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The fully resolved dynamics of an elliptical particle suspended in an upward flow in an expanding channel, with successively increasing degrees of freedom of motion from a completely fixed state to an eventually fluidized state, are examined using particle resolved direct numerical simulation. The signed distance function immersed boundary method (sdfibm), implemented in OpenFOAM, is validated against results from COMSOL for the case of a pinned ellipse. The aspect ratio of the ellipse (defined as the ratio of the major to the minor axis (γ=Ra/Rb) is held constant at 2, while the minor axis is kept as Rb=0.15 (with respect to the inlet width, Wi = 1). A particle Reynolds number defined as Rep=(2Ra)Vmax/ν (where Vmax = 1 is the centerline velocity) is varied up to 300. The simulations exhibit rich dynamical behavior with stable, steady solutions up to Rep around 67, above which vortex shedding begins, with the ellipse responding with its oscillatory motion in response to the stresses and torques acting on it by the fluid. For the case of free translation in the x direction, multiple oscillatory states are found, where the particle is confined in the left or right half of the flow domain, depending on the initial placement of the particle. The forces on the particle get progressively complicated as the degree of freedom of movement of the ellipse increases, indicating that traditional drag correlations with fixed particles may not be valid in situations when they have mobility if one desires higher fidelity in coarse-grained models.

Heat recirculating reactors have many potential applications as thermal oxidizers, combustors, and fuel reformers due to their extensive operating range. Low emissions and fuel flexibility make suc...

This study investigates the impact of pressure and dilution on combustion behavior of C2H6/air/N2 mixtures within an externally heated micro-tube using both experimental and analytical techniques. Experiments are conducted for pressures up to 10 bar at multiple dilution levels and document three flame regimes, i.e. weak flames, FREI and normal flames. Characteristic temperatures for flame stabilization, as well as ignition and extinction are obtained by applying a novel thin filament pyrometry method (TFP), which uses a thin silicon carbide (SiC) filament that enables temperature measurements for both reacting and non-reacting cases. Results show a decrease of characteristic temperatures with increasing pressure, which can be partially compensated by a decrease of O2 content. Further, regime transitions occur at lower velocities as the pressure increases, whereas weak flame positions are insensitive to dilution at a given pressure. Predictions from an analytical model compare favorably with experimental results. The most prominent finding is that as dilution increases, the transition from weak flame to FREI becomes less abrupt and is completely lost for high dilution levels (i.e. low O2 content). Both experimental and analytical results indicate that the unstable FREI combustion regime does not exist at high dilution levels, i.e. weak flame and normal flame regimes are no longer clearly separated. As FREI correspond to a cyclic transition from an ignition zone to a propagating flame, results for highly diluted conditions are interpreted as a combustion regime where classical flame propagation ceases to exist.

Characteristics of non-premixed flames such as flame height and lift-off height are affected by the presence of magnetic fields due to the paramagnetic properties of some combustion species. However, it is unknown whether magnetic fields can be used to reduce the emission of pollutants in non-premixed flames. In general, pollutant emissions are reduced in combustion systems if the mixing of combustion species is enhanced during the process. Since paramagnetic combustion species such as O2, O, OH, HO2, etc have a preferential motion direction in the presence of magnetic fields, there is a potential to harness this effect of mixing by imposing a magnetic field on the flame. This study seeks to provide some insights on the effect of magnetic field on pollutants generated in a laminar non-premixed flame numerically. The non-premixed flame is simulated using a detailed chemical mechanism for propane-air combustion and a modified Moss-Brookes soot model. To simulate the effect of magnetism on the paramagnetic chemical species, the species paramagnetic susceptibility is computed using the Curie relation. The non-premixed flame is placed at three different locations within the magnetic field. The computation predicted that the amount of average pollutants reduction is dependent on the location of the flames within the magnetic fields with respect to magnetic gradients. The mass weighted average of the soot volume fraction over the computational domain decreased when the non-premixed flame is located at certain locations within the magnetic field of the solenoid with respect to the absence of the magnetic fields, but increases in other locations.

