| Steam-water two-phase flow and boiling heat transfer are widely used in heat exchange equipment such as waste heat utilization in nuclear reactor,sewage treatment in chemical industry,aerospace thermal control system and so on.With the demand of new heat exchangers tending to be compact,modular and highly integrated,narrow rectangular channels have the advantages of compact structure,good safety,high allowable heat flux and high heat transfer per unit volume,which have attracted great interest and extensive attention from experts and scholars at home and abroad.At present,most of the research on flow and heat transfer in narrow rectangular channels are single-sided heating and double-sided heating.A series of closed cross-section heating channels with three-sided heating combination often appear in water-cooled radiators in engineering.Because of the narrow-sided effect,the thermodynamic balance and hydrodynamic balance of two-phase fluids in the channels are affected,and the thermodynamic performance of two-phase fluids is different.There are few reports on three-sided heating.Based on this research background,the research on the boiling heat transfer and flow characteristics of steam-water two-phase flow in three-side heated narrow rectangular channel has certain academic research significance and engineering application value.Firstly,an experimental study of steam-water two-phase flow in a three-side heated narrow rectangular channel is carried out,by combining the narrow rectangular channel heated on three sides with a high-pressure steam-water experimental platform in this paper.The slug flow in a three-side heated narrow rectangular channel is studied in depth,based on visual observation and high-speed camera.A new flow pattern was found under the condition of three-sided heating narrow rectangular channel.Four evolution behaviors of the slug flow were observed,which were the full development of slug flow,the appearance of sandwich-type slug flow,the behaviors of merging small slug into large slug and the behaviors of merging large slug into elongated slug.Secondly,experimental datas were obtained on the basis of the study of slug flow pattern.The influence of some thermal parameters,such as heat flux,mass flow rate and inlet subcooling,on the void fraction of slug flow section is analyzed.The results show that the influence of heat flux density on void fraction is basically the same as that of pipe flow.Mass flow velocity has a great influence on void fraction,while inlet subcooling has a relatively small influence on cross-section void fraction.At the same time,by introducing Reynolds number,a formula for calculating the void fraction of slug flow in narrow rectangular channel heated on three sides is obtained.Finally,saturated boiling starting point(FDB)conditions were obtained based on experiments.The effects of heat flux,inlet subcooling and mass flow rate on the starting point of saturated boiling were analyzed and compared with single-side heating channel.The results show that the moving trend of saturated boiling starting point of narrow rectangular channel heated on three sides is similar to that of narrow rectangular channel heated on one side.In addition,the wall superheat at the saturated boiling starting point was quantitatively analyzed.The results show that the inlet subcooling has no significant effect on the wall superheat at the starting point of saturated boiling,and the mass flow rate has a greater effect on the wall superheat at the starting point of saturated boiling.At the same time,by modifying the traditional formula of heat flux at the starting point of saturated boiling,a formula for calculating the heat flux at the starting point of saturated boiling in narrow rectangular channels with three-sided heating is obtained.Comparing the calculated results of the new formula with the existing experimental data,it is found that the new formula can be extended to a narrow rectangular channel heated on one side. |
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