Meso-scale Combustor Research Articles

Ultra-rich fuel dynamics of a holder-stabilized premixed flame in a preheated mesoscale combustor

The present work experimentally studies the laminar premixed flame dynamics at fuel ultra-rich condition under the synergistic effect of heat and flow recirculation, which take place in many practical combustors, such as aero engines. The normal, anomalous, and low speed blow-off phenomena are observed. Furthermore, the anomalous standoff distance between the flame root and the holder is found for the first time at fuel rich condition. Interestingly, with an increase in the equivalence ratio, the oscillating flame appears before blow-off at Reynolds number Re ≥ 60. The stability limit first increases and then decreases with a decrease in Re, which is also reported for the first time. If the equivalence ratio is increased further, the oscillating flame at Re ≥ 60 blows off with a small timescale, while the stable flame at Re < 60 gradually acquires a flat behavior, and then shifts downstream at a very slow speed.

Anomalous blow-off limit of methane-air premixed flame in a micro preheated combustor with a flame holder

The present work first reports the combustion characteristics of methane/air premixed fuel mixture (Lewis number Le ≈ 1.0) at different Reynolds numbers (Re) in a micro preheated combustor with a flame holder which can be used as heat source for the micro power generators. Unexpectedly, the previously reported anomalous blow-off of lean H2/CH4/air (Le < 1.0) premixed flame in the mesoscale combustor appears in the present micro combustor, which is observed for the first time. The flame dynamics in the micro combustor also presents other new features comparing with that in the similar mesoscale combustor. Three neighboring cases (Re = 40, 80, and 120) are adopted to reveal the underlying mechanisms of the anomalous blow-off in terms of the flow field, stretch effect, conjugate heat exchange, and preferential transport effect. A better heat recirculation effect results in the increase of blow-off limit from Re = 120 to 80. A worse flow recirculation effect, a larger heat loss ratio to the environment, and a worse preferential transport effect lead to the decrease of blow-off limit from Re = 80 to 40. In the end, the critical standoff distance between the flame root and flame holder presents a second-order function relation with the change of Re.

Anchoring mechanisms of a holder-stabilized premixed flame in a preheated mesoscale combustor

A very lean-premixed, laminar methane–air flame is demonstrated, experimentally, to be stable in a mesoscale combustor with a flame holder. Unlike the anchoring location of the flame tip, the anchoring location of the flame root is practically independent of the equivalence ratio, inlet velocity, and thermal conductivity of the solid wall material. When the mixture becomes leaner, both the flame root and tip can adaptively shift toward the locations with higher temperatures, and additionally, the anchoring temperature of the flame root is higher. Subsequently, by means of the three-dimensional computational mechanics, their anchoring mechanisms are thoroughly analyzed in terms of the flow recirculation, stretch effect, preferential diffusion, and conjugate heat exchange. A recirculation zone or a low-velocity zone formed behind/near the flame holder and combustion chamber wall can assist the balance between the flow velocity and the flame speed for the flame anchoring, and the flame root can adaptively shift to a zone of lower local velocity. The stretch effect is not responsible for the flame root anchoring, but this effect stabilizes the flame tip by increasing the local flame speed near the flame tip. Preferential diffusion significantly promotes the local equivalence ratio near the anchoring location of the flame root, thereby facilitating the stability of this flame root, though it stabilizes the flame tip only slightly. Furthermore, the conjugate heat exchange plays an important role in preheating fuel/air and intensifying combustion, which influences the stabilization of both the flame root and tip. The shorter distance between the flame tip and the combustion chamber wall results in a stronger flame–wall coupling. These results indicate that the anchoring mechanisms for the flame root and tip differ.