Within the area of combustion, externally heated microtubes have been introduced to study the combustion characteristics of fuels and fuel blends. Microreactors have advantages over other conventional fuel testing methods because of their potential to test small volumes (< 20 μl) at high throughput. In this work, a high-pressure microreactor is designed and implemented to test fuels up to a pressure of 20 bar where automated testing reduces test time substantially. The novelty of this device is its capability to operate at pressure exceeding the current state of the art of 12 bar. The combustion behavior of fuels is tested in an externally heated quartz tube, with a diameter less than the conventional quenching diameter of the fuel. The ultimate objective of the experiment is to investigate the impact of fuel on flame characteristics. The ability to reach engine relevant pressure conditions and its inherent small volume requirements make this device a potential candidate for measurements of laboratory transportation fuels and fuel blends. For initial validation, tests from an earlier intermediate pressure experiment with ethane/air and nitrogen mixtures are repeated. Chemiluminescence images are taken to evaluate the combustion characteristics in terms of the three classical flame regimes: weak flames, Flames with Repetitive Extinction, and Ignition (FREI) and normal flames. Previous results at intermediate pressure showed that as the pressure increases, the weak flame and FREI regimes shift towards lower velocities. Also, as dilution level increase (i.e. reducing oxygen concentration), the transition from the weak flame to FREI becomes less abrupt and is completely lost for marginal oxygen concentration. The objective of this study is to document flame dynamics at higher pressures.

Micro-combustion has shown significant potential to study and characterize the combustion behavior of hydrocarbon fuels. Among several experimental approaches based on this method, the most prominent one employs an externally heated micro-channel. Three distinct combustion regimes are reported for this device namely, weak flames, flames with repetitive extinction and ignition (FREI), and normal flames, which are formed at low, moderate, and high flow rate ranges, respectively. Within each flame regime, noticeable differences exist in both shape and luminosity where transition points can be used to obtain insights into fuel characteristics. In this study, flame images are obtained using a monochrome camera equipped with a 430 nm bandpass filter to capture the chemiluminescence signal emitted by the flame. Sequences of conventional flame photographs are taken during the experiment, which are computationally merged to generate high dynamic range (HDR) images. In a highly diluted fuel/oxidizer mixture, it is observed that FREI disappear and are replaced by a gradual and direct transition between weak and normal flames which makes it hard to identify different combustion regimes. To resolve the issue, a convolutional neural network (CNN) is introduced to classify the flame regime. The accuracy of the model is calculated to be 99.34, 99.66, and 99.83% for “training”, “validation”, and “testing” data-sets, respectively. This level of accuracy is achieved by conducting a grid search to acquire optimized parameters for CNN. Furthermore, a data augmentation technique based on different experimental scenarios is used to generate flame images to increase the size of the data-set.

Temperature measurements based on spectral radiation are often limited by prior knowledge of target surface properties and detector sensitivity. The present work describes a multispectral pyrometry method for temperature estimation that does not require information on surface emissivity or calibration. Digital camera simulations considering a silicon-based sensor and five bandpass filters are used to create synthetic images of targets. The detector sensitivity is compensated via high dynamic range imaging technique. The effect of natural and detector noise on temperature estimates is assessed via Monte Carlo simulations. Results from a wavelength-independent model (Gray) are compared with those from the Band model (with emissivity varying between constant spectral bands). The effect of detector dynamic range and spectral position of bandpass filters is discussed. Temperatures as low as 520 K are obtained, with an overall uncertainty of the order of 10 K. The proposed method is applicable to temperature measurements using commercial-grade CMOS/CCD cameras.

This study employs a simplified analytical model to obtain qualitative predictions of transitions between non-adiabatic combustion regimes found in non-catalytic tubular micro-flow reactors, i.e. normal stable flames, flames with repetitive extinction and ignition (FREI), and weak flames that are stabilized by wall heat transfer. Assuming large activation energy for single-step kinetics, a comparison of exact solutions for constant and variable wall temperature profiles reveals that while a non-zero wall temperature gradient introduces a temperature lag, it has no impact on the structure of the flame solution itself. This result is significant as it establishes a link to quenching characteristics within heated micro-tubes, and further facilitates a simplified micro-flow reactor analysis. Predictions for sweeps of wall temperatures are combined to S-curves, which allow for a combustion regime classification for a given combination of micro-flow reactor geometry and mixture properties. A variation of parameters such as tube diameter or mixture stoichiometry yields predictions of shifts in regime transitions, thus providing insights on interactions between heat transfer and first order effects of chemical kinetics. A comparison to experimental data illustrates that the theoretical framework, – within inherent limitations, – captures qualitatively correct trends that reflect experimental observations, thus providing valuable insights for micro-flow reactor design and operation.

