Experimental study of the combustion characteristic of circular transverse fuel jet in crossflow

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Abstract

Experimental studies of the combustion process of circular transverse methane jets in crossflow are carried out both at stable combustion conditions and approaching flameout. It is found that under stable combustion, a steady recirculation zone is formed downstream of the jet, and the mixing coefficient is basically constant under different conditions. Thus, the excess air coefficient in the recirculation zone is also basically constant, and the combustion temperature and combustion efficiency almost don't change with the overall excess air coefficient, especially in the back half of the recirculation zone, within the experiment extent. Under stable combustion, high combustion efficiency is observed at a short distance downstream of the jet. When the flame approaches to extinguished, the recirculation zone cannot exist steady, resulting in a sharp increase of the excess air coefficient and a sharp decrease of the combustion temperature in the combustion zone. Approaching flameout, the flame oscillates distinctly, dissipates heat and energy during the oscillation process, then goes out.

Introduction

The increasing requirements for combustor or afterburner in advanced multi-purpose power systems in the future include providing low aerodynamic resistance and high combustion efficiency in wide operating conditions [1], [2]. One of the possible solutions to meet these requirements is using a circular transverse jet as the flame stabilizer and fuel/air mixer, which has several advantages compared to traditional methods of organizing the working process in the combustor. Namely, low aerodynamic resistance, a big possibility of regulation, and great potential for high combustion efficiency [3], [4].

As one kind of special transverse jet, the circular transverse jet is issued from an axisymmetric circular gap on a tube, located at the center of the mainstream, as reported by Li. et al. [5]. Due to the wide application of transverse jet in steam pumps, gas turbines, combustion chambers, industrial burners, and jet components in certain automatic control systems [6], [7], [8], transverse jet in crossflow has been widely studied by researchers [9], [10], [11], [12]. Brzustowski T.A. [13], [14], Hasselbrink E [15], [16], and Bandaru [17] carried out in-depth studies on combustion flames of transverse jets, using straight-tube burners and different fuels (ethylene, propane, methane, and CO-hydrogen mixtures) to study the flames length, emission characteristics, and in-flame soot characteristics. A very wide range of velocity ratio (4 < R < 37) and momentum flux ratio (9 < q < 940) were explored in these studies, and the researchers observed that the flame lengths in the transverse jet were shorter than that of a comparable free jet flame, which indicates that the suction and mixing of the transverse jet is stronger than that of the free jet, which is consistent with the work of another group of researchers [14], [18]. Relatively higher unburned hydrocarbon and CO emissions were measured in cross-jet flame compared to that in quiescent environments, possibly due to fuel being swept out from the jet in the nearfield. Nitrogen oxide (NOx) emissions in transverse jet flames basically follow the same trend as in free jet flames [17].

The above studies on reactive transverse jets are all carried out under the conditions that the cross-flow velocity is very low. The research on reactive transverse jet in high-speed incoming flow, mainly focuses on the supersonic incoming flow with the ramjet and scramjet as the application background. At this time, the transverse jet is mainly used as a form of fuel injection, but not as the primary flame stabilizer under the single-hole or porous transverse jet, and the stability of the flame is mainly realized using concave cavity, plate, backward steps, etc. [19], [20].

The stabilization of flames in high-velocity flow is of significant practical importance and high relevance for an understanding of combustion processes. If we do not consider auxiliary devices such as pilot flames or external ignition sources, there are two major mechanisms left for anchoring and stabilizing flames: Flame propagation and autoignition [21]. In many cases, both mechanisms contribute to flame stabilization. But, for the combustion with low stagnation temperature, flame propagation is mainly used because it is difficult to reach the temperature required for autoignition. At this time, it is generally necessary to stabilize the flame by creating a recirculation zone or a shear layer. Micka and Driscoll [22] used high-speed photography to study the influence of fuel injection position on the stable combustion area under the condition of Mach number 2.2 at the inlet of the combustion chamber, and observed two flame stabilization modes: cavity stabilized combustion mode (at low stagnation temperature) and jet-wake stabilized combustion mode (at large stagnation temperature). Rasmussen et al. [23] explored the flame stabilization mechanism of ethylene injection at the bottom wall and rear edge of the cavity through PLIF and pointed out that high-temperature products in the cavity play an important role in flame stabilization. Zhang [24] in his research observed the flame stabilization modes are the typical cavity shear-layer stabilized combustion mode and the lifted-shear layer stabilized combustion mode, but they are difficult to distinguish by rule and line. Yueming Yuan [25] experimentally found that the combustor operates in a scramjet mode when the flame is stabilized in the cavity shear layer, and in a ramjet mode when the flame is in the jet-wake. The flame oscillation mode, observed only for a narrow range of Φ, was found to correlate with the combustor transition between the scramjet and ramjet modes. Kun Wu [26] studied the multiple flame stabilization modes of DLR hydrogen-fueled strut injection supersonic combustor and found that at relatively low stagnation temperatures, the flame is stabilized in an “attached flame” mode, which requires a low-speed recirculation zone behind the strut for radical production and a high-speed intense combustion zone for heat release, while at relatively high stagnation temperatures, the flame is stabilized in a “lifted flame” mode, in which the effect of the low-speed recirculation zone is negligible, rendering most reactions take place in supersonic flow, and at intermediate stagnation temperatures, blow-out was always observed and flame cannot be stabilized in the combustor even with initially forced ignition. Ye Tian [27] investigated the combustion and flame stabilization modes in a hydrogen fueled scramjet combustor by experiments and numerical simulations with the inflow conditions of Mach number of 2.0, static temperature of 656.5 K, and static pressure of 0.125 MPa, and find that the combustion mode of the reacting cases could be divided into two typical modes, the combustion mode was scramjet mode when the equivalence ratio of hydrogen was less than 0.23, and that was ramjet mode when the equivalence ratio of hydrogen was not less than 0.23. The flame stabilization mode of scramjet mode was cavity shear layer stabilized combustion, and that of ramjet mode was combined cavity shear layer/recirculation stabilized combustion. Wang [28] observed three flame stabilization modes: cavity assisted jet-wake stabilized combustion mode, cavity shear-layer stabilized combustion mode, and combined cavity shear-layer/recirculation stabilized combustion mode. It was found that the jet-wake stabilized combustion could not be achieved without a cavity.

