LNG空温式气化器换热机理及结霜工况下的换热计算

Mechanism and calculation of heat transfer of LNG-AAV under frosting condition

ILNG空温式气化器在运行时时常伴有结霜现象,霜层恶化了传热,影响了气化器的性能。探寻霜层在翅片管表面随时间变化的规律及对气化器换热产生的影响,对LNG空温式气化器的设计和安全稳定运行具有重要意义。为此,建立了霜层内部能量和质量守恒方程,并引入霜层物性参数经验公式,得到了封闭的方程组,进而计算出了冷表面结霜后的霜层热阻随时间的变化规律。然后再考虑霜层热阻的影响,计算LNG空温式气化器内外传热传质的相互耦合规律随时间变化的动态过程,模拟结霜工况下LNG空温式气化器的运行情况,获得了不同时刻翅片管内外物性参数沿管长的分布规律。结果表明:①霜层在翅片管表面的覆盖面积可以达到80%,霜层对LNG空温式气化器换热的影响不可忽略;②除了局部由于霜层的肋片作用使换热增强,绝大多数情况下霜层会使翅片管的换热效率大幅降低,最大可降低85%,结霜后的总传热系数为80~220 W/(m^2·K);③结霜后,如果环境适宜,气化器会在很短的时间内达到另一个稳态稳定运行。

During the operation of LNG AAV （Ambient Air Vaporizer）, frosting often occurs, which may deteriorate heat transfer and thereby affect the performance of LNG AAV. How the frost growth on the surface of the finned tubes changes with time and af- fects the heat transfer of the AAV was examined to provide theoretical guidance in the design and operation of LNG-AAVs. Based on the equations of mass and energy conservation and the empirical formulas of physical parameters for the frost layer, the closed equa tions were established to calculate the thermal resistance of the frost layer on the cold surface with time. Considering the influence of the frost thermal resistance, the dynamic process of mutual coupling of the heat and mass transfer changes with time inside and out- side the LNCr-AAV was calculated, the operation of the LNG-AAV under the frosting condition was simulated, and finally the dis- tribution of physical parameters along the length of the finned tube at different time was achieved. Research results show that the frost layer can cover 80G of the surface area of the finned tubes, which means that the influence of the frost layer on the heat trans- fer of the LNG-AAV cannot be ignored. In most cases, the frost layer reduces the heat transfer efficiency of the finned tubes, even by 85%, except the local areas where the heat transfer is enhanced because the frost layer can be treated as fins, and the total heat transfer coefficient ranges in 80 - 220 W/（m2 · K） after frosting. Moreover, the AAV will reach another steady operation in a short period of time after frosting when the surrounding environment is suitable.