In industry, fired heaters are used to heat process fluids and to generate steam. The efficiency of these devices is increased with air preheating. Gnerally, the flow through the air preheaters is driven by a centrifugal fan, via forced draft. Through a widening transition duct (large angle diffuser), air flows from the fan towards the air preheater. In terms of fluid mechanics, the geometry of such a duct is often unfavorable. This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied.