Cross-sectional Combustor Research Articles

Effects of inner cone length on continuous rotating detonation in a variable cross-section combustor

A variable cross-section combustion chamber with different inner cone length L is designed, composing of an annular part with divergent width and a cylinder part without inner cone. Ethylene-air and hydrogen-air continuous rotating detonation (CRD) experimental tests are conducted. The results show that the ethylene-air CRD wave can be achieved in the variable cross-section combustor with the short inner cone. The propagation velocity of ethylene-air CRD wave can be 1661.37 m/s with the equivalence ratio of 0.91 and L of 10 mm. With L increasing from 0 mm to 30 mm, the combustion mode varies in the single wave mode, the sawtooth wave mode, and the hybrid mode. The combustor width is not the main factor leading to the failure of detonation in this study. The acceleration of propellent mixture caused by the expansion of the annulus channel, and the downstream movement and reduction of recirculation zone around the inner cone both increase the difficulty in CRD realization. While the hydrogen-air CRD can self-sustain with the intensive reaction zone in the annular part in the critical configuration of ethylene-air CRD. This study will contribute to the understandings of self-sustaining mechanism of CRD in the divergent combustor and the combustor design of the hollow chamber.

Upgraded design methodology for airframe/engine integrated full-waverider vehicle considering thrust chamber design

An upgraded airframe/engine integrated design approach for a full-waverider vehicle considering thrust chamber design is proposed. Compared with the existing design approach of the full-waverider vehicle, the isolator, the combustor, and the nozzle are designed complementally. First, the airframe and inlet are generated using the full-waverider theory, the isolator with the centroid offset variable cross-section is designed using the geometric morphing method, and the maximum thrust nozzle is obtained by the method of characteristics. Coupled with a circular cross-section combustor and air rudders, a complete full-waverider vehicle including the thrust chamber is generated. Second, the correctness and the availability of the design method is verified through numerical simulations under the design condition. Finally, the aerodynamic performance of the full-waverider vehicle is analyzed under a wide-speed range in the moment equilibrium state. The optimal lift-to-drag ratio of the entire vehicle is 4.03 appearing at 4° angle of attack. Owing to the rudder deflection, the optimal lift to drag ratio reduces from 4.03 to 2.44, and the corresponding angle of attack changes as well. Moreover, the inlet unstart phenomenon occurs with the decrease of the Mach number and with the increase of the angle of attack.

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.

Study on combustion characteristics of premixed methane/oxygen in meso-combustors with different cross-sections

The combustion characteristics of premixed methane/oxygen in three kinds of meso-combustors with the same hydraulic diameter were compared. The upper flammability limits of the premixed gas in the combustors with different cross-sections were measured experimentally. And the temperature field, flow field and OH concentration distribution in the meso-combustors were analyzed by numerical simulation. In addition, the effects of equivalence ratio and inlet velocity on OH mass fraction and wall temperature of combustors were also investigated. The results showed that the upper flammability limits of premixed gases decreased in turn when the methane flow rate was same for the combustion process of circular, square and rectangular cross-sections combustors. Under the same inlet parameters, the intensity of gas-phase reaction in the circular cross-section combustor was greater than that in the combustor with square cross-section, and that in the combustor with rectangular cross-section was the least. When the equivalence ratio or inlet velocity was constant, the mean temperature of the “square” cross-section combustor was higher, but the temperature difference in the combustor with circular cross-section was smaller and the temperature was more uniform.

Continuous spin detonation of methane/hydrogen-air mixtures with additional injection of air to combustion products

Results of experimental investigations in a flow-type annular cylindrical combustor with an outer diameter dc = 503 mm are reported. The influence of air addition to products of continuous spin detonation of a CH4 + 8H2 - air mixture on detonation wave parameters, pressures in the combustor, and specific impulse is studied. The range of existence of continuous spin detonation in terms of the specific flow rate of the mixture through the combustor cross section is extended to 1360 kg/(s·m2), whereas the equivalence ratio is reduced to φ∑ = 0.4. It is shown that addition of air to detonation products up to the level of the air flow rate in the main injection system increases the velocity of continuous spin detonation, pressure in the combustor, and thrust, whereas the specific flow rate of the fuel decreases.

