Supersonic air ejectors are mechanical devices in which a supersonic primary air stream entrains a secondary air stream by creating suction. Because of their ability to use low-grade energy for their operation and the absence of moving parts, these ejectors are used in various sectors such as refrigeration and fluid transportation. The ejectors' performance is quantified using the entrainment ratio, which is the amount of secondary mass flow rate entrained per unit primary mass flow rate. Though many research studies evaluated the impact of operating conditions and geometrical configuration on the ejector entrainment ratio, few studies characterized the loss generation in these ejectors. Although these studies attributed viscous dissipation as the major irreversibility contributing mechanism, there is limited information about the turbulence driven irreversibilities, the influence of eddy viscosity on mean flow entropy generation, and impact of the shock strengths and shear layers on entropy generation. First, a one-dimensional analytical model is developed in the present study to estimate the entrainment ratio and the overall characteristics of the flow parameters within the ejector. This model is subject to some assumptions and cannot predict the flow intricacies in-depth. For detailed investigations, two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations are performed to quantify entropy generation in a supersonic air ejector operating at different stagnation pressure ratios for a range of area ratios subject to two different mixing chamber geometries, namely, constant-pressure and constant-area mixing chambers. Local entropy generation is dominated by viscous dissipation in the flow at locations corresponding to flow separation zones, shear layer instabilities, recirculation zones, and shock structures. The results show that the influences of the turbulence and gas dynamics on the ejector irreversibilities are significant; however, the latter dominates. In this thesis, several designs are proposed in which various arrangements of inclined plates are inserted into the diffuser aiming to reduce ejector entropy generation. These variants impact the diffuser shock structure, shock location, and ejector choking conditions. The results show that, at a stagnation pressure ratio of 3.5, an ejector modified with a single-inclined plate may reduce the entropy generation by 9% relative to its unmodified conditions.