The Characteristics And Mechanism Of Bubble Evolution In High Subcooled Flow Boiling In Vertical Narrow Channel

The characters of a narrow channel include compact structure and enhancing heat transfer, etc. So the narrow channel is widely used in moving nuclear reactor. According to the thermal-hydraulic analysis of nuclear reactor, the characteristics of bubble evolution in subcooled flow boiling in the narrow channels of reactor core is significant in heat transfer. In order to ensure the reactor safety, high subcooling at the exit of narrow channels is required. Most of the studies on bubble evolution characteristics in published literature were carried out under low pressure and conventional channel. In this study, visual investigation on high subcooled flow boiling in a vertical rectangular narrow channel is carried out under different system pressure. The purpose is investigating the bubble evolution characteristics and mechanism in high subcooled flow boiling in vertical narrow rectangular channel, proposing models to predict the bubble growth, bubble condensation and coalescence based on experimental and theoretical analysis, simulating the bubble growth, motion and coalescence.It is found that the system pressure has significant effect on bubble growth. Under low pressure (p≤0.3 MPa),the bubbles grow at nucleate site and then collapses without sliding. However, under higher pressure (p≥0.6 MPa), the bubble is sliding along the heating wall during growing and grows up to a bigger size. With increasing system pressure, the bubble growth rate decreases gradually. The bubble growth rate under 0.1MPa is about 10 times of that under 1.0 MPa in this experimental study. Accordint to the analysis on the forces, the difference between the bubbles behavior under different pressure is attributed to the change in bubble grwoth force which is affect by the bubble size and latent heat needed for unite bubble volume.Based on experimental and theoretical analysis on bubble growth in high subcooled flow boiling, a bubble growth model is proposed with the consideration of bulk subcooling, velocity and including the effect of Ja,Re, correction coefficient s. This model well predicts the experimental result with an uncertainty less than±25%. The correction coefficient s indicates the effect of bulk subcooling on bubble growth. With low wall superheat, the bulk subcooling has significant effect on the bubble growth, which leads to small value of s. With high wall superheat, the bulk subcooling has slight effect on the bubble growth, which leads to a great value of s. According to the heat balance during bubble condensation, the prediction model of bubble diameter during bubble condensation is proposed and the predicted result agrees well with the experimental result.Based on the experimental and theoretical analysis on sliding bubble, it is concluded that the bubble sliding distance increases with increasing heat flux and other identical conditions. However, the bubble sliding distance tends to decrease due to bubble coalescence if the heat flux is high enough. With other identical working conditions, the bubble sliding distance decreases with increasing bulk velocity and subcooling.Based on the analysis on each heat transfer mechanism, it is found that the disturbance induced by bubbles is the main factor of heat transfer enhancement, and the fraction of heat transfer due to evaporation of microlayer beneath the bubble is not significant. Based on heat balance, a formula for the estimation of transient conduction heat flux caused by sliding bubble and stationary bubble is proposed in this study. In different experimental conditions, the fraction of transient conduction,βtcs, caused by sliding bubble increase with increasing the heat flux, and it ranges from 0 to 0.35.βtcs is much greater than the fraction of transient conduction caused by stationary bubble,βtcf. The ratio ofβtcs toβtcf ranges from 1.8 to 10.3, which indicates that the bubble sliding along heating wall enhances the local heat transfer.In order to investigate single bubble evolution further, a phase change model in subcooled flow boiling is proposed, and the UDF (User Defined Function) interface in fluent platform is applied to realize this model. The VOF (Volume Of Fluid) model combined with phase change model is adopted to simulate the single bubble growth and movement. The effects of bulk velocity, liquid subcooling, wall superheat and vapor.liquid contact angle are considered in this model. The predicted bubble growth curve agrees well with the experimental result. Based on the analysis of bubble shape evolution and temperature field, it is found that the average bubble growth rate, bulk velocity and dynamic contact angle have great effect on the bubble shape during the bubble growth and movement; while the temperature gradient in superheated liquid does not change with bubble growing.The experimental result indicates that the bubble coalescence become notable with increasing heat flux and it is affected by the system pressure significantly. Under low pressure, with not high heat flux, the bubble coalescence among growing bubbles in adjacent nucleate site and coalesced bubbles condense rapidly. While the bubble dose not condense after coalescence with high heat flux and it slides along the heating wall. Under high pressure, with increasing heat flux, the bubbles from the same nucleate site coalesce during sliding, and the coalesced bubble keeps sliding. While coalescence happens among sliding bubbles from different nucleate sites with high heat flux. According to experimental and numerical analysis, the bubble coalescence is divided into five process:i) two bubbles approach to each other;ii) thin liquid layer forms between the two bubbles;iii) thin liquid layer becomes much thin and the interface of the two bubbles contacts;iv) the contact area of the two bubbles breakup; v) the vapor in the two bubbles mixes together. The process ii, iii and iv are the key process.