Flame stability and behavior inside meso-scale combustor with different flame holder

Flame stability and behavior inside meso-scale combustor with different flame holder was investigated experimentally. Three types of flame holder i.e. wire mesh, flat plate with circular holes and flat plate with narrow slits, were used to improve the flame stability inside the meso-scale combustor. Combustor with flat plate - narrow slits flame holder has the best flame stability, i.e. stable flame established inside the combustor at the highest reactant velocity compared to the other combustor with the different flame holder. Furthermore, combustor with wire mesh and flat plate with narrow slits have a relatively uniform flame colour at the combustor cross section, compared to flame visualization inside meso-scale combustor with flame holder of flat plate with circular holes. This phenomena is related to non-uniform reactant distribution on the combustor cross section.

Large-scale hydrogen–air continuous detonation combustor

A large-scale continuous detonation combustor (CDC) has been designed, fabricated and tested to study the effect of different design elements on the operation process and CDC propulsion performance. It has been shown experimentally that widening of the air-inlet slit in the annular combustion chamber from 2 to 15 mm leads to a decrease in the number of detonation waves (DWs) simultaneously circulating in the combustor from four to one and, finally, to transition to the operation mode with intermittent (pulse) longitudinal reaction waves resembling pulse detonations. The number of DWs and the thrust produced by the CDC can be increased by installing a shaped obstacle at the CDC exit nozzle providing the blockage of the combustor cross section. The maximum net thrust produced by the CDC attained 6 kN at the total mass flow rate of fuel components of 7.5 kg/s, whereas the maximum fuel-based specific impulse attained ∼3000 s.

Incipient thermal choking and stable shock-train formation in the heat-release region of a scramjet combustor. Part I: Shock-tunnel experiments

A series of experiments is performed in the High Enthalpy Shock Tunnel Göttingen to investigate the response of the HyShot II scramjet combustor to equivalence ratios close to the critical value at which the onset of thermal choking occurs. This critical equivalence ratio is first identified (for simulated Mach-8, 27-km altitude conditions) as 0.38–0.39. Subsequent experiments cover the range between this value and 0.50. As the HyShot II combustor has a constant-area cross section, the expected behavior in this equivalence-ratio range is the formation and upstream propagation of an unsteady shock train that would eventually lead to inlet unstart. Instead, however, the shock train is observed to become lodged in the heat-release zone of the combustion chamber, with a quasi-stable position that shifts upstream with increasing equivalence ratio; this behavior is distinct from the steady isolator shock trains seen in traditional dual-mode scramjet configurations. The shock-train development in the present experiments is characterized through fast-response surface pressure and heat-flux measurements, as well as simultaneous high-speed schlieren and OH∗ chemiluminescence visualization. Possible explanations for this unexpected behavior are proposed in terms of the increased heat-transfer to the combustor walls and a possible increased uniformity of heat release across the combustor cross-section caused by the presence of the shock train.

The Numerical Simulation and Experimental Research of Combustor in Micro Thermophotovoltaic Systems

With the establishment of the appropriate porous media model of the combustor, temperature contour map on combustor cross section were simulated under the condition of different flow rate and different porosity in the Fluent software, and experimented to verify the simulation. The results show that: Flame core position moves toward the export with the increase of flow rate, but when the flow increases to a certain amount, the outlet temperature rises significantly. temperature distribution is the best when flow rate is 120 mL/min; With the decrease of the porosity, the flame core position moves to the entrance. Wall average temperature of the combustor is the highest when porosity is 0.4.

Large Eddy Simulation-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions

The influence of the thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large eddy simulations are carried out using the dynamically thickened flame combustion model. Then system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with a 90 mm × 90 mm combustor cross section and nonadiabatic walls: (1) a combustor cross section similar to the reference case with adiabatic combustor walls, and (2) a different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in the flame shape and flow field. In particular, the FTF computed with an adiabatic wall boundary condition which produces a flame with a significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in the flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in the FTF on stability limits is analyzed with a low-order thermoacoustic model.

Optical and thrust measurement of a pulse detonation combustor with a coaxial rotary valve