Blow-off mechanism of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor

In a previous study, we fabricated a well-designed mesoscale combustor with a flame holder and preheating channels to improve the flame stability under extreme combustion conditions. This combustor can also act as a model for studying the synergistic effect of heat and flow recirculation on the flame dynamics. This paper systematically illustrates and discusses the blow-off mechanisms of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor via three-dimensional numerical simulation. The flame blow-off processes that have been observed experimentally at various Reynolds number (Re) are well reproduced. Pinch-off appears at large Re but is absent at small Re. First, the moving characteristics of the flame root and flame top in the blow-off process at small Re are discussed in detail. Then, the flame pinch-off mechanisms are carefully analyzed in terms of the flow field, conjugate heat exchange, and stretch effect. The results indicate that the influence of conjugate heat exchange, which exists between the flame holder and flame root, is not significant in the flame pinch-off phenomenon, but a larger recirculation zone (which helps to stabilize the flame root) and flame stretch (which splits the flame) both induce flame pinch-off. This work can aid in gaining insight into the flame blow-off dynamics when the effects of heat and flow recirculation work synergistically in many practical combustors.

Dynamics of a holder-stabilized laminar methane-air premixed flame in a preheated mesoscale combustor at ultra-lean condition

The flame dynamics at ultra-lean condition under the synergistic effect of heat recirculation and flow recirculation is experimentally studied in this work. The previously reported anomalous blow-off of lean H2/CH4/air premixed flame with Lewis number Le < 1.0 is obtained for the lean CH4/air premixed flame at Le ≈ 1.0, and the underlying mechanisms are analyzed in terms of heat recirculation, flow recirculation, preferential diffusion, and stretch effects. An anomalous standoff distance between the stable flame root and flame holder is also observed for the lean CH4/air premixed flame. The blow-off dynamics at large and small values of Reynolds numbers are quite different. The flame with local extinction and reignition appears in the blow-off process at large Reynold numbers (≥100), whereas it is absent at small Reynold numbers (<100). The results attained in this study can help to gain insights into flame dynamics in many practical combustors.

Mesoscale combustor-powered thermoelectric generator: Experimental optimization and evaluation metrics

The recent development of potable power sources is of considerable interest. A mesoscale combustor-powered thermoelectric generator integrated with a mesoscale stagnation-point reverse-flow combustor and a built-in heat exchanger was designed and optimized in this work to augment electric power and overall efficiency. The mesoscale combustor can run at a volumetric heat load of 72 MW/m3. The thickness of heat spreader, input power, equivalent ratio, fuel types, and heat exchanger channel types were explored to optimize the system performance. The optimized electric power of 30.7 W obtained at an overall efficiency of 3.21% was larger than the previous record of 18.1 W at an overall efficiency of 3.01% in 2015. The proposed thermoelectric generator, which was conducted with 13.2 kg of methane, reached an energy density of 300 Wh/kg. The novel design provided a concrete way to solve the contradiction between the availability of Bi2Te3-based thermoelectric module and its relatively low working temperature. Additionally, a new metrics, which is defined as (EFS/efTE,max) at Pin, was firstly proposed to evaluate various thermoelectric generators, in which different thermoelectric materials, combustion types, and input powers could be involved. The proposed metrics correlates the overall, running thermoelectric, maximum thermoelectric, combustion (reaction), and heat collection efficiencies under different input powers. The power of this metrics was comprehensively discussed by deducing the running thermoelectric efficiencies of various previous works and presenting the performance comparisons of different previous thermoelectric generators. The obtained metrics, that is, (75.9%/4.23%) at 957 W in this work was higher than those in previous studies.

Performance of cylindrical and planar meso­scale combustor with double narrow slit flame holder for micropower generator

This research compared the performance of a cylindrical mesoscale combustor against two planar mesoscale combustors, which include the shape of the flame front, temperature of the combustor axis and the combustor wall, and the resulting flammability limit. The combustor used has a circular, square and rectangular cross-section. All three combustors have the same cross-section area and combustion chamber volume. The flame holder used is a double narrow slit. The fuel used is liquefied petroleum gas with a pure oxygen oxidizer. The experiment results showed that the cylindrical combustor produces a more even flame shape that fills the combustion chamber and there is no clear separation between the sides of the flame on each side of the narrow slit. A high ratio of the entrance to average velocity results in a large adverse pressure gradient which generates vortex and recirculation behind the flame holder which gives the mixture a longer chance in the combustion chamber (prolonged residence time). The flame front shape affects the temperature of the combustor axis. The flame front shape that fills the entire combustion chamber has a higher flame temperature than the separate flame front shape. The circular combustor has the highest average axis temperature, but it has the lowest combustor wall temperature. This fact shows that the circular combustor has the smallest heat loss from the flame to the combustor wall. Furthermore, a circular mesoscale combustor has the most extensive stability map. For the same volume, the circular combustor has a lower surface area to volume ratio, thus the heat loss is also low. The dead zone area also becomes narrower, only at a low reactant rate. Rectangular combustors have the largest surface area to volume ratio, thus the losses are also the biggest. Despite the narrowest flammability limits, rectangular combustors have the highest average wall temperatures

