单气泡池沸腾传热中的重力效应数值模拟

Numerical simulation of gravity effect on heat transfer in single bubble pool boiling

本文数值模拟了不同重力条件下饱和状态的FC-72在厚度5 mm的SiO2固壁上的单气泡池沸腾传热现象及相应的气泡动力学和传热性能。固壁底面给定均匀过热度10 K,其瞬态热响应被考虑在内。通过多个气泡周期的计算,得到了准稳态的沸腾过程。结果表明,在小热流密度下,气泡脱落直径Db反比于重力的1/2次方,且与Fritz模型一致;气泡脱落频率f则正比于重力。传热特性存在2个明显不同的区域,即重力相关区和重力无关区,其分界位置约为0.03g0:高于该临界重力时,热流密度与重力相关,恒定过热度下热流密度与重力呈确定的指数函数关系;小于该临界重力时,热流密度与重力无关。临界重力值对应于基于Laplace长度的无量纲加热器长度约2~3之间,与Raj-Kim-McQuillen重力标度模型建议的2.1相近。重力相关区标度指数随着过热度增大而单调增大,但明显大于Raj-Kim-McQuillen重力标度模型预测结果。研究结果还表明固壁瞬态热响应对重力无关区传热性能的影响比重力相关区更为明显。

The bubble dynamics and heat transfer in single bubble pool boiling of saturated FC-72 at different gravity levels are studied numerically,having transient thermal response of the heated SiO2 wall with a thickness of 5 mm.A constant and uniform temperature corresponding to a superheat of 10 K is prescribed at the bottom surface of the solid wall.Multi-cycle simulations are conducted and quasi-steady state of boiling process is achieved.It is observed that the bubble departure diameter,inversely proportional to the square root of gravity,is consistent with the Fritz model,and the bubble frequency is proportional to the gravity.There are two regions with obvious differences in terms of heat transfer,namely a gravity-dependent region and a gravity-independent region.The corresponding transition or critical value of the gravity is about 0.03 g0.The heat flux has a power law relation with the gravity at constant superheat if the gravity is larger than the critical value,while this trend will break down when gravity is less than the critical value.The boundary between these two regions is located in the range of dimensionless heater length based on the Laplace length from 2 to 3,which is close to 2.1 proposed by Raj-Kim-McQuillen model.In the gravity related region,the exponent of the power law relation increases monotonously with the superheat,but its value is obviously larger than the prediction of Raj-Kim-McQuillen model.It is also found that the transient thermal response of the solid wall has a more obvious effect on the heat transfer performance in SDB region than in BDB region.