Thermal and Acoustic Performance of Al2O3, MgOâZrO2, and SiC

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Thermal and Acoustic Performance of AlO, MgO-ZrO, and SiC Porous Media in a Flow Stabilized Heterogeneous Combustor Anthony Carmine Terracciano, Samuel Timothy de Oliveira, Demetrius A. Vazquez-Molina, Fernando J. Uribe Romo, Subith S Vasu, and Nina Orlovskaya Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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Thermal and Acoustic Performance of Al2O3, MgO-ZrO2, and SiC Porous Media in a Flow Stabilized Heterogeneous Combustor

Anthony Carmine Terracciano1*, Samuel De Oliveira1, Demetrius Vazquez-Molina2, Fernando J. Uribe-Romo2, Subith S. Vasu 1,3, Nina Orlovskaya1 Corresponding author: Anthony Carmine Terracciano, email: [email protected], phone: 954-793-7563 Target Journal: Energy & Fuels

Keywords: Methane, Heterogeneous Combustion, Substrate, Spectroscopy

1

Department of Mechanical and Aerospace Engineering, University of Central Florida, 4000

Central Florida Blvd., Orlando, FL, 32816-2450, USA 2

Department of Chemistry, University of Central Florida, 4000 Central Florida Blvd., Orlando,

FL, 32816-2450, USA 3

Center for Advanced Turbomachinery and Energy Research, University of Central Florida,

4000 Central Florida Blvd., Orlando, FL, 32816-2450, USA

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ABSTRACT A comparative analysis of performance of three different porous ceramic materials used as a flame contained media within a flow stabilized combustor is presented within this work. The experiments were performed at a constant air flow rate and variable methane flow rate.

α-

Al2O3, MgO-ZrO2, and SiC highly porous ceramics were used as the porous media. All three porous media used in the study had equal dimensions and porosity, and were located in the same fixed position within the combustion chamber. Characterization of the structure of porous media before experimentation was done using multiple methods. During combustor operation, temperature profiles were collected along with optical and acoustic emissions and correlated with the composition of the gaseous methane/air mixtures. It was found that while SiC enables the highest temperatures within the combustion chamber at higher equivalence ratios, the ionically conducting MgO-ZrO2 porous media greatly expands the lean limit of combustor operation and is, thus, the preferred porous media material for lower equivalence ratio operation. 1. INTRODUCTION Heterogeneous combustion in porous media is an advanced combustion technique in which a solid reticulated foam placed within a combustion chamber is used to support the burning of a fuel-oxidizer mixture flowing through the medium [1]. Significant thermal coupling between the gaseous and solid phases enables local temperatures to exceed the adiabatic flame temperature within the combustion chamber, which enhances flame stability as well as the possibility of burning an ultra-rich and ultra-lean fuel/oxidizer mixtures inside of the porous media [2, 3], making heterogeneous combustion a viable means of combusting low calorific value biofuels [4]. A broad variety of solid media have been studied as porous support materials in heterogeneous combustion. Different porous ceramic materials such as Al2O3 [5]; Al2O3


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composites matricies which feature various other dopants [6], ZrO2 stabilized either with CaO [7], MgO, or Y2O3 [8]; ZrO2 toughened mullite [9]; SiSiC [10]; or pure SiC [11] have been used as flame holders to support and maintain combustion. Some of the more commonly utilized porous media materials, such as Al2O3, are catalytically inert and the combustion reactions which occur within the combustion chamber while using porous media of said composition are assumed to occur only in the gas phase. In such cases, the catalytically inert media simply serves as a means to store and redistribute heat throughout the combustion chamber enabling higher net reaction rates and improved combustion stability, in comparison with combustion in the absence of porous media [12]. Other ceramic media such as ZrO2 or even SiC are considered as catalytically active [13, 14], as their surfaces can enable additional chemical reaction pathways which facilitate increased combustion temperatures, lower light off temperatures, while simultaneously reducing the concentration of undesired gaseous species [2, 15, 16]. In this paper we report the performance of α-Al2O3, MgO-ZrO2, and SiC ceramics used as porous media for methane combustion within a specially designed combustion chamber [17, 18]. Although some variations may be expected in the dimensions, pore size, and pore structures of the three materials, the differences are insignificant and thus, the emphasis of this study was on the materials’ effect on the heterogeneous combustion of methane. As it has been shown that combustion instabilities have a wave like component [19], acoustic emissions profiles were a focus of this study. In addition to the measurement of acoustic emissions, temperature profiles at fixed locations within the combustion chamber, and radiative emissions in the visible spectrum from the exhaust were collected from the combustion chamber while operation was conducted at various equivalence ratios and a fixed air flow rate, using methane as fuel. By conducting such a


