A methodology for estimation of local heat fluxes in steady laminar boundary layer diffusion flames

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity ( U ∞ ) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition ( T s ) was varied between −10 and −80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions ( T s  = −80 to −60 °C) and temperatures in which anomalous self-preservation took place ( T s  = −30 to −10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T s  = −50 and −40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Boundary layer flashback of non-swirling premixed flames: Mechanisms, fundamental research, and recent advances

A thorough numerical and experimental investigation of laminar boundary-layer diffusion flames established over the surface of a condensed fuel is presented. By extension of the Reynold’s Analogy, it is hypothesized that the non-dimensional temperature gradient at the surface of a condensed fuel is related to the local mass-burning rate through some constant of proportionality. First, this proportionality is tested by using a validated numerical model for a steady flame established over a condensed fuel surface, under free and forced convective conditions. Second, the relationship is tested by conducting experiments in a free-convective environment (vertical wall) using methanol and ethanol as liquid fuels and PMMA as a solid fuel, where a detailed temperature profile is mapped during steady burning using fine-wire thermocouples mounted to a precision two-axis traverse mechanism. The results from the present study suggests that there is indeed a unique correlation between the mass burning rates of liquid/solid fuels and the temperature gradients at the fuel surface. The correlating factor depends upon the Spalding mass transfer number and gas-phase thermo-physical properties and works in the prediction of both integrated as well as local variations of the mass burning rate as a function of non-dimensional temperature gradient. Additional results from precise measurements of the thermal field are also presented.

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Estimation of local mass burning rates for steady laminar boundary layer diffusion flames

Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct

The influence of the flame reaction on the aerodynamic and thermal structures of the laminar, two-dimensional boundary layer over a flat plate has been studied experimentally The diffusion flame was produced by injecting fuel gas (methane or propane) from a porous plate surface into a uniform air stream flowing along the plate The results show that acceleration of the gas stream is observed, even at the station closeto the leading flame edge located just above the edge of the porous plate The temperature in the blue-flame zone is found to increase with the distance from this leading edge Changes in the velocity and temperature profiles are also observed in the boundary layer upstream of the leading flame edge The pressure distribution has been calculated using the measured velocity and temperatureprofiles The pressure distortion on the air-stream side of the flame zone can be atributed to the rapid increase in the thickness of the laminar boundary layer with a diffusion flame The velocity overshoot near the flame zone may be caused by this pressure distortion It is shown that the aerodynamic structure of the boundary layer changes considerably in the presence of a diffusion flame, and that even in the laminar boundary layer over a flat plate, the pressure cannot be considered to be uniform if a diffusion flame is established in it

Toshisuke Hirano

Papers

Aerodynamic and thermal structures of the laminarboundary layer over a flat plate with a diffusion flame