Ammonia Measurements with Femtosecond Two-photon

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Biofuels and Biomass

Ammonia Measurements with Femtosecond Two-photon Laser-induced Fluorescence in Premixed NH3/air Flames Jixu Liu, Qiang Gao, Bo Li, Dayuan Zhang, Yifu Tian, and Zhongshan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02121 • Publication Date (Web): 25 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Energy & Fuels

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Ammonia Measurements with Femtosecond Two-

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photon Laser-induced Fluorescence in Premixed

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NH3/air Flames

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AUTHOR NAMES Jixu Liu, a Qiang Gao, a Bo Li, a, * Dayuan Zhang, a Yifu Tian, a Zhongshan Li a, b

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AUTHOR ADDRESS

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a

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China

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b

Division of Combustion Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden

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KEYWORDS

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ammonia, femtosecond laser, two-photon laser-induced fluorescence, combustion.

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ABSTRACT: Ammonia (NH3), which can be a hydrogen-carrier, is a promising alternative to

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fossil fuels in the future carbon-free economy. In-situ techniques feasible for the remote and non-

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intrusive detection of NH3 will provide strong support for developing clean NH3 combustion. Here,

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we demonstrated a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for

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interference-free in-situ NH3 measurements in laminar premixed NH3/air flames. The two-head

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band of NH3 at ~565 nm was observed, which verifies the feasibility of fs-TPLIF for NH3

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measurements in a combustion environment. The single-shot NH3 fs-TPLIF images with efficient

2

signal-to-noise ratios (SNRs) were obtained. The variation of the thickness of the reaction zone

3

with the equivalence ratio of NH3 flames was also obtained. Due to the broad line-width of the

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femtosecond laser, the OH fluorescence was also observed together with the NH3 fluorescence.

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The potential of fs-TPLIF for simultaneous measurements of NH3 and OH with only one laser was

6

analyzed. The laser power dependence of NH3 and NH emissions were investigated. The radial

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distributions of NH3, NH, and OH in the flame were also discussed. This work is the first attempt

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of fs-TPLIF for NH3 measurements in a combustion environment, and the results indicate that fs-

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TPLIF is a feasible tool for NH3 measurements in combustion diagnostics.

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1. INTRODUCTION

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With the increasing energy demand and the ever-stringent restriction on conventional fossil fuels

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consumption, it is of significance to develop renewable fuels with low- or non-carbon. Ammonia

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(NH3), as a sustainable fuel, has recently attracted much attention with the direct combustion as

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the most efficient way for energy utilization. 1-4 Compared with other alternatives, NH3 has several

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advantages. Firstly, NH3 can be a hydrogen-carrier without carbon emissions, 5 whose utilization

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would largely mitigate the increasing global warming. Secondly, NH3 can be synthesized from

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renewable CO2-free energy sources, e.g., solar and wind energy,

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absence of carbon in its production process. Thirdly, NH3 has a high volumetric energy density

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and a low storage cost. 7 Due to the importance of NH3 as a fuel, it is highly required to understand

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the characteristics of NH3 combustion. As a consequence, it is an essential task to develop effective

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methods for interference-free in-situ NH3 measurements, especially in combustion flow fields.

6

thus ensuring the complete


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Energy & Fuels

NH3 measurement techniques can be divided into intrusive methods and non-intrusive ones. The 8-10

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former is based on NH3 sensors,

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response time of NH3 sensors might be too long for real-time measurements. 9 The latter, however,

4

are laser-based techniques that do not possess the drawbacks mentioned above. Laser-based

5

techniques include Raman scattering,

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absorption spectroscopy, TDLAS 14, 15), nonlinear optical techniques, 16-18 and fluorescence-based

7

techniques. 19-27 Among them, Raman scattering has a relatively simple experimental system, but

8

its signal is weak and susceptible to interference.

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sensitivity (e.g., 1 ppm 12), can achieve quantitative measurements, but it can only obtain line-of-

10

sight information. The nonlinear optical techniques, such as degenerate four-wave mixing (DFWM)

11

16, 17

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fluorescence-based techniques can achieve imaging measurements with high spatial resolutions,

13

which are commonly used for species detection.

and polarization spectroscopy,

which may interfere with the flow fields. In addition, the

11

18

absorption spectroscopy

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12, 13

(or tunable diode laser

Absorption spectroscopy, which has a high

have complicated experimental systems. Alternatively,

14

The fluorescence-based techniques with nanosecond (ns) lasers have been adopted for NH3

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measurements. They fall into two categories: one based on photofragmentation fluorescence 19 and

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the other laser-induced fluorescence (LIF).