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study, the optimal porous media for given ranges of equivalence ratios may be determined and evaluated as potential substrates for catalytic applications [20-22]. 2. EXPRIMENTAL PROCEDURES Porous media of α-Al2O3, MgO-ZrO2, or SiC were characterized, prior to investigation in combustion experiments, using Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). Combustion experiments were performed in a flow stabilized porous combustor described elsewhere [17, 18] at constant air flow rates with variable equivalence ratio, with simultaneous measurements of temperature profiles within the combustion chamber, the collection of optical images, as well as acoustic emission signals coming from the combustion chamber exhaust. Selected properties for the selected substrate materials are presented in Table 1 [23-31]. 2.1 Substrate Characterization The phase compositions of the powderized α-Al2O3, 3 mol% MgO partially stabilized ZrO2 (MgO-ZrO2), and SiC ceramic media; were characterized by Powder X-ray diffraction (PXRD) data was collected using a Rigaku MiniFlex300 θ-2θ diffractometer, (Tokyo, Japan) in BraggBrentano geometry with a 300 mm goniometer diameter, Ni-filtered CuKα radiation (λ = 1.5418 Å) at 600 W power (40 kV, 15 mA), equipped with a NaI(Tl) SC-70 scintillation detector, 5.0º incident and receiving Soller slits, a 0.625º divergent slit, a 1.25º scattering slit, a 0.3 mm receiving slit and a Ni-CuKβ filter. Samples were analyzed from 15º to 90º 2θ-degrees with 0.02º per step and a scan rate of 0.10º 2θ-degrees per min. To prepare samples for PXRD measurements, pieces of the α-Al2O3 and MgO-ZrO2 porous media were ground into a fine powder using mortar and pestle; while pieces of the SiC media were placed into a bag and subsequently crushed using a hammer. The surface morphologies of the porous media were -4ACS Paragon Plus Environment

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characterized using Scanning Electron Microscope (SEM) using Zeiss EVO, (Oberkochen, DE). Before SEM investigation, the surfaces were coated with a highly conductive Pd thin film. 2.2 Experimental Apparatus and Procedures A heterogeneous combustor [17, 18], consisting of the combustion chamber, reactant storage and metering systems, reactant delivery nozzle, exhaust sampling port, with externally placed microphone and CCD camera; was used in this work to measure temperature profiles, acoustic emissions and optical images of the exhaust. A mixture of methane and air at different equivalence ratios was burned during operation of the device. The porous α-Al2O3, MgO-ZrO2, or SiC ceramic matricies with dimensions of 50.0 mm in diameter and 50.4 mm in length and porosity of 10 ppin (pores per inch) was inserted inside of the combustion chamber at 40.0 mm from the inlet. A schematic presentation of the combustion chamber with porous media inside is shown in Fig. 1, with key dimensions as well as locations of the 14 thermocouples indicated, that were used to measure the temperature profiles during combustion experiments. The following naming convention was used to identify the thermocouples at various axial positions starting at the combustion chamber inlet all the way along the outer radius of the combustion chamber to the exhaust exit: TC0 (0.0 mm) was located at the combustion chamber inlet; TC1 (44.1 mm), TC2

(64.7 mm), and TC3 (85.4 mm) were located at the beginning, middle and end of the

porous media respectively, while TC4 (112.9 mm) was located at the end of the combustion chamber (Fig. 1). In addition to the use of thermocouples for the measurement of the temperatures within the chamber, images of the exhaust were also taken utilizing a CCD camera (Shenzhen Meikeda Electronic Co: Shenzhen, CN). Through the entire duration of the experiment, temperature measurements and images of the exhaust were recorded at 8 Hz and 1/60 Hz frequencies, respectively. -5ACS Paragon Plus Environment

