Abstract
Supercritical carbon dioxide (CO 2 ) has special thermal properties with better heat transfer and flow characteristics. Due to this reason, supercritical CO 2 is being used recently in air-condition and refrigeration systems to replace non environmental friendly refrigerants. Even though many researches have been done, there are not many literatures for heat transfer and flow characteristics of supercritical CO 2 . Therefore, the main purpose of this study is to develop flow and heat transfer CFD models on two different phases; vapour and supercritical of CO 2 to investigate the heat transfer characteristics and pressure drop in micro-channels. CO 2 is considered to be in different phases with different flow pressures but at same temperature. For the simulation, the CO 2 flow was assumed to be turbulent, nonisothermal and Newtonian. The numerical results for both phases are compared. From the numerical analysis, for both vapour and supercritical phases, the heat energy from CO 2 gas transferred to water to attain thermal equilibrium. The temperature of CO 2 at vapour phase decreased 1.78% compared to supercritical phase, which decreased for 0.56% from the inlet temperature. There was a drastic increase of 72% for average Nu when the phase changed from vapour to supercritical. The average Nu decreased rapidly about 41% after total pressure of 9.0 MPa. Pressure drop (Δ P ) increased together with Reynolds number ( Re ) for vapour and supercritical phases. When the phase changed from vapour to supercritical, Δ P was increased about 26%. The results obtained from this study can provide information for further investigations on supercritical CO 2 .
Highlights
Carbon dioxide (CO2) gas, which has zero ozone depleting potential (ODP) and zero effective global warming potential (GWP) was reintroduced as an environmental friendly gas, and used as working fluid in refrigerators and air conditioning systems
The temperature of CO2 at vapour phase decreased 1.78% compared to supercritical phase, which decreased for 0.56% from the inlet temperature
The length of the tube is not enough for supercritical CO2 to heat up the water. For both vapour and supercritical phases, the heat energy from CO2 gas transferred to water to attain thermal equilibrium, as shown is Figures 2 and 4
Summary
Carbon dioxide (CO2) gas, which has zero ozone depleting potential (ODP) and zero effective global warming potential (GWP) was reintroduced as an environmental friendly gas, and used as working fluid in refrigerators and air conditioning systems. As the supercritical CO2 reaches near to its critical point, the physical properties shows extremely rapid variations with a change in temperature and pressure, which makes it as the most important characteristics [1]. The current refrigerants, such as chlorofluorocarbons (CFCs) and hydro chlorofluorocarbons (HCFCs), which are being used in air conditioning and refrigeration systems, have high ozone depletion and effective global warming potentials.
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