Research On Bubbles Effection On Heat Transfer Characteristics For Subcooled Flow Boiling In Narrow Rectangular Channel

In narrow rectangular channel,the bubble can't develop freely because of the channel size.After detachment from the nucleation points,bubble will slide long the heating wall.In the process of growth,departure and sliding,bubbles will affect the heat transfer between the mainstream and the heating wall.This paper focuses on the role of bubbles in heat transfer characteristics of rectangular channel,and on the basis of experimental data,the impact of bubbles in heat transfer abilities is measured quantitatively.In experiments deionized water is used as the working fluid,and the test section is a visible one-side heating rectangular channel.The cross-sectional dimension is 40 mm × 2mm,the working fluid is flowing upward in the vertical channel.The high-speed photography is used to capture and record the bubble behaviors at both of atmospheric pressure and high pressure.A set of self-writing Matlab code and software are used to analyze videos,and bubble diameters are acquired.Based on the bubble behaviors characteristics,a compound boiling heat transfer model in a narrow rectangular channel is built,in which the effect of the sliding bubble is involved.The model includes bubble growth decalescence,transient conduction caused by departure and sliding,microlayer evaporation of sliding bubble as well as single-phase convection on the non-bubble area.Combined with experimental data,the impact of a single bubble in heat transfer is evaluated by the heat transfer model at both of atmospheric pressure and high-pressure.Further,the coupling impact between bubble cluster and heat transfer abilities on rectangular channel is analyzed quantitatively.Bubbles at atmospheric pressure with large diameter and short sliding distance are influenced strongly by main flow.Transient conduction caused by departure and sliding,microlayer evaporation of sliding bubble are the main heat transfer mechanism on the bubble area.Besides,the value of transient conduction and microlayer evaporation caused by sliding bubble increases with bubble departure diameters.Bubbles at high pressure have tiny diameters and do not lift off.After departing the nucleation site they slide along the heating wall with the increasing diameter and speed.Transient conduction caused by sliding is the main heat transfer mechanism on the bubble area.Besides,the value of transient conduction increases with sliding distance.For bubble cluster,more bubbles generate with higher heat flux,resulting in fiercer disruption on the thermal boundary layer,thus the heat transfer abilities is affected.Along with increasing the heat flux,the total projection area and average sliding speed increase,and the area affected by bubbles increases as well,which give a significant rise to heat transfer caused by sliding bubble cluster,and boost the share of the total heat exchange between the mainstream a nd the heating wall.At the high heat flux,heat transfer caused by sliding bubbles cluster is the main mechanism of heat transfer.In addition,compared to the pure single-phase convection,sliding bubbles do enhance the heat transfer capability between the main stream and the heating wall.