Two phase heat transfer and flow regimes of R134a and R410A during condensation in horizontal micro-fin tubes.

Abstract

Micro fin tubes are widely being used in condensers and evaporators of commercial air conditioners. These tubes enhance the heat transfer without causing the similar increase in refrigerant charge and pressure drop. The condensation heat transfer characteristics are strongly dependent on prevailing flow regimes. Conventionally, two phase heat transfer correlations are used for the design of condenser (Cavallini et al. (1999), Kedzierski & Gonclaves (1997) etc.). These correlations can pose serious errors, if these are inconsistent with the prevailing flow regime (Bell et al., 1970). The present study deals with the experimental investigations on local flow regimes and local condensation heat transfer coefficient at saturation temperatures of 35°C and 40°C respectively, in mass flux range of 200-400kg/m2-s & vapour quality range of 0.1-0.9 for R134a and in mass flux range of 200-300kg/m2-s & vapour quality range of 0.1-0.9 for R410A respectively using two horizontal micro fin tubes (9.54/9.52 mm OD) having helix angles of 15° and 18°, with fin height of 0.2mm each, apex angle of 44° & 55° and number of fins being 72 & 60 respectively. Various flow regimes encountered during the condensation of refrigerants R134a and R410A have been captured with high speed digital CMOS camera. Most of the flow regimes in the present study are observed to lie in Annular, Semi annular and stratified zone. Three existing flow regime maps, namely that of Breber, Tandon and Thome have been used to predict the flow regimes corresponding to different experimental conditions. It has been observed that the prediction capability of Tandon et al. (1982) flow regime map is very good as compared to Breber et.al (1980) and Thome et.al.(2003) flow regime maps, for both pure refrigerant R134a and mixture refrigerant R410A. The uncertainties in the measurement of refrigerant mass flow rate, test condenser water flow rate, refrigerant saturation temperature, wall temperature and water temperatures are ±0.002kg/s, ±5e-05m3/min, ±0.15°C, ±0.5°C & ±0.1°C respectively. The overall uncertainty in the measurement of local heat transfer coefficient was in the range of ±6-20% for R134a & 12-40% for R410A and that for average vapour quality was in the range of ±3-15% respectively.