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We examined in this work the combined effects of chemical reaction order n and the stoichiometric mixture fraction Zs on the propagation of nonpremixed edge flames in a counterflow configuration. W...

A novel thruster concept for CubeSats and other small satellites using hydrogen peroxide vapor as a propellant is presented and evaluated. This concept leads to the highest theoretical vacuum speci...

The propagation rates (Uedge) of edge-flames in premixed hydrocarbon-oxygen-inert mixtures were measured as a function of global strain rate (σ), mixture strength, and (by changing fuel and inert type) Lewis number (Le). Using a counterflow slot-jet burner with electrical heaters at both ends to anchor the flame edges, both advancing (positive Uedge) and retreating (negative Uedge) edge-flames were characterized. Results are presented for both twin (premixed gas against premixed gas) and single (premixed gas against cold inert gas) edge-flames in terms of the effects of a non-dimensional strain rate (e) and non-dimensional heat loss (κ) on a scaled propagation rate. Uedge showed a strong dependence on Le and flames images show that high (low) Le lead to weaker (stronger) edge-flame burning intensity. Uedge for single edge-flames scaled with the square root of the unburned to burned gas density ratio in a manner similar to nonpremixed flames whereas for premixed flames Uedge scaled linearly with density ratio. Edge-flames exhibited two extinction limits corresponding to a high-σ strain induced limit and a low-σ heat loss induced limit; a simple description of the low-σ limits was proposed and found to correlate well with experiments in twin-premixed, single-premixed, and nonpremixed edge-flames over more than a two-decade range of κ. Results are in good qualitative and reasonable quantitative agreement with simple theories except that retreating twin edge-flames in high-Le mixtures are predicted theoretically but were not observed experimentally.

The influences of stoichiometric mixture fraction (Zst) and global strain rate (σ) on the shapes and propagation rates (Uedge) of nonpremixed edge-flames in H2 N2/O2 N2 mixtures were investigated using a counterflow slot-jet apparatus. Both positive and negative Uedge were observed depending on dilution level, Zst and σ. At low Zst only continuous flames were observed whereas at sufficiently high Zst, where a shift from more oxygen-deficient to more fuel-deficient conditions at the reaction zone occurs, broken structures characteristic of low Lewis number premixed flames were observed which enabled combustion under conditions where no flames could be sustained at lower Zst, even for the same dilution level. At sufficiently high σ these broken structures could transition from advancing edge-flames to isolated, stationary flames, particularly for highly-diluted mixtures. These findings were in surprisingly good agreement with theoretical predictions. Appropriate scalings of these behaviors for different mixtures based on computed 1D extinction strain rates were identified. Nonpremixed H2 N2/O2 N2 edge-flames have profoundly different responses to Zst than corresponding hydrocarbon edge-flames, which is shown to be due to differences in the chemistry and Lewis numbers of the two fuels.

Micro combustion and power generation systems have increasingly been investigated as potential alternatives to electrochemical energy storage thanks to hydrocarbon fuel’s high energy density, but electrical componentry for pumping significantly limits the overall system efficiency. These components must be eliminated to allow for widespread adoption of micro combustion and power generation systems, and so the development of an alternative pumping technique is required. By taking advantage of the thermal transpiration phenomenon, small-scale pumping can be obtained in the presence of a temperature gradient. Initial work has been done to investigate the efficacy of this system, but a major issue has arisen due to the lack of low-cost thermal transpiration membranes with desirable pore characteristics. Research has revealed that vessel hyphae present in the roots of mushrooms (mycelium) form a network which could meet the requirements of an effective thermal transpiration membrane. Proper growing conditions could also allow for an application specific mycelium structure providing a highly effective and low-cost thermal transpiration membrane for micro combustion systems.

Motivated by experimental observations on premixed-gas flame propagation in Hele-Shaw cells, this work analyzes quasi-isobaric flame propagation between two adiabatic parallel plates using a simple quasi-2D formulation based on averaging the flow properties across the cell gap. Instabilities associated with thermal expansion, buoyancy, viscosity change across the front and differential diffusion of thermal energy and reactants are investigated with one-step chemistry, constant heat capacity and variable transport coefficients through time-dependent computations of the flame front evolution in large domains. These instabilities are found to induce flame wrinkling which increases flame surface area and thus propagation speeds in ways different from those associated with freely propagating flames. The simulations are compared with experiments in Hele-Shaw cells; very good qualitative and (in some cases) quantitative agreement is found.

