Evaluation of methods for determining air exchange rate in a naturally ventilated dairy cattle building with large openings using computational fluid dynamics (CFD)

Naturally ventilated dairy cattle buildings are a major source of ammonia and greenhouse gas emissions. Accurate estimation in gas emissions constitutes the first step towards reducing the negative impact of emissions on the local environment. The greatest uncertainty in the emission estimation from a naturally ventilated livestock building with large openings is the determination of the air exchange rate (AER) and the choice of the gas sampling positions for representative outlet gas concentration. To reduce the uncertainties in the emission estimation, the performances of three techniques – integrating volume flow rates (VFR), tracer gas decay (TGD) and constant tracer gas (CTG) for determination of ventilation rates were assessed in this paper by Computational fluid dynamics (CFD). In the developed CFD model, the animal occupied zone (AOZ) was treated as porous media and the resistance coefficient of porous zone was derived by pressure drops across AOZ using a sub-CFD model. The results showed that AERs predicted by VFR and TGD were in good agreement with each other within a large range of wind speeds. The large difference in AER estimation using CTG and VFR indicates that the mean CO2 concentration of the entire room may not represent the concentration at the air exit. It may be not suitable to calculate AER using mean concentration of internal sampling positions. When wind became stronger, the accuracy of CTG decreased. The gas sampling positions should be close to the openings or even in the openings; the gas sampling positions should be located adjacent to the openings or even in the openings. To reduce the uncertain introduced by wind direction, all the openings especially of different azimuths should possess sampling tubes. The maximum gas concentrations in the different openings could be the optimum value to represent the concentration in the exit air.

► Methods for calculating natural ventilation rate were assessed using a CFD model.
► Internal gas sampling may cause large errors to estimate ventilation rate.
► Gas sampling positions should be close to the sidewall openings.
► Maximum gas concentration in different openings may represent that in exit air.