent Variable(s)
Temperature difference
Ankle
Regression Equation
Variance
Ratio
Regression Equation
Variance
Ratio
Correlation
Intercept Gradient(s) Coefficient
F
Correlation
Intercept Gradient(s) Coefficient
F
9.48
- 1.06
-0.59
33 .2
11.59
- 0.98
-0.48
18.5
Nozzle velocity
- 5.66
24.23
0.54
24.8
- 5.31
27.04
0.53
23.4
Square root of nozzle velocity
- 18.32
36.19
0.54
24.5
- 19.40
40.34
0.53
23.1
Arrival velocity
-6.04
37.12
0.61
36.6
-7.78
46.33
0.67
50.5
Square root of arrival velocity
- 19.48
46.29
0.60
34.3
- 24.60
57.83
0.66
47.1
Temp. diff. x nozzle velocity
9.48
- 1.76
-0.68
52.5
11 .59
- 1.66
-0.56
27.8
Temp, diff. x sq. rt. of nozzle vel.
9.48
-1.48
-0.66
46.7
11.59
-1.38
- 0.54
24.9
Temp. diff. x arrival velocity
9.48
- 2.68
-0.70
60.2
11.59
-2.67
- 0.62
37.4
Temp. diff. x sq. rt. of arrival vel.
9.48
- 1.85
-0.67
50.7
11.59
- 1.78
- 0.57
29.2
Temp. diff. + nozzle velocity
- 5.66
- 1.06
0.81
56.2
- 5.31
-0.98
0.72
32. 1
0.72
31. 7
0.84
65.9
0.83
59.3
24.23
Temp. diff. + sq. rt. of nozzle vel.
- 18.32
- 1.06
27.04
0.81
55.6
- 19.40
Temp. diff. + arrival velocity
-2.20
1.06
0.87
90.9
-2.66
- 10.69
- 1.06
0.86
82.0
The heat flux changes, which are calculated from the regression constants of the nozzle velocity and a velocity of 0.5 mls
(100 fpm), were 6.46 and 8.2i W /m 2 (2.1 and 2.6 Btu/h·tt2),
respectively, at the back of the neck and the ankle. The measured
change at the ankle was slightly larger than that at the back of the
neck, when the changes were estimated from the difference in
the above forced convection heat transfer coefficients at the two
locations.
Natural convection heat transfer coefficients at various
heights of a heated vertical plate can be calculated by applying
a surface temperature and the ambient air temperature
(ASHRAE 2001) . This result can be applied for a vertical
cylinder when its diameter is sufficiently larger than the
boundary layer thickness (Holman 1986). Putting the case as
above, the theoretical natural convection heat transfer coefficients are 2.9 at the backofthe neck (height = 1.1 m, 3.7 ft) and
4.6 W /m 2 ·K (0.9 and 1.5 Btu/h·tt2) at the ankle (height =
0.1 m, 0.33 ft).
The proportion of the mean heat flux changes to the mean
temperature changes at 5 rum (0.2 in.) was - 3.85 at the back
of the neck and -4.06 at the ankle. The heat flux change for a
unit change of the air temperature at 5 rum was larger at the
ankle than at the back of the neck. The local airflow seemed to
- 13.00
-0.98
49.69
40.77
190
-0.98
50.92
41.76
Temp. diff. + sq. rt. of arrival vel.
-0.98
40.34
36.19
invade deeper into the natural convection at the ankle than at
the back of the neck.
Comparison between the
Back of the Neck and the Ankle
The boundary layer temperature changes at the back of
the neck and the ankle are compared in Figure 3. The diagonal
line in the figure is an equal change line. Most of the changes
were negative, and the comparison points were distributed
under the equal change line. This indicates that the boundary
layer temperature was changed more at the ankle than at the
back of the neck. The variance ratio was 98, as shown in
Table 4, so the comparison is significant at P < 0.005 .
The heat flux changes at the two locations are compared
in Figure 4. Most of the changes were positive, and they were
situated above the equal change line. So the heat flux changes
were larger at the ankle than at the back of the neck. Their
correlation coefficient was 0.82, and their variance ratio was
256. The critical region P was < 0.005.
DISCUSSION
The mean heat fluxes without the local airflow were
converted into natural convection heat transfer coefficients by
dividing them with the mean differences between the correASHRAE Transactions: Research
sponding surface temperatures and the air temperature in the
laboratory. They were 1.83 and 2.17 W /m 2 X (0.32 and
0.38 BtuIh·ft2.oF), respectively, at the back of the neck and at
the ankle. Natural convection heat transfer coefficients at various heights of a heated plate can be calculated by applying a
surface temperature and the ambient air temperature
(ASHRAE 2001) This result can be applied for a vertical
cylinder when its diameter is sufficiently larger than the
boundary layer thickness (Holman 1986). With the case
above, the theoretical natural convection heat transfer coefficients are 2.9 at the back ofthe neck (height = 1.1 m, 3.7 ft) and
4.6 W /m2X (0.5 I and 0.81 BtuIh·tt2.°F), respectively, at the
ankle (height = 0.1 m, 0.33 ft). The natural convection heat
transfer coefficients were smaller in the present measurement
than the theoretical values at the two locations. This may be
caused by the difference in the complicated shape of the manikin and the heated vertical plate.