The absorption spectra of vapors of concentrated hydrogen peroxide/water mixtures (without a carrier gas) were characterized at wavelengths from 1390 to 1470 nm utilizing a near-infrared diode laser. Low pressures were employed to examine these spectral features near the Doppler-broadened limit. An advantageous portion of the spectra near 1420 nm containing several distinct H2O2 peaks and one well-known H2O peak (for calibration) was identified and the cross-sections of these peaks determined. These cross section values can be employed to measure vapor-phase concentrations of H2O2 in propulsion, atmospheric chemistry, and sterilization applications.

A new type of jet-stirred chamber (JSC) incorporating multiple impinging turbulent jets is proposed as an apparatus for the study of turbulent premixed flames. ANSYS-FLUENT computations using a RANS - Reynolds Stress Model were used to simulate the flows inside the JSC and identify an optimal configuration of inlet jets and outlet ports providing the most nearly ideal flow, i.e. homogeneous and isotropic with large turbulence intensity compared to the mean velocity. A configuration of 8 jets, each surrounded by a concentric annular outlet, at the corners of an imaginary cube circumscribed by a spherical chamber, produced by far the most nearly optimal flow characteristics. The performance of this configuration, called Concentric Inlet And Outlet (CIAO), was compared quantitatively to two popular fan-stirred chamber (FSC) designs and the CIAO JSC was found to provide far more nearly ideal flow properties. The robustness of the CIAO design was demonstrated by intentionally misaligning the jets or mismatching their flow velocities. A comparison of simulated turbulent flames in CIAO and an FSC showed that CIAO enabled far more nearly spherical expanding flames with nearly the same inferred turbulent burning velocity ( S T ) regardless of the flame radius r f and the value of the mean progress variable ( c ¯ ) used to define the flame location, whereas in the FSC the flame was not nearly as spherical and there was considerable variation of S T depending on r f and c ¯ .

The propagation speeds (Uedge) of nonpremixed edge-flames were measured as a function of stoichiometric mixture fraction (Zst) and global strain rate (σ) for several fuel/oxidant/diluent combinations using a counterflow slot-jet burner. It was found that for fuel Lewis number (Lef) ≈ 1 and oxidant Lewis number (Leo) ≈ 1 with fixed σ, Uedge increases monotonically with increasing Zst. In contrast, for Lef >1 and Leo ≈ 1 with fixed σ, Uedge exhibits a minimum, typically at Zst ≈ 0.3, except for dimethyl ether which showed Uedge montonically decreasing with increasing Zst. These results indicate that Zst has both chemical and Lewis number effects on nonpremixed edge-flame speeds. For Lef ≈ Leo ≈ 1, chemical effects dominate over the whole range of Zst whereas for Lef> 1 and Leo ≈ 1, Lewis number effects become important at low Zst. It is shown that all observed Uedge vs. Zst trends are consistent with computed values of extinction strain rate (σext) of these mixtures in a 1D counterflow, thus σext serves as a simple surrogate for predicting edge-flame behavior.

An experimental and numerical study of the rates of decomposition of hydrogen peroxide vapor on catalytic surfaces was conducted. Hydrogen peroxide vapor and helium carrier gas flowed over a catalyst (platinum on alumina spheres of varying diameter) at a total pressure of

Motivated by experimental observations on premixed-gas flame propagation in Hele-Shaw cells, this work analyzes quasi-isobaric flame propagation between two adiabatic parallel plates using a simple quasi-2D formulation based on averaging the flow properties across the cell gap. Instabilities associated with thermal expansion, buoyancy, viscosity change across the front and differential diffusion of thermal energy and reactants are investigated with one-step chemistry, constant heat capacity and variable transport coefficients through time-dependent computations of the flame front evolution in large domains. These instabilities are found to induce flame wrinkling which increases flame surface area and thus propagation speeds in ways different from those associated with freely propagating flames. The simulations are compared with experiments in Hele-Shaw cells; very good qualitative and (in some cases) quantitative agreement is found.

Hydrocarbon fuels is roughly 50 times higher energy density compared to commercial batteries. However, scaling down electrical power generators for portable devices have been far from successful due to the difficulties of minimizing heat and friction losses. Hence, an alternate solution without moving parts incorporating a micro-tubular solid oxide fuel cell (mT-SOFC), thermal transpiration (gas pumping in a nanoporous medium from temperature gradient) and catalytic combustion on platinum surface is proposed. The concept involves having thermal transpiration membrane utilizes the temperature gradient created by mT-SOFC and catalytic combustion to supply reactants at high temperature to mT-SOFC for its operation. With this concept, catalytic combustion modeling and thermal transpiration membrane modeling were studied individually and integrated together for an airbreathing cylindrical chamber. The chamber design involves an aluminum cylinder wrapped with glass microfiber filter as transpiration membrane. Aluminum structure provides practically uniform temperature on inside surface of the membranes. Platinum mesh was used as a catalyst for rich butane-air combustion, the products of which supply the mT-SOFC. It was found that the temperature, fuel flow rate and equivalence ratio can be varied over mT-SOFC operation range. The integrated system demonstrates capability in self-sustaining power generation using hydrocarbon fuel without moving parts.

A simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices. A three-dimensional (3D) numerical model of spiral counterflow ‘Swiss roll’ combustors was then used to confirm and extend the applicability of the identified parameters. It was found that without property adjustment to maintain constant values of these parameters, at low Reynolds number (Re) smaller-scale combustors actually showed better performance (in terms of having lower lean extinction limits at the same Re) due to lower heat loss and internal wall-to-wall radiation effects, whereas at high Re, larger-scale combustors showed better performance due to longer residence time relative to chemical reaction time. By adjustment of property values, it was confirmed that four dimensionless parameters were sufficient to characterise combustor performance at all scales: Re, a heat loss coefficient (α), a Damkohler number (...

A self-sustaining thermal transpiration-based combustor and gas pump system having no moving parts and using readily storable hydrocarbon fuel is presented. Thermal transpiration was accomplished by meeting two essential conditions: (1) gas flow in the transitional or molecular regime using glass microfiber filters as transpiration membranes and (2) a temperature gradient through the membrane using catalytic combustion downstream of the membrane. Experiments showed that transpiration pumps with larger porosity and larger overall size exhibited better performance, though membrane pore size had little effect. These results were quantitatively consistent with theoretical predictions. By exploiting the temperature and fuel/oxygen concentrations within the transpiration pump, a novel, complete portable power generation system was created using a single-chamber solid-oxide fuel cell and its performance is reported.

A Thermal Partial Oxidation (TPOX) based “Swiss-Roll” Reformer is being developed to convert hydrocarbon fuels into hydrogen rich syngas for subsequent use in a solid oxide fuel cell (SOFC) for electrical power generation. The “Swiss-roll” reformer is an efficient countercurrent heat exchanger that recuperates heat from the hot reformate stream generated internal to the reformer and preheats the cold reactants prior to entrance into a central region where the high temperature reforming reactions occur. Effective heat recuperation enables super-adiabatic temperatures to be achieved within the reformer which drives the reformate composition towards chemical equilibrium without the need for catalysts and/or external energy. The simplicity, compactness, lack of catalysts and fuel flexibility of the reformer are attractive. In this work, experimental data obtained using a 3D printed Swiss-roll reformer is reported that shows the conversion of representative fuel (propane) to syngas. The associated temperature and species concentrations taken at different locations within the reformer are also reported. These measurements provide information on the effectiveness of the heat exchange process as well as the reforming reactions inside the compact TPOX reformer.

Three-dimensional (3D) numerical simulations of spiral counterflow Swiss roll heat-recirculating combustors were performed including gas-phase conduction, convection and chemical reaction of propane–air mixtures, solid-phase conduction and surface-to-surface radiation. These simulations showed that in 3D, results are surprisingly similar with or without a turbulence model activated because without turbulence, Dean vortices form in the curved channels which enhance heat transport and thus heat recirculation by nearly the same amount as turbulence does. Turbulence enhances the apparent viscosity to the point that, when the turbulence model is activated, the Dean vortices are not formed at the moderate Reynolds numbers studied in this work. Predictions of both 3D models are in good agreement with experiments with respect to extinction limits and temperature distributions. Comparing 3D to two-dimensional (2D) simulations employing a simple model for the out-of-plane heat losses, the turbulence model is found to be essential for accurate predictions in 2D because Dean vortices cannot form in 2D simulations. Predictions obtained using a 1-step chemical reaction model with the pre-exponential term adjusted to obtain agreement between model and experiments at one test condition are compared to a 4-step reaction model developed for flow reactors. The latter is found to provide good agreement with experiments with no adjustable parameters, which is argued to be plausible because the conditions inside heat-recirculating combustors are closer to those of flow reactors than propagating flames.

