A study of natural convection above a line fire

Abstract

The behaviour of a natural convection plume above a line fire is studied both theoretically and experimentally. In the theoretical treatment, a turbulent plume above a steady two-dimensional finite source of heated fluid in a uniform ambient fluid is investigated. By the use of the lateral entrainment assumption, a quadrature solution has been obtained for each of two separate ranges of a source Froude number, F < 1 or F > 1. In neither of these cases can the finite width line source be accurately represented by an equivalent mathematical line source at a lower level. Only the special case, F = 1, can be so represented and its solution is discussed.

In the experimental treatment, hot gases, resulting from the burning of a liquid fuel in a long channel burner, are driven upwards by buoyancy and gradually cooled down by the entrainment of ambient air. The average temperature along lines parallel to the channel burner was measured by a piece of resistance wire. For the case of a non-luminous diffusion flame, the effective radiation loss to the surroundings was assumed to be negligible, and,by a comparison of the energy flux supplied from the fuel and the energy flux contained in the plume, the characteristic turbulence entrainment coefficient is determined. By the alternate use of either an absorbing or a reflecting surface for the table-top surrounding a luminous flame, a measurement was made of the energy radiated from the flame that was intercepted by the fire surroundings and subsequently returned to the buoyancy plume by heating the ingested air. These measurements agree with estimates computed from such data as are available. The experimental results relating to the behaviour of the convection plume agree closely with the theoretical predictions in all cases.