An exploration of stabilization mechanisms in a novel vortex-tube combustor with localized stratified peculiarity

Experimental and numerical (via ANSYS FLUENT) studies have been conducted on the combustion stability and stabilization mechanisms in a localized stratified vortex-tube combustor (LSVC) under lean conditions. The stability limit and flame configuration were obtained under different combustion conditions. Combined with the flow field distribution, the formation mechanisms of the local stratification of species and the resultant flame configuration were analyzed. Results show that the local stratification peculiarity is responsible for the dual flame appearance. On the basis of the local stratification of species, the local equivalence ratio is close to stoichiometry in the vicinity of the flame front, while it is above 1.0 in the interior, enabling the achievement of stable combustion at a global equivalence ratio as low as 0.12 in the LSVC. The flow field can help the transport of the reactive species and yields an intensified combustion and a large density gradient. The peak heat release rate (HRR) of 0.5 W/mm³ in the LSVC is much higher than that of 0.1 W/mm³ in the rapidly mixed vortex-tube combustor (RMVC) at the global equivalence ratio of 0.6 and the maximum tangential velocity of 26.44 m/s. The flame–vortex interaction theory provides a new perspective to interpret the rapid flame propagation in vortex-tube combustors. Based on the pressure jump theory, the flame speed was obtained via a specific formula closely related to the density gradient and the injection velocity. It turns out that the flame speed in the LSVC is remarkably higher than that in the RMVC at a certain same combustion condition. Moreover, the decrease of local flow velocity resulted from the strong swirl provides a favorable guarantee for the balance with the local flame speed.     

A numerical study of the combustion and jet characteristics of a hydrogen fueled turbulent hot-jet ignition (THJI) chamber

Turbulent hot-jet ignition (THJI) is an advanced ignition enhancement technology which can potentially overcome the problem associated with lean burn combustion. The present study makes an effort on the comprehensive understanding of a hydrogen fueled THJI chamber with various pre-chamber spark locations. Computational fluid dynamics (CFD) simulations are performed using an in-house code based on the KIVA-3V release 2 program coupled with an in-house chemical solver. A detailed chemical kinetics mechanism with 10 species and 19 reversible reactions is used for the H₂/air mixture in both the pre-chamber and the main chamber. The results show that moving the spark ignition location farther from the orifice significantly reduces the $0?10\text{\%}$ mass fraction burn period. By analyzing the local Mach number, turbulence kinetic energy and turbulence length scale, the compressibility and turbulence level of the jet flow are evaluated. Further analysis of the OH mass fraction distribution identifies three regions in the hot jet, i.e. extinction region, just-igniting region and combustion region. A critical Damköhler number of 0.3 is determined to separate the extinction region from the other regions. Meanwhile, transition Damköhler numbers ranging from 0.3 to 0.6 are determined in the just-igniting region.     

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 mechanisms of flame stabilization and low NOx emission in an eccentric jet pulverized coal combustor

The mechanisms of flame stabilization and low NO_x emission features of an eccentric jet pulverized coal combustor were studied through numerical modelling and experimental investigation. The results show that the formation of the unique flowfield structure is closely related to the interaction among combustor configuration, the primary jet and the control jet; and that certain rules should be followed in order to obtain the optimum condition for flame stabilization. The distributions of temperature and concentrations of NO, O₂, CO and CO₂ inside the combustor were experimentally measured. The effects of structural and operational parameters on combustion and NO formation were studied. It was found that reduction of primary air, suitable use of control jet and reasonable uptilt angle of the primary jet all contributed to the reduction of NO_x at the combustor exit. A new hypothesis, that reasonable separation of oxygen and fuel within the fuel-rich zone is beneficial to further reduction of NO_x emission, is given. The study showed that good compatibility existed between the capability of flame stabilization and low NO_x emission for this type of combustor. This project was supported by the National Natural Science Foundation of China     

Multi-factor impact mechanism on combustion efficiency of a hydrogen-fueled micro-cylindrical combustor

