An annulus-type micro reforming system integrated with a two-staged micro-combustor

A new configuration of a micro reforming system integrated with a micro-combustor is studied experimentally and computationally. The micro-combustor as a heat source is a simple cylinder, which is easy to fabricate, but is two-staged (expanding downstream) to control ignition and stable burning. A micro-evaporator to vaporize methanol–water mixtures and a micro-reformer to convert the vaporized methanol–water mixtures to hydrogen are annuli, which are effective to transfer heat from the first and second-stage micro-combustors, respectively. The annulus-type micro reforming system is designed to produce 1–10 W (based on lower heating value, LHV) of hydrogen using the steam reforming method. The molar ratio of water to methanol, the feed rate of water–methanol mixtures, the micro-combustor inlet velocity of fuel–air mixtures and the micro-combustor materials substantially affect the performance of the designed micro reforming system. Under optimized design and operating conditions, the micro reforming system produces 6.9 W (based on LHV) of hydrogen with a conversion rate of 97.5%, an overall system efficiency of 39.7% and a carbon monoxide concentration of 6.7 ppm. Thus, the present configuration can be applied to practical micro reforming systems for use with fuel cells.     

Effect of micro-pin-fin arrays on the heat transfer and combustion characteristics in the micro-combustor

Micro-combustor is an important component elements of the micro-thermophotovoltaic (MTPV) conversion device. The combustion stability is critical to improve its thermal performance, and thus three kinds of combustors are compared by computational fluid dynamics (CFD), which includes single – channel combustor, alternate permutation combustor and in-line combustor. The influences of micro-pin-fin arrays on the performance of the micro-combustor are discussed. Results indicate that the maximum surface temperature of combustor with fins is about 100 K higher than that without fins and the mean temperature and heat flux of in-line combustor are always higher in magnitude than those of the alternate permutation combustor. Analysis in this paper reveals that comparing with single-channel combustor, the micro-combustor with fins greatly enhances the heat transfer process through the wall. There are low velocity zones in the tail of fins, which can gather the reactants and prolong the residence time which make the combustion more sufficient and improve the effect of stable combustion. Meanwhile, under calculated conditions, the influence of micro-pin-fin arrays on the combustion reaction is stronger as the flow rate increase. The fin array in micro-combustor does not only improve the wall temperature but also minimize the wall temperature difference along the axial direction. Moreover, when the inlet velocity is larger than 4 m/s, the hydrogen conversion ratios of micro-combustors with fins was not strengthened obviously with the further increase of inlet velocity.     

A micro reforming system integrated with a heat-recirculating micro-combustor to produce hydrogen from ammonia

In order to evaluate the potential of reforming ammonia as a carbon-free fuel in production of hydrogen, a new configuration of a micro reforming system integrated with a micro-combustor is studied experimentally. The micro-combustor as a heat source is a simple cylinder with an annular-type shield that applies a heat-recirculation concept. A micro-reformer to convert ammonia to hydrogen is an annulus, which is effective to transfer heat from the micro-combustor. The annulus-type micro reforming system is designed to produce 1-10 W (based on lower heating value, LHV) of hydrogen using various catalysts. The feed rate of ammonia, the micro-combustor inlet velocity of fuel-air mixtures and the catalyst materials substantially affect the performance of the designed micro reforming system. Under optimized design and operating conditions, the micro reforming system using ruthenium as a catalyst produces 5.4 W (based on LHV) of hydrogen with a conversion rate of 98.0% and an overall system efficiency of 13.7%. Thus, the present configuration can be applied to practical micro reforming systems, supporting the potential of using ammonia as a clean fuel.     

Effects of major parameters on micro-combustion for thermophotovoltaic energy conversion

For a micro-thermophotovoltaic (TPV) energy conversion device, high surface to volume ratio in the micro-combustor provides a great potential to achieve high surface radiation power output per unit energy input. This work investigated experimentally the effects of three major parameters on micro-combustion, namely hydrogen to oxygen mixing ratio, nozzle to combustor diameter ratio, and wall thickness to combustor diameter ratio. The results show that the high average wall temperature can be achieved at slightly oxygen rich mixing ratios. Nozzle to combustor diameter ratio affects both the magnitude and uniformity of wall temperature distribution. The newly designed thin wall combustor which yields a reduction of axial heat conduction loss is able to increase wall temperature more than 150 K. Optimized design of these parameters will have significant impact on the enhancement of radiation heat output in micro-TPV energy conversion.     