Hydrocarbons have 50-100 times higher energy per unit weight compared to commercially available batteries, thus harvesting only 10% of the energy from hydrocarbons could provide a far lighter energy source for portable electronic devices. With this motivation, the feasibility of using Polymer Electrolyte Membrane (PEM) fuel cells with propane fuel, operating at low temperatures (< 100˚C), was explored. Nafion® membranes were used as electrolytes and platinum black was used as catalyst. At a catalyst loading of 15 mg/cm2 at anode and 8 mg/cm2 at cathode, a maximum steady-state power density of 7.5 mW/cm2 was observed. However, with hydrazine treatment of platinum black catalyst nearly doubled the power density of propane fuel and increased power by about 50% with hydrogen fuel. It was interesting to note that there was a significant influence of the current history on the fuel cell performance. In particular, at higher current densities (> 24 mA/cm2) the power output gradually decreases then rapidly “extinguishes” (i.e., produces no power). However, by employing an unconventional operating mode (“load-interrupt”) where the current is shut off for a short period of time and then reapplied, the maximum average power density increased to 11.6 mW/cm2. For dimethyl ether (DME) and hydrogen, no significant effect of current history was observed. It is proposed that the fuel cell “extinction” is associated with the formation of inactive intermediate species on the anode, blocking active sites. A simple model was developed in an effort to interpret “extinction” behavior. This model assumes all sites are in either an “active” or “inactive” state with heuristic kinetic relations to describe the rates of conversion of active to inactive sites and vice versa. This model was found to be in good qualitative agreement with the experiments when the rate of formation of inactive sites was proportional not only to the number of active sites but also the number of already-inactive sites, i.e. in a self-accelerating manner. Possible physical mechanisms for this heuristic model are discussed.

We consider generalized flame balls which correspond to stationary spherical flames with a flow of hot inert gas, either a source or a sink, at the origin. Depending on the flow, these flames can have positive, zero, or negative burning speeds, with zero speeds characterizing the Zeldovich flame balls. A full analytical description of these structures and their stability to radial perturbations is provided, using a large activation energy asymptotic approach and a thermo-diffusive approximation. The results are also complemented by a numerical study. The number and stability of the generalized flame balls are identified in various regions of the l-M-h 0 space, where l is the (reduced) Lewis number, and M and h 0 the flow rate and its enthalpy at the origin, respectively. It is typically found that, when the flow is a source, there is a maximum value of the flow rate M ax depending on l and h 0, above which no stationary solutions exist, and below which there are two solutions characterizing a small stable...

The integration of an external combustion micro heat engine with a Swiss roll combustor to produce power is presented in this work. The micro heat engine, a micro-electro-mechanical-systems (MEMS) based engine fabricated with standard micro-fabrication techniques, consists of a cavity filled with a saturated, two-phase working fluid, and bounded on the top and bottom by thin membranes. The engine is coupled to the combustor through a thermal switch which controls heat conduction from the combustor to the engine. The engine can utilize heat over a wide range of temperatures to produce power. The Swiss roll combustor utilizes the high energy density of hydrocarbon fuels in order to provide the necessary heat transfer required to power the engine. The overall feasibility of this integration as well as the results of this endeavor are explored.

A two-dimensional numerical model of spiral counterflow heat recirculating combustors was developed including the effects of temperature-dependent gas and solid properties, viscous flow, surface-to-surface radiative heat transfer, heat conduction within the solid structure, one-step chemical reaction and heat loss from the combustor to its surroundings. A simplified model of heat loss in the 3rd dimension was implemented and found to provide satisfactory representation of such losses at greatly reduced computational cost compared to fully three-dimensional models. The model predicts broad reaction zones with structure decidedly different from conventional premixed flames. Extinction limits were determined over a wide range of Reynolds numbers (2 500, modeling of turbulent flow and transport was required to obtain such agreement. Heat conduction along the heat exchanger wall has a major impact extinction limits; the wall thermal conductivity providing the broadest limits is actually less than that of air. Radiative heat transfer between walls was found to have an effect similar to that of heat conduction along the wall. In addition to weak-burning extinction limits, strong-burning limits in which the reaction zone moves out of the combustor center toward the inlet were also predicted by the numerical model, in agreement with experiments. It is concluded that several physical processes including radiative transfer, turbulence and wall heat conduction strongly affect the performance of heat-recirculating combustors, but the relative importance of such effects is strongly dependent on Re.

The propagation rates (U{sub edge}) of nonpremixed ignition (advancing) fronts and extinction (retreating) edge flames were measured as a function of global strain rate ({sigma}), jet spacing (d), mixture strength, stoichiometric mixture fraction (Z{sub st}), and Lewis number (Le) using a counterflow slot-jet burner in which edge flames propagated along its long dimension. Electrical heaters at both ends of the slot 'anchored' the flames, allowing conditions resulting in negative values of U{sub edge} to be studied by triggering local extinction of anchored flames with an N{sub 2} jet. Results are presented in terms of the effects of a dimensionless flame thickness ({epsilon}) related to {sigma} and a dimensionless heat loss (k) on a scaled U{sub edge}. Propagation rates were markedly enhanced/retarded in mixtures with low/high Le. Propagation rates and extinction conditions were highly asymmetric with respect to Z{sub st}=0.5 despite the symmetry of flame location; mixtures with Z{sub st} greater/less than 0.5 behaved like stronger/weaker mixtures, apparently due to the relative locations of the radical production zone and maximum temperature zone for varying Z{sub st}. Two extinction limits were identified, corresponding to a high-{sigma} strain-induced limit that was strongly dependent on Le but nearly independent of k and a low-{sigma}more » heat-loss-induced limit that was strongly dependent on k but not Le. Most experimental findings were in good agreement with theoretical predictions; however, unlike predictions, 'tailless' triple flames were not observed; instead standard triple flames and 'short length' edge flames were found, and only for a very narrow range of experimental conditions. This is proposed to be due to the difference between the volumetric heat loss presumed in the models and the conductive transfer to the jet exits that dominates heat loss in the experiments. (author)« less

