A numerical and experimental investigation is undertaken for developing laminar flow in a duct with one opaque, uniformly heated wall and one transparent wall. In the numerical model, mixed convection, radiative exchange, as well as two-dimensional conduction in the substrate are considered. Experiments are conducted in a high-aspect-ratio rectangular channel using infrared thermography to validate the numerical model and visualize the temperature field on a heated surface. An extended parametric study using the validated model is also carried out to assess the impact of channel height, and thermal conductivity and thickness of the substrate. For a channel height of $H=6 mm$ and a heating power of $qs=257 W/m2$, as Re increases from 150 to 940 the fraction of heat transfer by convection from the heated surface rises from 65% to 79%. At $Re=150$, as $H$ increases from 6 mm to 25 mm, radiation from the heated surface increases from 35% to 70% of the total heating power. The influence of substrate conductivity and thickness on local flux distributions is limited to regions near the channel inlet and outlet. Over the entire parametric space considered, radiation loss from the interior duct surfaces to the inlet and outlet apertures is less than 2% of the total heat input and thus unimportant.

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