Science and Technology (KAUST) - Flame Structure...

www.flame-structure-2014.com Berlin Institute of Technology (TU Berlin) Prof. Dr. Frank Behrendt Fakultät III: Prozesswissenschaften, Institut für Energietechnik Chair Energy Process Engineering and Conversion Technologies for Renewable Energies (EVUR) Fasanenstr. 89 10623 Berlin Contact [email protected] [email protected]

Berlin Institute of Technology • Fasanenstr. 89 • 10623 Berlin

King Abdullah University of Science and Technology (KAUST) Awad Alquaity CCRC Thuwal, 23955-6900, Makkah, Saudi Arabia 28. Mai 14

Confirmation of paper submission Name: Awad Alquaity Email: [email protected] Co-author: - 2nd co-author: - 3rd co-author: - Title of Paper: Measurement of Positive Ions in Premixed

Methane-Oxygen Flames Program: Laminar flames Name of Institute: King Abdullah University of Science and

Technology


Measurement of Positive Ions in Premixed Methane-Oxygen Flames

Awad B.S. Alquaity, Nadim Hourani, May Chahine, S. Mani Sarathy, Aamir Farooq*

Clean Combustion Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia

*Corresponding author. Email: [email protected], Phone: +966-128026966

Abstract: Ions are formed as a result of chemi-ionization processes in combustion systems. Recently, there has been an increasing interest in understanding flame ion chemistry due to the possible application of external electric fields to reduce emissions and improve combustion efficiency by active control of combustion processes. In order to predict the effect of external electric fields on combustion plasma, the first step is to gain a good understanding of ion chemistry in flames by measuring flame ion concentration profiles. In this work, a Molecular Beam Mass Spectrometer (MBMS) is utilized to measure ion concentration profiles in premixed methane-oxygen-argon burner-stabilized flames as a function of height above the burner surface. Lean, stoichiometric and rich flames at atmospheric pressure are used to study the dependence of ion chemistry on equivalence ratio of premixed flames. The relative ion concentration profiles are compared qualitatively with previous methane-oxygen studies and showed good agreement. Information on the total positive ion concentration obtained using the Langmuir probe will be combined with mass spectrometric measurements to convert the relative ion concentrations to an absolute scale. The ion concentration data obtained in the present study can be used to validate and improve existing ion chemistry models for methane-oxygen flames.

1. Introduction

Flame is weak plasma containing various ions in low concentrations. In the past decade, there has been an increased interest in studying flame ion chemistry due to the potential of using external electric fields to actively control combustion processes for reduced emissions and increased efficiency. Electric field assisted combustion has been shown to reduce emissions [1], improve combustion efficiency, and extend the operability of combustion devices [2–4]. Nevertheless, the progress is restrained due to the limited understanding of the flame ionization processes, and the lack of experimental data on the evolution and distribution of ionic species in the flame. This necessitates extensive experimental efforts to investigate combustion plasma formation and flame ion composition.

There have been very few detailed experimental studies on the evolution of ionic species in premixed methane-oxygen flames [5–8]; all of which were carried out by one research group [7,8]. Even the most recent


numerical work [9] compare their results with the measurements of Goodings et al. [7,8] which were carried out almost three decades ago. Goodings et al. [7,8] conducted a detailed study of positive and negative ion chemistry by measuring ion concentrations in a premixed atmospheric-pressure methane-oxygen flames produced in a conical flame. However, they did not report flame temperatures as a function of height above the burner. The lack of temperature profile and two-dimensional flame structure make it difficult to perform a direct comparison between the simulation results and measured ion concentration profiles [9]. Additionally, since the ion concentration data for methane-oxygen flame is available only from a single research group, it is essential to conduct another study to evaluate the possible facility effects on the experimental data.

The present work is aimed at studying ion chemistry by measuring ion concentration profiles in methane-oxygen flames using Molecular Beam Mass Spectrometry (MBMS). The ion concentration measurements in the premixed flame are carried out in conjunction with a Langmuir probe, which provides a simple technique of measuring the total positive ion concentration in combustion plasma. The above study is expected to improve the understating of ion chemistry and the data collected herein will serve as a benchmark for testing of numerical models for simulating flame ion chemistry. In what follows, we give a brief description of our MBMS experimental setup followed by some preliminary results.

