The study presented herein describes and analyzes a detailed experimental investigation of the effects of density ratio on endwall thermal performance at varying blowing rates for a typical nozzle guide vane platform purge jet cooling scheme. An axisymmetric converging endwall with an upstream doublet staggered cylindrical hole purge jet cooling scheme was employed. Nominal exit flow conditions were engine representative and as follows: {rm Ma}_{Exit} = 0.85, {rm Re}_{Exit,C_{ax}} = 1.5 times {10}^6, and large-scale freestream Tu = 16%. Two blowing ratios were investigated corresponding to the upper and lower engine extrema. Each blowing ratio was investigated amid two density ratios; one representing typical experimental neglect of density ratio, at DR = 1.2, and another engine representative density ratio achieved by mixing foreign gases, DR = 1.95. All tests were conducted on a linear cascade in the Virginia Tech Transonic Blowdown Wind Tunnel using IR thermography and transient data reduction techniques. Oil paint flow visualization techniques were used to gather quantitative information regarding the alteration of endwall flow physics due two different blowing rates of high-density coolant. High resolution endwall adiabatic film cooling effectiveness, Nusselt number, and Net Heat Flux Reduction contour plots were used to analyze the thermal effects. The effect of density is dependent on the coolant blowing rate and varies greatly from the high to low blowing condition. At the low blowing condition better near-hole film cooling performance and heat transfer reduction is facilitated with increasing density. However, high density coolant at low blowing rates isn't adequately equipped to penetrate and suppress secondary flows, leaving the SS and PS largely exposed to high velocity and temperature mainstream gases. Conversely, it is observed that density ratio only marginally affects the high blowing condition, as the momentum effects become increasingly dominant. Overall it is concluded density ratio has a first order impact on the secondary flow alterations and subsequent heat transfer distributions that occur as a result of coolant injection and should be accounted for in purge jet cooling scheme design and analysis. Additionally, the effect of increasing high density coolant blowing rate was analyzed. Oil paint flow visualization indicated that significant secondary flow suppression occurs as a result of increasing the blowing rate of high-density coolant. Endwall adiabatic film cooling effectiveness, Nusselt number, and NHFR comparisons confirm this. Low blowing rate coolant has a more favorable thermal impact in the upstream region of the passage, especially near injection. The low momentum of the coolant is eventually dominated and entrained by secondary flows, providing less effectiveness near PS, near SS, and into the throat of the passage. The high momentum present for the high blowing rate, high-density coolant suppresses these secondary flows and provides enhanced cooling in the throat and in high secondary flow regions. However, the increased turbulence impartation due to lift off has an adverse effect on the heat load in the upstream region of the passage. It is concluded that only marginal gains near the throat of the passage are observed with an increase in high density coolant blowing rate, but severe thermal penalty is observed near the passage onset. / Master of Science / Gas turbine technology is used frequently in the burning of natural gas for power production. Increases in engine efficiency are observed with increasing firing temperatures, however this leads to the potential of overheating in the stages following. To prevent failure or melting of components, cooler air is extracted from the upstream compressor section and used to cool these components through various highly complex cooling schemes. The design and operational adequacy of these schemes is highly subject to the mainstream and coolant flow conditions, which are hard to represent in a laboratory setting. This experimental study explores the effects of various coolant conditions, and their respective response, for a purge jet cooling scheme commonly found in engine. This scheme utilizes two rows of staggered cylindrical holes to inject air into the mainstream from platform, upstream of the nozzle guide vane. It is the hope that this air forms a protective layer, effectively shielding the platform from the hostile mainstream conditions. Currently, little research has been done to quantify these effects of purge flow cooling scheme while mimicking engine geometry, mainstream and coolant conditions. For this study, an endwall geometry like that found in engine with a purge jet cooling scheme is studied. Commonly, an upstream gap is formed between the combustor lining and first stage vane platform, which is accounted for in this testing. Mainstream and coolant flow conditions can have large impacts on the results gathered, so both were matched to engine conditions. Varying of coolant density and injection rate is studied and quantitative results are gathered. Results indicate coolant fluid density plays a large role in purge jet cooling, and with neglection of this, potential thermal failure points could be overlooked This is exacerbated with less coolant injection. Interestingly, increasing the amount of coolant injected decreases performance across much of the passage, with only marginal gains in regions of complex flow. These results help to better explain the impacts of experimental neglect of coolant density, and aid in the understanding of purge jet coolant injection.