Research On Nucleate Pool Boiling Heat Transfer Characteristics Of Refrigerant-based Nanofluid

As a new type of refrigerant, the refrigerant-based nanofluids composed of nano-powders and conventional refrigerants have gained increased interest. The refrigerant-based nanofluids have been proved by the existing research to have larger thermal conductivities than the conventional refrigerants. As the phase change heat transfer performance is related to the heat conduction performance, the refrigerant-based nanofluids are expected to have higher boiling heat transfer performance than the conventional refrigerants. Therefore, it is necessary to investigate the boiling heat transfer of refrigerant-based nanofluids.In order to investigate the boiling heat transfer of refrigerant-based nanofluids, researchs were started with the boiling heat transfer by experiment and theoretical analysis. The main works and conclusions are reviewed as follows:(1) In order to investigate the nucleate pool boiling heat transfer charctersistics of refrigerant-based nanofluids, the experimental setup was designed and built. The nucleate pool boiling characteristics of the refrigerant-oil mixtures with spherical nanoparticles and CNTs were experimentally investigated. The influences of surfactants on the nucleate pool boiling of refrigerant-based nanofluids and refrigerant/nanolubricant mixtures were tested.(2) By the scanning electron microscope (SEM), the influences of spherical nanoparticles and CNTs on the heating surface characteristics were analyzed. The method for quantitatively predicting the nucleate pool boiling heat transfer of phase change fluid with the nano-scale solid component and additional non-phase change component was proposed. Based on this method, the nucleate pool boiling heat transfer coefficient correlations for refrigerant-oil mixtures with spherical nanoparticles and CNTs were proposed. By introducing the surfactant impact factor, the correlations for predicting the influences of surfactants on the nucleate pool boiling heat transfer of refrigerant-based nanofluids and refrigerant/nanolubricant mixtures were established.(3) The results showed that the presence of nano-powders enhances the nucleate pool boiling heat transfer of refrigerant/nanolubricant mixture. The higher thermal conductivity and the smaller diameter of spherical nanoparticles, and the smaller diameter and the larger length of CNTs are beneficial to the enhancement of the nucleate pool boiling heat transfer. The main reason for the enhancement of the nucleate pool boiling heat transfer is the formation of nano-scale porous layer on the heating surface, which leads to the increase of the cavitation density and the active nucleation site density of the heating surface. The nanopraticle size and concentration as well as the CNTs physical dimension have influences on the structure of nano-scale porous layer, and then affect the enhancement degree of the nucleate pool boiling heat transfer. The presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid or refrigerant/nanolubricant mixture on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The enhancement of the nucleate pool boiling heat transfer increases with the increase of surfactant concentration and then decreases, presenting a maximum value. The surfactant concentration corresponding to the maximum value increases with the increase of nanolubricant concentration. The order of the enhancement of nucleate pool boiling heat transfer for different types of surfactants is opposite to the order of surfactant molecular weight values, and the enhancement increases with the decrease of nanoparticle concentration, nanolubricant concentration and heat flux. The nucleate pool boiling heat transfer coefficient correlations for refrigerant-oil mixtures with spherical nanoparticles and CNTs are applicable to the spherical nanoparticles with different types and sizes as well as the CNTs with different physical dimensions, and have good accuracy. The nucleate pool boiling heat transfer correlations for refrigerant-based nanofluids and refrigerant/nanolubricant mixtures containing surfactants can accurately predict the influences of surfactants on the nucleate pool boiling heat transfer of refrigerant-based nanofluids and refrigerant/nanolubricant mixtures.