As it is important to achieve higher combustion efficiency for applications of micro-cylindrical combustor, the multi-factor impact mechanism on the combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor is investigated in this work. Firstly, six factors such as hydrogen/air equivalence ratio, inlet velocity, inlet temperature, wall thermal conductivity, wall emissivity and convective heat transfer coefficient of outer wall and five levels of each factor are determined. Orthogonal design table L₂₅(5⁶) is introduced to arrange cases. Secondly, grey relational analysis is adopted to investigate the effects of the six factors on combustion efficiency. Finally, the results of grey relational analysis are validated by analysis of variance. Based on grey relational analysis and analysis of variance, it is determined that the impact ranking from the largest to the smallest is hydrogen/air equivalence ratio, inlet velocity and inlet temperature, followed by the other three factors. The impact of wall thermal conductivity, convective heat transfer coefficient of outer wall and wall emissivity is considered to be equal due to their difference of impact on combustion efficiency is very small. This work provides us significant reference for optimizing combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor.     

Simulations of combustion oscillation and flame dynamics in a strut-based supersonic combustor

This paper numerically investigated the dynamic characteristics of combustion in a model scramjet. Three-dimensional compressible large eddy simulation was performed on a hydrogen fueled combustor and pressure fluctuations were recorded. The analysis of pressure data showed that the combustion processes are intrinsically unstable under supersonic air inflow conditions. Flame dynamics were convinced by the fluctuations in flame lift-off distance away from the strut base. Combined with the corresponding time interval, instantaneous flame speed was calculated. Results indicated that pressure oscillations at different locations show difference in amplitude, frequency, and the underlying control mechanism. Flame front oscillation analysis showed that the flame–shock interaction in the strut recirculation zone was responsible for the combustion instability. Flame dynamics were compared with low-speed turbulent lifted flames. The transition between flame propagation just after the strut and shock-induced combustion in the subsonic bubble at the intersection of two wall-reflected oblique shocks made for the flame stabilization.     

Effect of variation of inlet boundary conditions on the combustion flow-field of a typical double cavity scramjet combustor

The present research work deals with the numerical simulation of double cavity scramjet combustor by using two equation standard k-$\epsilon$ turbulence model and finite-rate/eddy-dissipation reaction models which is again coupled with Reynolds-Averaged Navier-Stokes (RANS) equations to investigate the influence of variation of inlet boundary condition of air as well as H₂ fuel on the combustion flow-field of scramjet engine subsequently. At the same time, the validations of the current computational approach have been completed against a standard experimental data which is available in the literature. An acceptable similarity is observed between present numerical approach with the experimentally attained schlieren photograph and the pressure distribution curve. In the present work, 8 different cases are studied. Among them, first four cases are investigated for the variation of inlet boundary condition of air and the remaining four cases are studied for the variation of inlet boundary condition of H₂ fuel. The obtained results show that the formation of high-pressure region around the cavities for case 3 and case 4 actually helps to push the greater amount of air to the cavity region where it is mixed with adequate amount of H₂ for proper and stable combustion whereas for case 6, it is observed that most of the combustion phenomena closely fitted into a small space of the combustor and mainly occurs near the cavity region.     

Design and performance of a pressurized cyclone combustor (PCC) for high and low heating value gas combustion

The combustion difficulties for low heating value (LHV) gases derived from biomass fuels via a gasification process have led to more investigations into LHV gas combustors. Cyclone combustors provide good air/fuel mixing with long residence times. In this study, a small-scale pressurized cyclone combustor (PCC) was designed and optimized using computational fluid dynamics (CFD) simulation. The PCC, along with a turbocharger-based, two-stage microturbine engine, was first characterized experimentally with liquefied petroleum gas (LPG) fuel and then with both LPG and LHV gas derived from biomass in dual-fuel mode. The combustor achieved ultra-low CO and NO_x emissions of about 5 and 7 ppm, respectively, for LPG fuel and of about 55 and 12 ppm, respectively, in dual-fuel mode at the maximum second-stage turbine speed of 26,000 rpm with stable turbine operation.     

