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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。
The boiling heat transfer test system with equal heat flux density was set up to simulate the boiling heat transfer characteristics of R134a working fluid with equivalent diameter of 1.28mm and 1.59mm saw-toothed flat tubes. The test parameters were as follows: mass flow rate of refrigerant Is 68.5 ~ 305.5kg / (m2 · s), the working saturation pressure is 0.27 ~ 0.46 MPa, and the heating heat flux density is 9 ~ 42kW / m2. The experimental results show that the channel with the same structure has the smaller equivalent heat transfer capacity, and the influence of heat flux and saturation pressure on boiling heat transfer is related to a dryness value. When the dryness is less than this value, the boiling heat transfer coefficient will increase with the increase of heat flux and saturation pressure. When the dryness value is higher than this value, the boiling heat transfer coefficient decreases sharply with the increase of dryness. Density and saturation pressure have little effect on heat transfer; the dryness value decreases with increasing heat flux or saturation pressure. The effect of mass flow rate on boiling heat transfer is related to the heat flux. As the heat flux density increases, the mass flow rate tends to affect the dry mass fraction. By analyzing the influence of each parameter on the boiling heat transfer, an empirical formula for calculating the boiling heat transfer coefficient of a jagged flat tube with a small size under a predictive test condition was established.