Printed circuit heat exchangers(PCHEs)are promising candidates for recuperators in supercritical CO2 Brayton cycles.A comparative study is given in this paper on the flow and heat transfer characteristics of PCHEs wit...Printed circuit heat exchangers(PCHEs)are promising candidates for recuperators in supercritical CO2 Brayton cycles.A comparative study is given in this paper on the flow and heat transfer characteristics of PCHEs with sinusoidal and zigzag channels.With mass flow rates of 0.6–1.8 kg/h and the bend angles of 15°–30°,the thermal-hydraulic performance of the PCHEs is discussed.Results show that the sinusoidal channel is superior to the zigzag channel in its comprehensive performance.Larger bend angles result in greater reductions in pressure drop if sinusoidal channels are used instead of zigzag channels and a maximum of 48.4%reduction can be obtained in the considered working conditions.Meanwhile,the inlet sections should be carefully optimized since these sections account for up to 31%and 17%of the total pressure drop in the sinusoidal and zigzag channels,respectively.The corner shape of the zigzag channel can be specially designed to further reduce the pressure drop.The nonuniform density and heat flux distributions in both channels are found to be related to the periodic changes of flow directions and the centrifugal forces should not be ignored when optimizing the sinusoidal and zigzag channels.展开更多
In this study,an experimental system was built to investigate the global performance of an 80-k W zigzag printed circuit heat exchanger(PCHE).It could meet the requirement of the pre-cooler for the supercritical carbo...In this study,an experimental system was built to investigate the global performance of an 80-k W zigzag printed circuit heat exchanger(PCHE).It could meet the requirement of the pre-cooler for the supercritical carbon dioxide(S-CO_(2))Brayton power cycle and the modified effectiveness considering the pinch point is between 61.5%and 79.3%.When the outlet S-CO_(2)temperature is near the pseudo-critical point,the thermo-physical properties have more effects on heat transfer performance compared to flow characteristics.For the local performance,the mass flow rates of both sides have crucial influences on the location where the peak of S-CO_(2)Nusselt number occurs while only the S-CO_(2)flow rate affects the variation of the peak value.In addition,the influence of the radius of curvature on the secondary-flow should not be ignored.In the end,new empirical correlations were proposed considering the drastic variations of the Prandtl number.展开更多
Three-dimensional numerical simulations and experiments were carried out to study the heat transfer characteristics and the pressure drop of air flow in a circular tube with Edgefold-Twisted Tape （ETT） inserts and w...Three-dimensional numerical simulations and experiments were carried out to study the heat transfer characteristics and the pressure drop of air flow in a circular tube with Edgefold-Twisted Tape （ETT） inserts and with classic Spiral-Twisted-Tape （STT） inserts of the same twist ratio. The RNG turbulence model for mildly swirling flows, the enhanced wall treatment for low Reynolds numbers, and the SIMPLE pressure-velocity method were adopted to simulate the flow and heat transfer characteristics. Within the range of Reynolds number from 2 500 to 9 500 and the twist ratio y from 5.4 to 11.4, the Nusselt number of the tube with ETT inserts is found to be 3.9% - 9.2% higher than that with STT inserts, and the friction factor of the tube with ETT inserts is 8.7% - 74% higher than that of STT inserts. The heat enhancement is due to higher tangential velocity and asymmetrical velocity profile with the increase and decrease of the periodic velocity within an edgefold length. It is found that main factors affecting the heat transfer of ETT inserts are the twist angle and the gap width between the tube and inserts. A larger twist angle leads to a higher tangential velocity, and larger Nusselt number and friction factor. The thermal-hydraulic performance slowly decreases as the twist angle increases. The gap width between tube and inserts has a significant influence on the heat transfer, while little influence on pressure drops. The thermal-hydraulic performance increases in average by 124% and 140% when the gap width reduces from 1.5 mm to 1.0 mm and 0.5 mm. The larger the gap width, the higher velocity through the gap will be, which would reduce the main flow velocity and tangential velocity. So a small gap is desirable. Comparing experimental and numerical results at variable air flow and tube wall temperature, the numerical results are found to be in a reasonable agreement with the experiment results, with difference of the Nusselt number in a range of 1.6% - 3.6%, and that of the friction factor in a range of 8展开更多