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measured indirectly through the detection of the fluorescence emitted from the fragments

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generated by laser photolysis. It was first demonstrated by Buckley et al.

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which is seeded in the premixed CH4/air flames, is photolyzed to generate excited NH with an ns

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laser at 193 nm, and the subsequent NH fluorescence from the ( A3  − X 3  − ) transition was

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detected at ~336 nm. Different from photofragmentation fluorescence, the NH3 measurement with

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LIF is based on the resonant excitation scheme. In this scheme, NH3 is measured directly through

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the detection of the fluorescence emitted from NH3 itself. Until now, several LIF excitation-

20-25

For photofragmentation fluorescence, NH3 is

19

In their work, NH3,


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detection schemes have been demonstrated, as listed in Table 1. For single-photon LIF

2

measurements of NH3, the resonant laser is located from the vacuum ultraviolet (VUV) region to

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~210 nm. 20 For example, the (X-A) transition of NH3 could be realized through a single-photon

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excitation process with a laser at ~210 nm, but the fluorescence quantum efficiencies are too low

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to be applied in complicated flow fields.

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two-photon excitation scheme (namely two-photon laser-induced fluorescence, TPLIF), where one

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NH3 molecule absorbs two photons simultaneously. The excitation wavelength of TPLIF is located

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at UV and the visible region, which can avoid the attenuation of laser energy resulted from strong

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absorption by air. Resonant NH3 measurements are mainly based on TPLIF. There are two notable

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transitions accessible for NH3 measurements with TPLIF: 22 one is to excite (X-B) at ~303 nm and

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to detect (B-A) at ~720 nm; the other is to excite (X-C’) at ~305 nm and to detect (C’-A) at ~565

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nm. It has been investigated that the fluorescence intensity from the (X-C’) transition is about two

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orders of magnitude stronger than that from the (X-B) transition. 22 Therefore, TPLIF with the (X-

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C’) excitation scheme was commonly used for NH3 detection. It was first demonstrated by Aldén

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et al. 22 in the combustion flow field. Brackmann et al. 25 also employed the same scheme for NH3

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measurements, and the single-shot NH3 images were obtained in NH3-seeded CH4/air flames.

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Table 1. Excitation and detection strategies of NH3 with LIF

Laser Scheme

ns

single-photon two-photon

fs

two-photon

20, 21

One alternative to solve this problem is using the

Excitation

Detection

Ref.

Wavelength

Transition Wavelength Transition

~210 nm

X-A

~210 nm

A-X

[20, 21]

303 nm

X-B

720 nm

B-A

[22]

305 nm

X-C’

565 nm

C’-A

[22-25]

305 nm

X-C’

565 nm

C’-A

[26]


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Energy & Fuels

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The techniques mentioned above are based on ns lasers. The development of ultrafast laser

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technology provides novel methods for NH3 detection. Compared with ns lasers, femtosecond (fs)

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lasers have high peak power that can satisfy a high efficiency of multi-photon excitation, and at

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the same time, the relatively low pulse energy can suppress the interferences from photolysis.

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Currently, fs laser-based techniques have been extensively developed for species detection 29 (such

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as CO, 28 CH4,

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been reported. 26, 27

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OH,

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H,

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28

and O 33). Recently, the NH3 measurements with an fs laser have

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Our group initiated the investigation of NH3 measurements using an fs laser. We performed

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indirect NH3 measurements with femtosecond laser-induced plasma spectroscopy (FLIPS), 27 and

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direct NH3 measurements with fs-TPLIF. 26 For indirect NH3 measurements, 27 an fs laser at 800

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nm was adopted to photolyze NH3, and the fluorescence from NH fragments was detected.

