Heat transfer measurements in water using a schlieren technique

A quantitative schlieren technique has been applied to the study of two-dimensional free convection heat transfer in asymmetrically heated vertical channels, using water as working fluid. As it is well known, the schlieren technique utilizes the refraction of light rays to display a pattern related to the temperature field. The schlieren system employed here makes use of a focal filament filter to identify regions of fluid that deflect light rays by the same amount. The schlieren images recorded enable local heat transfer coefficients to be reconstructed. This optical technique has been extensively used for the measurement of heat transfer coefficients in air, whereas its use in water was limited in the past, and restricted to qualitative observation like flow visualizations. The present paper is seemingly the first to present measurements of the local heat transfer coefficient along the heated wall of a vertical channel using water as working fluid. Experimental results are presented for two values of the channel aspect ratio (0.1 and 0.2) and Rayleigh number values, based on the channel height, of the order of 10^7 (laminar flow regime). Comparisons with numerical and theoretical data (for the isolated isothermal plate limit) are provided.