EXPERIMENTAL RESULTS OF A NOVEL, VARIABLE ROUGHNESS TECHNIQUE TO ENHANCE, ON DEMAND, HEAT TRANSFER IN A SINGLE-PHASE HEAT EXCHANGER

摘要

A novel, active concept for a variable-roughness, heat-exchanger- tube insert that can enhance heat transfer was developed. The final design consists of shape-memory-alloy (SMA) wire coils that ride along a support structure in close proximity to the inner-tube wall of a double-pipe heat exchanger. At low tube-wall temperature, the coils are close-coiled. An excessive tube-wall temperature (low heat transfer coefficient) results in coil extension which produces substantial heat transfer enhancement. This concept could also be applied to the outside surface of a heat-exchanger tube. 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 constant test-fluid inlet temperatures [30°C (86°F), 35°C (95°F), 40°C (104°F)]. Extension of the SMA coils does produce a heat transfer enhancement and a pressure drop penalty. The increase in heat transfer coefficient between compressed-coil and extended-coil states ranges from 28 to 64 percent. The corresponding increase in friction factor is between 18 and 25 percent. Most favorable results occur at a test-fluid flow rate of 0.14 kg/s (18.52 lb/min), Re～10000.