2. Approach

Molecular Beam Mass Spectrometry (MBMS) is a widely used technique for measuring species concentration profiles in premixed flames [10]. Its high sensitivity and ability to differentiate ions from neutrals make this technique attractive for studying ion chemistry in flames. Herein, a modified Hiden HPR-60 MBMS system equipped with a triple filter quadrupole mass spectrometer (EQP, 300 m/z range) is used (Figure 1a). A commercially available water-cooled 6 cm diameter McKenna burner (Holthuis & Associates) was used to produce a premixed methane-oxygen flat flame with argon as the diluent gas. The McKenna burner was mounted on a vertical translation stage with a minimum step size of about 100 µm. For the preliminary results presented in the next section, we used a 4.5 mm long nickel cone with an aperture of 0.05 mm for sampling the flame gases (Figure 1b). The flame gases were sampled along the centerline of the flat flame and transferred via a molecular beam to the mass spectrometer region using three stages of differential pumping. Electrical potentials applied to the first stage and second stage skimmer cones facilitated the extraction and transmission of ions via the molecular beam. In the third stage, the ions are transmitted to the EQP using ion optics. The nickel cone was mounted on a water-cooled front flange and the temperature of the front flange was monitored continuously to keep it below 350 K. The flame temperature was measured using an SiO2 coated platinum-30% rhodium/ platinum – 6% rhodium (Type-B, Omega) thermocouple with a bead size of about 0.4 mm and corrected for radiation errors. The voltage settings on the


MBMS were optimized using the MBMS software to obtain maximum detector signal for the flame ions.

(a)

(b)

Figure 1. (a) Photograph of the MBMS setup, (b) Photograph of the McKenna flat flame burner near the nickel sampling cone

3. Results and discussion

The preliminary results obtained with the MBMS system are presented here. Mass scans were carried out, over a mass to charge ratio (m/z) range of 1 to 300, in different regions of the flame to identify the dominant ions in a lean (φ = 0.5) premixed flame. The mass scans showed that C2H3O+, CH5O+, H3O+ ions and their hydrates were the most abundant ions, which is similar to the observations of Goodings et al. [7] in their premixed conical flame. These initial experiments were carried out to optimize the MBMS settings to obtain maximum detector signals for charged species.


Since the mass spectrometer used here is both a mass and energy analyzer, the mass scan obtained at a specific energy level does not give enough information to obtain relative concentrations of ions. The ion concentrations can only be compared once the integrated area of the ion energy distribution curves for each ion is measured and the mass discrimination of the system is known. Therefore, initially a mass scan was carried out at an optimum energy level to identify all ions present in the flame, which was followed by an energy scan for the dominant ions. The integrated energy enables the plotting of concentration profiles for ions of similar masses. However, relative ion concentration profile can be obtained after the mass discrimination of the system is measured. The mass discrimination of the system will be measured in the future, and this will enable conversion of the integrated ion signals presented below to relative ion concentrations [11].

Figure 2 shows the ion profiles for the dominant ions in fuel lean flame (φ = 0.5). As mentioned earlier, all the ions observed were either C2H3O+, CH5O+, H3O+ or their hydrates. The integrated signals measured for these ions were repeatable within 10% uncertainty. Although it is difficult to comment on the relative concentrations of these three parent ions and their hydrates without the knowledge of the mass discrimination of the system, a few trends can be observed. The ions C2H3O+, CH5O+ and their hydrates exist in the flame front region near the burner and decay beyond 2.5 mm away from the burner, whereas H3O+ hydrates exist across the entire region of the flame. Also, the concentration of H3O+ hydrates is higher than that of C2H3O+, CH5O+ and their hydrates in the measured region of the flame.


2 4 60

5000

10000

15000In

tegr

ated

sig

nal (

arb.

uni

ts)

Height above burner (mm)

m/z = 43 (C2H3O+)

m/z = 79 (C2H3O+.(H2O)2)

m/z = 97 (C2H3O+.(H2O)3)

m/z = 115 (C2H3O+.(H2O)4)

m/z =133 (C2H3O+.(H2O)5)

m/z = 51 (CH5O+.(H2O))

m/z = 69 (CH5O+.(H2O)2)

2 4 60

20000

40000

60000

80000

Inte

grat

ed s

igna

l (ar

b. u

nits

)

Height above burner (mm)

m/z = 55 (H3O+.(H2O)2)

m/z = 73 (H3O+.(H2O)3)

m/z = 91 (H3O+.(H2O)4)

m/z = 109 (H3O+.(H2O)5)

(a)

(b)

Figure 2 Dominant ions at different heights above the burner; a) Hydrates of C2H3O+ and CH5O+; (b) Hydrates of H3O+

4. Conclusions

A Molecular Beam Mass Spectrometer coupled to a flat flamer burner was used to measure ion signals in different regions of the flame. Ion signal profiles of all the dominant ions in lean premixed flames were also measured. Future work will contain measurements of relative concentration of all the major ions observed in lean/stoichiometric/rich premixed flames. The relative ion concentration data will be converted to an absolute scale using total positive ion concentrations measured by the Langmuir probe. The ion concentration data obtained in this study will be useful for validating numerical models of ion chemistry in methane-oxygen flames and will eventually help in improving the understating of ion chemistry in flames.

Acknowledgements

Research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST).

References


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