US20140260313A1

US20140260313A1 - Micro-mixer/combustor

Info

Publication number: US20140260313A1
Application number: US13/999,589
Authority: US
Inventors: Jihad A. Badra; Assaad R. Masri
Original assignee: University of Sydney; King Abdullah University of Science and Technology KAUST
Current assignee: University of Sydney; King Abdullah University of Science and Technology KAUST
Priority date: 2013-03-12
Filing date: 2014-03-11
Publication date: 2014-09-18

Abstract

A micro-mixer/combustor to mix fuel and oxidant streams into combustible mixtures where flames resulting from combustion of the mixture can be sustained inside its combustion chamber is provided. The present design is particularly suitable for diffusion flames. In various aspects the present design mixes the fuel and oxidant streams prior to entering a combustion chamber. The combustion chamber is designed to prevent excess pressure to build up within the combustion chamber, which build up can cause instabilities in the flame. A restriction in the inlet to the combustion chamber from the mixing chamber forces the incoming streams to converge while introducing minor pressure drop. In one or more aspects, heat from combustion products exhausted from the combustion chamber may be used to provide heat to at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber. In one or more aspects, an ignition strip may be positioned in the combustion chamber to sustain a flame without preheating.

Description

CLAIM OF PRIORITY TO RELATED APPLICATION

This application claims priority to co-pending U.S. provisional application entitled “MICRO-MIXER/COMBUSTOR” having Ser. No. 61/851,683, filed on Mar. 12, 2013, which is incorporated by reference as if fully set forth herein.

CROSS-REFERENCE TO RELATED DOCUMENTS

This application makes reference to and incorporates by reference the following paper as if it were fully set forth herein expressly in its entirety:
J. Badra & A. R. Masri (2012): Design of a Numerical Microcombustor for Diffusion Flames, Combustion Science and Technology, 184:7-8, 1121-1134 (attached hereto as Appendix B).

TECHNICAL FIELD

The present disclosure generally relates to micro-combustors, in particular for diffusion flames as an energy source.

BACKGROUND

The need for replacing batteries was stimulated by work at the Massachusetts Institute of Technology (MIT) Gas Turbine Laboratory, whose researchers were amongst the first ones to fabricate a miniaturized gas turbine generator. A. H. Epstein & S. D. Senturia, Science 276 (1997) 1211. Since then, the designs of micro-combustors have attracted attention.
Combustion is the rapid oxidation of fuels accompanied by the emission of energy, usually in the form of heat and light. Combustion occurring within sub-millimeter volumes is known as micro-combustion. The release of heat can result in the production of light in the form of either glowing or a flame. The advantage of micro-combustors is their ability to utilize hydrogen or hydrocarbon fuels which have extremely high specific energies of approximately 142 MJ/kg and 45 MJ/kg, respectively. The best batteries currently available, lithium sulfur, have an energy density of only 0.792 MJ/kg. Therefore, even if only 1% of the stored chemical energy of a hydrocarbon fuel were converted into useable power its power output would be competitive with that of batteries. If these efficiencies can be achieved, micro-combustion could lead to the development of power sources with high power-to-weight ratios. These would be attractive electronic and electrochemical devices where a key consideration is the size and weight of the power source. Micro-combustion also presents other advantages over batteries in that it could also reduce hazardous waste by eliminating battery production and disposal, as micro-combustion devices are refueled, not replaced.
Attempts to design a micro-combustor, particularly for diffusion flames, have suffered from some basic but critical outstanding issues, including fluid mixing and flame stability. The difficulty in flame stability is imposed by the small volume of the reactor and hence, the flames proximity to solid surfaces, resulting in significant quenching due to losses of heat as well as important reactive radicals. Mixing is limited by the narrow channels, the low velocities, and hence the laminar flows that result only in molecular mixing of species for the preparation of a combustible mixture. Efforts to date have been unsuccessful in enhancing mixing in micro-fluidity devices. As a result, efforts to construct micro-combustors have been largely limited to premixed flames to bypass the issues of mixing and to focus on combustion stability. Stability, however, remains an issue even for premixed flames, largely due to poor fuel conversion.
Accordingly, there is a need to address the aforementioned deficiencies and inadequacies.

