Numerical Investigation Of Bubble Dynamics And Heat Transfer In Subcooling Pool Boiling Under Microgravity

With the rapid development of science and technology,equipment in various high-tech fields is gradually developing towards high power,integration and miniaturization,which puts forward higher requirements for heat dissipation with high heat flux density.Nuclear boiling could achieve high heat fluxes at a moderate wall superheat,which is considered to be one of the most promising ways of heat dissipation.Due to the fact that bubble growth process involved with complex mass,momentum,energy transfer and interfacial interaction,it might be difficult to study boiling heat transfer.In terrestrial boiling,buoyancy caused by the gravity and large differences between liquid and vapor densities is often considered as the governing mechanism for bubble dynamics and boiling heat transfer.Comparing to the buoyancy,the other effects are smaller such as the inertia force and the surface tension,which are ignored.Those lead to an incomprehensive understanding of the mechanism of boiling heat transfer.Microgravity environments weaken buoyancy convection caused by gravity,reveal the bubble dynamics,and provide a reliable way to investigate boiling heat transfer mechanism.In view of the low level of microgravity and the inadequacy of long-term experimental requirements in short-term microgravity experiments on the ground,while carrying on space microgravity experiment would cost much,have fewer opportunities and take longer time to prepare in the early stage,it's difficult to carry out a large number of boiling experiments.As a method of low cost and high efficiency,numerical simulation is of great significance in revealing boiling heat transfer mechanism.In this article,the vapor-liquid interface is captured by the phase field method.A thin superheated layer and the Marangoni convection caused by the surface tension variation along the surface are considered.Boiling heat transfer and bubble dynamics such as bubble growth,departing bubble,non-departing bubble have been investigated in microgravity.In the first section,a two-dimensional axisymmetric model of the single bubble is performed.The surface tension and Marangoni force have been included in the momentum equation.In addition,the continuity equation and energy equation are modified to allow for the phase change.A thin superheated layer is considered in the numerical model.The vapor-liquid interface is captured by the phase field method.Numerical investigation of single bubble behavior has been performed with the coupled equations.Besides,the grid independence test has been carried with three different grid quantities.When the phase field method is applied to two-phase flows with large density ratios,it might occur interface smearing.Therefore,a mass loss is analyzed.The mathematical model repeats the results of microgravity physics experiments in the literatures qualitatively and quantitatively.When the subcooling increases to a certain value,the bubble would adhere to the heater surface and maintain its size constantly,testifying that the model is practical and correct to simulate bubble dynamics and heat transfer in subcooling pool boiling under microgravity.In the second section,based on the model mentioned above,a 2D axisymmetric single bubble dynamics and heat transfer have been simulated.The effects of gravity level,contact angle and wall superheat on the bubble growth,critical subcooling?the liquid subcooling under the condition that the evaporation rate of a bubble is equal to its condensation rate?,together with heat transfer have been investigated.The results show that the growth period and departure radius both reduce with the increase in gravity level,while the critical subcooling increases slightly.Large contact angle at the three-phase contact line augments the departure radius.However,the critical subcooling decreases as contact angle increases.With the wall superheat increasing,the growth period reduces rapidly,while the departure radius and the critical subcooling increase.The departing bubble is a highly efficient way of heat transfer which is involved with rewetting process and wake flow.In contrast,the non-departing bubble adhering to the surface would prevent heat transfer with a dry spot,which is an inefficient way of heat transfer.In the third section,considering that bubble could move on the heater surface randomly in the experiments,the model mentioned above has been modified partially to fit in with the real physical situation.Therefore,the axisymmetric condition is removed,and a 2D model of the single bubble is performed.A thin superheated layer and the Marangoni convection caused by the surface tension variation along the surface are also considered.The variations of bubble migration on the heater surface,bubble dynamics,velocity fields,temperature fields and heat flux can be obtained by simulation.The results show that the bubble changes from a hemisphere to an ellipsoid and eventually becomes a pear shape.Owing to no axisymmetric condition,the bubble could move on the surface randomly.Besides,the bubble is asymmetrical in shape during growth period.The bubble departure diameter and departure time are proportional to g^-0.488 and g^-1.113 respectively.And the average heat flux on the heater surface is proportional to g^0.229.In the present research,the numerical simulation of a single vapor bubble growth in subcooling pool boiling under different gravity conditions has been carried out.In the numerical model,a thin superheated layer and the Marangoni convection caused by the surface tension variation along the surface are considered.The results obtained in this paper will serve as a reference for the relevant physical experiments.