Bayindir H. Saracoglu

Experimental analysis on the effects of DC arc discharges at various flow regimes

This paper addresses the control of the boundary layer on a compression ramp by means of DC electrical arc discharges. The development and realization of the control system are first described and then assessed in the wind tunnel. The objective of the research was to control the supersonic flow using the minimum amount of energy. The array of electrodes was located at the base of a ramp, where a low momentum flow develops. The electrical discharge was generated by a custom designed electronic facility based on high-voltage ignition coils. The slanted tungsten electrodes were insulated by mounting them in a ceramic support. The discharge evolution was studied through high-speed flow visualizations, while electrical measurements at the high-voltage section of the circuitry allowed to estimate the energy release. The development of a high-speed short exposure Schlieren imaging technique, based on a very short duration laser pulse illumination and a double shot CCD camera, allowed to observe the macroscopic effects associated with the arc establishment between the electrodes (glow, sound wave and heat release). Due to the long residence time, the thermal perturbation spread along the streamwise direction. Cross correlation of Schlieren images with short time separation revealed that in supersonic conditions, the discharges led to an overall acceleration of the flow field underneath the oblique shock wave.

Blunt Trailing Edge Cooling Effects at Supersonic Regime

Vane shocks are a major contribution to the efficiency loss in HP turbines. Additionally vane shocks are the source of large pressure fluctuations that may result in low and high – cycle fatigue problems. High pressure bladings are internally cooled due to the large gas temperatures, well above the melting temperature. Part of the coolant flow is discharged into the main flow at the trailing edge or rear pressure side to refrigerate the rear part of the airfoil. In the present study, the effects of coolant blowing on the base pressure, and trailing edge shock patterns are investigated. For this purpose, series of numerical simulations in a blunt trailing edge at various coolant ejection regimes are performed at supersonic conditions. The numerical Schlieren data is post processed to determine the shock inclination and strength. The cooling ejection alters the shock waves, while modifying the base pressure, which may result in a significant reduction of the vane trailing edge loss.

Multi-stage nozzle-shape optimization for pulsed hydrogen–air detonation combustor:

Thermal engines based on pressure gain combustion offer new opportunities to generate thrust with enhanced efficiency and relatively simple machinery. The sudden expansion of detonation products from a single-opening tube yields thrust, although this is suboptimal. In this article, we present the complete design optimization strategy for nozzles exposed to detonation pulses, combining unsteady Reynolds-averaged Navier–Stokes solvers with the accurate modeling of the combustion process. The parameterized shape of the nozzle is optimized using a differential evolution algorithm to maximize the force at the nozzle exhaust. The design of experiments begins with a first optimization considering steady-flow conditions, subsequently followed by a refined optimization for transient supersonic flow pulse. Finally, the optimized nozzle performance is assessed in three dimensions with unsteady Reynolds-averaged Navier–Stokes capturing the deflagration-to-detonation transition of a stoichiometric, premixed hydrogen–air mixture. The optimized nozzle can deliver 80% more thrust than a standard detonation tube and about 2% more than the optimized results assuming steady-flow operation. This study proposes a new multi-fidelity approach to optimize the design of nozzles exposed to transient operation, instead of the traditional methods proposed for steady-flow operation.

Axial Bladeless Turbine Suitable for High Supersonic Flows

The development of future propulsion systems may rely on the extraction of power from high supersonic flows. In conventional supersonic turbines operating under such extreme flow conditions, the presence of blades induces important flow perturbations that result in severe aerodynamic losses and restrictions in the operating range, seriously affected by starting issues. These limitations present design opportunities for a bladeless turbine configuration. The present paper documents, for the first time in literature, an axial bladeless turbine concept, which is able to extract mechanical power from high supersonic flows through an annular channel. The tangential drag force exerted on the rotating inner and outer end-walls produces power, together with enhanced simplicity, coolability, and maintainability. Reynolds-averaged Navier–Stokes simulations allowed the detailed evaluation of the aerodynamic performance of the bladeless configuration. This unconventional axial turbine concept becomes a design choice ...

