Reduction of convective heat transfer from reacting flows by application of electric fields
The electric field-induced reduction of heat transfer from a rod-stabilized diffusion flame and a step-stabilized premixed flame was investigated. The fuel examined was propane. Inlet velocity for the diffusion mode was a nominal value of 3.4 m/s with nominal air/fuel ratios of 420, 320, and 270. Inlet velocities ranged from 4.5 to 9.9 m/s for the premixed mode with equivalence ratios of 0.65 to 1.03. Maximum applied voltages for the diffusion and premixed modes were 8.0 and 6.6 kVDC, respectively. The field was applied in a direction perpendicular to the flow.
Heat transfer amelioration was quantified using records of temperature versus downstream distance from the stabilizer acquired for the external surface of the heatloaded electrode which was exposed to the ambient environment. In addition, shadowgraphs and photographs were used to observe any alteration of flame position or of the bulk flowfield. These observations were used to investigate mechanisms potentially responsible for heat transfer reduction.
The rod-stabilized diffusion mode displayed some field-induced reduction in heat transfer. Both bulk flow alteration and reduction in radiation (associated with soot) were concluded to be responsible. Flame impingement on the heat-loaded electrode was reduced by a field-induced increase in flow along the surface. Flame luminosity was reduced by the electric field (presumably due to a field-induced modification of soot production and/or destruction). This caused a reduction in radiative heat transfer.
No heat-transfer amelioration was noted for the premixed step-stabilized mode. This was attributed primarily to a geometry not accommodating to field-induced heat transfer reduction. Higher velocities and a lower presence of soot than the diffusion mode and problems associated with flame impingement on both electrodes (reduces maximum voltages and distorts field), also contributed to the negative result. Limited displacement of the luminous portion of the reaction zone was noted.