Condensation of R134a inside a horizontal microfin and smooth tube: experimental heat transfer and flow pattern visualization

Microfin tubes have already been widely used for air-conditioning and refrigeration applications as they ensure a large heat transfer enhancement with a relatively low pressure drop increase. During condensation microfin tubes show a heat transfer enhancement, compared to equivalent smooth tubes under the same operating conditions, that is partly due to the mere increase in the effective exchange area, and additionally to the turbulence induced in the liquid film by the micro fins and to the surface tension effect on the liquid drainage. The performances of these microfin tubes during condensation of refrigerants have been studied experimentally and theoretically. In this work the authors present their own data when condensing R-134a inside two different tubes: a 9.5 mm outer diameter microfin tube (7.69 mm inside diameter at the fin tip and 60 fins with 0.23 mm fin height and 13° helix angle) and a plain 8 mm inner diameter tube. Many authors use the so-called Wilson-plot method to measure the heat transfer coefficient of tubes; here, the experimental data have been taken by direct measurement of the wall temperature in a wide range of operative conditions: mass velocities ranging from 100 to 800 kg/(m2s) and vapour quality from 20 to 80%; saturation temperature was kept constant at 40°C. The experimental tests were run in a new test section set up at the Dipartimento di Fisica Tecnica of the University of Padova. The test section is a counter flow tube-in-tube condenser, with the refrigerant condensing in the inner tube, against the cold water flowing in the annulus. The measuring sections are 300 mm long and they are instrumented with thermocouples embedded in the tube wall, in the middle of the tube for the microfin tube and at inlet and outlet for the smooth tube. The thermocouples are soldered along the circumference to draw a cross shape. Refrigerant temperatures at inlet and outlet of the test section are measured by means of adiabatic sections, using thermocouples inserted into both the refrigerant flow and the tube wall. The refrigerant flow can be independently controlled by a variable speed gear pump. Two digital strain gauge pressure (absolute and differential) transducers are connected to manometric taps to measure the refrigerant pressure upstream and downstream of the test tube. The refrigerant mass flow rate is measured by a Coriolis effect mass flow meter inserted downstream of the pump. The cooling water flow rate is measured by a magnetic flow meter and its temperature gain across the instrumented test tube is measured with a copper-constantan thermopile, installed into mixing chambers to assure perfect mixing of the water. There is agreement in the literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime. During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear when gravity is the controlling force. In a fully developed annular flow pattern, there is a thin condensate film on the entire tube wall, while the gas phase flows in the central core, and heat transfer is governed by vapour shear and turbulence. Very poor evidence about the effect of microfins on the flow patterns during condensation is given in the open literature. Thus, to investigate the two phase flow pattern during condensation a new test section was built up; it consists of a large brass chamber equipped with three glass windows expressly designed to light and to observe the fluid. The test tube ends up in the chamber, in this way the fluid outlet pattern can be analyzed and recorded with a high shutter digital video camera. The same operative conditions set for heat transfer coefficient measurements are reproduced in the visualization section in order to investigate the specific flow pattern at the outlet vapour quality. For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the visualization experimental data are analysed with reference to parameters like dimensionless vapour velocity, Martinelli parameter and void fraction, that are adopted in the main available flow pattern maps. The aim of this work is to link the heat transfer coefficients and in particular the heat transfer enhancement of the microfin tube with the flow pattern during the process. Microfin tube shows different heat transfer enhancement with different mass velocities. Since the available maps are designed only for smooth tubes, the results of this experimental work could be used as a starting point for drawing a new flow pattern map specifically dedicated to enhanced tubes.