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Although FLIPS can obtain a strong signal with a simple experimental system, the NH

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fluorescence from the ( A3  - X 3  - ) transition at ~336 nm is partially overlapped with the N2

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emission at ~337 nm. Hence, it is hardly possible to directly visualize NH with an ICCD camera

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mounted with a bandpass filter. Furthermore, our group developed NH3 measurements with fs-

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TPLIF, and we achieved the single-shot one-dimensional NH3 measurements in the NH3/N2

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mixtures with an fs laser at 305 nm.

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measurements in combustion flow fields has not been investigated.

26

However, the applicability of fs-TPLIF for NH3

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Here, the interference-free in-situ NH3 measurements in laminar premixed NH3/air flames using

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fs-TPLIF were demonstrated. Furthermore, the single-shot one-dimensional NH3 fs-TPLIF images

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were obtained. Also, the variation of the thickness of the reaction zone with the equivalence ratio

22

of NH3 flames was obtained. In addition, the observation of OH ( A2 + - X 2 i ) bands in the

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spatially resolved fluorescence spectra of fs-TPLIF was reported, and the potential of fs-TPLIF for


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simultaneous measurements of NH3 and OH was analyzed. The laser power dependence of NH3

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and NH emissions in the unburned region were also investigated. The radial distributions of NH3,

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NH, and OH in the flame were discussed. This work represents the first attempt of fs-TPLIF for

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NH3 measurements in a combustion environment, and the results indicate that fs-TPLIF is a

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promising tool for NH3 measurements.

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2. EXPERIMENTAL SECTION

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A schematic of the experimental setup together with a photo of a typical laminar premixed

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NH3/air flame are shown in Fig. 1. The fundamental wavelength at 800 nm of a femtosecond Ti:

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sapphire laser system (Spitfire Ace, Spectra-Physics) was used to pump an optical parametric

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amplifier (OPA, TOPAS-Prime, Light Conversion). Then, a laser output from the OPA at 305 nm

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26

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formed. The laser beam was focused at the center of the combustion flow field by a spherical lens

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(f=300 mm). Experiments were divided into two parts: spectral measurements and imaging

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measurements. For spectral measurements, the fluorescence was collected by a spectrometer

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(Acton, SP-2300i, Princeton Instruments, grating: 300 grooves/mm, blazed at 300 nm) through a

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condenser (f=100 mm), and then was captured by an ICCD camera (PI-MAX4: 1024i, Princeton

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Instruments) equipped at the exit port of the spectrometer. The slit of the spectrometer was parallel

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to the laser beam with its slit width of 250 μm. For imaging measurements, the NH3 fluorescence

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was imaged directly by an ICCD camera (Nikon, f=50 mm, f/1.2) mounted with a bandpass filter

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(peak transmittance: 65% at 565 nm, FWHM: 10 nm). The gate-width of the ICCD camera was 10

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ns, and the gate delay relative to the laser arriving at the combustion flow field was 0 ns. The

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flames were generated by a modified McKenna burner, which consists of two coaxial tubes. The

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central tube with an inner diameter of 3 mm was supplied with premixed NH3/air mixtures,

with a pulse energy of 20 μJ, a pulse duration of ~45 fs, and a repetition rate of 1 KHz was


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Energy & Fuels

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resulting in Bunsen flames. The outer co-flow from a water-cooled annular porous plug with an

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outer diameter of 60 mm was supplied independently with premixed CH4/air mixtures, resulting

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in a flat flame. The burned gas from the flat flame acted as a thermal co-flow to protect the inner

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Bunsen flames from the interferences of the ambient air. The equivalence ratios and the gas supply

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rates of the flames were controlled by mass flow controllers. The photos of typical laminar

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premixed NH3/air ﬂames with different equivalence ratios (0.8, 1.0, and 1.2) are shown in Fig. 2.

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Figure 1. The schematic of the experimental setup of femtosecond two-photon LIF (fs-TPLIF) of

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NH3 measurements. The inset is a photo of a laminar premixed NH3/air flame piloted by a flat

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premixed CH4/air flame stabilized on a modified McKenna burner.

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Figure 2. Photos of typical laminar premixed NH3/air ﬂames with different equivalence ratios (0.8,

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1.0, and 1.2).