SUMMARY

The present disclosure provides a micro-mixer/combustor to mix fuel and oxidant streams into combustible mixtures where flames resulting from combustion of the mixture can be sustained inside its combustion chamber. The present design is particularly suitable for diffusion flames. In an aspect the present design mixes the fuel and oxidant streams prior to entering a combustion chamber. The combustion chamber is designed in a way that does not allow excess pressure to build up within the combustion chamber, which build up can cause instabilities in the flame. A restriction in the inlet to the combustion chamber from the mixing chamber forces the incoming streams to converge while introducing minor pressure drop. In one or more aspects, a catalytic strip may be positioned in the combustion chamber to initiate reactions and to sustain a flame without preheating.
Briefly described, one embodiment, among others, provides a micro-mixer/combustor comprising:
a fuel inlet and an oxidant inlet;
a mixing chamber downstream from the fuel and oxidant inlets, and in communication with the fuel and oxidant inlets, designed to mix fuel and oxidant received from the fuel and oxidant inlets, the mixing chamber having walls that converge towards each other downstream from the fuel and oxidant inlets, the walls forming a restriction at the downstream end of the mixing chamber restricting the flow of fuel and oxidant out of the mixing chamber;
a combustion chamber downstream from the restriction and in fluid communication with the mixing chamber through the restriction, the combustion chamber including walls that diverge from each other from the restriction, the combustion chamber being wider than the restriction at an end of the combustion chamber downstream from the restriction; and
an exhaust outlet downstream from the combustion chamber for exhausting combustion products from the combustion chamber.
In one or more aspects, heat from combustion products exhausted from the combustion chamber may be used to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber. In a non-limiting aspect heat from the combustion products may provide preheating to at least 100° C.
In one or more aspects, the micro-mixer/combustor includes an outer wall encasing the micro-mixer/combustor, the outer wall formed of a material having a low thermal conductivity. In various aspects the micro-mixer/combustor may, but need not, be externally adiabatic.
In one or more aspects the micro-mixer/combustor may include an outer wall and two inner walls. The inner walls may be positioned inside of the outer wall and spaced apart from the outer wall. The inner walls may further be positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber in the space between the inner walls. The spacing between the inner walls and the outer wall may form the exhaust outlet. At least one of the inner walls may be formed of a thermally conductive material allowing heat from combustion products passing through the exhaust outlet to be transferred through at least one of the inner walls to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber. The inner walls may be positioned inside of the outer wall and spaced apart from the outer wall to form an exhaust outlet having at least two exhaust channels, an exhaust channel provided in the space between one of the inner walls and the outer wall and a second exhaust channel provided in the space between the second inner wall and the outer wall. At least one of the exhaust channels may be positioned on a side of an inner wall opposite at least one of the mixing chamber, restriction or combustion chamber.
In one or more aspects, the micro-mixer/combustor may include a catalytic ignition strip in the combustion chamber. The catalytic ignition strip may be a platinum strip or a piece of platinum positioned on or in the side of a wall forming the combustion chamber. The ignition strip may be positioned in the combustion chamber to initiate ignition and to sustain a flame without preheating.
In another aspect, a method of mixing and combusting a fuel and an oxidant is provided including the steps of:
providing the micro-mixer/combustor such as described in one or more aspects above, the micro-mixer/combustor including an outer wall and two inner walls, the inner walls positioned inside of the outer wall and spaced apart from the outer wall, the inner walls further positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber, the spacing between the inner walls and the outer wall forming the exhaust outlet, the exhaust outlet including an exhaust channel formed in a space between the outer wall and at least one of the inner walls and positioned on a side of the at least one inner wall opposite at least one of the mixing chamber, restriction or combustion chamber, wherein the at least one inner wall is formed of a thermally conductive material allowing heat from combustion products passing through the exhaust channel to be transferred through the at least one inner wall to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber;
introducing fuel into the mixing chamber of the micro-mixer/combustor through the fuel inlet;
introducing oxidant into the mixing chamber of the micro-mixer/combustor through the oxidant inlet;
mixing the fuel and the oxidant in the mixing chamber;
passing the mixture of fuel and oxidant through the restriction into the combustion chamber;
combusting the mixture of fuel and oxidant in the combustion chamber;
exhausting combustion products resulting from the combustion of the mixture of fuel and oxidant out of the combustion chamber and through the exhaust channel, such that heat from the combustion products passing through the exhaust channel is transferred through the at least one inner wall and into at least one of the mixing chamber, restriction or combustion chamber to preheat the fuel, the oxidant and/or the mixture of the fuel and oxidant in the micro-mixer/combustor.
In one or more aspects, the fuel, the oxidant and/or the mixture of the fuel and oxidant may be heated or preheated to at least 100° C. The outer wall may be formed of a material having a low thermal conductivity. In various aspects the micro-mixer/combustor may, but need not, be externally adiabatic.
Other systems, methods, features, and advantages of the present disclosure for a micro-mixer/combustor, in particular for diffusion flames, will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 depicts a 2D longitudinal cross-sectional view of an embodiment of a micro-mixer/combustor of the present disclosure.

FIG. 2 depicts a 3D perspective view of the micro-mixer/combustor of FIG. 1.

FIGS. 3 a-c depict back, right side and front views of the micro-mixer/combustor of FIGS. 1 and 2.

FIG. 4 depicts simulated color contours of mass fraction of methane for inflows of methane and air in an embodiment of a micro-mixer/combustor of the present disclosure.

FIG. 5 depicts a sequence of images tracing a computed evolution of temperature in the micro-mixer/combustor of FIG. 4 for various times after ignition.

FIG. 6 a depicts the computed steady-state temperature and selected mass fractions for steady-state cases when the fuel and air inlet streams were heated to 100° C. and 300° C.

FIG. 6 b depicts the computed contours of temperature and selected species mass fractions for three cases of material properties within the micro-mixer/combustor of FIG. 4.

FIG. 7 a depicts the temperature contours for a case where the inlet temperatures are set at 100° C. and the internal material is aluminum as an internally conductive material.

FIG. 7 b depicts an embodiment of the micro-mixer/combustor as a milliburner with a gap size of 10 mm.

FIG. 7 c depicts color contours of temperature of embodiments of the milliburner of FIG. 7 b with a gap size of 4 mm comprised of fused silica and stainless steel.

FIG. 7 d depicts color contours of temperature for the milliburner of FIG. 7 b with a gap size of 1.5 mm comprised of fused silica.

FIG. 8 a depicts an embodiment of the present disclosure incorporating a catalytic ignition strip in the combustion chamber.

FIG. 8 b depicts simulated temperature and mass fractions for the embodiment of FIG. 8 a.

