The characterization of flow regimes with power spectral density distributions of pressure fluctuations during condensation in smooth and micro-fin tubes

This paper presents an objective predictor of the prevailing flow regime during refrigerant condensation inside smooth-, micro-fin and herringbone tubes. The power spectral density (PSD) distribution of the fluctuating condensing pressure signal was used to predict the prevailing flow regime, as opposed to the traditional (and subjective) use of visual-only methods, and/or smooth-tube flow regime maps. The prevailing flow regime was observed by using digital cameras and was validated with the use of the conventional smooth-tube flow regime transition criteria, Froude rate criteria, as well as a new flow regime map that was developed for micro-fin tube condensation. Experimental work was conducted for condensing R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 300 to 800 kg/m² s, and with vapour qualities ranging from 0.85-0.95 at condenser inlet to 0.05-0.15 at condenser outlet. Tests were conducted with one smooth-tube condenser and three micro-fin tube condensers (with helix angles of 10°, 18°, and 37°, respectively). It is shown that the micro-fin tubes cause a delay in the transition from annular to intermittent flow by at least 19% (compared to the smooth tube), thus significantly contributing to the enhancement of heat transfer.