Time resolved flowfield, flame structure and acoustic characterization of a staged multi-injection burner

doi:10.1016/j.proci.2008.06.139

Proceedings of the Combustion Institute

Volume 32, Issue 2, 2009, Pages 2965-2972

https://doi.org/10.1016/j.proci.2008.06.139 Get rights and content

Abstract

The present work details the analysis of staging and multipoint injection effects on the flow field, flame structure and acoustic properties of a lean swirled injector, representative of a gas turbine burner. The study is carried out for a constant power and global equivalence ratio while the staging parameter α between pilot and multipoint stages is varied. Using PLIF-OH, it is shown that the flame structure consists in a large central recirculation zone of burnt gases surrounded by an annular conical jet of fresh gases. The OH spatial distribution indicates that increasing α significantly modifies the flame structure, which becomes more compact and more organized while the mean flame front angle increases. Using High Frequency PIV in the reactive situation, a temporal analysis of the velocity field is carried out, indicating that large coherent structures appear periodically within the combustion chamber. The structure frequencies are determined and compared with acoustic measurements. This comparison indicates that for α = 0%, the vortex frequency is slightly lower than the first eigenmode of the chamber, while for α = 30%, it corresponds to a secondary peak at a higher frequency of the spectrum for both p′ and q′, with dramatic consequences on the burner behavior in terms of stability. When α increases starting from zero, the level of p′ and q′ increases to reach a maximum value for α = 20% before decreasing for higher staging. This acoustic behavior can be compared with the evolution of the temperature in the injector, that also reaches a maximum value for α = 20%, proving that strong instabilities are associated with flame stabilization within the injector. Increasing the staging factor makes it possible to decrease the flame instability level while keeping the flame compact and robust.

Introduction

Lean premixed combustion devices have been widely developed to reach low NO_x level in industrial applications. However, in this combustion regime, strong instabilities may occur leading to mechanical damages or flame extinction [1], [2]. At the design stage, several solutions have already been proposed to improve flame stabilization in the combustion chamber. First, swirled flows permit to enhance the combustion process by improving the mixing rate between fuel and oxidant streams and the flame stabilization itself through the swirl-induced recirculation of hot products near the nozzle [3]. Nevertheless, swirling flames often feature large periodic coherent structures, which play a key role in the flame dynamics. Their interactions with acoustic modes of the combustor may cause undesirable thermo-acoustic combustion instabilities. Two other ways to improve flame holding or ignition is to use secondary fuel injections and fuel staging procedures, which can be efficient even when supplying only a small amount of fuel. In addition, it has been shown that for some conditions, this secondary fuel injection can reduce combustion instabilities [4]. However, its efficiency is strongly dependent on its location [5], the operating conditions and combustor geometry [6]. This is why, in order to achieve an efficient control of the combustion process, a detailed understanding of the mechanisms associated with flame stabilization and dynamics is necessary.

In the present work, an analysis of the combustion of a gas-turbine multipoint staged swirled injector is proposed. The effects of staging and multi-injections on the flame structure, on the reactive flow fields and on the burner acoustic behavior are detailed. The flame structure is first investigated by classical measurements of OH Planar Laser Induced Fluorescence (PLIF). The acoustic behavior of the burner is studied using microphones and a photomultiplier. High Frequency Particle Image Velocimetry (HFPIV) has been used to determine the velocity field characteristics and dynamical behavior. To our knowledge, this kind of measurements has never been done before in reactive, close to industrial applications, configurations. This information is usually available at a rate of about 10 Hz, which makes it possible to obtain statistical quantities only, or phase-locked data, when a periodic phenomenon is studied. For the last two decades, the development of more powerful PIV algorithms has considerably improved the efficiency of this diagnostic and increased its spatial resolution [7], [8]. Nevertheless, to better understand highly turbulent swirled flows, which are unsteady by nature, high-speed PIV is necessary. Thanks to recent technical improvements, commercial PIV systems working at 1 kHz are already available. If these systems can help in understanding phenomena occurring in many unsteady flows, they are not fast enough to analyze the behavior of strongly turbulent flames, whose study requires a higher acquisition rate. Recently, PIV systems operating at an acquisition rate up to 20 kHz have been developed to study the wake of bluff bodies [9] or to obtain velocity fields for both cold and hot flows [10]. The PIV system used in [9] consists in a copper vapor laser and a dump camera. This induces a heavy work of post-processing and seems difficult to adapt to the acquisition of large series of velocity fields. In the present paper, the combustor flow field is studied using a PIV system working at 12 kHz, to carry out the analysis of the interactions existing between the swirled flow field, the pressure oscillations and the heat release process.

The experimental burner and diagnostics are first described. Then the effects of staging on the flame structure, flow field and acoustical behavior are successively presented and discussed.

