Abstract
Results of an experimental investigation of transient natural convection flow and transport development adjacent to a suddenly heated, 1.27-m-high, vertical surface in cold water are given. These measurements determine the effects of the anomalous behavior of cold water on transient convective response and on the eventual steady-state heat transfer characteristics. The range of ambient temperatures studied is 1.2 ⩽ t ∞ ⩽ 6.8° C, for surface heating rates in the range 587–1870 W m −2. The transient and resulting eventual flows were visualized and local surface temperatures measured. For the lowest ambient temperature and heating conditions, t ∞ = 1.2° C and 587 W m −2, the established flow resembled a downward flowing boundary layer. As t ∞ and the heating rate are increased, the flow is not of simple boundary region form. A vigorous turbulent downflow, away from the surface, and intermittent upflow, very near the surface, was seen. Further increase in t ∞ and heating rate results in yet stronger inner upflows. The outer downflow existed at all times. However, it arose only intermittently for t ∞ = 3.1° C and heating rates of 1140 and 1870 W m −2. For these conditions, the flows resembled an upward flowing boundary layer. These changes in transport regime are confirmed by local surface temperature measurements. Surface height-averaged temperature, t L , was used in the evaluation of Nu L and Gr L near the extremum temperature t M . Average steady-state heat transfer data are best correlated by Nu L Gr L 1 4 = 1.35 ¦R − 0.26 ¦ 0.165 outside 0.23 < R < 0.29, where R = (t m − t ∞) (t L − t ∞) . Within 0.23 < R < 0.29 a minimum in heat transfer rate is observed. Details of flow and transport evolution during the transient, starting from quiescence were also studied. Early one-dimensional conduction transport is followed by entrainment development. Both unidirectional and bidirectional entrainment development were seen, depending upon the ambient and surface-heating conditions. Bidirectional transient flows were found to be unstable, resulting eventually in turbulent flow. Transition to turbulence occurred simultaneously at all downstream distances. The persistence of one-dimensional transport was determined, at various downstream locations, for all ambient and heating conditions. The measurements show the qualitative changes in flow regimes and also the actual variation of the Nusselt number in the density extremum region. These characteristics are quite different from those found in the well-studied Boussinesq behavior.
Published Version
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