Wind tunnel generation of horizontal roll vortices over a differentially heated surface
Wildland crown fires represent an extreme type of forest fire that advances from top to top of trees or shrubs. Some such fires generate energy comparable to several nuclear explosions. They occur most frequently in coniferous forests found in the United States and Canada, and in broad-leafed eucalyptus forests of Australia. Here we report visualization of wind tunnel experiments in which airflow along the perimeter of a simulated forest fire produced horizontal vortex pairs with axes in the downstream direction. These vortex pairs formed above a differentially heated surface simulating the thermal energy flux from a crown fire. The thermal energy was produced by an electrically heated surface ribbon at a temperature of ∼250°C and also, in a separate experiment, by an electrically heated embedded wire coil at a temperature of ∼825°C. Wind velocities ranged from 0.2 to 3.6 m s−1. Similar vortex action may cause the unburned tree-crown strips that are observed when crown fires, driven by a significant wind, burn over flat terrane.
An Experimental Study of Mixed, Forced, and Free Convection Heat Transfer From a Horizontal Flat Plate to Air'
The paper Wang described experiments in which a flat plate was heated nonuniformly across the width of a wind tunnel, although the data were treated as though the heating were uniform. A description of the experiments noted that a central heater. with a width of 40 percent of the channel width, was sup plemented by two guard heaters that were used to minimize the transverse heat transfer. Transverse temperature details were not given. Ordinarily this procedure is satisfactory for convective heat transfer, and a number of studies have produced acceptable results. Wang indicated that his data could be used to verify theoretical criteria that determine the conditions marking the onset of longitudinal vortex rolls in a horizontal Blasius flow. These criteria, developed by Wu and Cheng [14], apply to a flat plate heated uniformly and isothermally from below or cooled isothermally from above.
Turbulent free convection in a vertical slot
This paper is a continuation of the experimental study (Elder 1965), of free convection in a vertical slot, to unsteady and turbulent motion. The flow is specified by three dimensionless parameters: σ, the Prandtl number; h = H|L, the aspect ratio and $A = \gamma g \Delta TL^3|kv$, the Rayleigh number. The present experiments are generally for σ = 7 (water), h = 10–30 and A > 106. For A above about 8 × 108S|h3, travelling, wave-like motions grow up the hot wall of the slot and also down the cold wall. These waves grow most readily mid-way between the two ends. At higher values of the Rayleigh number when the wave amplitude is finite, the phase of successive wave fronts becomes increasingly random till near A = 1·0 × 1010|h3 an intense entrainment and mixing process commences between the wall region and the interior. The middle portion of the interior is then turbulent, the extent of the region growing further toward the ends as A increases. Measurements of the mean temperature and the probability distribution of temperature fluctuations of the turbulent flow are reported. Except within the thin wall layers and distant form the ends, the interior has a mean temperature, uniform to within 0·1%, superimposed on which is a nearly Gaussian fluctuation-field of variance of order 0·01ΔT. A comparison is made with recent theories of turbulent convection; moderate agreement is found with the similarity ideas of Priestley (1959). The wall layer is seen as a marginally unstable sublayer.