A Numerical Study on Heat Transfer Characteristics of CO₂-DME Mixture Fluid

In response to the call for environment protection, more and more researches have been finding new alternative refrigerants which are of zero Ozone depleting potential (ODP), low global warming potential (GWP) and will be efficient, inflammable, and nontoxic to replace the HCFCs and HFCs refrigerants. Non-azeotropic mixtures of suitable fluids are becoming the important candidates of that search race. In this paper, the flow and heat transfer performance of CO₂-DME (dimethyl ether) mixture refrigerants at 8.0 MPa in a horizontal tube was numerically investigated. In particular, this work has been focused upon the laminar heat transfer characteristics of CO₂-DME mixture refrigerant flow with Reynolds number ranging from 180 to 1800 and the mass fraction of DME ranging from 21% to 39%. The results show that the heat transfer performance of the mixture fluid can be improved when the mass fraction of DME increases to 30%. Considering both the best heat transfer performance and the safety, the optimum component of CO₂-DME (70/30, mass%) has been found in this study. And the mechanisms that are responsible for the heat transfer enhancement are obtained and they are: (1) The temperature gradient of CO₂-DME mixture fluid with high mass fraction of DME is larger than that of CO₂-DME mixture fluid with low mass fraction of DME; in addition, the temperature gradient increases with the mass fraction of DME; (2) The increase of thermal conductivity for CO₂-DME mixtures can enhance the heat transfer performance to a large extent. (3) The self-acceleration characteristic of the supercritical mixture fluid is also beneficial to the heat transfer enhancement. Furthermore, it is found that the heat transfer performance of the mixture fluid can also be improved with Reynolds number. The viscous boundary layer of the fluid becomes thinner when the Reynolds number increases, which leads to an enhancement in heat transfer. In addition, at high Reynolds number, the flow is tend to be turbulent, which is beneficial to the heat transfer performance of mixture fluid. With regard to the flow characteristic of mixtures, first the flowing changes depend on the temperature and the density of fluid to a large extent-the high value of velocity can occur when the fluid has a high temperature and a low density. And secondly, the supercritical CO₂-DME mixtures speed up along the tube and make themselves different with water flow. These results of this study have significant implications for the design of high-performance heat exchangers, using CO₂-DME mixture as the working fluid.