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 study for the combustion characteristics of Orimulsion fuel in a small-scale combustor

A numerical and experimental study has been made in order to figure out the combustion characteristics of Orimulsion fuel for the possible substitution of traditional fuels in existing boilers. To this end, a comprehensive computer model has been developed using Patankar’s SIMPLE algorithm for the complex process associated with the turbulent reaction of this fuel. The program includes several phenomenological models such as behavior of fuel droplet, multi-phase reaction and formation of pollutant species such as NO and SO₂ in radiation-participating, turbulent flows.The calculation results of species and temperature profiles have been successfully compared with the experimental data measured in a small-scale combustor. Comparing the combustion characteristics of Orimulsion with that of heavy fuel oil, one of the noticeable features of Orimulsion combustion is relatively low flame temperature together with the delayed appearance of the peak flame. This is considered to be attributed to the effect of moisture content and the increased flow rate of Orimulsion fuel on the basis of an equivalent calorific value. Further this feature of Orimulsion flame observed in calculation and experiment are consistent with other experimental findings reported in open literature.It is found that the flame characteristics of Orimulsion fuel can be changed to some extent by the variation of operation conditions such as the injection velocity of Orimulsion fuel and the swirl strength of air stream. Further, other parametric study has been made in terms of the type of atomizing fluid, equivalence ratio, radiation model and grid size. In general the results are physically acceptable and consistent. Therefore the computer program developed is considered as a viable tool for the advanced design and determination of optimal operating condition for the large scale of Orimulsion combustor.     

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.     

Modelling of combustion performances and emission characteristics of coal gases in a model gas turbine combustor

In recent years, gas mixtures are being used as alternative fuels in combustors. These gas mixtures are obtained by different methods. For instance, coal gasification and carbonization as coal have the largest reserves among fossil fuels. Gas mixtures obtained via coal gasification and carbonization are called water gas, generator gas, town gas and coke oven gas. These fuels contain various gases. As a result of this, heating values of fuels are also different. Therefore, combustion performances and emission characteristics of these fuels need to be investigated. In this study, combustion performances and emissions including CO, CO₂ and NO_X of water gas, generator gas, town gases, coke oven gas and methane were numerically investigated in a model gas turbine combustor. The numerical modelling of turbulent nonpremixed diffusion flames has been performed in this combustor. Mathematical models used in this study involved the k–ε model of turbulent flow, the PDF/mixture fraction model of nonpremixed combustion and P‐1 radiation model. A CFD code ANSYS Fluent was used for all numerical investigations. Temperature distributions of axial and radial directions were determined. A NO_X post‐processor was used for the prediction of NO_X emissions from the gas turbine combustor. Modelling was performed for 60 kW thermal power and different equivalance ratios (i.e. Ф = 0.91, Ф = 0.77 and Ф = 0.67). The studied type 1 model gas turbine combustor was modelled for Ф = 0.91 equivalance ratio. Then, Other equivalance ratios were analysed for type 2 model gas turbine combustor. The effect of dilution air on combustion performances and emission characteristics was also investigated. It is concluded that the coke oven gas, the town gas I, town gas II and the water gas are appropriate for usage as alternative fuel, whereas the generator gas is not suitable for gas turbine combustors. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance evaluation and outlet load improvement of a rotating detonation combustor with different outlet nozzles