Analysis of the flow field around the wing section of a FanWing aircraft under various flow conditions

A novel horizontal-axis open-rotor configuration for a distributed propulsion system was numerically investigated. URANS simulations were performed using the compressible flow solver of Fluent (ANSYS) 14 with Spalart-Allmaras turbulence closure. The effects of the rotational speed, aircraft cruise speed, geometrical parameters and angle of attack were investigated and their contributions to the optimum lift and thrust forces analyzed. The effects of the geometrical attributes such as wing chord, fan blade airfoil shape and rotation mechanism were also investigated in the analysis. The present paper discusses in detail an alternative high lift propulsion system by dissecting the aerodynamic attributes of its components.

Energy Analysis of Pulsating Coolant Ejection

High-pressure turbines are subjected to high expansion ratios which may result in supersonic vane or rotor outlet. In the supersonic regime, shock waves are formed at the vane trailing edges that periodically impact on the downstream rotor blades and the neighboring vane. Pulsating cooling was proposed to modulate the trailing edge shocks to diminish the detrimental efficiency abatement and structural problems. A large test section equipped with three airfoils, replicated the actual loading of a transonic vane. Tests were performed in a short duration compression tube facility at four Mach numbers (0.8, 0.95, 1.1 and 1.2) and two different Reynolds numbers (4 and 6 million). Coolant was fed to the trailing edge of the central airfoil through a siren valve, that allowed to study four different coolant blowing cases: no-blowing, continuous blowing at three pressure levels and a pulsating coolant blowing condition. Unsteady numerical simulations of the flow over the airfoil model were performed using ANSYS 14 (Fluent) flow solver. In order to understand the impact of the different steady and unsteady cooling schemes on the efficiency of high pressure turbine bladings, the individual contribution of trailing edge and profile losses were calculated. The coolant ejection generated a significant reduction of the trailing edge loss. The overall losses also diminished by the introduction of cooling as compared to no blowing case. Improvements in loss levels owing to pulsating cooling observed to be more pronounced for the engine representative cooling rates (∼3%). The trends in loss variation with respect to cooling scheme show that pulsating cooling may become superior for high blowing cases.Copyright

Pulsating Coolant Ejection Effects Downstream of Supersonic Trailing Edge

Abstract Trailing edge blowing is used in a great variety of applications. In particular, high-pressure turbines bleed about 3% of the engine core massflow through a rear slot of the inlet guide vanes. At transonic conditions the coolant flow alters substantially the base region characteristics, and therefore the wake and shocks. Certain pulsating coolant frequencies give rise to exotic vortical shedding structures, not observed when the coolant is continuous. A detailed analysis of the flow field near the trailing edge region has been performed using the in-house HybFlow code. Blunt and circular trailing edges, exposed to continuous blowing (at different rates) were compared by using steady simulations. Both shock intensity and wake loss are discussed in this paper considering pulsating coolant ejection at various frequencies and blowing ratios.

Pulsating Coolant Ejection Effects Downstream of a Transonic Rounded Trailing Edge

Trailing edge blowing is used in a great variety of applications. In particular, high-pressure turbines bleed about 3% of the engine core massflow through a rear slot. At transonic conditions the coolant flow alters substantially the base region, and therefore the wake and shocks. Certain pulsating coolant frequencies give rise to exotic vortical shedding structures, not observed when the coolant is continuous. A detailed analysis of the flow field near the trailing edge region has been performed using the in-house HybFlow code. A rounded trailing edge with null and continuous blowing (at different pressure ratios) have been compared with a blunt trailing edge geometry using steady simulations. Both shock intensity, and wake loss are discussed in this paper considering pulsating coolant ejection at various frequencies and blowing ratios.

Aeroacoustics of Flow over Rectangular Cavities

Unsteady Reynolds Averaged Navier Stokes (URANS) computations were performed to simulate flow past a rectangular cavity with free stream Mach number of 0.3. Deep cavity geometry was used with L/D ratio of 0.21. Frequency spectrum of the time accurate pressure data obtained using both URANS and LES simulations for closed cavity case. The dominant frequency of the resonance in the cavity was obtained as 16 kHz for the closed cavity case both with URANS and LES simulations. By injection of %1 of the main stream mass flow from the bottom of the cavity for the same free stream Mach number, dominant frequency reduced to the 8 kHz. The unsteady flow structures and effect of injection on the cavity flow instability mechanism were analyzed in detail in terms of flow physics. It was shown that injection effects vortex-acoustic feedback loop, by lifting up the vortices near the trailing edge of the cavity. Moreover, with the effect of injection, pressure fluctuations increased from 280 Pa to the 650 Pa.