Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multi-pass, heat transfer model with both radially inward and outward flow. Trip strips. skewed at 45 degrees to the flow direction, were machined on the leading and trailing surfaces of the radial coolant passages. A n analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages:coolant-to-wall temperature ratio, rotation number. Reynolds number and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from similar stationary and rotating models with smooth walls and with trip strips normal to the flow direction. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation and huoyancy. decreased to as low as 40 percenl of the value wthout rotation. However, the maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels previously obtained with the smooth wall model. It was concluded that (1) both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips. (2) the effects of rotation are markedly different depending upon the flow direction and (3) the heat transfer with skewed trip strips i s less sensitivity to buoyancy than the heat transfer in motleis with either smooth walls or normal trips. Therefore, skewed trip strips rather than normal trip strips are recommended and peometry-specific tests will be required for accurate design information.

The clearance gap between the stationary outer air seal and blade tips of an axial turbine allows a clearance gap leakage flow to be driven through the gap by the pressure-to-suction side pressure difference. The presence of strong secondary flows on the pressure side of the airfoil tends to deliver air from the hottest regions of the mainstream to the clearance gap. The blade tip region, particularly near the trailing edge, is very difficult to cool adequately with blade internal coolant flow. In this case, film cooling injection directly onto the blade tip region can be used in an attempt to directly reduce the heat transfer rates from the hot gases in the clearance gap to the blade tip. The present paper is intended as a memorial tribute to the late Professor Darryl E. Metzger who has made significant contributions in this particular area over the past decade. A summary of this work is made to present the results of his more recent experimental work that has been performed to investigate the effects of film coolant injection on convection heat transfer to the turbine blade tip for a variety of tip shapes and coolant injection configurations. Experiments are conducted with blade tip models that are stationary relative to the simulated outer air seal based on the result of earlier works that found the leakage flow to be mainly a pressure-driven flow which is related strongly to the airfoil pressure loading distribution and only weakly, if at all, to the relative motion between blade tip and shroud. Both heat transfer and film effectiveness are measured locally over the test surface using a transient thermal liquid crystal test technique with a computer vision data acquisition and reduction system for various combinations of clearance heights, clearance flow Reynolds numbers, and film flow rates with different coolant injection configurations. The present results reveal a strong dependency of film cooling performance on the choice of the coolant supply hole shapes and injection locations for a given tip geometry.

Experiments were conducted to determine the effects of model orientation as well as buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. Turbine blades have internal coolant passage surfaces at the leading and trailing edges of the airfoil with surfaces at angles which are as large as +/−50 to 60 degrees to the axis of rotation. Most of the previously–presented, multiple–passage, rotating heat transfer experiments have focused on radial passages aligned with the axis of rotation. The present work compares results from serpentine passages with orientations 0 and 45 degrees to the axis of rotation which simulate the coolant passages for the midchord and trailing edge regions of the rotating airfoil. The experiments were conducted with rotation in both directions to simulate serpentine coolant passages with the rearward flow of coolant or with the forward flow of coolant. The experiments were conducted for passages with smooth surfaces and with 45 degree trips adjacent to airfoil surfaces for the radial portion of the serpentine passages. At a typical flow condition, the heat transfer on the leading surfaces for flow outward in the first passage with smooth walls was twice as much for the model at 45 degrees compared to the model at 0 degrees. However, the differences for the other passages and with trips were less. In addition, the effects of buoyancy and Coriolis forces on heat transfer in the rotating passage were decreased with the model at 45 degrees, compared to the results at 0 degrees. The heat transfer in the turn regions and immediately downstream of the turns in the second passage with flow inward and in the third passage with flow outward was also a function of model orientation with differences as large as 40 to 50 percent occurring between the model orientations with forward flow and rearward flow of coolant.

Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multi-pass, heat transfer model with both radially inward and outward flow. Trip strips. skewed at 45 degrees to the flow direction, were machined on the leading and trailing surfaces of the radial coolant passages. A n analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages:coolant-to-wall temperature ratio, rotation number. Reynolds number and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from similar stationary and rotating models with smooth walls and with trip strips normal to the flow direction. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation and huoyancy. decreased to as low as 40 percenl of the value wthout rotation. However, the maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels previously obtained with the smooth wall model. It was concluded that (1) both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips. (2) the effects of rotation are markedly different depending upon the flow direction and (3) the heat transfer with skewed trip strips i s less sensitivity to buoyancy than the heat transfer in motleis with either smooth walls or normal trips. Therefore, skewed trip strips rather than normal trip strips are recommended and peometry-specific tests will be required for accurate design information.

Velocity data were obtained by laser-Doppler velocimetry in rotating coolant passages with a square cross section having skewed trips. Measurements were obtained at Reynolds and Rotation numbers of 25,000 and 0.24 to quantify the influence of Coriolis effects and to explain the heat transfer phenomena observed by Johnson, et al. (1994). With rotation, outward flow and trips skewed at −45° generates counter-clockwise swirl on the high pressure side and a corner recirculation zone at the inner corner of the low pressure side. Inward flow and trips skewed at +45° generates clockwise swirl on the high pressure side and a corner recirculation zone at the upper corner of the low pressure side. The implications on heat transfer were estimated from the velocity data by the Nu-Re0.8 correlation. Johnson, et al. (1994) show that, in a passage with skewed trips, rotation increases the heat transfer from the high pressure surface of the first passage by up to 300% (relative to smooth wall stationary reference). The estimated contributions from increases in streamwise velocity, swirl and re-attachment of cross-flow in the inter-ribbed region were, respectively, 48%, <100% and >125%.