Study On Performance Of Flow And Heat Transfer Of Supercritical Nitrogen In Airfoil Fin Printed Plate Heat Exchanger

The printed circuit heat exchanger(PCHE)is a new kind of high efficiency and compact plate heat exchangers,which can be used in extreme conditions,such as high pressure,high heat flux and volumetric limitation.In addition,supercritical pressure fluid has been widely used in the industrial field due to its favorable flow and heat transfer characteristics.Nowadays,how to improve the flow and heat transfer characteristics of PCHE and its internal characteristics has attracted the attention of many scholars at home and abroad.It is rare in the domestic studies that the supercritical fluid was applied for heat transfer in PCHE.This article adopts the method of experimental research and numerical simulation to deeply research the thermal hydraulic characteristics of PCHE with supercritical pressure nitrogen as the working fluid in different conditions and different fin arrangement of airfoil fin and to explore the better channel structure and working condition in airfoil PCHE flow passage.Therefore,the research of this topic has important theoretical and practical significance.This paper first introduces the supercritical fluid and its characteristics,shows the physical properties of supercritical pressure nitrogen,and fits the physical polynomial of supercritical pressure nitrogen.The special heat transfer characteristics of supercritical fluids are introduced from the aspects of mass flow,heat flow density and pressure.The special heat transfer characteristics are mainly related to the dramatic changes of physical properties.Then from the aspects of computational fluid dynamics,FLUENT introduction,flow and heat transfer basic control equations,turbulence model and numerical simulation settings,the numerical simulation method is described,which provides a basis for later research and analysis.The flow and heat transfer performance of supercritical pressure nitrogen in airfoil PCHE under different working conditions are studied experimentally.Firstly,the experimental system and experimental process are introduced in detail,including:experimental device principle,experimental procedure,measuring component and experimental data measurement,experimental data collection and processing.In the experiment,the inlet mass flux of liquid nitrogen was 203-228 kg/m~2s,the inlet pressure was 5-8 MPa and the inlet temperature was103-121 K.The effect of different mass flow,inlet temperature and heat flux on outlet temperature and pressure drop were analyzed.The experimental results show that under the same inlet pressure,the outlet temperature decreases with the increase of mass flow rate.At the same mass flow rate,the outlet temperature increases as the pressure increases.When the inlet temperature increases,the outlet temperature also gradually increases,and the increase is relatively uniform.Moreover,it is apparent that the heat flux of 70 kw/m~2 is higher than the outlet temperature of 66 kw/m~2 and the outlet temperature is increased by about 16.5°C.Furthermore,at the same mass flow rate,the pressure drop increases as the operating pressure decreases.As the inlet temperature increases,the pressure drop of the airfoil fin PCHE gradually increases and the slope of the pressure drop increases.Secondly,the flow and heat transfer characteristics of supercritical nitrogen in airfoil fin PCHE are studied numerically.The effects of mass flux,pressure and inlet temperature on the flow and heat transfer of airfoil PCHE are studied.The structure and arrangement of the airfoil fins in the flow channel simulated in this chapter are the same as experiment.The pressures in the simulated working conditions are 5 MPa,6 MPa,7 MPa,8 MPa and the mass flux is203-228 kg/m~2s.The initial temperature is 103 K.Based on the convective heat transfer coefficient h,Nusselt number and pressure drop of supercritical pressure nitrogen,the flow characteristics of supercritical pressure nitrogen under different working conditions are comprehensively analyzed,and new correlations for Nusselt number and Fanning friction factor are proposed.The simulation results show that the average convective heat transfer coefficient increases with the increase of mass flow rate under different inlet pressures.At the same mass flow rate,the average convective heat transfer coefficient of the airfoil PCHE increases with increasing inlet pressure.As the pressure decreases,the inflection point of the slope of the increase in Nusselt number appears at a larger Reynolds number.Different inlet temperatures also affect the flow and heat transfer of supercritical pressure nitrogen in the airfoil fin PCHE.Considering the comprehensive heat transfer performance,the pressure loss of the inlet temperature of 103 K is small and has strong convective heat transfer performance than the 127 K.Under supercritical pressure conditions,the Gnielinski and Dittuss-Boelter correlations failed to accurately predict the Nusselt and Fanning friction coefficients.This chapter proposes a new correlation between the Fanning friction factor and the Nusselt number.By comparison and verification,we find that the Nusselt number and the Fanning friction factor of the new correlation are in good agreement with the experimental and numerical simulation results.Finally,the flow and heat transfer performance of supercritical pressure nitrogen in airfoil fin PCHE with different fin arrangements are studied numerically.The effects of vertical distance and stagger distance between airfoil fins are studied.The operating pressure is 6 MPa,the mass flux is 227.7 kg/m~2s,and the inlet initial temperature is 103 K.The flow characteristics of different arrangement of airfoil fins are analyzed mainly from the aspects of velocity distribution,pressure distribution,overall convective heat transfer coefficient,Nusselt number and pressure drop.The comprehensive performance of the flow channel is compared by the performance evaluation index(PEC).The simulation results show that the velocity distribution becomes more and more uniform when the staggered distance increases from L_s=0 mm to L_s=1.2 mm.The flow velocity in the flow channel under the different fin arrangement is similar along the flow channel,and the flow rate increases more and more steeply.In addition,the magnitude of the pressure drop gradually increases in the direction of flow.Properly reducing the vertical distance between the two rows of fins and reducing the staggered distance can improve the convective heat transfer of the PCHE.In addition,the complete staggering of the fins and the proper increase of the vertical distance between the two rows of fins can reduce pressure losses.The channel of L_s=0.6mm,L_v=1.25 mm is compared with L_s=1.2 mm,L_v=1 mm and L_s=1.2 mm,L_v=0.8mm,the channel with L_s=1.2mm,L_v=1.25 mm has higher PEC value.This phenomenon is especially obvious when the Reynolds number is low.Therefore,the overall heat transfer performance of the fins can be improved by completely stagger arrangement and appropriately reducing the vertical distance between the fins.