Effect of fan-generated swirl on turbulent heat transfer and fluid flow in a pipe

• Fan-generated swirl.
• Decay of swirl in a round pipe.
• Swirl-based heat transfer enhancement.
• Swirl-related wall shear stress.

Fan-generated swirl can be characterized as chaotic and strongly mixed. It differs from controlled swirl with respect to those characteristics that tend to increase both the rate of heat transfer and the pressure drop. In the investigation described here, a rotating fan simulation, which takes full account of the rotation of the fan blades and of the true nature of the delivered flow, was implemented with high fidelity. The fan output is delivered directly to the inlet of a round pipe. It is believed that this is the first time that such a realistic approach has been used. The air delivered to the pipe has a temperature different from that of the pipe wall. The swirl component is shown to give rise to a significant enhancement of heat transfer, from a factor of two to 50% between x/D = 0 and 20. Similar increases of the wall shear were also found to result from the swirl. As a comparison case, a blower curve for the fan in question was used. Since blower curves omit the swirl component, only a uniform axial flow can be extracted. The swirl effects on heat transfer and shear stress were obtained by comparing the results for the actual rotating-fan-provided flow with those for the blower curve model. In the absence of swirl, the axial distributions of the local heat transfer coefficient and wall shear displayed a local undershoot, thereby supporting limited earlier reporting of such happenings in simpler situations.