甲烷液滴在其蒸气中自然对流的换热特性

Natural convection heat transfer characteristics of methane droplets in their vapor in the precooling process of an LNG tank

LNG液舱喷雾预冷过程中,液滴在其蒸气中自然对流蒸发换热模型计算结果与实验数据之间存在着较大的差异。为此,采用计算流体力学的方法,建立了静止饱和LNG单液滴在其蒸气中自然对流蒸发模型,据此研究不同温差下液滴的蒸发传热特性,并与低温球模型进行对比,定量分析喷雾预冷过程中"吹拂效应"对液滴自然对流蒸发换热过程的影响。研究结果表明:①随着温差的增大,液滴表面蒸气喷发速度、温度边界层厚度、液滴换热量线性均增大,努塞尔数线性减小;②随着角度的增大,液滴局部蒸气喷发速度、局部努塞尔数逐渐减小,温度边界层厚度逐渐增大;③吹拂效应的影响与蒸气喷发速度呈正比;④因为吹拂效应,同等条件下液滴的换热量和努塞尔数相比于低温球要更小,温度边界层厚度相比于低温球要更厚,并且吹拂效应的影响随着温差的增大而线性增大。结论认为,该研究成果有助于更加准确地预测LNG液舱预冷过程,对完善液滴蒸发理论模型及指导LNG液舱预冷操作都具有重要的意义。

The model calculation result of the natural convection and evaporation heat transfer of droplets in their vapor in the precooling process of an LNG tank by spray is more deviated from the experimental data. To solve this problem, this paper established a natural convection and evaporation model for the static saturated LNG single droplets by means of the computational fluid dynamics. Then, the evaporation heat transfer characteristics of droplets with different temperature differences were investigated in this model and compared with the calculation results of the cryogenic sphere model. Finally, the influence of the "blowing effect" on the natural convection and evaporation heat transfer of droplets in the precooling process by spray was analyzed quantitatively. And the following research results were obtained. First, with the increase of temperature difference, the vapor eruption velocity on the droplet surface, the thickness of the temperature boundary layer and the heat transfer of droplets increase linearly, while the Nusselt number decreases linearly. Second, with the increase of angle, the local vapor eruption velocity and Nusselt number of droplets decrease gradually, and the thickness of the temperature boundary layer increases gradually. Third, the influence of blowing effect is proportional to the vapor eruption velocity. Fourth, due to the influence of blowing effect, the proposed model presents lower droplet heat transfer and the Nusselt number and the thicker temperature boundary layer than the cryogenic sphere under the same conditions. The influence of blowing effect increases linearly with the increase of temperature difference. In conclusion, the research results are helpful to predict the precooling process of an LNG tank more accurately and of great significance to improving the theoretical model of droplet evaporation and guiding the precooling operation of an LNG tank.