Flame behavior inside constant diameter cylindrical meso­scale combustor with different backward facing step size

This research observes the behavior of the flame stability in a cylindrical meso-scale combustor at various backward facing step sizes. The backward facing step was varied by changing the size of the combustor inlet diameter while the size of the combustor outlet diameter was kept constant, keeping a constant contact area. Butane gas (C4H10) was used as fuel with air as the oxidizing agent. The results show that generally, the flame mode and area of the flame mode map are obtained for the conditions of the stable flame at combustor rim, stable flame in combustor, stable flame near the step, oscillating flame, oscillating spinning flame, spinning flame, flashback, and no ignition. Flame mode and flame mode map distribution depend on reactant flow velocity behavior, jet flow generating shear stress, vortex flow regulating wall-thermal interaction, and average flow generated by varying the backward facing step size at various equivalence ratio and reactant velocity in the test range. Jet flow destructs flame stability to be extinct due to strong shear stress. Vortex flow spins the flame while the transition from jet to vortex flow oscillates the spinning flame. Weak vortex at average flow plays an important role in wall-thermal interaction that keeps flame very stable. Decreasing the backward facing step size tends to widen the flame stability region, but the combustion process causes the flame to be flashed back. By setting the reactant velocity at a small backward facing step size to the condition where the weak vortex flow exists, flashback conditions could be avoided keeping the flame very stable. Stable flame tends to be performed around stoichiometric to the lean mixture and in the low to medium reactant flow velocity. At high reactant flow velocities, the flames tend to be unstable. However, at low to medium reactant flow velocity, the flame tends to be stable in the combustor

A numerical study on diffusion H2/air flames in Y-shaped mesoscale combustors

Recently, we experimentally investigated the combustion characteristics of non-premixed H2 and air in Y-shaped mesoscale combustors. In the present study, we aim at numerically explaining some experimental phenomena and tendencies, and revealing more details of flame propagation which cannot be well captured in the experiments. The numerical results show that the flame propagates in premixed flame mode in the downstream and shifts to edge flame mode in the upstream, which is a result of the fuel/air mixing effectiveness associated with the longitudinal distance. The inclination direction of flame might change during the propagation process, depending on its original direction after ignition. Moreover, the flame propagates faster in edge flame mode than in premixed flame mode. In addition, the edge flame length increases with the increase of combustor width because a longer distance is needed for a well mixing in the wider channel. Furthermore, the edge flame in the combustor of L = 100 mm is evidently longer compared to that of L = 200 mm when the nominal equivalence ratio is below unity (e.g., ϕ = 0.7). This is because the heat loss ratio under L = 100 mm is less than that of L = 200 mm.

Experimental investigation of flame propagation in a meso-combustor

The present work aims to study the quenching of propagating flames in meso-combustors for which dimensions are of the order of quenching distances of hydrocarbon fuels. Combustion of gaseous fuels and subsequent flame propagation in a meso-scale combustor duct of square cross-section is studied experimentally. Premixed mixtures of methane, propane, and ethylene with air are considered. Two different variants of flame propagation states are found to occur in the meso-combustor, viz., one undergoing flame propagation till the combustor entry and quenching at the step and the other undergoing wall quenching. Regime transitions across these flame states are mapped comprehensively over a wide range of operating conditions. The radius of curvature of the flame and the dead space between the flame and the wall are determined for those conditions with the aid of curve fitting and image processing techniques using Matlab software. The spatial and temporal variation of both these parameters show a drastic increase during quenching in the wall-quenched case, while it remains nearly constant in the step quenched case. With increasing duct Reynolds number, the flame propagates slower, and the heat conduction to the wall leads to a decrease in the dead space and flattening of the flame, particularly at equivalence ratios corresponding to lower flame speeds. This flame-wall interaction is found to be low for methane, resulting in more heat loss and thereby wall-quenched flames compared to propane and ethylene. None of the ethylene flames were found to suffer wall quenching thereby making it a suitable fuel for meso-/micro-combustors among the three fuels used in the present work.

