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The objective of this investigation was to improve the thermal environment for a turbine vane through reduction of passage secondary flows. This was accomplished by modifying the vane/endwall junction to include a leading edge fillet. The problem approach was to integrate optimization methods with computational fluid dynamics to optimize the fillet design. The resulting leading edge fillet was then tested in a large-scale, low speed cascade to verify thermal performance. A combustor simulator located upstream of the cascade was used to generate realistic inlet conditions for the turbine vane. Both computational and experimental results underscore the importance of properly modeling the inlet conditions to the turbine. Results of the computational optimization process indicate that significant reductions in adiabatic wall temperature can be achieved with a leading edge fillet. While the intent of the initial fillet design was to improve the thermal environment for the vane endwall, computational results also indicate thermal benefit to the vane surfaces. Flow and thermal field results show that a fillet can enhance coolant effectiveness, prevent formation of the leading edge horseshoe vortex, and preclude full development of a passage vortex. In experimental testing, four cascade inlet conditions were investigated to evaluate the effectiveness of the fillet in reducing endwall temperature levels. Two tested conditions featured a flush combustor/cascade interface, while the remaining two included coolant injection through a backward-facing slot. With the flush interface, fillet thermal performance was evaluated for two inlet total pressure profiles. For the design profile, the fillet had a positive impact on the endwall temperature distribution as well as on the passage thermal field. For the off-design profile, the fillet was observed to have a slightly detrimental impact on the endwall adiabatic temperature distribution; however, passage thermal field results indicate a thermal benefit for the vane suction surface. With the backward-facing slot, thermal tests were conducted for two slot coolant flow rates. For both slot flow rates, the fillet improved endwall thermal protection and prevented coolant lift-off. While increasing the flow rate of slot coolant enhanced endwall effectiveness, fillet thermal performance was similar for the two slot flow rates. / Ph. D.

This paper investigates the effect of blowing ratio on the film cooling performance of a showerhead film cooled first stage turbine blade. The blade was instrumented with double-sided thin film heat flux gages to experimentally characterize the Nusselt number and film cooling effectiveness distributions over the surface of the blade. The blade was arranged in a two-dimensional, linear cascade within a transonic, blowdown type wind tunnel. The wind tunnel freestream conditions were varied over two exit Mach numbers, Me=0.78 and Me=1.01, with an inlet freestream turbulence intensity of 12% , with an integral length scale normalized by blade chord of 0.26 generated by a passive, mesh turbulence grid. The coolant conditions were varied by changing the ratio of coolant to freestream mass flux, blowing ratio, over three values, BR=0.60, 1.0, and 1.5 while keeping a density ratio of 1.7. Experimental results show that ingestion of freestream flow into the showerhead cooling plenum can occur below a blowing ratio of 0.6. Film cooling increases Nusselt number over the uncooled case and increasing the blowing ratio also increases Nusselt number. At a blowing ratio of 1.5 and Me=1.01 a large drop in effectiveness just downstream of injection on both the pressure and suction surfaces is evidence of jet liftoff. The blowing ratio of 1.0 was found to have superior heat load reduction over the blade surface at both freestream conditions tested. The blowing ratio of 1.0 reduced the heat load by as much as 39% and 32% at Me=0.78 and 1.01, respectively. / Master of Science

Firebrands, small pieces of burning vegetation or debris generated by fires, are one of the primary ways wildfires ignite structures. Due to their small size, firebrands can be carried several kilometers by high winds before landing on combustible surfaces such as decks or roofs and potentially igniting homes. Until recently, little has been known about the heat transfer capabilities of firebrands to the surfaces on which they land. Understanding the heat transfer from firebrands is an essential step in engineering for greater fire resilience. In the first phase of this research, heat transfer from individual firebrands to horizontal surfaces was investigated using oak firebrands made from commercially available lumber. The firebrand shape, wind speed, and wind direction were varied to see how these variables affect the heat transfer. A method of inverse heat transfer analysis based on infrared thermographs was used to measure distributed heat fluxes from firebrands to the surfaces through time. This measurement technique provided spatial resolutions of < 0.5 