Heat transfer enhancement phenomena and pressure drop characteristics in two-phase pulsating flow using R-134A

The heat transfer and pressure drop aspect of a saturated two-phase flow imposed to periodic inlet mass flow rate were studied using both experimental and modeling approaches. The two-phase flow of R-134 was tested in a single pass, horizontal smooth copper tube coil with a 6.2 mm (0.244-in.) inner diameter. The test section uses aluminum plain fins 0.30 m (11.8-in.) long, 0.03 m (1.18-in.) wide and 0.3 mm (0.012-in.) thick. Heat was applied to the test section using external air flow at ambient temperature. The test parameters varied as follows: mass flux, 75 – 250 kg/m2-s (55 – 184 klbm/ft2-hr); heat flux, 2-12 kW/m2 (600-3800 Btu/hr-ft2); vapor quality, 10-98 percent; saturation temperature 15 oC (59 oF); flow pulsation period (2-24 sec). The temporal pressure drop data was recorded and used as the basis of a newly proposed close-form model for predicting the heat transfer and pressure drop for pulsating two-phase flow based on the quasi-steady state assumption. The enhancement of heat transfer in saturated boiling pulsating two-phase flow was found to be higher in shorter pulsation periods. It was also found that for low inlet vapor qualities and short pulsation periods, reduction in the pressure drop and enhancement in heat transfer coefficient could be achieved at the same time, which can be potentially beneficial to the system COP. Furthermore, the flow regime, which is a widely recognized factor having dominant influence on the heat transfer of two-phase flow, was also captured and analyzed in this study using high speed camera. Synchronized flow regime images and pressure drop data were also presented to demonstrate the relation between flow regime evolution and pressure drop variation with time as a potential means to identify the flow regime using the pressure drop characteristics