sink and the housing because of the configuration of the present device the gases at the exit are travelling slightly slower than at the ring as they pass through aperture 74 the exit velocity through the thermal transfer gap 90 can be reduced is further by increasing the area of the thermal transfer gap 90 while keeping the distance between the support surface 30 and the lower surface of the heat sink 12 constant in understanding the processes occurring within the combustion chamber this might more easily be explained and understood as a fluid dynamics process the cooling air element illustrated by the flow pattern 102 desirably travels upwardly at approximately the same speed as the combination of flame and combustion gases the exit speed of the combined gases through the thermal transfer gap 90 is reduced slightly to allow the heat to remain as long as possible adjacent the base of the heat sink the swirl generated inside the combustion chamber causes the flame and combustion gases to remain substantially within the central portion of housing 16 thereby concentrating the greatest portion of the heat centrally of the under side surface of the heat sink upon impinging the heat sink the heat is therefore substantially uniformly diffused on the surface thereof in the device of the present invention the maximum temperature measured inside the combustion chamber when operating with wood as the fuel was 950 u00b0 c at the centre of support surface 30 while the invention has been described in connection with specific embodiments thereof it will be understood that it is capable of further.