To improve detailed chemical kinetics models, the oxidation of methanol was investigated behind reflected shock waves in shock tubes. Ignition delay times of methanol–air mixtures, with Ar as diluent, were studied between 940 and 1540 K in a heated shock tube, for pressures up to 14.9 atm and for equivalence ratios of 0.5, 1.0, and 2.0. Water profiles were measured by utilizing a laser absorption technique in the 1350-to-1600-K temperature range, at an average pressure of 1.3 atm and for similar equivalence ratios. The present study shows the ignition delay times of methanol to be in very good agreement with results from the literature (Fieweger et al., 1997), whereas the other conditions have never been investigated before. The ignition delay time data are also in good agreement with modern detailed kinetics mechanisms such as the AramcoMech 3.0 model. The water time-history profiles were modeled using well-known literature mechanisms. Discrepancies were observed between these kinetics mechanisms, and poor predictions were observed for the lower temperatures investigated. Sensitivity and rate-of-production analyses were performed using 3 literature mechanisms (namely, AramcoMech 3.0, Princeton, and JetSurfII). Discrepancies were found among the models when predicting important reactions dominating the oxidation of methanol as well as the rate-of-production of H2O.

Glycerol is available in high volumes and at low costs as a byproduct of bio-diesel production. While combustion characteristics are poor, it is appealing to investigate potential reforming processes to recover its hydrogen and carbon in the form of synthesis gas, i.e. a combustible mixture of hydrogen, carbon monoxide, methane and other small hydrocarbon species. This study pursues a non-catalytic approach, where reactions occur in the gas-phase at elevated temperatures favored by chemical kinetics. Detailed glycerol reaction chemistry involves many species and reactions, which makes a careful selection of numerical approaches critical. In this work, Eulerian and Lagrangian viewpoints for numerical calculations of a non-catalytic reforming process at intermediate temperatures are studied. All approaches are validated by propane partial oxidation, where non-catalytic experimental results are available in literature. Testing the numerical approaches for a wide range of operating conditions, it is illustrated that the Lagrangian approach outperforms other models in terms of efficiency. Using a transient zero-D Lagrangian technique, glycerol reforming characteristics are discussed at various wall temperatures and mixture conditions. The impact of fuel stoichiometry is investigated based on the oxygen ratio, which is a metric for mixture stoichiometry that accounts for the amount of oxygen bound in the fuel molecules. Results clearly indicate that wall temperatures higher than 1173 K and oxygen ratios in the range of 0.3–0.45 represent conditions that are favorable for glycerol reforming. It is further shown that excess methanol – a commonly found contaminant in crude glycerol – does not affect the reforming performance. As there is a lack of information on non-catalytic glycerol reforming in literature, qualitative comparisons between non-catalytic (from this work) and catalytic (from literature) glycerol reforming are discussed.

Ignition delay times of a gasoline surrogate (iso-octane/toluene/n-heptane/1-hexene at 55/25/15/5% by liquid volume, developed in McCormick et al. 2017) with high-level bioblendstock-gasoline surrogate blends (50% and 85% biofuel by liquid volume of each ethanol and methyl acetate) were collected behind reflected shock waves. Post-reflected-shock temperatures ranged from 968 to 1361 K at pressures of about 4, 10, 25, and 50 atm. Data were collected for real fuel–air mixtures at fuel lean and stoichiometric conditions (φ = 0.5, 1.0) with focus on the more-practical, fuel-lean conditions. Ignition delay times were measured from OH* chemiluminescence around 307 nm using an endwall diagnostic. The RON and MON of the surrogate were 90.3 and 84.7, respectively. Ethanol was chosen due to its wide use in flex-fuel vehicles and availability, while also providing an increased octane rating. Methyl acetate was chosen for its especially high octane rating to investigate the effect of an extreme case. To validate the gasoline surrogate, the data are compared to real gasoline (RD387) and several gasoline surrogate experiments from the literature. Using the wide range of pressures studied, the gasoline surrogate’s pressure dependence was quantified to account for test-to-test variations using regression analysis. Similarly, a global correlation for gasoline and its surrogates was developed using all available data from the literature. Two modern chemical kinetics models targeting gasoline and its surrogates are compared to the ignition delay time measurements. These new high-pressure, high-bioblendstock concentration tests provide required chemical kinetic data for optimizing fuel and engine design.