Outlet nozzles for a rotating detonation combustor were designed to meet a downstream turbine and reduce the high pressure and heat load caused by the oblique shock wave at the outlet. The effects of the rotating detonation combustor with two types of outlet nozzles were studied, and the performance and outlet parameters of the combustor were measured at an elevated chamber pressure and preheating temperature based on gas turbine conditions. The results showed that the outlet nozzles could cause changes in the wave collisions and folding of the weak flame front in the detonation formation process, but the basic propagation process was similar to that without a nozzle. The pressure ratio changed from 1.427 in the original model to 1.392 and 1.304 with the two types of outlet nozzles. Meanwhile, the outlet load was greatly improved. The peak values of the static temperature at the outlet dropped by 22.423% and 27.572% with the two types of outlet nozzles compared to the original model. In addition, the peak static pressures dropped by 75.737% and 83.722%, respectively. In addition, the outlet nozzles significantly reduced the unevenness of the outlet static temperature and static pressure distributions. This created a better outlet operating environment, thereby improving the performance of the rotating detonation combustor.     

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.     

Numerical investigations of mixed supersonic and subsonic combustion modes in a model combustor

Flame dynamics and statistics of mixed supersonic and subsonic combustion modes under different air inflow and global equivalence ratio conditions in a hydrogen-fueled model combustor are numerically studied. Three methods including spanwise-averaged Mach number, spanwise-averaged Mach number conditioning on the local heat release, and fraction of heat release are proposed to identify supersonic and subsonic combustion modes. The probability distributions of supersonic and subsonic combustion modes are also analyzed based on the statistics on multiple instantaneous snapshots of the numerical results. The critical global equivalence ratio for thermal choking in a range of supersonic inflow conditions is derived theoretically based on a one-dimensional duct flow with heat addition. Furthermore, it is found that the flame lift-off distance in both supersonic and subsonic flows decreases with increased air inflow velocity, but increases with global equivalence ratio. The fraction of supersonic heat release and its oscillation increase with increased air inflow velocity.     

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.     

Computational realization of turbulent combustion in a scramjet combustor stabilized by a lobed strut

Currently, there are theoretical and technical challenges to organize stable flame in scramjet, especially for variable flight conditions. This paper applies the numerical simulation to investigate and analyze the combustion characteristics in a lobed strut-stabilized scramjet combustor under different conditions of Mach number (Ma). By comparing with the experiment date of hydrogen mole fraction, the reliability of large eddy simulation solver used in this paper is verified. On this basis, the simulations of mixing and combustion cases at Ma 1.5–3.0 are carried out separately. The numerical results show the streamwise vortices induced by the lobed strut enhance the mixing and are conducive to the subsequent stable combustion. The turbulent flame behaves lifted characteristics, and the auto-ignition provides the initial active radicals for its stabilization. There are two chemical reaction modes in the flow field, namely auto-ignition, and flame. Further comparison of the cases of different Mach numbers indicates that the lifted height of the flame depends on the Mach number. Analysis of probability density function (PDF) reveals that Mach number influent the competitive relationship between the auto-ignition mode and the flame mode. The auto-ignition and the flame coexist downstream in high ma cases, while the flame mode is the dominant in low Ma cases.     

Design and numerical analysis of a 3 kWe flameless microturbine combustor for hydrogen fuel

In this work, a new 3 kWe flameless combustor for hydrogen fuel is designed and analyzed using CFD simulation. The strategy of the design is to provide a large volumetric combustion for hydrogen fuel without significant rise of the temperature. The combustor initial dimensions and specification were obtained from practical design procedures, and then optimized using CFD simulations. A three-dimensional model for the designed combustor is constructed to further analysis of flameless hydrogen combustion and consideration that leads to disappearance of flame-front and flameless combustion. The key design parameters including aerodynamic, temperature at walls and flame, NO_X, pressure drop, combustion efficiency for the hydrogen flame is analyzed in the designed combustor. To well demonstrate the combustor, the NO_X and entropy destruction and finally energy conversion efficiency, and overall operability in the microturbine cycle of hydrogen flameless combustor is compared with a 3 kWe design counterpart for natural gas. The findings demonstrate that hydrogen flameless combustion is superior to derive the microturbines with significantly lower NO_X, and improvements in energy efficiency, and cycle overall efficiency with low wall temperatures guaranteeing the long-term operation of combustor and microturbine parts.