Numerical comparison of premixed H2/air combustion characteristic of three types of micro cavity-combustors with guide vanes,bluff body,guide vanes and bluff body respectively

Micro-scale combustion is facing the problems of ignition difficulty, combustion instability, and low combustion efficiency. Therefore, it is necessary to improve the combustion characteristics in micro-combustor to expand the application range of micro-combustor. Focusing on the problem of the weak preheating effect of the micro cavity-combustor, the guide vanes are constructed to enhance the combustion near the cavity, and further combine with the bluff body to enhance combustion. The combustion characteristics of three types of cavity combustors with guide vanes (CCGV), bluff body (CCBB), bluff body, and guide vanes (CCGB) respectively are compared and analyzed under different inlet velocities (8–32 m/s), equivalence ratios (0.6–1.4), and wall materials (quartz glass, steel, and SiC). Results show that the guide vanes can greatly improve the combustion intensity near the cavity and improve the combustion stability of the cavity combustor. The combustion efficiency of CCGV and CCGB are increased by 43.04% and 85.96% respectively than CCBB when the inlet velocity is 32 m/s. The reaction heat of CCGV is 244.5 W when inlet velocity is 32 m/s, which is 0.55 times and 1.57 times that of CCGV and CCBB, respectively. The temperature uniformity and mean temperature of the outer wall of CCGV and CCGB both are better than that of CCGB. The combustion efficiency of CCGB is the highest among three combustors under the same equivalence ratio, especially when the equivalence ratio is less than 1. The reaction intensity in the cavity of the CCGV and CCGB with steel wall material is highest than that of the combustor with the other two wall materials. Wall materials with high thermal conductivity have a better preheating effect. Compared with quartz as the wall material, the mean temperature of the external wall of CCGV and CCGB using steel and silicon carbide as the wall material both increase by more than 130 K, and the wall heat loss both increase by more than 50%.     

Numerical study of methane and air combustion inside a small tube with an axial temperature gradient at the wall

A numerical study on CH₄ and air premixed combustion inside a small tube with a temperature gradient at the wall was undertaken to investigate the effects of inlet velocity, equivalence ratio and combustor size on combustion characteristics. The simulation results show that the inlet velocity has a significant influence on the reaction zone, and the flame front shifts downstream as the inlet velocity increases. The results also show that, the inlet velocity has no obvious effects on the flame temperature. The highest flame temperature is obtained if the equivalence ratio is set to 1. It is disclosed that the combustor size strongly influences the combustion characteristics. The smaller the combustor size is, the more difficult it is to maintain the steady combustion. The smallest combustor size that the stable flame can be sustained is determined mainly by the wall temperature of the micro-combustor under the given conditions. The higher the wall temperature is, the smaller the smallest combustor size. Therefore increasing wall temperature is an effective way to realize flame stabilization for a given combustor size.     

Effects of channel length on propagation behaviors of non-premixed H2-air flames in Y-shaped micro combustors

In the present study, dynamics of non-premixed hydrogen-air flames in two Y-shaped cylindrical micro combustors of different horizontal channel lengths (L = 100 and 200 mm) were experimentally compared. The inner diameters of the micro-combustors are 2 mm. Unburned mixture was ignited by heating the near-exit wall with a butane torch. The results show that six and three flame propagation modes in the 200-mm and 100-mm micro-combustors were observed, respectively. Moreover, it is found that the flame oscillation duration is much longer with a larger noise intensity in the 200-mm micro-combustor. As a result, the mean propagation speed under L = 100 mm is much larger. In addition, the edge flame is longer on the lean side under L = 100 mm and almost identical on the rich side for the two combustors. Furthermore, the luminosity of edge flame in the 100-mm micro-combustor is much brighter. Numerical analysis reveals that the deflection of propagating flame in the Y-shaped micro-combustor is determined by the stoichiometric line. In summary, the short combustor has a smaller heat loss ratio and a stronger flame-wall thermal coupling, which can enhance the combustion intensity and increase the flame propagation speed.     

