Combustion Characteristics of Premixed Hydrogen/Air in an Undulate Microchannel

Pedro R Resende,Alexandre M Afonso,Carlos Pinho,Leandro C Morais

doi:10.3390/en15020626

Abstract

This work reports a numerical investigation of microcombustion in an undulate microchannel, using premixed hydrogen and air to understand the effect of the burner design on the flame in order to obtain stability of the flame. The simulations were performed for a fixed equivalence ratio and a hyperbolic temperature profile imposed at the microchannel walls in order to mimic the heat external losses occurred in experimental setups. Due to the complexity of the flow dynamics combined with the combustion behavior, the present study focuses on understanding the effect of the fuel inlet rate on the flame characteristics, keeping other parameters constant. The results presented stable flame structure regardless of the inlet velocity for this type of design, meaning that a significant reduction in the heat flux losses through the walls occurred, allowing the design of new simpler systems. The increase in inlet velocity increased the flame extension, with the flame being stretched along the microchannel. For higher velocities, flame separation was observed, with two detected different combustion zones, and the temperature profiles along the burner centerline presented a non-monotonic decrease due to the dynamics of the vortices observed in the convex regions of the undulated geometry walls. The geometry effects on the flame structure, flow field, thermal evolution and species distribution for different inlet velocities are reported and discussed.

Highlights

The technology advances in the last years allowed the development of integrated system devices at microscale, including both electro and mechanical components designated by MEMS (Micro Electro-Mechanical Systems) [1]
The dynamics of the flame structure are analyzed by comparing with other cases carried out in previous works
The impact of the inlet velocity on the flame structure for the stable cases can be observed in Figure 4, presenting the contour maps of the heat release rate distribution Q

Summary

Introduction

The technology advances in the last years allowed the development of integrated system devices at microscale, including both electro and mechanical components designated by MEMS (Micro Electro-Mechanical Systems) [1]. Raimondeau et al [4] presented a numerical study on flammability limitations that relied on simple models to analyze the radical and thermal effects in the flame quenching using a methane/air mixture They established that both effects were caused by the wall, the thermal quenching resulted from the high heat losses through the combustor walls, leading to a situation were the auto-sustainable combustion is no longer possible, and the radical quenching was due to a deficit homogeneous chemistry caused by the radicals absorption at the wall and their subsequent recombination. This establishes the importance of a good choice of the wall proprieties to achieve a stable flame. Norton and Vlachos [5] performed a numerical study to analyze the impact of different combinations of the wall thermal conductivity and thickness in the external heat losses, changing the dimensions of the combustion chamber and operation conditions

Methods

Results

Conclusion