Extinction of laminar jet diffusion microflames

Abstract

This paper presents a theory to predict the extinction limit of laminar jet diffusion microflames, defined as flames established on submillimeter-diameter burners. The classical Burke–Schumann (BS) theory is first extended to include the effect of one-step, finite-rate chemistry. Then, a theory of diffusion-flame extinction is applied using activation-energy asymptotics to predict the extinction limit. The present theory correctly reproduces experimental observations, i.e., u L ∼ d −2, where u L is the fuel jet velocity at the extinction limit (lower limit) and d the burner diameter. According to the BS theory, the gradient of mixture fraction at the flame-sheet location is infinite at the burner rim, and it decreases with increasing axial distance to the minimal value at the flame tip. Therefore, local extinction initiates at the burner rim, and extinction occurs when the mixture-fraction gradient at the flame tip is greater than a critical value. This view of microflame extinction is supported by the results of experiments and numerical simulations. It is found that the present theory can be applied for various types of fuels, those are, methane, propane, and butane.