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After ignition and warm up periods during operation of the combustion chamber, similarly as it was done in different experiments using this device [17, 18], a constant equivalence ratio and reactant flow rate was maintained until steady state operation was achieved. In this work, the steady state operation of the combustor was defined when all the thermocouples within the combustion chamber exhibit a maximum temperature difference of 0.4 ~ 0.5 °C/min over a twominute interval. Once steady state of combustion is achieved, a 5 second acoustic sound sample at 192 kHz and 24-bit depth was collected using an externally placed microphone (AKG Acoustics: Vienna AT). Following the acoustic measurement, the air flow rate was maintained constant while the methane flow rate was reduced such that the equivalence ratio, denoted by φ, was reduced by ∆φ=0.02 in a stepwise manner. Such decrease in methane flow rate was repeated until the flame could no longer be visible via CCD camera or recorded by acoustic measurements. 2.3 Modeling A CFD analysis examining swirl and recirculation of incoming gases at the inlet of the combustion chamber was performed to verify that recirculation would not overlap the space within the combustion chamber where the combustion media would be placed. Additionally, modeling was performed to find characteristic stable flow structures which are presumed to be the source of specific acoustic signals collected by the microphone. A flow domain was generated and analyzed using Siemens NX 8.5 (Siemens: Munich, DE) which replicated the combustion chamber consisting of 1.8 million tetragonal quadratic elements and a nominal wall surface roughness of 115 µm. The inlet boundary condition imposed at the coupler assumed dry air at 26.85°C with a mass flow rate of 60 SLPM, which is the maximum flow rate permissible for the existing instrumentation [17, 18], and when the recirculation region would occupy the -6ACS Paragon Plus Environment

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largest volume within the combustion chamber. At the exit boundary condition, an atmospheric criterion was assumed. 3. Results and Discussion Results presented within this work include characterization of the porous media surfaces as well as evaluation of combustion performance when either α-Al2O3, MgO-ZrO2, or SiC ceramics were used as a porous media. The characterization was done using collected temperature profiles collected as functions of time or equivalence ratio during operation of the combustor. The optical images of exhaust along with acoustic emissions spectra were also collected during the combustion process. 3.1 Characterization of Phase Composition and Surface Morphologies of α-Al2O3, MgO-ZrO2, and SiC Porous Media X-ray diffraction patterns of α -Al2O3, MgO-ZrO2, and SiC ceramics are presented in Fig. 2 A-C. As one can see from Fig. 2A, the α-Al2O3 matrix ceramic consists of phase pure hexagonal α-Al2O3, where no extra peaks and no texture was observed in the XRD pattern. The intensity of the tetragonal (111) peak of the ZrO2 is miniscule in comparison to the monoclinic (111) ZrO2 and (111ത) ZrO2 peaks, thus the amount of tetragonal phase may be evaluated using ܺ௠ = ഥ)೘ ூ(ଵଵଵ)೘ ାூ(ଵଵଵ ഥ)೘ ାூ(ଵଵଵ)೟ ூ(ଵଵଵ)೘ ାூ(ଵଵଵ

[32]. Indeed, the calculation revealed that the majority of ZrO2 phase is

monoclinic (93.6 vol%) with the rest of the ZrO2 having some other lattice structure. Such high presence of monoclinic phase is easily explained by the preparation route to make samples of the porous media for XRD. As it is only 3 mol% of MgO that was used to stabilize ZrO2, the tetragonal ZrO2 is very easily transformed into monoclinic ZrO2 during grinding, when the applied stress from the mortar and pestle initiated a stress induced t → m phase transition. Therefore, while the XRD pattern of MgO-ZrO2 shown in Fig. 2B consists mainly of m-ZrO2, -7ACS Paragon Plus Environment

