Direct Measurement of Heat Transfer Coefficient Augmentation at Multiple Density Ratios

Abstract

Knowing the heat transfer coefficient augmentation is imperative to predicting film cooling performance on turbine components. In the past, heat transfer coefficient augmentation was generally measured at unit density ratio to keep measurements simple and uncertainty low. Some researchers have measured heat transfer coefficient augmentation while taking density ratio effects into account, but none have made direct temperature measurements of the wall and adiabatic wall to calculate hf/h0 at higher density ratios. This work presents results from measuring the heat transfer coefficient augmentation downstream of shaped holes with a 7 deg forward and lateral expansion at DR = 1.0, 1.2, and 1.5 on a flat plate using a constant heat flux surface. The results showed that the heat transfer coefficient augmentation was low while the jets were attached to the surface and increased when the jets started to separate. At DR = 1.0, hf/h0 was higher for a given blowing ratio than at DR = 1.2 and DR = 1.5. However, when velocity ratios are matched, better correspondence was found at the different density ratios. Surface contours of hf/h0 showed that the heat transfer was initially increased along the centerline of the jet, but was reduced along the centerline at distances farther downstream. The decrease along the centerline may be due to counter-rotating vortices sweeping warm air next to the heat flux plate toward the center of the jet, where they sweep upward and thicken the thermal boundary layer. This warming of the core of the coolant jet over the heated surface was confirmed with thermal field measurements.

Influence of inhomogeneous porosity on effusion cooling

“…The adiabatic cooling effect was applied to evaluate the cooling effect of the film on the porous surface [33]. The solid zone in the numerical model was deleted to erase the heat conduction and heat convection in the porous sample.…”

“…The porous sample was cooled only by the film on the porous surface. The adiabatic cooling efficiency was defined as [33]:…”

“…However, discrete film holes can't provide a full-coverage film on the blade surface which leads to an inhomogeneous cooling effect. The film cooling efficiency will even decrease when injecting excessive amount of coolant due to the detachment of film [2]. Effusion cooling has many rows of holes fully covered on the surface compared with film cooling.…”

Length to diameter ratio effect on heat transfer performance of simple and compound angle holes in thin-wall airfoil cooling

“…Heat transfer coefficient augmentation using compound angle injection hole for film cooling has been measured by [12][13][14][15][16][17][18][19][20]. Sen et al [12] examined the effect of blowing ratio, hole spacing, and hole shape on the heat transfer coefficient with compound angle injection film hole.…”

“…They found that the laterally averaged heat transfer coefficient of compound angle hole is close to that of simple angle hole at low blowing ratios, while the heat transfer coefficient drastically increases 10D downstream of injection hole at high blowing ratios due to the asymmetric vortex motion under the jets. Recently, Boyd et al [19] and Schreivogel et al [20] used new methods to measure two dimensional heat transfer coefficient distribution on shaped hole and trenched hole, respectively. Some recent works include hydrodynamic measurements [21] and combined investigation of film effectiveness and heat transfer coefficient [22].…”