Abstract: An enclosure containing rotating blades for mechanical stirring of gaseous fluids may be a heat exchanger, flue gas energy recovery device, gas generator or combustion device. A high rotational stirring velocity increases the convective heat transfer rate, rendering it independent of the lower throughput velocity of primary fluid which can be controlled independently of the stirring speed. Primary fluid is isolated from a secondary fluid by a thermal transfer surface which is continuously swept by the swirling primary fluid and which may be the peripheral wall of the enclosure or a flow tube located proximate the peripheral wall. In one embodiment, swirling is achieved by radially-extending blade assemblies secured to a rotatable shaft at longitudinally spaced locations to establish discrete swirling zones in which fluid recirculates from the shaft outwardly, along the wall, and back toward the shaft.