No abstract

CO 2 is a promising alternative to hazardous, ozone-depleting and global-warming refrigerants. It is more suitable to the ejector-refrigeration cycle than to the vapor compression cycle. However, shock waves significantly reduce the ejector-nozzle efficiency. Therefore, they must be characterized to improve the coefficient of performance and the nozzle efficiency. This paper elucidates the types of CO 2 shock waves and their relation to nozzle inlet conditions and two-phase thermodynamic states. Shock waves in supersonic liquid-vapor flows through the diverging sections of rectangular converging-diverging nozzles with the same divergence angle were investigated. They increased with supercritical inlet entropy. Equilibrium shock waves were calculated in the nozzles. The pressure behind these waves increased with decreasing diverging-section length. Two-phase-flow equilibrium shock waves, which both sides are two-phase fluid, were calculated in long nozzles. Equilibrium shock waves were very strong and thin but they were not experimentally observed. Instead, relaxation phenomena and both weak pseudo-shock waves and dispersed shock waves were experimentally observed.

A mathematical model of a heat-driven ejector refrigeration system that uses geothermal energy as the heat source was established. Philippine low-enthalpy geothermal resources were investigated and became the bases in computing for the heat at the generator part of the ejector refrigeration system. Analysis and comparison of the performance of the cycle considering working fluids like ammonia (R717) and R134a as the refrigerants were conducted. The properties of those fluids were based on an available thermodynamic database of various refrigerants. The governing principles and conservation equations for energy, mass and momentum were successively applied to control volume of ejector components. The properties for both fluid and flow were solved iteratively for isentropic and irreversible processes wherein entropy generation and frictional losses were accounted for. This included simulation of flows in two-phase region. Input parameters were set like the generating temperature and condensing temperature. The range of 60 to 100 °C available geothermal fluid temperature could produce 50 to 90°C of generating temperature for the fluid refrigerant. This range of generating temperature yielded an evaporating temperature of 8 to 25 °C at a fixed condensing temperature of 40 °C. After numerical analyses, the determined coefficient of performance was at the range of 0.21 to 0.39, while nozzle and ejector efficiencies were from 94% to 99%. The geometric profiles of the ejector were also projected along with the varying generating temperature for both fluids. From the calculation, ammonia offers higher performance and efficiencies and lower evaporating temperatures suitable for larger cooling needs.