This study focuses on a new image processing based color capturing technique for the quantitative interpretation of liquid crystal images used in convective heat transfer studies. The present method is highly applicable to the surfaces exposed to convective heating in gas turbine engines. The study shows that, in single-crystal mode, many of the colors appearing on the heat transfer surface correlate strongly with the local temperature. A very accurate quantitative approach using an experimentally determined linear hue versus temperature relation is possible. The new hue-capturing process is discussed in detail, in terms of the strength of the light source illuminating the heat transfer surface, effect of the orientation of the illuminating source with respect to the surface, crystal layer uniformity, and the repeatability of the process. The method uses a 24-bit color image processing system operating in hue-saturation-intensity domain, which is an alternative to conventional systems using red-green-blue color definition. The present method is more advantageous than the multiple filter method because of its ability to generate many isotherms simultaneously from a single-crystal image at a high resolution, in a very time-efficient manner. The current approach is valuable in terms of its direct application to both steady-state and transient heat transfer techniques currently used for the hot section heat transfer research in air-breathing propulsion systems.

SUMMARYCommercially available elements of a composite consisting of a plastic sheet coated with liquid crystal, another sheet with a thin layer of a conducting material (gold or carbon), and copper bus bar strips were evaluated and found to provide a simple, convenient, accurate, and low-cost measuring device for use in heat transfer research. The particular feature of the composite is its ability to obtain local heat transfer coefficients and isotherm patterns that provide visual evaluation of the thermal performances of turbine blade cooling configurations. Examples of the use of the composite are presentee.

Local heat-transfer coefficients were experimentally mapped along the midchord of a five-times-size turbine blade airfoil in a static cascade operated at room temperature over a range of Reynolds numbers. The test surface consisted of a composite of commercially available materials: a mylar sheet with a layer of cholesteric liquid crystals, which change color with temperature, and a heater sheet made of a carbon-impregnated paper, which produces uniform heat flux. After the initial selection and calibration of the composite sheet, accurate, quantitative, and continuous heat-transfer coefficients were mapped over the airfoil surface. The local heat-transfer coefficients are presented for Reynolds numbers from 2.8×105 to 7.6×105. Comparisons are made with analytical values of heat-transfer coefficients obtained from the STAN5 boundary layer code. Also, a leading-edge separation bubble was revealed by thermal and flow visualization.

This study focuses on a new image processing based color capturing technique for the quantitative interpretation of liquid crystal images used in convective heat transfer studies. The present method is highly applicable to the surfaces exposed to convective heating in gas turbine engines. The study shows that, in single crystal mode, many of the colors appearing on the heat transfer surface strongly correlate with the local temperature. A very accurate quantitative approach using an experimentally determined linear hue versus temperature relation is possible. The new hue capturing process is discussed in detail, in terms of the strength of the light source illuminating the heat transfer surface, effect of the orientation of the illuminating source with respect to the surface, crystal layer uniformity and the repeatability of the process. The method uses a 24 bit color image processing system operating in hue-saturation-intensity domain which is an alternative to conventional systems using red-green-blue color definition. The present method is more advantageous than the multiple filter method because of its ability to generate many isotherms simultaneously from a single crystal image at a high resolution, in a very time efficient manner. The current approach is valuable in terms of its direct application to both steady state and transient heat transfer techniques currently used for the hot section heat transfer research in air breathing propulsion systems.