Autoignition in stratified mixtures for pressure gain combustion [original]

Fatma Cansu Yücel, Fabian Habicht, Myles D. Bohon, Christian

Oliver Paschereit

Autoignition in stratified mixtures for pressure gain

combustion

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Citation details

Yücel, F. C., Habicht, F., Bohon, M. D., & Paschereit, C. O. (2021). Autoignition in stratified mixtures for

pressure gain combustion. In Proceedings of the Combustion Institute (Vol. 38, Issue 3, pp. 3815–3823).

Elsevier BV. https://doi.org/10.1016/j.proci.2020.07.108.

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Autoignition in Stratified Mixtures for Pressure Gain Combustion

Fatma Cansu Y¨

ucela,∗, Fabian Habichta, Myles Bohona, Christian Oliver Paschereita

aTechnical University of Berlin, Institute of Fluid Dynamics and Technical Acoustics, M¨uller-Breslau-Strasse 8, Berlin and 10623, Germany

Abstract

The reliable generation of quasi-homogeneous autoignition inside a combustor fed by a continuous air flow would

represent a milestone in realizing pressure gain combustion in gas turbines. In this work, the ignition distribution in-

side a stratified fuel–air mixture is analyzed. The ability of precise and reproducible injection of a desired fuel profile

inside a convecting air flow is verified by applying tunable diode laser absorption spectroscopy in non-reacting mea-

surements. High-speed, static pressure sensors and ionization probes allow for simultaneous detection of the flame and

pressure rise at several axial positions in reactive measurements with dimethyl ether as fuel. A second, exchangeable

combustion tube enables optical observation of OH∗intensity in combination with pressure measurements. Experi-

ments with three arbitrary fuel profiles show a set of ignition distributions that vary in shape, homogeneity, and the

number of simultaneous autoignition events. Although the measurements show notable variation, a significant and

reproducible influence of the fuel injection on the ignition distribution is observed. Results show that uniform au-

toignition leads to a coupling of the reaction front with the pressure rise and, therefore, induces a greater aerodynamic

constraint than non-uniform ignition distributions, which are dominated by propagating deflagration fronts.

Keywords:

homogeneous autoignition, fuel stratification, pressure gain combustion, shockless explosion combustor

∗Corresponding author:

Email address: [email protected]

(Fatma Cansu Y¨

ucel)

Preprint submitted to Proceedings of the Combustion Institute May 16, 2022

1. Introduction

Replacing the conventional isobaric combustion of the

gas turbine cycle by pressure gain combustion is a

promising concept to achieve improvements in cycle ef-

ficiency. Different approaches have been investigated,

such as pulse detonation combustion (PDC) [1] and ro-

tation detonation combustion (RDC) [2]. Both concepts

utilize propagating detonation wave(s), the high tem-

peratures and pressures of which present significant en-

gineering challenges. An alternative approach, called

shockless explosion combustion (SEC) [3], overcomes

these challenges by using quasi-homogeneous autoigni-

tion to achieve approximately constant volume combus-

tion without the presence of a detonation wave or me-

chanical constraints.

Homogeneous autoignition occurring in well mixed

combustible mixtures was originally referred to as ther-

mal explosion by Zel’dovich [4] as a concept of sponta-

neous flames, leading to an increase in pressure sim-

ilar to constant volume combustion. However, non-

uniformities in the mixture, such as variations in tem-

perature, pressure or equivalence ratio, can cause devi-

ations in ignition delay time and result in a propagating

autoignition front instead. The propagation velocity of

this autoignition front uai is inversely proportional to the

spatial gradient of the ignition delay time τai. Assum-

ing constant temperature Tand pressure pacross the

mixture, the ignition delay time τai is a function of the

equivalence ratio ϕsuch that uai can be expressed as:

uai = ∂τai

∂x!−1

= ∂τai

∂ϕ

∂x!−1

.(1)

Comparing uai to the speed of sound aleads to the di-

mensionless parameter ξ, where

ξ=a

uai

=a∂τai

∂ϕ

∂x.(2)

This allows for the description of different combustion

modes [4, 5]. A subsonic propagation of the reaction

front occurs when ξ > 1. For ξ=1 the propagation

velocity of the autoignition front is equal to the speed

of sound, allowing amplification by coupling and en-

abling deflagration-to-detonation transition (DDT). The

ideal process of thermal explosion, which occurs for

ξ=0, is not practical in experiments due to unavoid-

able perturbations of the initial conditions and mixture

inhomogeneity, leading to gradients in reactivity. How-

ever, ξ < 1 results in a quasi-homogeneous autoignition

leading to an approximate constant volume combustion

while avoiding DDT. Similar behavior in pressure rise

quasi-homogeneous

refilling of the tube

burned gas

stratified fuel-air mixture

burned gas

air-buffer quasi-homogeneous autoignition

p>p0

τ=const.

expansion wave

pressure wave

autoignition

Figure 1: Sketch of the single tube SEC test rig

can be observed in the case of multiple separate igni-

tion sources that ignite quasi-simultaneously [6]. This

regime of quasi-homogeneous autoignition and/or many

distributed ignition points is the objective for the imple-

mentation of SEC.