Flame stability studies in a hydrogen-air premixed flame annular microcombustor

Flame stability in an annular heat recirculating microcombustor burning stoichiometric hydrogen-air mixture was explored by means of a rigorous thermal analysis. The analysis is based on computational fluid dynamics model of reacting fluid flow accounting for interactions in flow, species, and conjugate thermal field in fluid and solid. Consideration of thermal diffusion effects in the model was necessary for realistic predictions in all the cases. Flame stability under different inlet velocity and wall thermal conductivities was studied. Results showed that a stable flame could stabilize in this combustor in the velocity range of 3-35 m/s. However, the upper stability limit widened for lower wall thermal conductivity. Low velocity flashback and high velocity blowout bounded the stability region with respect to inlet velocity for lower thermal conductivity wall material. Lower flame stability limit was influenced by thermal design of the microcombustor that prevented flame extinction and ability of flame to stabilize at the heated wall even at higher inlet velocity controlled the upper flame stability limit. Flame established well within the combustor for the lowest wall thermal conductivity without blowout and approached flashback for the highest conductivity when wall thermal conductivity was varied at constant inlet velocity. Relative importance of axial and radial wall heat conduction in flame stabilization was explored at the extremes of operating conditions. Both the components played equally important roles in flame stabilization by influencing heat recirculation and losses within the microcombustor. A suitable combination of structural materials could provide a stable flame with high surface temperatures in a lightweight system.     

Fabrication and performance test of catalytic micro-combustors as a heat source of methanol steam reformer

Two different types of H₂ catalytic micro-combustors were fabricated and evaluated as a heat source of methanol steam reformer through MEMS fabrication technology with photosensitive glass wafers. In a packed-bed micro-combustor design, ceramic foam coated with Pt was the catalyst bed. In the thin-film-coated combustor, Al₂O₃ was used as catalyst supports and coated on the combustion chamber wall. Pt was coated on the Al₂O₃ thin-film, which was constructed on the wall. The preparation of Al₂O₃ coating solution and coating process was set up based on sol–gel method. Both combustors had a combustion chamber whose height was 1 mm and the external volume of combustors was 1.8 cm³. Catalytic combustion of H₂ was stable with both combustors. H₂ conversions were over 90% for packed-bed micro-combustor and over 99% for Pt/Al₂O₃ coated micro-combustor. Both combustors burned 80 ml/min of H₂. The catalytic micro-combustors fabricated were applicable to the methanol steam reforming system for 20 W level PEMFC.     

Thermal performance improvement of premixed hydrogen/air fueled cylindrical micro-combustor using a preheater-conductor plate

A micro-combustor is the key component of the micro thermo photovoltaic power generation system. The energy conversion efficiency of the system strongly depends on the outer wall temperature and the exergy efficiency of the micro-combustor. A novel preheater-conductor plate was designed to be inserted into a cylindrical micro-combustor to improve the outer wall temperature and the exergy efficiency by preheating premixed hydrogen/air mixture via the preheater plate and conducting out the heat in the combustion chamber via the conductor plate. The impact of the width of preheater plate, the gradually increased and decreased thickness with divergent angle and convergent angle of conductor plate, and the inserted number of conductor plate on the thermal performance of the micro-combustor were investigated. It is found that the gradually varying thickness of the conductor plate has a slight influence on micro-combustor performance compared to other factors. Inserting a preheater-conductor plate with the 0.5 mm-wide preheater plate and the three 0.5 mm-thick conductor plates improve the mean outer wall temperature and the exergy efficiency of the micro-combustor by 173 K and 22.9% respectively. It is indicated that the proposed preheater-conductor plate is capable of considerably improving the thermal performance of premixed hydrogen/air fueled cylindrical micro-combustor.     

Effect of a crossed-semicircular-plate on thermal performance of micro-combustor fueled by premixed hydrogen-air mixture

A novel crossed semicircular plate has been presented to enhance the thermal efficacy of a micro-combustor. A whirling stream can be generated when the fuel-air mixture is passed around the plate, which boosts the turbulence intensity and residence time of combustion gas. The heat transfer capacity from the flame to the outer wall of the micro-combustor can be promoted by heat conduction of the plate. Moreover, the plate also plays the role of preheating the hydrogen-air mixture. Effects of the design parameters including the mounting position, crossing angle, thickness and thermal conductivity of the plate and the mass flow rate on the thermal performance of a cylindrical micro-combustor with the plate fueled with premixed hydrogen-air mixture were numerically investigated. The findings show the outer wall temperature decreases with the increase of the distance from the inlet to the plate and increases with the increase of the crossing angle, thickness and thermal conductivity of the plate. The maximum outer wall temperature of 1259 K is obtained at a distance of 7 mm, the crossing angle of 120° and the thickness of 0.5 mm. This study offers a conceptual direction for the design and optimization of a micro-combustor.     