To attain (forced) heat transfer coefficients under the
effect of the local airflows, the following procedure was
followed . The mean heat flux changes by the local airflows
;p'
0
55
local airflow temp . diff.
Deg.C
L....J
Q)
3C
I:
tV
~
.-5
Q5
I:
tV
010
0
~
N
45
Ol
~
00
Q)
I>D
•
~
.atV
•
~
.-10
1il
~
were divided with the same temperature differences as above
and added to the respective natural convection heat transfer
coefficients. The forced convection heat transfer coefficients
were 3.02 and 3.77 W /m2·K (0.53 and 0.66 BtuIh·ft2 .oF),
respectively, at the back of the neck and at the ankle. The
local convection heat transfer coefficients were measured at
the body parts of a thermal manikin with whole body covering airflows by Oguro et al. (2002). From this measurement,
a condition where the manikin was naked and in a seated
position, and the airflows were directed from the back of the
manikin, was chosen to compare the present experimental
results with this results. The convective heat transfer coefficient was 4.74 W /m 2 ·K (0.83 Btulh·ft2.oF) at the head, and
was 7.17 W /m 2 X (1.26 BtuIh·ft2 .oF) at the foot at an air
velocity 0[0.5 mls (100 fpm) , which corresponds to the mean
velocity of the present experiment. In Oguro 's experiment,
the convective heat transfer coefficient was 1.5 times larger
at the foot than at the head. In the present experiment, the
same proportion was 1.3 . When a local airflow stimulates a
CD
35
•
W
•
~25
CD
.£:
.-5
0
~
....
""
Q)
15
00
~
a.
05
CD
E
I
Ol
5
Q)
010
.oJ
-5
-10
-5
-10
Figure 3
Comparison of temperature changes at back of
neck and ankle.
Table 4.
15
5
temperature change at neck [OC] 0
25
35
45
55
Heat flux ohange at neok [W/ m2]
Figure 4
Comparison of heat flux changes at back of neck
and ankle.
Comparison of Influences of Local Airflows on Back of Neck and Ankle
Mean of Changes
Changes in air temperature
Changes in heat flux
ASHRAE Transactions: Research
Linear Regression Equation
Variance
Ratio
Samples
Ankle
Neck
Intercept
Gradient
Correlation
Coefficient
F
25
-2.94°C
- 2.46°C
-0.80
0.87
0.86
98
2.20
1.03
0.89
256
60
11.93
W·m2
9.48
W·m 2
191
surface, its effect is influenced by the strength of the natural
convection by the metabolic heat. These proportions indicated this influence. It seems reasonable that a local airflow
has a stronger influence on an ankle than on the back of a
neck.
In the existing research reports of subjective inquiry on
the thermal sensitivities of the two locations, some reports
claim that ankles are more sensitive for thermal stimulation,
and the others claim the opposite. From this experiment, the
following two reasons can be indicated for this contradiction.
One of them is the influence of the natural convection. It may
be easier for a local airflow to penetrate through the weak and
thin natural convection layer at the ankle than the strong and
thick one at the back of the neck. The sensitivity comparison
may be influenced by the experimental conditions whether the
natural convection was shielded or not. The second reason is
that physiological sensitivities to thermal or airflow receptors
may not be equally distributed over the body surface. This will
be studied with human subjects in the future.
CONCLUSIONS
The heat flux change related more strongly to the temperature of the local airflows at the back of the neck than at the
ankle. But it was influenced almost equally at the two locations by the velocity of the local airflow.
At the back of the neck, the variance ratio between the
heat fluxes and the nozzle velocities increased by about 1.4
times upon replacing the latter with the arrival velocity from
the nozzle velocity. At the ankle, the variance ratio of the
arrival velocity increased to about twice that of the nozzle
velocity. The local airflow seemed to break through the natural
convection layer more easily at the ankle.
Both the changes in the boundary layer air temperatures
and the heat fluxes were more strongly influenced at the ankle
than at the back of the neck (p < 0.005).
Present practice in an office or vehicle cabin is to locate
an air-conditioning outlet outside the natural convection layer
of an occupant. To design more effective air conditioning, the
concurrent structure of these two airflows and the sensitivity
distribution of thermal receptors on a body surface must be
studied further in greater detail.
