17. Visualization of Convective Heat Transfer

This chapter introduces optical techniques for the measurement of temperature and species concentration in heat and mass transfer processes occurring in fluids. Measurement is initiated by a light source and changes in the attributes of light emerging from the test cell are recorded by a suitable detector. The fluid medium is taken to be transparent to the passage of light. The light intensity distribution and contrast generated during the measurement can arise either due to variations in the refractive index of the fluid or by scattering from preselected optically active particles. Among the refractive index methods, interferometry, schlieren (monochrome and color), and shadowgraph are discussed. Liquid crystal thermography is presented as an example of a scattering technique. Infrared measurements are also included for completeness. Specific advantages of optical methods are that they are nonintrusive, image a flow cross section, and are inertia-free. A continuous sequence of optical images can be recorded in a computer using CCD cameras, thus extending measurements to the time domain. Optical images can be analyzed and methods of extracting quantitative data from such images are discussed. Several applications and representative images recorded on a laboratory scale are also presented.