Background Oriented Schlieren characterization of the thermal boundary layer over a vertical heated plate in free convection

Background Oriented Schlieren (BOS) is an optical technique that allows an indirect measurement of the temperature gradient in a flow field characterized by a not uniform distribution of the refractive index (Schlieren object). The operating principle is based on the dependence of this optical property on temperature. In particular, the refractive index gradient can be measured by comparing the images of a background pattern taken with and without the Schlieren object. This method is particularly attractive, due to its simpler set-up and easier data reduction compared to other techniques based on the same principle. As a first step toward the application of BOS to combustion problems and with the aim to verify its accuracy, a simple case has been investigated. The two dimensional thermal field over a vertical heated plate subjected to free convection has been measured and compared to the numerical solution obtained by Ostrach. A speckle pattern has been artificially generated and used as a background image. Temperature gradient has been measured by evaluating the speckle displacement occurring in the distorted background image. To this aim a cross-correlation algorithm used in PIV measurements has been applied. The temperature distribution obtained in the thermal boundary layer is in very good agreement with the Ostrach solution, while the deviation in the convective heat transfer coefficient lies within about 10%.