This study investigated the effects of zigzag-flow channel bending angle in printed circuit heat exchangers(PCHEs) using a computational fluid dynamics method with ANSYS-FLUENT simulation.The three-dimensional model o...This study investigated the effects of zigzag-flow channel bending angle in printed circuit heat exchangers(PCHEs) using a computational fluid dynamics method with ANSYS-FLUENT simulation.The three-dimensional model of PCHE with a 15° curved,zigzag channel was conducted for preliminary validation.The comparisons between the CFD simulation results and the experimental data showed good agreement with some discrepancies in the heat transfer and pressure drop results.In addition,different bending angle configurations(0°,3° to 30°) of zigzag channels were analyzed to obtain better thermal-hydraulic performance of the zigzag channel PCHE under different inlet mass flow rates.The criteria of heat transfer and frictional factor were applied to evaluate the thermal-hydraulic performance of the PCHE.The results showed that the 6° and 9°bending channel provided good thermal-hydraulic performance.New correlations were developed using the 6°and 9° bending channel angles in PCHE designs to predict the Nusselt number and friction factor.展开更多
This work is experimental investigation on the impact of rectangular baffles on the thermo-hydraulic performance of a solar thermal collector.The study was conducted under moderate weather conditions and six cases wer...This work is experimental investigation on the impact of rectangular baffles on the thermo-hydraulic performance of a solar thermal collector.The study was conducted under moderate weather conditions and six cases were studied,including 2M135,6M135,10M135,14M135,18M135,and a smooth plate case.The results showed that the number and placement of baffles plays a crucial role in improving heat transfer and the best cases in terms of thermal performance were cases V and IV,with maximum values of h=15.84 W/m2.k,and h=17.46 W/m2k.The study also found that the heat transfer was greatest at the beginning of the channel and decreased towards the end.The highest thermal efficiency was recorded in case 18M135 with a maximum value ofȠ=0.73.展开更多
In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louve...In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louvered fin and flat tube heat exchangers(LFHEs).After validation of the LF side pressure dropΔP and heat transfer coefficient hLFof the baseline by experiments,the numerical method is applied to investigate the influential mechanisms of the RWVG parameters(the number N(7 to 15),attack angleβ(30°to 90°),height H_(VG)(0.8 mm to 2 mm)and width W_(VG)(0.8 mm to 1.2 mm))on the performance of the LFHE in the velocity range of 3 m/s to 10 m/s.Results show that thermal-hydraulic performance of the LFHE is significantly impacted by the RWVGs,and according to the performance evaluation criteria(PEC),the LFHE achieves its optimal thermal-hydraulic performance when N=7,β=45°,H_(VG)=1.8 mm and W_(VG)=1 mm.Compared to the baseline,the maximum,minimum and average increments of PEC for the optimal case are 13.85%,4.67%and 8.39%,respectively.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.51822606,51806249)Hunan Provincial Natural Science Foundation of China(Grant No.2019JJ50801)。
文摘Printed circuit heat exchangers(PCHEs)are promising candidates for recuperators in supercritical CO2 Brayton cycles.A comparative study is given in this paper on the flow and heat transfer characteristics of PCHEs with sinusoidal and zigzag channels.With mass flow rates of 0.6–1.8 kg/h and the bend angles of 15°–30°,the thermal-hydraulic performance of the PCHEs is discussed.Results show that the sinusoidal channel is superior to the zigzag channel in its comprehensive performance.Larger bend angles result in greater reductions in pressure drop if sinusoidal channels are used instead of zigzag channels and a maximum of 48.4%reduction can be obtained in the considered working conditions.Meanwhile,the inlet sections should be carefully optimized since these sections account for up to 31%and 17%of the total pressure drop in the sinusoidal and zigzag channels,respectively.The corner shape of the zigzag channel can be specially designed to further reduce the pressure drop.The nonuniform density and heat flux distributions in both channels are found to be related to the periodic changes of flow directions and the centrifugal forces should not be ignored when optimizing the sinusoidal and zigzag channels.
基金supported by the National Natural Science Foundation of China(No.51606191)the National Key Research and Development Program-China(2017YFB0601803)Key deployment project of Chinese Academy of Sciences(Y7220112H1)。
文摘In this study,an experimental system was built to investigate the global performance of an 80-k W zigzag printed circuit heat exchanger(PCHE).It could meet the requirement of the pre-cooler for the supercritical carbon dioxide(S-CO_(2))Brayton power cycle and the modified effectiveness considering the pinch point is between 61.5%and 79.3%.When the outlet S-CO_(2)temperature is near the pseudo-critical point,the thermo-physical properties have more effects on heat transfer performance compared to flow characteristics.For the local performance,the mass flow rates of both sides have crucial influences on the location where the peak of S-CO_(2)Nusselt number occurs while only the S-CO_(2)flow rate affects the variation of the peak value.In addition,the influence of the radius of curvature on the secondary-flow should not be ignored.In the end,new empirical correlations were proposed considering the drastic variations of the Prandtl number.
