Influence of multi-structure optimization on the comprehensive performance of micro-cylindrical combustor inserting with spiral fin by using grey relational analysis and analysis of variance

Targeted at improving the energy output of thermophotovoltaic system, a novel micro combustor with spiral fin is proposed. The multi-structure factors of spiral fin concluding spiral fin length, spiral fin pitch, spiral fin number and spiral fin opening size impact the thermal and energy performance of micro combustor are investigated in this study. The method of Taguchi experimental design is introduced to determine the testing cases, Orthogonal design table ${\text{L}}_{16}\left(4^4\right)$. Then, grey relational grade is adopted to obtain the influence of four factors on performance parameters. The evaluation indexes of each testing case include mean outer wall temperature, outer wall temperature uniformity and pressure drop, which are closely connected with the thermal and energy performance of micro combustors. Moreover, the results calculated by grey relational grade are verified by analysis of variance. Results show that the optimal combination of the structure is micro combustor inserting with spiral fin, in which the spiral fin is 16 mm, the spiral fin pitch is 1.57 mm, the spiral fin number is 8 and spiral fin opening size is 0.2 mm. Meanwhile, the spiral fin length has the greatest impact on the comprehensive performance of the micro combustor, with a contribution of 48.522%.     

Effects of rectangular rib on exergy efficiency of a hydrogen-fueled micro combustor

In order to further optimize the working performance of micro-cylindrical combustor, the combustion chamber of the micro-cylindrical combustor is inserted in a rectangular rib. Extensive numerical investigations are conducted to compare the exergy efficiency of non-ribbed and rectangular-ribbed micro combustors under various hydrogen mass flow rates and hydrogen/air equivalence ratios. Moreover, the effects of dimensionless rib positions and heights on the exergy efficiency of micro-cylindrical combustor are also widely investigated. Results suggest that the exergy efficiency of the rectangular-ribbed micro combustor is significantly higher than that of the non-ribbed micro combustor under different inlet conditions. Moreover, the exergy efficiency of the rectangular-ribbed micro combustor is significantly affected by the dimensionless positions l. The optimum dimensionless l is increased with the increase of hydrogen mass flow rate. This work offers us significant reference for optimizing the micro combustor in energy utilization.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

Influence of hole size and number on pressure drop and energy output of the micro-cylindrical combustor inserting with an internal spiral fin with holes

Targeting at optimizing the energy output and thermal performance of the micro combustor in the application of micro-thermophotovoltaic (MTPV) systems, but the introduction of spiral fins brings higher pressure loss. Thus, a novel design of the micro combustor with the spiral fin opening is developed. The influence of inlet velocities, the hole size and hole number of the spiral fin on the pressure drop and thermal characteristic and energy characteristic are numerically investigated. It's illustrated that the spiral fin opening is conductive to decrease the pressure loss and optimize the outer wall temperature distribution, but has a negative influence on increasing the mean outer wall temperature of the micro combustor and energy output in the MTPV systems. With the increase of the hole size and hole number of the spiral fin, the pressure loss decreases and the outer wall temperature uniformity increases significantly, while the mean outer wall temperature drops and total energy output decreases. The better performance obtains when the micro combustor with spiral fin with four 0.5 mm holes.     

Impact of parametric variation on combustion characteristics of hydrogen-fueled strut based scramjet combustor at supersonic speed

Supersonic combustion ramjet engine is more fascinating among all the air-breathing engines. Due to its higher thrust to weight ratio, researchers are more interested to get the superior combustion performance at the optimum boundary conditions. The flow field characteristics and combustion performance have been analysed with the help of Ansys 14.0 software. Generic scramjet combustor of German Aerospace Center (DLR) has been taken into consideration for comparison purpose and off design analysis has been conducted to investigate and analyse the changes. Two dimensional compressible Reynolds Averaged Navier-Stokes (RANS) turbulence model has been opted with the finite-rate/eddy-dissipation reaction model. K-ε two equation turbulence model has been selected to reach up to reasonable accuracy. Validation of the present work has been done with the help of both non-reacting and reacting type data from open literature. To choose the appropriate meshing of the computational model three different types of mesh elements, that is, coarse, medium and fine has been analysed and also grid independence analysis is performed. The present article objective is to get optimum boundary condition by changing the incoming air temperature and pressure at constant Mach number to connect the bridge between incoming air temperature and pressure to the change in velocity throughout the combustion chamber. The detailed understanding and explanation have been done by varying the temperature range of incoming air because of its major impact on combustion performance. Nonetheless, a small variation of air pressure will also discuss to observe the parameters which majorly influence while doing performance analysis. At the end the Optimum boundary condition for the present computation work is observed to be at 833 K temperature with 115 299 Pa pressure.     

