Swiss-roll Combustor Research Articles

Effects of channel height on the combustion efficiency and thermal performances of a micro Swiss-roll combustor

Micro combustors can be used as the heat sources for micro power generation systems. Recently, we developed a micro Swiss-roll combustor for non-premixed fuel and air. In the current work, the impacts of channel height (H2 = 7.5, 7.0, 6.5, and 6.0 mm) on the combustion efficiency of CH4, and the surface radiation efficiency of this combustor were numerically investigated under constant flow rates of the fuel and air. The results showed that both combustion efficiency and radiation efficiency varied non-monotonically with the decrease of channel height. The optimal channel height was found to be 6.5 mm under which a highest combustion efficiency of 99.05 % and a highest radiation efficiency of 27.03 % were obtained. These variation rules were theoretically analyzed in terms of the heat recirculation effect on fresh reactants, the buoyancy effect on flame structure and chemical reaction, and the non-dimensional residence time of the reactants (DaL). It was unraveled that the combustor with H2 = 6.5 mm achieved a best heat recirculation effect. In addition, the buoyancy effect was weakened as the channel height was decreased, resulting in a more symmetric flame structure in the vertical direction and a more intense chemical reaction. Furthermore, when the channel height descended from H2 = 7.5 mm to 6.5 mm, the region of DaL > 1 was broadened due to the intensified reaction, but narrowed as the channel height was further reduced to 6.0 mm due to increased flow velocity. The combined actions of these factors led to non-monotonic variations of combustion efficiency and radiation efficiency. The discoveries of the present study provided a guideline for optimal design of such kind of micro Swiss-roll combustors.

Experimental and numerical investigation on non-premixed CH4/air combustion in a micro Swiss-roll combustor

This paper reports the experimental investigation and numerical analysis of the non-premixed CH4/air combustion in a micro Swiss-roll combustor. Six flame patterns were observed in the experiment, namely, single flame, twin flames, pulsating flame, obviously asymmetric flame, extinction and blow-off. A global map is drawn for these combustion regimes based on systematic experimental observation. The lower combustible limits of the nominal equivalence ratio (ϕ) and inlet methane flow rate (FM) are 0.4 and 0.02 L/min, respectively. Moreover, the blow-off limit increases significantly as the equivalence ratio is increased from 0.7 to 0.8. Furthermore, it is found that both the maximum and average surface temperature of the combustor cover show a non-monotonic variation versus the nominal equivalence ratio, and their maximum and minimum values appear at ϕ = 0.9 and 0.8, respectively. Numerical simulation can reproduce the flame shapes successfully. In addition, the asymmetric flame structures in the vertical direction were also discovered through numerical simulation. The comparative analysis demonstrates that the buoyancy effect leads to asymmetric flow field in the vertical cross section of the combustor. As a result, the reaction heat and fuel conversion ratio in the combustion chambers change non-monotonically with the equivalence ratio, which leads to a similar variation manner of the surface temperature. In summary, the present study provides a deep insight into the characteristics of non-premixed combustion in micro Swiss-roll combustors.

The application of an innovative integrated Swiss-roll-combustor/Stirling-hot-end component on an unpressurized Stirling engine

An innovative integrated Swiss-roll-combustor/Stirling-hot-end component is proposed in this paper. A power generation system in conjunction with a Stirling engine is also built around the component and tested. The Stirling engine is a compact-size, unpressurized, γ-type engine that uses air as working gas. The Swiss-roll combustor is laden with small catalytic beads to decrease the ignition temperature and improve combustion efficiency. Experiments respectively burning dimethyl ether and syngas were conducted to better understand the potential of the system on tapping into green biomass energy. The marriage between the Swiss-roll combustor and the Stirling engine’s hot end offers many advantages as follows: The structural support from the spiral-channel wall in the Swiss-roll combustor enables the bottom wall of the Stirling engine to be made very thin, which improves the rate of heat transfer from the heat source to the engine significantly. Meanwhile, the system inherits those advantageous features in combustion from the Swiss-roll combustor, including a broader range of fuel/air ratio, higher combustion efficiency, and higher efficiency in harvesting the heat of flue gas. Compared to the predecessor of the present engine, the incorporation of the proposed component is proven to improve engine’s performance remarkably. In the experiment of burning dimethyl ether, the system produced 18.7 W of electric power and achieved 2.4% thermal to electric efficiency under a temperature difference of 254 °C. Such performance is remarkable among atmospheric-pressure Stirling engines. The proposed component has accomplished its design goal and would hopefully inspire new ideas for the development of green-energy Stirling-engine power generation systems in the future.

