Study On Thermal Hydraulic Performance Of Supercritical LNG In Printed Circuit Heat Exchanger

With the shortage of energy and the intensification of environmental pollution around the world,the development and utilization of renewable clean energy,and the improvement of energy efficiency have become the trend of China’s energy development.Natural gas is a high-quality clean energy,which has the advantages of abundant resources,high combustion calorific value and no pollution after burnout.Many countries,including China,take natural gas as an important alternative energy source.With the increase of offshore natural gas development,the development of highly compact,efficient and reliable gasification equipment has become a key issue in the natural gas gasification system.Printed circuit heat exchanger(PCHE)has the advantages of compact structure and high effectiveness.Studying the convective heat transfer characteristics of supercritical natural gas in microchannels,and developing new types of efficient and compact PCHE structure are of great significance for the efficient use of energy and sustainable development.In this thesis,firstly,the convective heat transfer of supercritical LNG in a single airfoil channel was studied by numerical simulation.The effects of inlet velocity and operating pressure of supercritical LNG on its flow and heat transfer characteristics were analyzed using velocity field,temperature field,Nusselt number,and friction coefficient.The results showed that,under constant inlet velocity conditions,the flow velocity and turbulence intensity of the fluid increase as the working pressure of supercritical LNG decreases,resulting in stronger convective heat transfer performance.As the pressure increases,the specific heat near the pseudo-critical temperature decreases,leading to reduced local heat transfer performance with increasing pressure.Supercritical LNG has better heat transfer performance at lower pressures.When the pressure decreases from 9.0 MPa to 7.0 MPa,the average heat transfer performance of the PCHE gradually increases,and the average Nusselt number at 7.0 MPa increases by 1.4%and 2.7%compared to that at 8.0 MPa and 9.0 MPa,respectively.However,the increased heat transfer performance is accompanied by an increase in pressure drop as the pressure decreases,which needs to be considered in practical applications.Secondly,the flow and heat transfer characteristics of supercritical LNG in PCHE with different airfoil channels was studied by numerical simulation,and the heat transfer enhancement mechanism was revealed through the secondary flow analysis.In addition,based on the simulation results,empirical correlation formulas for flow and heat transfer were summarized,and various airfoil channels were evaluated comprehensively using the identical mass flow rate criterion to select the optimal airfoil structure.The results showed that as the inlet flow velocity increases,the heat transfer performance of the airfoil also increases.At the same inlet velocity,the Nu number of NACA 0024 increases by 14.4%,7.3%,and 6.2%compared to NACA 63,NACA 23021,and NACA 2421,respectively.The comparison of all airfoil configurations indicates that symmetrical airfoils provide better heat transfer performance than asymmetrical airfoils.Furthermore,a suitable airfoil structure could reduce the influence of the boundary layer,increase the disturbance of internal flow,facilitate the formation of secondary flow,and strengthen heat transfer by changing the temperature gradient and velocity vector.The friction performance of NACA 0024 airfoil is the worst,while that of NACA 63 is the best.Finally,using the identical mass flow rate criterion,the comprehensive performance of four airfoils was compared,and the performance of airfoil channels was found to be superior to that of straight channels.The heat transfer rate of all airfoils increases with increasing Reynolds number,and NACA 0024 provides the best heat transfer performance.At last,Finally,the influence of structural parameters on the flow and heat transfer of supercritical LNG in the airfoil channel was analyzed,and a new non-uniform arrangement method was proposed.The flow and heat transfer performance of supercritical LNG in the PCHE with non-uniform arrangement was analyzed through numerical simulation.Five new airfoil arrangements and traditional airfoil arrangement methods were evaluated using performance evaluation criteria to obtain the PCHE with the best comprehensive performance.The results showed that staggered arrangement and appropriate sparse arrangement are more conducive to flow and heat transfer.Under the condition of the same staggered distance(L_s=4mm),the vertical distance L_v=5 mm increases by 25.3%,14.5%,and 5.9%compared with L_v=2 mm,L_v=3 mm,and L_v=4 mm,respectively.Secondly,for the PCHE with airfoil fins,upstream denser airfoil fin arrangement was a good choice to improve overall heat transfer performance.The best airfoil arrangement in these six cases was upstream dense arrangement(Case 3).Within the range of inlet velocity studied,compared with the average arrangement(Case 1),the average relative deviation of PEC values of upstream denser arrangement 1(Case2),upstream denser arrangement 2(Case 3),downstream denser arrangement 1(Case 4),downstream denser arrangement 2(Case 5),and middle thinner arrangement(Case 6)were4.6%,6.2%,-6.9%,-6.1%,and-4.5%,respectively.In this thesis,research on convective heat transfer performance of supercritical LNG is gradually carried out in the order of“single airfoil channel,airfoil selection,and structural optimization”It analyzes the influence of different airfoil structures and arrangement methods on flow and heat transfer,and introduces a novel non-uniform airfoil arrangement method.The innovative non-uniform airfoil arrangement proposed in this paper and the comprehensive analysis of various airfoil arrangements provide valuable insights for the study of supercritical LNG flow behavior and heat transfer mechanisms,as well as for the optimization design of channels in PCHE.