Experimental study of evaporation heat transfer of refrigerant HCFC22 inside an internally grooved horizontal tube

The characteristics of flow pattern, heat transfer and pressure drop of evaporation of HCFC 22 inside a horizontal tube are experimentally revealed. The experiment is conducted with a double-tube evaporator, where the refrigerant flows inside the inner tube and the heating water flows countercurrently in the surrounding annulus. The inner tube is an internally grooved copper tube having a 9.52 mm o.d. and an 8.72 mm mean i.d. The ranges tested for the refrigerant are as follows: mass velocity of 110 to 220 kg/(m²s), vapor pressure of 0.4 to 0.65 MPa and heat flux of 5 to 35 kW/m². The flow pattern observed through sight glasses changes from the wavy-annular type to the annular type and then to the mist flow type as the evaporation progresses along the tube. These transitions of flow patterns occur at much lower values of vapor qualities than those predicted from the Baker map modified by Scott for a smooth tube. The heat transfer coefficients in the wavy-annular and annular flow regions are about 2 to 4 times higher than those of smooth tubes, and are expressed as a simple function of the Lockhart-Martinelli parameters. The empirical equations for the heat transfer coefficient in the mist flow region and in the single phase heat transfer region are also presented. The accuracy of these equations along with the value of the transition quality is confirmed in a comparison between the design calculations and the experimental data.