Abstract
Mixed-flow fans (MFF) are widely used to reduce the heat stress in dairy cows in summer. Our research team developed MFFs with a newly shaped diffuser with the length of 250 mm and the circumferential angle of 150°, which have better performance in terms of maximum flow flux and energy efficiency. However, how the elevation angle of the diffuser influences the performance of MFFs and how the optimal fan perform in the field experiment has not been studied yet. In this paper, the diffuser was optimized by CFD (Computational Fluid Dynamics) simulation of the fan and a laboratory prototype test. An orthogonal test showed no interaction among length, circumferential angle, and elevation angle. The diffuser with an elevation angle of 10° performed better than that with an elevation angle of 0°, showing increased jet lengths, flow flux, and energy efficiency by 0.5 m, 0.69%, and 1.39%, respectively, and attaining greater axial wind speeds and better non-uniformity coefficients at the dairy cattle height. Then, through on-site controlled trials, we found that the 10°/150°/250 mm diffusers increased the overall average wind speeds by 9.4% with respect to the MFFs without a diffuser. MFFs with the newly shaped diffuser were used for field tests, and their effectiveness in alleviating heat stress in dairy cows was evaluated by testing environmental parameters and dairy cows’ physiological indicators. Although the temperature–humidity indexes (THIs) in the experimental barn with the optimized fan at different times were lower than those in the controlled barn, the environmental conditions corresponded to moderate heat stress. However, this was not consistent with cow's respiratory rate and rectal temperature. Finally, on the basis of the CFD simulation of a dairy cow barn, the equivalent temperature of cattle (ETIC), which takes into account the effect of air velocity, showed that the environment caused moderate heat stress only at 13:00, but not at other times of the day. This shows that ETIC is more accurate to evaluate heat stress.
Highlights
In summer, the high air temperature in open cowsheds causes significant heat stress in dairy cows, which results in decreased milk production and increased reproductive problems [1,2,3]
The analysis of variance showed that the circumferential angle θ had the greatest influence on the jet length, and the interaction between the factors had no significant effect on the jet length and could be ignored
The test showed p = 0.36, i.e., >0.05. This indicates that there was no significant difference between simulated wind speed and measured wind speed according to the level of alpha = 0.05
Summary
The high air temperature in open cowsheds causes significant heat stress in dairy cows, which results in decreased milk production and increased reproductive problems [1,2,3]. Mixed-flow fans (MFFs) are commonly installed inside naturally ventilated dairy barns to provide sufficient airflow to cool the animals in high-temperature environment [5]. Ventilation combined with evaporative cooling is often used to mitigate the effects of heat stress [6]. Gebremedhin et al [7] indicated that evaporation is the dominant form of heat transfer for dairy cows in stressful hot environments, and Energies 2019, 12, 4315; doi:10.3390/en12224315 www.mdpi.com/journal/energies. Improving the air velocity by MFFs in open cowsheds can increase heat dissipation and relieve heat stress in dairy cows
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