Research On Flow And Heat Transfer Characteristics Of Compact Heat Exchanger In Intercooled Recuperated Engine

Intercooled recuperated cycle engine has attracted much attention for its advantages of reducing fuel consumption,emission of pollutants,noise and so on.The intercooler is located in the bypass fan-passage between the middle pressure compressor and high pressure compressor,for the purpose of reducing the compressor inlet temperature and thus decreasing the power consumption of high pressure compressor.The recuperator is located in the nozzle downstream the high pressure turbine outlet.It is acted as a heat exchanger to exploit the thermal energy of the external high-temperature exhaust gas for pre-heating the internal air which is drawn from compressor outlet and then forced into combustor.Therefore,the utilization of intercooled recuperated cycle is capable of decreasing the fuel delivery of combustion chamber and improving the thermal efficiency for a given turbine inlet temperature.However,the introduction of the intercooler and recuperator as well as the corresponding pipeline system would inevitably lead to the increase of internal and external bypass pressure losses as well as the total weight,which will seriously affect or even offset the advantages of intercooled recuperated cycle engine.So that,a light-weight and low-loss compact heat exchanger is essential in the practical application of intercooled recuperative aero-engines.Focused on the two technical indicators of pressure loss and heat exchange efficiency,the model experiments and corresponding numerical calculation are carried out in the presnt paper to research the internal and external flow and heat transfer characteristics of intercooler and regenerator.The main research contents and conclusions of the present work are the followings:Model experiments and numerical simulations are carried out for a typical double-U-shaped tube bundle heat exchanger which is regarded as one of the most promising heat recuperator geometries in the aero-engines.Firstly,single double-U-shaped tube bundle heat exchanger embedded inside a rectangular passage is taken into consideration in the modeled experiment.The influences of tube type,installation angle,air inlet mode and temperature on the internal and external pressure drops and heat exchange efficiency are studied.Secondly,tandem double-U-shaped-tubes heat exchangers embedded inside a nozzle are taken into consideration in the modeled experiment.The influence of typical arrangement modes on external pressure drop and heat exchange efficiency of the tubes is compared and analyzed.Based on the model experiments,the corresponding numerical calculation models are established to analyze the velocity field and temperature field to reveal more details of flow and heat transfer features.Due to the large tube number and closed bundle arrangement of the U-shaped tube bundle heat exchanger,a simplified approach utilizing an isotropic porous medium model is adopted on the modeling of the flow through a compact exchanger.Simuilaneously,three-dimensional CFD(3-D CFD)method is also adopted for the direct computation of flow field and heat transfer performances.The results show that the mounting angle of double-U-shaped-tubes heat exchanger has a more obvious effect on the pressure drop compared to tube type.The influences of mounting angle and air inlet mode are behaved more significantly than the tube type on the heat exchange efficiency.The mounting angle of the regenerator should be decreased gradually along the air flow direction to ensure the relative uniformity of heat exchange efficiency.It is suggested that the isotropic porous medium assumption is capable of making precise pressure drop prediction given the reasonable pressure-velocity relationship,however,unable to make precise simulation of the detailed flow features in the channel with a built-in double-U-shaped tube bundle heat exchanger.Model experiments are carried out for a typical straight-shaped tube bundle heat exchanger in V-type arrangement,which is regarded as one of the most promising intercooler geometries in the aero-engines.The influence of tubes angle on the internal and external pressure drop as well as heat exchange efficiency is comparatively analyzed.The corresponding numerical calculation model based on the model experiments is also established to investigate the influences of tube position and air along the process of pipe on the internal tube flow rate distribution,the influence of tube angle on the external flow field as well as the influences of flow rate distribution mode and tube diameter on coupled heat transfer.The results show that the tube angle has a significant effect of on the external tube pressure drop but a limited effect on the internal tube pressure drop as well as the intercooled efficiency.As the bundle angle increases,the comprehensive performance is improved gradually.The criterion correlation of external Nusselt number changed with external Reynolds number was summarized and compared with the empirical formula of the air flow crossed tubes according to experimental data,provide theoretical basis for the V-type arrangement of intercooler.In generally,reduction of the tube diameter and increase of the tube rows number can make the heat transfer coefficient and heat exchange area increased significantly,thus improving intercooling effect.Based on the model experiments of intercoolers and recuperators as well as the design-needed structural and aerodynamic parameters for a specific Intercooled recuperated cycle engine,the designing calculation program and checking calculation program for the intercoolers and recuperators are written by Matlab.The structural parameters and heat exchange area which could meet the required intercooled and regenerated performance are obtained,and the correspondingly numerical study on the really operational state is carried out.For the intercooler,the structures of plate-type and tube-type intercoolers are respectively analyzed.The results show that the tube structured intercooler has more advantages in heat transfer compared with the plate structured intercooler.The relativelyideal arrangement of multiple U-shaped tube bundle recuperators is presented according to the model experimental results.