竖直上升气液泡状流数学模型封闭的研究

Comparative investigation on closure models for the simulation of vertical gas-liquid bubbly upflow

竖直上升管气液两相流广泛应用于相变传热、核反应堆等工业过程。本文以竖直上升气液两相流为研究对象,运用欧拉双流体模型,针对表观液速为0.45m/s、表观气速分别为0.015m/s和0.1m/s的泡状流数值模拟过程中的升力、壁面润滑力、湍流扩散力、气泡诱导湍流(BIT)等封闭模型,开展数值模拟比较研究。模拟发现:1低气速泡状流中,升力和壁面润滑力的同时加入能够改善壁面附近的气含率,气泡在这两个力作用下在径向上达到一个相对平衡,得到与实验气含率类似的壁面峰,模拟的液相速度较合理;低气速时,BIT的影响可以忽略。2高气速泡状流中,BIT对气-液两相流的模拟结果影响比较明显,湍动耗散源项的加入能使液速分布的模拟结果得到改善,Troshko模型相对Sato模型更能反映气泡诱导湍流对液相湍流的作用。3高气速时升力的引入使气含率产生壁面峰,加入湍流扩散力能使峰值略微降低,但仍没有解决高气速时引入升力出现的气含率壁面峰问题,说明在径向上湍流扩散力还不足以抵抗升力。

Gas-liquid upflow in vertical pipes was widely used in the phase change heat transfer and nuclear reactors. In this paper,the Euler-Euler two-fluid model was applied in the simulation of the gas-liquid upflow with the emphasis on the selections of the closure models. The lift force,wall lubrication force,turbulent dispersion force and bubble induced turbulence（BIT） were studied at a superficial liquid velocity 0.45m/s and superficial gas velocities,0.015m/s and 0.1m/s,respectively. The simulation results showed that：①At low superficial gas velocity,the near wall gas holdup can be optimized with both the lift force and wall lubrication force taken into account. The bubbles reached a relative balance under the effect of this two forces with the result of wall peak of gas fraction and the reasonable simulated liquid velocity;The effect of BIT can be negligible at low superficial gas velocity. ②At high superficial gas velocity bubbly flows,the influence of BIT on the simulation results of gas-liquid two phase flow was evident. The simulation results can be improved with the turbulence dissipation source term included,and Troshko model prevailed over Sato model in describing the effect of bubble induced turbulence on the liquid turbulence. ③At high superficial gas velocity,the wallpeak of gas holdup occurred with the lift force included,then the peak value can be reduced after the turbulent dispersion force taken into account. While the issue of wall peak still can＇t be solved,indicating that turbulence dispersion force was not strong enough to overcome the lift force.