| High-power compressor is needed in continuous transonic/supersonic wind tunnel to keep the wind tunnel run steadily. The power provided by the compressor is changed into thermal energy totally,and the air flow is heated. Because of this high power input, the air downstream of the compressor must be cooled to remove the heat of compression. Most modern transonic/supersonic wind tunnels utilize water-cooled heat exchanger located in the tunnel circuit to provide the required cooling. As a critical element of transonic/supersonic wind tunnel, both heat transfer design and structural design of the heat exchanger system can have a significant impact on wind tunnel performance. The heat exchanger must be properly designed to attain the required aerodynamic and thermodynamic performance. The design objective is to ensure the heat removal capacity of heat exchanger must be maximized while minimizing the air-side pressure drop and improving temperature uniformity of the outflow of heat exchanger.In order to design a heat exchanger with high performance, this paper focuses the research work on the finned elliptic tube heat exchanger. These jobs include engineering calculation and numerical simulation of air flow and heat transfer. The major influence factors are also discussed in this paper so as to afford the designers the major design considerations to match both the heat transfer rate and friction requirements.In engineering calculation, the structural size of a group of heat exchangers is initially given based on the work conditions and characteristics of the wind tunnel heat exchanger. Then, the heat transfer and pressure drop performance is studied using experimental formulae suited for this case. A plan of numerical simulation is presented based on the foregoing research work. The physical geometry and the pattern of the flow/thermal solution of the finned tube heat exchanger have a periodically repeating nature. There is no need to simulate the whole heat exchanger model. Periodic boundary conditions are used to solve such question in FLUENT, so only one periodic region of heat exchanger is needed simulating. Consequently, the grid amount is reduced greatly, the convergence is speeded up and the simulation results have a better precision. There is certain difference in the numerical value between engineering calculation and numerical simulation. But the structural parameters and the flow conditions have the same influence trend. Factors which most affect the performance of finned heat exchanger are tube type, fin parameters, arrangement of heat transfer tubes and so on. Numerical results also indicate that thermal conductivity of fin material has a significant impact on thermal performance but changes total pressure drop little.Heat exchanger will influence the uniformity of air flow. It is necessary to make it clear. A theoretic model is developed to study temperature distribution along the tubes in the outflow section of in-line heat exchanger. It indicates temperature grads augments along with the increase of tube row number. In order to meet the temperature uniformity requirements, the tubes should be connected in a way that water flows in the first row from top to bottom, in the next row from bottom to the top. This counterflow principle holds for the total rows of the heat exchanger. And the variation of outflow temperature can be controlled within 1 K. Theoretic analysis results are consistent with numerical simulation results. Based on the numerical simulation results, the variation range of velocity and temperature will de reduced in the downstream flow because of large outflow turbulence.Finally, a design method of heat exchanger is put forward based on the research results and the working conditions of 0.6m transonic/supersonic wind tunnel. The performance parameters of this heat exchanger are calculated out. The uniformity of outflow temperature is analyzed too in this paper. The results are found to meet the anticipated design specification. |
