As a result the actual (not ideal) refrigeration and power cycles can be presented on equivalent temperature versus enthalpy variation diagrams. The present paper proposes to overcome these shortcomings by replacing the actual processes of expansion and compression by combinations of two thermodynamic paths: isentropic and isobaric. Second, from the point of view of rigorous thermodynamics, the proposed ratio gives the temperature dimension for the isobaric processes only. First, the approach is not applicable for the processes of expansion and compression particularly for the isenthalpic processes taking place in expansion valves. Despite the usefulness of this approach two important shortcomings should be emphasized. It is also possible to draw the paths of working fluids in steady-state, steady-flow thermodynamic cycles on this diagram using the definition of " the equivalent temperature " as the ratio between the variations of enthalpy and entropy in an analyzed process. With enthalpy variation (heat) as the abscissa and the Carnot factor as the ordinate the area between the curves representing the heat exchanging media on this diagram illustrates the exergy losses due to the transfer. The Carnot factor versus enthalpy variation (heat) diagram has been used extensively for the second law analysis of heat transfer processes. The developed 1-D thermodynamic model of the ejector showed that the ejector operates in the subcritical mode when the generating pressure is below the Cr optimum point, while operates in critical mode at or above this optimum generating pressure. It was observed that there is an optimum value of the generating pressure that simultaneously maximizes the ejector Cr and minimizes the suction pressure.
Then, the influence of the generating pressure on the ejector’s performance was studied using the filtered experimental data, for different condenser temperatures ranging from 20 to 24 ☌. The Moving average filtering method eliminated the periodic noise from the signals and improved the experimental precision. Signal processing techniques were applied to analyse the acquired signals of an R245fa ejector refrigeration system and revealed the existence of periodic noise in the experimental data. The present paper studied the impact of generating pressure on Cr for the ejector installed in an ERS. The highest compression ratio (Cr) of an ejector refrigeration system could guarantee the high efficiency for such systems. Examples of its application for isentropic and irreversible acceleration/deceleration of a perfect gas are provided and their results are analysed and compared. The model uses a fixed polytropic efficiency (rather than the fixed isentropic efficiency used in previous studies) to simulate the acceleration and deceleration processes thus taking into account the effect of the pressure ratio during off-design operation. The paper also describes a method for determining the off-design performance of a fixed geometry ejector which reproduces the experimental relations between the entrainment ratio, the compression ratio and the inlet conditions of the two fluids. The maximization of the back pressure is obtained subject to constraints imposed by the 2nd law of thermodynamics and the requirements that the flow must be subsonic at the diffuser entrance, that the mixing efficiency must be positive but smaller than one and that the length to diameter ratio for the constant area duct must be between fixed limits recommended in previous studies. This paper proposes a 1-D thermodynamic model for determining the critical pressure ratio, the mixing efficiency and all the dimensions of an optimum ejector providing the highest possible compression ratio for fixed inlet conditions and mass flowrates of the motive and suction fluids.
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