Heat transfer of supercritical carbon dioxide in mini/micro tubes.

摘要

As a non-flammable and non-toxic natural fluid, CO₂ has a zero ODP (ozone depleting potential) and a zero effective GWP (global warming potential). Thus, it will become a primary candidate for the next-generation environmentally benign refrigerant in automobile air-conditioning, railway air-conditioning, residential air-conditioning and heat pumps. Cycles using carbon dioxide as the refrigerant usually have to operate in a transcritical cycle. As such, the heat rejection takes place at supercritical pressures in a so-called gas cooler. Use of mini/micro channels in the gas cooler not only allows one to handle high pressures effectively without excessive wall thickness and material weight, but also makes the refrigeration systems extremely compact. The objective of this thesis was to study the convective heat transfer characteristics of supercritical CO₂ in mini/micro tubes. There are two major challenges in this work: (i) thermophysical properties of the fluid exhibit rapid variations for the problems under investigation; and (ii) the flow passages are extremely small.; The thesis began with the development of a cycle simulation model that predicts and optimizes the COP (coefficient of performance) of transcritical CO₂ Cycles. Based on the cycle simulation, correlations of the optimal heat rejection pressure in terms of appropriate parameters were obtained for specific conditions. The cycle simulation also showed that the COP of the transcritical CO₂ cycle increases significantly with the decrease of the outlet temperature of the gas cooler. Therefore, to design a gas cooler with high heat transfer efficiency is crucial to transcritical systems.; Laminar convective heat transfer of supercritical CO₂ flowing in vertical min/micro tubes was then investigated numerically in this thesis. Typical velocity profiles, temperature profiles, Nusselt numbers, and skin-friction coefficients for circular tubes having diameters of 0.5, 0.7, 1.4 and 2.16 mm under both cooling and heating conditions with and without gravity were obtained.; Finally, turbulent convective heat transfer and pressure drops of supercritical CO₂ flowing in horizontal and vertical mini/micro circular tubes under cooling and heating conditions were investigated experimentally. Six stainless steel circular tubes having inside-diameters of 0.50 mm, 0.70 mm, 1.10 mm, 1.40 mm, 1.55 mm, and 2.16 mm were tested. Measurements were carried out for pressures ranging from 74 to 120 bar, temperatures ranging from 20 to 120°C, and mass flow rates ranging from 0.02 to 0.2 kg/min. It is found that the buoyancy effect was still significant, although supercritical CO ₂ was in forced motion at Reynolds numbers up to 105. The experimental results also indicate that the Nusselt number decreased with the reduction in tube diameter. Based on the experimental data, correlations were developed for the Nusselt number and pressure drop in terms of appropriate dimensionless parameters for forced convection of supercritical CO₂ in mini/micro tubes. The results are of significance for the design of mini/micro channel gas coolers in the transcritical CO₂ cycles.