Investigation of flow characteristics in supersonic combustion ramjet combustor toward improvement of combustion efficiency

Supersonic combustion ramjet (scramjet) is a variant of ramjet in which the combustion takes place at supersonic velocity. The flow physics inside the scramjet combustor is quite complex due to the fact that the mixing and completion of the combustion take place in a short time, which is of the order of milliseconds. This study focuses on flow characteristics within the combustion chamber of the scramjet engine that is designed to improve energy efficiency by enhancing combustion efficiency. The effect on combustion performance and thereby the energy efficiency on using strut‐based flame stabilizer is evaluated at different positions. Reynolds averaged Navier‐Stokes equations are solved with the Shear Stress Transport k‐ω turbulence model. Single strut configuration is used to validate with the experimental data. Single strut is then compared with three‐strut configuration. In the three‐strut configuration, the location of the primary strut is kept constant, and the secondary struts are relocated in x and y directions. Combustion performance was evaluated for the cases of flow from primary strut only and through three struts. It was found that the placement of secondary strut in a three‐strut configuration plays a vital role in improving energy efficiency. A maximum of 33.86% improvement in combustion efficiency was observed in comparison to the single strut combustor. A reduction in unburned fuel was observed, making the system more energy efficient. If the struts are not placed optimally, the combustion performance of the combustor was observed to be lower than that of a single‐strut configuration. The shock reflection and expansion fans within the primary combustion zone and the secondary strut region enhance the combustion efficiency. The wall static pressure was observed to increase with the addition of secondary struts. For certain strut configurations, flow separation was seen on the combustor walls. If the secondary strut was placed close to the primary strut, combustion efficiency was found to enhance. It was seen that combustion efficiency was also enhanced for the cases of reacting flow from primary strut only. It could also help to increase fuel efficiency, as additional fuel is not supplied to the secondary strut, making the overall system energy efficient. As the secondary strut is introduced, total pressure loss also increases. It could also be noted that if the combustor length was increased, there could be further increased in combustion efficiency.     

Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.     

Investigation on hydrogen-fueled combustion characteristics and thermal performance in a micro heat-recirculation combustor inserted with block

Numerical investigation on the premixed H₂/air combustion in a micro heat-recirculation combustor inserted with/without block is conducted. Effects of block setting, heat-recirculation, and flow rate on combustion characteristics and thermal performance are depicted and analyzed. The results demonstrate that the block enhances the flame stability and preheating effect, which also reduces the heat loss via exhaust gas, while it shortens reactants residence time. The combustor setting with a transverse block gains a better thermal performance than that inserted with a longitudinal block. With the increase of transverse block height, the high-temperature zone is broadened and radiation is improved. However, the block with a height of 10 mm separates the fluid field and weakens the effects of heat recirculation, leading to a lower outer wall temperature. Furthermore, the appropriate block insertion method and height contribute to the significant improvement of heat transfer, radiant efficiency and further optimization of micro power generator.     

Effects of operating conditions and fuel properties on emission performance and combustion efficiency of a swirling fluidized-bed combustor fired with a biomass fuel

This work reports an experimental study on firing 80 kg/h rice husk in a swirling fluidized-bed combustor (SFBC) using an annular air distributor as the swirl generator. Two NO_x emission control techniques were investigated in this work: (1) air staging of the combustion process, and (2) firing rice husk as moisturized fuel. In the first test series for the air-staged combustion, CO, NO and C_xH_y emissions and combustion efficiency were determined for burning “as-received” rice husk at fixed excess air of 40%, while secondary-to-primary air ratio (SA/PA) was ranged from 0.26 to 0.75. The effects of SA/PA on CO and NO emissions from the combustor were found to be quite weak, whereas C_xH_y emissions exhibited an apparent influence of air staging. In the second test series, rice husks with the fuel-moisture content of 8.4% to 35% were fired at excess air varied from 20% to 80%, while the flow rate of secondary air was fixed. Radial and axial temperature and gas concentration (O₂, CO, NO) profiles in the reactor, as well as CO and NO emissions, are discussed for the selected operating conditions. The temperature and gas concentration profiles for variable fuel quality exhibited significant effects of both fuel-moisture and excess air. As revealed by experimental results, the emission of NO from this SFBC can be substantially reduced through moisturizing rice husk, while CO is effectively mitigated by injection of secondary air into the bed splash zone, resulting in a rather low emission of CO and high (over 99%) combustion efficiency of the combustor for the ranges of operating conditions and fuel properties.     

