Buoyant Unstable Behavior of Initially Spherical Lean Hydrogen-Air Premixed Flames

Zuo-Yu Sun,Hong-Meng Li,Guo-Xiu Li,Yue Zhai,Zi-Hang Zhou

doi:10.3390/en7084938

Abstract

Buoyant unstable behavior in initially spherical lean hydrogen-air premixed flames within a center-ignited combustion vessel have been studied experimentally under a wide range of pressures (including reduced, normal, and elevated pressures). The experimental observations show that the flame front of lean hydrogen-air premixed flames will not give rise to the phenomenon of cellular instability when the equivalence ratio has been reduced to a certain value, which is totally different from the traditional understanding of the instability characteristics of lean hydrogen premixed flames. Accompanied by the smoothened flame front, the propagation mode of lean hydrogen premixed flames transitions from initially spherical outwardly towards upwardly when the flames expand to certain sizes. To quantitatively investigate such buoyant instability behaviors, two parameters, “float rate (ψ)” and “critical flame radius (Rcr)”, have been proposed in the present article. The quantitative results demonstrate that the influences of initial pressure (Pint) on buoyant unstable behaviors are different. Based on the effects of variation of density difference and stretch rate on the flame front, the mechanism of such buoyant unstable behaviors has been explained by the competition between the stretch force and the results of gravity and buoyancy, and lean hydrogen premixed flames will display buoyant unstable behavior when the stretch effects on the flame front are weaker than the effects of gravity and buoyancy.

Highlights

With the concerns upon the depletion of fossil fuels and the deterioration of the global environment, investigations on alternative fuels have become a hot topic in the energy science field
According to the previously reported results [10,11,12,13], the flame front of premixed hydrogen-air flames will be continuously wrinkled towards cellular instability during the propagation
As mentioned above and discussed in the previous subsections, when the lean hydrogen premixed flames have begun to be lifted up is essential to the variation trends of the buoyant instability behavior; we propose the term ―critical flame radius, Rcr‖ as the indicator to the onset of the buoyant instability

Summary

Introduction

With the concerns upon the depletion of fossil fuels and the deterioration of the global environment, investigations on alternative fuels have become a hot topic in the energy science field. Due to its preeminent advantages in burning velocity and emitted products, hydrogen gas has been considered as a promising alternative fuel for the future [1,2] and has been employed in spark-ignition (SI) internal combustion engines throughout the world [3,4,5]. For a SI internal combustion engine, the combustion performance is strongly affected by the propagation process, and an in-depth understanding of the characteristics of propagating hydrogen-air premixed flames is essential for designing and optimizing hydrogen fuelled SI engines [6,7], and it is widely believed that the intrinsic instabilities are of the utmost importance for premixed flames [8,9]. There exist three intrinsic factors to induce an unstable premixed flame—hydrodynamic effects, thermal-diffusive effects, and buoyant effects [14]

Methods

Results

Discussion

Conclusion