Experimental studies on flame stabilization in a three step rearward facing configuration based micro channel combustor

Highlights

• Three rearward step micro channel combustor with methane-air mixtures.

• Increasing the number of steps increases flame stability limits.

• The increase in first and second step length improves the lower and upper limits.

• Increase in third step length affects the flame stability limit at high flow rates.

• Power input for stable flame in combustor is lower than other combustors reported.

Abstract

In this study a detailed experimental analysis on the characterization of flame stability behaviour in a 2.0 mm support diameter micro-combustor with three rearward facing steps has been reported. Premixed mixture of methane–air has been used as a fuel for present investigations. Maximum and minimum diameter in the micro-combustor was maintained at 2 mm and 6 mm respectively. The effect of change in number of steps, length of steps, mixture equivalence ratios (ϕ) and flow rates on stability limits of flame and flame position has been investigated. It was observed that the zone of recirculation created due to the sudden expansion at the rearward step aids in stabilizing the flame inside the micro-combustor and improves the limits of flame stability significantly. The increase in the first and second step length helps in improving the lower and upper flame stability limits. The increase in the third step length affects the flame stability limit at higher flow rates only. Pollutants measurement shows that no NO_x emissions were produced and CO emissions increase as the equivalence ratio (ϕ) increases.

Introduction

Recently there has been considerable research and developments towards miniaturization of electronic devices due to increased demand of micro power production devices with long life, low recharge time, low weight and high power density as compared to electrochemical batteries [1], [2], [3], [4]. Combustion-based device has high power density which results in reduced weight, increased lifetime of micro-scale electronic and mechanical systems (MEMS) [2]. Energy density of electricity producing devices based on hydrocarbon fuel is 20–50 times higher than the advanced Li-ion concept electrochemical batteries. Therefore, a miniaturized power-generating device even operating at very low efficiencies would provide a good alternate to conventional batteries. The increased amount of surface to volume ratio and radical quenching on wall makes it difficult to reach an efficient and steady combustion at small scales [5], [6]. Radical quenching is due to adsorption of radicals on combustion chamber walls and recombination, which eventually leads to reduction in homogenous chemistry. In 1977 Epstein and Senturia [7] proposed a concept of micro heat engine. Since then, considerable amount of research has been done for the application and development of small devices to generate power based on the combustion of hydrocarbon fuels.

Kim and Kwon [8] computationally and experimentally studied flame propagation characteristics in a two staged micro combustor with premixed propane–air mixtures. They observed that recirculation plays an important part in stabilizing the flame, and aids in increasing the flame stability limits. In an experimental study on single step based micro combustor by Pan et al. [9], it was found that nozzle to combustor diameter ratio plays a vital role in stabilizing the flame. They studied effect of different parameters on flame stability limits including nozzle to combustor diameter ratio, wall thickness to combustor diameter ratio and hydrogen to oxygen mixing ratio. It was also observed that nozzle to combustor diameter ratio changes uniformity and magnitude of temperature distribution on wall. Flame structure and stability behaviour in a radial micro channel (externally heated) burning premixed methane–air mixtures have been extensively studied by Kumar et al. [10], [11], [12]. It is also proposed that the configuration of combustor studied, could be used as a micro power generation system as in case of disk-type combustion chamber for micro gas turbines [13]. In different studies by Kumar et al. [10], [11], [12] various non-stationary flame structures were observed, which were in addition to the conventional circular stable flame structure. Non-stationary flame structures include travelling flames, rotating pelton-like flames, and so forth. Later, Khandelwal et al. [14], [15], [16] have also reported similar studies on the effect of heat and flow recirculation on flame dynamics, stability and structure. Different configurations of combustors were studied and reported include single step micro combustor [14], double step micro combustor [15] and diverging micro channel combustor [16]. These combustors can be used as small scale combustion chambers to be used in micro power production devices and systems.

Zhang et al. [17] experimentally studied combustion of hydrogen assisted preliminary flame ignition and flame dynamics. It is observed that preliminary flame ignition assisted by hydrogen helps in expanding the flame stability limits for steady combustion of methane–air mixtures. It is also observed that there is no substantial distinction between hydrogen combustion and hydrogen added to the hydrocarbon combustion in the configuration studied. The improvement in the performance is in fact due to presence of platinum metal inside the combustor which acts as a catalyst. Similar other studies of flame behaviour, stabilization and combustion characteristics in different small-scale systems are reported in literature, such as free piston knock engine [18], micro thermo-photo voltaic system [9], radial channel combustors [10], [11], [12], [19], [20], [21], miniaturized combustors [22], [23], [24], Swiss-roll combustor [4], cylindrical combustors [25], [26] and staged combustors [8], [14], [15], [27]. Presently there is no literature available on safety concern of combustion based batteries, but if manufactured appropriately such devices would pose no risk.

The aim of the present study is to experimentally examine the effect of changing the flow field by abrupt flow expansion with help of a rearward facing step on flame stability limits and micro combustor performance. To examine the effect of a rearward facing step in improving the flame stability limits and stabilizing the flame at such minute scales, a 2.0 mm inlet diameter combustion chamber is selected. Several rearward facing steps have been integrated into the combustion chamber which is having a maximum diameter of 6.0 mm keeping the base configuration. Methane–air mixture is selected in the present study due it is extensive use by combustion researchers around the world for understanding the combustion phenomena. Methane is also commercially available in form of compressed natural gas which is used for various industry applications. The present work would help in taking a step forward for improving the design and characteristics of the micro-combustors, predominantly by employing a reorganization of the flow field by a sequence of rearward facing steps.