Convection heat transfer at the curved bottom surface of a square to rectangular transition duct using a new hue capturing based liquid crystal technique

Accurate determination of convective heat transfer coefficients on complex surfaces with high resolution is essential in the aero-thermal design and analysis of air breathing propulsion system components. Complicated three dimensional flows may exist in internal/external flow configurations having laminar, transitional and turbulent flow regimes simultaneously. This study focuses on the implementation of a recently developed hue capturing technique for the quantitative interpretation of liquid crystal images obtained from the bottom surface of a square to rectangular transition duct. The interpretation includes the use of a linear hue versus temperature relation as an accurate temperature measuring tool, a color image analysis system recently introduced for liquid crystal studies and a transient heat transfer model for the conversion of time accurate temperature information into heat transfer coefficient maps. The heat transfer experiments with ambient temperature air in a transition duct are reported in which the model is preheated using a custom designed electric heater. The video record in natural color is then processed frame by frame by using the new hue capturing method. The curved bottom surface of the model is coated with three separate liquid crystals responding at different temperature bands. The three crystals are mixed and sprayed simultaneously to form a thin temperature sensitive layer on the surface. A novel calibration procedure for the liquid crystal mixture is discussed in detail. The image analysis system uses a 24 bit color recognition technique employing a hue-saturation-intensity approach which is a new alternative to conventional systems using red-green-blue color definition. Two dimensional surface distribution of heat transfer coefficients are presented with a high spatial resolution. However, the current results immediately show that the surface heat transfer distributions on the bottom wall are controlled by highly three dimensional organized vortical structures in the duct. There are significant variations of heat transfer coefficients along the main flow direction and the transversal direction caused by these structures. The new sets of data are very valuable in understanding complex transition duct flows. The study also presents unique sets of well documented heat transfer data for further computational heat transfer validation studies which are under consideration.