Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.
Due to increasing demand for clean and green energy, a need exists for fuels with low emissions, such as synthetic gas (syngas), which exhibits excellent combustion properties and has demonstrated promise in low-emission energy production, especially at microscales. However, due to complicated flame properties in microscale systems, it is of utmost importance to describe syngas combustion and comprehend its properties with respect to its boundary and inlet conditions, and its geometric characteristics. The present work studied premixed syngas combustion in a two-dimensional channel, with a length of 20 mm and a half-width of 1 mm, using computational approaches. Specifically, a fixed temperature gradient was imposed at the upper wall, from 300 K at the inlet to 1500 K at the outlet, to preheat the mixture, accounting for the conjugate heat transfer through the walls. The detailed chemistry of the ignition process was imitated using the San Diego mechanism involving 46 species and 235 reactions. For the given boundary conditions, stoichiometric premixed syngas containing various compositions of carbon monoxide, methane, and hydrogen, over a range of inlet velocities, was simulated, and various combustion phenomena, such as ignition, flame stabilization, and flames with repeated extinction and ignition (FREI), were analyzed using different metrics. The flame stability and the ignition time were found to correlate with the inlet velocity for a given syngas mixture composition. Similarly, for a given inlet velocity, the correlation of the flame properties with respect to the syngas composition was further scrutinized.
With the increasing global concerns about the impacts of byproducts from the combustion of fossil fuels, researchers have made significant progress in seeking alternative fuels that have cleaner combustion characteristics. Such fuels are most suitable for addressing the increasing demands on combustion-based micro power generation systems due to their prominently higher energy density as compared to other energy resources such as batteries. This cultivates a great opportunity to develop portable power devices, which can be utilized in unmanned aerial vehicles (UAVs), micro satellite thrusters or micro chemical reactors and sensors. However, combustion at small scales—whether premixed or non-premixed (diffusion)—has its own challenges as the interplay of various physical phenomena needs to be understood comprehensively. This paper reviews the scientific progress that researchers have made over the past couple of decades for the numerical investigations of diffusion flames at micro scales. Specifically, the objective of this review is to provide insights on different numerical approaches in analyzing diffusion combustion at micro scales, where the importance of operating conditions, critical parameters and the conjugate heat transfer/heat re-circulation have been extensively analyzed. Comparing simulation results with experimental data, numerical approaches have been shown to perform differently in different conditions and careful consideration should be given to the selection of the numerical models depending on the specifics of the cases that are being modeled. Varying different parameters such as fuel type and mixture, inlet velocity, wall conductivity, and so forth, researchers have shown that at micro scales, diffusion combustion characteristics and flame dynamics are critically sensitive to the operating conditions, that is, it is possible to alter the flammability limits, control the flame stability/instability or change other flame characteristics such as flame shape and height, flame temperature, and so forth.
Combustion characteristics at small scales have been studied continuously due to the potential applications in portable power devices. It is known that heat release impacts at small scales result in different flame behavior as compared to conventional scales. The impacts of geometry, stoichiometry, flow rates, wall temperatures, etc., are widely studied in the literature. However, dilution impacts still need to be further studied due to its important role on controlling the flame behavior and subsequent pollutants emissions at these scales. In this work, premixed hydrogen/air combustion is simulated at an axis-symmetric microchannel (with diameter D = 0.8 mm and length L = 10 mm), where detailed chemical kinetics are implemented in simulations (32 species and 173 reactions). The heat transfer on the wall is considered by imposing a hyperbolic temperature profile on the wall, where the wall temperature increases from 300 K at the inlet to 1300 K at the outlet. With this setup, a range of equivalence ratios including a typical fuel-lean regime (ϕ = 0.7), stoichiometric regime (ϕ = 1.0), and two cases at an ultra-rich regime (ϕ = 2.0 and ϕ = 3.0) are investigated. For each equivalence ratio, excess dilution (using N2) is introduced to the mixture, and its impact is compared with other cases. With that, the impacts of dilution variations on the combustion characteristics of premixed hydrogen/air are investigated for different equivalence ratios. More specifically, several incidents such as flame dynamics, flame stabilization, extinctions, and NOx emissions are studied for the aforementioned operating conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.