Computational evaluation of air jet cooling from a perforated air ducting system to mitigate heat stress of cows in free stalls

Abstract

Alleviating the heat stress suffered by dairy cows in order to increase their productivity depends on an effective ventilation system. For this reason, this study evaluated the heat mitigating performance of a perforated air ducting (PAD) system with special attention given to significant design parameters (e.g., the shape and size of the orifices and the speed and flow rate of the air jets) having to do with the jet flow pattern and its corresponding cooling efficiency. More specifically, this work aimed to evaluate, using numerical simulation, the effects produced by four different types of air-jet orifice (i.e., three-circular orifice, single-circular orifice, rectangular orifice, and slot orifice) and also the effects produced by different jet flow rates and by the bulk movement of indoor air with respect to the jet properties and their cooling performance in relation to cows reclining in free stalls. The outcomes show that the jet emitted from the slot orifice distributes air more effectively over the surface of a reclining cow and produces a more even range of skin temperatures. The jet cooling performance achieved with a jet flow rate of 470.4 m3 h−1 (50% of the recommended ventilation rate for 625 kg cow) and a jet temperature of 28 °C was equivalent to the cooling performance achieved by a 2 m s−1 wind stream of the same air temperature passing over the model cow’s body. Additionally, the convective heat dissipation rate of the reclining cow increased from 63.47 to 91.27 W m−2 as the jet flow rate increased from 212.0 to 564.5 m3 h−1, indicating that increasing the flow rate of a jet emitted through the slot orifice could improve the convective heat dissipation rate, but the rate of the increase in the convective heat transfer decreased simultaneously. The bulk movement of the indoor air (>0.2 m s−1) negatively affects jet flow and the corresponding cooling performance. Therefore, a PAD system equipped with slot orifices will most likely be more effective when atmospheric conditions are less agitated (such as those in naturally ventilated barns with minimal cross wind speeds or inside enclosed barns). In short, the findings of this study should warrant using the PAD system to ventilate free-stall dairy barns to mitigate heat stress of dairy cows.