Abstract
Nowadays, global environmental events such as thinning of the ozone layer and climate changes are increasing. These types of events are not only affecting all creatures living on the Earth, but also decreasing the quality of life. For this reason, natural refrigerants which are not harmful to environment, have been preferred in cooling systems. In this study, heat transfer coefficient of CO2 was investigated during the evaporation process in smooth tube and designed microfin tube. Features of the tube used in evaporator of these cooling systems directly affect to the heat transfer coefficient. The geometric parameters of the microfin tubes are an outer diameter of 9.52 mm, number of fins 50, apex angle of 38°, helix angle of 20° and fin height of 0.12 mm. Theoretical model was created on MATLAB environment. The heat transfer coefficient was investigated based on vapor quality. When theoretical results obtained for microfin tube were compared with experimental results, 6% approach was seen. A theoretical model was created for smooth and microfin tube by selecting heat flux as 10 kw/m^2 and mass flux as 380 kg/m2s and heat transfer coefficients in different evaporation temperatures were compared based on vapor quality. It has been concluded that heat transfer coefficients of microfin tube at 5°C, 0°C and -8°C were 52%, 44% and 34% higher than that of smooth tube, respectively.
Highlights
Conventional refrigerants such as CFC, HCFC and HFC refrigerants used in air conditioning systems, cause to depletion of ozone layer and global warming
The change of heat transfer coefficient depending on the vapor quality was theoretically modeled during CO2 evaporation in microfin tube
In this paper, the change in the heat transfer coefficient of CO2 depending on vapor quality was theoretically investigated in microfin and smooth tube
Summary
Conventional refrigerants such as CFC, HCFC and HFC refrigerants used in air conditioning systems, cause to depletion of ozone layer and global warming. Cho et al [5] investigated the heat transfer coefficient of CO2 in smooth and microfin tube during boiling process. When results taken from experiments were compared to theoretical results, close to 50% difference between the results was seen For this reason, for CO2 microfin tube at low evaporation temperature, necessity to development of new correlations was stated. Yoon et al [11] investigated the evaporation heat transfer coefficient of CO2 at low temperature of -30 to -20°C in a horizontal smooth tube. The evaporative heat transfer coefficient of CO2 was theoretically examined in smooth and microfin tubes. In smooth and microfin tubes, heat transfer coefficients of CO2 were compared, at different evaporation temperatures such as -8°C, 0°C and 5°C.
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