Study on Themo-Fluiddynamic Coupling involved in Supercritical Droplet Combusion and Vaporization/Combustion Characteristics Prediction

研究了发生在液体燃料临界混合面附近的热-流耦合现象对液滴汽化和液体燃料射流的影响。(1)我们之前对超临界液滴汽化的研究得到了扩展，揭示了在这种气化状态下液滴经历连续相变的新特征，表明(a)气化寿命变得非常短，与液滴的温度松弛时间相当。(b)液滴的加热是过渡不可缺少的。(c)由于失去相平衡约束，燃料的扩散率大大增加。(d)无表面的运动液滴变形以扩大其表面积，从而提高汽化和混合速率。(e)被限制在室中的喷雾可能经历声共振汽化模式。(2)用线性稳定性分析和TVD模拟的方法研究了液体燃料注入压力和温度超过燃料临界值的惰性气体中的雾化、汽化和混合特性。主要发现有：(a)在高压下降低表面张力需要形成更薄的液体碎片，为此瑞利不稳定性有效地使液体雾化。(b)对射流表面附近剪切流线性稳定性的无粘分析表明，在接近临界混合条件时，由于密度变化和表面张力消失，最大增长率迅速增加到饱和值。(c)超临界流体流动的数值模拟必须考虑可压缩性（超压缩性）的影响。超临界流体流动是一种雷诺数和马赫数都可能很大的新流型。

The effects of thrmo-fluidynamic coupling phenomena which take place near the critical mixing surface of liquid fuel have been examined for droplet vaporization and liquid fuel jet flow.(1) Our previous study on supercritical droplet vaporization has been extended to reveal new features which intrinsically appear in such a gasification regime that the droplet experiences a transition to continuous phase change, showing that (a) The gasification lifetime becomes very short and comparable to the temperature relaxation time of the droplet. (b) The heating of droplet is indespensable for the transition. (c) The fuel diffusivity is greatly increased by the loss of phase equilibrium constraint. (d) A moving droplet without surface deforms to enlarge its surface area, thus enhancing vaporization and mixing rates. (e) Sprays confined in a chamber may experience an acoustically resonant vaporization mode.(2) The atomization, vaporization and mixing characteristics of a liquid fuel issued into an inert gas whose pressure and temperature exceed the critical values of the fuel have been examined by means of linear stability analysis and TVD simulation. Main findings are ; (a) The reduction of surface tension at elevated pressures requires the formation of thinner liquid fragments, for which Rayleigh instability effectively works to atomize liquid. (b) The inviscid analysis conducted for the linear stability of shear flow in vicinity of the jet surface shows that the maximum growth rate rapidly increases to a saturated value due to density variation and vanishing surface tension as the critical mixing condition is approached. (c) Since the sound speed takes a small value near critical conditions, the effect of compressibility (supercompressibility) must be taken into account for the numerical simulation of supercritical fluid flow- a new flow regime in which both Reynolds number and Mach number may take large values.