Most flow visualizations and flow measurements to understand particle mobility in porous media are typically performed in transparent microfluidic devices (micro-models) with 2D pore-throat networks. Nano-particle mobility studies to date have been limited to micro-models made of transparent thermoplastic or silicone-based materials. In an effort to fabricate materials close to reservoir rock, ceramic micro-model has been designed and micro fabricated by our group to study nano-particle transport in rock-based ceramic micro-model. A Confocal Micro-Particle Image Velocimetry (C-μPIV) technique augmented with associated post processing algorithms [1] is used in obtaining 3D distributions of nano-particle velocity and concentration at selected locations of the ceramic micro-model. Furthermore, a novel in-situ, nondestructive method of measuring 3D geometry of non-transparent ceramic micro-model is described and validated. The particle experiment uses 860 nm fluorescence labeled polystyrene neutrally buoyant, and electrically neutral nano-particles. The data was acquired using confocal laser-scanning microscope to quantify 3D particle transport at selected observation locations. In addition, fluorescence microscope was used to measure in-situ geometry of porous media. Results of detailed 3D measurements of nano-particle velocity and particle concentration from experiment conducted at a constant flow rate of 30 nL/min in the rock-based micro-model are presented and discussed. Particle velocities range from 0 to 20.93 μm/sec in magnitude, and average concentration range from 6.02 × 103 to 6.79 × 103 particles at inlet channel while velocities range from 0 to 73.63 μm/sec and concentration range from 4.9 × 101 to 1.45 × 103 particles at selected observation locations of the ceramic micro-model. 3D velocity fields at selected locations also indicate that mean velocity closer to the top wall is comparatively higher than bottom wall, because of higher planar porosity and smooth pathway for the nano-particles closer to the top wall. The three dimensional micro-model geometry reconstructed from the fluorescence data can be used to conduct numerical simulations of the flow in the as-tested micro-model for future comparisons to experimental results after incorporating particle transport and particle-wall interaction models.

A ceramic-based micromodel was fabricated with batching of green alumina ceramics mixed with polymer binders, extrusion of the green alumina tapes, and hot embossing of the green tapes with a metal mold. The metal mold fabricated using optical lithography of SU8 and electroforming of nickel contained 2.5D pore network geometry in 13 layers of a rock, Boise sandstone. The hot embossing process enabled the generation of the pore network geometries with a minimum feature size of 25 μm and for distinct formation of the 13 layers of the 2.5D pore geometry of the rock. The green ceramic micromodels were processed with solvent extraction, thermal debinding, and sintering. The sintered micromodels showed significant shrinkages at all directions of the micromodels, which were 17.6% in x, 17.5% in y, and 14.6% in z. The sintered, 2.5D rock-based ceramic micromodel was capped with a thin glass cover slide and used for flow visualization with a fluorescent dye and fluorescent nano-particles. The dye-filled micromodel showed good flow connectivity and fluorescence signal intensity dependence on depth. It was observed that the peak particle concentration close to the observation window and gradual decrease in particle concentration along the depth. The higher velocities were measured in the low flow resistance region with velocity variations along the depth. The microfabricated 2.5D ceramic micromodels will allow resistance to harsh experimental conditions such as high temperature and pressure, and opportunity for investigation of the complex flow patterns in 3D.

