5208533 NMR machine with low field and dynamic polarization

Abstract

Pulsating behaviors of flame spread over n-butanol fuel are investigated with various initial temperatures under quasi-quiescent condition. The flame pulsating is an alternate process of diffusion combustion (crawling phase) and premixed combustion (jumping phase). Non-dimensional pulsating velocity Vfmax-VfminVf varies directly proportionally with the square root of non-dimensional initial temperature (THb-T0)/THb. The fundamental mechanisms of flame pulsation are physically interpreted based on heat and mass transfer in both liquid- and gas-phase flows. In liquid phase, a thermal vortex that is driven by the combined effects of Marangoni force and buoyancy force is formed ahead of the flame tip to enhance fuel evaporation rate. In gas phase, the existence of a gas-phase recirculation cell is indispensable for flame pulsation. Counter-current buoyancy dominates hot gas expansion effect, which produces a gas-phase recirculation cell ahead of the flame tip in pulsating flame spread regime. Otherwise, the uniform flame spread regime is achieved. The critical temperature between the uniform and pulsating flame spread regimes of n-butanol is predicted to be 27.9 °C, which is acceptably consistent with the experimental value of 30.2 °C. Finally, the coupling process between flame tip and subsurface convection flow front is achieved. Three steps are included in the coupling process of both movements: (a) flame crawling and subsurface convection flow crawling, (b) flame jumping and subsurface convection flow crawling, and (c) flame crawling and subsurface convection flow jumping.