An optimization approach for improving the exergetic efficiency in mesoscale combustor

The optimum designs of combustor are traditionally based on trial and error method. In this paper, we propose a systematic optimization method for the optimum design of this kind of reactor. The optimization procedure is introduced by taking a premixed methane/air combustion as an illustrating example. We show the efficiency of the combustion process is dominated by the entropy generation of the reaction and heat transfer and the viscous dissipation of the fluid flow in the reactor. The minimum of entropy generation of the process is thus taken as the objective function and the conservation equations and constant viscous dissipation as the constraints. A solution procedure for the optimization problem based on the calculus of variation is proposed. To validate the proposed method, we show that the optimization procedure can provide a thermodynamic limit for the combustion process by comparing with different internal structures of combustion chamber.

Soot formation and combustion characteristics in confined mesoscale combustors under conventional and oxy-combustion conditions (O2/N2 and O2/CO2)

This work investigated soot formation and combustion characteristics in confined mesoscale combustors of two diameters 4 and 6 mm between conventional and oxy-combustion conditions using high resolution transmissions electron microscopy, thermogravimetric analysis and exhaust gases analysis. Results demonstrated that by increasing O2 concentration, the combustion performance can be improved significantly. In 4 mm combustor, both ethylene conversion and mean wall temperature for N2 diluted flame were larger for higher O2 concentration because the reduction of inert diluted gas decreased the heat loss and more O2 content contributed to reactions of more fuel. For N2 diluted flames in 6 mm combustor, soot nanostructure under 21% O2 exhibited amorphous structures but typical fullerene-like structures with more organized fringes could be observed under 30% and 40% O2. The soot carbonization degree increased notably with higher O2 concentration. For the same O2 concentration and combustor, ethylene conversions and temperatures for CO2 diluted flames were always less than those from N2 diluted flames due the larger specific capacity of CO2 than N2. Higher CO concentrations were also detected in exhaust gas from the flame with CO2 dilution than with N2 because CO2 directly participated in the reaction CO2+H⇄CO+OH.

The effect of copper flame holder application on butane combustion characteristics in meso-scale combustor

This experimental research aims at determining the characteristics of butane combustion in Mesocombustor made of a quartz glass tube with a diameter of 3.5 mm and an outer diameter of 5 mm. The findings revealed that the flammability limit became wider with sharp curve narrowing. The narrowing of flammability limit did not occur in the other flame holder made of mesh stainless steel. Different from stainless steel conductivity, copper has declining thermal conductivity in each of temperature increase. The wider flame section was due to the increase of temperature in each flow velocity.

The Experimental and Kinetic Study on Ignition Characteristics of n-Butane/Oxygen-Enriched Air Mixtures in a Heated Meso-Channel

ABSTRACT Oxygen-enriched combustion is expected to be introduced into small scale to improve the characteristics of micro- or meso-scale combustion. To fully understand the advantages of oxygen-enriched combustion, the ignition characteristics of n-butane/oxygen-enriched air mixtures are studied by the experimental and numerical methods in a heated quartz tube with an inner diameter of 5 mm. The experimental results show that the oxygen enrichment level, equivalence ratio, and flow rate have an important influence on the ignition temperature of the mixed gas in the tube. The ignition temperature decreases with the increasing oxygen enrichment and is strongly affected by oxygen concentration at higher equivalence ratio and lower oxygen enrichment level. In the meanwhile, higher equivalence ratio and lower flow velocity also significantly reduce the ignition temperature in the experimental cases. Furthermore, the increase in the oxygen enrichment level significantly enhances the effect of the equivalence ratio on the ignition temperature while higher flow velocity slightly weakens the effect. The numerical results show that oxygen enrichment is beneficial to the production of OH radicals in pre-ignition period and significantly promotes the oxydehydrogenation reactions of intermediates.

