The transient liquid crystal technique is nowadays widely used for measuring the heat transfer characteristics in gas turbine applications. Usually, the assumption is made that the wall of the test model can be treated as a flat and semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thickness of the wall and to the radius of curvature is small. However, those two assumptions are not always respected for measurements near the leading edge of a blade. This paper presents a rigorous treatment of the curvature and finite wall thickness effects. The unsteady heat transfer for a hollow cylinder has been investigated analytically and a data reduction method taking into account curvature and finite wall thickness effects has been developed. Experimental tests made on hollow cylinder models have been evaluated using the new reduction method as well as the traditional semi-infinite flat plate approach and a third method that approximately accounts for curvature effects. It has been found that curvature and finite thickness of the wall have in some cases a significant influence on the obtained heat transfer coefficient. The parameters influencing the accuracy of the semi-infinite flat plate model and the approximate curvature correction are determined and the domains of validity are represented. INTRODUCTIONThe demand for gas turbines with improved efficiency and higher power output has led to a continuous increase of the turbine inlet temperature. Consequently, turbine components such as blades and platforms are exposed to ever-higher thermal loads. To guarantee safe operating conditions and acceptable blade life the designers have to optimize the cooling systems and to predict with sufficient precision the blade temperatures. To fulfill those tasks, detailed and accurate knowledge of the heat transfer characteristics of the blades are required.Transient techniques have been successfully and widely used to measure the heat transfer coefficients on both internal and external surfaces of turbine blades (e.g. Clifford et al. (1983), Guo et al. (1995) and Drost and Bölcs (1998)). Ireland and Jones (1985) used for the first time liquid crystals together with the transient technique. They measured the heat transfer coefficient in blade cooling passages with high spatial resolution, and showed the importance of the method for gas turbine applications. The transient experiment is usually generated by a step change in the gas temperature. The evolution of the surface temperature during the transient test is measured and compared with the predictions of a one-dimensional heat conduction model for a prescribed heat transfer coefficient. This heat conduction model generally considers the wall of the test object as a flat, semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thickness of the wall and to the radius of curvature is small. Schulz and Jones (1973) evaluated that the semi-infinite model holds if the d...

This study examines experimentally the cooling performance of narrow impingement channels as could be cast-in in modern turbine airfoils. Full surface heat transfer coefficients are evaluated for the target plate and the sidewalls of the channels using the transient liquid crystal technique. Several narrow impingement channel geometries, consisting of a single row of five cooling holes, have been investigated composing a test matrix of nine different models. The experimental data are analyzed by means of various post-processing procedures aiming to clarify and quantify the effect of cooling hole offset position from the channel centerline on the local and average heat transfer coefficients and over a range of Reynolds numbers (11,100–86,000). The results indicated a noticeable effect of the jet pattern on the distribution of convection coefficients as well as similarities with conventional multi-jet impingement cooling systems.

The transient liquid crystal technique is nowadays widely used for measuring the heat transfer characteristics in gas turbine applications. Usually, the assumption is made that the wall of the test model can be treated as a flat and semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thickness of the wall and to the radius of curvature is small. However, those two assumptions are not always respected for measurements near the leading edge of a blade. This paper presents a rigorous treatment of the curvature and finite wall thickness effects. The unsteady heat transfer for a hollow cylinder has been investigated analytically and a data reduction method taking into account curvature and finite wall thickness effects has been developed. Experimental tests made on hollow cylinder models have been evaluated using the new reduction method as well as the traditional semi-infinite flat plate approach and a third method that approximately accounts for curvature effects. It has been found that curvature and finite thickness of the wall have in some cases a significant influence on the obtained heat transfer coefficient. The parameters influencing the accuracy of the semi-infinite flat plate model and the approximate curvature correction are determined and the domains of validity are represented. INTRODUCTIONThe demand for gas turbines with improved efficiency and higher power output has led to a continuous increase of the turbine inlet temperature. Consequently, turbine components such as blades and platforms are exposed to ever-higher thermal loads. To guarantee safe operating conditions and acceptable blade life the designers have to optimize the cooling systems and to predict with sufficient precision the blade temperatures. To fulfill those tasks, detailed and accurate knowledge of the heat transfer characteristics of the blades are required.Transient techniques have been successfully and widely used to measure the heat transfer coefficients on both internal and external surfaces of turbine blades (e.g. Clifford et al. (1983), Guo et al. (1995) and Drost and Bölcs (1998)). Ireland and Jones (1985) used for the first time liquid crystals together with the transient technique. They measured the heat transfer coefficient in blade cooling passages with high spatial resolution, and showed the importance of the method for gas turbine applications. The transient experiment is usually generated by a step change in the gas temperature. The evolution of the surface temperature during the transient test is measured and compared with the predictions of a one-dimensional heat conduction model for a prescribed heat transfer coefficient. This heat conduction model generally considers the wall of the test object as a flat, semi-infinite solid. This assumption is correct as long as the penetration depth of the heat compared to the thickness of the wall and to the radius of curvature is small. Schulz and Jones (1973) evaluated that the semi-infinite model holds if the d...

This study examines experimentally the cooling performance of integrally cast impingement cooling channels which provide increased heat transfer area compared to traditional impingement configurations. For the evaluation of the heat transfer coefficient, the transient liquid crystal method was used. Full surface heat transfer coefficient distributions on the target plate and the side walls of the channel have been measured by recording the temperature history of liquid crystals using a frame grabber. Several impingement cooling geometries have been tested composing a test matrix of nine different geometrical configurations. The experimental data are analyzed by means of various post-processing procedures and aim to clarify and quantify the effect of hole staggering on the overall cooling performance, a variable which has been little addressed in the open literature. The experiments were carried out in a low speed wind tunnel over a wide range of Reynolds numbers between 15,000 and 100,000. The results indicated similarities with convectional multi-jet impingement cooling systems as well as a noticeable effect of the cooling hole pattern. Finally, an error propagation analysis of the experimental uncertainties was performed providing information for the significance of scatter on repeated experiments.

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