Study On Performances Of Heat Exchanger In Bypass Stream Of Aero-engine

In a turbine with high thrust-weight ratio, the cooling enhancement of high temperature componentsmeets very prominent contradiction: on the one hand, the high temperature components demandstrengthen cooling ability because of the rising temperature before turbine section; on the other hand,the quality of cooling air reduces due to higher pressure ratio in compressor section. Although thiscontradiction can be alleviated by using the high temperature resistive material, but improving thecooling air quality is a practical and effective approach. At present, using by-pass air to air heatexchanger is a kind of relatively simple and reliable way to reduce the cooling air temperature.Numerical simulation was conducted by using the commercial code FULENT in this paper to studythe flow and heat transfer characteristics of different structure of heat exchanger. On this basis, usingthe ANSYS software to analysis the thermal stress of the typical structure of heat exchanger thermalstress simulation and analysis, obtains the related stress distribution.First of all, according to the study of several different shaped-tube heat exchangers, Flat-ellipse tubewith good streamline gets minimal pressure loss in by-pass. But inside the tube, Flat-ellipse withnarrow channel and high flow resistance gets biggest pressure loss coefficient. The pressure losscoefficient caused by circle tube is only75.5%of flat-ellipse tube. Compared with the ordinarysmooth tube, slotted tube increased the tube fluid turbulence, which causes heat transfer enhanced.But at the same time this kind of turbulence has brought greater flow loss, thereby causing the tube airmass flow reduction, which weakened the whole heat exchange capability. The increase of blockageratio effectively improving overall cooling ability of heat exchanger, but the pressure loss of by-passincrease sharply, when pipe spacing ratio change from H/D=8to H/D=5.5the by-pass pressureloss coefficient amplitude is amounted to101%.Secondly, structure design on the enhancement of heat transfer of the heat exchanger has beenanalyzed. Fin structure based on smooth tube effectively extends the heat exchanger area, so that itenhanced the heat exchange ability. With the increase of the density of fin arrangement, heatexchanger capacity gradually improved, the by-pass pressure loss also increases accordingly, but theincrease trend of pressure loss is on the decrease. Under the same condition slotted fin gets higherheat transfer capability than smooth fin, mainly because of fluid in each slit forming a small eddy.These eddy effectively increasing the disturbance and improve the heat exchange effect, but the by-pass pressure loss also increases accordingly. The model with slot width=1mm and slotted spacingratio s₂/s₁=3gets the best heat transfer effect, which formed strong and coherent vortex around thefins.Finally, thermal stress of heat exchanger structure has been analyzed in this paper. Results show thatthermal stress maximum positions of smooth pipe structure are in pipe bends; The integral finstructure limits the tube deformation, so thermal stress concentration phenomenon is obvious, themaximum stress position in the connection between tube and fin, resulting in damage of rube at thesame time; The structure, cut the integral fin apart, is a well relief to the contradiction between heattransfer enhancement and stress concentration.