【作者】 黄丹；

【导师】 李蔚； Bengt Sunden；

【作者基本信息】 浙江大学 ， 热能工程， 2016， 博士

【摘要】 超临界压力下,流体的物性在拟临界温度附近变化迅速,粘度随着温度的升高不断下降,导致流体流速增大,湍流度增大；流体比热也迅速增大,直到最大值,使得相同温升下流体能够吸收更多热量。这些物性的变化都使得超临界压力下流体的换热常常得到强化。因此,研究者们对超临界压力下流体的换热性能进行了大量的研究。以往的研究对象通常是水、二氧化碳等无机物,近年来随着高超声速飞行器,火箭以及导弹发动机等的发展,超临界压力下碳氢燃料的传热研究越来越多。目前超高声速飞行器所面临的一大棘手的问题就是发动机等壁面的冷却问题。而再生式冷却系统--发动机燃料(如航空煤油)作为冷却剂,通过自身的比热容、气化潜热以及化学裂解吸热,储存和转移发动机燃烧室壁面的热量,再在燃烧室通过燃烧释放出来--被很多研究者认为是一种既有效又可行的冷却方法。本文以航空煤油为研究对象,在现有的实验设备下,通过实验和数值模拟相结合的方法,研究各因素(质量流量,热流密度,压力,入口温度,管径和流动方向等)的变化对超临界压力下航空煤油在竖直光滑管内换热的影响,并分析这些因素影响煤油换热的机理。通过数值模拟探究煤油在超临界压力下的传热恶化现象,并分析传热恶化产生机理。最后探究煤油在强化管内的换热特性,并与同工况下竖直光滑管内的换热特性进行比较,以期得到进一步强化煤油换热和有效抑制传热恶化的方法。实验的结果表明：换热系数随着质量流量和入口温度的增大而增大；而热流密度对煤油换热的影响比较复杂,流体温度升高带来的换热强化效果与壁面温度升高带来的换热恶化效果相互竞争；而压力对煤油换热的影响较小,不同压力下流体温度和壁面温度几乎相同,换热系数的差别也不是很大。而数值模拟的结果表明：相同工况下(相同入口雷诺数,壁面热流密度等),换热系数随着管径的减小而增大；煤油在1.8 mm管道中时,竖直向下流中的换热要强于竖直向上流的换热。在1 mm管道中时,竖直向上流和竖直向下流的换热没有明显差别；煤油在较低质量流量,较高热流密度,较低压力以及较高入口温度时,容易发生换热恶化现象。本论文中,观察到了两类换热恶化现象。第一类热恶化现象发生在壁面温度超过拟临界温度时,第二类换热恶化现象发生在流体温度超过拟临界温度时。当质量流量足够大或热流密度足够小或压力足够大时,换热恶化现象就会消失；强化管内由于不对称肋片的存在,壁面温度显著降低,且换热系数远高于相同工况下光滑管内换热系数。更多还原

【Abstract】 The thermo-physical properties of fluids at supercritical pressures vary significantly at pseudo-critical point. The viscosity drops dramatically, thus the fluid velocity increases and the turbulent intensity increases; the heat capacity also increases continually to a maxium value, which allows the fluid to absorb more heat for a given temperature. These variations of thermo-physical properties benefit the heat transfer of supercritical fluids and thus researchers focused on the heat transfer of supercritical fluids. However, early studies mainly focused on heat transfer performance of supercritical water and carbon dioxide. In recent years, with the development of advanced hypersonic aircraft, rocket and missile engines etc, the interest on heat transfer characteristics of hydrocarbon fuels at supercritical pressures is also increasing. In order to improve the cooling efficiency of heat transfer systems, the regenerative cooling system, where engine fuel (e.g. aviation kerosene) works as coolants and travels through the cooling tubes along the chamber wall, is developed as an effective thermal management technique.In this paper, the effects of important parameters (such as mass flow rate, heat flux, pressure, inlet temperature, tube diameter and flow direction) on heat transfer of aviation kerosene flowing in a vertical smooth tube were studied by expereimental and numerical methosds. The parametric effects were analyzed by experimental data and simulation results. The heat transfer deterioration was also numerically investigated. Then, the heat transfer of aviation kerosene flowing in enhanced tubes was studied, and the simulation results were compared with that flowing in smooth tubes at the same working condition.The experimental results show that heat transfer coefficient increases with increasing mass flow rate and inlet temperature. The effects of heat flux on heat transfer are complicated. The enhanced effects caused by fluid temperature compete with the deteriorated effects caused by wall temperature. The effect of pressure on heat transfer is rather small. The difference of fluid temperature, wall temperature and heat transfer coefficient under different pressures is small.The simulation results show that the heat transfer coefficient increases with decreasing tube diameter when the working condition is the same (e.g. the same inlet Reynolds number, the same heat flux, pressure and inlet temperature). The heat transfer in downward flow is better than that in upward flow in a 1.8 mm tube. However, there is no much difference in heat transfer coefficient between downward flow and upward flow in a 1 mm tube. Heat transfer deterioration was observed at low mass flow rates, high heat fluxes, low pressures and high inlet tempreratures. Two kinds of heat transfer deterioration were observed. The first kind of heat transfer deteorioration occurrs at where the inner wall temperature is higher than the pseudo-critical temperature, while the second kind of heat transfer deterioration occurrs at where the fluid temperature is higher than the pseudo-critical temperature. When the mass flow rate is high enough, or the heat flux is small enough or the pressure is high enough, the heat transfer deterioration would disappaear. With the existence of asymmetric fins, the heat transfer coefficient in the enhanced tubes is much higher that in the smooth tube.更多还原