Development and testing of a novel, variable-roughness technique to enhance, on demand, heat transfer in a single-phase heat exchanger

A novel enhancement concept involves a variable-roughness heat-exchanger-tu be insert that can repeatedly fluctuate between enhanced and unenhanced hea t transfer states. The desired response of the heat-exchanger-tube insert i s 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 correlat ion was used to determine the material deformation required to produce "eff ective" enhancement, a roughness change that would produce at least a 10 de greesC reduction in tube-wall temperature. Shape-memory alloys (SMA's) can be used to develop a variable-roughness, heat-exchanger-tube insert that ca n respond to an excessive tube-wall temperature. The final design consists of Nickel Titanium (NiTi) shape-memory-alloy wire coils that ride along a s upport- structure insert in close proximity to the tube wall. At low temper ature, the shape-memory coils are close-coiled and produce little heat tran sfer enhancement. In response to an excessive tube-wall temperature, the co il extends and produces substantial heat transfer enhancement. Experiments were conducted at three test-fluid flow rates [0.14 kg/s (18.52 lb/min), 0. 30 kg/s (39.68 lb/min), 0.58 kg/s (76.72 lb/min)] while maintaining constan t test-fluid inlet temperatures [30 degreesC (86 degreesF), 35 degreesC (95 degreesF), 40 degreesC (104 degreesF)]. Extension of the SMA coils does pr oduce heat transfer enhancement and a pressure drop penalty. The increase i n heat transfer coefficient between compressed-coil and extended-coil state s ranges from 28 to 64 percent. The corresponding increase in friction fact or is between 18 and 25 percent.