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3. RESULTS AND DISCUSSION

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To investigate the feasibility of fs-TPLIF for NH3 measurements in combustion environments,

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we recorded an fs-TPLIF spectral curve from a laminar premixed NH3/air flame with its

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equivalence ratio of 1.0. Figure 3 shows the fluorescence spectrum in the wavelength range 400-

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800 nm in the unburned region of the flame, and the inset is the spectrum with a higher spectral

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resolution in the wavelength range 552.5-577.5 nm. The spectral resolution is 0.15 nm in the

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current work. The spectral curve was averaged over 20000 laser pulses. As presented in the spectral

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curve, only the NH3 fluorescence from the (C’-A) (2-2) transition at ~565 nm can be observed,

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which indicates that the photolysis interference is not a problem in our NH3 fs-TPLIF

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measurements. Furthermore, the two-head band of NH3 in the inset is recognized as the thermally

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equilibrated Q and R branches to the shorter wavelength, and the P branch to the longer wavelength.

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Similar results have also been observed in NH3-seeded flames using ns-TPLIF,

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reacting NH3/air mixtures using fs-TPLIF.

16

al.

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pressure, and at room temperature. In addition, they estimated that the NH3 LIF intensity from the

18

X-B transition (at ~720 nm) is about two orders of magnitude weaker than that from the X-C’

19

transition (at ~565 nm). In the current work, the NH3 fluorescence from the (B-A) (7-7) transition

20

at ~720 nm was not observed. It might be the poor detector quantum efficiency at 720 nm to blame,

21

which is only 10% as sensitive as that at 565 nm. In addition, different from their measurement

22

environment, the combustion flow field in the current work might exacerbate the fluorescence

23

quenching. As a result, the NH3 fluorescence at ~720 nm was not observed in this work. As

22

26

22

and in non-

Besides the fluorescence at ~565 nm, Westblom et

also observed the NH3 signal at ~720 nm in a cell filled with pure NH3 gas at atmospheric


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Energy & Fuels

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discussed above, we concluded that fs-TPLIF is feasible for NH3 measurements with the detection

2

of its fluorescence at ~565 nm in NH3/air flames.

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Figure 3. The NH3 fs-TPLIF spectrum in the unburned region of the laminar premixed NH3/air

5

flame with its equivalence ratio of 1.0.

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After verifying the feasibility of fs-TPLIF for NH3 measurements, we investigated its imaging

7

capability in different flame conditions. The NH3 imaging measurements using fs-TPLIF were

8

performed in different flames with equivalence ratios ranging from 0.4 to 1.8 by an ICCD camera

9

mounted with a bandpass filter. Figure 4 shows the relationship between the signal-to-noise ratio

10

(SNR) of the single-shot NH3 fs-TPLIF images and the equivalence ratio of the flames. The SNR

11

is defined as: SNR=

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in Fig. 4 are the experimental data obtained by averaging 100 measurements, and the error bars

13

are the standard deviations. The error bar denotes the experimental uncertainty. In addition, the

14

solid red line is the linear fitting of the experimental results. As shown in Fig. 4, the SNR shows a

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linear dependence on the equivalence ratio with an R-square coefficient of 0.971, and the SNR

16

increases with the increasing equivalence ratio. This variation can be explained as follows. With

Mean NH3 signal-Mean noise in the background . The scatters (black square) Noise standard deviation


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the increasing equivalence ratio, there are more NH3 molecules in the NH3/air mixtures, which

2

causes more NH3 molecules excited by the laser pulse at 305 nm. Hence, the stronger fluorescence

3

intensity promotes the better SNR of images. Moreover, the single-shot NH3 fs-TPLIF images

4

could obtain efficient SNRs within a wide range of equivalence ratios. Even if in the lean flame

5

with an equivalence ratio of 0.4, its SNR is still up to ~15. The inset in Fig. 4 is the single-shot

6

NH3 image in a rich flame with an equivalence ratio of 1.8, where the SNR is ~35. As a

7

consequence, these results demonstrated that fs-TPLIF could be applied to visualize NH3 in

8

NH3/air flames within a wide range of equivalence ratios.

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Figure 4. The relationship between the signal-to-noise ratio (SNR) of the single-shot NH3 fs-

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TPLIF images and the equivalence ratio of the flames.