基金Project supported by the National Basic Research Program of China (973 Program, Grant No. 2007CB206903)
文摘Three-dimensional numerical simulations and experiments were carried out to study the heat transfer characteristics and the pressure drop of air flow in a circular tube with Edgefold-Twisted Tape （ETT） inserts and with classic Spiral-Twisted-Tape （STT） inserts of the same twist ratio. The RNG turbulence model for mildly swirling flows, the enhanced wall treatment for low Reynolds numbers, and the SIMPLE pressure-velocity method were adopted to simulate the flow and heat transfer characteristics. Within the range of Reynolds number from 2 500 to 9 500 and the twist ratio y from 5.4 to 11.4, the Nusselt number of the tube with ETT inserts is found to be 3.9% - 9.2% higher than that with STT inserts, and the friction factor of the tube with ETT inserts is 8.7% - 74% higher than that of STT inserts. The heat enhancement is due to higher tangential velocity and asymmetrical velocity profile with the increase and decrease of the periodic velocity within an edgefold length. It is found that main factors affecting the heat transfer of ETT inserts are the twist angle and the gap width between the tube and inserts. A larger twist angle leads to a higher tangential velocity, and larger Nusselt number and friction factor. The thermal-hydraulic performance slowly decreases as the twist angle increases. The gap width between tube and inserts has a significant influence on the heat transfer, while little influence on pressure drops. The thermal-hydraulic performance increases in average by 124% and 140% when the gap width reduces from 1.5 mm to 1.0 mm and 0.5 mm. The larger the gap width, the higher velocity through the gap will be, which would reduce the main flow velocity and tangential velocity. So a small gap is desirable. Comparing experimental and numerical results at variable air flow and tube wall temperature, the numerical results are found to be in a reasonable agreement with the experiment results, with difference of the Nusselt number in a range of 1.6% - 3.6%, and that of the friction factor in a range of 8
基金supported by the School of Mechanical,Institute of Engineering,Suranaree University of Technology (Thailand),Mechanical and Process System Engineering Program,and Vithedbundit Scholarship,Institute of Engineering,Suranaree University of Technology (Thailand)。
文摘This study investigated the effects of zigzag-flow channel bending angle in printed circuit heat exchangers(PCHEs) using a computational fluid dynamics method with ANSYS-FLUENT simulation.The three-dimensional model of PCHE with a 15° curved,zigzag channel was conducted for preliminary validation.The comparisons between the CFD simulation results and the experimental data showed good agreement with some discrepancies in the heat transfer and pressure drop results.In addition,different bending angle configurations(0°,3° to 30°) of zigzag channels were analyzed to obtain better thermal-hydraulic performance of the zigzag channel PCHE under different inlet mass flow rates.The criteria of heat transfer and frictional factor were applied to evaluate the thermal-hydraulic performance of the PCHE.The results showed that the 6° and 9°bending channel provided good thermal-hydraulic performance.New correlations were developed using the 6°and 9° bending channel angles in PCHE designs to predict the Nusselt number and friction factor.
文摘This work is experimental investigation on the impact of rectangular baffles on the thermo-hydraulic performance of a solar thermal collector.The study was conducted under moderate weather conditions and six cases were studied,including 2M135,6M135,10M135,14M135,18M135,and a smooth plate case.The results showed that the number and placement of baffles plays a crucial role in improving heat transfer and the best cases in terms of thermal performance were cases V and IV,with maximum values of h=15.84 W/m2.k,and h=17.46 W/m2k.The study also found that the heat transfer was greatest at the beginning of the channel and decreased towards the end.The highest thermal efficiency was recorded in case 18M135 with a maximum value ofȠ=0.73.
基金supported by the National Natural Science Foundation of China(51875238)。
文摘In this paper,a novel composite heat transfer enhancement technique comprised of louvered fins(LFs)and rectangular wing vortex generators(RWVGs)is proposed to improve the LF side thermal-hydraulic performance of louvered fin and flat tube heat exchangers(LFHEs).After validation of the LF side pressure dropΔP and heat transfer coefficient hLFof the baseline by experiments,the numerical method is applied to investigate the influential mechanisms of the RWVG parameters(the number N(7 to 15),attack angleβ(30°to 90°),height H_(VG)(0.8 mm to 2 mm)and width W_(VG)(0.8 mm to 1.2 mm))on the performance of the LFHE in the velocity range of 3 m/s to 10 m/s.Results show that thermal-hydraulic performance of the LFHE is significantly impacted by the RWVGs,and according to the performance evaluation criteria(PEC),the LFHE achieves its optimal thermal-hydraulic performance when N=7,β=45°,H_(VG)=1.8 mm and W_(VG)=1 mm.Compared to the baseline,the maximum,minimum and average increments of PEC for the optimal case are 13.85%,4.67%and 8.39%,respectively.