DETAILED DESCRIPTION

Described below are various embodiments of the present systems and methods for a micro-mixer/combustor. Although particular embodiments are described, those embodiments are mere exemplary implementations of the system and method. One skilled in the art will recognize other embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. Moreover, all references cited herein are intended to be and are hereby incorporated by reference into this disclosure as if fully set forth herein. While the disclosure will now be described in reference to the above drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure.
The present disclosure is directed to a micro-mixer/combustor that provides suitable mixing of fuel and oxidant streams and stable combustion of the mixed streams. In particular, in various aspects, it provides a stable diffusion flame of the mixture of the fuel and oxidant streams. In general, the micro-mixer/combustor includes separate inlets for delivering fuel and oxidant to a mixing chamber. A restriction is provided at an outlet from the mixing chamber downstream from the fuel and oxidant inlets, the restriction leading to a combustion chamber. The combustion chamber includes one or more outlets to exhaust combustion products from the combustion chamber. In various aspects the mixing chamber is designed to gradually optimize the mixing of the fuel and oxidant streams prior to the mixture entering the combustion chamber. The combustion chamber may be designed to minimize build-up of the pressure of combustion products in the combustion chamber.
A non-limiting embodiment of our present micro-mixer/combustor is illustrated in FIGS. 1-3. FIG. 1 depicts a 2D longitudinal cross-sectional view of an aspect of our micro-mixer/combustor. In this aspect, our micro-mixer/combustor 10 includes an outer wall 12 having sides 14, 15, and 16. As depicted the external surfaces of sides 14, 15, and 16 form a generally U-shaped outer wall 12. The outer wall 12 need not, however, have a generally U shaped configuration. Other configurations may be possible. As seen by reference to one or more examples below, in one or more aspects our micro/mixer-combustor may be sub-decimetric, meaning that the dimensions of outer wall 12, including its sides, may have dimensions less than a decimeter.
The micro-mixer/combustor 10 further includes inner walls or baffles 22, 26 positioned inside of the outer wall 12 and spaced inwardly and apart from the outer wall 12. A splitter wall 22 a is positioned inwardly and spaced apart from wall or baffle 22. The spacing between inner wall 22 and splitter wall 22 a provides a channel 24 serving as a fuel inlet 25. As depicted in FIG. 1 inner wall 22 is generally linear having straight sides, though other configurations may be possible. Inner wall or baffle 26 is spaced apart from and opposed to inner wall 22 and also spaced apart and separate from outer side wall 16. The spacing between inner wall or baffle 26 and splitter wall 22 a forms a gap that serves as an oxidant inlet channel 27.
Inner wall/baffle 26 includes surfaces 26 a-c spaced apart from but facing inner wall 22, including a portion 26 a that narrows the spacing between walls 22 and 26 forming a restriction 32. The space between the inner walls/baffles 22 and 26 and between the restriction 32 and the end of the oxidant channel 27 closest to the restriction 32 forms a mixing area or chamber 34 within the micro-mixer/combustor 10. On the opposite side of restriction 32 the space between inner walls/baffles 22, 26 and between restriction 32 and sidewall 15 forms a combustion chamber 36 in which fuel and oxidant mixed in the mixing chamber 34 and passing through restriction 32 can be combusted. One or more exhaust channels are provided to exhaust combustion products from the combustion chamber 36. In the aspect depicted in FIG. 1, two exhaust channels 38 a, b are depicted. Exhaust channel 38 a is formed in the space between outer wall 14 and inner wall 22, and exhaust channel 38 b is formed in the space between outer wall 16 and inner wall 26. It should be understood that other arrangements for exhaust of the combusted materials can be provided.
In FIG. 1, it can be seen that the oxidant inlet channel 27 is larger in cross-section than the fuel inlet channel 25. In an aspect the size of the inlet streams can be proportioned close to the stoichiometric ratio of fuel/oxidant when the same inlet velocity is maintained for both streams. Mixing of fuel and oxidant occurs relatively quickly in the mixing chamber 34 as the flow approaches the restriction 32, and perfect mixing can be achieved within the mixing chamber 34 downstream from the end of fuel channel 24 leading into the mixing chamber 34. The restriction 32 allows the upstream fuel and oxidant streams to converge and mix prior to entering the reaction chamber 36. In an aspect the inner surface 26 b of wall/baffle 26 between the oxidant inlet 27 and restriction 32 converges toward wall 22, narrowing the spacing between walls/baffles 22 and 26 forming restriction 32. In an embodiment inner surface 26 b of wall/baffle 26 can curve towards splitter wall 22 a and wall 22. In an embodiment, splitter wall 22 a can have a curved inner surface (opposite to and facing surface 26 b of wall/baffle 26) leading to a point at its end closest to restriction 32. The inner curved surface of splitter wall 22 a and the inner curved surface 26 b of wall/baffle 26 can cooperate to direct the flow of the oxidant into the flow of the fuel stream inducing mixing of the streams. The restriction 32 introduces a pressure drop, though it can be relatively small.
In the aspect depicted in FIG. 1 the combustion chamber 36 is formed by diverging opposed walls/baffles 22 and 26. In an aspect the combustion chamber 36 may have a generally triangular cross-section formed between inner wall 22 and the downstream portion 26 c of the inner surface of wall/baffle 26. The cross-section of the combustion chamber 36 downstream of the restriction 32 allows for gas expansion due to heat release and reduces pressure buildup within the combustion chamber 36. The widest point at the downstream end of the reaction chamber 36 leads to an exhaust outlet for the combustion chamber. In an aspect the exhaust outlet includes two side channels 38 a, b for exhausting combustion products from the combustion chamber.
In various aspects exhaust channels 38 a, b can wrap around the body of the burner 10, overlapping inlet streams 25, 27 to provide heating or preheating of the incoming fuel and oxidant gases and the mixing chamber 34 by conduction of heat from the combustion products exiting exhaust channels 38 a, b through walls/baffles 22 and/or 26. Surface 26 d of wall/baffle 26 and the opposed inner surfaces 16 a and 16 b of outer wall 16 can form an exhaust channel 38 b designed to drive the combustion products along the outlet channel 38 b such that the hot combustion products travel closer to the cold reactants so that heat stored in the combustion products is used in an efficient way to preheat the reactants by conduction through wall/baffle 26. In the non-limiting example depicted in FIGS. 1-3 exhaust channel 38 b runs generally parallel to surfaces 26 a-c, though this is not necessary. Similarly heat in combustion products passing through exhaust channel 38 a can be used in an efficient way to preheat the reactants by conduction through wall 22.
A 3D view of the micro-mixer/burner is presented in FIG. 2 and the back, right side and front views of the micro-mixer/burner are shown in FIGS. 3 a, 3 b and 3 c, respectively. The flow channels (fuel channel 24, oxidant channel, mixing chamber 34, restriction 32, combustion chamber 36 and outlet channels 38 a, 38 b are grooved within a block of low thermal conductive material. The depth of the groove may depend on the manufacturing material. The depth of the groove is a important parameter that can be related to the quenching distance below which the flame cannot be sustained. The flow channels are confined by back wall 17 (shown) and a front wall opposite back wall 17 (cut away and not shown) which may serve as additional source of heat loss. However, in one or more aspects back wall 17 and the front wall may be adiabatic. The distance between the inside surface 17 a of the back wall 17 and the inside surface of the front wall opposing surface 17 a is sometimes referred to as the “gap size” of the micro-mixer/combustor 10.
Suitable fuels for use in our present micro-mixer/combustor include organic compounds typically used in or for combustion. The organic compounds can include hydrocarbons, such as methane, ethane, propane, butane and propylene. Other suitable fuels include hydrogen and dimethyl ether. The fuel may be in gas or liquid phase in the storage container. However when entering the micro-mixer/combustor through the inlets 25 and 27 the fuel is preferred to be in vapor phase. By changing the velocities of the fuel and oxidant streams 25, 27 we can reach any mixture we want prior to entering the combustion chamber 36. Therefore this design may provide the user full control over the stoichiometry of any fuel/oxidant mixtures by adjusting the flow rates of the fuel and oxidant accordingly.
Suitable oxidants include those typically used in or for combustion, for example oxygen and air. Other suitable oxidants include ozone, hydrogen peroxide, fluorine and chlorine; however air and oxygen are the safest and cheapest oxidants. Also, possible oxidants include mixtures of oxygen and other inert gases such as nitrogen and argon. This mixing method can be utilized to control the temperature of the combustion process so that no material failure due to high temperatures is observed.
Suitable materials of construction for our micro-mixer/combustor include materials that would maintain the micro-mixer/combustor almost externally adiabatic, thus minimizing if not preventing heat loss or heat transfer from the combustion chamber 36 outwardly through the outer walls (including wall 12, back wall 17 and the front wall opposite back wall 17) and through inner walls/baffles 22 and 26 (26 c) that are in contact with the combustion chamber 36 that can lead to flame quenching. Preferred materials for the outer walls include materials having low thermal conductivity. Suitable materials having a low thermal conductivity include ceramic materials such as quartz and fused silica. Other materials may be used, however, particularly if combined with thermal insulation provided on the outside of outer walls to minimize or prevent heat loss. Suitable materials are also materials that can withstand high temperatures without degradation. Materials for inner walls or baffles 22 and 26 can include any materials that can withstand the temperatures of combustion also without degradation. The selection of material for inner walls/baffles 22 and 26 may depend upon whether heat conduction is to be provided from the exhaust streams 38 a, b to heat or preheat one or both of the inlet fuel and oxidant streams 25 and 27 and/or mixing chamber 34. Further considerations for materials for the present micro-mixer/combustor are materials that are surface treated to minimize the quenching of radicals such as annealed polycrystalline alumina which is baked in a high temperature oxygen environment.
In one or more further aspects, a metal strip may be provided on or in an inner wall of the combustion chamber 26 to serve as an ignition source for igniting the fuel/oxidant mixture. As an example, a catalyst strip can be added along a portion of surface 17 a (FIG. 2) of inner wall 17 that constitutes a portion of the combustion chamber 36. Also, a metal strip may be provided along at least a portion of surface 26 c of inner wall/baffle 26 that forms a portion of combustion chamber 36. A metal strip may also be provided on or in at least a portion of the inner surface of wall 22 that forms combustion chamber 36 in conjunction with surface 26 c. As an example, a platinum strip can be provided along at least a portion of a wall forming the combustion chamber 36 that will provide ignition of a fuel+hydrogen/oxidant mixture entering the combustion chamber 36. Platinum can serve to ignite such a mixture at or above ambient temperature. By doing so no external ignition source is needed. This may facilitate its implementation in practical systems.

