An introduction to heat transfer

Introduction to heat transfer

Geometry: shape, size, aspect ratio and orientation Flow Type: forced, natural, laminar, turbulent, internal, external Boundary: isothermal (Tw = constant) or isoflux (q̇w = constant) Fluid Type: viscous oil, water, gases or liquid metals Properties: all properties determined at film temperature Tf = (Tw + T∞)/2 Note: ρ and ν ∝ 1/Patm ⇒ see Q12-40 density: ρ ((kg/m) specific heat: Cp (J/kg ·K) dynamic viscosity: μ, (N · s/m) kinematic viscosity: ν ≡ μ/ρ (m/s) thermal conductivity: k, (W/m ·K) thermal diffusivity: α, ≡ k/(ρ · Cp) (m/s) Prandtl number: Pr, ≡ ν/α (−−) volumetric compressibility: β, (1/K)

Convection Heat Transfer

Fifty-five full-scale steady-state experiments were conducted to study the flow induced by a simulated pool fire in a compartment under conditions characteristic of the developing fire period. The mass flow rate through the door or window opening and bounds on the fire plume entrainment rate are presented as a function of opening geometry, fire strength, and fire location. The characteristics of the measured opening flow rates are explained by a simple hydrostatic model based on temperature distribution. A good correlation between the measured results and the idealized flows, taking into account the complete temperature distribution, is demonstrated. Entrainment results for fires near walls are in reasonable agreement with results from free-standing plume models. Except for the smallest openings, fires in other locations entrain at a rate two to three times the rate predicted by these models. This phenomenon is attributed to room disturbances caused by the opening flow and is similar to the behavior of a fire plume in a cross wind.

Flow induced by fire in a compartment

Proceedings of the royal society.

Compartment fire phenomena under limited ventilation

A thermally thick slab of polymethyl methacrylate was used to study the effects of the inclination angle of a fuel surface on upward flame spread. While investigation of upward spread over solid fuels has typically been restricted to an upright orientation, inclination of the fuel surface from the vertical is a common occurrence that has not yet been adequately addressed. By performing experiments on 10 cm wide by 20 cm tall fuel samples it was found that the maximum flame-spread rate, occurring nearly in a vertical configuration, does not correspond to the maximum fuel mass-loss rate, which occurs closer to a horizontal configuration. A detailed study of both flame spread and steady burning at different angles of inclination revealed the influence of buoyancy-induced flows in modifying heat-flux profiles ahead of the flame front, which control flame spread, and in affecting the heat flux to the burning surface of the fuel, which controls fuel mass-loss rates.

Experimental study of upward flame spread of an inclined fuel surface

This work revisits the classical problem of flame spread on solid fuels, to incorporate finite width effects. It is argued that in addition to the excess pyrolyzate [ P.J. Pagni, T.M. Shih, Proc. Combust. Inst. 16 (1978) 1329–1343.], a fraction of the fuel also diffuses to the sides and changes the amount of fuel available to participate in flame spread as excess pyrolyzate. This diffusion ( M ˙ Z ′ ) is significant for narrow fuel samples (width M ˙ Z ′ is calculated as a function of xp. The significance of M ˙ Z ′ in changing the flame height, and flame spread rate is explained. Experimentally measured flame heights show good agreement with the current theory. Previous work in two-dimensional flame spread theory and correlations are compared with the current analysis.

Upward flame spread on a vertically oriented fuel surface: The effect of finite width

The extinction and suppression of diffusion flames is examined theoretically. The effects of oxygen reduction and external heat flux are examined compared to data in the literature. An application of extinction in compartment fires is also examined. The theory developed is based on a critical flame temperature, and that theory includes transient effects and the addition of water as well.

A theory for flame extinction based on flame temperature

http://www.gollnerfire.com/wp-content/uploads/2012/07/2011_upwardspread_candf.pdf

Ali S. Rangwala

Papers

Compartment fire phenomena under limited ventilation

Experimental study of upward flame spread of an inclined fuel surface

Upward flame spread on a vertically oriented fuel surface: The effect of finite width

A theory for flame extinction based on flame temperature

Upward Flame Spread Over Corrugated Cardboard