Oguz Uzol

Experimental Investigation of Unsteady Flow Field Within a Two Stage Axial Turbomachine Using Particle Image Velocimetry

Detailed measurements of the flow field within the entire 2 nd stage of a two stage axial turbomachine are performed using Particle Image Velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a “lattice of wakes”, and the resulting wakewake and wake-blade interactions cause major flow and turbulence non-uniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average-passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential non-uniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The non-uniformities in the lateral velocity component due to the interaction of the rotor blade with the 1 st stage rotor-stator wakes, result in 13% variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the nonuniformities in flow structure have significant effects on the overall performance of the system. NOMENCLATURE k Turbulent kinetic energy kdet Deterministic kinetic energy Ls Stage length starting from the rotor leading edge N Total number of instantaneous samples NaI Sodium Iodide t Time T Blade passing period u Instantaneous axial velocity Utip Velocity of rotor blade tip v Instantaneous lateral velocity

Aerodynamic Loss Characteristics of a Turbine Blade With Trailing Edge Coolant Ejection: Part 2—External Aerodynamics, Total Pressure Losses, and Predictions

Investigation of the internal fluid mechanic losses for a turbine blade with trailing edge coolant ejection was present in Uzol et al. (2000). The current study is a detailed experimental investigation of the external subsonic flowfield near the trailing edge and the investigation of the external aerodynamic loss characteristics of the turbine blade with trailing edge coolant ejection system. Particle Image Velocimetry experiments and total pressure surveys in the near wake of the blade are conducted for two different Reynolds numbers and four different ejection rates. Two different trailing edge configurations with different cut-back lengths are also investigated. Numerical simulations of the flowfield are also performed for qualitative flow visualization purposes. Two-dimensional, incompressible, and steady solutions of Reynolds-averaged Navier ‐Stokes equations are obtained. A two-equation standard k‐e turbulence model coupled with an Algebraic Reynolds Stress Model is used for the simulation of the turbulent flowfield. The results show that the aerodynamic penalty levels in the wake region near the trailing edge are increased due to the mixing of the coolant and mainstream flows for 0‐3 percent ejection rates. However, after a threshold level (5 percent ejection rate), the ejected coolant flow has enough momentum to fill the wake of the blade, which in turn results in a decrease in the aerodynamic penalty levels. @DOI: 10.1115/1.1351817#

Heat Transfer, Pressure Loss and Flow Field Measurements Downstream of Staggered Two-Row Circular and Elliptical Pin Fin Arrays

We present the results of heat transfer, total pressure loss, and wake flow field measurements downstream of two-row staggered elliptical and circular pin fin arrays. Two different types of elliptical fins are tested, i.e., a Standard Elliptical Fin (SEF) and a fin that is based on NACA four digit symmetrical airfoil shapes (N fin). The results are compared to those of a corresponding circular pin fin array. The minor axis lengths for both types of elliptical fins are kept equal to the diameter of the circular fins. Experiments are performed using Liquid Crystal Thermography and total pressure probe wake surveys in a Reynolds number range of 18 000 and 86 000 as well as Particle Image Velocimetry (PIV) measurements at Re D =18 000. The pin fins had a height-to-diameter ratio of 1.5. The streamwise and the transverse spacings were equal to one circular fin diameter, i.e., S/D =X/D =2

Flow Nonuniformities and Turbulent “Hot Spots” Due to Wake-Blade and Wake-Wake Interactions in a Multi-Stage Turbomachine

