Investigation of Hydrocarbon Two-phase Flow for Charge Reduced Heat Exchangers

Abstract

Hydrocarbons off er an alternative to the HVAC industry as a natural working fluid with minimal environmental impact. The main challenge with hydrocarbons utilization is flammability which can be mitigated by reduction of charge in systems. Internally enhanced tubes provide a powerful tool for designing a more efficient heat exchanger leading to reductions in both volume and charge. Microfinned tubes are the most commonly utilized internally enhanced tubes that increase the heat transfer coefficient on the refrigerant fluid side in heat exchangers. Thanks to the higher heat transfer coefficients, the internal volume of the heat exchangers can be reduced. One of the main challenges for using microffinned tubes has been the design of the heat exchangers. This is caused by a lack of reliable predictive methods. In this context, the present thesis presents experimental results for characteristics of two-phase flow of hydrocarbons. Propane (R290), isobutane (R600a), and propylene (R1270) were studied since they are commonly used in HVAC applications. Three tubes with an outer diameter of 5 mm were tested, one smooth and two microffinned. The two microffinned tubes differed in the number of fins and helix angle, causing a different increase in the available heat exchange area. One test rig was used to obtain both condensation and evaporation characteristics, which was possible due to the design of the rapidly interchangeable test sections. In evaporation tests, the effects of fluid properties, heat flux, mass flux and saturation temperature were studied in addition to the effect of internally enhanced tubes. Condensation tests were focused on fluid properties, mass flux and internal enhancement of tubes. Additionally, the data obtained for the heat transfer coefficient and pressure drop were compared against predictive methods to find the most reliable correlations. Finally, this data was used for numerical simulation of fin-and-tube heat exchangers in different environmental conditions to compare the charge with other types of heat exchangers.