Abstrakt

A new method of infrared (IR) camera use in convective heat transfer studies has been presented. Until now, IR cameras were utilized for temperature field visualizations of flat heated surfaces of solid objects only. Because liquids and gasses do not emit radiation in the infrared range of wave lengths, detection of temperature field in the air was conducted with the use of plastic mesh, which, after heating by hot air, emits infrared radiation visible by IR camera. As the mesh is made from a low conductivity material, it therefore maintains the sharp contours of isotherms without blurring. The radiation from lateral surface of the grid (perpendicular positioned to the heated surface and parallel to gravity) can be recorded by IR camera set perpendicularly to the mesh. From the obtained temperature field and the temperature of heated surface, also measured by the IR camera, temperature gradients on the wall ∂T/∂y|y=0 and their distribution on surface can be found. From these measured gradients and its distribution, the local heat transfer coefficients αx and local Nusselt numbers Nux on the heated surface can be calculated. By integrating the local values of Nux the average value of the Nusselt number Nu, representative for the entire surface, can be obtained. Verification of accuracy of the presented new method has been conducted through three dimensional temperature field visualization studies of free convective heat transfer from an isothermal heat plate. This configuration was deliberately chosen because the vertical plate was the earliest and most thoroughly studied case and its rich scientific documentation is best known and therefore the most reliable. The results, obtained by the use of IR camera, presented in the form of Nusselt–Rayleigh relation Nug = 0.571 Ra^0.25 have a good correlation with the literature data, which testifies the positive verification and utility presented in this new method of convection heat transfer investigation.

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