挡风板对低屋面横向通风牛舍内空气流场影响的PIV测试

低屋面横向通风(low profile cross ventilated,LPCV)牛舍内的空气流场由于受到舍内建筑设施的影响而分布不均匀。为了研究舍内挡风板、矮墙、入口风速、奶牛等对舍内气流的影响,分析目前LPCV牛舍内气流分布不均匀的原因,该文按模型/原型=1/15的比例制作了LPCV牛舍和奶牛的模型。计算结果表明原型牛舍在正常通风情况下的雷诺数为4.92×10~5,只要模型入口风速大于2.56 m/s,欧拉数不再随着雷诺数的增加而改变,此时空气流动已经进入自动模拟区,根据近似模型法理论,原型和模型中的气体流动已经进入了自动模拟区,两者的气体流动是相似的。根据挡风板、矮墙的设置情况、不同的入口风速等设计了6种不同工况,采用粒子图像测速(particle image velocimetry,PIV)技术分别对模型内6种工况下的空气流场进行测试。结果表明:LPCV牛舍内挡风板和矮墙的同时设置是造成舍内空气流向发生偏转的根本原因,挡风板和矮墙单独设置时都不会造成舍内气流分布的不均匀,但挡风板能够增加舍内下方奶牛活动区域的气流速度。舍内奶牛的存在和入口风速的大小都不会对舍内气流的分布造成影响。该研究可为LPCV牛舍内挡风板优化设置提供参考。

Influence of baffle on air flow field in low profile cross ventilated cattle barn analysis based on PIV test

Abstract: Low profile cross ventilated (LPCV) cattle barn has been widely used in large-scale dairy farms in China in recent years. The effective control of the indoor environment of cattle barn has significantly improved the production of the cattles. However, there are problems with airflow pattern in practice, such as uneven air distribution inside the barn. Particle image velocimetry (PIV) technology has been widely used in indoor airflow pattern investigation and similar aspects, and it was applied to study the influences on the indoor airflow pattern from the baffle, parapet, inlet air velocity and cows in this study. Based on the similitude theory, the influence of fluid viscous force is relatively weakened for viscous flow when the reynolds number increases to a specific value, after that, although the reynolds number is continuously increased, it will not affect the airflow pattern, the resulting viscous effect can be the same regardless of the changes in the reynolds number, this phenomenon is called automatic modelling of fluid. Based on this principle, the reynolds number of both the prototype and model cattle barn were calculated to fit the criteria of automatic modelling of fluid which meant that the airflow in the prototype and model could enter the automatic simulation zone and the airflow pattern of the prototype and model cattle barn was similar. A model/prototype ratio in 1/15 was selected and applied in this study, and the scaled models of LPCV cattle barn and cows were to investigate the airflow pattern inside the cattle barn. The airflow patterns under 6 working conditions were tested by particle image velocimetry using this scaled model. The tested cases included the cases with/without the baffle, cases with/without the parapet, cases with/without the cow models, and cases under 3 inlet air velocities 3.4, 3.8, and 4.0 m/s. Due to the limited capture area of the camera, the zone in a dimension of 280 mm(280 mm close to the parapet where was considered to be the most critical area was chosen for the investigated region. It was found that the baffle and the parapet in the LPCV were the cause of the deflection of the airflow in the cattle barn, the airflow entered the animal occupied area in the lower part of the barn under the guidance of the baffle, then the airflow direction was deflected to the upper space inside the barn with the influence from the parapet. Meanwhile, a low-velocity region was formed in the rear of the parapet, and this phenomenon happened only when both the baffle and parapet were installed simultaneously. When the baffle and the parapet were installed singly, the uneven distribution of the airflow in the barn was not observed, the baffle increased the airflow velocity at the lower part of the barn where the animal occupied. The presence of the cow models in the barn and the inlet wind speed had no significantly influence on the flow direction of the airflow in the whole barn, However, due to the presence of cows, when the air flow went around the cow''s back, the high-speed area of the air flow shifted upward, and the average speed of the air flow increased with the increase of the inlet speed. This study provides references for the optimal design of the airflow pattern inside the LPCV cattle barn.