mm, approximately 10 times higher than previous experiments in this field. Results showed that localized heat transfer was significantly higher than had previously been reported, reaching as high as 80 kW/m2 in some cases. It was also found that wind speed, wind direction, and firebrand shape all affected the heat transfer from individual firebrands. Firebrands have also been shown to accumulate in piles on decks or roofs creating complex systems that have different ignition capabilities than individual firebrands. Potentially many factors could influence the heat transfer from firebrand piles including wood moisture content, wood type (hardwood or softwood), wood density, wood state (live, dead, or artificial), wind speed, pile mass, firebrand diameter, and firebrand length. The second phase of this research used the same method of high-resolution heat transfer measurement to assess which of these factors significantly impacted the heat transfer from firebrand piles. Design of experiments was used to develop the test matrices and a rigorous statistical framework was employed to evaluate results at the α=0.05 level. It was found that wind speed, firebrand length, and an interaction between firebrand length and diameter were important. Additionally, it was found that there was a difference between the heat transfer from piles made with artificial and real firebrands, suggesting that using dowels as surrogate firebrands may produce higher heat fluxes than expected from real firebrands. Pile mass did not appear to significantly impact the heat flux from firebrand piles. The last phase of this research developed a simple engineering model to predict the ignition of common building materials by firebrand piles. The model used time-varying heat transfer data from firebrand pile tests and material properties developed by testing on select building materials in a cone calorimeter. The model predicted the surface temperature rise of the material due to an exposure heat flux with ignition being predicted when the surface temperature exceeded the ignition temperature of the material. The model was used to predict ignition for a number of pile/fuel combinations and experiments were run to validate the predictions. It was found that the model did an excellent job in predicting ignition for materials which did not melt. Together this research provides an important step in understanding heat transfer from firebrands and firebrand piles, predicting ignition, and engineering for greater fire resilience. / Master of Science / Uncontrolled wildfires burning close to human civilizations result in hundreds of deaths, the destruction of thousands of structures, and billions of dollars in economic damages each year. One of the primary ways wildfires ignite structures is through firebrands: small pieces of burning vegetation or debris generated by the fire. These firebrands can be carried great distance by strong winds, eventually landing on decks or roofs and potentially igniting homes. Until recently, little has been known about the heat transfer from firebrands to the surfaces on which they land. Understanding firebrand heat transfer will allow building materials to be selected that are resistant to ignition by firebrands and reduce the number of structures destroyed by wildfires. In the first phase of this research, heat transfer from individual firebrands was investigated. The firebrand shape, wind speed, and wind direction were varied to see how these variables affect the heat transfer. A high-resolution measurement technique was used, allowing heat transfer to be measured with approximately 10 times higher resolution than previous experiments. Results showed that localized heat transfer was significantly higher than had previously been reported and indicated that wind speed, wind direction, and firebrand shape all affected the heat transfer from individual firebrands. Firebrands have also been shown to accumulate in piles on decks or roofs creating complex systems that have different ignition capabilities than individual firebrands. Potentially many factors could influence the heat transfer from firebrand piles including wood moisture content, wood type (hardwood or softwood), wood density, wood state (live, dead, or artificial), wind speed, pile mass, firebrand diameter, and firebrand length. It was found that wind speed, firebrand length, and an interaction between firebrand length and diameter were important. Additionally, it was found that there was a difference between the heat transfer from piles made with artificial and real firebrands. The last phase of this research developed a simple engineering model to predict the ignition of common building materials by firebrand piles. The model used time-varying heat transfer data, and material properties developed by experimental testing. The model was used to predict ignition of select building materials with different firebrand piles, and experiments were run to validate the predictions. It was found that the model did an excellent job in predicting ignition for materials which did not melt.