192
REFERENCES
ASHRAE. 2001. 2001 ASHRAE Handbook- Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
Fanger, P.O. , and N.K. Christensen. 1986. Perception of
draught in ventilated spaces. Ergonomics 29(2):215235.
Fiedorowicz, J. 1975. Boundary layer of air on a nude man
(manikin) caused by free convective heat exchange.
Internal report, Thermal Insulation Laboratory, Technical University of Denmark.
Fishman, D.S. 1978. Subjective effects of air movement
around the feet. Paper presented at the Chartered Institute of Building Services symposium "Man, Environment and Buildings." pp. HI-HIO.
Gonzalez, R.G., et al. 1976. Effect of cool environments on
local thermal sensation, discomfort and clothing selection. ASHRAE Transactions 82(1):76-86.
Holman, J.P. 1986. Heat Transfer, 6th ed. New York:
McGraw-HilI.
Homrna, H. 200 I. An experimental study of convection heat
transfer of a body disturbed by local air flow. ASHRAE
Transactions 107(2) :406-414.
Homrna, H. , et al. 1988. Examination of free convection
around occupant's body caused by its metabolic heat.
ASHRAE Transactions 94(1):104-124.
Houghten, FC., et al. 1938. Draft temperature and velocities
in relation to skin temperature and feeling of warmth.
Transactions ofASHVE, pp. 289-308.
Jia, W., et al. 2002. Data processing of larg-scale particle
image velocity measurements around the human body.
ASHRAE Transactions 108(1):243-250.
Lewis, et al. 1969. Aerodynamics of the human micro environment. The Lancet, 28 June.
Melikov, A., and G. Zhou. 1996. Air movement at the neck
of the human body. Proceedings of 1ndoor Air, Vol. I,
pp. 209-214.
Oguro, M., et al. 2002. Convective heat transfer coefficients
and clothing insulations for parts of the clothed human
body under airflow. Journal of Archil. Plann. Environ.
Eng. 11(561):21-30.
Wyon, D.P., et al. 1969. Thermal comfort during surgical
operations. J.1.H. V.R. 37(Oct.): 150-168.
ASHRAE Transactions : Research
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4800
An Experimental Study of Convective Heat
Transfer on a Body Disturbed by Local
Airflow-Part 2: Stimu lation Structure of
Local Airflow Through Natural Convection
Listiani Nurul Huda, DrEng
Hiroshi Homma, TeknDr
Nohomi Matsu bara
Member ASHRAE
ABSTRACT
To stimulate a thermal or contact receptor on a body
surface, a local airflow must pass through the natural convection that rises around a body from its own metabolic heat. The
effect of local airflows on local convective heat transfer was
statistically examined using thermal manikin . experimental
results. The effect ofthe local airflows was evaluatedfrom the
changes in the natural convection boundary layer temperature
and the changes in the body surface heat fluxes . The effects of
nozzle surface velocities and arrival velocities after traveling
the distance between the nozzle and a body surface were
compared. Though the boundary layer temperature changes
differed irregularly between the back ofthe neck and ankle, the
surface heatflux changes were significantly larger at the ankle
than at the back ofthe neck. The results indicated that the local
airflows affected the ankle more strongly than the neck.
INTRODUCTION
When economy is sought for summer air conditioning in
rapidly developing Asian societies, a higher air velocity than
the presently applied upper limit seems to be acceptable. This
may allow an air conditioner to supply air at a higher temperature than the present practice, which may improve the performance of an air conditioner. Thermal comfort standards such
as ASHRAE Standard 55-2004 (ASHRAE 2004) and 1SO7730 (ISO 1994), suggest a limit for the allowable air velocity.
The table of optimum and acceptable ranges of operative
temperature for a sedentary occupant indicates that the mean
air speed should be less than 0.15 mls (30 fpm), and the air
speed must be no greater than 0.25 mls (50 tpm), even if a
measure is provided for the affected person to control the
velocity. These standards were developed for relatively cool
regions. For summer air conditioning, the ASHRAE standard
indicates that thermal comfort can be attained at an air temperature above the comfort zone if the heat transfer is adjusted by
increasing the air velocity, but the air velocity is limited to less
than 0.8 mls (160 fpm) to avoid blowing of papers. However,
in Southeast Asia, it is believed that not only does a higher air
velocity than this not disturb thermal comfort but also that it
causes a sense of freshness and encouragement for the room
occupants.
Provided that such an increase in an air velocity is
accepted for an occupied zone, it is impractical to distribute
the higher air velocity evenly throughout the occupied space.
