INVESTIGATION AND VISUALIZATION OF FLOW PATTERNS INSIDE MICRO-FINNED TUBES

The combination of refrigerants possessing low values of Global Warming Potential (GWP) and enhanced tubes promoting high heat transfer coefficient can promote the transition to sustainable, efficient air conditioning. Among the low GWP fluids, HydroFluoroOlefins are emerging in the market thanks to their low GWPs. R1234ze(E) being one of the refrigerants at the center of attention belongs to this category of refrigerants, and it has an exceptionally low value of GWP (lower than one). R1234ze(E) is a promising candidate to replace the vastly implemented R134a, whose GWP is higher than 1300. Currently, research on R1234ze(E) is incomplete and mostly limited to heat transfer coefficient, pressure drop, and COP analysis. While few authors explored the flow regimes of R1234ze(E) inside smooth tubes, the literature lacks the presence of flow regime analysis of R1234ze(E) inside micro-finned tubes. Furthermore, since R1234ze(E) is classified as mildly flammable (A2L flammability class), heat exchangers with small diameter tubes are requested in the market to reduce the refrigerant charge of the system. Therefore, micro-finned tubes with reduced diameters could bolster the aforementioned industrial demand. In this scenario, the following research presents some preliminary data of flow regime observations during condensation of R1234ze(E) inside micro fin tubes with outer diameters of 5.0, 7.0, 4.0, and 3.0 mm. The experimental campaign was conducted at the MicroGeometries Laboratory of the Department of Industrial Engineering of the University of Padua. The micro-finned tubes are diagonally cut and located in a test section with a visualization window: this setup permits direct visualization of the flow regime. Before the test section, a pre-section, made by a homemade water-cooled condenser, is located to permit the setting of the vapor quality inside the test section. The vapor quality derives from an energy balance between the refrigerant and the waterside in the pre-section. The vapor quality varies from about 0.01 to 0.9, with mass velocities ranging from 50 to 400 kg m^(-2) s^(-1) with a saturation temperature of 30 °C. Right after the pre-section, two T-type thermocouples were attached to the external wall of the micro fin tube, one on the bottom side and one on the top side. A high-speed camera is implemented to record the two-phase flow that assists with the identification of the flow regime at the various operating conditions. In addition, intending to link the visualized flow regime to the trends of the upper and lower wall temperatures measured, two thermocouples are utilized. The collected observations of the flow regime, being quite rare in the literature with such small-sized micro fin tubes, are of great importance to validate, and if necessary to develop accurate flow regime maps. The outcome of the research is divided into three main sections, Firstly, the observed flow regime types are identified and illustrated on a mass velocity versus vapor quality graph (G-x )for each tube diameter. At this stage, possible links between two graphs of the standard deviation of the upper wall temperature and upper-lower wall temperature difference and flow regime type have been explored. Secondly, after a succinct review of flow regime maps in the literature developed based on theoretical concepts and experiments on condensation, two of the existing suggestions for flow regime-type transition lines by Doretti et al and Chen et al with the former being constructed on a non-dimensional gas velocity versus Martinelli parameter ( J_G-X_tt) graph and the latter based on modified Weber number versus Martinelli parameter (We*-X_tt) graph were compared to the results acquired by the experiments. The comparison was simply achieved by plotting the collected data points on the quoted maps. Based on acquired results, future research implications are also provided and thoroughly discussed.