Heat Transfer Enhancement in a Pipe Using a Flow Inverter

Flow inversion, transferring a liquid from the wall region toward the center of the pipe or from the axis toward the heat transfer surface, improves heat transfer in the laminar flow regime. While a fully developed velocity profile is quickly established, a thin thermal boundary layer is preserved for a considerable distance in the pipe behind the flow inverter for highly viscous liquids. Thus the pressure drop is increased only locally (by the inverter itself), while heat transfer enhancement is also seen in a long straight section of the pipe. Two original flow inverter designs were tested in a long pipe (3 m in length) heated by condensing steam, using starch molasses as a working medium. Experiments carried out in the range of Reynolds number 4-60 and Graetz number 150-700 resulted in an increase of 20-35% in the heat transfer coefficient, accompanied by a 30-40% increase in the pressure drop. The experimental results confirm the numerical model prediction (within an extended Reynolds number range starting from 0.1). Thus an almost 40% increase in heat transfer can be expected at the optimal Graetz number in the range 50-100, using only one flow inverter located in the middle of a sufficiently long pipe.