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the MgO-ZrO2 is likely to have t-ZrO2 phase which maintains sufficient structural stability when used as a support material for combustion processes. The X-ray diffraction pattern of the SiC porous matrix ceramic is shown in Fig. 2C. A mixture of many SiC polytypes were found [3335] by analyzing the X-ray diffraction pattern, along with the peaks which belong to various phases of SiO2 [36-38]. SEM micrographs of surfaces of the three ceramic compositions are shown in Fig. 2 D-E. The micrograph of the α-Al2O3 surface reveled typical hexagonal faceted grains of α-Al2O3 clearly visible in Fig. 2D. The size of individual alumina biaxial grains is 2~5 µm. The small elongated lamellars are visible on the surface of MgO-ZrO2 porous ceramic (Fig. 2E). Such lamellars are characteristic of the tetragonal twins [39], which could undergo t → m phase transformation easily under applied stress [40]. One can estimate that the thickness of the lamellars is on the order of 0.4~0.5 µm, while the length is 2~3 µm. The surface of SiC porous material (Fig. 2F) appears smoother in comparison with both α-Al2O3 and MgO-ZrO2 ceramics with some very fine porosity, and estimated grain size on the order of 0.3~0.5 µm. 3.2 Temperature Profiles of Combustion Chamber The time and position dependent temperature measurements characterizing thermal performance of the porous combustor where three different α-Al2O3, MgO-ZrO2, and SiC ceramics were used as a porous ceramic media are shown in Fig. 3, 4, and 5, respectively. The complete details of the experiments presented in Fig. 3 and 4, where α-Al2O3, and MgO-ZrO2 ceramics were used as a porous media are described elsewhere [20], and the similar experiments with SiC ceramics used as a porous media is reported in this paper (Fig. 5.). The maximum measured temperatures over the whole span of measurement as a function of equivalence ratio for three different ceramics used as porous media are shown in Fig. 6. From Fig. 6, as -8ACS Paragon Plus Environment

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equivalence ratio is reduced it can be seen that temperatures within the combustion chamber decreases regardless of the media composition. At 0.53±0.02≤φ≤0.75±0.02 SiC porous ceramic media outperforms both MgO-ZrO2 and α-Al2O3 porous media, as it facilitates the highest temperature of the flame; and from Fig. 5B it may be seen that this maximum temperature occurs at TC1. However, at φ=0.51±0.02, the last equivalence ratio at which a SiC media still supports a flame, the maximum temperature drops dramatically. No flame can be maintained at equivalence ratios lower than φ=0.51±0.02 when SiC is used. While both α-Al2O3 and MgO-ZrO2 ceramics were able to maintain comparable maximum temperatures at the same position inside the combustor (Fig. 3B and 4B). Slightly higher maximum temperatures were recorded for MgOZrO2 ceramics for 0.51±0.02≤φ≤0.75±0.02 as compared to α-Al2O3 ceramics. Unlike the SiC porous matrix, both MgO-ZrO2 and α-Al2O3 media are still able to support a flame at φ=0.49±0.02, however at this equivalence ratio maximum temperatures within the combustion chamber are significantly lower than at φ=0.51±0.02, indicating that there is some critical equivalence ratio at which the rate of chemical reactions are insufficient to enable complete combustion. At φ=0.47±0.02 only MgO-ZrO2 media is capable of supporting a flame. The comparison of α-Al2O3, MgO-ZrO2, SiC porous media thermal performances at different equivalence ratios φ=0.75; 0.61; 0.51; or 0.49±0.02 for ܸሶ air=47.56 SLPM is presented in Fig. 7AD. SiC ceramic media outperforms both MgO-ZrO2 and α-Al2O3 for φ=0.75±0.02 (Fig. 7A). The highest temperature measured was found to be Tmax=804.1±6.5°C for the SiC matrix, whereas Tmax= 762.6±6.1 °C for MgO-ZrO2 matrix and Tmax= 728.8±5.9 °C for α-Al2O3 matrix at TC1 (44.1 mm). As the porous media is located 40mm from the inlet (indicated as 0 at x Axial position axis), then the highest temperature of the flame is located about 4mm deep inside the porous media from the inlet side. This indicates that the flame at this location is the most intense,