Chemical Explosive Mode Analysis (CEMA) is used to illustrate how source terms and reactions associated with state variables drive the combustion process across a large database of lean, premixed H 2 /air flames with detailed chemistry. Based on an eigenanalysis of chemical sources terms, chemical source modes and associated reaction modes are distinguished. The dominant mode amplitude is used to determine whether source terms within the flame structure are promoting or counteracting the dominant chemical source mode. Mode sources are then defined from source terms to further differentiate dominant sources (i.e. species or temperature) that promote or counteract a chemical source mode. Similarly, mode participation distinguishes promoting reactions from counteracting ones. Based on mode sources and mode participation, transition temperatures are introduced to track dominant species and reactions within the flame structure. Concepts are detailed for one reference case ( ϕ = 0.99 and T in = 302 K ), which is then applied to a large database of simulated test cases with distributed inlet temperature (300–800 K) and equivalence ratio (0.1–1). In all cases, transitions correlate with the adiabatic flame temperature for a simulation. Results show a perfect collapse of temperatures associated with the definition of the flame sheet in the context of CEMA. Furthermore, transitions within the flame structure show trends that clearly illustrate that flames with similar adiabatic temperatures share a comparable chemical structure.

The objective of this study is to investigate the choice of thermoplastic binders in a two-step debinding process involving solvent extraction (SE) and thermal debinding (TD) of samples produced using polymer (co-)extrusion. Two-step debinding uses a binder system combining soluble binders and thermoplastic binders, which are removed by SE and TD, respectively. In prior work, it was demonstrated that the two-step protocol eliminates debinding defects that are commonly observed in TD (Sharmin and Schoegl, 2014 [1] ). The primary goal of this follow-up study is to establish that two-step debinding eliminates debinding defects irrespective of the choice of thermoplastic binders; a secondary goal is to investigate how the binder choice affects the co-extrusion process. In total, five batches using alumina powder and a binder system with a 1:1 ratio of thermoplastic binder and polyethylene glycol (PEG) as water soluble binder were tested. Three grades of polyethylene butyl acrylate (PEBA) and two grades of polyethylene vinyl acetate (EVA) with different melt flow indices were investigated. In all cases, extraction tests resulted in 80 wt% removal of the original PEG content. However, PEBA samples showed a significantly faster PEG removal compared to EVA samples, which is attributed to a better dispersion of PEG within EVA. Tests with 4h extracted PEBA and 6h extracted EVA samples resulted in 60 wt% removal of PEG and a defect-free specimen after TD and sintering. Also, microstructures of sintered PEBA and EVA samples are comparable, which confirms that the choice of thermoplastic binders has no effect on the two-step debinding process. Co-extrusion tests were performed for selected EVA and PEBA blends, where results showed better performance for co-extrusion of EVA blends.

Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.

The objective of this study is to assess the feasibility of solvent extraction (SE) for partial binder removal in the context of polymer co-extrusion with a thermoplastic binder component. Polymer co-extrusion is able to produce multilayered, functionally graded and/or textured structures in an efficient manufacturing process, but requires a polymer binder system with suitable flow characteristics. Traditionally, the binder is removed by thermal debinding (TD), which, however, is prone to form cracks or blisters, both of which are attributed to a lack of initial pore space that allows pyrolysis products to escape. The primary focus of this work is to demonstrate that a binder system with a high soluble binder content is suitable for conventional polymer co-extrusion and to document that a two-step binder removal process involving both SE and TD eliminates debinding defects. The overall fabrication process is documented for the extrusion of solid ceramic rods and co-extrusion of tubes, where alumina powder was batched with polyethylene butyl acrylate (PEBA) as backbone polymer and polyethylene glycol (PEG) as water soluble binder. SE for specimen with varying PEBA:PEG ratios was tested in water at three different temperatures for various times. The 1:1 mixture showed a PEG removal up to 80 wt.% of the original PEG content after 6 h extraction; after subsequent thermal debinding, rods and tubes sintered successfully without defects, demonstrating the viability of the process.

Temperature and concentration distributions of a horizontal section through a simulated stationary test flame were reconstructed with the help of computer tomography. The study uses temperature and concentration distributions equal to published time-averaged data for the Sandia D flame. Projection values are obtained by simulated line-of-sight integration for two water vapor (H2O) absorption lines in the 6930–6940 cm−1 range, where direct absorption spectroscopy and wavelength modulation spectroscopy with second harmonic frequency detection are considered. The projected values obtained from line-of-sight measurements are deconvoluted by tomographic methods, where Abel inversion and filtered backprojection are investigated. The tomographic reconstruction yields spatially resolved spectroscopic data, which is then used to calculate distributions for temperature and concentrations. Results illustrate that tunable diode laser absorption spectroscopy artifacts are caused by relatively small errors in reconstr...

