Research On Flow Uniformity Of Supercritical Working Fluid In Parallel Tubes Bundles

Working fluid in superheater and reheater are in supercritical state in supercritical circulating fluidized bed boiler.The physical parameters of the working fluid especially specific heat capacity change dramatically at the critical point,which have a great impact on the flow of the working fluid in the tubes,which will lead to the tubes over temperature or even burst.To investigate flow uniformity of working fluid in parallel tubes in supercritical condition,an experimental system of parallel tubes bundles in supercritical pressure was designed by using R-134 a to simulate supercritical water according to the fluid-to-fluid modelling method.The experimental system were verified by particle tests and pure air flow tests.The flow uniformity of working fluid in the parallel tubes was studied by changing the parameters of working fluid,the parameters of circulating fluidized bed and the inlet and outlet of tubes.The main research contents and results are as follows:(1)The flow uniformity of the supercritical working fluid in parallel tubes is influenced by the heat transfer inside the tubes and outside the tubes and the pressure drop inside the tubes.The effect on both sides of the tubes is bigger.(2)The change of working fluid parameters affects the heat transfer between working fluid and tubes.The flow uniformity in the tubes decreases with the increase of inlet temperature.The increase of inlet pressure has less effect on the flow uniformity when the pressure of working fluid is higher than the critical pressure.The flow uniformity increases with the increase of the mass flow rate of the working fluid.(3)The change of parameters in circulating fluidized bed mainly changes the solid suspension density in the furnace,which affects the heat transfer between the tubes and the gas-solid phase.The flow uniformity of working fluid increases with the increase of solid suspension density.The flow uniformity of working fluid in the tubes decreases with the increase of cross section air speed when the solid suspension density is guaranteed to be constant.The flow uniformity increases first and decreases with the decrease of the secondary air rate when the secondary air speed is constant.The flow uniformity decreases with the increase of the secondary air rate when the cross section air speed is constant.The empirical formula of the flow uniformity coefficient was summarized.(4)Different inlet and outlet structures of tubes cause different pressure drop in the tubes.Due to the low flow rate of working fluid and the small size of tubes,the pressure drop change was small,the influence of pressure drop on the flow uniformity is not as great as that of heat transfer.