摘要
为了探索一种高效且运行成本低的牛舍降温方式,该试验通过设计合理的风管布置和开口,采取上置置换通风的模式对肉牛舍降温。该设计方案用计算流体力学(computational fluid dynamics,CFD)的方法验证其可行性,并通过实际测试设定风机开启和关闭的适宜时间,试验表明:处理舍系统产生的冷风大量流向牛活动区域,温湿度梯度比对照舍明显,达到了局部降温的节能目的。该系统开启后,10:00-18:00期间,处理舍牛的平均热负荷指数(heat load index,HLI)比对照舍降低7.4(P<0.01),平均呼吸频率降低12次/min(P<0.01),平均日增质量提高0.37 kg/d(P<0.01),缓解了肉牛的热应激,提高了肉牛的养殖效益。处理舍由于相对封闭,故牛活动区域平均相对湿度、CO2和NH3浓度分别比对照舍高26.8%、252 mg/m3、0.54 mg/m3(P<0.01),但牛舍环境空气质量符合相关行业标准的要求。该试验为冷风机-风管通风降温系统设计提供了一些参考参数,有利于该系统的优化,并发挥更好的降温效果。
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.
出处
《农业工程学报》
EI
CAS
CSCD
北大核心
2014年第19期240-249,共10页
Transactions of the Chinese Society of Agricultural Engineering
基金
公益性行业(农业)科研专项基金(201303145)
关键词
降温
温度
射流
上置置换通风
肉牛舍
冷风机
纤维风管
cooling
temperature
jets
displacement ventilation with up-fixing diffusers
beef cattle barn
cooling fan
fabric air dispersion