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Besides the SNR, the variation of the thickness of the reaction zone with the equivalence ratio

13

of NH3/air flames was also investigated. Here, the reaction zone of an NH3/air flame is defined by

14

the distribution of NH2 through observing its chemiluminescence, and its edge is determined by

15

using the FWHM of NH2 distribution. The chemiluminescence spectra of flames with different

16

equivalence ratios ranging from 0.6 to 1.8 with a step of 0.2 were measured, and the variation of


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Energy & Fuels

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the thickness of the reaction zone was obtained, as shown in Figure 5, in which the data are

2

obtained by averaging 50 measurements, and the error bars are the standard deviations. As

3

presented in Fig. 5, the thickness reaches its minimum around an equivalence ratio of 1.2. In our

4

experimental conditions, the variation of the thickness from its minimum to maximum is estimated

5

to be ~10%. This tendency is the same as that of hydrocarbon fuels, 34 which can be explained by

6

the fact that a higher burning velocity near the slightly rich flame leads to a thinner reaction zone.

7

35, 36

8 9 10

Figure 5. The relationship between the thickness of the reaction zone and the equivalence ratio of NH3/air flames.

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We tested the single-shot imaging capability of NH3 fs-TPLIF at different heights in the

12

premixed NH3/air flame with an equivalence ratio of 1.0. The ratio of y/d was chosen to denote

13

different heights in the flame, 26 where y is the distance between the laser and the burner surface,

14

and d is the inner diameter of the central tube of the McKenna burner (d=3 mm). The experiments

15

were performed at y/d=2, 3, 4, and 5 (denoted by P1, P2, P3, and P4, as marked in the inset of Fig.

16

6) in the flame, respectively. Shown in Fig. 6 is the composite single-shot NH3 images at different


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heights in the flame, in which the dotted white curve outlines the layer of the reaction zone of the

2

flame corresponding to that in the inset. From the four images, we can recognize that the

3

distribution of NH3 at different heights is consistent with the conical structure of the laminar

4

premixed flame, as shown in the inset. Furthermore, there is an ignorable difference between SNRs

5

of the images at different heights, where SNRs are ~24, which indicates that fs-TPLIF for NH3

6

imaging could be applied to different heights in flames with little impact on SNRs.

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Figure 6. The single-shot NH3 fs-TPLIF images at different heights in the laminar premixed

9

NH3/air flame with its equivalence ratio of 1.0. The inset is the photo of the corresponding flame,

10

and the dotted yellow lines indicate the measurement positions, and the dotted white curve outlines

11

the layer of the reaction zone.

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To acquire more spectral details, we measured the spatially resolved fs-TPLIF spectra in the

13

laminar premixed NH3/air flame with an equivalence ratio of 1.0 in the wavelength range 200-800

14

nm. Figure 7a shows a spatially resolved fs-TPLIF spectral graph with the y-coordinate

15

representing radial positions across the flame. Figure 7b-d show the spectral curves integrated from

16

the burned region, the reaction zone, and the unburned region of the flame corresponding to that

17

in Fig. 7a, respectively. All the lines in Fig. 7b-d are recognized. In the unburned region (Fig. 7d),

18

except for the NH3 emission at ~565 nm from the (C’-A) (2-2) transition, we also observed the NH

19

emissions at ~336 nm and ~337 nm, and the N2 emission at 337 nm. In the reaction zone (Fig. 7c),


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except for the NH and N2 emissions similar to that in the unburned region, the OH emissions at

2

~308 nm and ~310 nm show up, which also appears in the burned region (Fig. 7b). The origins of

3

all the lines in the spectra will be discussed as follows. The NH fluorescence originates from the

4

( A3  - X 3  - ) transition, which includes (0-0) band at ~336 nm and (1-1) band at ~337 nm. The

5

NH ( A3  ) might have two origins: one is from the fs laser-induced photolysis of the parent NH3

6

molecules, resulting in the (0-0) and (1-1) bands;

7

excitation of the natural NH radicals in the flame, resulting in the (0-0) band. 37, 38 The N2 emission

8

is assigned to the ( C 3  u − B 3  g ) (0-0) transition, which is generated from the laser-induced

9

photochemical reactions between N2 in the air and the fs laser. Similar results for the observation

10

of N2 fluorescence have been reported by Xu et al. 39, 40, and Talebpour et al. 41 OH is a common

11

and crucial intermediate in combustion flow fields, which can be utilized to visualize the outer

12

reaction zone and the burned region of flames, and it is also a key indicator to evaluate the heat

13

release of combustion. Here, the observation of OH fluorescence is of great interest.

