Design and cooling effect of cooling fan-duct displacement ventilation system with up-fixing diffusers in beef cattle barn

Abstract: In order to search for an effective cooling system that is energy saving with low running cost in beef cattle barns, this study investigated the displacement ventilation with up-fixing diffusers system to reduce heat stress of grain-fed beef cattle using an evaporative cooling air conditioner combining fabric air dispersion with rational layout and opening design. Four wet curtain cooling fans with air volume at 12 000 m3/h and 1 kW of electric power were used as the cooling source. Along the wall, four fabric ducts were connected to one cooling fan paralleled to the longitudinal wall. The diameter of the duct was 0.8 m and the bottom of the duct was 2.1 m from the floor. The duct was designed with twenty rows of big holes with 4.0 mm of diameter and 10 degrees to the vertical direction, twenty rows of small holes with 2.0 mm of diameter and 10mm from the big hole respectively. Both small and big holes directed the cooling air to the cattle bodies. As a result of the designing, the wind velocity gradient was developed at the same distance from the openings, which could avoid large jet entraining and thereby facilitated the displacement ventilation. The design of this experiment was validated with numerical simulation based on computational fluid dynamics (CFD), and the time of running and stopping the cooling system was set according to experimental measurement. Wind speed around the cattle was 0.5 m/s faster than the wind speed of feeding aisle at average. However, wind speed near the floor was less than 0.5 m/s which fits the requirement of displacement ventilation. The temperature and humidity difference of the vertical height between 2 m and 3 m from the floor in treatment barn were (1.11±0.11)℃ and6.1%±0.5% respectively, while the difference in the control barn between 2 and 3 m from the floor were (0.6±0.05)℃ and 3.9%±0.3% respectively. The results showed that cooling air from the fabric air dispersion mainly flowed to the area where the cattle stood, and that the temperature and humidity gradient in the treatment barn was larger than the control barn with using the ceiling fan, thus the system achieved locally cooling and reduced the energy waste. In addition, during 10:00-18:00, the average temperature and humidity of treatment barn were (31.5±0.7)℃ and 78.6%±4.4% respectively，whereas those of the control barn were (36.4±1.4)℃ and 51.8%±10.3% respectively. There was a significant difference of average temperature and humidity in both the treatment and control barns (P<0.01). Compared with the control barn, during 10:00-18:00, the heat load index (HLI) was 8.1 lower in the treatment cattle barn, the respiration rate and average daily body weight gain was significantly different by reducing 12 breaths per minute (P<0.01) and increasing 0.37 kg (P<0.01) in the treatment cattle barn, respectively, which demonstrated this design of ventilation could be effective to alleviate heat stress and improved the production performance of the beef cattle. However, the relative humidity and the concentration of CO2 and NH3 in the treatment barn was higher compared to the control barn, as a result of the shortage of air circulation outside, but the air quality in the treatment barn still could meet the requirement of relative industry standards. This study could provide parameters and suggestions for cooling fan-duct cooling system design, thus optimize and improve the cooling effect of the system.