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just after the entry region of the porous media. Very similar trend in the temperature distribution is observed when the equivalence ratio is lowered to φ=0.61±0.02 (Fig. 7B). However, as expected, at this equivalence ratio, the maximum temperatures across the whole combustion chamber for three ceramic media was decreased in comparison with such measurements performed at φ=0.75. For φ=0.61±0.02, Tmax= 711.8±5.8°C for SiC, Tmax= 668.3±5.5°C for MgO-ZrO2 and Tmax= 651.8±5.4°C for α-Al2O3 porous media. When the equivalence ratio is further decreased to φ=0.51±0.02 the temperatures measured were significantly lower inside the combustor when SiC porous media was used as compared to the experiments when either MgOZrO2 or α-Al2O3 porous media were employed. As previously stated, φ=0.51±0.02 is the lowest possible stable equivalence ratio for which SiC media would still support a flame, where Tmax=392.3±3.9°C was measured. At this same equivalence ratio, Tmax=587.4±5.0°C for MgOZrO2 and Tmax=559.7±5.0°C for α-Al2O3 porous media. In all three cases the temperature profiles were approximately uniform, measured across the 40 to 90 mm span of the combustion chamber which coincides with the position of the porous media, thus indicating the existence of a quasiisothermal profile within the span of the porous media. Such isothermal behavior of the porous media is supported by analytical models [41, 42], and simultaneously indicate that a more distributed flame within the combustion chamber exists at this lower equivalence ratio φ=0.51±0.02 in comparison with either φ=0.75±0.02 or φ=0.61±0.02. When equivalence ratio is further lowered to φ=0.49±0.02, which is the lowest equivalence ratio at which α-Al2O3 still maintained a flame, the maximum temperatures measured were Tmax=325.3±4.0°C for α-Al2O3 and for MgO-ZrO2 porous media Tmax=408.8±4.4°C. From the presented temperature profile, important characteristics may be inferred about the flame behavior within α-Al2O3, MgO-ZrO2, and SiC porous matrices. The equation describing


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the change in the maximum temperature as a function of equivalence ratio can be written as డ்೘ೌೣ డఝ ௏ሶୀ௖

= ‫ܥ‬ଵ , for equivalence ratios between 0.53±0.02 ≤φ≤0.75±0.02, where ‫ܥ‬ଵ is a constant

which depends on the ceramic material properties, airflow rate, and fuel composition [43]. As it is assumed that for the 0.53±0.02≤φ≤0.75±0.02 range of equivalence ratios the predominant channel for chemical reactions is gas phase, combustion performance could be considered to be dictated by the ability of the substrate to efficiently transport heat through the combustion chamber. Such a claim is well supported as the thermal conductivity of SiC is roughly 10x that of MgO-ZrO2 and 6x that of α-Al2O3, respectively. Furthermore from Table 1 it may be seen that the normal emissivity ᆅn of SiC is approximately twice that of α-Al2O3 and MgO-ZrO2. Such emissive properties are particularly important for heat transfer in porous media as it has been shown that for porous media of a lower linear pore density, the influence of radiative heat transfer between pores dramatically increases the apparent heat transfer rate of the porous material [44]. Therefore, it is right to expect that SiC would outperform both MgO-ZrO2 and αAl2O3 because of its significantly better thermal properties. It is known that at some equivalence ratio, for a given fuel-oxidizer and substrate combination, in heterogeneous combustion there is a limit at which equation

డ మ ்೘ೌೣ డఝ మ ௏ሶୀ௖

≠0

occurs, which has a strong correlation to the lean limit of a flame [45]. When such conditions are reached, then the temperatures within the combustion chamber will rapidly decrease. For equivalence ratios coinciding with, and below, this transition, it is assumed that surface based chemical reactions occurring on the substrate become significantly more important as a reaction channel, which sustains the flame [46]. Thus, if the material on the surface of the porous media is catalytically active, temperatures through the combustion chamber increase as a result of the catalytic reactions enabling the formation of an increased fraction of complete combustion -11ACS Paragon Plus Environment

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byproducts. Of the three porous media compositions studied here, it is thought that MgO-ZrO2 is the only substrate that enables catalytic reactions to occur. The surface of SiC may oxidize and SiO2 will form on the substrate surface [47], which is not catalytically active but will enable water vapor adsorption, then the flame extinguishes below φ=0.51±0.02. α-Al2O3 is also unable to promote significant reactivity of the combustion species as the surface is inert [13]. Due to the spectral emissivity of α-Al2O3 a significantly smaller quantity of heat is discharged via radiation compared to SiC, enabling the flame to be stable at the equivalence ratio φ=0.49±0.02. Therefore, the temperatures measured when the flame is supported by α-Al2O3 are lower in comparison with MgO-ZrO2 ceramics. It was reported that ZrO2 surfaces exhibit both mildly basic and acidic sites which enables catalytic reactions to occur [14], as it has been shown that the surface of the MgO-ZrO2 media features many lamellar structures it is expected that there are linear and point defects which are unique to the interfaces between the lamellars and the adjacent features of the porous media where adsorbates could radially bond via uniquely exposed electronic orbitals present within the vicinity of said defect sites [48, 49]. At the same time MgO partially stabilized ZrO2 is also an ionic conductor, where oxygen vacancies are present in the ZrO2 lattice and at the grain interfaces may enhance oxygen absorption, exchange, and O2- ion transport within the surface of the ceramic media; thus significantly enhancing, modifying, and facilitating the chemical reaction pathways [14, 50]. 3.3 Visual Characterization of the Porous Burner at the Exhaust Optical images of the exhaust pane of the combustion chamber show light emitted from the porous media which is at elevated temperatures due to occurring combustion reactions at various equivalence ratios and a fixed air flow rate of ܸሶ air=47.56 SLPM (Fig. 8). As one can see from Fig. 8, emitted wavelengths of light from the porous media vary, as indicated by the distinct -12ACS Paragon Plus Environment