Examples

In order to test out the present design, an exemplary micro-mixer/combustor was simulated having overall external dimensions of 2.5 mm for the length of outer wall 15 and 4.6 mm for the lengths of outer walls 14 and 16 with a fuel inlet channel 24 having a width of 0.08 mm and the oxidant inlet 27 having a width of 0.75 mm. Methane was assumed as the fuel and pure air (21% oxygen and 79% nitrogen) as the oxidant. The size of the fuel and oxidant inlet streams was thus proportioned close to the stoichiometric ratio of methane/air when the same inlet velocity is maintained for both inlet streams. Color contours of the mass fraction of methane are shown for inflows of methane and air at 0.5 ms in 27° C. on the left side of FIG. 4. In this simulation mixing of methane and air occurs relatively quickly as the flow of the streams approaches restriction 32. Perfect mixing of both streams is achieved within mixing chamber 34 downstream from fuel inlet channel 24, as is evident from the false contours shown on FIG. 4. The narrowest passage at the neck of the restriction is 0.1 mm. The introduction of the restriction 32 allows the upstream methane and air streams to converge and mix prior to entering the combustion chamber 36.
For the 2D burner configuration shown in this simulation, the volume flow-rate of air is 9.375 times that of methane, leading to an overall stoichiometric composition. This is achieved just before the restriction inlet to the combustion chamber 36 so that a mixture with Ø≈1 enters the combustion chamber 36, as seen from the color contours of FIG. 4 corresponding to a stoichiometric mass fraction of methane of ˜0.053 filling the remainder of the mixing chamber 34. This burner design (or a variation thereof), particularly in regards to the fuel and oxidant inlets, mixing chamber, restriction and combustion chamber, is used here for all subsequent calculations and is later extended to a 3D configuration as described below. As can be seen variations include variation in the exhaust channels 38 a, b.

