Research Papers

J. Turbomach. 2018;140(9):091001-091001-12. doi:10.1115/1.4040853.

Motivated by the recent advances in additive manufacturing, this study investigated a new turbine end-wall aerothermal management method by engineered surface structures. The feasibility of enhancing purge air cooling effectiveness through a series of small-scale ribs added onto the turbine end-wall was explored experimentally and numerically in this two-part paper. Part I presents the fundamental working mechanism and cooling performance in a 90 deg turning duct (part I), and part II of this paper validates the concept in a more realistic turbine cascade case. In part I, the turning duct is employed as a simplified model for the turbine passage without introducing the horseshoe vortex. End-wall heat transfer and temperature were measured by the infrared thermography. Computational fluid dynamics (CFD) simulation was also performed using ANSYS fluent to compliment the experimental findings. With the added end-wall rib structures, purge air flow was observed to be more attached to the end-wall and cover a larger wall surface area. Both experimental and numerical results reveal a consistent trend on improved film cooling effectiveness. The practical design optimization strategy is also discussed in this paper.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091002-091002-11. doi:10.1115/1.4040854.

Motivated by the recent advances in additive manufacturing, a novel turbine end-wall aerothermal management method is presented in this two-part paper. The feasibility of enhancing purge air cooling effectiveness through engineered surface structure was experimentally and numerically investigated. The fundamental working mechanism and improved cooling performance for a 90 deg turning duct are presented in Part I. The second part of this paper demonstrates this novel concept in a low-speed linear cascade environment. The performance in three purge air blowing ratios is presented and enhanced cooling effectiveness and net heat flux reduction (NHFR) were observed from experimental data, especially for higher blow ratios. The Computational fluid dynamics (CFD) analysis indicates that the additional surface features are effective in reducing the passage vortex and providing a larger area of coolant coverage without introducing additional aerodynamic loss.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091003-091003-10. doi:10.1115/1.4040860.

Inlet distortion often occurs under off-design conditions when a flow separates within an intake and this unsteady phenomenon can seriously impact fan performance. Fan–distortion interaction is a highly unsteady aerodynamic process into which high-fidelity simulations can provide detailed insights. However, due to limitations on the computational resource, the use of an eddy resolving method for a fully resolved fan calculation is currently infeasible within industry. To solve this problem, a mixed-fidelity computational fluid dynamics method is proposed. This method uses the large Eddy simulation (LES) approach to resolve the turbulence associated with separation and the immersed boundary method (IBM) with smeared geometry (IBMSG) to model the fan. The method is validated by providing comparisons against the experiment on the Darmstadt Rotor, which shows a good agreement in terms of total pressure distributions. A detailed investigation is then conducted for a subsonic rotor with an annular beam-generating inlet distortion. A number of studies are performed in order to investigate the fan's influence on the distortions. A comparison to the case without a fan shows that the fan has a significant effect in reducing distortions. Three fan locations are examined which reveal that the fan nearer to the inlet tends to have a higher pressure recovery. Three beams with different heights are also tested to generate various degrees of distortion. The results indicate that the fan can suppress the distortions and that the recovery effect is proportional to the degree of inlet distortion.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091004-091004-15. doi:10.1115/1.4041035.

An experimental investigation of the geometrical parameter effects on the film cooling performance of a fan-shaped hole was conducted on a low speed flat-plate facility. The pressure sensitive paint (PSP) technique and steady liquid crystal (SLC) technique were employed to determine the adiabatic film cooling effectiveness and heat transfer coefficients, respectively, for a blowing ratio ranging from 0.3 to 3 and a density ratio of DR = 1.5. Several geometrical parameters were investigated, including lateral expansion angle, length-to-diameter ratio, and hole entrance shape. Local, laterally averaged, and area-averaged adiabatic film cooling effectiveness, heat transfer coefficients, and net heat flux reduction (NHFR) were shown to provide a comprehensive understanding on the geometrical parameter effects on the thermal performance. A novel method was proposed for designing a fan-shaped hole with short length-to-diameter ratio to design to achieve high film cooling performance. The original and optimized fan-shaped holes were compared in terms of adiabatic film cooling effectiveness, heat transfer coefficients, and NHFR. Results showed that the optimized fan-shaped hole with short length-to-diameter ratio, large lateral diffusion angle, and slot hole entrance shape obtained highest overall thermal performance. It demonstrated the feasibility of adopting the proposed design method to design fan-shaped holes applied in thin wall gas turbine blades.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091005-091005-10. doi:10.1115/1.4040997.

