Numerical Study On Subcooled Flow Boiling In Inclined Tube

Due to the existence of convection,heat transfer and phase transition in the subcooled flow boiling,the flow heat transfer characteristics in the tube are very complicated.At present,researches of subcooled flow boiling mainly focus on the analysis of macroscopic characteristics,such as heat transfer coefficient and void fraction.The mechanism of flow and heat transfer cannot be accurately obtained only by analyzing the macroscopic characteristics.It is necessary to study the microscopic phenomena of subcooled flow boiling in combination with the behavior of bubbles.On the other hand,in practical engineering applications,subcooled flow boiling under inclined condition may be encountered.Under the condition of inclination,the characteristics of flow and heat transfer will change greatly.Therefore,it is necessary to study the subcooled flow boiling in inclined tube.Using the VOF model,this paper conducts a numerical study on the subcooled flow boiling phenomena in vertical and inclined tube.Numerical model of subcooled flow boiling in a circular tube and a subchannel was established.The characteristics of flow and heat transfer were analyzed in terms of void fraction,physical parameters field distribution and bubble behaviors.The simulation results are in good agreement with the experimental data,which proves the accuracy of the numerical model.In subcooled flow boiling,complex bubble behavior is an important factor affecting the flow and heat transfer mechanism.When bubbles are formed at the wall surface,the local heat transfer coefficient increases suddenly and the wall temperature drops drastically.With the increase of height,the sectional void fraction increases gradually,and the bubble behaviors become more complex.There are several bubble behaviors in the upper section,such as sliding,detachment from the wall,entrainment,coalescence and rupture.Under the influence of the bubble behavior,the flow velocity of the fluid in the tube increases and the heat transfer is enhanced.In the inclined tube,the bubble behaviors change under the influence of the radial component of buoyancy force.On one hand,the departure time of the bubble from the wall changes.On the other hand,the bubble will move laterally in the tube.Due to the change of bubble behaviors,the flow and heat transfer characteristics in inclined tube are different from those in vertical tube.The transverse movement accelerates the condensation of the bubbles,which leads to the reduction of the cross-section void fraction.When the inclined angle of the tube is large,the wall at the higher segement will be covered by bubbles,the heat transfer at the wall will deteriorate,and the wall temperature increases significantly.Due to the influence of the geometric structure,the distribution of vapor and liquid in the subchannel is quite different from that in the circular tube.In the subchannel,the bubbles only exist near the heating surface,there is no bubbles in the bulk flow and the connected areas.Compared with the vertical sub-channel,the bubble in the inclined subchannel will slip laterally along the heating surface.In addition,the transverse flow fields in the vertical and the inclined subchannel are different.In the vertical condition,the secondary vortices at the cross section appear with the generation of the bubbles,while in the inclined condition,there are secondary vortices in the single-phase flow region,and the position of the secondary vortices changes with the generation of bubbles.