Optimal Design Of Carbon Dioxide Microchannel Gas Cooler And Molecular Dynamics Simulation

At present, with the rapid development of economy, energy and environment issues have become a major problem restricting the sustainable development of human society and the economy, such as: the haze problem in recent years. Therefore,natural refrigerants are paid more and more attention to promote in international refrigeration field by international refrigeration industry. The researches have been focused on the development of substitutes because Freon(HCFC and CFC are contained) refrigerants destroy the ozone layer and aggravate the greenhouse effect.Natural refrigerant carbon dioxide as a non-flammable, non-toxic natural fluid has zero ODP(ozone depletion potential) and lower effective GWP(global warming potential.) Carbon dioxide has advantages in security, flammability, excellent thermos-physical properties and so on, therefore,carbon dioxide is the first choice in natural refrigerants.Mirco-channel parallel flow gas cooler has better performance in trans-critical CO2 automobile air conditioning system with its structural advantages and high heat transfer efficiency.Therefore, in this article based on the theory and numerical analysis, the heat transfer performance of carbon dioxide in the micro-channel parallel flow gas cooler is researched and also analyzes its structure optimization. At the same time, by using of molecular dynamics calculations the microstructure and transport properties of carbon dioxide. For solving the above problems, in this paper we carried out research in the following parts:1. The advantages and disadvantages of the integral finned heat exchanger are analyzed. The height, thickness and pitch of the fin are used as input variables and the airside minimal pressure drop and the biggest heat exchange as the output optimization goals. The mathematical model built with the CFD software ANSYS isimported into the optimization software mode FRONTIER, which is calculated with the multi-objective genetic algorithm to be optimized. The optimized results show that the height of the fin should be 5.7-6.3mm, the thickness 0.15-0.25 mm and the fin pitch 1.3-1.4mm, when the airside pressure drop and the property of the heat exchanger achieve the best combination.2. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are analyzed using CFD numerical simulation, focuses on the influence of structural parameters on the heat exchanger pressure drop and heat transfer coefficient, the influence of the geometry parameters such as the fin height,pith and thickness on the heat exchanger pressure drop and heat transfer coefficient are analyzed. And we conclude that with the decreasing of fin height, increasing of thickness and decreasing of fin pitch, the pressure drop and heat transfer coefficient will increase.3. For studying the microscopic structure of carbon dioxide, we use the most powerful molecular modeling software Materials Studio. First, we study about all the key techniques of molecular dynamics simulations, including ensemble, force field and step length and so on; then we simulate the density of supercritical carbon dioxide as well as the self-diffusion coefficient; finally, we compare the density value calculation results with Refprop. Analysis of the parameters derived molecular dynamics simulation settings accurately, we find it can accurately calculate the physical properties of supercritical carbon dioxide. We also study the trend of self-diffusion coefficients of carbon dioxide at different temperatures and pressures.