No abstract

Experiments were performed to measure the heat transfer augmentation and film cooling effectiveness on a film-cooled annular surface subjected to unsteady passing wakes. The wakes can have a profound influence on the effectiveness of film cooling and heat transfer characteristics and it is the objective of an ongoing study to quantify that influence. As part of the study, three blowing ratios (M=0.25, 0.5, 0.75) were tested with discrete film injection (p/D=3) in this paper. The tests were performed for two wake Strouhal numbers (S=0.15, 0.3). The baseline cases involved a steady mainstream flow (S=0). Heat transfer augmentation was measured with passing wakes and with film cooling separately and then with combination of both. A numerical model replicating the annular geometry was used to predict film cooling effectiveness for the steady mainstream cases (S=0) and one transient case was attempted (M=0.5,S=0.3). The computations were performed with pressure-based Reynolds-Averaged Navier-Stokes solver and the realizable k-ε turbulence model. The results from the experiment and computations are compared with relevant published literature. The uncertainties in the experimental values are calculated to be ± 0.03 (absolute) for film cooling effectiveness, ± 3% for velocity measurements, and ± 6.5% for heat transfer coefficient ratio respectively. The passing wakes increased the heat transfer coefficients as high as 11% for the highest wake passing frequency for no film injection (M=0, S=0.3). The influence of the passing wakes was more significant with film injection with a heat transfer augmentation of 37% approximately for M=0.75,S=0.3. The displacement thickness to the film hole diameter ratio at the injection location was observed to be a pertinent parameter that dictates the heat transfer augmentation for the film injection experiments. The centerline film cooling effectiveness was greatly affected by the passing wakes with a maximum decrease of 15% observed for M=0.5,S=0.3. NomenclatureA = area (m 2 ) D = film cooling hole diameter (8mm) D = wake rod diameter (19mm) h = heat transfer coefficient (W/m 2 K) I = momentum flux ratio k = thermal conductivity (W/mK) M = blowing ratio (mass flux ratio) N = angular velocity of wake generator (RPM) n = number of wake generator rods p = pitch (mm) q′′ = heat flux (W/m 2 ) R = resistance of foil heater (ohm) Re = Reynolds number based on entry length; film hole diameter S = wake Strouhal number T = temperature (°C/K) Tu = turbulence intensity U = mean velocity (m/s) V = voltage (V) VR = velocity ratio w = width of heater strip (31 mm)Greek letters α = streamwise inclination angle of film hole (35°) β = compound angle of film hole (0°) δ = disturbance/boundary layer thickness (mm) ε = emissivity η = film cooling effectiveness θ = pitchwise distance θ * = momentum thickness (mm) ρ = density (kg/m 3 ); resistivity of heater material ߢ = ratio of specific heats (c p /c v )

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. Nomenclature c = speed of sound D = depth of the cavity f = resonance frequency L = cavity length m = mode of the oscillations M = Mach number U =main stream velocity

Stator-rotor systems are commonly used in many different types of turbomachinery applications to supply an air for secondary air flows. Commercial CFD codes with variety of turbulence models are widely used in order to estimate the amount of flow supplied by the preswirl stator-rotor system. CFD investigations can provide detailed information about the local flow field which is extremely difficult to obtain from rotating rig due to the measurement limitations in rotating frame, however the accuracy of CFD needs to be investigated by conducting experiments. In this study the purpose is to evaluate how accurate CFD simulations with different turbulence models can predict the flow rate supplied by the system. An experimental rig composed of a stationary preswirler, a rotating disk with an internal flow path and a stator-rotor cavity with a rim seal was used in this study. Air is supplied to the stator from the ambient due to the suction provided by the rotor which can rotate at up to 3100 rpm. Incoming air first flows through annular preswirl guide vanes located inside the stator then discharges into the stator-rotor cavity located downstream of the preswirl guide vanes. Some fraction of the flow induced into the rotor by the help of inlet guides which are attached to the rotor face and angled to match the flow angle in rotating frame. Remaining part of the flow passes through rim seal and discharges out to the ambient. Two experimental cases, one with preswirl guide vanes without endwall contouring and the other with endwall contouring were been investigated at 3100 rpm. Mass flow rate at the inlet was 14.6% higher for the case with endwall contoured configuration compared to the case without endwall contouring. For both of the cases approximately 90% of the inlet flow was purged through rim seal while remaining 10% flows through the radial rotor disk passages. CFD analysis of the rotating rig were conducted using commercial code STAR CCM+. Turbulence models of k-ε, k-ω, Reynolds stress (RST) and Spalart-Allmaras were used and the mass flow rate drawn into the system was compared with experiments. The mass flow rate into the rig from experimental measurements was 7.4% higher compared to the best CFD prediction given by RST Linear. Among all turbulence models k-w was the worst performer by predicting mass flow 13% lower compared to the experimental value. Different sub-options of these turbulence models were also investigated. This study provided significant information for preswirl stator-rotor system designers in terms of the amount of flow rate that can be obtained and how well can it be predicted by CFD.