Heat transfer and pressure drop of condensation from superheated vapor to subcooled liquid

摘要

The focus of this paper is heat transfer and pressure drop during condensation in a horizontal smooth round tube. R134a in 6.1 mm inner diameter is used as an example for heat transfer measurements operated in a range of mass fluxes from 50 kg m~(-2) s~(-1) to 200 kg m~(-2) s~(-1) and heat fluxes from 5 kW m~(-2) to 15 kW m~(-2). The paper also presents pressure drop measurements with mass flux from 100 kg m~(-2) s~(-1) to 200 kg m~(-2) s~(-1) showing the effect of mass flux and heat flux on the heat transfer coefficient (HTC) and pressure drop. All the measurements are taken at a constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 ℃. By connecting heat transfer results with flow characterizations, the behavior of HTC is explained from the perspectives of both mathematics and physics. The result shows that for fixed heat flux and different mass flux, even though the liquid film for higher mass flux is much thinner in the condensing superheated (CSH) region, the HTC is not affected by mass flux. In the two-phase (TP) region, however, higher mass flux clearly yields higher HTC. Opposite behavior is found when heat flux is varied and mass flux is fixed. In the CSH region, HTC increases with heat flux, while in TP region, HTC does not change with heat flux. For both cases, a peak of HTC is presented at quality one, which seems to imply some counter-acting factors that always put heat transfer largest at quality one. The counter-acting factors, however, should not exist based on flow characterization. After mathematically explaining the behavior of HTC in CSH region, film HTC (HTC_f) is proposed and ready to serve as a tool in a unified heat transfer model throughout the entire CSH and TP regions. The result of film HTC shows that the film HTC goes to infinite at the onset of condensation, which is physically correct. Also, the film HTC increases with increasing mass flux. The effect of heat flux, or wall temperature on film HTC is still under discussion. In this study, higher heat flux yields lower film HTC only at high qualities, which is due to the difference in void fraction. In addition, the result of pressure drop measurement also shows that the presence of liquid film tends to increase the pressure drop, which suggests that the wavy structure of vapor-liquid interface increases the friction of the flow. The peak of the pressure drop, however, occurs at lower enthalpy for higher mass flux. The reason could be the later onset of condensation and thinner film, which delays the peak of vapor-liquid interaction at higher mass flux.