F. Malak

Computational Study of Geometric Parameter Influence on Aggressive Inter-Turbine Duct Performance

Modern direct-drive turbofan engines typically have the fan turbine designed at significantly higher diameter than the gas producer turbine. Furthermore, the gas turbine industry is being pushed to shorten engine length with the goal of reducing weight. This results in a need to design very aggressive inter-turbine-ducts (ITD’s) that have high endwall slopes. The gas turbine design cycle typically begins with conceptual design where many engine configuration iterations are made. During conceptual design, there usually is little firm geometric definition or time for detailed CFD studies on aggressive ITD’s. This can cause a large amount of risk to the engine development schedule and cost if the space allocated for the ITD during conceptual design is found to be insufficient later in the design cycle. Therefore, simple analytical tools for accurately assessing the risk of an ITD in conceptual design are important. The gas turbine industry is familiar with the Sovran and Klomp annular diffuser performance chart [1] as a conceptual design tool for assessing ITD’s. However, its applicability to modern gas turbine ducts with high endwall slope is limited. The location of the maximum pressure recovery for a given length, the Cp* line, considers only two geometric parameters: area ratio and normalized length. The chart makes no distinction of risk of flow separation regarding the level of slope or the pitch-wise turning in the duct. However, intuition would suggest that a high slope duct would have more risk of separation than an equivalent area ratio duct with low slope. Similarly, a duct that turns the flow from axial to radial would be expected to be riskier than a pure axial duct. To help assess the interaction of duct slope and pitch-wise turning with area ratio and length, an analytical Design of Experiments (DOE) was run using approximately sixty different duct configurations. The DOE was carried out using 3D, steady CFD analysis. The results of the DOE are presented with insights provided into how the Cp* line may shift as a function of duct slope. Of particular interest is that slope by itself does not work particularly well as a risk indicator. However, a combination of new area ratio-length and slope-length parameters was found to segregate ducts between separated and non-separated cases.Copyright

Review of Platform Cooling Technology for High Pressure Turbine Blades

With the relatively large surface area of the platform of the gas turbine blades being exposed directly to the hot, mainstream gas, it is vital to efficiently cool this region of the blades. This region is particularly difficult to protect due to the strong secondary flows developed at the airfoil junction (formation of the leading edge horseshoe vortex) and circumferentially across the blade passage (strengthening passage vortex moving from the pressure side to the suction side of the passage). Over the past decade, researchers and engine designers have attempted to combat the enhanced heat transfer to the blade platform by implementing both frontside and backside novel cooling techniques. This paper presents a review of platform cooling technology ranging from frontside film cooling via stator-rotor purge flow, mid-passage purge flow, and discrete film holes to backside cooling achieved via impinging jet arrays or cooling channels. To gain a full understanding of state-of-the-art cooling technology, recent patents, journal articles, and conference proceedings are included in this review.Copyright

Enhanced Film Cooling Effectiveness With Surface Trenches

In this study, cylindrical and fan shaped film cooling holes are evaluated on the blade surface numerically, using the Computational Fluid Dynamics (CFD) tool ANSYS-CFX, with the objective of improving cooling effectiveness by understanding the flow pattern at the cooling hole exit. The coolant flow rates are adjusted for blowing ratios of 0.5, 1.0 & 1.5 (momentum flux ratios of 0.125, 0.5 & 1.125 respectively). The density ratio is maintained at 2.0. New shaped holes viz. straight, concave and convex trench holes are introduced and are evaluated under similar operating conditions. Results are presented in terms of surface temperatures and adiabatic effectiveness at three different blowing ratios for the different film cooling hole shapes analyzed. Comparison is made with reference to the fan shaped film cooling hole to bring out relative merits of different shapes. The new trench holes improved the film cooling effectiveness by allowing more residence time for coolant to spread laterally while directing smoothly onto the airfoil surface. While convex trench improved the centre-line effectiveness, straight trench improved the laterally-averaged and overall effectiveness at all blowing ratios. Concave trench improved the effectiveness at blowing ratios 0.5 and 1.0.Copyright

Novel Turbulators to Enhance Turbine Blade Internal Heat Transfer Rates

Turbine blades are driven by hot gases from the combustor. The heat transfer from the hot gases produces substantial thermal load and can affect the performance of the turbine blades. In previous designs, cavities inside the blades were created to pass the coolant. Such cooling designs helped to increase the thermal performance of the blades by taking away the turbine blades’ heat. The cooling effect was further enhanced by increasing the turbulence in the flow of coolant. To increase the turbulence in the cavities, various turbulator designs were proposed. However, most of the designs have also introduced wake area while increasing the turbulence. This reduces the heat exchange between the coolant and the blade. The current paper discusses new designs of tabulators for turbine blades that increase the heat transfer rates of the cooling surface by increasing the turbulence of the coolant flow while minimizing the wake area.Copyright