Transient plasmas that exist during the formative phase of a pulse-ignited atmospheric pressure discharge were studied for application to ignition of quiescent and flowing fuel-air mixtures. Quiescent methane-air mixture ignition was studied as a function of equivalence ratio, and flowing ethane-air mixture was studied in a pulse detonation engine (PDE). The transient plasma was primarily comprised of streamers, which exist during approximately 50 ns prior to the formation of an equilibrated electron energy distribution. Results of significant reduction in delay to ignition and ignition pressure rise time were obtained with energy costs roughly comparable to traditional spark ignition methods (100-800 mJ). Reduction in delay to ignition by factors of typically 3 in quiescent mixes to >4 in a flowing PDE (0.35 kg/s), and other enhancements in performance were obtained. These results, along with a discussion of a pseudospark-based pulse generator that was developed for these applications, will be presented.

A simple first-principles model of counter current heat-recirculating combustors is developed, including the effects of heat transfer from the product gas stream to the reactant stream, heat loss to ambient, and heat conduction in the streamwise direction through the dividing wall (and heat transfer surface) between the reactant and product streams. It is shown that streamwise conduction through the wall has a major effect on the operating limits of the combustor, especially at small dimensionless mass fluxes (M) or Reynolds numbers that would be characteristic of microscale devices. In particular, if this conduction is neglected, there is no small-M extinction limit because smaller M leads to larger heat recirculation and longer residence times that overcome heat loss if M is sufficiently small. In contrast, even a small effect of conduction along this surface leads to significantly higher minimum M. Comparison is made with an alternative configuration of a flame stabilized at the exit of a tube, where heat recirculation occurs via conduction through tube wall; it is found that the counter-current exchanger configuration provides superior performance under similar operating conditions. Implications for microscale combustion are discussed.

The thermodynamic theory governing the absolute maximum efficiency of energy conversion by thermoelectric devices that operate as part of the heat recycle in regenerative burners is examined. Comparison with a series of elementary Carnot cycles helps to address the question of whether higher system efficiencies are realizable by rejecting the unconverted heat to the cold surroundings or to the incoming reactants as part of the recycle. While for the second law (Carnot) heat engine cycles the maximum power that can be extracted is independent of the layout, in the case of irreversible thermoelectric assemblies a particular combination of both in a novel configuration is shown to be most advantageous. This heat exchanger/thermoelectric converter configuration consists of a coaxial assembly of many annular thermoelectric elements in series and a section in which heat is rejected to the incoming reactants that is followed by a second section, which discards unconverted heat to the cold surroundings. It is shown that the efficiencies of such devices could substantially exceed the maximum efficiencies of the best present-day thermolectric of such devices could substantially exceed the maximum efficiencies of the best present-day thermoelectric conversion systems, and the theory suggests practical designs for small, combustion-driven, power supplies.

The limits to self-sustaining catalytic combustion in a microscale channel were studied computationally using a cylindrical tube reactor. The tube, 1 mm in diameter, 10 mm long, and coated with Pt catalyst, was assumed to be thermally thin, and the boundary condition on the wall was set to be either adiabatic or non-adiabatic with fixed heat transfer coefficient. Methane/air mixtures with average velocities of 0.0375–0.96 m/s (corresponding to Reynolds number, Re, ranging from 2.5 to 60) were used. When the wall boundary condition was adiabatic, the equivalence ratio at the extinction limit monotonically decreased with increasing Re. In contrast, for non-adiabatic conditions, the extinction curve exhibited U-shaped dual limit behavior, that is, the extinction limits increased/decreased with decreasing Re in smaller/larger Re regions, respectively. The former extinction limit is caused by heat loss through the wall, and the latter is a blow-off-type extinction due to insufficient residence time compared to the chemical timescale. These heat-losses and blow-off-type extinction limits are characterized by small/large surface coverage of Pt(s) and conversely large/small numbers of surface coverage of O(s). It was found that by diluting the mixture with N2 rather than air, the fuel concentration and peak temperatures at the limit decreased substantially for mixtures with fuel-to-oxygen ratios even slightly rich of stoichiometric because of a transition from O(s) coverage to CO(s) coverage. Analogous behavior was observed experimentally in a heat-recirculating “Swiss-roll” burner at low Re, suggesting that the phenomenon is commonplace in catalytic combustors near extinction. No corresponding behavior was found for non-catalytic combustion. These results suggest that exhaust-gas recirculation rather than lean mixtures are preferable for minimizing flame temperatures in catalytic microcombustors.