On the contrary, if a body site where comfort is promoted by
local airflow is found, comfort can be improved by concentrating the local airflow to it. This may result in more economical air conditioning. In this context, if a draft-sensitive
location is found on the body surface, comfort air conditioning
may be established by avoiding a high velocity or cold air at
it. Draft has been treated as a difficult factor to control in an
indoor environment. However, once the stimulation of a local
airflow is well understood, it may be a possible to utilize it,
rather than to try to avoid it, for air-conditioned buildings or
vehicle cabins.
Metabolic heat dissipation causes natural convection
around a human body. The properties and magnitude of this
convection were well explained and were suggested to playa
considerable role in the indoor environment (Lewis et aI. 1969;
Homma et al. 1988). In a subjective assessment of thermal
comfort by office workers in Britain, Fishman (1978)
mentioned that once the draft was sufficiently high to interfere
with the natural convection over the body, it became important. Thus, there is an inconsistency in thermal comfort condi-
Listiani Nurul Huda is a lecturer in the Department of Industrial Engineering, University of Sumatera Utara, Medan, Indonesia. Hiroshi
Homma is a professor and Nohomi Matsubara is a student in the Department of Architecture and Civil Engineering, Toyohashi University
of Technology, Toyohashi, Japan.
© 2005 ASHRAE.
185
tions for local air movement. On one hand, higher intolerance
at the back ofthe neck for cold draft is emphasized compared
to the ankle (Houghten et al. 1938; Fanger and Christensen
1986), while on the other hand, the importance of the temperature in the lower part of a room is suggested (Wyon et aI.
1969; Gonzalez and Nishi 1976). This inconsistency may be
caused by regard or disregard of the natural convection in the
experiments of the two parties (Horoma et aI. 1988). The interaction ofthe natural convection and forced airflow around the
neck of a thermal manikin was investigated by Melikov and
Zhou (1996). The heat flux at the neck was found to increase
by more than 30% compared to the case of natural convection
when an airflow at a velocity of 0.3 mls was directed from the
back. An experimental study on the interaction of natural
convection and room airflow by an air conditioner was
conducted by Jia et al. (2002), using the particle image velocimetry technique.
However, another reason may account for this inconsistency. The higher tolerability of the ankle than the back ofthe
neck was explained as a result of acclimatization of the location to the room environment of the air-conditioning practices
of the 1930s, which produced a large vertical temperature
gradient (Houghten et aI. 1938). However, the back ofthe neck
may have more sensitive receptors for temperature and air
movement than the ankle for some physiological reason. The
mechanism of the effect of draft may be divided into two
stages from a standpoint of thermal comfort. The first stage is
how local airflow reaches a body surface; this is a physical part
of heat transfer. The second stage is physiological, involving
the sensitivity distribution ofthermal and contact receptors by
location. Therefore, we chose to analyze this inconsistency by
separating the physical stimulation of local airflow from the
physiological response.
To examine the physical side ofthis mechanism, an experiment was designed to study the influence of local airflow on
various body locations. In this design, the natural convection
around a body was considered as a protective, or resistance,
factor in metabolic heat transfer, while any impinging local
airflow was considered to be a penetrating factor from the
environment into the natural convection air layer. The general
characteristics of this influence have been reported in
ASHRAE Transactions (Homma 2001). In the present report,
the detail of this influence is examined statistically.
natural convection boundary layer and ofthe surface heat flux
by the local airflows were measured. These changes were used
to describe how the local airflow penetrated the natural
convection boundary layer and stimulated the surfaces at the
two locations. The thermal manikin was warmed to a normal
temperature for a human body, so it was enveloped by the natural convection.
The thermal manikin used had the posture of a seated
male. It consisted of nine segments of cast aluminum hollow
shells. The inside of each segment was regulated to have a
temperature of36±0.5°C (96.8±0.9°F) by circulating hot air in
it. The resulting surface temperatures were 31 °C and 30°C
(87.8°F and 86.0°F), respectively, at the back of the neck and
at the ankle. The unclothed manikin was placed on an office
chair, from which the back rest and its support had been
removed.
Two local airflow blowers were constructed to supply the
airflow for this test; its largest temperature difference was
10°C (l8°P) from the room temperature, and its highest velocity was 1.5 mls (300 fpm) from a nozzle with a diameter of
50 mm (2 in.).
The natural convection boundary air layer has a thickness
of 8 to 25 rom (0.3 to 1.0 in.) beside the ankle, and about
50 rom (2 in.) in front ofthe head. The thickness is supposed
to be about 0.2 m (8 in.) at its best developed upper body side
(Fiedorowicz 1975; Horoma et al. 1988). To detect the influence of local airflows of various velocities and temperatures,
the position of the temperature sensor must be as close as
possible to the object's surface, and its precise position must
be measured. For these reasons, the sensors for the boundary
layer temperature change measurement were positioned at a
distance of 5 rom (0.2 in.) from the surface of the back of the
neck and the side of the ankle. The temperature sensors were
0.2 rom (8 mils) diameter copper constantan thermocouples.
