Experimental investigation on heat transfer characteristics of supercritical carbon dioxide inside a horizontal micro-fin copper tube during cooling process

Experimental study on cooling heat transfer of CO₂ and CO ₂-PAG mixture flowing in a horizontal micro-fin tube under the supercritical pressure conditions was performed. The test tube having the average inner diameter of 4.76 mm, the fin height of 0.245 mm, the number of fins of 50, the area expansion ratio of 2.08 was conducted in this study. The microfins of the test tube were treated in straight along the axes of the tube on the heat transfer surface. The heat transfer coefficients of pure CO ₂ were measured at conditions of 8 and 10 MPa in pressure and 340 - 660 kg/(m2 s) in mass velocity. The measured heat transfer coefficients of the micro-fin tube showed 1.7 times higher than that of smooth tube with 4.42 mm inner diameter. The effect of PAG on heat transfer characteristics were investigated experimentally at 10 MPa in pressure, 510 kg/(m2 s) in mass velocity and 0.06 to 2.26 % in the mass percent concentration of PAG in CO ₂-PAG mixture. The measured heat transfer coefficients of CO ₂ -PAG mixture decreased about 50% as compared with that of pure CO₂ at the pseudocritical temperature.