Microgravity experiments of flame spread over thermally thick fuels were conducterd using foam fuels to obtain low density and low thermal conductivity and thus large spread rate (Sf) compared to dense fuels such as polymethylmethacrylate. This scheme enabled meaningful results to be obtained even in 2.2 s drop tower experiments. It was found that, in contrast to conventional understanding, steady spread can occur over thick fuels in quiescent microgravity environments, especally whenm a radiatively active diluent gas such as CO2 is employed. This is proposed to be due to radiative transfer from the flame to the fel surface that can lead to steady spread even when conductive heat transfer from the flame to the fuel bed is negligible, Radiative effects are more significant under microgravity conditions because the flame thickness is larger and thus the volume of radiating combustion products is larger at microgravity. The effects of oxygen concentration and pressure are shown and the transition from thermally thick to thermally thin behavior with decreasing bed thickness is demonstrated. A simple semiquantiative model of radiationdriven flame spread rates is consistent with experimental observations. Radiative flux measurements confirm the proposed effects of diluent type and gravity level. These results are particularly noteworth considering that the International Space Station employs CO2 fire extinguishers; our results suggest that helium may be a better inerting agent on both mass and mole bases at microgravity even though CO2 is much better on a mole basis at earth gravity.

This study examines the successful development of a combustion-driven thermal transpiration-based combustor and a self-sustaining gas pump system having no moving parts and using readily storable hydrocarbon fuel. A stacked configuration was then integrated into the combustor creating a self-sustaining power generation system. In recent years, power generation devices employing hydrocarbon fuels rather than electrochemical storage as energy feedstock have been studied extensively due to the much higher energy densities of hydrocarbon fuels than the best available batteries. While many devices have been proposed including internal combustion engines and gas turbines, they all require the use of air to obtain a higher energy density so that only one reactant (fuel) need be carried. Thermal transpiration was accomplished by meeting two essential conditions: (1) gas flow in the transitional or molecular regime using glass microfiber filters as transpiration membranes and (2) a temperature gradient through the membrane using catalytic combustion downstream of the membrane. A cubic combustor was designed to house the thermal transpiration membrane and develop into a self-sustaining gas pump system. Fuel/Air would feed through an inlet into a mixing chamber that would flow into the thermal guard containing the thermal transpiration membrane. The thermal guard was developed from a high thermal conductivity stainless steel made into a cubic formation by using a 3D printing process. This configuration allowed both fuel and air to be transpired through the membrane meaning it was not possible for any reactant flow to occur as a result of the fuel supply pressure and only the membrane could draw reactants into the device. In addition to pumping, a single-chamber solid-oxide fuel cell (SC-SOFC) was incorporated into combustion driven thermal transpiration pumps to convert chemical or thermal energy into electrical energy for a self-contained portable power generation system. Experiments showed that transpiration pumps with larger porosity and larger overall size exhibited better performance, though membrane pore size had little effect. These results were quantitatively consistent with theoretical predictions. By exploiting the temperature and fuel/oxygen concentrations within the transpiration pump, the SOFC achieved a maximum power density of 40 mW/cm2. Despite being far lower than necessary for a power source to be competitive with batteries, this preliminary study signifies an on-going positive efficiency that has potential for improvement through optimizing SOFC technology.

It is proposed that concurrent-flow flame spread over solid fuel beds will be steady under conditions where heat and momentum losses to the sides of the fuel samples and/or surface radiative losses are significant. These losses are argued to be unavoidable because the flame length will grow until these losses balance the heat and momentum generation rates. Approximate relations (with no adjustable parameters or necessity for supplemental measured quantities such as heat fluxes or pyrolysis times) are derived for steady spread rates in the presence of these losses for laminar and turbulent flow, buoyant and forced convection, and thin and thick fuels. Experimental tests of these relations were conducted for upward flame spread over thermally thin fuels. Varying pressures, oxygen mole fractions, and diluents were employed to cover a seven-decade range of the Grashof number. These experiments generally support the validity of the proposed mechanisms.

