超临界压力下碳氢燃料在竖直圆管内换热特性

以国产航空煤油RP-3为对象研究了超临界压力下热流密度和进口温度对碳氢燃料在竖直向上管和竖直向下管的换热特性的影响.实验中热流密度变化范围为300~600kW/m²,进口温度变化范围为293~723K,压力及流量分别保持为5MPa以及3g/s.研究表明:在所有实验工况下,实验进口处将首先出现换热恶化现象,之后随着热边界层的充分发展换热逐渐增强;当管内流体状态从超临界压力液态过渡到超临界状态,由于物性的显著变化将导致换热沿管程方向得到显著强化.当进口油温超过其拟临界温度后,由于碳氢燃料吸热能力迅速降低导致管内出现了换热恶化.对于竖直向上流与竖直向下流,即使浮升力判断因子的值小于10^-5,浮升力的影响仍然不能忽略.最后,在实验结果基础上,提出了超临界压力碳氢燃料在微细管内流动的强迫对流换热经验关系式.

Heat transfer characteristics of hydrocarbon fuel at supercritical pressure in vertical circular tubes

Experimental results of convection heat transfer to supercritical hydrocarbon fuel (RP-3) in heated vertical tubes were reported in this paper.The effects of heat flux,inlet temperature and flow direction (upward and downward) on heat transfer were studied.In the experiments,the pressure and mass flow rate were maintained at 5MPa and 3g/s the respectively;the heat flux changed from 300kW/m² to 600kW/m² and the inlet temperature ranged from 293K to 673K.Test results indicate that heat transfer deterioration appears firstly at the inlet of the heated test section and then increases after thermal boundary layer is developed totally.In addition,heat transfer is enhanced near the critical point of the fuel due to the significant changes of thermalphysics properties;when the inlet temperature of RP-3 exceeds its pseudo-critical temperature,heat transfer deterioration occurs again due to the heat capacity of RP-3 decreases rapidly,Furthermore,the heat transfer coefficient of RP-3 in downward flow is greater than that in upward flow due to the buoyancy effect even if the value of determining factor is lower than 10^-5.Based on the experimental data,correlations are developed for the local Nusselt number of convection heat transfer to supercritical hydrocarbon fuel in vertical miniature heated tubes.