Effects of gravity on lean hydrogen/air flame instability: From linear scaling law to nonlinear morphology evolution

TL;DR

This study investigates how gravity influences lean hydrogen/air flame instability, revealing a universal scaling law and contrasting effects on small and large-scale flame structures through detailed simulations.

Contribution

It establishes a universal gravity sensitivity scaling law and elucidates gravity's opposite effects on small and large-scale flame morphology in lean hydrogen flames.

Findings

Gravity most affects flames under ultra-lean, low-temperature, high-pressure conditions.

A universal scaling law between gravity sensitivity and Froude number is established.

Gravity inhibits small-scale cellular splitting but promotes large-scale finger structures.

Abstract

The instability characteristics of lean hydrogen/air flames have attracted considerable research attention, yet the effect of gravity remains insufficiently understood. In this study, time-resolved two-dimensional simulations with detailed chemistry and transport are conducted to investigate the influence of gravity-induced Rayleigh-Taylor (RT) instability on the linear growth rate of disturbances and nonlinear morphology evolution of cellular flame fronts at different length scales. In the linear regime, a parametric study is performed across various equivalence ratios, initial temperatures and pressures; in each case, the dispersion relation is calculated for various gravity levels. The influence of gravity is most pronounced under ultra-lean, low-temperature, and high-pressure conditions, and a universal scaling law between gravity sensitivity and the Froude number is established. In the nonlinear regime, gravity has opposite effects on the large-scale and small-scale structures of lean hydrogen flames. On the one hand, gravity inhibits the splitting of small-scale cellular structures through a baroclinic torque mechanism; on the other hand, it promotes the development of large-scale finger-like structures, thereby increasing the total surface area and the global consumption speed of the flame. The effects of gravity on the probability distributions of cell size, displacement speed, Karlovitz number, and local curvature are also analyzed. The results and findings of the present study should advance the fundamental understanding of hydrogen flame dynamics under varying gravity conditions and provide insight for relevant applications, including fire safety and space propulsion.