Analysis And Research On Condensation Flow And Heat Transfer Characteristics In Printed Circuit Heat Exchanger

Printed circuit heat exchanger（PCHE）is a compact,efficient and highly integrated plate heat exchanger,which has been successfully applied to the LNG regasification project,is the core equipment of the floating storage and regasification unit（FSRU）.The core of PCHE is composed of many mini/micro-channels.Compared with the traditional macro-channels,the flows in mini/micro channels are affected by different forces,exhibiting different flow characteristics and transformation mechanisms.In addition,the two-phase flow characteristics and flow patterns can strongly affect heat transfer.At present,PCHE has been studied by a large number of researchers,but rarely involving two-phase flow.Therefore,it is necessary to study the characteristics of two-phase condensation flow and heat transfer in PCHE minichannels,which is of great significance for enhancing heat transfer.In this paper,the condensation flow and heat transfer performance of refrigerant R22 in PCHE hot side minichannels were studied by experiment and numerical simulation.Firstly,the pressure drop and heat transfer performance of refrigerant R22 in PCHE hot side minichannels were studied experimentally.We designed and set up a liquid nitrogen-R22 experimental loop with PCHE as the core equipment and measured some parameters such as temperature,pressure and mass flow rate.The measured experimental data were processed to research the pressure drop and heat transfer coefficient of R22 in minichannels under different pressures,different inlet temperatures and different mass flow conditions.Through analysis,it is found that the pressure drop and heat transfer coefficient of R22 in the minichannel increase with the increase of mass fluxes when the pressure is kept at 0.65 MPa.The pressure drop and heat transfer coefficient are relatively higher at lower pressures when the mass flux is 10.52 kg m^-22 s^-1.The variation trends of pressure drop and heat transfer coefficient were analyzed by the thermo-physical properties of R22 under different conditions.In addition,the existing correlations for pressure drop and heat transfer are evaluated and analyzed by comparing with the experimental data.Secondly,due to the variability of the experimental conditions,it is impossible to completely control the influence variables of each operating point,so that the pressure drop and heat transfer characteristics of R22 in the channel only show a general trend without obvious regularity.In addition,the condensation rate and flow pattern of R22 in the channel are different under different working condition and the experimental study cannot visualize the internal flow information of the PCHE channels,so a simplified minichannel model of PCHE hot side was established to explore the R22 condensation flow and heat transfer characteristics in minichannels through numerical simulation.Firstly,the condensation flow regularity and flow patterns of R22 in the channel were explored by two-phase couters.It was found that during condensation process,the liquid phase first appeared at the sharp corners where the upper and lower surfaces of the channel met,and then extended to the middle along the surface.Smooth-annular,wavy-annular,slug and bubbly flow patterns were found at different refrigerant mass fluxes and plotted as a flow pattern map.Secondly,the local heat transfer coefficient and pressure along the channel under different inlet vapor qualities and different mass fluxes were studied in depth.It is found that the heat transfer coefficient increases with decreasing inlet quality and the pressure drop varies irregularly with the inlet vapor quality.Through comprehensive analysis,the heat transfer performance is optimal at the inlet vapor quality of 0.7.The local heat transfer coefficient and pressure drop increase with rising R22 mass flux.Under the same mass flux,the heat transfer coefficient increases along the flow direction,indicating that the intermittent flow enhances heat transfer.Finally,the new local Nusselt number and Fanning friction factor correlations were proposed for this study,and the proposed correlations were verified to have good predictability by comparation with numerical data.Finally,the simplified model was used to simulate the experimental conditions.The differences between experiments and simulations were analyzed by comparing the experimental values of the Nusselt number（Nu）and friction pressure gradient with the numerical ones.Furthermore,the influences of pressure and mass flux on the Nu as well as the friction pressure gradient was analyzed in depth through condensation flow regimes to explore the underlying mechanism giving the results.The results show that the numerical Nu values are higher than the experimental Nu values,which can be attributed to the error in the experimental calculation process.The values of experimental friction pressure gradient are higher than those of the simulation due to the use of an ideal numerical model and the inlet and outlet pressure drop are neglected in the calculation of the experimental friction pressure drop.The variation trends were analyzed through the condensation regime.The Nu values increase with the increasing mass fluxes.Because at a low mass flux,R22 condenses faster and even subcools,while at a high mass flux,the R22 is always in the two-phase flow state.As the mass flux increases,the frictional pressure gradient increases and the flow pattern transforms from intermittent to annular flow.At lower pressures,the Nu and friction pressure gradient values are higher.Because at lower pressures,the position of complete condensation is closer to the outlet,the phase-change heat transfer is more sufficient and the vapor-liquid velocity difference increases with decreasing pressure.