A novel, variable-roughness technique to enhance, on demand, heat transfer in a single-phase heat exchanger

A novel, active-enhancement concept to develop a variable-roughness heat exchanger-tube insert that can repeatedly fluctuate between enhanced and unenhanced heat transfer states was developed. The desired response of the heat-exchanger-tube insert is dictated by an excessive tube-wall temperature. A well-known correlation for the heat transfer coefficient due to roughness in turbulent pipe flows was applied to the tube-side of a double-pipe heat exchanger. This correlation was used to determine the material deformation required to produce "effective" enhancement, a roughness change that would produce at least a 10 degrees C reduction in tube-wall temperature. Shape-memory alloys can be used to develop a variable-roughness, heat-exchanger-tube insert that can respond to an excessive tube-wall temperature. The final design here consists of Nickel Titanium (NiTi) shape-memory-alloy wire coils that ride along a support structure in close proximity to the tube wall. At low temperature, the shape-memory coils are close-coiled and produce little heat transfer enhancement. In response to an excessive tube-wall temperature, the coil extends and produces substantial heat transfer enhancement.