The temperatures were measured with a precision of 0.1 °c
(0.2°F).
For the heat flux measurement, heat flux sensors were
attached to the surface of the manikin with an adhesive layer.
The average sensitivity of the heat flux sensors was 0.0124
millivolts for a heat flux of 1 W /m 2 (0.32 Btu/h ft2) .
EXPERIMENTAL PROCEDURE
In the present study, only the average changes in the
boundary layer temperatures or the heat fluxes with and without local airflows on the centerline of the local airflow are
treated.
Horizontal local airflows were directed through the natural convection boundary layers at the back of the neck and the
side of the ankle of a life-sized thermal manikin. At the back
of the neck, the centerline of the local airflow nozzle was
aligned to the front-back centerline of the manikin. At the side
of the ankle, it was arranged perpendicular to the protruding
surface of the ankle. The horizontal local airflow was regulated to produce various temperature differences from the air
temperature in the experimental space, and its velocity was
also changed. The resulting changes of the temperatures in the
The laboratory used for the measurement was not to influence this change. The measurement of the changes in the
boundary layer temperature was executed by changing the
temperature differences of the local airflows from the room
temperature to - 10°C, -5°C, O°C, SoC, and 10°C (- 18°F, - 9°F,
O°F, 9°F, and 18°F) and also by changing the velocity to 0.25 ,
0.50, 0.75, 1.00, and 1.25 mls (50, 100, 150, 200, and 250
fpm) . The distance between the nozzle outlet and the object
surfaces was fixed to 0.4 m (16 in). The total number of the
measured conditions was 25 .
186
ASHRAE Transactions: Research
In the heat flux change measurement, the temperature of
the local airflow was changed to the same temperature differences as the boundary layer temperature measurement. The
velocity was changed to 0.25, 0.50, 0.75, and 1.00 mls (50,
100, 150, and 200 fpm) . Further, the distances between the
nozzles and the object surfaces were changed to 0.25 , 0.40,
and 0.60 m (0.82, 1.33, and 2.0 ft). The decayed velocities
after traveling these distances in an almost unlimited space are
called the "arrival velocity" and are shown in Table I. A total
of 60 conditions of local airflows were examined.
The details of the experimental arrangements and the
direct results of this study were reported in an earlier paper
(Homrna 2001).
RESULTS
The influence of the local airflow on the boundary layer
temperature changes and heat flux changes was examined
statistically using correlation coefficient (R) and variance ratio
(F) tests.
Table 1.
Centerline Velocity Decay
Arrival Velocity
Distance from Nozzle (m)
Nozzle Velocity
0.25
0.40
0.60
1.00
0.25
0.113
0.083
0.024
0.010
0.50
0.339
0.205
0.098
0.024
0.75
0.509
0.326
0.179
0.041
1.00
0.693
0.498
0.306
0.084
1.25
0.891
0.640
0.404
0.140
Temperature Changes in the
Natural Convection Boundary Layer
Figures la and Ib show the temperature changes at the
back of the neck and at the ankle, respectively. The means of
the temperature changes induced by the 25 local airflows were
-2.46 and -2.94, respectively, at the back ofthe neck and at the
ankle. Their standard deviations were 2.696 and 2.604, respectively. The correlation coefficients at the two locations were
0.636 and 0.707, and the variance ratios were 18 and 27,
respectively. The relation between the local airflow temperature differences and the boundary layer temperature changes
were significant at P < 0.005 . These statistics all show that the
temperature changes were larger, and the relation to the local
airflows was stronger at the ankle than at the back of the neck.
Two velocities were evaluated: the velocity at the nozzle
and the arrival velocity. Heat transfer on a surface is expressed
as proportional to the velocity or to the square root of the
velocity (ASHRAE 2001) according to the range of the velocity. The results of the tests are shown in Table 2. The relation
between the nozzle velocities and the boundary layer temperature changes showed a negative gradient. The correlation
coefficients of the back of the neck and the ankle were 0.606
and 0.418, respectively, and were significant with P < 0.005
and P < 0.025, respectively.
The relation between the temperature changes and the
nozzle velocity, arrival velocity, and their square roots had
almost the same variance ratios of 15, and they were significant with P < 0.005 . However, the nozzle velocity had a
slightly larger variance ratio than the arrival velocity. The
correlation coefficients at the ankle were -0.46, and the average variance ratio of the above four variations of velocity
terms was 6.9. The average variance ratio was larger at the
back of the neck than at the ankle. At the back ofthe neck, the
variance ratios indicated that the temperature differences had
2
2
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f::J
~
~
0.,-,
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+'
'-
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>-c
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~(J
-6
"c
-8
as
5
-~
~1.0
-5
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~o
-6
"c
-8
::J
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.!l
___ 1.25
-10
-10
as as
as
~O.75
.!l
5
10
local airflow temperature difference [OC]
(a) back of neck
Figure 1
-2
0.......0
i fa -4
il>-c111 -4
-~
0
'~
0.,-,
~0
. 75
~1
.0
___ 1.25
-10
-10
-5
0
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10
local airflow temperature difference [OC]
(b) ankle
Boundary layer temperature changes by local airflows.
