• Energy Research
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Abstract Transpiration cooling is one of the most efficient cooling technologies to protect hot section components such as turbine airfoils, missile heads and shells of rockets or space craft. This external cooling method has much higher efficiency than film cooling when consuming the same amount of coolant, due to the uniformity of coolant distribution. However, pore plugging, which frequently occurs during the operation of transpiration cooled components, has limited its long term stability and prevented its application in industrial components. Dust deposition is one of the main reasons causing plugging of pores for transpiration cooling. Although a lot of effort has been devoted into explaining dust deposition and erosion mechanisms of transpiration cooled components, reducing plugging impact remained difficult as the plugging caused by dusts was unpredictable for traditional porous media. Additive manufacturing, with capability to precisely construct structures in small scales, has emerged as considerable new tool to enhance the controllability of porous media, and furthermore, to achieve a good solution to minimize the plugging disadvantage. The present study selected a transpiration cooling configuration perforated by straight holes with an additive manufacturable diameter of 0.4 mm. Computational Fluid Dynamics (CFD) methods were employed to model the pore plugging and its effect on heat transfer. A scripting code in addition to the ANSYS CFX solver was utilized to simulate the random plugging conditions of the holes. Two hundred numerical cases with four different plugging probabilities were calculated and statistically evaluated to quantify the disadvantage of pore plugging on the cooling effectiveness. A theoretic model with convolution functions was developed to predict the local cooling effectiveness. Results obtained from the numerical analysis indicated that the overall plugging ratio was a dominating parameter for the cooling effectiveness but this single parameter was not adequate to scale the cooling effectiveness for all locations. On the contrary, the unique pair of discrete convolution parameters indexing all other transpiration holes in the array developed in this study had a significantly higher accuracy in predicting the cooling effectiveness than the overall plugging ratio. The present study was among one of the earliest to use convolution modeling method to predict transpiration cooling and related plugging disadvantages. This effort could provide a quantitative understanding of the random plugging on the specific transpiration cooling configuration, and could benefit further optimization effort to reduce the plugging disadvantage of transpiration cooling using additive manufacturing.

Abstract Transpiration cooling is one of the most efficient cooling technologies to protect hot section components such as turbine airfoils, missile heads and shells of rockets or space craft. This external cooling method has much higher efficiency than film cooling when consuming the same amount of coolant, due to the uniformity of coolant distribution. However, pore plugging, which frequently occurs during the operation of transpiration cooled components, has limited its long term stability and prevented its application in industrial components. Dust deposition is one of the main reasons causing plugging of pores for transpiration cooling. Although a lot of effort has been devoted into explaining dust deposition and erosion mechanisms of transpiration cooled components, reducing plugging impact remained difficult as the plugging caused by dusts was unpredictable for traditional porous media. Additive manufacturing, with capability to precisely construct structures in small scales, has emerged as considerable new tool to enhance the controllability of porous media, and furthermore, to achieve a good solution to minimize the plugging disadvantage. The present study selected a transpiration cooling configuration perforated by straight holes with an additive manufacturable diameter of 0.4 mm. Computational Fluid Dynamics (CFD) methods were employed to model the pore plugging and its effect on heat transfer. A scripting code in addition to the ANSYS CFX solver was utilized to simulate the random plugging conditions of the holes. Two hundred numerical cases with four different plugging probabilities were calculated and statistically evaluated to quantify the disadvantage of pore plugging on the cooling effectiveness. A theoretic model with convolution functions was developed to predict the local cooling effectiveness. Results obtained from the numerical analysis indicated that the overall plugging ratio was a dominating parameter for the cooling effectiveness but this single parameter was not adequate to scale the cooling effectiveness for all locations. On the contrary, the unique pair of discrete convolution parameters indexing all other transpiration holes in the array developed in this study had a significantly higher accuracy in predicting the cooling effectiveness than the overall plugging ratio. The present study was among one of the earliest to use convolution modeling method to predict transpiration cooling and related plugging disadvantages. This effort could provide a quantitative understanding of the random plugging on the specific transpiration cooling configuration, and could benefit further optimization effort to reduce the plugging disadvantage of transpiration cooling using additive manufacturing.