This experimental study provides striking examples of the complex flow and turbulence structure resulting from blade-wake and wake-wake interactions in a multi-stage turbomachine. Particle image velocimetry (PIV) measurements are performed within the entire 2 nd stage of a two-stage turbomachine. The experiments are performed in a facility that allows unobstructed view of the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. This paper contains data on the phase-averaged flow structure including velocity, vorticity and strain-rate, as well as the turbulent kinetic energy and shear stress, at mid span, for several orientation of the rotor relative to the stator. Two different test setups with different blade geometries are used in order to highlight and elucidate complex phenomena involved, as well as to demonstrate that some of the interactions are characteristic to turbomachines and can be found in a variety of geometries. The first part of the paper deals with the interaction of a 2 nd -stage rotor with the wakes of both the rotor and the stator of the 1 st stage. Even before interacting with the blade, localized regions with concentrated mean vorticity and elevated turbulence levels form at the intersection of the rotor and stator wakes of the 1 st stage. These phenomena persist even after being ingested by the rotor blade of the 2 nd stage. As the wake segment of the 1 st -stage rotor blade arrives to the 2 nd stage, the rotor blades become submerged in its elevated turbulence levels, and separate the region with negative vorticity that travels along the pressure side of the blade, from the region with positive vorticity that remains on the suction side. The 1 st -stage stator wake is chopped-off by the blades. Due to difference in mean lateral velocity, the stator wake segment on the pressure side is advected faster than the segment on the suction side (in the absolute frame of reference), creating discontinuities in the stator wake trajectory. The nonuniformities in phase-averaged velocity distributions generated by the wakes of the 1 st stage persist while passing through the 2 nd -stage rotor. The combined effects of the 1 st -stage blade rows cause 10-12 deg variations of flow angle along the pressure side of the blade. Thus, in spite of the large gap between the 1 st and 2 nd rotors (compared to typical rotor-stator spacings in axial compressors), 6.5 rotor axial chords, the wake-blade interactions are substantial. The second part focuses on the flow structure at the intersection of the wakes generated by a rotor and a stator located upstream of it. In both test setups the rotor wake is sheared by the nonuniformities in the axial velocity distributions, which are a direct result of the discontinuities in the trajectories of the stator wake. This shearing creates a kink in the trajectory of the rotor wake, a quadruple structure in the distribution of strain, regions with concentrated vorticity, high turbulence levels and high shear stresses, the latter with a complex structure that resembles the mean strain. Although the hot spots diffuse as they are advected downstream, they still have elevated turbulence levels compared to the local levels around them. In fact, every region of wake intersection has an elevated turbulence level.

The Effect of Inlet Guide Vanes Wake Impingement on the Flow Structure and Turbulence Around a Rotor Blade

The flow structure and turbulence around the leading and trailing edges of a rotor blade operating downstream of a row of inlet guide vanes (IGV) are investigated experimentally. Particle image velocimetry (PIV) measurements are performed in a refractive index matched facility that provides unobstructed view of the entire flow field. Data obtained at several rotor blade phases focus on modification to the flow structure and turbulence in the IGV wake as it propagates along the blade. The phase-averaged velocity distributions demonstrate that wake impingement significantly modifies the wall-parallel velocity component and its gradients along the blade. Due to spatially non-uniform velocity distribution, especially on the suction side, the wake deforms while propagating along the blade, expanding near the leading edge and shrinking near the trailing edge. While being exposed to the nonuniform strain field within the rotor passage, the turbulence within the IGV wake becomes spatially nonuniform and highly anisotropic. Several mechanisms, which are consistent with rapid distortion theory (RDT) and distribution of turbulence production rate, contribute to the observed trends. For example, streamwise (in rotor frame reference) diffusion in the aft part of the rotor passage enhances the streamwise fluctuations. Compression also enhances the turbulence production very near the leading edge. However, along the suction side, rapid changes to the direction of compression and extension cause negative production. The so-called wall blockage effect reduces the wall-normal component.

Experimental Investigation of Unsteady Flow Field Within a Two-Stage Axial Turbomachine Using Particle Image Velocimetry

Detailed measurements of the flow field within the entire 2nd stage of a two-stage axial turbomachine are performed using particle image velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a lattice of wakes, and the resulting wake-wake and wake-blade interactions cause major flow and turbulence nonuniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential nonuniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The nonuniformities in the lateral velocity component due to the interaction of the rotor blade with the 1st-stage rotor-stator wakes, result in 13 percent variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the nonuniformities in flow structure have significant effects on the overall performance of the system.

Average Passage Flow Field and Deterministic Stresses in the Tip and Hub Regions of a Multistage Turbomachine

