Effects of heat transfer on flame stability limits in a planar micro-combustor partially filled with porous medium
Introduction
Since Epstein and Senturia proposed the concept of ‘micro machinery’ in 1991, combustion-based micro power devices have attracted remarkable attention of scientists and engineers all over the world [1]. Various types of micro power devices have been prototyped, developed and tested [2]. As the key component of the above-mentioned micro power devices, the micro-combustor plays a key role in determining the overall efficiency and system performance. The reduced physical dimensions of the micro-combustor give rise to shorter residence times and higher heat loss fractions, and therefore flame stability and flammability limits are the main challenges to the design and operation of micro-combustors. Achieving self-sustained and stabilized flames in micro-combustors is the primary requirement for downstream power generation or conversion devices, for example, the micro thermophotovoltaic (TPV) [3] or the thermoelectric (TE) systems [4].
Theoretical analyses [5], [6] indicate that heat recirculation is crucial to sustaining stabilized combustion in micro-combustors. A good example is the Swill-roll micro-combustors [7], [8] which allow the exhaust gases to transfer heat to the unburned mixture through a non-contact counter-flow structure. In the meanwhile, heat recirculation through the solid matrix of the porous medium, commonly used in filtration combustion (also known as ‘porous medium combustion’), provides a clue to further enhance heat recirculation in micro-combustors. Li et al. [9] inserted some porous medium made of stainless steel (SS) mesh into a planar micro-combustor and conducted the experiment on its performance as a flame holder. Their further studies were carried out on the flame stability limits of such a configuration under different filling conditions [10], [11]. Unlike its macro-scale counterpart [12], [13], [14], filtration combustion in micro-combustors exhibits a wider range of stability limits. Yang et al. [15] and Fursenko et al. [16] studied filtration combustion of premixed CH4/air in high-porosity micro-fibrous porous medium. Interestingly, they found a regime within which standing-wave combustion modes were observed. Similar results were obtained by Wan et al. [17] in both their experiment and numerical simulations.
When a micro-combustor is fully or partially filled with porous medium, besides the combustor wall, a new pathway for heat recirculation emerges, that is, the solid matrix. In our previous study [18], the detailed methodology to quantify the two pathways of heat recirculation was developed. However, it is realized that heat recirculation alone is insufficient to understand the flame stability limits in a porous micro-combustor. Another important factor, that is, heat loss in the flame zone, should be considered in order to reveal the underlying mechanism. Thus, the primary objective of the present study is to identify the effects of heat transfer on the flame stability limits in porous micro-combustors. A planar micro-combustor partially filled with porous medium is chosen to be experimented and modelled, because this configuration has been studied extensively [9], [10], [11], [18], [19]. It has the advantages of controllable flame positions and lower flow resistances. The numerical model and subsequent analysis presented in this paper could be readily extended to micro-combustors fully filled porous medium, and therefore the present study is not limited by the specific configuration considered. In a recent publication, Sirotkin et al. [20] pointed out that the discrete structure of the solid matrix had some strong impact on the flow, heat and mass transfer inside the porous medium, and a continuum model could fail to capture some intrinsic features, especially when the pore size and the channel height are comparable. Having realized that problem, the continuum model is still employed in the present study mainly because of its simplicity and low computational cost. What is more, the porous structure of the folded SS mesh is unknown, making a pore-scale model unfeasible at this moment of time. As such, the approach of our modelling and analysis has a rigorous theoretical basis, while the numerical results should only be taken as qualitatively accurate.
Section snippets
Modelling
The numerical model is established based on a planar micro-combustor with the channel height (H) of 1 mm and the combustor length (L) of 21 mm. Figure 1 shows the direct photo, design features, physical model and boundary conditions of the micro-combustor. The combustor is made of SS 316 L, and the wall surface is highly oxidized after some time of experiment, as shown in Fig. 1a. The aspect ratio of the combustor is 10, as shown in Fig. 1b, giving the majority of the flow to be two-dimensional
Model validation
In order to validate the numerical model, an experiment of premixed H2/air combustion within a range of velocities and equivalence ratios was conducted. The porous medium with a width of 5 mm is inserted into the middle part of the planar micro-combustor, as schematically shown in Fig. 3a. The porosity is estimated to be around 0.87 [19]. Wall temperature profiles along the centerline of the combustor (see Fig. 1b) were acquired by an OPTRIS® high-precision infrared thermometer (Model
Conclusions
Both experimental and numerical studies have shown that extended standing-wave combustion regimes exist in micro-combustors filled with porous medium. In order to reveal the underlying mechanism that dictates the critical flame stability limits, numerical simulations are carried out for a planar micro-combustor partially filled with porous medium. Within the porous medium, two zones, namely, the preheat zone and the heat loss zone, are delineated and defined. Preheating and heat loss are then
Acknowledgment
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51776136).
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