Flame structure and fuel reaction on a non-premixed shuttlecock-like conical burner

Highlights

• The structure of a shuttlecock is applied for a new burner design.

• A novel burner is presented to enhance the efficiency of non-premixed combustion.

• The designed burner naturally introduces air co-flow to replace non-premixed combustion.

• The mixing mechanisms of a shuttlecock-like conical burner have been investigated.

• The reaction characteristics of a shuttlecock-like conical burner have been examined.

Abstract

To promote the efficiency of non-premixed combustion, we designed slot structures with a simple geometry on the side of a conical burner named a shuttlecock-like conical burner. The slot structure which can introduce the air co-flow naturally into the cone instead of using other and complicated geometry is inspired from the particular shape of a shuttlecock serving as a missile in badminton sport. Therefore, the air co-flow not only produces a recirculation zone on the edge of the burner but also moves directly into the burner. The combustion condition transforms from non-premixed to partially premixed combustion; the mixing and reaction of fuel and oxidant are hence greatly enhanced. We observed the structures of the flame and the flow of a shuttlecock-like conical burner, and examined its reaction characteristics with measurements of the chemiluminescence of OH* and the flame temperature. The observation of an isothermal flow field demonstrates that the air co-flow moves directly into the burner via the slots. Double pairs of vortex structures are observed above the exit of the burner because the introduced air co-flow decreases the strength of the outer vortex, allowing the central jet to form an inner vortex. The reaction zone of a shuttlecock-like conical burner hence approaches not only the central region of the flame but also the burner exit. The reaction intensity of the fuel of a shuttlecock-like conical burner is also stronger than that of an ordinary conical burner (without the slots). The designed shuttlecock-like conical burner is significantly useful to serve as a reference to improve the combustion efficiency of non-premixed combustion.

Introduction

A traditional shuttlecock that serves as a missile between players of badminton has a conical shape and gap structures, formed by feathers inserted into a cork base. Those special structures, the conical shape and gaps, induce interesting aerodynamic phenomena in the wake region when an air current passes through a shuttlecock. The effects of the conical shape and gaps on the aerodynamic characteristics of a shuttlecock have been widely investigated [[1], [2], [3], [4]]. Cooke [1] examined the aerodynamics of a shuttlecock when an air current passed through it; the air jet through the gap structure interacted with the outer flow, forming an unsteady and irregular wake pattern. The outer flow tended to curl inward, while the inner air jet tended to curl outward. Those curl structures impacted each other in the wake; the air jet dissipated downstream behind the wake. Lin et al. [2] investigated the effects of gaps on the flow properties of a conical structure; the gaps caused the introduction of an air jet that diminished the wake of recirculation type behind the bluff body. The effect of a bluff body hence decreases with a larger gap structure. Kitta et al. [3] and Hasegawa et al. [4] visualized the flow pattern of the wake region of a shuttlecock; they found that the flow passed the gaps and proceeded downstream along the axis of a shuttlecock when it had gaps; the flow through the gaps suppressed the roll-up vortex in the near region of the trailing edge of a shuttlecock. The strong voticies were maintained, however, even far downstream because of the entrainment of the fluid. A large vortex exists near the leeside of a shuttlecock with gaps, and typically sheds downstream, but a roll-up vortex is not observed when a shuttlecock lacks gaps. As the gap structures on a conical body maintain the strong vortices in the wake and allow an introduction of the co-flow into the interior of the cone, the mixing of fuel and air might be thereby enhanced when this structure is applied to the design of a combustor.

Non-premixed combustion is widely used in many practical combustion systems, such as gas turbines, boilers and furnaces. In a non-premixed flame, the reactants remain separated from the oxidant; both mixing and reaction occur only at the interface between the fuel and oxidant [5]. Flash-back phenomena hence do not occur during a non-premixed combustion process, but its combustion efficiency is less than that of premixed or partially premixed combustion because of a lack of oxidant or an insufficient mixing of fuel and oxidant. A recirculating flow is beneficial for the mixing of fuel and oxidant; a combustor is thus generally designed to form a recirculation zone [[6], [7], [8]]. According to the aerodynamic principle, a zone of low pressure induced by a bluff body or a swirl generator causes a reverse flow and a recirculation zone at the burner exit, which increases the mixing and extends the residence period of fuel and oxidant [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. The efficiency and stability of non-premixed combustion are thereby improved.

Other useful designs to form a recirculation flow include the diffuser [6,14,16], the conical burner [[17], [18], [19], [20], [21], [22], [23]], the double-concentric-pipe burner [24], and a new method using a circular transverse jet [25]. The cone shape of a conical burner is similar to a shuttlecock; the major difference between a conical burner and a shuttlecock is the gap structure. Domkundwar et al. [14] reported that the strength and size of the recirculation zone increase with an increasing angle of divergence; the swirl required to form the recirculation can be significantly decreased at a large angle of divergence. The pressure loss in the flow field hence was kept small. The strength and size of the reversal zone, however, decrease with a secondary mass flow passing through the walls of the diffuser and with combustion. Mansour designed a concentric-flow conical-nozzle burner (CFCN) [17] and concentric flow slot burner (CFSB) [18] in which the non-premixed fuel achieved a partially premixed condition through the spatial structure of the burner. For a CFCN burner, the flame stability was greatly affected by the cone angle because an increasing cone angle caused a large entrainment of air, breaking the core of stabilization and then impeding the flame stability [19]. Li et al. [20] experimentally and numerically studied a partially premixed flame in a conical burner; they found that the flame is stabilized inside the cone above the nozzle exit because of the propagation of the premixed flame front. The flow reverses near the wall of the cone, and depends on the cone angle. Mansour et al. [21] employed a fused-silica conical nozzle to undertake the measurement with particle-image velocimetry (PIV); they reported that a recirculation zone was produced at the near region inside the cone, heating the entrained air from the surroundings. The increasing reverse velocity increased the rate of spread, but decreased the intensity of turbulence inside the cone, and decreased the thickness of the boundary layer. When an air co-flow was applied to the CFCN burner [22], the added co-flow caused the formation of two vortices at the cone exit, resulting in the stabilization of the flame anchored to the nozzle tip; the reaction shifted toward a decreased turbulent intensity at the side of the cone. The stability of a flame is improved on adding a co-flow of air. The modified CFCN swirl burner improves flam stability and extends the blowout limits over a wide range of partial premixing levels [23]. As a conical burner with a simple flow geometry makes feasible the formation of a reversal flow structure, it is suitable to serve as a reference for the design of a burner for non-premixed combustion.

Accordingly, the aim of this work was to apply slot structures to an ordinary conical burner. We designed a shuttlecock-like conical burner that had slot structures on a conical body; those slots introduced a co-flow of air into the cone of the designed burner. The mixing condition of the fuel and the air co-flow thereby transformed naturally from a non-premixed to partically premixed. The simple geometry of the designed burner is adaptable for industrial applications to improve the efficiency of non-premixed combustion. The flame structure, flow structure, chemiluminescence of OH radical and distribution of flame temperature were experimentally examined and are discussed in what follows.