Investigation On Supercritical Carbon Dioxide Cycle Based Thermoelectric Conversion System For Scramjet

With the development of hypersonic vehicle,the thermal protection technology of scramjet has become one of the key issues limiting its long-time flight.Meanwhile,it is also urgent to develop an efficient power generation system to meet the power demand of hypersonic vehicle.The supercritical carbon dioxide Brayton cycle is treated as one of the most promising power systems to with high efficiency,good stability and safety,which has a good potential application in hypersonic vehicle.Thus,a supercritical carbon dioxide Brayton cycle based thermoelectric conversion system for scramjet,which considers the particularity of heat and cold sources,is put forward.In order to evaluate the cooling performance and power generation capacity of the system,the following researches are carried out.A quasi-one-dimensional model of combustor coupled with active cooling channel is developed to obtain the wall heat flux and temperature distribution satisfying the heat transfer process of combustor and the active cooling channel.The results show that the working fluid and the inlet temperature of cooling channel has a great effect on the wall heat flux and the inlet pressure has little effect.Namely,the thermal environment will be affected by the cycle parameters and it is necessary to take the change of cycle parameters into account heat flux calculation.Then based on the first law of thermodynamics,the effects of the parameters on thermal performance of recompression cycle are analyzed.There is an optimal value of split ratio to obtain the highest thermal efficiency and the optimal split ratio decreases with the increase of the maximum cycle pressure.Due to the existence of pseudo-critical point of supercritical carbon dioxide,the optimal inlet pressure of main compressor usually appears near the corresponding pseudo-critical pressure.Three typical layouts of supercritical carbon dioxide cycle are employed to build the thermoelectric conversion system of scramjet and the main performance parameters are defined.A coupling analytical model of supercritical carbon dioxide cycle and active cooling combustor is established to investigate the influence factors on pressure drop in cooling channel,maximum wall temperature and fuel consumption for cooling.The results show that the maximum cycle pressure P2 has a great influence on the pressure loss in cooling channel,a higher P2 leads to a smaller pressure loss.The maximum wall temperature is mainly affected by the turbine inlet temperature T3 and a higher T3 leads to a higher wall temperature.The fuel consumption for cooling is mainly affected by the fuel outlet temperature,not the thermal efficiency.Different from the conventional applications of supercritical carbon dioxide cycle,in order to obtain a higher fuel outlet temperature,it is necessary to maintain the main compressor inlet temperature above the critical temperature.Besides,the simple cycle has the best cooling performance due to its highest fuel outlet temperature,though recompression cycle has the highest thermal efficiency.However,the reduction of fuel consumption and power output is relatively low and the improvement measures need to be taken.Two improved cycle based on recompression cycle are proposed by changing the location of split point to enhance the cooling performance,named high-temperature split cycle and low-temperature split cycle.Compared with the original recompression cycle,the working fluid inlet temperature of fuel heat exchanger is improved and the fuel outlet temperature will also increase,then the fuel consumption for cooling will decrease.Although the low-temperature split cycle has the best cooling performance,the hightemperature split cycle can maintain higher reduction of fuel consumption and power output in the whole split ratio range.Hence the high-temperature split cycle is more suitable for the application of hypersonic vehicle in the view of system stability.The off-design model is developed for high-temperature split cycle to analyze the off-design performance characteristics of the thermoelectric conversion system.The turbine and the main compressor are managed by the same shaft.Based on the off-design performance prediction models of compressor and turbine,the off-design parameters can be calculated by matching the mass flow rate in compressor and turbine.An exploration of off-design cooling performance with the change of carbon dioxide mass flow rate and equivalent ratio is conducted.The results indicate that the system can obtain higher reduction of fuel consumption and power output under off-design conditions when select the average mass flow rate as design-point mass flow rate.Although choosing the low equivalent ratio as design point can get a better off-design performance,the maximum cycle pressure will increase with the rise of equivalent ratio and might exceed the capacity of the system.Therefore,the selection of design-point equivalent ratio should also consider the change of maximum pressure and it may be better to choose a smaller equivalent ratio within the allowable pressure range.