score will also be accustomed to high fodder intakes and are likely to have a higher rate of metabolic heat generation, compounding their ‘softness’ under heat stress. Variation of
response with fat score is one area where controlled environment room research is needed.
3.4 Thermal Modelling
3.4.1 Overview
A relation between the air speed and the critical core wet bulb difference has been developed to eventually be applied to different classes of animal. The earlier heat transfer
analysis has been adapted with parameters reconfigured to allow determination of the critical core wet bulb difference. The thermal model incorporates radiation, convection forced and
natural, evaporation where applicable forced and natural and respiratory heat rejection. These are balanced against animal metabolic rate. Heat transfer relations for a number of
conditions were calculated to assess sensitivities. These sensitivities indicated where further development of the model is required.
3.4.2 Details
For a series of wet bulb temperatures, thermal equations were set up to determine the relevant heat transfer component as described below.
Radiation – this is a function of skin temperature and ambient conditions. It is assumed that the ambient dry bulb is the mean radiant temperature of the surroundings. The
average skin temperature is used as the radiating temperature. Convection – two components of convection are assessed, forced and natural. For low
air velocity through the pen, natural convection will dominate. Convective heat transfer is driven by the difference between the skin temperature and ambient air temperature. It
was assumed that forced convection would be relevant to a proportion of animal surface i.e. across the animals back, while natural convection would dominate on the
remainder. This seems a fair assumption as air velocities underneath and on sides of animals would be low. The bulk air movement would mostly be in the space above the
animals.
Evaporation – two components of evaporation are assessed, forced and natural. As for convective heat transfer, the forced component would only act on a proportion of the
animal surface. Respiration – this is a function of breathing rate, breath air condition and ambient air
condition. Other factors such as heat storage and direct conduction to surfaces were not assessed.
Heat storage effects are not significant if conditions change slowly. Conductive heat transfer would be small compared to the other heat transfer components. The above components
were summed and equated to the metabolic heat generation rate. For a series of wet bulb temperatures at a given relative humidity, the equations were balanced by firstly changing
the animal skin temperature and then changing the air speed across the animal. The skin temperature was allowed to rise to an upper limit of 1
o
C below core temperature. The air speed was then increased if necessary until thermal balance was achieved.
Hence, for a range of ambient wet bulb temperatures and hence critical core wet bulb difference the air velocity required to achieve thermal balance can be estimated. Given a
known limiting critical core wet bulb difference, the required velocity to maintain appropriate heat transfer can be estimated.
A number of sensitivities are being assessed including the effect of changing humidity ratio, metabolic rate and the split between forced and convective cooling.
Project: LIVE.116 – Development of a Heat Stress Risk Management Model Revision F
Maunsell Australia Pty Ltd Page 37 of 129
Final Report December 2003
3.5 Validation from a Voyage
