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In this study, numerical simulation of flow and heat transfer of S-CO2 (supercritical CO2) in the inclined upward and downward smooth tubes with an inner diameter of 10 mm and a heating length of 2000 mm at an inclination angle of 45° were conducted. The SST k - ω low Reynolds number turbulence model was used, and the accuracy of the calculation model was verified by the experimental data of S-CO2 flowing upward in the vertical smooth tube. The variation characteristics of the axial and circumferential inner wall temperature T w,i and the convective heat transfer coefficient h were analyzed. The S-CO2 would undergo the phase change from a liquid-like to the vapor-like at the pseudocritical temperature T pc by assuming supercritical pseudo-boiling. Based on this assumption, the reasons for a difference in the wall temperature distribution at the top generatrix during S-CO2 flowing in different directions were studied. By the detailed analysis of the velocity distribution, thermophysical properties distribution, including specific heat capacity at constant pressure cp and thermal conductivity λ , and the turbulent kinetic energy k distribution in the cross-sections of the inclined circular tube, the heat transfer mechanism related to the wall temperature difference at upward and downward was studied. The results showed that the thickness of vapor-like film, axial velocity u , turbulent kinetic energy k , and the thickness of viscosity sub-layer δ y +=5 were the main factors affecting the difference in the wall temperature distribution at the top generatrix in different flow directions.

The objective of this paper is to provide the optimal choice of single-reheating or double-reheating when considering residual flue gas heat in S-CO2 coal fired power system. The cascade utilization of flue gas energy includes three temperature levels, with high and low temperature ranges of flue gas heat extracted by S-CO2 cycle and air preheater, respectively. Two methods are proposed to absorb residual flue gas heat Qre in middle temperature range. Both methods shall decrease CO2 temperature entering the boiler T4 and increase secondary air temperature Tsec air, whose maximum value is deduced based on energy conservation in air preheater. The system is analyzed incorporating thermodynamics, boiler pressure drop and energy distribution. It is shown that at a given main vapor temperature T5, the main vapor pressure P5 can be adjusted to a value so that Qre is completely eliminated, which is called the main vapor pressure adjustment method. For this method, single-reheating is only available for higher main vapor temperatures. The power generation efficiency for single-reheating is obviously higher than double-reheating. If residual flue gas heat does exist, a flue gas heater FGC is integrated with S-CO2 cycle, which is called the FGC method. Both single-reheating and double-reheating share similar power generation efficiency, but single-reheating creates less residual flue gas heat. We conclude that single-reheating is preferable, and the pressure adjustment method achieves obviously higher power generation efficiency than the FGC method.

It is great significance to understand the mechanism of heat transfer deterioration of supercritical CO2 for heat exchanger design and safe operation in the supercritical CO2 Brayton cycle. Three-dimensional steady-state numerical simulation was performed to investigate the behavior of supercritical CO2 heat transfer in heated vertical up-flow tube with inner diameter di = 10 mm and heated length Lh = 2000 mm. Based on the characteristics of inverted-annular film boiling at subcritical pressure, the heat transfer model of supercritical CO2 flowing in the heated vertical tube was established in this paper. The mechanisms of heat transfer deterioration (HTD) and heat transfer recovery (HTR) for supercritical CO2 were discussed. Numerical results demonstrate that HTD is affected by multiple factors, such as the thickness and property of vapor-like film near the wall, the turbulence intensity near the interface between liquid-like and vapor-like, and in the liquid-like core region as well as the distribution of radial velocity vector. Among the above factors, the change of turbulent kinetic energy caused by the buoyancy effect seems to be a more important contributor to HTD and HTR. Furthermore, the influences of heat flux and mass flux on the distribution of wall temperature were analyzed, respectively. The reasons for the difference in wall temperature at different heat fluxes and mass fluxes were explained by capturing detailed thermal physical properties and turbulence fields. The present investigation can provide valuable information for the design optimization and safe operation of a supercritical CO2 heat exchanger.

With the development of low-carbon economy and the capacity increase of ultra-supercritical units, the optimization design of boiler furnace structure and scientific selection of combustion mode are seriously technical problems in the power industry. This paper makes analysis and provides the resolution scheme for the problems aiming at the three main combustion modes of pulverized coal boiler, thus tangential circle firing, opposed wall firing and “W-shaped flame firing”, in the process of boiler scaling-up development, besides, makes the analysis of the three main combustion modes and combined with coal quality and factors existing in the overall layout of Π-type boiler and tower layout programs, discusses the of the selection principles for 600MW and 1000MW ultra-supercritical units. The main problems in the process of boiler scaling-up development includes heat deviation increase caused by the burner arrangement and the regulation of aspect ratio caused by the boiler capacity increase, and the resolution scheme of mixed arrangement of tangential oval and opposed wall has great advantages in the opposed wall firing of ultra-supercritical units using sliding-parameter operating and adapting spiral tube water wall. Combustion mode selection of pulverized coal boiler mainly depends on coal characteristics and thermal characteristics parameter of furnace, meanwhile it is also related to the milling system, the burner, the performance design and arrangement of air-distribution device, the coal fineness, the air temperature and the air distribution condition. The choice of combustion mode of ultra-supercritical units isn't also invariable, but the cooperation of tower layout and four comer tangential circle combustion modes is the ideal choice to wholly resolve the gas heat deviation for ultra-supercritical units. With the further research of tower layout with eight-square double tangential circle and the theory of Π-type with tangential oval combustion mode, the technical problem of combustion mode selection of ultra-supercritical units will be radically resolved, and the scientific selection of combustion mode will cooperate with the energy saving and emission reduction of boilers.