Higher turbine inlet temperatures (TIT) are required for higher overall efficiency of gas turbine engines. Due to the constant push towards achieving high TIT, the heat load on high pressure turbine components has been increasing with time. Gas turbine airfoils are equipped with several sophisticated cooling technologies which protect them from harsh external environment and increase their operating life and reduce the maintenance cost. The turbine airfoils are coated with thermal barrier coatings (TBCs) and the external surface is protected by film cooling. The internals of gas turbine blades are cooled by relatively colder air bled off from the compressor discharge. Gas turbine internals can be divided into three broad segments – Leading edge section, (2) mid-chord section and (3) trailing edge section. The leading edge of the airfoil is subjected to extreme heat loads due to hot main gas stagnation and high turbulence intensity of the combustor exit gases. The leading edge is typically cooled by jet impingement which cross-over the rib turbulators in the feed chamber. The mid-chord section of the turbine airfoils have serpentine passages connected via. 180° bends, and they feature turbulence promotors which enhance the heat exchange rates between the coolant and the internal walls of the airfoil. The trailing edge section is typically cooled by array of pin fins. On one hand, the coolant routed through the internal passages of turbine airfoil help maintain the airfoil temperatures within safe limits of operation, the cooled air comes at a cost of loss of high pressure air from the compressor section. The aim of this study is to develop internal cooling concepts which have high thermal hydraulic performance, i.e. to gain high levels heat transfer enhancement due to cooling concepts at lower pumping power requirements. Experimental and numerical studies have been carried out and new rib turbulator designs such as Criss-Cross pattern, compound channels featuring uniquely organized ribs and dimples, novel jet impingement hole shapes have been developed which have high thermal-hydraulic performance. Further, gas turbine blades rotate at high rotational speeds. The internal flow routed thought the serpentine passages are subjected to Coriolis and centrifugal buoyancy forces. The combined effects of these forces results in enhancement and reduction in heat transfer on the pressure side and suction side internal walls. This leads to non-uniformity in the heat transfer enhancement which leads to non-uniform cooling and increase in the sites of high and low internal wall temperatures. Development of cooling concepts which have high thermal hydraulic performance under non-rotating conditions is important, however, under rotation, the heat transfer characteristics of the internal passages is significantly different in an unfavorable way. So the aim of the turbine cooling research is to have concepts which provide highly efficient and uniform cooling. The negative effects of rotation has been addressed in this study and new orientation of two-pass cooling channels has been presented which utilizes the rotational energy in favor of heat transfer enhancement on both pressure and suction side internal walls. Present study has led to several new cooling concepts which are efficient under both stationary and rotating conditions. / Ph. D. / Higher turbine inlet temperatures lead to higher overall efficiency of gas turbines. Hence, the high pressure stages of turbine sections, which are downstream of the combustor section, have significant thermal load. The turbine inlet temperatures can be as high as 1700°C and turbine airfoil material melting point temperature is around 1000°C. In order to protect the blade for the harsh environment, relatively colder air (~700°C) bled off from the compressor discharge is routed through the internal cooling passages of turbine airfoils. The coolant bled from the compressor section contributes the reduction in the performance of the engine. Hence, the aim of the turbine cooling research is to achieve high rates of heat transfer at relatively lower pumping power requirements. In order to enhance the heat transfer rates from between the hot internal walls of airfoil and the coolant, turbulence promotors are typically installed in the mid-section of the airfoil which features serpentine passages interconnected by 180° bends. Present study is focused on development of highly efficient concepts for internal flows in turbine airfoils. The other aspect of internal cooling research is focused on characterization of heat transfer under rotating conditions. Coriolis force and centrifugal buoyancy forces lead to non-uniform cooling and the heat transfer rates are significantly different under rotating conditions compared to non-rotating conditions. Present study utilizes detailed measurements of heat transfer coefficients under rotating conditions for the development of cooling designs for two-pass ribbed channels where rotational effects can be used in favor of heat transfer enhancement, leading to enhanced and more uniform cooling of internal walls.

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Vita. / Digitized by Kansas Correctional Industries