Effects of discharge angle of jet from a slot orifice on cooling performance for a perforated air ducting system in dairy cattle barn

Abstract

By employing a type of ventilation system that relies on perforated air ducting (PAD) and slot orifices, dairy farmers can help their cows avoid heat stress and in so doing increase milk production. However, the air jets from the orifices would be delivered at an angle, namely the discharge angle, which is the angle between the outflowing jet direction and the axis of the air duct. The discharge angle can affect the system’s efficiency and the jet will fail to deliver a sufficient amount of air if the angle is such that the jet does not properly target the cow. Consequently, by means of computational fluid dynamic simulation, this study sought to optimize the discharge angle on the cooling efficiency of PAD system, first by examining the factors (such as airflow rate and duct diameter) that affect the discharge angle and then by evaluating two types of air jet deflector (rectangular duct and rectangular plate) and also the effects produced by varying the spacing between the slot orifices. Results showed that the first orifice (counting from the inlet to the end of the duct) was mostly affected by the two investigated factors. Increasing the airflow rate from 376.3 to 564.5 m3 h−1 and the duct diameter from 0.8 m to 1.0 m had a limited effect on the discharge angle (around a 2° difference in the discharge angle). Also, as the number of sequential orifices increased, the discharge angle increased gradually, and the discharge angle of the last orifice remained at a constant 90°. Of all the discharge-angle mitigation tested, the rectangular-duct deflector produced the highest mean-convection heat transfer rate of the skin surface (98.37 W m−2) and the lowest standard deviation (±1.25 W m−2). The convection heat transfer rate registered by the cow positioned under the first orifice would increase by as much as 27.5% after the spacing between the orifices was modified (in comparison with the original case), while both an increase and a decrease in the convective heat transfer rate were observed to occur among the other cows. Given these findings, it was concluded that the PAD system’s performance could best be improved by adding a deflector with a rectangular duct to adjust the discharge angle.