Stability analysis of partially premixed methane-air flames in a novel micro-mixing micro-combustor

Abstract

Background

Micro-mixing is an innovative technique adopted in large-scale combustion to enhance flame stabilization. Flame stability enhancement is achieved as the flame structure is transformed from diffusion to partially premixed type. The present work is the first attempt to introduce micro-mixing in micro-scale combustion. A slot (micro-mixing slot) is provided in the centrally slotted bluff body to enable a small quantity of air gets mixed with fuel which subsequently enhances the flame stabilization in a micro-combustor. The angle of the micro-mixing slot is optimized by considering varied inlet velocity and equivalence ratio range. Moreover, the local mixing characteristics, variation in flame dynamics at different inlet conditions and lift-off dynamics were investigated in the present study.

Methods

The analysis was conducted numerically by adopting steady state and transient (higher velocity and lift-off conditions) laminar simulations at different inlet velocities and equivalence ratios.

Significant Findings

The local equivalence ratio at the central slot interface (φ_inter) plays the key role in anchoring the flame to bluff body as well as in the transformation of flame structure. The effect of φ_inter (cold flow) was investigated for different micro-mixing slot at varied inlet conditions and a new criterion to predict stable flames using cold flow itself, was developed. The range obtained is 2 < φ_inter < 5 for cold flow to achieve stable flames. The transformation of flame structure from diffusion to partially premixed type and the subsequent flame stability enhancement was analyzed by comparing combustors with and without micro-mixing. The magnitude of micro-mixing slot angle was optimized as ‘60°′ by analyzing the flame stabilization characteristics and thermal performance of the combustor at different inlet conditions. The trend in flame dynamics and wall temperature distribution by varying inlet velocity (at φ_g = 1.0) and global equivalence ratio (at V_air = 1m/s) were analyzed and furthermore the lean stability limit was determined. The lift-off mechanism was investigated at stoichiometric and lean global equivalence ratios - φ_g = 1.0, 0.8 and 0.5. Even at the lift-off condition, heat transfer from flame to bluff body tip was prominent, which enabled enhanced flame stabilization. As the mixture became leaner, the reaction zone expanded, and the flame shifted radially toward the fuel side.