车用小通道平行流换热器传热流动试验与分析

小通道平行流换热器是燃料电池汽车的主要散热部件。吸收了电堆废热的冷却液(50%乙二醇溶液),流过小通道换热管,由换热器外侧空气冷却。在进液温度、进风温度、冷却液流量以及风速变化的试验工况下,测试了换热器的传热流动性能。引入量纲一参数k,评估了各工况参数对换热量、阻力影响的强弱。接着,分析液侧努谢尔数Nu和摩阻系数f随雷诺数Re的变化趋势,结果显示:在小通道内(当量直径D=2.685 mm),冷却液从层流到湍流的转浪点Re_c=1 750,介于微尺度与常规尺度的临界值之间。在此基础上,通过多元回归法,拟合得到层流和湍流的液侧换热系数,摩阻系数的关联式,以及空气侧阻力f_a公式。Nu和f的计算值与试验值误差分别在-7.06%,5.93%]和-3.95%,4.11%]内,f_a的误差在-2.22%,3.62%]内。基于这些关联式,建立数学模型,可在广泛多变的运行条件下,对换热器的运行性能进行理论预测和评估。

Experimental and Theoretical Analysis for Heat Transfer and Flow Performance of Mini-channel Parallel Flow Heat Exchanger in Vehicle

The parallel flow heat exchanger with mini-channel is used as the major radiator for fuel cell vehicle (FCV) to dissipate the waste heat, product by fuel cell stack, from the coolant, a glycol-water mixture with volume concentration of 50%, to the air. Under the operating conditions with varied inlet temperatures of air and coolant, coolant volume flow rate and air velocity, the flow and heat transfer characteristics of the heat exchanger is investigated experimentally. Then, a dimensionless parameter,κ, is defined to quantify the influences of the condition parameters on heat transfer and pressure drop. The trends of coolant-side Nusselt numberNu and friction factorf with Reynolds numberRe are investigated. It is observed that the flow transition from laminar to turbulent in the mini-channels (D=2.685 mm) of the heat exchanger occurred at the critical Reynolds number ,Rec=1 750, which is lower than that in conventional channels and higher than that in micro-channels. Furthermore, the coolant-side correlations are developed in different forms for laminar and turbulent flows to predict the heat transfer coefficients and friction factors. The correlation of air-side friction coefficientfa is also present. Great agreements between the predicted and experimental values are observed. The deviations ofNu and fare within ?7.06%,5.93%] and ?3.95%,4.11%]. And the errors offa ranged from?2.22% to 3.62%. Base on these correlations, a mathematical model is built to predict performance of FCV radiators under various operating conditions, especially some extreme but significant conditions that could not realize at labs.