Development of a Standalone, Liquid Fuelled Miniature Power Generation System

Abstract This paper aims at developing a novel mesoscale combustion-based thermoelectric power generator as an alternate to the electrochemical batteries. Most of the micro and mesoscale combustors investigated till date are based on external fuel and air supply systems, which may not be beneficial for a practical system. The proposed design is a standalone system which makes use of the heat conducted through the combustor walls, as an energy source to evaporate the liquid fuel stored in a surrounding tank and supply the vaporized fuel to the combustor. The high momentum fuel (vapor) jet is designed to entrain the ambient air in appropriate proportion so as to form a combustible mixture. The fuel/air mixture is fed to a mesoscale combustor and the flame is stabilized by facilitating hot gas recirculation regions. The hot combustion products then flow over a ceramic plate on the other side of which the hot side of a thermoelectric module (TEM) is attached, while ensuring a near uniform temperature, to generate electricity. The cold side of the TEM is maintained at relatively lower temperature and the heat is designed to be rejected using fins to the ambient. The prototype is designed to produce an electrical power output of 12 W with an overall efficiency of about 2.4% (heat load of 500 W) and endurance of 1.5 h in a single fuel refill. The paper presents detailed description of the constituent components and the CFD analysis which evaluates the performance of the system.

Experimental study on the electrospray and combustion characteristics of biodiesel-ethanol blends in a meso-scale combustor

The spray and combustion characteristics of biodiesel-ethanol blends were experimentally investigated by using electrospray atomization in a meso-scale combustor. Phase Doppler anemometry measurements were employed to obtain droplet size distribution. Optical measurements were carried out to quantify the electrospray structures. With the addition of ethanol, the spray angle increased and the droplet size decreased from 128 μm to 12 μm. The polydisperse spray of the biodiesel-ethanol blends with a low ethanol blending ratio yielded the bimodal distribution in droplet size due to Coulomb fission. Relative standard deviation of monodisperse droplets is below 15%. The combustion of the blends was performed in a meso-scale combustor with a 12 mm inner diameter. The flame can be stabilized near the mesh in the combustor without any external heating. It was found that the spray structures have a direct effect on the flame structures in spray combustion. 40% ethanol addition (BE40) has the best combustion performance compared to other blends. The results show that the flame of BE40 has a broader stabilization limit both in flow velocity and equivalence ratio. The reduction of the droplet size for the blends with high ethanol blending ratio results in the decrease of CO emission.

The effects of aspect ratio on CH4/air flame stability in rectangular mesoscale combustors

The effects of aspect ratio (α = 1, 2, 3 and 4) of mesoscale rectangular channels on CH4-air flame stability were numerically studied. The results show that at relatively high inlet velocity, the flame is convex (with respect to the upstream) when α ≤ 3; instead it shifts to a W-shaped appearance at α = 4. In addition, the flame blowout limit exhibits a non-monotonic variation and it peaks at α = 3. Our analysis reveals that when α is raised from 1 to 3, both of the heat loss ratio and hydrodynamic perturbation rate are reduced, which favors a larger blowout limit. However, for the channel with α = 4, the perturbation rate of hydrodynamic instability is significantly increased at the flame tip because of the increased wave number, which results from the W-shaped flame front. Consequently, the intensified hydrodynamic instability will lead to a reduction in the flame blowout limit of the channel with α = 4.