Along these principles, the realization of homoge-

neous charge compression ignition (HCCI) has been in-

vestigated intensively in the past [7]. Preventing engine

knock is a major challenge in HCCI and has been ad-

dressed by applying different methods, such as modifi-

cation of oxidizer and fuel properties, equivalence ra-

tio, exhaust gas recirculation, or engine parameters [8].

However, the implementation of this concept in a gas

turbine cycle for pressure gain combustion is a new ap-

proach.

The SEC is based on a periodic combustion process

as sketched in Fig. 1. The cycle begins with a strati-

fied, autoignitable fuel–air mixture throughout the com-

bustor (Fig. 1a). This stratification has been tailored

to compensate for the gradient in residence time, re-

sulting in a quasi-homogeneous autoignition (Fig. 1b).

The pressure rise during combustion induces a pressure

wave propagating downstream which is reflected as an

expansion wave when reaching the acoustically open

combustor outlet (Fig. 1c). When this wave reaches the

combustor inlet, the refilling process begins (Fig. 1d)

and the cycle restarts.

The objective of this work is to investigate the igni-

tion processes within a stratified fuel–air mixture. First,

the ability of injecting a defined mixture profile in a con-

vecting air flow is analyzed. Subsequently, the homo-

geneity of measured autoignition times and pressure rise

as a function of the fuel stratification are examined. Fi-

nally, the processes of autoignition homogeneity, flame

propagation, and process variability are studied in high-

speed images of OH∗chemiluminescence.

injection station

vaporizer

pressure regulator

FAT1P1P2P3P4P5

I1I2I3I4I5I6I7

preheater restriction convection tube combustor exhaust tube

solenoid valve

optical access combustor

1 2 34

Figure 2: Sketch of the test rig. Sensors: low-speed, static pressure sensors (FA, FF), thermocouples (T1, T2), high-speed, static pressure sensors

(P1–P5), ionization probes (I1–I8). The inset subfigure b) shows the exchangeable version of the combustor tube with optical access.

2. Experimental Setup and Measurement Procedure

A sketch of the test rig for the experimental investiga-

tion of autoignition of an axially stratified fuel–air mix-

ture in a convecting flow is shown in Fig. 2. The test

rig is composed of several sections, including reactant

injection, convection (0.5 m), combustor (0.5 m), and

exhaust (1 m) sections. All sections have an inner di-

ameter of 40 mm. The rig was originally designed by

Bobusch et al. [3, 9] and later used by Reichel et al. [10].

However, in these works, reproducibility was limited

and consistent homogeneous autoignition was difficult

to achieve. The control of the injection process has since

been improved by Y¨

ucel et al. [11].

In the reacting cases, a preheater is used to raise the

temperature of the constant air flow to 1023 K measured

at T1. Downstream of the preheater, the air flow is

forced through a restriction in order to prevent back-

flow of hot gases into the preheater due to ignition.

Fuel is injected via ten radial ports with 1 mm diam-

eter each, which are individually controlled by high-

speed solenoid valves (Staiger VA 204-716). A dome-

loaded pressure regulator (Swagelok RD6) is installed

upstream of the injection station to control the fuel

supply pressure. Two static pressure sensors FAand

FF(Festo SPTW) are installed to monitor the air and

fuel supply pressures. The fuel injection duration is

∆tinj =50 ms, and is divided into ten time windows,

each with a length of 5 ms. The number of open valves

is individually set for each time window defining the in-

jected fuel profile.

The modular setup allows for exchanging the stain-

less steel combustor for a quartz tube (Fig. 2b) in order

to achieve optical access. This configuration is used for

fuel concentration measurements and OH* chemilumi-

nescence imaging of the ignition distribution.

2.1. Fuel Concentration Measurements

Fuel concentration measurements using near-infrared

tunable diode laser absorption spectroscopy (TDLAS)

are conducted as proposed by Li et al. [12]. These

measurements are used to validate the control of the

injection geometry to achieve a desired mixture pro-

file within a defined time frame. This technique has

been used previously for time-resolved fuel concentra-

tion measurements in a similar configuration [10, 13].

While the combustion experiments in this work are con-

ducted with dimethyl ether (DME) as fuel, the concen-

tration measurements are done with methane to match

the absorption features around a wavelength of 1654 nm

utilizing the available laser. In the scope of this work,

we expect the variation in the injected mixture fraction

profile to be primarily controlled by turbulent diffusion

and mixing. Since turbulent fluctuations scale with the

Reynolds number, it is considered as the dominant mix-

ing parameter rather than molecular diffusion (which

is of the same order for both fuels). However, when

comparing the Reynolds numbers for the non-reacting

(approx. 48000) to reacting (approx. 6000) cases, it is

expected that the non-reacting cases will exhibit much

greater turbulent diffusion and a blurring of the mixture

profile. Lastly, the residence time is kept constant for all

measurements by matching the flow velocities, allowing

an equal amount of time to diffuse. Considering this, it

is reasonable to conclude that for the reacting cases the

resulting gradients are steeper than TDLAS measure-

ments reveal. While this prevents quantifying the exact

local equivalence ratio for the reacting DME cases, it

does allow for a qualitative measure of the reproducibil-

ity and accuracy of the injection scheme.