The study compares the performance of different pathways for gas-phase (non-catalytic) fuel reforming between 600 and 1000 °C. Specifically, the conversion of propane to hydrogen-rich syngas was investigated numerically and experimentally for pyrolysis (Py), steam reforming (SR), partial oxidation (POx), and autothermal reforming (ATR). Experiments were conducted in a tubular quartz reactor, where temperatures were imposed externally; reactants were diluted with nitrogen to reduce the impact of endothermic/exothermic reactions on the variation of gas-phase temperatures. In experiments, product concentrations of hydrogen, carbon monoxide, carbon dioxide, methane, and a range of hydrocarbon species were measured at predetermined operating conditions. The performance of each homogeneous reforming process was evaluated and compared by assessing propane conversion and production efficiencies for hydrogen and other species of interest. At 600 °C, propane conversion was low, but increased substantially with temperature; complete conversion was achieved at 1000 °C. Furthermore, findings show improved hydrogen production efficiencies of POx/ATR when compared to Py/SR. Experimental results are substantiated by numerical simulations with detailed chemical kinetics; numerical results are in good agreement with experiments at identical operating conditions. Experimental and numerical results for non-catalytic propane reforming at all tested temperatures (600–1000 °C) imply a negligible impact of steam addition to the process, as results for SR resemble Py results, and ATR closely follows POx characteristics. As such, results clearly show that steam does not play an active role in gas-phase reforming of propane at intermediate temperatures.

Combustion-based personal power systems have great potential for applications that demand high energy density, compact, and durable components. Through the use of heat recirculation, increased combustor firing rates and extension of flammability limits can be achieved. This study examines the combustion of lean methane/air reactant mixtures below the conventional flammability limit in a mesoscale heat-recirculating counter-flow reactor. An analytical model, previously developed for reforming in the counter-flow reactor, was adapted and applied to study lean operation. Results qualitatively describe the operating principle of the reactor and its behavior in response to changes in inlet conditions. Experiments determined the stable operating range of the reactor. Peak reactor wall temperature measurements indicate superadiabatic operation. Exhaust emissions of CO and NO x range from 35 to 143 ppm and 5 to 25 ppm, respectively, and emission indices as a function of firing rate are presented. Unburned hydrocarbons were detected at the lowest firing rates.

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.

Recirculation of heat has been used for decades to react mixtures beyond the conventional flammability limits. One means of obtaining this recirculation is through counter-flow heat exchange. In contrast to filtration combustion in which a reaction front propagates through a packed bed, counter-flow heat exchange results in stationary reactions zones. The objective of this study is the numerical investigation of reaction zone characteristics of ultra-rich methane/air combustion with counter-flow heat exchange. The flow channels of a counter-flow heat exchanger are modeled in two dimensions, and detailed reaction chemistry is described by the kinetics mechanism GRI 2.11. The results show that combustion starts in the vicinity of the channel walls, resulting in tulip-shaped reaction zones. The impact of operating conditions on combustion characteristics is studied in terms of inlet velocity and equivalence ratio, and findings are related to recently published experimental data.

Although hydrogen is abundant in nature, it is primarily bound into other molecules such as hydrocarbons. The conversion, or reforming, of a hydrocarbon into a hydrogen-rich syngas may be accomplished through either catalytic or non-catalytic processes. In this paper, we examine a non-catalytic reactor for syngas production that consists of multiple parallel channels with alternating flow directions. Heat recirculation between channels with opposing flow directions promotes reactions of rich mixtures beyond the conventional flammability limit. Experimental results with propane/air mixtures show that stable reaction fronts are obtained at a wide range of equivalence ratios and inlet velocities. The performance of the reactor with propane/air is compared to previously published results with methane/air mixtures, where significant differences are attributed to the more complex reaction chemistry of rich propane combustion.

