Research On Thermal-Hydraulic And Technology Characteristics For Offset Strip Fin Cooler

From a long time, the fundamental research is still scare on offset strip fins taken liquid fluid as working media, which is limited to independent development and innovation of domestic offset strip fin coolers. The purpose of the thesis is to get the design theory and method of heat exchangers applicable to domestic producing condition, synthetically considering each factor affecting heat exchangers. The thesis has integrated domestic heat exchanger processing technology closely and carried out the experimental and simulated research on several kinds of offset strip fins. The experimental correlations to predict the thermal and hydraulic performance on offset strip fin coolers are obtained. The simulated results on fin performance are used to instruct the processing tolerance design of important geometries for offset strip fins. The effects of two important pressing technologies, fin pressing technology and stainless steel vacuum braze, on heat exchanger performance are focused on, and the quantitatively correction methods are discussed. Lastly, the performance of plate fin oil coolers adopted offset strip fin is simulated and predicted.(1) Taken the single layer fin as test specimen, the test rig to measure the heat transfer and friction characteristics is set up. The CD15W/40 lubricant oil commonly used in engine lubricating system is taken as working media. The 23 kinds of HPD (High Pressure Direction) type, 26 kinds of LPD (Low Pressure Direction) type and 16 kinds of X (Inclined Direction) type offset strip fins are experimented respectively at actual working conditions and the effects of geometrical parameters on fin performance are analyzed. The experimental correlations on each type of offset strip fin are gained to predict the thermal and hydraulic characteristics of offset strip fin coolers, which can be used in practical engineering design.(2) Based on computational fluid dynamics simulation technique, the performance on the HPD type offset strip fins has been simulated. Taguchi method is developed by Dr. Taguchi according to statistical theory, which adopts design of experiment to select the optimal value and the combination of component parameters affecting the quality system of values, in order to make the system quality value fluctuate smallest and the stability best. Taguchi method is introduced to carry out parametrical study on the HPD type offset strip fins, and the optimum scheme is obtained, which is originated from the geometrical parameter combination based on the optimal combination of performance and cost. The multiple quality-cost loss function is introduced. Taken the heat transfer quantity and the pressure drop as two quality targets, and the whole loss of two variables as the optimum object function, the processing tolerances of fin height and fin wavelength are designed, which instructs the tolerance control in fin practical processing.(3) Based on the fin-base plate brazed joint formation mechanism and the minimum energy principle during the solder melting and balance period, the fin-base plate brazed joint topology is predicted. The correlation is quantitatively obtained on the solder thickness and geometrical topology of fin brazed joint. The finite element analysis method is adopted to compare the thermal contact resistances of fin-base plate brazed joints having different geometrical parameters. It is concluded that the thicker the solder thickness is, the smaller the thermal contact resistance is, and it is beneficial to increase the fin heat transfer efficiency. The thermal and hydraulic characteristics of fins with different brazed joints at different solder thicknesses have been simulated by CFD, and the correction methods are quantitatively gained on fin heat transfer factor and friction factor at different solder thicknesses, which is helpful to get the high performance and low cost products.(4) At different pressing times for cold pressing die made of Cr12MoV, 11 specimen offset strip fins are selected. The heat transfer and friction characteristics of 11 specimen fins are experimental researched. The effect of mould on fin performance is analyzed. The experimental results show that with the variation of mould pressing times, the heat transfer and friction performance varies obviously corresponding to the three mould wear stages, early wear stage, normal wear stage and excessive wear stage. The performance of fins pressed at excessive wear stage deteriorates sharply. The cold mould operational lifetime is recommended at the end of mould normal wear stage and the Cr12MoV cold mould lifetime is about 1,050,000 pressing times. The relations of friction factor, pressing times and Reynolds numbers are obtained at each mould wear stage, which is used to correlate the fin friction performance calculation. (5) The metallographic microscope is used to observe the longitudinal and transverse sections for 11 specimen fins pressed at different pressing times. When the mould wear stage is the excessive wear stage, fin surface burrs increase and the surface roughness becomes larger. From the transverse section observations, with the mould pressing times increasing, the fin transverse sections of'rectangle'shape become'meniscus'shape.'The area transform factor'is defined to judge the area variation of fin transverse sections, and the judging criterion of mould wear stage is put forward.(6) Based on CFD simulation technique, the porous media model is adopted to simulate oil side core of plate fin oil coolers adopted offset strip fin, and the thermal and hydraulic characteristics of oil coolers are simulated. From the distribution principles of mass flow rate and heat transfer quantity at each layer core, the optimum rule for core layer numbers is obtained. Considering the effects of pressing die and braze technology on fin performance, the focus parameter model is adopted, and the heat transfer and friction characteristics for plate fin oil coolers are predicted. The comparisons between calculated results and experimental results show that the method, combined the experimental correlations and the relevant correlated methods, is met the requirement for performance prediction of plate fin oil coolers adopted offset strip fin.As a whole, based on the research means combined experiment with simulation, the fundamental research on offset strip fins provides an academic foundation for the independent development of offset strip fin coolers. The domestic processing technology is considered closely, which makes the fundamental research results transform faster and more effectively and accelerates the independent brands building for domestic automobile parts.