ASH RAE Transactions : Research
187
Forced convection heat transfer coefficients were calculated
to be 8.6 and 1O.3W/m 2 X (2.7 and 3.3 Btu/h if), respectively,
for the neck and the ankle at an air velocity of 0.5 mls (100 fpm)
from the equation for a single cylinder (ASHRAE 2001). The
temperature changes at a nozzle velocity of 0.5 mls (100 fpm),
which were calculated from the regression constants, were l.28°e and -2.lO o e (2.3°F and 3.8°F), respectively, for the back
of the neck and the ankle. Ifa temperature change was proportional to the forced convection heat transfer, the difference
between the measured temperature changes at the two locations
seemed too large. The strength of the natural convection at the
two locations seemed to influence this difference.
showed a large variance ratio at the ankle. The correlation
coefficients and variance ratios showed the same tendency.
The variance ratios between the surface heat flux changes
and the products of the temperature differences and nozzle
velocities were slightly larger at the back of the neck than the
ankle. Even when the arrival velocity was used in the product,
the change in the variance ratio was slight. The correlation
coefficients showed that the back of the neck had a stronger
relation with the product than the ankle. This difference was
shown more clearly in the variance ratios. As the arrival velocity was nearly proportional to the reciprocal of the travel
distance to the nozzle velocity, as can be seen from Table 1, a
local airflow index I, calculated as the product of the local
airflow temperature difference IlT and the nozzle velocity V,
divided by the travel distance d, as
Change in Heat Flux
Figures 2a and 2b show the surface heat flux changes at
the two locations with the temperature difference of the local
airflow on the abscissa.
1 = ~T
The intercepts, gradients, correlation coefficients, and
variance ratios with the conditions of the local airflow are
shown in Table 3. In the single regression analysis, the gradient of the heat flux changes to the local airflow temperature
differences had a slightly larger negative value at the back of
the neck than the ankle. The gradients ofthe four variations of
the velocity terms were, on average, larger at the ankle than at
the back ofthe neck. At the back of the neck, the average of the
variance ratios of the nozzle velocity and its square root was
30, and that of the arrival velocity and its square root was 42.
The corresponding values at the ankle were 34 and 52, respectively. The proportion of the variance ratio of the arrival velocity to that of the nozzle velocity was considerably larger in the
heat flux changes than in the boundary layer temperature
changes; however, the square root of the arrival velocity
60
55
50
x
d
V
'
was employed as an element of heat transfer. The correlation
coefficients and the variance ratios for this index varied
slightly when they were compared with the results of the
products of the local airflow temperature difference and the
arrival velocity at the back of the neck but showed clear
improvement at the ankle.
In the multiple regression analysis, the average variance
ratio was 55 at the back of the neck (P < 0.005) when nozzle
velocity was applied; it increased to 86 when the arrival velocity was applied. These values were 32 and 63, respectively, at
the ankle (P < 0.005). These indicate that the arrival velocity
had a stronger influence than the nozzle velocity in the heat
flux changes. The opposite relations were found in the boundary layer temperature changes.
distance [m] x
velocity [m/s]
~
""""~: ~
,--,45
~40
2S 35
~30
~ 25
I'..
~
" "'"
"'.""
""...
110
5
o
'
. ~
~
,
..\
~:.
~
- .• - o.40xO.25
""'-
---
t- : , .
"
........
- •• - OAOxO.50
- ... - o.40x1 .00
". :- .' .-~
o
-5
. .... · O.60xO.25
(a) neck
10
5
local airflow temperature difference
Figure 2
- .• - o.40xO.75
r---....
~
. . • . · o.60xO.50
-5
-10
r--....
.... .~ .... -.
t-~
o.25xO.50
- + - 025XO.75
_ _ 0.25x1 .00
.:~
. ."\.,
20
0=
... 15
o.25xO
_
["C]
-10
-5
0
5
1( ..... ·0.60x075
local airflow temperature difference ["C]
(b) ankle
•• - • ·060x1.00
Surface heatflux changes with the temperature difference a/the local airflow an the abscissa.
AS HRAE Transactions: Research
189
Table 3.
