Heat Transfer Characteristics Of Supercritical Water In A Non-uniform Heated Internal Rifled Tube

Energy is an important foundation of modern economic and social development.With the characteristics of high heat transfer rate,low combustion temperature and small emission of pollutants,CFB technology has been rapidly promoted internationally.It is one of the most effective means to reduce pollutant emission per unit power generation and improve coal power efficiency to develop high parameter and large capacity ultra supercritical units.However,in the actual engineering equipment,the non-uniform distribution of heat flux on the water wall can lead to the temperature difference of the tube wall,and the deviation of the wall temperature is very easy to cause tube explosion or even shutdown,threatening the safe operation of the boiler.To ensure the operation safety of water wall in supercritical state has become an important development topic of supercritical circulating fluidized bed boiler.In this paper,supercritical water is taken as the research object.The numerical simulation method is used to explore the heat transfer characteristics of the fluid in the internal rifled tube.According to different types of heat flux distribution,the influence of operation conditions on heat transfer characteristics is analyzed.The main research contents are as follows:(1)Under the condition of constant heat flux applied to the internal rifled tube,the numerical simulation of supercritical water convection heat transfer in vertical rising internal rifled tube is studied,and the effects of operating pressure,mass flux and inlet temperature on supercritical water heat transfer are analyzed.The effects of operating parameters such as inlet temperature,mass flux rate and pressure on the heat transfer of supercritical water are similar under the condition of axial uniform and non-uniform heating.Therefore,the correlation initially developed under the condition of uniform heating also shows acceptable performance under the condition of non-uniform heating.(2)Under the condition that the axial non-uniform heat flux is applied to the internal rifled tube,the complete axial distribution based on the local heat flux and local heat transfer coefficient is obtained.At high q/G,due to the strong buoyancy,the axial heating has a significant effect on the flow and heat transfer.However,the sensitivity of the minimum heat transfer coefficient to the heat flux distribution is low,which can not be improved by delaying the appearance of the maximum heat flux.When forced convection and natural convection act together,the axial heating only affects the localheat transfer coefficient.(3)Under the condition of half-circumferential heating of inner rifled tube,the different heat transfer rules of applying uniform heat flux(axial and circumferential)on vertical upward inner rifled tube is compared and analyzed.The swirling effect of non-uniform heating strengthens the mixing of fluids with different temperatures.The effect of secondary flow makes the fluid near the heating surface and the low temperature fluid near the non-heating surface exchange in time,so that the heat transfer effect of the heating surface is relatively good.(4)In the case of non-uniform heat flux,the heat transfer characteristics of supercritical water are quite different from that of uniform heating.In the case of axial non-uniform heating,the axial heating mode has a significant effect on the flow and heat transfer.When the circumferential non-uniform heating,the fluid is heated more evenly,and the circumferential wall temperature difference decreases along the axial direction.Because the heat flux is no longer uniform,the existed heat transfer correlations based on the constant heat flux can not effectively predict the heat transfer.A new correlation is proposed to predict the heat transfer of supercritical water under non-uniform heating.The average relative deviation of HTC predicted by correlation is 0.5%,and the relative standard deviation is 10.5%.