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colors, according to the spectral emissivity of the media’s material and the temperature of the media.. While at the highest evaluated equivalence ratio of φ=0.75±0.02 all three porous media are hot enough to emit a sufficient quantity of electromagnetic radiation in the visible spectrum such that no features of the porous media are discernable within the photograph; the hue of the emissions from the porous media change depending on the media composition. Where the αAl2O3 media is primarily yellow-while emissions from the MgO-ZrO2 and SiC media are pinkwhite and violet-white, respectively. The brightness of imaging artifacts surrounding MgO-ZrO2 media is the highest, which is a white region within the center of the pane surrounded by a glow (Fig. 8), while the brightness of the imaging artifacts from the SiC media is the lowest as the extent of glow surrounding the exhaust port is nearly negligible. From the images of the SiC porous matrix, it can be seen that as the equivalence ratio is reduced, the temperatures within the combustion chamber decrease; and as a result there is a reduction in the amount of intensity in the imaging artifacts surrounding the exhaust port, revealing the structure of the porous media at the outer radius of the combustion chamber. However, in the case of media comprised of αAl2O3 or MgO-ZrO2, a competing process causes the quantity of light emitted from the combustion chamber exhaust to increase when the position of the chemically reactive span within the combustion chamber approaches the exhaust when equivalence ratio is reduced. As the equivalence ratio is decreased to φ=0.53±0.02, radiative emissions from the α-Al2O3, MgO-ZrO2 and SiC media become significantly more pronounced (Fig. 8). While MgO-ZrO2 porous media maintains a significant brightness, the intensity of light observed across the exit pane of the combustion chamber is significantly reduced in cases when either α-Al2O3 and SiC porous media are used (Fig. 8). Both for α-Al2O3 and SiC a rather large portion of the porous media’s structure becomes discernable. When the equivalence ratio is further reduced to


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φ=0.51±0.02, which is the lowest permissible equivalence ratio for SiC porous media, there is no radiative emission observed by the detector and the image is completely dark. A faint glow is still visible for α-Al2O3 porous media at this equivalence ratio. However, light emitted from the exhaust pane when a MgO-ZrO2 porous matrix is used, is bright enough at φ=0.51±0.02 that the media structure is not yet discernable and is comparatively the brightest. If the equivalence ratio is decreased further, considering the spectral images (Fig. 8) and temperature measurements (Fig. 7), both SiC and α-Al2O3 matrices in the combustor did not project sufficient quantities of thermal radiation in the visible spectrum. 3.4 Abstractions from Flow Simulation and Relation to Acoustic Spectra From the simulations performed analyzing gaseous flow through the combustion chamber, it is known that there will be three types of stable modes of the flow within the span between the combustion chamber inlet and the beginning of the porous media (Fig 9A). The three distinct flow modes are: (i) an axial core flow, having velocity components in the axial and radial direction (Fig. 9B); (ii) stabilized recirculation, in which gasses near the combustion chamber wall will flow against the direction of the axial core flow as a result of the sudden expansion (Fig. 9C); and (iii) swirl, which is characterized by azimuthal and radial velocity components (Fig. 9D). The three distinct flow schemes are components of the acoustic spectra emitted from the combustion chamber exhaust. Additionally, it is reasonable to expect that other stable modes of flow, which are inherently coupled, will exist within the span of the porous media and between the porous media and combustor exhaust [51, 52]. While the primary source of the acoustic spectra is the gaseous flow modes within the combustion chamber, it is also important to understand that the local rates of volumetric heat release from combustion are also strongly coupled with the acoustic spectra [53] as local -14ACS Paragon Plus Environment