Recent models of straight diffusion flame edges are extended to consider the effect of a curved edge forming the perimeter of an axisymmetric ‘hole’, where a burning flame surrounds a quenched inner region. For ‘free’ flame-holes (without a heat sink near the axis), at small Damkohler number (Da), the holes grow if the initial radius is large but shrink if it is small. For large Da, the holes shrink for any initial radius. Thus, free flame-holes are not stable for any Da, which is consistent with experimental observations. When the flame-hole is ‘anchored’ by a heat sink near to the axis, stationary holes of finite radius can exist for sufficiently high Da, but the solutions revert to ‘free’ hole behaviour for radii sufficiently larger than the heat sink radius. Based on these results, it is suggested that quasi-stationary flame-holes are not likely to be a common feature of turbulent diffusion flames, except possibly when large lateral gradients of Da are present due to intense vortices passing through t...

Flames subject to temporally and spatially uniform hydrodynamic strain are frequently used to model the local interactions of flame fronts with turbulent flow fields (Williams, 1985; Peters, 1986; Bradley, 1992). The applicability of laminar flamelet models in strongly turbulent flows have been questioned recently (Shay and Ronney, 1998) because in turbulent flows the strain rate (sigma) changes at rates comparable to sigma itself and the scale over which the flame front curvature and sigma changes is comparable to the curvature scale itself. Therefore quasi-static, local models of turbulent strain and curvature effects on laminar flamelets may not be accurate under conditions where the strain and curvature effects are most significant. The purpose of this study is to examine flames in spatially-varying strain and compare their properties to those of uniformly strained flames.

A numerical study was conducted to examine the structure and dynamics of steady, source-free spherical premixed flames (“flame balls”), which have been observed in microgravity experiments. A time-dependent spherically symmetric code was used with detailed chemical, transport, and radiation submodels. Steady properties, stability limits, and dynamics of flame balls are computed for H 2 -air mixtures. The chemical and radiation models used were found to affect flame ball properties substantially. The special and unusual role of thermal radiation from the combustion products is described. In particular, the far-field radiative loss is found to affect the behavior of flame balls in a manner very different from propagating planar flames. One new feature was identified: mixtures capable of exhibiting both stable flame balls and steadily propagating flames depending on the initial condition. Numerical results are compared to theoretical predictions, prior steady-state numerical calculations, and prior experimental results on flame ball size. Additional experiments were performed to measure flame ball radiant emission. Qualitative agreement with theory and experiment is found; however, quantitative agreement with experiment is only fair, indicating the need for improved microgravity conditions, e.g., in orbiting spacecraft.

Near-infrared (IR) and ultraviolet (UV) emission profiles of flame balls at microgravity conditions in H 2 -O 2 -diluent mixtures were measured in the JAMIC 10 s drop-tower and compared to numerical simulations and supplemental KC135 aircraft μg experiments. Measured flame ball radii based on images obtained in the JAMIC, KC135, and recent space experiments (IR only) were quite consistent, indicating that radius is a rather robust property of flame balls. The predicted IR radii were always smaller than UV radii, whereas the experiments always showed the opposite behavior. Agreement between measured and predicted flame ball properties was closer for UV radii than IR radii in H 2 -air mixtures but closer for IR radii in H 2 -O 2 -CO 2 mixtures. The large experimental IR radii in H 2 -air tests is particularly difficult to interpret even when uncertainties in chemical and radiation models are considered. Experimental radii would be consistent with a chemiluminescence reaction of the form HO 2 + HO 2 → H 2 O 2 + O 2 producing an excited state of H 2 O 2 , since HO 2 is consumed at large radii through this reaction and its exothermicity is sufficient to create excited states that could emit at the observed wavelengths, however, no appropriate transition of H 2 O 2 ∗ could be identified.© 1998 by The Combustion Institute

Turbulent-flame speeds in methane-air mixtures were measured in a Taylor-Couette apparatus with counter-rotating cylinders, used to generate turbulence that is nearly homogeneous and isotropic over many integral length and time scales. While laminar-flame propagation is found to be influenced by the Darrieus-Landau instability and heat loss to the walls of the apparatus, turbulent-flame propagation in high-intensity turbulence is found to be uninfluenced by these effects. A decreasing sensitivity of the turbulent-flame speed to increases in turbulence intensity is found to occur beyond turbulence intensities of approximately 2.5 times the laminar-flame speed. This is possibly due to a transition to a nonflamelet combustion regime where flame propagation is influenced by both small-scale flame-structure modification and large-scale flame-front wrinkling. Results are compared with those obtained by earlier investigators using other experimental apparatuses and with theoretical predictions.