Annular jet, opposed jet and circular transverse jet can be used as the flame holder without the assistance of cavity or strut, etc. Fabio J.W.A. Martins [29] experimentally studied the characteristics of the complex flow fields that arise from annular turbulent non-reacting and reacting jets dispersed through the SPP1980 SpraySyn-burner. Juan Zhang[30] investigated three various annular jet nozzles for injection of the sonic hydrogen jet at supersonic air crossflow with Mach-4, and found that the nozzle with 3-lobe configuration has 25% more fuel mixing performance than other configurations and the mixing performance of annular lobe-injector is about 15% more than simple one for cases with 2-lobe and 4-lobe injectors. Luis Cifuentes[31] investigated the local entrainment velocity of the enstrophy interfaces of a methane-air turbulent premixed turbulent annular jet flame stabilized on a bluff-body burner using a high-fidelity flame-resolved three-dimensional simulation. But the flame stabilization with annular jets has big instability. Karol Wawrzak [32] numerically studied the annular non–swirling jets by LES, revealed the formation of the spiral structures located in the inner and outer mixing layers and found that the structures are the result of the instability that leads to the precession of the recirculation region formed in the near-field.

Shaffer and Cambel were the first to study the technology of using opposed jet flow as a flame stabilization. They achieved flame stabilization by ejecting air jet in the opposite direction of high-speed incoming flow, and published research results on the flame stabilization mechanism and extinction boundary of opposed jet in a series of papers [33], [34], [35]. E. Masterakos et al. [36] studied the influence of dilution of combustion products on reactants and preheat on flame stability. It is found that there is a critical temperature of about 1550 K. When the combustion products are higher than 1550 K, the equivalence ratio of 0.2 can still stabilize the combustion, while below this temperature, the chemical reaction cannot sustain itself. B. Bohm et al. [37] studied the characteristics of turbulent opposed jet flame during transient flameout by using PIV and PLIF methods at the same time, and found that vortex structures would accumulate near the flame during flameout, and high-stress areas would be generated near the flameout area. But, in the research of Kosterin V.A. [38], it was found that the flameout boundary of the opposed jet flame stabilizer was very sensitive to the inclination angle of the incoming flow, and the flame stability was insufficient.

While, the axisymmetric circular transverse jet can be used as a stable flame stabilizer and fuel injector in crossflow without any auxiliary devices like cavity or strut, which have already been reported in the research works of Li. et al. [3], [4], [5]. At present, results of computational and experimental studies of the gas dynamics of flow and the mixing processes when axisymmetric circular transverse jets interact with the crossflow [3], [4], [5], [39], [40] are published. However, the research on the characteristics and laws of stable combustion and approaching flameout, as well as the influence of the flow field structure and the composition distribution in the combustion zone on the combustion characteristics, is not sufficient.

The goal of this paper is to study the combustion mechanism and characteristics of circular transverse methane jet in the mainstream at stable combustion conditions and unstable combustion conditions. The main task is to reveal the characteristics of flame stabilization and flameout of the circular transverse jet in the mainstream, explore the influence of the overall excess air coefficient on the temperature and combustion efficiency of the combustion zone, summarize the distribution and correlation relationship of the temperature, combustion efficiency and local excess air coefficient in the combustion zone, and analyze the mechanism of the distribution characteristics.

Section snippets

Materials and methods

The test system and structure of the studied circular transverse jet injection device are described in reference [3], [4]. The main geometric parameters of the test section and the flow-field scheme are shown in Fig. 1.

The component of the cross mainstream is air, while the jet is methane. The operating parameters of the mainstream and jet provided by the test system are shown as follows:

– maximum flow rate of the mainstream 0.12 kg/s;

– maximum flow rate of the jet 1.2 g/s;

During the test, the

Experimental results

In the cases indicated in Table 1, the temperature and concentration of main components of the combustion products at these measuring points of the core combustion zone were obtained.

The distributions of temperature in the core combustion zone of 4 different cases are obtained, as shown in Fig. 4 (a – case 1, b – case 2, c – case 3, d – case 4).

In all cases, the temperature reaches its maximum value on the symmetric axis of the combustion zone and decreases along the radial direction.

Conclusions

Based on the experimental results obtained, the following conclusions can be obtained within the range of these experimental parameters:

1. At the stable combustion conditions, the temperature and combustion efficiency in the core combustion zone is uniformly distributed, and change slightly along with the overall excess air coefficient αall; when close to flameout, the temperature and combustion efficiency in the core combustion zone decrease sharply.

2. Under stable combustion conditions, a

CRediT authorship contribution statement

Investigation and writing, Z. L. and Y.Y., review and editing, V.L.V., W.Y., H. Z., W. L. and P.H.D. All authors have read and agreed to the published version of the manuscript.

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.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (NO. 50876104), and the Chinese-Russian cooperation and exchange project of the Chinese Academy of Sciences.

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