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Due to their simplicity, jet-ejector cooling systems have been investigated for different applications despite their lower thermal COP th in comparison to alternative thermally-driven cooling technologies. The present paper aims at the derivation of a simple method of representation of the operation characteristics of a jet-ejector cooling system for changing boundary conditions. Thereto, a jet-ejector cooling system operating with the refrigerant R134a has been experimentally investigated in double choking operation of the jet-ejector. The jet-ejector operation is characterized by the entrainment ratio μ in dependence of the adequate stagnation pressure ratios. The results are discussed in a non-dimensional graph as a three dimensional surface from which characteristic lines may be deduced. In several graphs, the motive, condenser and evaporator pressures are linked with the entrainment ratio μ , the cooling capacity Q ˙ 0 , the thermal COP th and the hydraulic COP h allowing a comprehensive determination of the system operation. [ABSTRACT FROM AUTHOR]

The geometrical simplicity of a jet-ejector is in stark contrast to the complexity of the flow phenomena occurring in ejector operation. The available flow models range from empirical models to models based on computational fluid dynamics, differing considerably in complexity and thus explanatory power. The one-dimensional flow models of semi-empirical nature are based on experiments at comparatively low motive pressures and thus on ideal gas equations. In contrast, a one-dimensional, experimentally validated model of a jet-ejector in critical double choking operation which includes real gas effects and relies on a single physically interpretable loss coefficient is introduced in the present paper. Real gas effects impact on the state quantities and the critical mass flow of the expansion in the supersonic nozzle in particular which should be considered in the determination of the entrainment ratio or the absolute motive and suction mass flows of the ejector and in ejector design. [ABSTRACT FROM AUTHOR]

This study presents an experimental investigation of the heat transfer occurring during reflux condensation of refrigerant R134a in an inclined micro-fin tube (0.008 m OD, 0.5 m length). The experiments were carried out at a system pressure of 0.74 MPa and at inclination angles of 30°, 38°, 45°, 60° and 90° from the horizontal. The experiments indicated that the inclination angle has a significant effect on heat transfer during reflux condensation. The maximum reflux heat transfer coefficient was found at 30° inclination angle. At this arrangement, the heat transfer increased by a factor of 2.45 compared to the vertical case. A comparison with previous studies suggests that the heat transfer in a micro-fin tube is 2.2 times better than that in a plain tube, and 1.34 times better than that in a plain rectangular channel both at 30° inclination. [ABSTRACT FROM AUTHOR]