Study On Performance Of Airborne Muti-stream Plate-fin Heat Exchanger

To satisfy the requirements of increasing amount of cooling for aircraft equipments, multi-stream plate fin heat exchanger(MPFHE) with high efficiency and compact structure becomes a new trend. Due to its complex structure of passages and so many influencing factors to flow temperature in each passage, fin bypass effect as well as phenomenon such as temperature cross, heat internal friction will happen under certain conditions. On the other hand, cross flow of ram-air and other fluid or phase change under aircraft condition will make heat transfer mechanism more complicated. Accurate numerical method and optimum structural design are investigated in this paper. Contents are as follows:(1) Numerical calculation method for MPFHEThe finite volume method is adopted, and MPFHE is divided to sub-cell heat exchangers with number of W×L. Mathematical model of MPFHE with cross type is established for the first time with sub-cell heat exchanger size independent from fin distance and transverse energy transfer independent from flow method between sub-cells, which can be calculated using programs compiled by VC++. In addition, for MPFHE with periodic channel arrangements, cycle closed control equations are established based on the characteristic that heat transfer amount is equal between plates up and down with a periodic channel arrangements, which reduces the calculation workload. Considering the influence of fin surface characteristics to calculation accuracy, optimal experimental correlations are selected for specific fin structure using error analysis for experiment data of foreign authority.(2) Performance analysis of aircraft three-stream plate fin heat exchangerBased on numerical method for MPFHE, temperature distributions of each flow and fins of three-stream plate fin condenser under two passage arrangements are conducted. The relationship between energy distributions and max heat transfer amount of target fluid is analyzed. It is considered that single-phase fluid heat transfer will be restricted due to big heat capacity of phase change fluid and high heat transfer coefficient on phase change surface. Utilization of heat transfer area is different under different arrangements, which means utilization of heat transfer area is higher when heat transfer area is allocated to fluids with large heat capacity and high heat transfer coefficient under same driving force of temperature difference, leading to high volume of heat transfer. Otherwise, it can be found from structure optimizing design of existing heat exchanger that if overlapping passages with same fluid are considerd as one passage while ratio of heat load is close to which of passage distribution, it is more beneficial for operating condition of heat exchanger to be close to design condition.(3) Experimental investigation on aircraft three-stream plate fin heat exchangerExperimental research on heat transfer characteristics of cross type three-stream plate fin heat exchanger with and without phase change have been proposed on thermodynamic experiment bench and evaporation refrigeration cycle experiment bench, verifying the accuracy of numerical method. Taking the cross type three-stream plate fin heat exchanger under experimental conditions as an example, it is analyzed for the feasibility of neglecting horizontal heat transfer as well as subcell size chosen. Intensive meshes do not have any influence on numerical results from the aspect of predicting outlet flow temperature, however will reduce calculating efficiency. Results of experiment and numerical calculation are compared demonstrating numerical calculation method can meet the requirements of high precision calculation in aircraft condition. Meanwhile it is found that the ununiformity of flow distribution inside the heat exchanger as well as calculation of surface heat transfer coefficient with phase change flow have large influence on accuracy of numerical calculation.(4) Investigation on the influence of flow distribution characteristics to heat transfer performanceModels of shell cover and single passage are established by Fluent, and then flow distribution characteristics in the passage and shell cover are investigated. It is considered that arrangement of shell cover and inducer will decide the flow distribution trend inside and between passages while flow distribution ununiformity will be effected by mass flow rate and passage resistance. Flow distribution model under specific structure is established based on CFD simulation results, meanwhile the influence of flow distribution ununiformity to heat transfer performance is investigated according to calculating program of heat exchanger for the first time. It is concluded that under different inlet parameters and physical property parameters, the influence of flow distribution ununiformity to heat transfer characteristics is different leading heat transfer performance increasing or decreasing. Passage resistance affects flow distribution ununiformity, which means flow distribution becomes more ununiformity under small flow resistance, even reducing heat transfer performance of each stream.(5) Investigation on evaluation methods of passage arrangements for MPFHEEvaluation methods of passage arrangements for MPFHE are concluded. The reasonability of using mean square error of cumulative heat load to evaluate passage arrangements under the condition of a certain heat load and heat load evenly distribution of each passage is demonstrated. Considering variation of heat load and lateral heat transfer between passages, mean square error of cumulative heat load is nondimensionalized while equations of temperature uniformity factor are improved. Moreover, the applicability of each evaluation method is verified taking three-stream plate fin condenser as an example under the condition of different arrangement, different inlet parameters as well as different flow method. Under the condition of considering lateral heat transfer of heat exchanger, to reduce calculating work of applying optimization algorithm to optimize passage arrangement, a new method applying sub-cell heat exchanger arrangement characteristics to search optimal passage arrangements is proposed for the first time. Meanwhile the method of selecting inlet parameters for sub-cell heat exchanger is investigated under the condition of different flow method, inlet parameters as well as fluid parameters.