Local heat transfer downstream of an asymmetric abrupt expansion and cavity in a circular tube

摘要

This study is concerned with measuring local heat transfer downstream of an asymmetric abrupt expansion and an asymmetric abrupt expansion followed by an asymmetric abrupt contraction (called "asymmetric cavity") in a circular tube at a uniform wall temperature. The effects of geometry and three-dimensionality of the flow caused by asymmetric expansion on heat transfer characteristics are also examined. The flow just upstream of the expansion is unheated and fully developed at the entrance to the heated asymmetric abrupt expansion region. Local heat transfer coefficients are measured using a specially designed isothermal heat flux sensor. Measurements for the asymmetric abrupt expansion are made at a small to large diameter ratio of d/D = 0.4 and 0.533 for Reynolds numbers of Re_D = 17,300 and 21,900, respectively. The eccentricities of the tube axis (e/D) are 0.25 and 0.17 for d/D = 0.4, and 0.195 and 0.065 for d/D = 0.533. For the asymmetric cavity, all tests are made at d/D = 0.4 and Re_D = 17,300 with various cavity lengths for e/D = 0 and 0.25, respectively. For both cases, the variations of local Nusselt number are observed along the wall of downstream circular tube at several angular positions around the tube circumference. In general, the local Nusselt numbers downstream of an asymmetric abrupt expansion are substantially higher than the fully developed values for the range of Reynolds numbers, diameter ratios and eccentricities investigated, due to high turbulence and mixing action in the recirculation region. And the maximum Nusselt numbers occur between 10 and 15 step heights from the expansion step. The Nusselt number distributions for the asymmetric cavity show a dramatic increase to the maximum values as the downstream region of the cavity is approached. This behavior is attributed to a periodic vortex shedding, subsequent impingement on the downstream corner region of the cavity and three-dimensionality effects which cause an increase in turbulence intensity.