Current advances in mobile devices bring about power demands that are quickly outpacing available battery technologies. Portable power systems based on fuel cells promise higher power densities, but face challenges with respect to the storage of suitable fuels. Hydrocarbon fuels offer high power densities and can be reformed into a hydrogen-rich syngas, which, once purified, can be used to power hydrogen operated fuel cells. In this paper, we present experimental results for a novel non-catalytic mesoscale fuel reformer concept, which is based on heat recirculation between parallel channels with opposing flow directions. The experimental results substantiate earlier analytical results by the authors predicting stationary combustion zones viable beyond the conventional rich flammability limit, where conditions are favorable for syngas production. Tests of a reactor prototype with four channels, 4 mm high, 91.5 mm long and 33.6 mm wide, showed the feasibility of the non-catalytic fuel reformer concept. Stationary combustion fronts were established for premixed methane/air mixtures at equivalence ratios between 1.8 and 2.5. For constant equivalence ratios, operation was possible for a wide range of firing rates, which is equivalent to a large turn-down ratio. In addition, superadiabatic temperatures substantially increase reaction rates, which result in a high power density of the fuel reformer.

The conversion of liquid heptane to syngas in a porous medium reactor consisting of a packed bed of alumina pellets is investigated numerically and experimentally. In experiments, the exhaust gas was analyzed for hydrogen, carbon monoxide, carbon dioxide, methane, and hydrocarbon species over equivalence ratios from 1.4 to 3.8 and a range of mixture inlet velocities. The efficiency of the noncatalytic fuel reformer regarding hydrogen production, carbon monoxide production and energy conversion is assessed. At constant inlet velocity, hydrogen production increases with increasing equivalence ratio, whereas hydrogen conversion efficiency reaches its peak value around an equivalence ratio of 3.0. Tests at a constant equivalence ratio of 2.5 show that conversion efficiency increases with mixture inlet velocity, and experimental values in excess of 80% are obtained for the highest tested velocity of 80 cm/s. Similar trends are observed for carbon monoxide conversion and energy efficiencies, where peak values exceed 90% and 80%, respectively. Some discrepancies are noted between the experimental and numerical results in the ultrarich regime. Overall, the results indicate favorable conditions for fuel reforming between equivalence ratios of 2.5 and 3.5 and show that the inlet velocity has a significant effect on the performance of noncatalytic fuel reforming. Substantial efficiency gains are observed for increased inlet velocities, which are attributed to increases in the reactor temperature.

Classic theories of combustion rely upon the principle that the maximum temperature attainable in a reaction is predicted by the adiabatic equilibrium temperature. In certain burner configurations, however, the maximum temperature may locally exceed this value by a significant amount due to heat recirculation, which has led to the concept of superadiabatic combustion. A striking aspect of this type of combustion is a broadening of the limits of flammability due to the accelerating effect of temperature increases on chemical reaction rates. In this paper, a simple analytical model is developed to study the superadiabatic performance of a combustor consisting of two parallel channels of finite length that are divided by a conducting wall. Assuming equal flow rates in the individual channels, the co-flow configuration is equivalent to combustion in conducting tubes, whereas the counterflow configuration is conceptually similar to a Swiss-roll burner. In both cases, the characteristics of superadiabatic combustion of a fuel-rich premixed fuel/air mixture are studied in terms of wall conductivity, heat transfer, and geometry of the combustor.

In this work, millimeter-scale tubular combustion channels were fabricated from ceramic precursor materials. Co-extrusion of structured feedrods holds promise for the development of multi-layered, functionally graded and/or textured combustor walls, but requires a polymer binder that is difficult to remove before structures can be sintered to full density. In conventional thermal debinding, cracking is a major issue, where crack formation is attributed to a lack of pore space for outgassing of pyrolysis products. The main focus of this study is to validate a manufacturing process that uses a combination of solvent de-binding and thermal debinding, which is applied to a simple combustor geometry. Alumina powder was batched with a mixture of polyethylene butyl-acrylate (PEBA) and polyethylene glycol (PEG) in a torque rheometer. A 19mm feedrod, consisting of a carbon-black/binder mixture as core, and a surrounding ceramic/binder mixture forming the wall, was extruded through a 5.84 mm die. The binder removal involves two processing steps, where the PEG content was removed by solvent extraction (SE) to initiate pore formation, after which thermal de-binding by pyrolysis removes the remaining binder and carbon-black. Solvent extraction was performed in water at three different temperatures for various times. The 1:1 mixture of PEG:PEBA showed the highest PEG removal of 80wt% for 6 hrs extraction. The thermal de-binding cycle was designed based on thermo-gravimetric analysis (TGA) and successfully performed with a ramping rate of 1.25°C/min to 1000°C without any crack formation. After de-binding, samples were sintered at 1600°C for 1 hr. SEM analysis showed some void spaces in the solvent extracted samples but confirmed that solvent extraction followed by thermal de-binding yielded the best results. The viability of sintered ceramic tubes was tested for conditions typical for thermal cycling in a combustion environment.Copyright © 2013 by ASME

