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In laminar, or stream line flow, the elements of the fluid flow parallel to each other, without any large amount of mixing, and the resulting laminar boundary layer stays thick. All heat transfer is done by conduction, and the heat transfer resistance is high. In turbulent flow, the fluid elements mix, setting up eddy motion and stripping the boundary layer from the wall, thereby ensuring that the boundary layer is thin and the resistance to heat transfer is reduced.
Turbulent or laminar flows are defined by Reynolds Number. The Reynolds Number is a dimensionless number and is a function of the fluid velocity inside the tube, tube diameter and viscosity of the liquid. A number below 2500 indicates laminar flow and one above 2500 indicates development to or fully developed turbulent flow. Fluid around the heat exchanger inside tanks, without work loads added, are completely at rest, with the boundary layer as thick as the entire tank-- surely very poor conditions for heat transfer. As the heat exchanger warms, so does the liquid adjacent to the tubes. The density of the fluid close to the heat exchanger walls changes, the heated fluid rises, some flow pattern develops and natural buoyancy creates a slowly developing free convection heat transfer pattern. The heat transfer from the outside surface of the tubes to the fluid can be greatly improved without waiting for the conversion from free to forced convection to occur. This is accomplished by introducing mechanical means to move the fluid inside the tank with the use of pumps, agitators or airspargers. For enhanced outside heat transfer, MBA Manufacturing has introduced a heat exchanger unit containing an attached air sparge. This demonstrates the versatility that can be accomplished with tubular in-tank heat exchangers.
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