Influence of Local Airflows on Heat Flux Changes
Back of Neck
Independent Variable(s)
Temperature difference
Ankle
Regression Equation
Variance
Ratio
Regression Equation
Variance
Ratio
Correlation
Intercept Gradient(s) Coefficient
F
Correlation
Intercept Gradient(s) Coefficient
F
9.48
- 1.06
-0.59
33 .2
11.59
- 0.98
-0.48
18.5
Nozzle velocity
- 5.66
24.23
0.54
24.8
- 5.31
27.04
0.53
23.4
Square root of nozzle velocity
- 18.32
36.19
0.54
24.5
- 19.40
40.34
0.53
23.1
Arrival velocity
-6.04
37.12
0.61
36.6
-7.78
46.33
0.67
50.5
Square root of arrival velocity
- 19.48
46.29
0.60
34.3
- 24.60
57.83
0.66
47.1
Temp. diff. x nozzle velocity
9.48
- 1.76
-0.68
52.5
11 .59
- 1.66
-0.56
27.8
Temp, diff. x sq. rt. of nozzle vel.
9.48
-1.48
-0.66
46.7
11.59
-1.38
- 0.54
24.9
Temp. diff. x arrival velocity
9.48
- 2.68
-0.70
60.2
11.59
-2.67
- 0.62
37.4
Temp. diff. x sq. rt. of arrival vel.
9.48
- 1.85
-0.67
50.7
11.59
- 1.78
- 0.57
29.2
Temp. diff. + nozzle velocity
- 5.66
- 1.06
0.81
56.2
- 5.31
-0.98
0.72
32. 1
0.72
31. 7
0.84
65.9
0.83
59.3
24.23
Temp. diff. + sq. rt. of nozzle vel.
- 18.32
- 1.06
27.04
0.81
55.6
- 19.40
Temp. diff. + arrival velocity
-2.20
1.06
0.87
90.9
-2.66
- 10.69
- 1.06
0.86
82.0
The heat flux changes, which are calculated from the regression constants of the nozzle velocity and a velocity of 0.5 mls
(100 fpm), were 6.46 and 8.2i W /m 2 (2.1 and 2.6 Btu/h·tt2),
respectively, at the back of the neck and the ankle. The measured
change at the ankle was slightly larger than that at the back of the
neck, when the changes were estimated from the difference in
the above forced convection heat transfer coefficients at the two
locations.
Natural convection heat transfer coefficients at various
heights of a heated vertical plate can be calculated by applying
a surface temperature and the ambient air temperature
(ASHRAE 2001) . This result can be applied for a vertical
cylinder when its diameter is sufficiently larger than the
boundary layer thickness (Holman 1986). Putting the case as
above, the theoretical natural convection heat transfer coefficients are 2.9 at the backofthe neck (height = 1.1 m, 3.7 ft) and
4.6 W /m 2 ·K (0.9 and 1.5 Btu/h·tt2) at the ankle (height =
0.1 m, 0.33 ft).
The proportion of the mean heat flux changes to the mean
temperature changes at 5 rum (0.2 in.) was - 3.85 at the back
of the neck and -4.06 at the ankle. The heat flux change for a
unit change of the air temperature at 5 rum was larger at the
ankle than at the back of the neck. The local airflow seemed to
- 13.00
-0.98
49.69
40.77
190
-0.98
50.92
41.76
Temp. diff. + sq. rt. of arrival vel.
-0.98
40.34
36.19
invade deeper into the natural convection at the ankle than at
the back of the neck.
Comparison between the
Back of the Neck and the Ankle
The boundary layer temperature changes at the back of
the neck and the ankle are compared in Figure 3. The diagonal
line in the figure is an equal change line. Most of the changes
were negative, and the comparison points were distributed
under the equal change line. This indicates that the boundary
layer temperature was changed more at the ankle than at the
back of the neck. The variance ratio was 98, as shown in
Table 4, so the comparison is significant at P < 0.005 .
The heat flux changes at the two locations are compared
in Figure 4. Most of the changes were positive, and they were
situated above the equal change line. So the heat flux changes
were larger at the ankle than at the back of the neck. Their
correlation coefficient was 0.82, and their variance ratio was
256. The critical region P was < 0.005.
DISCUSSION
The mean heat fluxes without the local airflow were
converted into natural convection heat transfer coefficients by
dividing them with the mean differences between the correASHRAE Transactions: Research
sponding surface temperatures and the air temperature in the
laboratory. They were 1.83 and 2.17 W /m 2 X (0.32 and
0.38 BtuIh·ft2.oF), respectively, at the back of the neck and at
the ankle. Natural convection heat transfer coefficients at various heights of a heated plate can be calculated by applying a
surface temperature and the ambient air temperature
(ASHRAE 2001) This result can be applied for a vertical
cylinder when its diameter is sufficiently larger than the
boundary layer thickness (Holman 1986). With the case
above, the theoretical natural convection heat transfer coefficients are 2.9 at the back ofthe neck (height = 1.1 m, 3.7 ft) and
4.6 W /m2X (0.5 I and 0.81 BtuIh·tt2.°F), respectively, at the
ankle (height = 0.1 m, 0.33 ft). The natural convection heat
transfer coefficients were smaller in the present measurement
than the theoretical values at the two locations. This may be
caused by the difference in the complicated shape of the manikin and the heated vertical plate.
