An experimental study of the ignition of Jet-A fuel sprays by an isothermal hot surface was conducted in a vertical axisymmetric duct. The ranges of flow conditions under which ignition was investigated were: 1) freestream velocity, 1-5 m/s; 2) boundary-layer momentum thickness, 3-20 mm; 3) freestream air temperature, 40-250°C; 4) fuel concentration, ignitability limits; and 5) droplet size (SMD), 20-200 /im. In addition to measurements of the wall temperature necessary for ignition under the above conditions, local measurements of velocity, "turbulence" intensity, fuel concentration, and the fraction of fuel vaporized were measured in the boundary layer at surface temperatures just below that required for ignition. The results exhibited vapor ignition trends for most of the flow conditions, with some exceptions where single-droplet ignition appeared to be present. The experimental data are compared with existing vapor ignition theory.
A PHIPr Re x SMD T ign y y* Nomenclature = [(4PrkL c p 00 c 00 )/U QO ] (c^hr/CpT^), parameter used by Toong = specific heat at constant pressure = c p associated with the freestream flow = enthalpy of combustion per unit mass = rate constant in an Arrhenius expression for the global reaction rate = length of heated surface = characteristic length in Toong's A parameter = bulk equivalence ratio in the flow = Prandtl number = freestream Reynolds number based on x -Sauter mean diameter of the droplets in the spray = temperature = spontaneous ignition temperature of the mixture = wall temperature at ignition = freestream temperature = gas velocity in the boundary layer = freestream velocity = 20.45 Lv/y* 2 Pr 2A = streamwise distance from the start of the momentum boundary layer = streamwise distance from the start of the momentum boundary layer to the start of the heated section = distance normal to the wall = distance from the wall to the isotherm corresponding to the spontaneous ignition temperature = thermal boundary-layer thickness = kinematic viscosity = mass density in the freestream flow
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