The use of heat circulator for flammability in mesoscale combustor

The mesoscale combustor is a part of the micropower electric generator. The function of the mesoscale combustor is to convert hydrocarbon to become thermal energy through combustion reaction. It is difficult to maintain the flame stability of a mesoscale combustor due to its millimetre-scale size.This study aims to determine the performance and recognize mesoscale combustor phenomena that have stainless steel heat recirculators. This study is to test the combustion characteristics of liquid and gas fuels in meso-combustors which use heat recirculator. The heat circulator is made of stainless steel tube with an inner diameter of 3.5. The parameters observed were flammability limits, temperature distribution and flame visualization.It is confirmed that the stainless steel heat recirculator, is useful for liquid fuel preheating and evaporating inside of mesoscale combustor. The flame of liquid fuels can be stabilized at an equivalence ratio of 0.9 to 1.25, and up to about 900 centigrade Celsius. Thus recommend for liquid fuel micropower generator. It is noted that when the heat recirculator is too close to the flame, excessive flame cooling occurs and causes the flame extinguished. The meso-combustor, which has no heat recirculator, and designed for gas fuel only, can stabilize flame at an equivalence ratio of 0.7 to 1.5. It is also confirmed that the inaccurate selection of the material of thermal recirculator risks reducing the flame stability. It is important to note that when the gas fuel exits the storage tube, there is an expansion and a decrease in temperature which can affect flammability limits

Development of planar meso­scale combustor with double narrow slit flame holder and various aspect ratios for micropower generator

We have investigated the effects of the aspect ratio of the rectangular mesoscale combustor with a narrow slit flame holder on the flame stability limit, flame behavior and uniformity of combustor wall temperature. The combustor was made of copper with a cross-section area of 6 mm2. The combustor aspect ratio (AR) was varied as 1, 1.5, 2.67, and 6. LPG and pure oxygen were premixed and the experiment was conducted at a limited flow rate. Pure oxygen is selected as an oxidizing agent with the intention of revealing in detail the range of flame stability within a very narrow quenching distance. All observed flames were inside the combustion chamber, not outside the channel. This research used a new type of flame holder namely double narrow slit flame holder as a flame stabilizer. The flame holder with double slit and a kind of bluff body in the center helps recirculate the flow and prolong the residence time to make the flame more stable. The use of double narrow slit flame holder successfully extended the stability map to a very lean equivalence ratio (f). However, there was a dead zone near stoichiometry condition due to very high flame propagation speed. Aspect ratio had an important role for the non-circular combustor. The aspect ratio gave a great effect to determine the limit of the stability map that can be achieved at the rich mixture. The combustor with AR=1.5 had the widest range of flammability limit, while AR=6 hadthe narrowestflame stability limits. However, the latter reached the most uniform wall temperature, which is important to obtain high efficiency thermal to electric energy conversion. The results of this study can be used to determine the right fuel mixture on the mesoscale combustor as a heat source on the micro power generator/thermal electric system

Effect of backward facing step on combustion stability in a constant contact area cylindrical meso­scale combustor

This experiment investigates the effect of backward facing step size variation on combustion stability in the cylindrical meso-scale combustor with the constant contact area. The backward facing step was varied by changing the combustor inlet diameter while the combustor diameter at the combustion zone was kept constant, i.e. has a constant contact area. Butane gas (C 4 H 10 ) was used as fuel with air as an oxidizing agent. The result shows that the backward facing step has an important role in the combustion stabilization mechanism. Stable flame could be stabilized inside the meso-scale combustor with the backward facing step. Without the backward facing step, the flame blows out, then stable at the combustor rim. Recirculation flow occurs in the area behind the backward facing step. The increasing backward facing step size leads to an increase in reactant inlet velocity, recirculation flow size and shear stress in the area near the backward facing step. At large backward facing step size, the high reactant inlet velocity together with the large shear stress quenches the flame while the heat recovered by recirculation flow is less sufficient to stabilize flame so that the flame drifts to the downstream position. Hence bigger backward facing step size causes narrower flame stability limit area. The smaller the backward facing step size the wider the flame stability limit which shifts more toward lower equivalence ratio and high reactant velocity regions. Decreasing the backward facing step size decreases the reactant velocity into the combustion reaction zone as well as decreases the recirculation flow and the shear stress, so that the quenching effect decreases. The smaller recirculation flow has a better function for the flame holder to increase flame stability inside the meso-scale combustor. Therefore, small backward facing step size has a very important role in recovering heat energy stabilizing the flame in the meso-scale combustor