Numerical Setup

The commercial Fluent 12 (Fluent 12, Ansys, 2009) CFD package was used for all calculations presented here. The Tri-pave meshing scheme is adopted, which allows us to control the aspect ratio and refine the mesh where needed. The computed species concentrations as well as temperature profiles at various locations in the domain are compared for various grid sizes to ensure that that a grid-independent solution is presented. The flow, reactants, and energy equations are solved first so as to provide a good starting point for the more complicated case where gaseous reactions dominate the solution. The results presented herein are obtained from the non-iterative time advancement unsteady state part of the solver for a time step of 1.0 μs, and then the steady laminar solver is turned on to ensure that the solution is fully converged. A second-order discretization scheme has been utilized for all the equations solved, and the under-relaxation parameters have been modified slightly to help converge and stabilize the solution.
The Smooke mechanism (Smooke et al., 1986) with the corresponding thermo-dynamic database file was used for the volumetric reactions. The GRI2.11 transport database file was used with the selected mechanisms to account for chemical reactions with mass, heat, and thermal conductivity diffusivity. Since the flow is laminar, the full multicomponent diffusion model must be enabled for the careful treatment of chemical species diffusion in the species transport and energy equations. Thermal diffusion is solved as well, and detailed gas chemistry is implemented using the ISAT algorithm where the ISAT error tolerance of 1e-6 was used.

Mixing Issues

In order to understand the mixing mechanism of fluid flow in micro-combustors, we examined the conservation equations of momentum, energy, and species normalized by the characteristic length and other relevant parameters of the device (Fernandez-Pello, 2002) as shown below:
$\begin{matrix} \frac{l_{c}}{t_{c} u_{c}} \frac{\partial \overline{u}}{\partial \overline{t}} + \overline{u} \frac{\partial \overline{u}}{\partial \overline{x}} = - \frac{p_{c}}{ρ_{c} u_{c}^{2}} \frac{\partial \overline{p}}{\partial \overline{x}} + \frac{1}{Re} \overline{v} \frac{\partial^{2} \overline{u}}{\partial \overline{x^{2}}} + \frac{g l_{c}}{u_{c}^{2}} & (1) \\ \frac{l_{c}}{t_{c} u_{c}} \frac{\partial \overline{T}}{\partial \overline{t}} + \overline{u} \frac{\partial \overline{T}}{\partial \overline{x}} = \frac{1}{Pe} \overline{α} \frac{\partial^{2} \overline{T}}{\partial \overline{x^{2}}} + Da \frac{Q}{{\overline{C}}_{{pT}_{c}}} {\overline{\dot{w}}}^{″} & (2) \\ \frac{l_{c}}{t_{c} u_{c}} \frac{\partial {\overline{y}}_{i}}{\partial \overline{t}} + \overline{u} \frac{\partial {\overline{y}}_{i}}{\partial \overline{x}} = \frac{1}{LePe} \overline{D} \frac{\partial^{2} {\overline{y}}_{i}}{\partial \overline{x^{2}}} + Da \frac{1}{y_{ic}} {\overline{\dot{w}}}^{″} & (3) \end{matrix}$
The above equations are derived for a continuum fluid, which is still a valid assumption for the flow considered here. Reference should be made to the Nomenclature (Appendix A) for explanatory details about the symbols. Equation (1) describes the momentum of the flow normalized by the parameters of the device, where the first term on the left-hand side is the unsteady acceleration, the second term on the left-hand side is the convective acceleration, the first term on the right-hand side is the pressure gradient, the second term on the right-hand side is the viscosity effect, and the third term on the right-hand side is the gravity body force.
Details about a number of dimensionless numbers are included: Reynolds number, Péclet number, Damköhler number, and Lewis number. The Péclet number, Pe, is defined as the ratio of the rate of advection of the flow to the rate of diffusion:
$\begin{matrix} Pe = \frac{l_{c} u_{c}}{α_{c}} & (4) \end{matrix}$
The Damköhler number, Da, is defined as characteristic mixing time or the ratio of time it takes for a fluid to travel a certain characteristic distance to the time it takes for the chemical reaction to complete:
$\begin{matrix} Da = \frac{{\dot{w}}_{c}^{″} l_{c}}{ρ_{c} u_{c}} & (5) \end{matrix}$
The Lewis number, Le, is defined as the ratio of thermal diffusivity to mass diffusivity:
$\begin{matrix} Le = \frac{α_{c}}{D_{c}} & (6) \end{matrix}$
In Equations (1)-(3), the diffusive terms are multiplied by the inverse of Reynolds number in Equation (1) and the inverse of Péclet number in Equations (2) and (3). Therefore, in the case of a large mixing system, viscous and diffusive terms are small relative to advection terms. As the characteristic lengths of the components become smaller (such is the case with microburners), the values of Reynolds and Peclet numbers decrease since the flow laminarizes and the advection terms become negligible. Mixing in such small devices is harder to achieve since it is largely driven by molecular processes, and hence a judicious design of the mixing streams is needed. The next section presents results for two microburner designs: the first leads to inadequate mixing, while the second is an almost perfect mixer.