Section snippets

Experimental setup

Figure 1 shows a schematic diagram of the experimental injector. The setup is composed of a two-staged swirled injector supplied with propane and air. The primary stage consists in a central duct fed with pure fuel and a swirler (18 vanes, swirler angle: 42°) supplied with air. The fuel and air are mixed in a specific zone downstream the injections of propane and air. Fuel is delivered in the secondary stage by 15 holes (Ø = 0.8 mm), which are located on a circular hollow part fed with propane.

Operation map of the burner

To analyze the influence of fuel injection on combustion, the staging factor, α, which is the fuel percentage injected through the primary stage, is varied between 0% and 100%, while the power, P, and the airflow rate, Q_air, are kept constant, respectively equal to, 74 kW and 105 Nm³/h. This implies that the global equivalence ratio, Φ, is kept at a constant value: 0.67. Figure 2 presents the evolution of the primary and secondary stage equivalence ratios Φ_p and Φ_s and the injector temperature, T

Flame structure

To visualize the flame, qualitative OH PLIF images are acquired in the axis of the combustion chamber, close to the injection plane. Figure 3 shows typical OH single shot LIF distributions (left) and averaged OH LIF images (right). These images display the region x^∗ = [0; 1.2] and y^∗ = [−0.65; 0.65], with x^∗ = x/D and y^∗ = y/D, x and y being, respectively, the axial and vertical directions and D the diameter of the outlet divergent. Averaged images are obtained by processing 200 instantaneous images.

Mean velocity fields in reactive cases

An analysis of the flow field within the combustor is performed using HFPIV measurements, acquired on the axis of the combustion chamber. A rectangular observation area (0 ⩽ x^∗ ⩽ 0.4 and 0 ⩽ y^∗ ⩽ 0.4) permits to visualize the distribution of velocity between the top and the bottom parts of the chamber very close to the injection plane. Unfortunately, no HFPIV measurements could be carried out for case A₂₀ because the instability level was so high that it caused seeding troubles that could not be

Dynamical behavior of the burner

The principal advantage of HFPIV is its high acquisition rate that makes a temporal analysis of the velocity field evolution possible. Case A₀ is first studied. A succession of six instantaneous velocity vectors and their associated vorticity fields are displayed on top of Fig. 6. Only the top part of the combustion chamber is shown. This set of frame has been chosen because it is representative of the phenomena observed. The velocity field dramatically changes from one image to the following

Effects of staging on combustion instabilities

Pressure oscillations and CH^∗ emissions are simultaneously recorded for cases A₀, A₂₀ and A₃₀. CH^∗ emissions can be divided into two parts, a mean part and a fluctuating part, which can be considered at the first order as proportional to the heat release oscillations, q′ [14]. Only this second part is analyzed here. To determine the oscillation frequency, the PSD of the microphone located in the semi-length of the chamber and PMT signals are calculated and reported in Fig. 7.

For case A₀ (solid

Concluding remarks

The present work details the analysis of staging effects on the flow field, flame structure and acoustics of a lean swirled injector, representative of a gas turbine burner. The study is carried out for a power of 74 kW, a global equivalence ratio of 0.67 and values of staging varying between 0% and 30%. First, PLIF-OH is applied to visualize the flame structure that consists in a central recirculation of burnt gases and a surrounding annular conical jet of fresh gases. The OH spatial

Acknowledgments

Séverine Barbosa benefits from a Ph.D. grant from Délégation Générale pour l’Armement (DGA) and this work is supported by SAFRAN/SNECMA through the INCA initiative.

References (16)

S. Candel
Proc. Combust. Inst.
(2002)
N. Syred
Prog. Energy Combust. Sci.
(2006)
J.G. Lee et al.
Proc. Combust. Inst.
(2000)
J. Shinjo et al.
Combust. Flame
(2007)
B. Higgins et al.
Fuel
(2001)
A. Nauert et al.
Exp. Fluids
(2007)
G.M. Choi et al.
Proc. Combust. Inst.
(2004)
R.J. Adrian
Annu. Rev. Fluids Mech.
(1991)

There are more references available in the full text version of this article.

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Time resolved flowfield, flame structure and acoustic characterization of a staged multi-injection burner

Abstract

Introduction

Section snippets

Experimental setup

Operation map of the burner

Flame structure

Mean velocity fields in reactive cases

Dynamical behavior of the burner

Effects of staging on combustion instabilities

Concluding remarks

Acknowledgments

Proc. Combust. Inst.

Prog. Energy Combust. Sci.

Proc. Combust. Inst.

Combust. Flame

Fuel

Exp. Fluids

Proc. Combust. Inst.

Annu. Rev. Fluids Mech.