27

the other is from laser-induced resonant


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Figure 7. The NH3 fs-TPLIF spectra: (a) spatially resolved spectral graph in the laminar premixed

3

NH3/air flame with its equivalence ratio of 1.0; (b-d) integrated spectral curves in the burned region,

4

the reaction zone, and the unburned region of the flame, respectively. Upper: burned region;

5

Middle: reaction zone; Bottom: unburned region.

6

There have been ample efforts devoting to the OH detection in combustion research with LIF

7

techniques. To verify that the OH fluorescence was generated through LIF, we performed an

8

excitation scan of OH in the NH3/air flame with an equivalence ratio of 1.0, where the laser

9

wavelength was tuned from 295 nm to 320 nm with a step of 1 nm, and the spectrum is shown in

10

Fig. 8. The excitation spectrum of OH is a broad band with two peaks at ~308 nm and ~310 nm,

11

which is in agreement with the works by Koh et al.

42

, and Car et al.

43

In their works, the OH


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fluorescence was recognized as the ( A2 + - X 2 i ) transition. Here, the observation of OH

2

fluorescence with strong intensity might be attributed to the following reasons: firstly, an fs laser

3

has a much broader line-width compared with an ns laser. As presented in Fig. 8, the measured fs

4

laser centering at 305 nm has a FWHM of ~4 nm. This property facilitates the OH excitation

5

involved in the NH3 excitation centering at 305 nm. Secondly, the ( A2 + - X 2 i ) transition of

6

OH is a single-photon excitation process, which is more effective than the two-photon excitation

7

process of NH3. As a consequence, though the excitation wavelength at 305 nm is not the optimal

8

excitation wavelength of OH (at ~308 nm and ~310 nm), the intensity of OH LIF is still strong,

9

even stronger than that of NH3 TPLIF. It indicates that the technique developed in this work has

10

the potential for simultaneous measurements of NH3 and OH in NH3/air flames, where NH3 can be

11

seen as the indicator for the unburned region while OH the indicator for the reaction zone, i.e.,

12

simultaneous visualization of both the unburned region and the reaction zone with only one laser.

13 14

Figure 8. An excitation scan of OH in the premixed NH3/air flame with its equivalence ratio of

15

1.0 (solid black line), and a measured fs laser profile centering at 305 nm (dotted red line).


15

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

1

The characterization of laser power dependence of NH3 and NH emissions were also investigated.

2

Figure 9 shows the laser power dependence of NH3 and NH fluorescence in the unburned region

3

of an NH3/air flame with an equivalence ratio of 1.0. In Fig. 9, the experimental data of NH3 are

4

denoted by red triangles, while NH is denoted by black squares. The laser energy was varied from

5

1 to 22 μJ/pulse. Figure 9a shows the relationship between the signal intensity and the laser energy

6

on a linear scale, and Figure 9b shows the relationship on a log-log scale. As shown in Fig. 9a,

7

both NH3 and NH signals are not saturated significantly in our experimental conditions, even if

8

the laser energy is up to 22 μJ/pulse. For the power dependence of NH3, the solid red line in Fig.

9

9b is the linear fitting of its experimental data, which has a slope of 1.780 with an R-square

10

coefficient of 0.993. Essentially, the NH3 TPLIF signal follows roughly a quadratic dependence

11

on the laser energy, which was also reported by Backmann et al.

12

dependence is also almost consistent with the two-photon process of fs-TPLIF. For the power

13

dependence of NH, the NH signal shows a clear cubical dependence on the laser energy, which

14

means that the generation of NH through photolysis of NH3 is a three-photon process, which is

15

firstly discovered by us in this work. It can be seen in Fig. 9a, when the laser energy is below 7

16

μJ/pulse, we could hardly observe the NH fluorescence. Hence, when the laser energy is low, the

17

photolysis of NH3 might be negligible, which favors the quantitative measurement of NH3.

25

and Ashfold et al.

