Heat Transfer and Film Effectiveness Study on a Pitchwise Curved Surface with Unsteady Wake Interaction

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 )