Thermodynamic and combustion study on blends of hydrocarbons and carbon dioxide as alternative refrigerants

• Online Access: http://hdl.handle.net/2047/D20324058

Alternative refrigerants are an essential subset in reducing the effects of human-made climate change. Among the next-generation refrigerants, hydrocarbons (HCs) are natural and efficient, but their flammability restricts their widespread usage for fire-safety reasons. This research proposes a zeotropic mixture of hydrocarbons with carbon dioxide (CO2), as refrigerants which have low flammability risks. The objective of this research is to evaluate the thermodynamic performance of vapor compression refrigeration cycle using binary mixtures of HC + CO2 and to measure the laminar burning speed of one selected HC + CO2 mixture with air. Laminar burning speed is measured to evaluate the safety of these refrigerants during an accidental leak. In this thesis, potential hydrocarbons were identified based on thermophysical properties. Thermodynamic performance parameters, like the coefficient of performance (COP) and volumetric refrigeration capacity, were compared for all selected blends at different operating conditions. Further analysis was performed to determine the non-dimensional irreversibilities in various components of the cycle. Different methods for performing thermodynamic analysis were discussed and evaluated, and a novel approach was developed, including temperature glide matching of zeotropic mixtures. The following four hydrocarbons can be classified as having best performance parameters in blends with CO2: Propylene, Dimethyl ether, Propane, and Isobutane. Propane was selected for laminar burning speed study as it is a widely available hydrocarbon and has excellent performance as a refrigerant. Combustion took place at various CO2 concentrations (0% to 80%), different equivalence ratios (0.7 < ϕ < 1.2) and the laminar burning speed was measured over a range of temperatures (298 K to 420 K) and pressures (0.5 atm to 6.2 atm). Only laminar, spherical, and smooth flames were considered in measuring laminar burning speeds. Pressure rise data as a function of time during the flame propagation was the primary input of the multi-shell thermodynamic model for calculating the laminar burning speed of propane + CO2 + air mixtures. Results show that the addition of CO2 decreases the laminar burning speed of the blends and the COPs of blends are higher to the COPs of pure refrigerants at lower concentrations of CO2.