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perturbations which are the source of the acoustic emissions dramatically enhanced localized species mixing. Within this current work, it was observed that there is a strong dependence of the acoustic spectra on changes in equivalence ratio as at different steady states of combustion, the spectral distribution and intensity of acoustic energy changes in a characteristic manner (Fig. 10). Additionally, it was found that porous media composition is also able to contribute to variations within the acoustic spectra during combustion. 3.5 Coupling of Simulated Flow Rates and Acoustic Spectra Acoustic profiles taken at the exhaust of the combustion chamber during steady state operation at different equivalence ratios for all three porous media are shown in Fig. 10. In the experiment the constant air flow rate of ܸሶ air=47.56 SLPM was used for all equivalence ratios. As one can see from Fig. 10, the acoustic emission spectra (peak intensities, positions, and full width at half maximum) varies based on both equivalence ratio and porous media composition. Regardless of the material used as porous media, acoustic spectra exhibits several peaks within the 200-500 Hz measured range. For spectra collected, when either Al2O3 or MgO-ZrO2 media were used, four distinct peaks were found at the equivalence ratio φ=0.75±0.02. These peaks are identified/numbered as peaks (1), (2), (3), and (4) in Fig. 10. However, the acoustic spectra recorded when SiC ceramics was used as porous media contained only three characteristic acoustic peaks, numbered (1), (2’), and (4) (Fig. 10) were recorded across the entire range of probed 200-500 Hz at φ=0.75±0.02 equivalence ratio. For acoustic spectra when either the αAl2O3 or MgO-ZrO2 media were utilized in the combustion chamber peaks (2) and (3) coalesce into (2’), which occurs at φ=0.53±0.02 and φ=0.57±0.02 for the α-Al2O3 and MgO-ZrO2 media, respectively. However, for when the SiC porous media was immersed in the combustion


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chamber, even at the highest evaluated equivalence ratio φ=0.75±0.02, the acoustic signature revealed only three peaks across the same frequency range: (1), (2’), and (3). Several distinct features of acoustic profiles can be resolved for each of the combinations of equivalence ratio and media composition. In all cases, regardless of media used, the intensity of all acoustic peaks decrease with decreasing value of equivalence ratio and, as a result coincide with decreasing temperatures within the combustion chamber. In addition, there is a shift of the acoustic peaks toward lower frequencies, which occurs simultaneously with rapid decay of the peaks’ amplitude upon decrease in equivalence ratio of the mixture in combustion. Such shift to the lower frequencies is especially pronounced at low equivalence ratios starting from φ=0.57±0.02 or lower. Although there is further study required to understand the absolute relationship between the emitted acoustic spectra and the processes occurring within the combustion chamber, such correlations between intensity and position of the acoustic peaks across the given frequency range could imply that acoustic spectroscopy can be used as a powerful ex-situ measurement technique to characterize heterogeneous combustion. The acoustic spectra collected during heterogeneous combustion over three different media at φ=0.75±0.02 and φ=0.53±0.02 are shown in Fig. 11 with an enlarged span of the frequency axis. While there is a plethora of information presented within Fig. 11, an attempt was made to identify key details from the numbered peaks which were found to be comprised of sets of subpeaks. For flows within porous media it is reasonable to expect characteristic structures of short range turbulence, confined to single voids of the porous media, and long range turbulence in which the turbulent flow of multiple voids interact. Such turbulent flows are suspected to be a major cause of the acoustic peaks. When such turbulence is well pronounced, a relatively high intensity of acoustic emissions occurs enabling a high signal to noise ratio. Such high intensity of


