Book excerpt: Impinging jets have been studied in great depth due to their high rates of heat transfer and wide range of application. Some applications of impinging jets include electronic equipment cooling, metal annealing, furnace heating, and many others. The goal of this research was to understand the effects of the nozzle-to-plate spacing and volumetric quality on the Nusselt number of two-phase, free surface impinging jets and submerged impinging jets. The Nusselt number of two-phase, free surface and submerged impinging jets were obtained at several nozzle-to-plate spacings and the trends of the stagnation pressure and Nusselt number were compared. The Nusselt number of the submerged jet was also compared to that of the free surface jets with the same conditions. The two working fluids of the jet were water and air. The nozzle-to-plate spacing ranged from H/d = 0.03 - 8.5 and the experiment was done at five volumetric qualities. The Nusselt number and stagnation pressure decreased exponentially in both free surface and submerged jets for all volumetric qualities in Region I, the jet deflection region. In Region II, the transition region, the Nusselt number and stagnation pressure of the free surface jet remained constant with some fluctuation. The Nusselt number and stagnation pressure of the submerged jet continued to decrease linearly. In Region III, the free jet region, the Nusselt number and stagnation pressure increased linearly in the free surface jet and decreased linearly in the submerged jet. Comparing the values of the free surface and submerged jet, the submerged jet had a higher Nusselt number for low nozzle-to-plate spacings, and a lower Nusselt number for higher nozzle-to-plate spacings.