Temperature and concentration distributions of a simulated flame were reconstructed with the help of computer tomography and tunable diode laser absorption spectroscopy (TDLAS). Reconstructions were based on the simulated numerical values of temperature and concentration of a stationary flame. Integrated absorption measurements along the line-of-sight (LOS) across the flames due to absorption by water vapor (H2O) in the near infra-red (NIR) region, specifically the 6930–6940 cm−1 range, were simulated to obtain the projection values for tomography. Spectroscopic parameters for the absorptions transitions, such as line-strengths, transition wavenumbers, collisional broadening coefficients and coefficients for their temperature dependency were selected from the HITRAN 2004 database. Simulated LOS data are inverted using a multiplicative algebraic reconstruction technique (MART), which are known to outperform traditional filtered back projection methods for cases with limited numbers of views. Based on spatially resolved reconstructions of spectroscopic data, temperature and concentration distributions are calculated using the wavelength modulation spectroscopy with second harmonic detection (WMS-2f) technique. A parametric study based on the number of views, orientation of views and number of rays per view required by the ART is performed in order to assess requirements for an acceptable reconstruction.Copyright © 2012 by ASME

Temperate and concentration distributions of a horizontal section through a stationary test flame were reconstructed with the help of computer tomography. Simulated reconstructions are based on temperature and concentrations distributions equal to published time-averaged data of the Sandia D flame. Based on property distributions, line integrated absorption due to water vapor (H 2 O) in the 6930-6940 cm 1 range was simulated to obtain projection values for tomographic reconstructions, where spectroscopic parameters for absorption features were obtained from the HITRAN 2004 1 database. Simulated projection values were calculated for direct absorption spectroscopy (DA) and wavelength modulation spectroscopy with second harmonic frequency detection (WMS-2f). The resulting projections were de-convoluted by tomographic methods, where Abel inversion and filtered backprojection were investigated. Based on the reconstructed spatial distribution of spectroscopic data, temperature and concentration profiles were obtained. Results highlight the impact of uncertainties introduced by measurement noise, deviations of reconstructed values, and tomographic artifacts. The comparisons of reconstructed results indicate that WMS-2f in conjunction with tomography is a viable method to reconstruct spatially resolved temperature and concentration distributions for stationary flames.

The application of tunable diode laser absorption spectroscopy (TDLAS) to flames with non-homogeneous temperature and concentration fields is an area where only few studies exist. Experimental work explores the performance of tomographic reconstructions of concentration and temperature profiles from wavelength-modulated TDLAS measurements within the plume of an axisymmetric McKenna burner. Water vapor transitions at 1391.67 nm and 1442.67 nm are probed using calibration free wavelength modulation spectroscopy with second harmonic detection (WMS-2f). A single collimated laser beam is swept parallel to the burner surface, where scans yield pairs of line-of-sight (LOS) data at multiple radial locations. Radial profiles of absorption data are reconstructed using Tikhonov regularized Abel inversion, which suppresses the amplification of experimental noise that is typically observed for reconstructions with high spatial resolution. Based on spectral data, temperatures and concentrations are calculated point-by-point. Here, a least-squares approach addresses difficulties due to modulation depths that cannot be universally optimized due to a non-uniform domain. Experimental results show successful reconstructions of temperature and concentration profiles based on two-transition, non-optimally modulated WMS-2f and Tikhonov regularized Abel inversion, and thus validate the technique as a viable diagnostic tool for flame measurements., Comment: This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/AO.53.008095. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law