Investigation On The Adaptive Flow Path Recuperator Of Supercritical Carbon Dioxide Brayton Cycle And The Heat Transfer Characteristics

Improving energy efficiency,reducing emissions and pollution has become an important goal of global energy utilization.With supercritical carbon dioxide(SCO2)as working medium,the SCO2 Brayton cycle has the advantages of high thermal efficiency,compact system,cheap and easy availability of working medium,etc.,which has a great application prospect in the fields of nuclear power,renewable energy,waste heat utilization and power transportation.The flow and heat transfer characteristics of the recuperator have an important effect on the SCO2 Britton cycle.The SCO2 is in the quasi-critical region at the inlet of the high-pressure side of the recuperator,and the pinch point caused by the dramatic change of physical properties with temperature may seriously worsen the heat transfer of the recuperator.In order to alleviate this problem,the SCO2 recompression Breton cycle(SCO2-RBC)is generally adopted to improve the cycle efficiency.Based on low-temperature recuperator affected by pinch point as the research focus,the adaptive flow path is used to solve the pinch point of SCO2,therefore,this paper studies the heat transfer of SCO2 in an upward vertical ordinary tube,analyzes the heat transfer of SCO2 in an upward adaptive tube,and then investigates the performance of adaptive flow path recuperator and relative issues in practical application,finally,the effects of low-temperature recuperator from aspect of heat transfer and flow characteristics on the performance of SCO2-RBC are analyzed.The heat transfer deterioration may occurs in an upward vertical ordinaty tube,therefore,the heat transfer of SCO2 in an upward vertical ordinaty tube are studied by experiment and simulation.The results show that when the mass flow is larger,the range of heat transfer deterioration becomes smaller.The heat transfer coefficient decreases and range of heat transfer deterioration increases when heat flux increases.In the area where heat transfer deterioration occurs,the influence of pressure on the outer wall temperature and heat transfer coefficient is weak.The change of inlet temperature will change the location of heat transfer deterioration.Due to the buoyancy,the velocity gradient as well as turbulent kenetic energy in the near-wall area first decrease and then increase,and the velocity distribution develops from U-shaped to gradually flat and then to M-shaped and at last to U-shaped,and the heat transfer deteriorates first and then recovers.Based on the heat transfer deterioration in an upward ordinary tube,the adaptive flow path is adopted to solve this issue,and the experiment study on an adaptive tube is conducted.The results indicate that,compared with ordinary tube,the wall temperature is remarkably low and shows a monotonically increasing trend,the peak of temperature does not appear and heat transfer deterioration is avoided,and the local heat transfer coefficient increases remarkably for adaptive tube.As the inlet temperature rises and the maximum of heat transfer coefficient shifts to the inlet and the wall temperature of the downstream section ascends remarkably.As the bulk SCO2 of the adaptive tube decreases in density and increases in volume during the heating process,it flows towards the inner wall surface and enhances the disturbance in the near wall area,heat transfer coefficient rises remarkably.The rise of mass flow leads to the rise of heat transfer coefficient.In the pseudo-critical region,mass flow exerts a more remarkable impact on the heat transfer coefficient,and as heating power declines or pressure declines,heat transfer coefficient rises remarkably.Outside the pseudo-critical region,heating power and pressure exert a relatively weak effect on the heat transfer coefficient.The performance of adapative flow path recuperator is investigated by experiment.The results demonstrate that the adaptive flow path recuperator can not only improve the heat transfer,decrease the pressure drop,but also improve pinch point problems.When mass flow ratio increases,the heat transfer effectiveness drops to its bottom and then rises.When the mass flow ratio reaches about 0.6,the heat transfer effectiveness reaches its bottom.However,when mass flow ratio ascends,the overall heat transfer coefficient reaches its peak and then declines.When the mass flow ratio is about 0.65,the overall heat transfer coefficient reaches its maximum.As the total mass flow rises,the heat transfer effectiveness declines and the overall heat transfer coefficient ascends.The heat transfer effectiveness and overall heat transfer coefficient rise when the pressure of cold side declines or the pressure of hot side ascends,and the heat transfer also improves when the inlet temperature of hot side ascends.The flow direction has a significant effect on the heat transfer.When the cold side is upward flow,the heat transfer effectiveness is the minimum;when the cold side is downward flow,the heat transfer effectiveness is the maximum.When cold side is upward flow,the overall heat transfer coefficient is about 9%lower than that of horizontal flow;when cold side is downward flow,the overall heat transfer coefficient is about 6%higher than that of horizontal flow.Because the size of the existing printed circuit plate recuperator is limited by the processing conditions,it is a simple and effective way to increase the number of recuperators to meet the thermal load.When the recuperators are stacked in parallel,there is interface thermal resistance.Therefore,this paper studies the interface thermal resistance.The results demonstrate that the interface thermal resistance has a remarkable effect on the heat transfer of the parallel stacked recuperators.When the interface are filled with grease,the heat transfer can be increased by at least 50%under the experimental operating conditions.Reducing the surface roughness can remarkably reduce the interface thermal resistance.When the pressure increases,the interface thermal resistance first decreases sharply,and then gradually becomes stable.The effect of surface roughness on interface thermal resistance enlarges as the pressure declines,and the effect of pressure on interface thermal resistance declines as the surface roughness declines.Compared with thermal pads and carbon-based materials,the effects of surface roughness and pressure on interface thermal resistance of phase change materials(PCM)and low melting point alloys(LMPA)are relatively small due to the good filling ability.The interface thermal resistance of the thermal pads is little affected by the temperature in the test range(30～70℃).The interface thermal resistance of the carbon-based material decreases slightly with the increase of temperature.When the interface temperature exceeds the phase change temperature,the flow capacity of the PCM and LMPA increases remarkably,and the interface thermal resistance decreases remarkably.According to the ability of enhancing heat transfer(including filling ability and thermal conductivity),the order from strong to weak is as follows:LMPA,PCM,carbon-based materials and thermal pads.If the surface roughness is small,the use of thermal pads or carbon-based materials may hinder the interface heat transfer.In the range of experimental pressure,the thermal pads does not reach the best performance.Therefore,it is recommended to use thermal pads when the assembly pressure is high.Because of the vertical arrangement of carbon fibers,the carbon-based materials can achieve the best performance when the compression ratio is 30%-40%.Finally,the influence of low-temperature recuperator on the performance of SCO2-RBC is analyzed from the heat transfer and flow characteristics of low-temperature recuperator,and the influence rule under different operating parameters is obtained.The results indicate that the thermal efficiency,specific work and exergic efficiency rise when pressure loss declines or effectiveness rises.Under the same conditions,when pressure loss decreases from 5%to 0.5%,thermal efficiency increases by about 0.04,specific work increases by about 14.0%,exergic efficiency increases by about 0.06;When the effectiveness increases from 0.82 to 0.88,thermal efficiency increases by about 0.02,specific work increases by about 1.2%,exergic efficiency increases by about 0.03.As the outlet pressure of the main compressor declines,the pressure loss has a more remarkable impact on efficiency.As the inlet temperature of the main compressor rises,the effect of pressure loss on efficiency shows a similar trend.As the inlet temperature of the main compressor declines,the effect of the effectiveness on the specific work is greater.As the inlet temperature of the main compressor changes,the trends of efficiency are opposite with that of specific work.The overall optimization of the cycle is carried out.When the thermal efficiency and exergic efficiency are greater,the total cost of the cycle is higher and the specific work is smaller.The efficiency,specific work and cost should be comprehensively considered in designing the cycle conditions.