Combustion in 2D

Solutions are now presented for three reacting cases in 2D with the following conditions: (i) an adiabatic, nonconductive burner with inlet streams at 27° C. leading to an extinguishing flame, (ii) an adiabatic, nonconductive micro-mixer/combustor (burner) with inlet streams at 100° C. and 300° C. leading to a stable flame, and (iii) an adiabatic burner with internal conduction and inlet streams at 300° C. leading to a stable flame. In this latter case, the outer walls 14-16 of the burner are adiabatic, but the inner walls/baffles 22 and 26 are conductive (non-adiabatic), enabling heating of the inlet channels 25 and 27 and the mixing and combustion chambers 34 and 36.
1) Extinguishing Case: Adiabatic, Nonconductive with Inlet at 27° C.
A sequence of images tracing the computed evolution of temperature in the 2D micro-mixer/combustor 10 is shown in false color contours in FIG. 5. Results are shown for various times after ignition for the case where both methane and air streams enter the burner at a velocity of 0.5 m/s and a temperature of 27° C. First, a solution for the non-reacting case is obtained showing almost complete mixing before the restriction 32, as shown in FIG. 4. This is used as a starting point for the reacting case. Ignition is initiated by introducing in the first iteration a hot patch that has a temperature of 2500 K and dimensions of 0.66 mm×0.85 mm centered in the middle of the triangular combustion chamber 36 downstream of the restriction 32, as shown in the first plate for a time of 10 μs. The case considered here is fully adiabatic.
It is evident from the sequence shown in FIG. 5 that the flame cannot be sustained for these inlet conditions and extinguishes with the initial combustion products, washing off through the left and right exhaust outlets 38 a, b. It can be seen from the images at 100 μs that the flame is initiated at the hot spot and propagates back to consume the unburned mixture of methane and air. The flame extinguishes, as indicated by the decreased peak temperature, and the combustion products gradually start to flush out of the combustion chamber 36 through the side exhaust outlets 38 a, b as shown from the contours of the 1 to 6 ms. The combustion products continue to exit the combustion chamber 36 until the fully non-reacted solution is recovered at times >8 ms.
2) Burning Case: Adiabatic, Nonconductive with Inlet at 100° C. and 300° C.
Given that the previous case extinguished, the inlet mixture temperature is increased here from 27° C. to 100° C. and then to 300° C. for the same adiabatic case, and a stable flame was obtained in both cases, albeit for a different position within the chamber. The computed steady-state temperature and selected species mass fractions (OH, O₂, and CO) for the steady-state case are shown in FIG. 6 a for both cases of 100° C. and 300° C. Note that the solid sections of the burner, namely outer wall 12 and inner walls/baffles 22 and 26, which are assumed here to have zero conductivity, are shown to be at the same temperatures of the entering mixture since no heat transfer is allowed here. It is evident from these results that, as the temperature of the incoming mixture increases, the peak mass fraction of OH increases and the flame front moves upstream closer to the restriction 32. This is consistent with the corresponding increase in flame speed at the hotter conditions. Oxygen is fully consumed in both cases, and CO forms right on the reaction zone in the combustion chamber 36 and gets consumed quickly to form CO₂(not shown here), which exists in higher quantities for the hotter inlets as a result of stronger reaction zone.
3) Burning Case: Adiabatic with Internal Heat Conduction and Inlet at 300° C.
In this third case, our calculations now allow for heat conduction within the inner core of the burner but no external heat losses through outer wall 12 so that the overall burner remains adiabatic. In this case heat from the combustion products exiting through exhaust channels 38 a, b is allowed to pass through inner walls/baffles 22 and 26 to the inlet channels to allow preheating of the methane and air inlet streams 25 and 27 and also the mixing chamber 34. Results are shown here for three materials, namely aluminum, steel, and fused silica for which relevant properties are shown in Table 1. For these materials, a flame cannot be stabilized when the temperature of the mixture is 27° C., so calculations are shown here for a temperature inlet of 300° C. where a stable flame is obtained.
FIG. 6 b shows the computed contours of temperature and selected species mass fractions (OH, O₂, and CO) for three cases where the material properties within the core of the burner (namely, the inner walls/baffles 22 and 26) change from aluminum to steel to fused silica. It is clear that the flame stability, as marked by the peak temperature and the maximum levels of OH formed, improves with the decreasing conductivity of the material. With fused silica, which has a low conductivity of 1.3 W/m·K, the flame front has actually moved upstream of the neck, and some reaction has occurred at the tip of the splitter plate 22 a (FIG. 1) separating the fuel and air streams where the temperature is 2100 K, and some CO as well as OH have formed. Aluminum and steel are very similar in terms of flame temperature and composition, but both are significantly different than fused silica. When using fused silica, the flame becomes hotter in the initial stages, and peak temperatures of 2700 K are observed at the restriction 32, the flame passes the neck and starts burning on top of the splitter 22 a as shown in FIG. 6 b where richer methane/air mixtures exist (see FIG. 4). This explains the lower flame temperature when using fused silica compared to aluminum and steel, where the flame sits at the neck and almost stoichiometric methane/air mixtures are burned.

TABLE 1

Material properties as adopted in the current calculations

	Density	Specific heat	Thermal conductivity
Material	(kg/m³)	(Cp) (J/kg · K)	(W/m · K)

Aluminum	2719	871	202.4
Steel	8030	502.48	16.27
Fused silica	7203	740	1.3

Combustion in 3D

The 3D version of micro-mixer/combustor is slightly modified from its 2D version, where the products travel closer to the reactants for a longer time to allow better heat exchange between hot products and reactants. The micro-mixer/combustor is sandwiched between two solid plates (wall 17) that are 1 mm apart. Only half of the domain is modeled due to symmetry along the third dimension, and the domain is meshed using 85,000 triangular cells and ran for mixing only; the results for mixing were the same as for the 2D case. FIG. 7 a shows the temperature contours for a case, where the inlet temperatures of fuel and air are set to T_jet=100° C.; the velocities of both stream are 0.5 m/s and the material used here is aluminum. Only internal heat transfer is allowed here, so the burner is externally adiabatic.
As can be observed from the computed temperature contours shown in FIG. 7 a, the flame is stable and the reaction zone sits close to the restriction. The internal heat exchange with the combustion products has allowed the entering reactants to heat even further, reaching a temperature of 1400 K. Further investigation on the 3D micro-burner will be carried out later to include a catalyst to assist ignition and stabilize the flame inside the burner.

