Vertical temperature distributions have been measured in slowly moving, horizontal liquid layers that were irradiated from above. Radiation was concentrated in the spectral range from 350 to 5000 nm, and the low wavelength (λ < 1300 nm) values of the liquid and substrate radiative properties were varied. Due to the effects of infrared radiation absorption, thermal conditions within the liquid did not exhibit a strong dependence on radiative properties. Conditions were characterized by a stably stratified upper layer, a core region of nearly uniform temperature, and a thin bottom layer of large, unstable temperature gradient. Satisfactory agreement was obtained between the measurements and predictions based on three-dimensional, laminar flow and one-dimensional, radiative transfer models. A weak secondary flow, which involves longitudinal vortices and/or ascending-descending thermals, was predicted by the model and was responsible for maintaining the nearly isothermal core conditions.

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