Experimental investigation on two-phase heat transfer in an aluminium test section made via additive manufacturing and other geometries

Processes involving vaporization and condensation of a fluid are commonly encountered in various energy applications, particularly in refrigeration, heat pump technologies, and power systems. The phase-out of conventional refrigerants like chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) due to their environmental impact has emphasized the need for alternative, sustainable refrigerants, such as for instance natural refrigerants and hydrofluoroolefins (HFOs). Hydrofluoroolefins (HFOs) emerge as promising pure fluid substitutes, offering advantages such as low flammability, low global warming potential (GWP), and a reduced atmospheric lifetime. When considering the use of HFOs in a vapour compression cycle, some studies have indicated that the volumetric cooling capacity and coefficient of performance in vapour compression cycles using HFOs may be lower compared to R134a. Therefore, a thorough design of the system and its components is essential when dealing with HFOs. In addition, many of the currently available alternatives introduce challenges related to flammability or toxicity. Consequently, special attention should be paid to the design of these systems. Minichannel heat exchangers emerge for their enhanced heat transfer coefficients and compact designs in comparison to traditional heat exchangers. This allows for more efficient heat transfer and a reduction in the amount of refrigerant needed in the system. The compact design promotes their compatibility with natural refrigerants, considering that they can be adopted with extremely high pressure refrigerants, such as carbon dioxide, and also with flammable or even toxic fluids, given the reduced refrigerant charge. Nevertheless, incorporating alternative and sustainable refrigerants into minichannel heat exchangers introduces certain challenges, considering that there is still a lack of knowledge regarding two-phase heat transfer. Achieving a comprehensive understanding of the phase change process within these channels becomes essential for their effective design when employing alternative refrigerants. To address these challenges, a new test section was designed to measure the local refrigerant heat transfer coefficient during two-phase heat transfer. Several CFD simulations were carried out to design the test section and, given the geometry complexity, the test-section was built by additive manufacturing. The test-section was then instrumented with 16 wall thermocouples and 16 thermocouples in the channel of the secondary fluid. Experimental tests were conducted to investigate both single-phase and condensation heat transfer, employing two low-GWP fluids with different thermal properties, R1234ze(E) and R1233zd(E), which can be employed in distinct applications. The aim was to locally measure the condensing-side heat transfer coefficient and assess the local impact of the main parameters such as vapour quality and mass flux. Additionally, a glass tube was incorporated into the test section to visually observe the flow regimes during the experiments, providing valuable insights into the flow patterns and enhancing the understanding of the involved heat transfer mechanisms. As previously observed for mini-channels, in the field of thermal engineering, brazed plate heat exchangers are also becoming more and more interesting due to their compactness, low internal volume and high efficiencies and they find extensive applications in various industries, ranging from refrigeration and air conditioning systems to power production and thermal management. Even in this case, the comprehension of the involved heat transfer mechanisms becomes essential to enhance system efficiency and reliability. To mitigate the environmental impact of vapour compression systems, the adoption of brazed plate heat exchangers instead of traditional tubular heat exchangers has emerged as a viable strategy.