The effects of atmosphere composition on the rate of opposed-flow flame spread (Sf ) over thin solid fuel beds at microgravity (μg) were measured and compared to earth gravity (lg) results and theoretical predictions. Two modifications to standard atmospheres were considered. First, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH4) were measured and compared to an existing theoretical model that was extended to ug conditions. The agreement between the model and experiment is reasonable considering the simplicity of the model. Notably, both model and experiment show that the effect of added gaseous fuel is greater at μg than lg. Secondly, the effect of diluent type on Sf -was studied by comparing results using He, N2, Ar, CO2 and SF6 diluents. It was found that, in agreement with prior studies in N2 diluent, for He, N2 or Ar diluents, Sf was larger at 1g than μg. In contrast, for CO2 diluent, Sf was slightly lower at 1g than at μg and for SF6 diluent, Sf was much lower at 1...

The effects of various diluent gases on steady, source-free spherical premixed flames (“flame balls”) were modeled using a time-dependent numerical code with detailed chemical, transport, and radiation submodels. The diluent gas affects the Lewis number ( Le ) and radiative properties of the flames. Numerical solutions for the steady properties and stability limits were obtained for lean H 2 -air, H 2 −O 2 , H 2 −O 2 −CO 2 , and H 2 −O 2 −SF 6 mixtures. When results were nondimensionalized by the properties at the dynamic stability limit, all results collapsed onto one of two curves, depending only on whether the dominant source of radiative loss is from the product H 2 O, whose concentration decays to zero in the far-field, or diluent gas, whose concentration is constant in the far-field. No significant Le effect was found. Numerical predictions were compared with recent Space Shuttle experiments. For CO 2 and SF 6 diluents, experimental results lie between computational predictions obtained with diluent radiation included and with diluent radiation artificially suppressed, indicating that radiation models including reabsorption effects are needed in these cases. Significant influences of the chemical mechanism were found, even for mechanisms that properly predict the burning velocities of H 2 -air mixtures away from extinction limits. Sensitivity analysis showed that the discrepancies are due mainly to differences in the H+O 2 +H 2 O→HO 2 +H 2 O rate parameters for these mechanisms.

Nonpremixed flames subject to steady but spatially varying straining flows were studied to examine one aspect of nonpremixed flames in strongly turbulent flows or near quenching conditions (e.g., near a burner rim), where strain-rate gradients are present and local strain rates may be high enough to cause local flame-front extinguishment. The spatially varying straining flow were created using an opposed slot-jet burner with slightly nonparallel jet exits. The most significant observation was that steady flame “edges” could be created where the flame would exist in the low-strain region but would be extinguished in the high-strain region. The local strain at the location of the stationary flame edge was almost always lower than the strain required to extinguish flames in the same mixture subject to a spatially uniform strain. The strain rate at the edge-flame location was independent of the strain-rate gradient and gradual transitions from edge-flame behavior to uniformly strained flame behavior were not observed, indicating that conventional nonpremixed flames and edge flames are quite distinct structures yet each has well-defined properties. At the flame edge, interferometer images indicate a region of locally intense burning and an abrupt transition to nonburning conditions away from the edge. These observations were found to be qualitatively consistent with recent theoretical models of flame edges. These results indicate that “laminar flamelet” models of nonpremixed turbulent combustion may require reevaluation at conditions approaching those where local flame quenching occurs. It is proposed that these models could be improved by adding edge-flame libraries to existing laminar flamelet libraries.

Premixed gas flames in mixtures of CH4, O2, N2, and CO2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically thin radiation models, particularly when CO2, a strong absorber, is present in the unburned gas, Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H2O and CO2, most of the radiation from product H2O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum Of CO2 is broader at flame temperatures than ambient temperature: thus, some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO2 Via both mechanisms, some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison with experiment demonstrates the importance of reabsorption in CO2 diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emission-absorption spectra of the reactant and product -gases and their temperature dependence and cannot be predicted using gray-gas or optically thin model parameters. Applications to practical flames at high pressure, in large combustion chambers, and with exhaust-gas or flue-gas recirculation are discussed.

A review of research on the effects of gravity on combustion processes is presented, with an emphasis on a discussion of the ways in which reduced-gravity experiments and modeling has led to new understanding. Comparison of time scales shows that the removal of buoyancy-induced convection leads to manifestations of other transport mechanisms, notably radiative heat transfer and diffusional processes such as Lewis number effects. Examples from premixed-gas combustion, non-premixed gas-jet flames, droplet combustion, flame spread over solid and liquid fuels, and other fields are presented. Promising directions for new research are outlined, the most important of which is suggested to be radiative reabsorption effects in weakly burning flames.