Numerical Study Of Flow And Heat Transfer Of Supercritical Water In Vertical Tube Heated By One Side

Fluids have better heat carrying capacity under supercritical pressure.The(ultra)supercritical circulating fluidized bed(SCCFB)power generation technology uses supercritical water as the heat transfer working medium,with advantages such as high coal power efficiency and low pollutant emissions from coal combustion,and has been widely used at domestic and foreign.However,when the unit operates under supercritical pressure,the physical properties of the working medium change dramatically,and the flow and heat transfer of the working medium in the water wall are very complex,making it difficult to monitor and prevent the deterioration of heat transfer.Therefore,as an important component of thermoelectric systems,studying and improving the hydrothermal performance of water cooled walls is the key to ensuring the efficient operation of SCCFB power generation units.Smooth tube and internally ribbed tube have been widely studied as the two most common water wall technologies.However,a large number of research results have been obtained under the condition of full cycle uniform heating,which is inconsistent with the actual operation of non-uniform heating on one side of the water wall.Therefore,this article takes smooth pipe and internally ribbed tube as the research objects,uses SST k-co Turbulence model,calls the REFPROP dynamic library to achieve variable physical input of the working medium,numerically studies the flow and heat transfer characteristics of supercritical water in a tube under half cycle non-uniform heating.The main research contents are as follows:(1)Under different heat flux densities,a numerical simulation study was conducted on the convective heat transfer of supercritical water flowing vertically upward in a half cycle nonuniformly heated smooth tube.The overall and local heat transfer performance,temperature distribution in the fluid and solid regions of the tube were analyzed,and three dimensionless buoyancy criteria for predicting heat transfer deterioration were evaluated.The heat transfer coefficients of the cold and hot side walls vary greatly,and the overall heat transfer performance is intermediate.The cold side heat transfer performance is positively correlated with heat flux density,while the heat transfer performance of the hot side and the overall heat transfer performance will deteriorate due to the influence of buoyancy at high heat flux densities.The temperature distribution in the fluid and solid domains is very uneven,and the fluid mixing on the hot and cold sides is poor.The initial threshold values of the in-tube buoyancy criterion numbers BoM and BoJ should be corrected to 1.6 × 10-4 and 8 × 10-4;Bop is not applicable to the calculation of dimensionless buoyancy in this study.There is an asynchronous relationship between the decrease in heat transfer performance and the emergence of buoyancy in the tube.Only when the buoyancy increases to a certain extent,does the overall heat transfer performance begin to decline.(2)The flow and heat transfer characteristics of vertically ascending supercritical water in a half cycle non-uniformly heated internally ribbed tube and a full cycle uniformly heated internally ribbed tube were compared under conditions above and below the pseudo critical heat flux.The maximum inner wall temperature of the half cycle non-uniformly heated internally ribbed tube is 10 K lower than that of the full cycle uniformly heated internally ribbed tube,and the circumferential and radial heat transfer performance is significantly improved.Half cycle heating can better suppress buoyancy,maintain forced convection heat transfer in the tube,and improve the turbulence intensity in the logarithmic rate region(y+>30),thereby enhancing heat transfer.The dangerous point analysis and quantitative prediction of the temperature distribution on the inner wall of a half cycle non uniformly heated internally ribbed pipe were carried out.The maximum inner wall temperature does not always occur at the center point of the heating side,but is most likely to occur at the intersection of the bottom of the screw near the center point of the heating side and the windward side of the screw rib.Combining the empirical correlations of Jacson and Mokry,a calculation formula for predicting the maximum inner wall temperature in a pipe is proposed.(3)The effects of internal screw structure parameters on the dynamic heat transfer performance of vertically rising supercritical water flow in a half cycle non-uniformly heated internal ribbed tube were investigated under conditions of above and below the pseudo critical heat flux.The temperature of the cold side wall is less affected by the structural parameters,while the heating side wall has a significant dependence on the structural parameters.Compared with low heat flux density,the influence of structural parameters on the heat transfer performance of supercritical water at high heat flux density is more obvious.At high heat flux,the wall temperature decreases as the screw height increases and the screw pitch decreases.Unlike full cycle heating,the geometric shape of the screw thread has little impact on heat transfer,and rectangular internally threaded tubes have better heat transfer performance.The average swirling intensity at cross sections varies significantly with different threaded structural parameters,but only the swirling intensity in the area of 30<y+<100 can correctly reflect the impact of structural parameters on heat transfer performance.Enhancing the turbulence intensity in this area is an important path for optimizing the water cooled wall of internally threaded tubes.