Effects of design conditions and irreversibilities on the dimensions of ejectors in refrigeration systems

A thermodynamic model for the design of ejectors is described, validated and applied for conditions prevailing in refrigeration systems. Contrary to previous models the present one determines all the dimensions of the ejector and uses polytropic (instead of isentropic) efficiencies thus taking into account the effects of the pressure ratio on the entropy increase during the irreversible acceleration and deceleration processes. The results include dimensions and fluid properties for a base case as well as a parametric study which analyzes the effect of inlet and outlet conditions on the dimensions and efficiencies of the acceleration, deceleration and mixing processes. The parametric study coupled to recommended constraints from the literature leads to the determination of design conditions for which the axial evolution of pressure, temperature and velocity are determined. The effects of the polytropic efficiency on the ejector dimensions and the efficiencies of the processes taking place in the ejector are also presented and analysed. It is also shown that the total exergy losses increase linearly when the mixing efficiency decreases.