To attain (forced) heat transfer coefficients under the
effect of the local airflows, the following procedure was
followed . The mean heat flux changes by the local airflows
;p'
0
55
local airflow temp . diff.
Deg.C
L....J
Q)
3C
I:
tV
~
.-5
Q5
I:
tV
010
0
~
N
45
Ol
~
00
Q)
I>D
•
~
.atV
•
~
.-10
1il
~
were divided with the same temperature differences as above
and added to the respective natural convection heat transfer
coefficients. The forced convection heat transfer coefficients
were 3.02 and 3.77 W /m2·K (0.53 and 0.66 BtuIh·ft2 .oF),
respectively, at the back of the neck and at the ankle. The
local convection heat transfer coefficients were measured at
the body parts of a thermal manikin with whole body covering airflows by Oguro et al. (2002). From this measurement,
a condition where the manikin was naked and in a seated
position, and the airflows were directed from the back of the
manikin, was chosen to compare the present experimental
results with this results. The convective heat transfer coefficient was 4.74 W /m 2 ·K (0.83 Btulh·ft2.oF) at the head, and
was 7.17 W /m 2 X (1.26 BtuIh·ft2 .oF) at the foot at an air
velocity 0[0.5 mls (100 fpm) , which corresponds to the mean
velocity of the present experiment. In Oguro 's experiment,
the convective heat transfer coefficient was 1.5 times larger
at the foot than at the head. In the present experiment, the
same proportion was 1.3 . When a local airflow stimulates a
CD
35
•
W
•
~25
CD
.£:
.-5
0
~
....
""
Q)
15
00
~
a.
05
CD
E
I
Ol
5
Q)
010
.oJ
-5
-10
-5
-10
Figure 3
Comparison of temperature changes at back of
neck and ankle.
Table 4.
15
5
temperature change at neck [OC] 0
25
35
45
55
Heat flux ohange at neok [W/ m2]
Figure 4
Comparison of heat flux changes at back of neck
and ankle.
Comparison of Influences of Local Airflows on Back of Neck and Ankle
Mean of Changes
Changes in air temperature
Changes in heat flux
ASHRAE Transactions: Research
Linear Regression Equation
Variance
Ratio
Samples
Ankle
Neck
Intercept
Gradient
Correlation
Coefficient
F
25
-2.94°C
- 2.46°C
-0.80
0.87
0.86
98
2.20
1.03
0.89
256
60
11.93
W·m2
9.48
W·m 2
191
surface, its effect is influenced by the strength of the natural
convection by the metabolic heat. These proportions indicated this influence. It seems reasonable that a local airflow
has a stronger influence on an ankle than on the back of a
neck.
In the existing research reports of subjective inquiry on
the thermal sensitivities of the two locations, some reports
claim that ankles are more sensitive for thermal stimulation,
and the others claim the opposite. From this experiment, the
following two reasons can be indicated for this contradiction.
One of them is the influence of the natural convection. It may
be easier for a local airflow to penetrate through the weak and
thin natural convection layer at the ankle than the strong and
thick one at the back of the neck. The sensitivity comparison
may be influenced by the experimental conditions whether the
natural convection was shielded or not. The second reason is
that physiological sensitivities to thermal or airflow receptors
may not be equally distributed over the body surface. This will
be studied with human subjects in the future.
CONCLUSIONS
The heat flux change related more strongly to the temperature of the local airflows at the back of the neck than at the
ankle. But it was influenced almost equally at the two locations by the velocity of the local airflow.
At the back of the neck, the variance ratio between the
heat fluxes and the nozzle velocities increased by about 1.4
times upon replacing the latter with the arrival velocity from
the nozzle velocity. At the ankle, the variance ratio of the
arrival velocity increased to about twice that of the nozzle
velocity. The local airflow seemed to break through the natural
convection layer more easily at the ankle.
Both the changes in the boundary layer air temperatures
and the heat fluxes were more strongly influenced at the ankle
than at the back of the neck (p < 0.005).
Present practice in an office or vehicle cabin is to locate
an air-conditioning outlet outside the natural convection layer
of an occupant. To design more effective air conditioning, the
concurrent structure of these two airflows and the sensitivity
distribution of thermal receptors on a body surface must be
studied further in greater detail.
192
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ASHRAE Transactions : Research