Effect of mixture velocity for given equivalence ratio on flame development in Swiss roll combustor

Small-scale power generation using heat energy from hydrocarbon fuels is a proven technology. In this study, we analyzed 2-D flame development in meso-scale Swiss roll combustor. A mixture of 60% butane and 40% propane was used (0.25?0.55 L per minute). During all the analyses, equivalence ratio (1.1) was kept constant by adjusting air quantity against fuel quantity. The effect of increase in the mixture velocity on the development of flame shapes/patterns was monitored. We found different patterns of flame, e. g., planar, concave, and conical, with the increase in mixture velocity. Increase in combustion chamber temperature was also noted. No flashback was observed and blowout was observed with very high mixture velocity. Combustion chamber temperatures were found to be increasing with the increase in mixture velocity at the same equivalence ratio. Elongation of the flame was observed because of the increased flow velocity. Heat re-circulation to the reactants enhances flame characteristics.

Experimental investigation on non-premixed CH4/air combustion in a novel miniature Swiss-roll combustor

In the past decades, combustion in miniature Swiss-roll combustors has widely investigated for applications to micro power generation systems. However, all the existing Swiss-roll combustors are designed for premixed combustion mode, in which the flame is prone to flash back or become extinguished at low flow rates. In the present work, a miniature Swiss-roll combustor was designed specifically for non-premixed combustion, which has two inlets for counterflows of methane and air, and two outlets for exhaust gas. Preliminary experimental investigation was conducted on a model combustor with a transparent quartz cover, which demonstrated that this new configuration brings about several merits. First, the flame structure and outer wall temperature distribution under most conditions are almost symmetric. Second, this combustor can run at a very low equivalence ratio (<0.3) and a very small fuel flow rate (˜0.029 L/min). As a result, the wall temperature level is relatively low, which makes this combustor a potential heat source for miniature thermoelectric devices using low-temperature materials such as Bi2Te3. Furthermore, the exhaust gas can be controlled at a very low temperature level (˜390–450 K), which means this micro combustor has a good heat recirculation performance.

Experimental investigations of effect of depth of Swiss roll combustor on its thermal performance as a heat generator

ABSTRACTHydrocarbon fuels have high energy density which is used to run small-scale mechanical systems. The present work emphasises on experimental investigations carried out on thermal characteristics of a small-size one turn Swiss roll combustor. Parameters investigated were namely equivalence ratio, flow velocity, combustors depth, flame stability, flame locations (flash back, stable and blowout), combustor surface temperatures, temperatures in reactants channel and temperatures in products channel. Flame stability was observed at the highest flow velocity of 32.75 m/s and at the maximum heat loss of 39.16 W. Values of volume flow rates of liquefied petroleum gas used during tests were 0.25 LPM, 0.3 LPM and 0.45 LPM. Combustor model with four depths was tested. Flame stability with higher heat loss and higher velocity was observed for the combustor of D21 and D5 models, respectively. The present combustor can be used as a heat source input to the thermoelectric device.Abbreviations:LPG: Liquefied petroleum gas; MLPG: Volume flow rate of LPG, LPM; MAIR: Volume flow rate of air, LPM

Experimental Research on the Effect of Materials of One Turn Swiss Roll Combustor on Its Thermal Performance as a Heat Generating Device

