Experimental study of two-phase heat transfer with R1234ze(E) inside a brazed plate heat exchanger

This thesis aims to experimentally measure and study the performance of a brazed plate heat exchanger (BPHE) used both as condenser and evaporator, employing a hydrofluoroolefin refrigerant. The phase-out of traditional refrigerants such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) is a consequence of recent regulations, the Kigali Amendment to the Montreal Protocol and the Fluorinated Gases Regulation (F-gas), due to their high environmental impact. As result, new alternatives have been introduced to enhance environmental sustainability, such as natural refrigerants (e.g., Carbon dioxide) and hydrofluoroolefins (HFOs), also known as “fourth-generation refrigerants”. The Hydrofluoroolefins offer the advantage of a low global warming potential (GWP), due to their short atmospheric lifetime, and a low flammability. However, when employed in vapor-compression systems, HFOs may exhibit lower volumetric cooling capacity and a lower coefficient of performance (COP) compared to traditional refrigerants such as R134a (HFC). For this reason, it is important design the system and its components according to the selected refrigerant in order to maximize the efficiency. The experimental tests presented in this work are conducted at the two-phase heat transfer laboratory of the department of Industrial Engineering at the University of Padua, using the R1234ze(E) as the refrigerant. The study focuses on the analysis of key operating parameters affecting the thermal performance of the heat exchanger. These include temperature approaches, mass flow rates, heat fluxes, vapor quality, and subcooling and superheating conditions. In addition, comparison with literature correlations are implemented to better understand the experimental analysis. The experimental results obtained include the heat transfer coefficients during condensation and vaporization, as well as pressure drops. These results are then compared with the predictions from correlations available in the literature, in order to evaluate how the real operating conditions of the plate heat exchanger matches the values of theoretical models. The outcomes provide useful guidelines for the plate geometry design and for the optimization of the heat exchanger in the industrial field.