Experimental and analytical study of heat transfer and turbulent flow field in tangentially injected swirl flow.

摘要

Enhancement of heat transfer in a tube has been experimentally studied when fluid is injected tangentially. The experiments were conducted with electrically heated tubes of different diameters and air as the test fluid. The effects of tube diameter, injector diameter, momentum flux ratio and Reynolds number were studied parametrically. The results show that with tangential injection, significant enhancement in heat transfer is possible.;To obtain understanding of the mechanisms by which heat transfer is augmented and the rate at which swirl decays, additional experiments were conducted by injecting air through injectors placed on the periphery of an 88.9mm inside diameter and 2.5m long acrylic tube. Four injectors of 15.88mm inside diameter and six injectors of 22.23mm inside diameter were used in the two sets of experiments. Tangential to total momentum flux rate ratio of 7.84 and 2.67 respectively were obtained in the two sets of experiments.;Using a single rotated straight hot wire and a single rotated slanted hot wire anemometer, profiles for mean velocities in the axial and tangential directions, as well as the Reynolds stresses were obtained. Axial velocity profile shows existence of flow reversal region in the central portion of the tube and an increased axial velocity near the wall. Tangential velocity profiles have a local maximum, the location of which moves radially inwards with distance. Reynolds stress data show an anisotropy in eddy viscosity. Furthermore, a separate set of experiments was conducted with the test section heated with a heating tape wrapped around the test section. No significant difference in mean velocities and Reynolds stresses was found between the adiabatic experiments and diabatic ones.;Two major mechanisms for enhancement of heat transfer are identified: (1) high maximum axial velocity near the wall produces higher heat flux from the wall; (2) high turbulence level improves mixing and thus the rate of heat transfer. Furthermore, it is observed that both the kinetic energy of the mean flow and the turbulence level decrease as swirl decays. However, during decay process the high turbulence-energy-production from Reynolds stresses is necessary to transfer the kinetic energy of the mean flow to the turbulence energy. This high turbulence-production, in turn, slows down the rate of decrease of the turbulence level. As a result, the swirl and the heat transfer enhancement are preserved for several tens of hydraulic diameters downstream.