Design and Optimization of High Temperature Heat Exchanger in Aero-engine based on Wall Temperature Control Method

This paper proposes an optimization approach with genetic algorithm (GA) for the air-fuel tubular heat exchanger which is applied for multiple working conditions in the ventilation pressurizing system of high-speed aircraft. A new segmented design method of variable transverse tube pitch to control the wall temperature to avoid coking in the high temperature heat-exchange unit and enhance the heat transfer rate in the low temperature heat-exchange unit is presented in this work. The sparse tube wall pitch layout is adopted to control the wall temperature in the high temperature heat-exchange section, and the dense tube wall pitch layout is employed to enhance the heat transfer rate. Based on the logarithmic mean temperature difference (LMTD) method and the segmentation design method, the heat-exchange tube bundles are divided into N heat-exchange units along the direction in which the air flows, and the heat transfer performance, flow resistance performance, and the weight of heat exchanger under different working conditions can be evaluated. Under the given performance constraints, tube diameter, transverse tube pitch, tube height, heat exchanger width, and number of heat-exchange units are selected as optimization variables, and the standard power-to-weight ratio is taken as the objective to optimize the structure parameters of the heat exchanger. The results show that using the design method of variable transverse tube pitch is efficient in controlling the wall temperature and enhancing the heat transfer ability of the heat exchanger. In the optimization process, the objective function and heat transfer coefficient increase significantly while the heat transfer area and weight keep decreasing, and it is difficult to get an optimal solution that can reach the boundary of all constraints at the same time, which means the main factor limiting the further increasing of standard power-to-weight ratio is changing continually. Compared with the initial structure, the convective heat transfer coefficient at both the air side and fuel side can be improved greatly, and the wall temperature of the heat-exchange tube and the pressure loss inside and outside the tube increase within the allowable range. By using the optimization method, the standard power-to-weight ratio of the heat exchanger increases by 99.69%, from 28.28 W/(kg·K) to 56.47 W/(kg·K), and the weight decreases by 44.11%, from 2.14 kg to 1.20 kg. To sum up, a more efficient and compact tubular heat exchanger that meets the requirement of wall temperature is designed, which could provide insight into the field of aeronautical high temperature heat exchanger design.