Combustion in 3D (Scaled-Up Version)

In the micro-mixer/combustor, and for the non-adiabatic case, a stable flame cannot be sustained due to heat losses through the side and back walls exceeding the heat generated by the flame. Various scaled-up versions of micro-mixer/combustor are tested here with various materials and thermal conductivities in an attempt to find a threshold beyond which a stable flame is obtained. The external heat transfer coefficient is 20 W/m²·K, and a free stream temperature of 300 K. The velocities of the fuel and air streams are 0.5 m/s. FIG. 7 b shows the temperature contours for a milliburner with a 10 mm gap size between the two solid plates (wall 17) using steel and fused silica. This corresponds to a scaling-up of 10 times in all three dimensions of the micro-mixer/combustor (the overall external dimensions being 25 mm (2.5 cm) for outer wall 15 and 46 mm (4.6 cm) for outer walls 14 and 16; the overall external dimensions thus being sub-decimetric).
As can be noticed from FIG. 7 b the flame is stabilized even when heat transfer is allowed from all the burner walls and as the thermal conductivity decreases, the temperature increases and the flame front shifts closer to the neck. However, the material with low thermal conductivity (fused silica) has a disadvantage of having local hot spots that might lead to material failure. These hot spots on the back surface of the burner disappear when a material with higher conductivity (such as steel) is used, as can be seen from FIG. 7 b.
The effect of the gap size between the two solid plates (wall 17) was investigated by decreasing the distance between the constraining plates to 4 mm while keeping the thickness of the covering plates (wall 17) unchanged (2 mm) so the conduction heat loss remains constant. As the gap size decreases, hence the surface-to-volume ratio increases, more heat is lost from the flame, which moves up as a result of the lower laminar flame speed. When the gap size is reduced to 4 mm (see FIG. 7 c) (instead of 10 mm), the flame is stabilized further down-stream closer to the exhaust vents. When using steel, the flame is sitting almost at the top of the combustion chamber and is likely to extinguish with a slight increase in heat losses. The local hot spot that might cause material failure still exists for the low conductive material such as fused silica.
Decreasing the gap size between the two solid plates (wall 17) further to 1.5 mm, which is lower than the quenching distance of methane/air mixture at ambient temperature (2 mm), causes a loss of the flame, even when using fused silica, because of the higher heat losses. Preheating the fuel and air streams to 100° C. is found to be necessary for all burner material used here (fused silica) when the gap size is 1.5 mm and no catalytic ignition strip is present. However, when the gap size is larger, for example 4 mm or larger as in the prior examples, then no preheating is required even in the absence of a catalytic ignition strip.
FIG. 7 d shows color contours of temperature for the fused silica burner with a gap size between the two solid plates (wall 17) of 1.5 mm. The flame is stabilized closer to the exhaust ports 38 a, b, but no hot spots are observed with a maximum temperature of 1000° C. at the inner side of the back plate (surface 17 a).
As can be seen from the foregoing the present disclosure provides a design of a micro-mixer/combustor that mixes separate fuel and oxidant streams and stabilizes a diffusion flame. An optimum design has been achieved to perfectly mix the fuel and air to stoichiometric mixtures before entering the combustion chamber 36. Flame stabilization inside the micro-mixer/combustor is numerically achieved using both 2D and 3D geometries. It was found that for a totally adiabatic micro-mixer/combustor, a flame could be sustained if the incoming gases are heated to at least 100° C., and as the mixture temperature increases, the flame moves upstream because of the increased laminar flame speed. When heat transfer is allowed within the reactor, without allowing heat transfer to the surroundings, the flame becomes more stable and stabilized further upstream within the combustion chamber. Further decreasing the thermal conductivity results in a flame traveling beyond the neck and sitting on top of the splitter 22 a. 3D simulations back up the 2D calculations for the externally adiabatic cases. Studies on a scaled up version of the micro-burner and the effect of the gap size showed without preheating the fuel/air streams a gap size more than 1.5 mm is required, and as the gap size decreases the flame weakens due to higher heat losses as a result of higher surface to volume ratio.
Combustion in 2D (Micro-Burner with Catalytic Ignition)
The previous discussion about the 2D micro-burner showed that the flame cannot be stabilised if any heat is lost from that particular 2D burner. Therefore, as illustrated for example in FIG. 8 a, the micro-burner may be turned into a Catalytically Stabilised (CST) micro-burner by adding a catalytic ignition strip 40, for example a platinum strip. The addition of the catalytic strip, such as platinum, benefits from a higher surface to volume ratio since the catalytic surface acts as a heat source not a heat sink. Also, since CH₄or any of the hydrocarbons do not ignite on platinum without an external heat source, hydrogen is used within the fuel mixture to help the ignition of the fuel/air mixture on platinum. Hydrogen self-ignites on platinum at ambient temperatures, as reported by (Deutschmann et al. 1996). The consumption of hydrogen on the platinum surface provides enough heat to initiate the ignition of other hydrocarbons that are mixed with the hydrogen as a fuel. An embodiment of a catalytic platinum plate that is inserted into the combustion chamber 36 of the micro-burner is shown in FIG. 8 a.
In a simulated example, a volume of 60% H₂and 40% CH₄is fed into the fuel inlet at 27° C. and 0.447 m/s. The oxidant used is air and is fed at 27° C. and 0.5 m/s. If the fuel and air mix perfectly within the micro-burner, an equivalence ratio of 0=0.5 will be achieved.
When steel is used as a heat conducting material in the micro-burner, and chemical reactions are enabled, it is observed reactions at the platinum plate start and result in higher temperatures recorded on the monitor points next to the plate. However, the temperatures on the monitor points start to decrease until they reach ambient temperatures. The reason for this is because the heat generated by the platinum strip consuming H₂is conducted through the wall 22, as indicated by the arrows in FIG. 8 a, and the heat loss terminates the surface reactions. Low thermal conductive materials, such as fused silica, are simulated and the reaction is sustained at the platinum plate as shown in FIG. 8 b. As can be observed from FIG. 8 b, the temperatures are much lower (850K) and the reactions that take place purely on the platinum surface produce a negligible amount of OH and CO at the leading edge of the plate.
It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

