An experimental study on flame front evolution and self-acceleration of hydrogen spherically expending flames was conducted on a constant volume combustion chamber over a wide range of Lewis number at elevated pressures. Flame morphology, onset of flame acceleration and flame self-similar acceleration were analyzed. The results shows that the critical radius for onset of flame acceleration increases with the increase in effective Lewis number and the decrease in initial pressure. And the critical Peclet number increases nonlinearly with the increasing Markstein length. Thus, the onset of flame acceleration is controlled by the couple effects of hydrodynamic and diffusional-thermal instabilities. Furthermore, the self-similar acceleration exponent increases with the decrease in the effective Lewis number (or Markstein number) and almost keeps constant with initial pressure. It is concluded that the self-similar acceleration exponent is mainly affected by diffusional-thermal instability.
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