The emerging renewable energy market calls for more advanced prediction tools for turbine transient operations in fast startup/shutdown cycles. Reliable numerical analysis of such transient cycles is complicated by the disparity in time scales of the thermal responses in fluid and solid domains. Obtaining fully coupled time-accurate unsteady conjugate heat transfer (CHT) results under these conditions would require to march in both domains using the time-step dictated by the fluid domain: typically, several orders of magnitude smaller than the one required by the solid. This requirement has strong impact on the computational cost of the simulation as well as being potentially detrimental to the accuracy of the solution due to accumulation of round-off errors in the solid. A novel loosely coupled CHT methodology has been recently proposed, and successfully applied to both natural and forced convection cases that remove these requirements through a source-term based modeling (STM) approach of the physical time derivative terms in the relevant equations. The method has been shown to be numerically stable for very large time steps with adequate accuracy. The present effort is aimed at further exploiting the potential of the methodology through a new adaptive time stepping approach. The proposed method allows for automatic time-step adjustment based on estimating the magnitude of the truncation error of the time discretization. The developed automatic time stepping strategy is applied to natural convection cases under long (2000 s) transients: relevant to the prediction of turbine thermal loads during fast startups/shutdowns. The results of the method are compared with fully coupled unsteady simulations showing comparable accuracy with a significant reduction of the computational costs.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091006-091006-11. doi:10.1115/1.4039936.

This work, a continuation of a series of investigations on the aerodynamics of aggressive interturbine ducts (ITD), is aimed at providing detailed understanding of the flow physics and loss mechanisms in four different ITD geometries. A systematic experimental and computational study was carried out by varying duct outlet-to-inlet area ratios (ARs) and mean rise angles while keeping the duct length-to-inlet height ratio, Reynolds number, and inlet swirl constant in all four geometries. The flow structures within the ITDs were found to be dominated by the boundary layer separation and counter-rotating vortices in both the casing and hub regions. The duct mean rise angle determined the severity of adverse pressure gradient in the casing's first bend, whereas the duct AR mainly governed the second bend's static pressure rise. The combination of upstream wake flow and the first bend's adverse pressure gradient caused the boundary layer to separate and intensify the strength of counter-rotating vortices. At high mean rise angle, the separation became stronger at the casing's first bend and moved farther upstream. At high ARs, a two-dimensional separation appeared on the casing and resulted in increased loss. Pressure loss penalties increased significantly with increasing duct mean rise angle and AR.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(9):091007-091007-11. doi:10.1115/1.4040852.

This study was motivated by the difficulties to assess the aerothermodynamic effects of heat transfer on the performance of turbocharger turbine by only looking at the global performance parameters, and by the lack of efforts to quantify the physical mechanisms associated with heat transfer. In this study, we aimed to investigate the sensitivity of performance to heat loss, to quantify the aerothermodynamic mechanisms associated with heat transfer and to study the available energy utilization by a turbocharger turbine. Exergy analysis was performed based on the predicted three-dimensional flow field by detached eddy simulation (DES). Our study showed that at a specified mass flow rate, (1) pressure ratio drop is less sensitive to heat loss as compared to turbine power reduction, (2) turbine power drop due to heat loss is relatively insignificant as compared to the exergy lost via heat transfer and thermal irreversibilities, and (3) a single-stage turbine is not an effective machine to harvest all the available exhaust energy in the system.

Commentary by Dr. Valentin Fuster

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