The impact of equivalence ratio oscillations on combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane-air flames are investigated in an atmospheric pressure, atmospheric inlet temperature, lean, premixed backward-facing step combustor. We modify the location of the fuel injector to examine the impact of equivalence ratio oscillations arriving at the flame on the combustion dynamics. Simultaneous pressure, velocity, heat-release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When the fuel is injected far upstream from the step, the equivalence ratio arriving at the flame is steady and the combustion dynamics are controlled only by flame-vortex interactions. In this case, different dynamic regimes are observed depending on the operating parameters. When the fuel is injected close to the step, the equivalence ratio arriving at the flame exhibits oscillations. In the presence of equivalence ratio oscillations, the measured sound pressure level is significant across the entire range of lean mean equivalence ratios even if the equivalence ratio oscillations arriving at the flame are out-of-phase with the pressure oscillations. The combustion dynamics are governed primarily by the flame-vortex interactions, while the equivalence ratio oscillations have secondary effects. The equivalence ratio oscillations could generate variations in the combustion dynamics in each cycle under some operating conditions, destabilize the flame at the entire range of the lean equivalence ratios, and increase the value of the mean equivalence ratio at the lean blowout limit.     

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO₂, owing to the reduced amount of carbonaceous fuel.     

Numerical investigation on implication of dual cavity on combustion characteristics in strut based scramjet combustor

The present study investigates the implication of a dual cavity on combustion and mixing characteristics of strut based combustor numerically. A scramjet combustor with and without cavity are considered to evaluate the influence of cavity on combustion characteristics. Moreover, the effect of spacing between the cavities on combustion performance is also discussed. A separate study is carried out to understand the interplay of the Mach number and the spacing between the cavities on combustion characteristics. Our study reveals that the introduction of a dual cavity in scramjet combustor enhances the mixing and improves combustion performance. Further, it is found that the mixing length is lesser and the penetration height is more when a dual cavity is employed in a combustor. It can also be observed that the size of the recirculation region is found to be larger for the combustor with dual cavity among all configurations at the spacing S₂ = 12.7 mm. Further, the combustion efficiency and fuel penetration height are found to be maximum at Mach 2.5 with a spacing S₂ = 12.7 mm.     

Effect of multiple bluff bodies on hydrogen/air combustion characteristics and thermal properties in micro combustor

The bluff body is commonly used to improve micro combustion. The micro combustor with multiple rectangular bluff bodies in a single row was proposed. The effects of bluff bodies on H₂/air combustion characteristics were numerically studied. The temperature distributions, ignition position, combustion efficiency and blow-out limit were investigated via changing the total width and number of bluff bodies. The results show that the combined use of multiple bluff bodies can further expand the blow-out limit of H₂/Air. The effect of high temperature and viscous force on the flow velocity is main factors for the flame morphology. When the total width of bluff bodies is 2 mm, the blow-out limit decreases with the increase of bluff body number. When the total width of bluff bodies is 4 mm and 6 mm, the blow-out limit increases with the increase of the number of bluff bodies. With the increase of inlet velocity, the complete combustion efficiency decreases. The combustion efficiency in the combustor with wider blow-out limit decreases more slowly. It indicates that the combustor with multi-bluff bodies is more suitable for the operation conditions with high flow velocity.     

Effect of convex platform structure on hydrogen and oxygen combustion characteristics in micro combustor

The combustion characteristics of the micro combustor with a convex platform were simulated and the effects of the height of the convex platform and the inlet velocity on the combustion process were analyzed. The results show that the setting of convex platform can significantly increase the maximum velocity and reduce the outlet velocity. When the height of the boss continues to increase, the maximum velocity is more significant, but has little effect on the outlet velocity. At the same time, the increasing height of the convex platform increases, the turbulent kinetic energy and reduces the intensity of combustion on the axis. However further increase in the height does not reduce the effect significantly. The fuel conversion rate increases significantly, but the velocity decreases. In the micro combustor with a convex platform, increasing the inlet velocity increases the axial temperature, the fuel conversion rate decreases.     

