Study On The Mechanism Of The Influence Of Forced Convection And Nanofluid On Downward-facing Boiling Heat Transfer

The heat dissipation problem of the RPV of nuclear reactors in severe accident has always been the important issue of researchers.IVR-ERVC uses the latent heat of the phase change generated by boiling heat transfer to effectively dissipate heat.It is an important strategy for reactor severe accident management.Widely concerned,among them CHF as a sign of deterioration of heat transfer is an important parameter in nuclear safety design.How to effectively improve CHF is the key for improving the nuclear safety.This experiment mainly discusses the mechanism of enhance the heat removal capabilities by different forced convection conditions and different concentrations of TiO2 nano fluid to the boiling heat transfer.The experiments are set up respectively,and the heating units are made of stainless steel SS-304,and the boiling of the lower heating plane is carried out.In the forced convection cases,the effects of different flow rates and flow impinging distances were mainly discussed.In the nanofluid pool boiling cases,the influence of the concentration of nanofluids and its heat removal capabilities were mainly discussed.The experimental results show that the boiling time of all forced convection conditions is longer than the time of pool boiling conditions,and the time of CHF occurs later.Increasing the flow rates or reducing the flow impinging distances could increase the heat flow of the entire boiling process to a certain extent.Density,CHF,and changes in surface flow fields are the main reasons for forced convection conditions to enhance the CHF.Through the surface experimental phenomena taken by high-speed cameras,the sliding and deformation of bubbles can be clearly observed.Regarding the pool boiling experiment of TiO2 nanofluid,different concentrations of TiO2 nanofluid have a certain delay and enhancement effect on CHF,which will change by different nanofluid concentrations,there is a maximum value of 0.005wt%,and CHF has been enhanced about 33%.The enhancement mechanism of nanofluids on CHF is mainly formed by the formation of a hydrophilic layer on the heating surface,which will adsorb the coolant easier and delay the time for the vapor film to cover.The SEM characterization can be used to observe the deposition of TiO2 nanoparticles.Through the measurement of contact angle and surface roughness,it can be concluded that the deposition of TiO2 nanoparticles will change the contact angles and surface roughness to enhance CHF.The change in contact angles plays the leading role.