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The following study presents an experimental and computational investigation on the effects of implementing a dual cavity slashface configuration and varying slashface coolant leakage mass flow rate on the thermal performance for a 1st stage nozzle guide vane axisymmetric converging endwall. An upstream doublet staggered cylindrical hole jet cooling scheme provides additional purged coolant with consistent conditions throughout the investigation. The effects are measured in engine representative transonic mainstream and coolant flow conditions where Mexit = 0.85, Reexit = 1.5 × 106, freestream turbulence intensity of 16%, and a coolant density ratio of 1.95. Four combinations of slashface Fwd and Aft cavity mass flow rate are experimentally analyzed by comparing key convective heat transfer parameters. Data is collected and reduced using a combination of IR thermography and a linear regression technique to map endwall heat transfer performance throughout the passage. A flow visualization study is employed using 100 cSt oil-based paint to gather qualitative insights into the endwall flow field. A complimentary CFD study is carried out to gather additional understanding of the endwall flow ingestion and egression behavior as well as comparing performance against a conventional cavity configuration. Experimental comparisons indicate slashface mass flow rate variations have a minor effect on passage film cooling coverage. Instead, coolant coverage across the passage is primarily driven by upstream purge coolant. However, endwall heat transfer coefficient is reduced as much as 20% in mid-passage areas as leakage flow decreases. This suggests that changes in leakage flow maintains a first order correlation in altering passage aerodynamics that, despite relatively consistent film cooling coverage, also leads to significant changes in net heat flux reduction in the passage. Endwall flow behavior proves to be complex along the gap interface showing signs of ingestion, egression, and tangential flow varying spatially throughout the gap. CFD comparisons suggests that a dual cavity configuration varies the gap static pressure distribution closer to the mainstream pressure throughout the passage in high speed applications compared to a single cavity configuration. The resulting decelerating flow creates a more stable endwall flow profile and favorable coolant environment by reducing boundary layer thinning and shear interaction in near gap endwall tangential flow. / Master of Science / Gas turbines are often exposed to high temperatures as they convert hot, energetic gas streams into mechanical motion. As turbines receive higher temperature gases, their efficiency increases and reduces waste. However, these temperatures can get too hot for turbine parts. To survive these high temperatures, turbine components are often assembled with a gap in between to allow the part to expand and contrast when it heats and cools. Relatively cold air is also fed into the gap to help prevent hot gases from entering. This cold air can also feed into other pathways to flow onto the turbine component's surface and act as an insulating layer to the hot gas and protect the component from overheating. The study presented investigates an assembly gap, referred to as a slashface gap, found in the middle of a vane located immediately after gas combustion with cold air leaking through. One unique aspect of this study is that there are two pathways for cold air, or coolant, to leak through when, typically, there is only one. The slashface gap lies on a wall which the vanes are attached to, referred to as the endwall. Multiple small holes on the endwall in between the combustor and vanes jet out coolant to try and protect the endwall from hot gases. These holes, called jump cooling holes, point out towards the vanes and angled more shallowly so that the holes do not face directly up from the endwall. The holes are angled as they are meant to gracefully spray coolant to cover and insulate the endwall instead of mixing with the hot air above. The experiments found that changing how much coolant is leaked through the slashface has little effect on how much coolant from jump cooling holes covered the endwall. However, smaller slashface leaks better protect the endwall from the hot gas by forcing it to move smoother and give off less heat across the endwall rather than a tumbling like manner. The experiment is modeled on a computer simulation to determine the differences of a slashface gap with the typical one coolant pathway and the coolant dual pathway configuration that is tested in the experiments. This simulation discovered that having two coolant pathways significantly reduces how much hot gas and jump cooling coolant enters and leaves the slashface gap. This makes the surrounding airflow along the endwall travel more smoothly and does not give off as much heat as if a single coolant pathway configuration is used instead.