Looking at the present need of small scale power generation to run devices like small thrusters, cellular phones, laptops etc. attempts were made to develop small scale power generating devices. But issues increases with downsizing of the small scale power generators. Present work mainly focuses on the experimental investigations carried out on different materials (i.e. Aluminum, copper, stainless steel and brass) which were manufactured as one turn Swiss roll combustor. Studies were carried out on the Swiss roll combustor for fixed depth of 20mm. Reactants channel dimension and Products channel dimensions were 2mm and 3mm respectively. During the experiments conducted parameters varied were equivalence ratio and materials of the combustor. Parameters measured during the experiments were flame stability limits, external surface temperatures, combustion space/room temperatures. External surface heat loss to atmosphere was calculated for different values of equivalence ratio. Flame was observed stable successfully during all the experiments conducted. Heat transferred on all the external surfaces from the combustion space can be used as heat input to thermal electric devices. Higher heat content values were observed for SS material and lower values for Br material. Equivalence ratio equal to 1.068 was found to be the best at which maximum temperatures and heat content observed for all the materials. Maximum total heat content of 39.21W was observed for SS material. Lean limit temperatures were observed to be higher compared to rich limit temperatures.

Experimental investigations on effect of different materials and varying depths of one turn exhaust channel swiss roll combustor on its thermal performance

More energy density of hydrocarbon fuels compared to advanced batteries available in the market demands for development of systems which will use hydrocarbon fuels at small scale to generate power in small quantity (i.e. in few watts) and device efficiency should be reasonably good, but the basic requirement is to generate heat from the fuels like methane, propane, hydrogen, LPG and converting into power. Swiss roll combustor has proved to be best combustor at small scale. Present work is carried out on one turn exhaust channel and half turn of inlet mixture channel Swiss roll combustor. Purpose of keeping exhaust channel length more than the inlet mixture channel to ensure sufficient time for heat exchange between burned and unburned gases, which is not reported in earlier studies. Experimental study mentions effects of different design parameters like materials of combustor, various depths, equivalence ratio, mass flow rates of liquefied petroleum gas (LPG), volume of combustion space and environmental conditions (with insulation and without insulation to combustors) on fuel lean limit and fuel rich limit, temperature profile obtained on all external surfaces, in the main combustion chamber, in the channel carrying unburned gas mixture and burned gas mixture, heat loss to atmosphere from all the walls of combustor, flame location. Different combustor materials tested were stainless steel, Aluminum, copper, brass, bronze, Granite. Depths considered were 22mm, 15mm, 10mm and 5mm. It was observed that flame stability inside the combustion chamber is affected by materials, depths and flow rates. Unburned mixture carrying channel was kept below quenching distance of flame to avoid flash back. Burned gas carrying channel dimension was more than the quenching distance. Considerable temperature rise was observed with insulation to combustors. But combustors with more thermal conductivity showed more heat loss to atmosphere which led to instability of flame.

Numerical investigation on flame blow-off limit of a novel microscale Swiss-roll combustor with a bluff-body

A micro Swiss-roll combustor with a bluff-body was proposed for the first time. It is demonstrated that this improved design can greatly extend flame blow-off limit by reducing the flame stretch effect. The flame stabilization ability of this new combustor made of quartz, stainless steel (SS) and silicon carbide (SiC) was numerically investigated using a three dimensional, steady state, k-epsilon turbulence model. The results show that at an equivalence ratio of 0.5, the blow-off limits of quartz combustor, SS combustor and SiC combustor are 40 m/s, 50 m/s and 35 m/s, respectively. Comprehensive analyses were conducted under an inlet velocity of 20 m/s. It was revealled that the heat loss rates for quartz, SS and SiC combustors were 153 W, 108 W and 185 W, respectively. The SS combustor has the best heat-recirculation effect, followed by quartz combustor and SiC combustor. The lengths of recirculation zones are 4.35 mm, 3.4 mm and 3.37 mm for SiC combustor, quartz combustor and SS combustor, respectively. In summary, the heat loss rate is the chief factor responsible for the flame blow-off limit of this novel Swiss-roll combustor. It acts together with the heat-recirculation effect and flow-recirculation effect to determine the flame blow-off limit.

