This paper describes the development of a numerical model that couples the thermal interaction between ice particles, water droplets, and the flowing gas of an icing wind tunnel for simulation of NASA Glenn Research Center's Propulsion Systems Laboratory (PSL). The ultimate goal of the model is to better understand the complex interactions between the test parameters and have greater confidence in the conditions at the test section of the PSL tunnel. The model attempts to explain the observed changes in test conditions by coupling the conservation of mass and energy equations for both the cloud particles and flowing gas mass. The model uses isentropic relations to relate gas temperature, velocity, density and pressure with respect to the PSL geometry. Measurements were taken at the PSL during wind tunnel tests simulating ice-crystal and mixed-phase icing that relate to ice accretions within turbofan engines in May 2015. The model was compared to experimentally measured values, where test conditions varied gas temperature, pressure, velocity and humidity levels, as well as the cloud total water content, particle initial temperature, and particle size distribution. Wet-bulb temperatures were generally within a few degrees of freezing. The model showed good agreement with experimentally measured values, to within approximately 30% of the measured change in gas temperature and humidity at the tunnel test section. The model did reasonably well in predicting melt content (liquid mass to total mass) at the test section, especially for clouds with larger particle sizes. In addition, the model predicted particle size at the tunnel exit with good agreement, however, the comparison was limited to clouds consisting of a small particle size distribution. One of the key findings from this work is that there was a nearly constant but slight increase in total wet-bulb temperature when the spray cloud was activated for every test and simulation. In addition, the total wet-bulb temperature in the tunnel plenum was a large factor in determining cloud phase.