Heat transfer inside and downstream of cavities using transient liquid crystal method

Abstract

Local heat transfer coefficient distributions are investigated inside and downstream of various cavities on a flat surface. The effect of cavity size, depth, and shape are studied using a transient liquid crystal image method. Liquid crystals are sprayed on the test surface and a hot mainstream is imposed, suddenly causing a color change. The time of color change is obtained using an image processing system. An increase in cavity size for the same depth increases heat transfer coefficients on the test surface. An increase hi cavity depth increases downstream heat transfer coefficients. Five cavity shapes are studied to compare local heat transfer behavior for the effect of various shapes. Nomenclature cp = specific heat of test surface material D = hydraulic diameter of cavity d = cavity depth h = local convection heat transfer coefficient with cavity h0 = local convection heat transfer coefficient without cavity k = thermal conductivity of test surface material Rex = Reynolds number based on axial distance, Ree = momentum thickness Reynolds number, T = temperature Tu = freestream turbulence intensity t = time of liquid crystal color change U = mainstream velocity x = axial distance along the mainstream z = spanwise distance a = thermal diffusivity of test surface material A = temperature step 8 = boundary-layer thickness 8* = displacement thickness 6 = momentum thickness of the boundary layer IJL = fluid dynamic viscosity p = fluid density r = time step