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the acoustic peaks provides a possibility to distinguish sub-peaks of the acoustic spectrum, which otherwise will not be resolvable. Thus, when φ=0.75±0.02 the sub-peaks of the numbered peaks are well discernable (Fig. 11 A-C). However, when φ=0.53±0.02 (Fig. 11 D-F), the acoustic signal intensities are significantly reduced and the number of sub peaks observable decreases; It is assumed that a reduction of the magnitude of turbulence within the flow field and intensity of combustion reactions is the cause of such decrease. 4. Conclusions A comparative study of performance of alumina (α-Al2O3), magnesia stabilized zirconia (MgOZrO2), and silicon carbide (SiC) highly porous media within a flow stabilized combustor at a constant air flow rate and variable methane flow rate was performed. It was determined that at high equivalence ratio of φ=0.75±0.02, SiC porous media significantly outperformed both MgOZrO2 and α-Al2O3 oxide porous media as a promotor of combustion, as significantly higher temperature profile was obtained. It was also established that there is a linear dependence between measured maximum temperature within the combustion chamber and the equivalence ratio at equivalence ratios φ ≤0.75 until a certain lean limit is reached, which is defined by the porous media’s chemical composition and heat loss from the combustion chamber. In addition, at lower equivalence ratios where gas phase chemical reactions are insufficient to maintain a stable flame the catalytic and O2- conductive MgO-ZrO2 porous matrix preformed optimally enabling the highest temperatures and the ability to maintain a stable flame at the lowest possible equivalence ratio of the evaluated media. Moreover, acoustic spectroscopy was used as an additional tool to characterize combustion in which it was shown that distinct acoustic peaks, comprised of many sub peaks, are dependent upon media type and equivalence ratio. The coupling between measured acoustic spectra and temperature profiles within the combustion


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chamber was speculated to be related to the turbulent flow of gaseous fluid within the porous media during combustion. Acknowledgements This research was supported in part by NSF IIP project “1343454” Technology Translation – Superadiabatic Combustion in Porous Media for Efficient Heat Production; and NSF MRI project “133775” Development of a Multi-Scale Thermal-Mechanical-Spectroscopic System for in-Situ Materials Characterization, Research and Training. The authors would also like to thank Robert Surace and Zafir Abdo at Siemens Energy, Inc. for assisting with SEM acquisition and characterization.


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Tables

Table 1:Thermal properties of the materials comprising porous foams at selected temperatures [23-31]. α-Al2O3 MgO-ZrO2 SiC ࢑ࡶ 0.76 0.72 0.71 C (20° C) [ ൗ࢑ࢍ · ࡷ] ࢑ࡶ C (1000° C) [ ൗ࢑ࢍ · ࡷ]

1.26

0.78

1.24

k (20° C) [ࢃൗ࢓ · ࡷ] k (1000° C) [ࢃൗ࢓ · ࡷ] ᆅn (1000° C)

24.0

4.0

70.0

6.1

2.1

21.5

0.46

0.38

0.87


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FIGURES

Fig. 1: A model schematic presentation of combustion chamber [30] and photographs of MgOZrO2 SiC, and α-Al2O3 porous media.


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Fig. 2:X-ray diffraction patterns of substrate powders (A) α-Al2O3 (B) MgO-ZrO2, and (C) SiC. SEM micrographs of (D) α-Al2O3 (E) MgO-ZrO2, and (F) SiC.


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Fig. 3: Temperature profile of combustion chamber with α-Al2O3 ceramic media as (A) functions of time at various thermocouple locations, and (B) axial position at various equivalence ratios.


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Fig. 4: Temperature profile of combustion chamber with MgO-ZrO2 ceramic media as (A) functions of time at various thermocouple locations, and (B) axial position at various equivalence ratios.


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Fig. 5: Temperature profile of combustion chamber with SiC ceramic media as (A) functions of time at various thermocouple locations, and (B) axial position at various equivalence ratios.


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Fig. 6: Plot of the maximum temperatures measured by axially mounted thermocouples during combustor operations at different equivalence ratios for the α-Al2O3, MgO-ZrO2, and SiC porous media.


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Fig. 7: Comparison of axial temperature profiles measured by thermocouples during combustor operation using α-Al2O3, MgO-ZrO2, and SiC porous media at various equivalence ratios.


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Fig. 8: Photographs of the combustor exhaust taken with various compositions of porous media within the combustion chamber during operation at selected equivalence ratios.


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Fig. 9: Proposed types of stable flow modes between the combustion chamber inlet and porous media.


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Fig. 10: Acoustic spectra recorded at steady state operating conditions during combustion.


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Fig. 11: Acoustic spectra emitted from the combustor with various substrates within the combustion chamber showing major peak divisions and fine details. (A through C) at φ=0.75±0.02, (D through F) at φ=0.53±0.02.


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Energy & Fuels

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Thermal and Acoustic Performance of Al2O3, MgOâZrO2, and SiC

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Thermal and Acoustic Performance of Al2O3, MgOâZrO2, and SiC