J. von Wolfersdorf

Film cooling on a convex surface with zero pressure gradient flow

Abstract The adiabatic film cooling effectiveness and heat transfer increase due to film injection was investigated on a convex surface at zero pressure gradient flow. Film cooling results were obtained using thermochromic liquid crystals to measure the local wall temperature distribution. Five different injection configurations, three with cylindrical and two with shaped holes were examined. Coolant injection with cylindrical holes indicates decreased film cooling effectiveness and increased heat transfer at moderate and high blowing rates. Significant higher film cooling effectiveness values and lower heat transfer increase due to film injection were achieved by shaped hole injection compared to cylindrical hole injection.

The transient liquid crystal technique: Influence of surface curvature and finite wall thickness

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.

An experimental investigation of film cooling on a convex surface subjected to favourable pressure gradient flow

Abstract The film cooling performance on a convex curved surface subjected to accelerated free-stream flow was investigated. The thermochromic liquid crystals technique was applied to determine adiabatic film cooling effectiveness values and heat transfer coefficients on the test surface. Five different injection geometries, three with cylindrical holes and two with shaped holes were investigated. The experimental results were compared to the results of a previous investigation obtained with constant free-stream velocity to determine the influence of free-stream acceleration. The free-stream acceleration caused decreasing adiabatic film cooling effectiveness values compared to constant free-stream velocity, especially further downstream of the point of injection. For the in-line cylindrical injection configurations, an increase in adiabatic film cooling effectiveness in the near hole region can be obtained for higher blowing rates. It is assumed that this effect is due to the reduced tendency of jet separation in the accelerated free-stream. The influence of the free-stream acceleration on the Stanton number ratio is generally small.

The Transient Liquid Crystal Technique: Influence of Surface Curvature and Finite Wall Thickness

Keywords: Heat Transfer ; Measurement Techniques ; Turbomachinery ; GTT ; LTT Reference LTT-CONF-2004-003 Record created on 2007-04-18, modified on 2016-08-08

A novel transient heater-foil technique for liquid crystal experiments on film cooled surfaces

Keywords: Heat Transfer ; Measurement Techniques ; Turbomachinery ; GTT ; LTT Reference LTT-ARTICLE-2003-002doi:10.1115/1.1578501View record in Web of Science Record created on 2007-04-18, modified on 2017-05-10

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

The paper presents an application of the transient heater foil measurement technique using thermochromic liquid crystals (TLC) to endwall heat transfer and film cooling investigations in a transonic turbine vane cascade. The film cooling configuration consists of an upstream slot, representing the leakage flow area between the interface of the combustor and the turbine, and several rows of cylindrical and fan-shaped holes within the passage. With the transient method chosen, the heat transfer and adiabatic film cooling effectiveness distributions can be obtained simultaneously together with the local heat release within the heater foil. Therefore, the heat release in the foil is not required to be uniform, and the foil can contain discrete holes in the film cooling configuration. Some new developments are presented, which are directed towards improved application of the transient heater foil method for such a complex configuration. This includes tailoring the foil heat-release distribution towards the expected heat transfer patterns, supported by numerical Finite-Element computations and the use of a double-TLC mixture for improved time-wise TLC indications. Additionally, CFDsimulations were used to evaluate the recovery temperature distribution through the vane cascade without film cooling. The experiments were performed in the linear cascade facility at the EPFL-Lausanne. A compressor provides a continuous air flow at near-ambient temperature regulated with heat exchangers. Carbon dioxide is used as coolant in order to achieve engine-representative density ratio between coolant and main flow. Multiple experiments with the same main and coolant flow settings but varying heat flux levels and coolant injection temperatures have been performed and simultaneously analysed using nonlinear regression analysis. The time required between successive experiments to return to homogenous initial conditions, as required by the transient method, has been analysed using an analytical solution for heat-on-heat-off conditions. This permits the model assumption of onedimensional conduction within a semi-infinite wall with a heat releasing layer on the top. Example results for cases without cooling, with film cooling from rows of discrete holes and the addition of slot film cooling are used to illustrate the benefit of the new approaches for the investigated vane cascade.