REFERENCES

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Claims

1. A micro-mixer/combustor comprising:

a fuel inlet and an oxidant inlet;

a mixing chamber downstream from the fuel and oxidant inlets, and in communication with the fuel and oxidant inlets, designed to mix fuel and oxidant received from the fuel and oxidant inlets, the mixing chamber having walls that converge towards each other downstream from the fuel and oxidant inlets, the walls forming a restriction at the downstream end of the mixing chamber restricting the flow of fuel and oxidant out of the mixing chamber;

a combustion chamber downstream from the restriction and in fluid communication with the mixing chamber through the restriction, the combustion chamber including walls that diverge from each other from the restriction, the combustion chamber being wider than the restriction at an end of the combustion chamber downstream from the restriction; and

an exhaust outlet downstream from the combustion chamber for exhausting combustion products from the combustion chamber.

2. The micro-mixer/combustor of claim 1, wherein heat from the combustion products is used to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber.

3. The micro-mixer/combustor of claim 1, further comprising an outer wall encasing the micro-mixer/combustor, the outer wall formed of a material having a low thermal conductivity sufficient to reduce heat loss and sustain a combustion flame in the combustion chamber.

4. The micro-mixer/combustor of claim 2, wherein heat from the combustion products provides preheating to at least 100° C.

5. The micro-mixer/combustor of claim 1, further including a catalytic ignition strip in the combustion chamber.

6. The micro-mixer/combustor of claim 5, wherein the catalytic ignition strip is a platinum strip.

7. The micro-mixer/combustor of claim 1, further comprising an outer wall and two inner walls, the inner walls positioned inside of the outer wall and spaced apart from the outer wall, the inner walls further positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber, the spacing between the inner walls and the outer wall forming the exhaust outlet.

8. The micro-mixer/combustor of claim 7, wherein at least one of the inner walls is formed of a thermally conductive material allowing heat from combustion products passing through the exhaust outlet to be transferred through the at least one inner wall to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber.

9. The micro-mixer/combustor of claim 8, wherein the inner walls are positioned inside of the outer wall and spaced apart from the outer wall to form the exhaust outlet having at least two exhaust channels, an exhaust channel provided in the space between one of the inner walls and the outer wall and a second exhaust channel provided in the space between the second inner wall and the outer wall.

10. The micro-mixer/combustor of claim 9, wherein at least one of the exhaust channels is on a side of an inner wall opposite at least one of the mixing chamber, restriction or combustion chamber.

11. The micro-mixer/combustor of claim 1, wherein the micro-mixer/combustor provides a diffusion flame.

12. A method of mixing and combusting a fuel and an oxidant, comprising:

providing the micro-mixer/combustor of claim 1, the micro-mixer/combustor including an outer wall and two inner walls, the inner walls positioned inside of the outer wall and spaced apart from the outer wall, the inner walls further positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber, the spacing between the inner walls and the outer wall forming the exhaust outlet, the exhaust outlet including an exhaust channel formed in a space between the outer wall and at least one of the inner walls and positioned on a side of the at least one inner wall opposite at least one of the mixing chamber, restriction or combustion chamber

wherein the at least one inner wall is formed of a thermally conductive material allowing heat from combustion products passing through the exhaust channel to be transferred through the at least one inner wall to provide heat to preheat at least one of fuel passing through the fuel inlet channel, oxidant passing through the oxidant inlet channel, the mixing chamber, or the combustion chamber;

introducing fuel into the mixing chamber of the micro-mixer/combustor through the fuel inlet;

introducing oxidant into the mixing chamber of the micro-mixer/combustor through the oxidant inlet;

mixing the fuel and the oxidant in the mixing chamber;

passing the mixture of fuel and oxidant through the restriction into the combustion chamber;

combusting the mixture of fuel and oxidant in the combustion chamber;

exhausting combustion products resulting from the combustion of the mixture of fuel and oxidant out of the combustion chamber and through the exhaust channel, such that heat from the combustion products passing through the exhaust channel is transferred through the at least one inner wall and into at least one of the mixing chamber, restriction or combustion chamber to preheat the fuel, the oxidant and/or the mixture of the fuel and oxidant in the micro-mixer/combustor.

13. The method of claim 12, wherein the combustion provides a stable diffusion flame.

14. The method of claim 12, wherein the fuel, the oxidant and/or the mixture of the fuel and oxidant is heated to at least 100° C.

15. The method of any of claim 12, wherein the outer wall is formed of a material having a low thermal conductivity sufficient to reduce heat loss and sustain a combustion flame in the combustion chamber.

16. The micro-mixer/combustor of claim 2, further comprising an outer wall encasing the micro-mixer/combustor, the outer wall formed of a material having a low thermal conductivity sufficient to reduce heat loss and sustain a combustion flame in the combustion chamber.

17. The micro-mixer/combustor of claim 2, further comprising an outer wall and two inner walls, the inner walls positioned inside of the outer wall and spaced apart from the outer wall, the inner walls further positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber, the spacing between the inner walls and the outer wall forming the exhaust outlet.

18. The micro-mixer/combustor of claim 3, further comprising an outer wall and two inner walls, the inner walls positioned inside of the outer wall and spaced apart from the outer wall, the inner walls further positioned opposite and spaced apart from each other, the inner walls providing the walls forming the mixing chamber, restriction and combustion chamber, the spacing between the inner walls and the outer wall forming the exhaust outlet.

19. The method of claim 14, wherein the outer wall is formed of a material having a low thermal conductivity sufficient to reduce heat loss and sustain a combustion flame in the combustion chamber.