44

This


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 2

Figure 9. The laser power dependence of NH3 and NH fluorescence in the unburned region of an

3

NH3/air flame with its equivalence ratio of 1.0. (a, the relationship between the signal intensity

4

and the laser energy on a linear scale; b, the relationship on a log-log scale).

5

Using the spatially resolved fs-TPLIF spectra mentioned above, we also studied the radial

6

distributions of several species in the flame. Figure 10 shows the radial distributions of NH3 (black),

7

NH (red), and OH (blue), where the region of interest was from the centerline of the flame (r=0

8

mm) to the burned region (r=6 mm) (indicated by the rectangular white box in the inset). From the

9

distribution of NH3 TPLIF signal, we observed that its fluorescence intensity drops sharply from

10

r=0 to r=2 mm, which might be attributed to the increasing temperature, but mostly to the thermal

11

decomposition of NH3 molecules. 45, 46 From the distribution of NH, the NH signals are very weak

12

in the reaction zone, which might be attributed to its generation by LIF. Although there have NH

13

in a large quantity in the NH3 combustion process, NH LIF is a single-photon process, whose

14

intensity is proportional to the laser energy. The fs laser used has a pulse energy at the order of μJ.

15

Besides, the excitation laser centering at 305 nm is not the optimal excitation wavelength of NH.

16

The two points combined must result in very weak NH LIF signals in the reaction zone. In addition,

17

the OH radicals are distributed in the outer reaction zone and the burned region. Also, we compared


17

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

1

the radial distribution of OH in the NH3/air flame (blue curve) with that in the blank flame without

2

NH3 (purple curve). As shown in Fig. 10, the maximum intensity of OH in the NH3 flame (blue

3

curve) is estimated to be ~1.3 times stronger than that in the blank flame (purple curve). Although

4

Fig. 10 can not exclude the contribution from the blank flame, we are confident that a substantial

5

part is from the NH3 flame. In short, the technique developed here is capable of detecting OH in

6

NH3 flames.

7 8

Figure 10. The radial distributions of NH3 (black), NH (red) and OH (blue) in the premixed

9

NH3/air flame with its equivalence ratio of 1.0, and the radial distribution of OH (purple) in the

10

blank flame without NH3.

11

4. CONCLUSIONS

12

In this paper, we demonstrated the feasibility of the femtosecond two-photon laser-induced

13

fluorescence (fs-TPLIF) technique for interference-free in-situ NH3 measurements in laminar

14

premixed NH3/air flames for the first time. The single-shot one-dimensional NH3 fs-TPLIF images

15

with efficient signal-to-noise ratios (SNRs) were obtained, which indicates that fs-TPLIF is a


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

promising tool for visualizing NH3 in a combustion environment. The variation of the thickness of

2

the reaction zone with the equivalence ratio of NH3 flames was obtained, which is similar to that

3

of hydrocarbon fuels. The spatially resolved fluorescence spectra were also collected. The OH

4

( A2 + − X 2 i ) bands at ~308 nm and ~310 nm generated by the fs-LIF were also recognized,

5

which is attributed to the broad line-width of the fs laser. We demonstrated that fs-TPLIF has the

6

potential for simultaneous measurements of NH3 and OH with only one laser. Also, the laser power

7

dependence of NH3 and NH emissions in the unburned region were investigated. The NH3 TPLIF

8

signal follows roughly a quadratic dependence on the laser energy. The NH signal shows a clear

9

cubical dependence, which indicates that the generation of NH through photolysis of NH3 is a

10

three-photon process, which is firstly discovered by us in this work. Furthermore, the radial

11

distributions of NH3, NH, and OH in the flame were obtained. This work is the first attempt of fs-

12

TPLIF for NH3 measurements in a combustion environment, and the results are beneficial for

13

promoting NH3 combustion research.

14

AUTHOR INFORMATION

15

Corresponding Author

16

*E-mail: [email protected] (B.L.)

17

Author Contributions

18

The manuscript was written through contributions of all authors. All authors have given approval

19

to the final version of the manuscript.

20

Notes

21

The authors declare no competing financial interest.

22

ACKNOWLEDGMENT


19

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

This work was supported by the National Natural Science Foundation of China (NSFC)

2

(91741205, 51776137).

3

REFERENCES

4 5 6 7 8 9 10

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Ammonia Measurements with Femtosecond Two-photon

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