Temperature And Pressure Sensitive Coatings

As part of the ongoing development of flapping-wing micro air vehicle prototypes at Cranfield University (Defence Academy Shrivenham), a model of insect-like wing aerodynamics in hover has been developed and implemented as MATLAB code. The model is intended to give better insight into the various aerodynamic effects on the wing, and therefore is as close to being purely analytical as possible. The model is modular, with the various effects treated separately. This modularity aids analysis and insight, and allows refinement of individual parts. However, it comes at the expense of considerable simplification, which requires empirical verification. The model starts from quasi-steady inviscid flow around a thin 2D rigid flat wing section, accounting for viscosity with the Kutta–Joukowski condition, and the leading edge suction analogy of Polhamus. Wake effects are modelled using the models of Kussner and Wagner, on a prescribed wake shape, as initially used by Loewy. The model has been validated against experimental data from Dickinson’s Robofly and found to give acceptable accuracy. Some empirically inspired refinements of the Polhamus effect are outlined, but these need further empirical validation.Separation control is also an important issue in biology. During the landing approach of birds and in flight through very turbulent air, one observes that the covering feathers on the upper side of bird wings tend to pop up. The raised feathers impede the spreading of the flow separation from the trailing edge to the leading edge of the wing. This mechanism of separation control by bird feathers is described in detail. Self-activated movable flaps (= artificial bird feathers) represent a high-lift system enhancing the maximum lift of airfoils up to 20%. This is achieved without perceivable deleterious effects under cruise conditions. Several data of wind tunnel experiments as well as flight experiments with an aircraft with laminar wing and movable flaps are shown. 1 Movable flaps on wings: artificial bird feathers The issue of artificial feathers on wings, has an almost anecdotal origin. Wolfgang Liebe, the inventor of the boundary layer fence once observed mountain crows in the Alps in the 1930s. He noticed that the covering feathers on the upper side of the wings tend to pop up when the birds were on landing approach or in other situations with high angle of attack, like flight through gusts. Once the attention of the observer is drawn to it, it is comparatively easy to observe this behaviour in almost any bird (see, for example, the feathers on the left-hand wing of a Skua in Fig. 1). Liebe interpreted this behaviour as a biological high-lift device [2]. Later, in 1938, Liebe worked as a young scientist at the former German Aeronautical Establishment (DVL—Deutsche Versuchsanstalt für Luftfahrt, the predecessor of the present DLR). Liebe attached a piece of leather on the upper side of one wing of a fighter aircraft (see Fig. 2), a Messerschmitt BF 109. Take-off and flight of this specially outfitted aircraft were satisfactory, but landing turned out to be tricky. At high angles of attack, the lift distribution on the wings was asymmetrical. Therefore, the pilot had to land the aircraft at a low angle of attack and at a very high speed. Much later, Liebe presented his ideas in a journal article [2]. Liebe’s original idea was that once separation starts to develop on a wing, reversed flow was bound to occur in the separation regime. Under these locally reversed flow conditions, light feathers would pop up. They would www.witpress.com, ISSN 1755-8336 (on-line)

The Effects of Vortex Generator Arrays on Heat Transfer and Flow Field

The effect of arrays of single-body, delta shaped vortex generators on heat transfer and flow field has been investigated numerically using RANS (Reynolds averaged Navier-Stokes) methods. The Reynolds number based on the hydraulic diameter of the channel in which the vortex generators are positioned is fixed at 300,000. For the closure of the equation system of the flow field a full differential Reynolds stress model has been used to capture the anisotropic effects of the induced vortex structures. To gain realistic results for the heat transfer the common approach for the closure of the Reynolds-averaged energy equation using a turbulent Prandtl number has been abandoned for explicit algebraic models which deliver more realistic results for complex flows. Simultaneously to the calculations measurements have been performed on some of the geometries to validate the numerical results.