锯齿扁管内沸腾换热试验

建立以等热流密度方式进行试验件加热的沸腾换热试验系统,分别对当量直径为1.28mm和1.59mm锯齿扁管内R134a工质的沸腾换热特性进行研究,试验参数范围:制冷剂质量流率为68.5~305.5kg/(m2·s),工作饱和压力为0.27~0.46 MPa,加热热流密度为9~42kW/m2。试验结果表明:相同结构的通道,当量直径小换热能力更强;热流密度和饱和压力对沸腾换热的影响与一个干度值有关。当干度小于此值时,沸腾换热系数会随着热流密度及饱和压力增大而增大;而当干度大于此值时,沸腾换热系数随着干度增大而急剧下降,热流密度和饱和压力对换热的影响较小;该干度值会随着热流密度或饱和压力增大而逐渐变小。质量流率对沸腾换热的影响与热流密度有关,随着热流密度增大,质量流率的影响趋向大干度区域。通过分析各参数对沸腾换热的影响,建立了一个预测试验工况下微小尺寸锯齿扁管的沸腾换热系数计算经验公式。

Test of boiling heat transfer in rectangular channels with offset fins

A boiling heat transfer test system is built to investigate the boiling heat transfer characteristics of two kinds of rectangular channels with offset fins,with R134a as the working medium.The hydraulic diameters of the channels are 1.28 mm and 1.59 mm.The experiments are performed at mass flow rate of refrigerant between 68.5 and 305.5 kg/(n2 · s),heat flux between 9 and 42 kW/m2,and saturation pressure between 0.27 and 0.45 MPa,with constant heat flux heating mode.The results show that the channel with smaller hydraulic diameter has stronger heat transfer capability.The influences of heat flux and saturation pressure on boiling heat transfer are related to a value of vapor quality.If the vapor quality is less than this value,the increase of heat flux or saturation pressure would lead to the increase of heat transfer coefficients.If the vapor quality is greater than this value,the boiling heat transfer coefficient will sharply decrease with the increase of vapor quality,and the influences of heat flux and saturation pressure will be much weaker.The value will gradually decrease with the increase of heat flux or saturation pressure.The effect of mass flow rate on boiling heat transfer is related to heat flux.With the increase of heat flux,the influence of mass flow rate tends to the region with greater vapor quality.On the basis of the analysis of these parameters,a new correlation is proposed to predict the boiling heat transfer coefficients of the small sized channels with offset fins under the experimental conditions.