This paper continues our effort to study the dynamics of deterministic stresses in a multistage turbomachine using experimental data. Here we focus on the tip and hub regions and compare them to mid span data obtained in previous studies. The analysis is based on data obtained in PIV measurements performed in the second stage of a two-stage turbomachine. A complete data set is obtained using blades and fluid with matched optical index of refraction. Previous measurements at mid span have shown that at mid span and close to design conditions, the deterministic kinetic energy is smaller than the turbulent kinetic energy. The primary contributor to the deterministic stresses at mid span is the interaction of a blade with the upstream wakes. Conversely, we find that the tip vortex is the dominant source of phasedependent unsteadiness and deterministic stresses in the tip region. Along the trajectory of the tip vortex, the deterministic kinetic energy levels are more than one order of magnitude higher than the levels measured in the hub and mid-span, and are of the same order of magnitude as the turbulent kinetic energy levels. Reasons for this trend are explained using a sample distribution of phase-averaged flow variables. Outside of the region affected by tip vortex transport, within the rotorstator gap and within the stator passages, the turbulent kinetic energy is still 3-4 times higher than the deterministic kinetic energy. The deterministic and turbulent shear stress levels are comparable in all spanwise locations, except for the wakes of the stator blades, where the turbulent stresses are higher. However, along the direction of tip vortex transport, the deterministic shear stresses are about an order of magnitude higher than the turbulent shear stresses. The decay rates of deterministic kinetic energy in the hub and mid-span regions are comparable to each other, whereas at the tip, the decay rate is higher. The decay rates of turbulent kinetic energy are much smaller than those of the deterministic kinetic energy. The paper also examines terms in the deterministic kinetic energy transport equation. The data indicate that “Deterministic Production” and a new term, called here “Dissipation due to Turbulence” are the dominant source/sink terms. Regions with alternating signs of Deterministic Production indicate that the energy transfer between the phase-averaged and average-passage flow fields can occur in both directions. The divergence of the PressureVelocity correlation, obtained from a balance of all the other terms, is dominant and appears to be much larger than the deterministic production (source/sink) term. This trend indicates that there are substantial deterministic pressure fluctuations in the flow field, especially within the rotor-stator gap and within the stator passage.

Aerodynamic Loss Characteristics of a Turbine Blade With Trailing Edge Coolant Ejection: Part 1—Effect of Cut-Back Length, Spanwise Rib Spacing, Free-Stream Reynolds Number, and Chordwise Rib Length on Discharge Coefficients

The internal fluid mechanics losses generated between the blade plenum chamber and a reference point located just downstream of the trailing edge are investigated for a turbine blade trailing edge cooling system. The discharge coefficient C d is presented as a function of the free-stream Reynolds number, cut-back length, spanwise rib spacing, and chordwise rib length. The results are presented in a wide range of coolant to free-stream mass flow rate ratios, The losses from the cooling system show strong free-stream Reynolds number dependency, especially at low ejection rates, when they are correlated against the coolant to free-stream pressure ratio. However, when C d is correlated against a coolant to free-stream mass flow rate ratio, the Reynolds number dependency is eliminated. The current data clearly show that internal viscous losses due to varying rib lengths do not differ significantly. The interaction of the external wall jet in the cutback region with the free-stream fluid is also a strong contributor to the losses. Since the discharge coefficients do not have Reynolds number dependency at high ejection rates, C d experiments can be performed at a low free-stream Reynolds number. Running a discharge coefficient experiment at low Reynolds number (or even in still air) will sufficiently define the high blowing rate portion of the curve. This approach is extremely time efficient and economical in finding the worst possible C d value for a given trailing edge coolant system.

Decomposition of the spatially filtered and ensemble averaged kinetic energy, the associated fluxes and scaling trends in a rotor wake

Particle image velocimetry data obtained in the rotor wake of a turbomachine are used for examining elements of the ensemble averaged and spatially filtered kinetic energy. These two distinct averaging processes decompose the kinetic energy into four parts, consisting of the mean-flow resolved, mean-flow subgrid, fluctuating resolved, and fluctuating subgrid parts. Their evolution equations include energy flux terms among these parts. The results elucidate the fundamental difference between the filtered turbulence (Reynolds) production and the ensemble averaged subgrid scale (SGS) dissipation rates. Each of these terms consist of three energy fluxes, but only one of them is common to both, the flux from the mean-flow resolved to the fluctuating subgrid kinetic energy parts. The other two elements of the SGS dissipation are the fluxes from the mean-flow resolved to the mean-flow subgrid parts and the fluctuating resolved to the fluctuating subgrid parts. Likewise, the other two contributions to the turbule...

Experimental and Computational Visualization and Frequency Measurements of the Jet Oscillation inside a Fluidic Oscillator

PIV measurements and computational simulations (2D, unsteady Navier-Stokes) are performed to visualize the inherently unsteady jet oscillation inside a fluidic oscillator. Both the measurements and computations are obtained for a jet exit Reynolds number of 321, based on the maximum velocity and the nozzle width at the jet exit plane. The computed jet oscillation frequency is in close agreement with the measured PIV frequency. Formation of a pressure gradient across the jet is observed from the computations. The variation of the jet oscillation frequency with jet exit Reynolds number is also determined by single sensor hot-wire measurements inside the oscillation chamber.