Experimental and Numerical Studies of Flame Propagation and Stability in Microchannels

微小通道內火焰傳播及穩定性的實驗和數值研究

Student thesis: Doctoral Thesis

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Award date24 Jul 2020

Abstract

Due to the great development of the technologies on micro-fabrication and integrated circuit, the rapid prototyping and batch-manufacturing techniques for microelectromechanical system (MEMS) become realizable. In present, various types of chemical batteries are generally used as the energy source of MEMS systems, but due to their low volume energy density, there is a large bottleneck in miniaturization. Compared with chemical batteries, the energy density of hydrocarbon fuels is more than ten or even dozens of times higher. Therefore, the micro-scale combustion of hydrocarbon fuel powered MEMS theoretically has significant advantages, such as high energy ratio and rapid recharge. As a micro-energy technology with great potential, micro-scale combustion technology has been extensively researched and developed rapidly since the 1990s.

However, unlike conventional macro-scale combustion, micro-scale combustion is often carried out at millimeters or even smaller scales. Due to the scale effect, the interaction between the flame and the wall surface is enhanced, including the effect of heat and mass transfer between the flame and the wall surface, which makes the flame become vulnerable to instability and extinction. Therefore, research on the flame propagation behavior and stability of micro-scale combustors is crucial for its safe and efficient application in MEMS systems. In view of the above issues and challenges in micro-scale combustion, this paper conducts experimental and simulation studies on the upward propagation of flames at low flow rates and the stability of flames at high flow rates in micro channels.

As regards to flame propagation, the flame propagation process of methane/air premixed flames were studied with micro-channels with rectangular cross-sections. The flame characteristics such as flame propagation speed, mode and structure were obtained using high-speed camera and laser induced fluorescence (PLIF) system. The influence of the parameters such as the height of the micro channel, the inlet flow rate and the equivalent ratio on the above flame characteristics is analyzed. In addition, considering the soot deposition in practical applications, the study investigated the effect of soot coating on the inner wall of the microchannel on flame propagation and flame structure. The experimental results show that the flame in a narrow channel is prone to flame wrinkling due to the hydrodynamic instability, resulting in sharp fluctuations in propagation velocity. The soot coating has a significant effect on the flame propagation speed, and its effect is related to the equivalent ratio.

In terms of flame stability, a bluff body was embedded in the cavity-type microchannel combustion chamber, and the effect of the bluff body structure on improving flame stability and combustion efficiency was studied. Based on the analysis of the effect of the post-bluff body on the flow and component transport on the combustion rate, a dimensionless parameter defined by the ratio of the convective mass transport rate and the reaction rate, the local Damköhler number (DaL) It was found that the variation of combustion efficiency along the stream wise direction is determined by the value of DaL

In addition, based on the above research, a bluff body-cavity coupling flame stabilizer is further proposed. The numerical simulation results show that the proposed flame stabilizer can significantly improve the flame stability of the original single bluff body or cavity. And through the analysis of the interaction between the flow and combustion in the area near the stabilizer in the micro channel, the physical mechanism of the synergy between the bluff body and the cavity of the coupled stabilizer is revealed, which provides the theoretical basis for further structural optimization of the coupled stable combustion structure.

    Research areas

  • Micro-scale combustion, Microchannel, Flame propagation, Stability, Bluff body, Cavity, Soot coating