1 PRESSURE SENSITIVE PAINT FOR ANALYSIS OF FILM-COOLING EFFECTS ON A GAS TURBINE BLADE TIP
This paper deals with the implementation of Pressure Sensitive Paints (PSP) in turbomachinery and other aero-thermal research studies. A specific calibration setup developed at the Pennsylvania State University and the applications of PSP are discussed in detail. PSP is an experimental technique for mapping pressure distributions on any aerodynamic body. It requires coating the surface of the body with a special paint that exhibits luminescence when it is exposed to light of appropriate frequency. The intensity of the exposed light is then used as a measure of the pressure at that point. In order to draw up a reference criteria, calibration of the paint was carried out in a specially designed chamber. The paper provides details about the calibration procedure as well as the setup. Although in the recent years PSP has been used for the investigation of high speed flows, its low-speed applications are also being investigated. The current effort aims at using PSP technology in exploring film-cooling effectiveness and the pressure loss characteristics associated with the use of flat as well as squealer tip rotor blades in an axial flow turbine. In conclusion, this paper attempts at establishing PSP as a simple, cost effective and highly efficient means of experimental verification of computational results along with proposing this technique as a substitute for the expensive wind tunnel testing of various aerodynamic bodies. Introduction Any increase in the thermal efficiency of a gas turbine engine is contingent on a corresponding increase in the turbine inlet temperature. However, at the same time an increased turbine inlet temperature leads to enhanced heat transfer and thermal stresses on the blade surface. In order for the blade to withstand such adverse environment and avoid any material failure, it becomes essential to provide an arrangement for cooling the system. Of the various cooling mechanism available, film-cooling has come to be widely accepted as one of the more useful active cooling methods. In case of film-cooling, relatively cooler air is passed through several small discreet holes onto the surface of the blade. This generates a protective film between the hot mainstream flow and the blade surface which protects the surface of the blade by keeping it at a lower temperature. The coolant used in film cooling is actually bled air from the compressor. Hence, although it protects the turbine components and allows for an increased turbine inlet temperature, excessive use of the coolant leads to a decrease in the overall thermal efficiency of the system as we have a higher percentage of compressor air being bled and greater mixing of cooler air into the hot mainstream air taking place. Thus the best configuration would have to be the one yielding the maximum increase in the turbine inlet temperature with the expense of least coolant possible. Many research efforts have been Paper Ref. # 335
Turbine Tip and Shroud Heat Transfer
Unshrouded blades of axial turbine stages move in close proximity to the stationary outer seal, or shroud, of the turbine housing. The pressure difference between the concave and convex sides of the blade drives a leakage flow through the gap between the moving blade tip and adjacent wall. This clearance leakage flow and accompanying heat-transfer are of interest because of long obvious effects on aerodynamic performance and structural durability, but understanding of its nature and influences has been elusive. Previous studies indicate that the leakage through the gap is mainly a pressure-driven flow whose magnitude is related strongly to the airfoil pressure loading distribution and only weakly, if at all, to the relative motion between blade tip and shroud. A simple flow and heat-transfer model incorporating these features can be used to estimate both tip and shroud heat transfer provided that reasonable estimates of the clearance gap size and clearance leakage flow can be made. The present work uses a numerical computation of the leakage flow to link the model to a specific turbine geometry and operating point for which a unique set of measured local tip and shroud heat fluxes are available. The resulting comparisons between the model estimates and measured heat-transfer are good. The model should thus prove useful in the understanding and interpretation of future measurements, and should additionally prove useful for providing early design estimates of the levels of tip and shroud heat transfer that need to be compensated for by active turbine cooling.Copyright © 1990 by ASME
Noctilucent cloud observations over Greenland by a Rayleigh lidar
We report on the detection of noctilucent clouds (NLCs) by a Rayleigh lidar system located on the west coast of Greenland during operations in July and August 1994. The lidar detected NLCs in the zenith on three nights (July 28–29, July 31–August 1, and August 4). The lidar measurements are presented at temporal (1 minute) and spatial (192 meter) resolutions exceeding previously reported lidar measurements of NLCs and reveal significantly more structure in the clouds. Unique cloud features are observed for each night NLCs were detected. Because of the large backscatter ratio associated with NLCs, the volume backscatter coefficient for the cloud particles can be determined more accurately from the lidar signal than the backscatter ratio as the upper mesospheric molecular number density is negligible in the volume backscatter coefficient calculation. The volume backscatter coefficient values determined from the three nights of NLC observations would be classified by current models as moderate to extreme. The extreme backscatter from NLC measurements on August 4 tests the assumptions used in existing models.