Transition of transient vertical natural-convection flows in water

Abstract

Experimental results and interpretations are given for transient natural convection adjacent to a suddenly heated flat vertical surface in quiescent water. The 1.24 m high surface resulted in laminar, transition and turbulent regimes downstream, in transients and in steady state, over a wide range of surface-energy input rates. Flows were visualized and velocity and temperature measurements made at various downstream locations, after imposing a uniform internal-energy generation rate within the very thin surface. Upflow development from quiescence to steady state was found to depend strongly on the downstream location x and imposed input heat flux. Laminar flow persisted into steady state, for short downstream distances. Further downstream, the flow became turbulent during the transient. Relaminarization at later time occurred only for lower flux inputs. Local measurements across the fluid layer show that the transient disturbances close to the leading edge of the surface are confined to within the final steady boundary layer. Downstream, they extend much further into the ambient. First disturbances always arose before the leading-edge-effect propagation estimates. The trend of data was in agreement with theory for a non-dimensional time τ < 85. For larger τ, turbulence instead terminated the one-dimensional transport regime simultaneously, at all downstream locations. This single value of τ also amounts to a single value of the non-dimensional thermal energy of the flow, ETT = 19.7. Disturbance frequency data early in the transient suggest the presence of a strongly selective amplification mechanism, very similar to that found in steady flows. The non-dimensional times at which local steady state was achieved were best correlated by a Fourier number, over a wide range of energy input conditions. Turbulence arising during the transient enhances the thermal transport significantly. Local convection coefficients then were found to be as much as 40% higher than the eventual steady values.