Thermophotovoltaic power conversion from a heat-recirculating micro-emitter

Power generation using a novel configuration of a 1–10 W micro-thermophotovoltaic (micro-TPV) device is studied experimentally. A micro-emitter as a thermal heat source is a simple cylinder with an annular-type shield that applies a heat-recirculation concept and an expanded exhaust outlet that facilitates ignition, which provides stable burning in the small confinement and uniform distribution of temperature along the wall. The micro-emitter is surrounded by a chamber with cooling fins, the inner wall of which has an installation of gallium antimonide photovoltaic cells (PVCs). The performance of the micro-TPV device is most favorable at reduced length of the cooling fins unless the temperature on the PVCs is higher than the operating limit temperature for the GaSb cells. The relative position of the micro-emitter to the PVCs also affects the performance of the micro-TPV device. These observations imply that the net amount of heat irradiation onto the PVCs is more dominant than the PVC temperature in determining the TPV performance. Under optimized operating conditions, the micro-TPV device produces 2.4 W with an overall efficiency of 2.1%, indicating that the efficiency up to the PVC surface is 21% at least if a PVC efficiency of 10% at most is assumed.     

Two-dimensional analytical investigation of conjugate heat transfer in a finite-length planar micro-combustor for a hydrogen-air mixture

This study aims to investigate the conjugate heat transfer in a finite-length planar micro-combustor by using a two-dimensional analytical model. The primary objective is to thermally analyze the effects of ambient, solid wall, and gas mixture properties on wall and gas temperature profiles and heat recirculation via the micro-combustor wall. The micro-combustor is divided into three sections composed of pre-flame, reaction, and post-flame zones. The energy equation is solved in each zone for both wall and gas mixture regarding the matching boundary conditions. As a result, an appropriate correlation that is a combination of hyperbolic tangent and linear functions for estimating the wall temperature profile is developed. Moreover, since the absolute amounts of positive wall temperature gradient are larger than the negative ones, heat is conducted intensely toward the inlet side of wall which results in heat recirculation at pre-flame zone. These findings can be used to propose a simple approach for preliminary micro-combustor design.     

Numerical study on methane/air combustion characteristics in a heat-recirculating micro combustor embedded with porous media

Micro-combustor is a portable power device that can provide energy efficiently, heat recirculating is considered to be an important factor affecting the combustion process. For enhancing the heat recirculating and improving the combustion stability, we proposed a heat-recirculating micro-combustor embedded with porous media, and the numerical simulation was carried out by CFD software. In this paper, the effect of porous media materials, thickness and inlet conditions (equivalence ratio, inlet velocity) on the temperature distribution and exhaust species in the micro combustor are investigated. The results showed that compared with the micro combustor without embedded porous media (MCNPM), micro-combustor embedded with porous media (MCEPM) can improve the temperature uniformity distribution in the radial direction and strengthen the preheating capacity. However, it is found that the embedding thickness of porous media should be reasonably arranged. Setting the thickness of porous media to 15 mm, the combustor can obtain excellent comprehensive capacity of steady combustion and heat recirculating. Compared the thermal performance of Al₂O₃, SiC, and ZrO₂ porous media materials, indicating that SiC due to its strong thermal conductivity, its combustion stabilization and heat recirculating capacity are obviously better than that of Al₂O₃ and ZrO₂. With the porous media embedded in the micro combustor, the combustion has a tempering limit of more than 10 m/s, and the flame is blown out of the porous media area over 100 m/s. The reasonable equivalence ratio of CH₄/air combustion should be controlled within the range of 0.1–0.5, and “super-enthalpy combustion” can be realized.     

Characterization of wall temperature and radiation power through cylindrical dump micro-combustors

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. An experimental study on the wall temperature and radiation power through the wall of a series of cylindrical dump micro-combustors was carried out. The effects of combustor diameter (d), combustor length (L), step height (s), flow velocity (u₀) and fuel-air equivalence ratio (Φ) on the wall temperature distribution were investigated. ‘Emitter efficiency’ was defined and quantified based on the measured wall temperature. It was demonstrated that for this particular configuration, that is, a cylindrical micro-combustor with a backward-facing step, the two dimensionless ratios - L/d and s/d, are sufficient to determine the emitter efficiency, provided the flow velocity (U_e) and Φ are known. Based on this result, the effects of the dimensionless step height (s/d) on the emitter efficiency were examined. It was shown that such a sudden flow expansion in the dump combustors does not favor the radiation through the outer wall. Finally, the position of the highest wall temperature and its variation with the flow Reynolds number were discussed. It was noted that the Reynolds number and the relative flow expansion (s/d) alone are inadequate to determine the relative position of the highest wall temperature.     

Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

An analytical model for flame stabilization in meso-scale channels is developed by solving the two-dimensional partial differential equations associated with heat transport in the gas and structure and species transport in the gas. It improves on previous models by eliminating the need to assume values for the Nusselt numbers in the pre and post-flame regions. The effects of heat loss to the environment, wall thermal conductivity, and wall geometry on the burning velocity and extinction are explored. Extinction limits and fast and slow burning modes are identified but their dependence on structure thermal conductivity and heat losses differ from previous quasi one-dimensional analyses. Heat recirculation from the post-flame to the pre-flame is shown to be the primary mechanism for flame stabilization and burning rate enhancement in micro-channels. Combustor design parameters like the wall thickness ratio, thermal conductivity ratio, and heat loss to the environment each influence the flame speed through their influence on the total heat recirculation. These findings are used to propose a simple methodology for preliminary micro-combustor design.     

Fast Ignition and Stable Combustion of Coarse Coal Particles in a Nonslagging Cyclone Combustor

A combustion set-up of an innovative nonalagging cyclone combustor called “Spouting-Cyclone Combustor(SCC)”，,with two-stage combustion,organized in orthogonal vortex flows,was established and the experimental studies on the fast ignition and stable combustion of coarse coal particles in this combustor were carried out.The flame temperature versus ignition time and the practical fast ignition the temperature fields in SCC were obtained.These results whow that it is possible to obtain highly efficient and clean combustion of unground coal particles by using this technology.     

The effect of the blockage ratio on the blow-off limit of a hydrogen/air flame in a planar micro-combustor with a bluff body

We recently developed a micro-combustor with a bluff body, which has a demonstrated 3- to 5-time extension in the blow-off limit. In the present work, the dimension effect of the bluff body on the blow-off limit (indicated by the blockage ratio, ζ) was investigated with a detailed H₂/O₂ reaction mechanism. The results indicate that the blow-off limits for ζ = 0.3, 0.4 and 0.5 are 20, 31 and 36 m/s, respectively. Analyses reveal that for ζ = 0.3, flame blowout occurs due to insufficient recirculation zone size. However, flame blowout occurs due to the stretching effect in the shear layers when ζ = 0.4 and 0.5. Calculations indicate that the three cases have negligible differences in heat loss because the high temperature zones are located in the combustor centers; therefore, their effects on the combustor walls are mitigated.     

Experimental and Numerical Study of High-Altitude Ignition of a Turbojet Combustor

This paper aims at contributing to the methodology used for the numerical prediction of ignition inside a combustion chamber. For this purpose, experiments are carried out in a model combustor with improved optical access. Laser tomography and high-speed video give a first insight into the unsteady airflow and the flame structure. Laser Doppler anemometry is used to measure the gas flow velocity field, and the nonreactive two-phase flow is studied in detail using particle Doppler analysis. The velocity field of the burning spray is measured using particle image velocimetry. Ignition tests are performed to evaluate the minimum global equivalence ratio. This in-depth database is used to validate RANS simulations conducted in parallel using the ONERA computational fluid dynamics (CFD) code CEDRE. The numerical model for transient, spherical kernel ignition, proposed in previous work, has been improved and fully implemented in CEDRE. A first parametric study has been conducted on a basic configuration consisting of three validation cases: a gaseous mixture, a monodisperse spray, and a polydisperse spray. These validation cases are inspired from previous studies found in the literature and give a better understanding of the basic phenomena involved in the first stages of flame propagation. This model is then used in combination with CEDRE to estimate the ignition probability of given spark-plug positions in a more realistic configuration: the MERCATO combustor.     

Flame propagation in metal slurry sprays

An analytical and computational study of vaporizing and burning liquid hydrocarbon-metal slurry droplet streams injected into a hot gas is presented. The objective is to investigate the mass and energy interactions between the slurry droplet streams and the gas flow. An idealized configuration consisting of parallel droplet streams is used. The governing gas-phase equations are analytically integrated by using the Green's function approach and a resulting set of first order nonlinear ordinary differential equations is numerically solved. The slurry droplet model includes transient heating and particle drag, a shell-bubble formulation, heating and ignition of the metal agglomerate subsequent to the vaporization of the liquid fuel, and vapor-phase burning of the metal. Results show that, at different combustor locations, interacting and distinct premixed and diffusion type reaction zones are present. For the metal particle to be ignited, at a given metal loading, there exists a minimum inlet gas temperature requirement which depends upon the equivalence ratio. The heating and burning times of the metal agglomerate are found to be much larger in comparison to the liquid fuel vaporization times, and they increase with increasing metal particle size and metal loading of the slurry droplets.