2.2. Reactive Measurements

For reactive measurements, the initial temperature is

monitored via two Type-K thermocouples. Five water-

cooled, high-speed pressure sensors are installed in the

combustor with a distance of 100 mm to record the static

pressure variation. The flame is detected via 8 ioniza-

tion probes that are mounted in the combustor and the

exhaust tube. Figure 2 shows the naming convention for

each sensor.

The temperature at the injection station remains con-

stant during the measurements. At the beginning of each

measurement, a gradient in wall temperature of about

50 K between sensors T1and T2is observed. Heat-

ing during the run increases T2by approximately 50 K.

However, the measurement data show no correlation be-

tween the ignition time throughout the measurement re-

gion and the measured wall temperature. Therefore, it

is reasonable to assume the impact of the transient wall

temperature on the ignition process to be negligible.

DME is used as fuel resulting in ignition times in

the range of 60 ms to 80 ms for the applied conditions

(p=1 atm, T=1023 K and 1 ≤ϕ≤2). This assures

autoignition of the convecting mixture inside the com-

bustor. The fuel supply pressure is FF=5.7 bar and the

equivalence ratio is controllable from ϕ=0 (all valves

closed) to ϕ=2 (ten valves open). The average fuel

mass flow rate was measured under steady state con-

ditions using a Coriolis mass flow meter. To assure a

gaseous state, the fuel is vaporized and guided through

a heated pipe (330 K) before injection.

The ignition behavior of DME is characterized by a

negative temperature coefficient (NTC) region, which

is studied in more detail by Burke et al. under high

pressure conditions [14]. However, calculating the rel-

evant ignition delay times with Cantera [15] for a zero-

dimensional constant volume reactor using the mech-

anism AramchoMech2.0 that has been validated for

DME-kinetics in previous works, reveal that all tests

were conducted outside the NTC region of DME. Op-

erating in the NTC region of DME would require ac-

counting for additional non-linear behavior of DME au-

toignition, and is therefore avoided.

The optically accessible section is composed of a se-

ries of four quartz tubes, each 120 mm long, supported

by stainless steel flanges fitted with one pressure sen-

sor each. An optical band-pass filter (CWL =310 nm,

FWHM =10 nm), an intensifier (Lambert Instruments

HiCATT) and a high-speed camera (Photron Fastcam

SA-Z) are used to detect the reaction zones by light

emission of OH∗intensity. The recorded high-speed im-

ages allow for observation of the ignition distribution at

87500 fps and a spatial resolution of 2.7 px/mm.

3. Results and Discussion

The results will be broken into two sections. First, the

control of the fuel injection profile will be investigated,

and three example contours will be discussed. The sec-

ond section will then examine the autoignition charac-

teristics of these profiles, focusing on the pressure rise

(as representing aerodynamic confinement) and corre-

0 20 40 60

tin ms

number of open valves

a) valve commands

0 20 40 60 80

tin ms

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

XCH4

b) measured injection curves

V-curve

Λ-curve

u-curve

Figure 3: Injection curve commands for V-, Λ- and u-curve (a) and

measured methane concentration 50 mm downstream of the combus-

tor inlet averaged over 150 cycles (b).

late the variation in pressure rise with direct observa-

tions of the homogeneity of autoignition.

3.1. Fuel Injection

The mass flow rates of fuel and air are set to match

the mixture bulk flow velocity for reacting experiments

(ubulk =18 m/s). Each control sequence is injected for

150 cycles with an operating frequency of 5 Hz. Three

different injection profiles are investigated at ambient

pressure and temperature: (i) Λ-curve, (ii) V-curve and

(iii) u-curve. The control sequences and the respec-

tive TDLAS measured, cycle averaged fuel concentra-

tion for the three trajectories are shown in Fig. 3.

The averaged results clearly show the capability of

replicating a desired fuel profile within the given time

span ∆tinj. The measured fuel profiles of individual cy-

cles show a standard deviation (std) of less than 5 %

throughout the injection. There is a clear smoothing ef-

fect, especially in the regions of high gradient. This is

expected and can be attributed to two effects: (i) turbu-

lent diffusion and (ii) shear layer effects. Diffusion will

smooth the sharp features of the injection profile (begin-

ning and end of injection period). The shear layer effect

near the wall causes a variation in the velocity profile

through the tube and induces a spatial distortion of the

injection profile. This phenomenon can only be mea-

sured as an integrated value across the tube through the

line-of-sight measurement. It is also important to men-

tion that due to inertia of the valves, there is a hysteresis

to the valve response when opening and closing, the ef-

fects of which are difficult to account. Currently, there

is no way to avoid these effects, and must instead be

accounted for when interpreting the reacting results.

For reacting tests, it is important to maintain a con-

stant cycle-averaged fuel flow rate, otherwise differ-

ences in pressure rise might occur due to variations in

total heat release. For this, the total valve-open time

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