This work describes numerical simulations applied for the investigation of vapor injection into subcooled flow in a capillary jet loop (CJL) system. The improvement of the injector (or ejector) design is discussed here by examining several parameters influencing its performance. Consequently, this paper covers the influence of the entrainment ratio γ (the ratio of the mass flow rate of the low-pressure secondary flow to the mass flow rate of the high-pressure primary flow), the characteristics of the working fluid, and the geometry on the pressure rise through the mixing region, the criterion chosen here to measure the injector efficiency. Considering that full condensation is expected to occur, part of the simulation will also cover the numerical aspects related to phase change to establish the most appropriate choices for obtaining a reliable solution for full condensation cases. Here, condensation is studied for two different injector designs: one with a vapor nozzle inclined to the direction of subcooled flow and the other with an inline orientation with respect to the secondary flow. From computational fluid dynamics (CFD) simulations performed using the commercial software Ansys Fluent (versions 19.4 and 2023R1), a parametric study indicated that increasing the injection angle had a negative impact on the pressure increase. The same negative impact was observed for γ. Regarding the working fluid, the use of methanol and acetone appeared to increase the pressure rise in the vapor injection region (in comparison to R1233zd), inducing faster condensation and, consequently, affecting the injector efficiency in a positive manner.