Effect of Catalyst Length and Hydrogen Addition on the Combustion Characteristics of n-Butane in a Catalytic Swiss-Roll Combustor

The effect of catalyst length and hydrogen addition on the combustion characteristics of n-butane in a catalytic Swiss-roll combustor was studied experimentally. The results show that the 4.5-mm ceramic with Pt catalyst slightly outperforms the 3.0- and 6.0-mm ceramics. However, changing only the catalyst length does not expand the lower flammable limits to lean fuel. Further experiments on the combustor with the 4.5-mm ceramic show that the addition of hydrogen to flammable mixtures can expand the lower flammable limits of such mixtures to lower than 0.5. Analyses on exhaust gas and stable combustion temperature indicate that hydrogen supplies heat through complete combustion and enhances the n-butane conversion ratio. A high amount of hydrogen is favorable for flammable range expansion. Moreover, a higher amount of hydrogen can further reduce the lower limits.

Catalytic Combustion of n-C4H10 and DME in Swiss-Roll Combustor with Porous Ceramics

An experimental study on combustion in Swiss-roll combustor with Pt-coated catalyst porous ceramics was conducted. For n-butane/air or dimethyl ether (DME)/air mixtures, both extinction limits and temperature distributions on an external surface of combustor were determined. The experimental results show that the lean limits of the fuels were actually in fuel rich conditions, and the flammable range of DME is larger than that of n-butane at low Reynolds numbers. The component analysis for the exhaust gas showed that only catalytic reactions occurred in the combustor. The surface temperature distributions indicate that a reaction zone is always in the center region where the catalyst is placed.

Experimental study on premixed CH 4/air mixture combustion in micro Swiss-roll combustors

Excess enthalpy combustion is a promising approach to stabilize flame in micro-combustors. Using a Swiss-roll combustor configuration, excess enthalpy combustion can be conveniently achieved. In this work, three types of Swiss-roll combustors with double spiral-shaped channels were designed and fabricated. The combustors were tested using methane/air mixtures of various equivalence ratios. Both temperature distributions and extinction limits were determined for each combustor configuration at different methane mass flow rates. Results indicate that the Swiss-roll combustors developed in the current study greatly enhance combustion stability in center regions of the combustors. At the same time, excess enthalpy combustors of the Swiss-roll configuration significantly extend the extinction limits of methane/air mixtures. In addition, the effects of combustor configurations and thermal insulation arrangements on temperature distributions and extinction limits were evaluated. With heat losses to the environment being significant, the use of thermal insulations further enhances the flame stability in center regions of the Swiss-roll combustors and extends flammable ranges.

Demonstration of an external combustion micro-heat engine

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

Flame stabilization and emission of small Swiss-roll combustors as heaters

The characteristics of small Swiss-roll combustors were investigated experimentally in detail. Three types of Swiss-roll combustors of different designs and two cases of heat transfer conditions were studied. The effects of design parameters on the performance of these combustors were examined. Each combustor consisted of a combustion region at the center (called the combustion room) and double spiral-shaped channels, the widths of which were smaller than the minimum quenching distance of a propane premixed flame at a normal state. Flames could be stabilized successfully for a wide range of equivalence ratios and mean velocities by using the recirculated heat from the burned gas, and blow-off was not observed. Temperature distributions of the combustors, variation of gas temperature, and the concentrations of the exhaust gas from the combustors were also investigated. Mean temperatures of the combustors were found to be governed by both the radiant heat loss from the combustors and the total chemical energy liberated by the combustors. Efficiencies of the combustors as heaters were evaluated. As a combustor became smaller, its thermal efficiency as a heater increased and its NO x emission decreased, while the emission of CO increased. By adding a catalytic reactor at the exhaust port, it was found that the emission of CO could be eliminated. This study provides new experimental results for a small Swiss-roll combustor, which represents an essential step toward the development of a microcombustor.