The effects and characteristics of hydrogen in SNG on gas turbine combustion using a diffusion type combustor

Converting coal to natural gas may be one of the alternative solutions for satisfying the demand for natural gas. However, synthetic natural gas (SNG) has not been proven effective in natural gas-fired power plants. In this research, several combustion tests using a diffusion type combustor were conducted to determine the effect of hydrogen content in SNG on gas turbine combustion. Three kinds of SNG with different H₂ content up to 3%vol were used for the combustion tests. Even a small amount of hydrogen in SNG affects the flame structure: it shortened the flame length and enlarged the flame angle slightly. However, hydrogen content up to 3% in SNG did not affect the gas turbine combustion characteristics, which are emission performance and combustion efficiency. Due to a similarity with real gas turbine combustor conditions for power generation, a high pressure combustion test helped us verify the ambient pressure combustion tests conducted to determine the effect of hydrogen in SNG. In the high pressure combustion test, the pattern factors were identical even though the hydrogen content was varied from 0% to 3%.     

Designing parameter optimization of a Parallel-Plain Fin heat sink using the grey-based fuzzy algorithm with the orthogonal arrays

In this paper, a grey-based fuzzy algorithm with the orthogonal arrays is employed to find the optimal designing parameters' setting for a heat sink with the Parallel-Plain Fin (PPF) on the multiple thermal performance characteristics. The proposed algorithm, coupling the grey relational analysis with the fuzzy logic, obtains a grey–fuzzy reasoning grade to evaluate the multiple performance characteristics according to the grey relational coefficient of each performance characteristic. In the present study, the designing parameters of the heat sink include the height of fin, the width of gap between fins, the width of slot, the number of slot and the air speed. The design of experiment (DOE) adopts the L₁₆(4⁵) orthogonal arrays table which is four levels and five factors type of factorial design. The average convective heat transfer coefficient, the thermal resistance and the pressure drop are considered as the multiple thermal performance characteristics and explored in the experiment. In addition, the response table, response graph and the analysis of variance (ANOVA) are used to find the optimal settings and the influence of designing parameters on the multiple performance characteristics. The results of confirmation tests with the optimal settings of designing parameters have obviously shown that the multiple thermal performance characteristics are effectively improved through these procedures.     

Numerical investigation of a novel micro combustor with a central and bilateral slotted blunt body

To improve the combustion stability at micro scale, the micro combustor with a central and bilateral slotted blunt body (MCSB) exhibiting significant combustion performance improvement is designed. The new one can not only effectively prevent the flame tip opening, but also exhibit higher combustion efficiency and blown-off limit. Its blown-off limit can reach 756 cm³/s at the flow rate ratio of 0.2 and the bluff body angle of 90°, which is 61.5% higher than that of the conventional one with the blown-off limit of 468 cm³/s. The combustion efficiency improves with the growth of the flow rate ratio, while the blown-off limit of MCSB increases first and then decreases. The blown-off limit of MCSB with the bluff body angle of 90° reaches to the peak value of 792 cm³/s at the flow rate ratio of 0.15. Moreover, the increase of the bluff body angle provides better combustion efficiency and stability.     

Effects of different channels on performance enhancement of a nozzle hydrogen-fueled micro combustor for micro-thermophotovoltaic applications

Under the background of the international energy crisis, it is urgent to develop a micro-thermophotovoltaic system using hydrogen as combustion energy. In order to optimize the micro-combustor in the system, the nozzle micro-combustors with five different channels are designed. All nozzle micro-combustors are numerically studied by using the mechanism of 9 components and 19 elementary reactions within the ANSYS Fluent 20.0, and their advantages and disadvantages in thermal performance, flame performance and chemical reaction are compared. It is concluded that the nozzle micro-combustor with constriction-expansion channel has the best performance among the five micro-combustors because of its reasonable segmented structure. Then, a new type of nozzle micro-combustor with segmented channel is designed, and the numerical study of segmented micro-burners and non-segmented micro-combustors with different inlet velocities and hydrogen/air equivalence ratios shows that the thermal performance of segmented micro-combustors is much better than that of non-segmented micro-burners. Therefore, compared with non-segmented nozzle micro-combustors, segmented nozzle micro-combustors have better application potential in micro-thermophotovoltaic applications.     

Experimental study on combustion characteristics of compost using New‐type combustor

The aim of the present study is to develop the biomass furnace combustor, which can effectively use the compost as a fuel. Here, the compost that is made from pig's waste and has the calorific value of 2000 kcal/kg is employed here. Emphasis is placed on the optimum conditions of fuel and air flow rates and moisture content of the compost and the corresponding combustion gas components and combustion gas temperature in the combustor. It is found from the study that (i) except 40% of the compost's moisture content, the self‐combustion of compost as the fuel takes place, (ii) the combustion gas concentrations are affected by gas temperature, and (iii) the optimum value of the air‐to‐fuel ratio is obtained based on the gas temperature and the concentration of combustion gases. Copyright © 2016 John Wiley & Sons, Ltd.