Self-excited noise generation from laminar methane/air premixed flames in thin annular jets
Introduction
Recent demands for high combustion loads and stringent emission requirements frequently induce combustion instabilities. Since these are undesirable sources of noise and can sometimes cause disastrous destruction of combustion equipment, active control of combustion instabilities becomes one of the key interests in combustion research. In order to develop effective control technologies, it is required to understand the mechanisms and the influencing factors governing combustion instabilities.
Previous studies on unsteady combustion instabilities have frequently focused on propulsion systems such as rocket motors [1], [2], [3], jet engine afterburners [4], [5], and ramjets [6], [7], [8], [9]. These phenomena are coupled by complex, feedback-type interactions between flow, acoustics, and combustion processes. Recently, attention has been focused on the source of combustion noise by investigating relatively simple and well-defined flow geometries [10], [11], [12]. The investigation of laminar jet flames impinging on a cool plate has shown that the source of sound generation is from periodic sudden extinction of large portion of the flame interacting with the cool plate [13]. The response of burner-stabilized flat flames to acoustic velocity perturbations has also been investigated numerically and analytically [14]. These studies utilized the forcing of the flow fields by acoustic speakers. Studies of self-excited combustion noise generation in simple laminar flow geometries are rather limited.
Although multi-coaxial burners are widely used in the field of material synthesis such as fine particle production, not many experimental studies on the characteristics of the related flames have been conducted [15], [16]. In this study, the characteristics of various flames that can be formed in a double coaxial burner were investigated, and the structure and stability of the flame were investigated.
This type of flow geometry has been investigated previously in terms of flame stabilization [17]. Self-excited combustion noise has been generated which depends on the equivalence ratio and jet velocity [18], [19]. The present study focuses on the self-excited noise generation in the burner, especially the regimes and overall characteristics of noise generation.
Section snippets
Experimental apparatus
The apparatus consisted of an annular jet burner and flow controllers, a visualization setup, and a sound pressure measurement system. The annular jet burner, shown schematically in Fig. 1, was composed of double concentric tubes, which were made of brass. The inner tube is 400 mm long with i.d. 10.1 mm and o.d. 10.9 mm. The outer tube was 300 mm long with i.d. 12.8 mm and o.d. 13.8 mm. The resonance frequency of the inner tube was 425 Hz. The annular slit width was 0.95 mm and the exit area of
Observed flames
Various flame types have been observed depending on the equivalence ratio ϕ and mixture velocity Up. Direct photographs of typical flame types are shown in Fig. 2 for the air velocity in the inner tube Ua = 0. The Reynolds number is defined as Up⋅De/ν, where ν is the kinematic viscosity, De is the hydraulic diameter based on the flow rate, that is, De = Dout − Din, Dout is the i.d of the outer tube, and Din is the o.d. of the inner tube. The Strouhal number is defined as f⋅De/Up, where f is the
Concluding remarks
Premixed methane/air flames in the laminar thin annular jet burner have been studied experimentally to investigate the characteristics of self-excited noise generation. The flame in the annular jets has various flame shapes, including ring shaped, near conical shaped, crown shaped, and oscillating crown shaped depending on equivalence ratio, mixture velocity and air velocity. The regimes of the existence of these flames were identified. Self-excited noise with an SPL of about 70 dB was
CRediT authorship contribution statement
Seong-Ho Jin: Methodology, Writing - original draft, Investigation, Writing - review & editing. Jae-Hoon Joung: Investigation. Suk-Ho Chung: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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