Errors associated with interferometric measurement of convective heat transfer coefficients

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Abstract
Errors caused by refraction and end effects in interferometric systems are studied analytically. Both errors are estimated for Mach-Zehnder and differential interferometers when these instruments were used to measure convective heat transfer coefficients in air and water. Both errors were found to be larger for the Mach-Zehnder interferometer than for the differential interferometer. This result suggests that the differential interferometer is potentially a more suitable and accurate device than the Mach-Zehnder interferometer, if one wishes to measure heat transfer coefficients. Convective heat transfer coefficients measured with a differential interferometer are presented. The experimental study used a specially designed test section, which was inserted into the interferometer. The test involved heat dissipated from a vertical plate by free convection to water. The measured results were found to compare satisfactorily with published results at Rayleigh numbers less than 104. At higher Rayleigh numbers, refraction caused sufficient bending of the light rays that a zone of no information next to the heated surface resulted in significant errors.