To meet the performance goals of advanced fossil power generation systems, future coal-gas fired turbines will likely be operated at temperatures higher than those in the current commercial natural gas-fired systems. The working fluid in these future turbines could contain substantial moisture (steam), mixed with carbon dioxide, instead of air or nitrogen in conventional gas turbines. As a result, the aerothermal characteristics among the advanced turbine systems are expected to be significantly different, not only from the natural gas turbines but also will be dependent strongly on the compositions of turbine working fluids. Described in this paper is a quantitative comparison of thermal load on the external surface of turbine airfoils that are projected to be utilized in different power cycles the U.S. Department of Energy plans for the next 2 decades. The study is pursued with a computational simulation, based on the three-dimensional computational fluid dynamics analysis. While the heat transfer coefficient has shown to vary strongly along the surface of the airfoil, the projected trends were relatively comparable for airfoils in syngas and hydrogen-fired cycles. However, the heat transfer coefficient for the oxyfuel cycle is found to be substantially higher by about 50–60% than its counterparts in syngas and hydrogen turbines. This is largely caused by the high steam concentration in the turbine flow. Results gained from this study overall suggest that advances in cooling technology and thermal barrier coatings are critical for developments of future coal-based turbine technologies with near zero emissions.

To meet the performance goals of advanced fossil power generation systems, future coal-gas fired turbines will likely be operated at temperatures higher than those in the current commercial natural gas-fired systems. The working fluid in these future turbines could contain substantial moisture (steam), mixed with carbon dioxide, instead of air or nitrogen in conventional gas turbines. As a result, the aerothermal characteristics among the advanced turbine systems are expected to be significantly different, not only from the natural gas turbines but also will be dependent strongly on the compositions of turbine working fluids. Described in this paper is a quantitative comparison of thermal load on the external surface of turbine airfoils that are projected to be utilized in different power cycles the U.S. Department of Energy plans for the next 2 decades. The study is pursued with a computational simulation, based on the three-dimensional computational fluid dynamics analysis. While the heat transfer coefficient has shown to vary strongly along the surface of the airfoil, the projected trends were relatively comparable for airfoils in syngas and hydrogen-fired cycles. However, the heat transfer coefficient for the oxyfuel cycle is found to be substantially higher by about 50–60% than its counterparts in syngas and hydrogen turbines. This is largely caused by the high steam concentration in the turbine flow. Results gained from this study overall suggest that advances in cooling technology and thermal barrier coatings are critical for developments of future coal-based turbine technologies with near zero emissions.

Future advanced turbine systems for electric power generation systems, based on coal-gasified fuels with CO2 capture and sequestration, are aimed for achieving higher cycle efficiency and near-zero emission. Most promising operating cycles being developed are hydrogen-fired cycle and oxy-fuel cycle. Both cycles will likely have turbine working fluids significantly different from that of conventional air-based gas turbines. In addition, the oxy-fuel cycle will have a turbine inlet temperature target at approximately 2030K (1760°C), significantly higher than the current level. This suggests that aerothermal control and cooling will play a critical role in realizing our nation’s future fossil power generation systems. This paper provides a computational analysis in comparing the internal cooling performance of a double-wall or skin-cooled airfoil to that of an equivalent serpentine-cooled airfoil. The present results reveal that the double-wall or skin cooled approach produces superior performance than the conventional serpentine designs. This is particularly effective for the oxy-fuel turbine with elevated turbine inlet temperatures. The effects of coolant-side internal heat transfer coefficient on the airfoil metal temperature in both hydrogen-fired and oxy-fuel turbines are evaluated. The contribution of thermal barrier coatings (TBC) toward overall thermal protection for turbine airfoil cooled under these two different cooling configurations is also assessed.

Future advanced turbine systems for electric power generation systems, based on coal-gasified fuels with CO2 capture and sequestration, are aimed for achieving higher cycle efficiency and near-zero emission. Most promising operating cycles being developed are hydrogen-fired cycle and oxy-fuel cycle. Both cycles will likely have turbine working fluids significantly different from that of conventional air-based gas turbines. In addition, the oxy-fuel cycle will have a turbine inlet temperature target at approximately 2030K (1760°C), significantly higher than the current level. This suggests that aerothermal control and cooling will play a critical role in realizing our nation’s future fossil power generation systems. This paper provides a computational analysis in comparing the internal cooling performance of a double-wall or skin-cooled airfoil to that of an equivalent serpentine-cooled airfoil. The present results reveal that the double-wall or skin cooled approach produces superior performance than the conventional serpentine designs. This is particularly effective for the oxy-fuel turbine with elevated turbine inlet temperatures. The effects of coolant-side internal heat transfer coefficient on the airfoil metal temperature in both hydrogen-fired and oxy-fuel turbines are evaluated. The contribution of thermal barrier coatings (TBC) toward overall thermal protection for turbine airfoil cooled under these two different cooling configurations is also assessed.