We developed a rotary valve for a pulse detonation engine (PDE), and confirmed its basic characteristics and performance. In a square cross-section combustor, we visualized a multi-shot of a pulse detonation rocket engine (PDRE) cycle at an operation frequency of 160 Hz by using a high-speed camera (time resolution: 3.33 μs, space resolution: 0.4 mm) and a Schlieren method. The propellant filling process and the purge process were confirmed, and each process was modeled. Moreover, we confirmed the processes of detonation wave generation and burned gas blowdown. In addition, we investigated the impact of shortening the passage width of a combustor and negative-time ignition (ignition time is earlier than the end-time of the propellant filling process) on the deflagration-to-detonation transition (DDT) distance and time. The DDT distance did not depend on the passage width of a combustor and decreased under the negative-time ignition condition. With a passage width of 20 mm, the DDT distance decreased by 22% under the negative-time ignition condition to a minimum value (76 ± 8 mm). The DDT time from spark time reached a minimum value (69 ± 14 μs) under the condition of a passage width of 10 mm and negative-time ignition. The detonation initiation time and the DDT distance were represented by the time until the flame expanded toward the tube-axis one-dimensionally from ignition (characteristic time). We also carried out thrust measurement using a PDRE system composed of a circular cross-section combustor and the newly developed valve. We obtained a stable time-averaged thrust in a wide range of operation frequency (40–160 Hz) and confirmed the increase of specific impulse due to a partial-fill effect. At a maximum operation frequency of 159 Hz, we achieved a maximum propellant-based specific impulse of 232 s and a maximum time-averaged thrust of 71 N.

Fluidized bed combustion of pelletized biomass and waste-derived fuels

The fluidized bed combustion of three pelletized biogenic fuels (sewage sludge, wood, and straw) has been investigated with a combination of experimental techniques. The fuels have been characterized from the standpoints of patterns and rates of fuel devolatilization and char burnout, extent of attrition and fragmentation, and their relevance to the fuel particle size distribution and the amount and size distribution of primary ash particles. Results highlight differences and similarities among the three fuels tested. The fuels were all characterized by limited primary fragmentation and relatively long devolatilization times, as compared with the time scale of particle dispersion away from the fuel feeding ports in practical FBC. Both features are favorable to effective lateral distribution of volatile matter across the combustor cross section. The three fuels exhibited distinctively different char conversion patterns. The high-ash pelletized sludge burned according to the shrinking core conversion pattern with negligible occurrence of secondary fragmentation. The low-ash pelletized wood burned according to the shrinking particle conversion pattern with extensive occurrence of secondary fragmentation. The medium-ash pelletized straw yielded char particles with a hollow structure, resembling big cenospheres, characterized by a coherent inorganic outer layer strong enough to prevent particle fragmentation. Inert bed particles were permanently attached to the hollow pellets as they were incorporated into ash melts. Carbon elutriation rates were very small for all the fuels tested. For pelletized sludge and straw, this was mostly due to the shielding effect of the coherent ash skeleton. For the wood pellet, carbon attrition was extensive, but was largely counterbalanced by effective afterburning due to the large intrinsic reactivity of attrited char fines. The impact of carbon attrition on combustion efficiency was negligible for all the fuels tested. The size distribution of primary ash particles liberated upon complete carbon burnoff largely reflected the combustion pattern of each fuel. Primary ash particles of size nearly equal to that of the parent fuel were generated upon complete burnoff of the pelletized sludge. Nonetheless, secondary attrition of primary ash from pelletized sludge is large, to the point where generation of fine ash would be extensive over the typical residence time of bed ash in fluidized bed combustors. Very few and relatively fine primary ash particles were released after complete burnoff of wood pellets. Primary ash particles remaining after complete burnoff of pelletized straw had sizes and shapes that were largely controlled by the occurrence of ash agglomeration phenomena.

Fireside Corrosion of Superheater Materials in Chlorine Containing Flue Gas

Corrosion resistance of three types of candidate materials for superheater sections under simulated waste incineration conditions was evaluated. A 9Cr1Mo steel, an AISI 310SS, and the Ni-based alloy Sanicro 28 were tested on a laboratory and on a pilot scale with different flue gas compositions (up to 2500 mg/Nm3 of HCl and 1500 mg/Nm3 of fly ash). Laboratory tests were carried out in a furnace up to 200 h. Metal and gas temperature were kept constant at 500 °C. Pilot scale tests were carried out by using a 0.3 × 0.3 m cross-sectional combustor, with flue gas velocity of 5 m/s. Air-cooled probes, designed to operate at a metal temperature of 500 °C and facing gas temperatures as high as 600 °C, were used for 200 h as maximum test time. Qualitative correspondence was found between results obtained by the two sets of experimental tests, but quantitative values were not comparable. Metallographic evaluations, metal loss measurements, and weight loss analysis evidenced as the most suitable alloy Sanicro28. Maximum metal loss observed was 240, 182, and 107 µm, respectively, for 9Cr1Mo, AISI310SS, and Sanicro 28 under the most aggressive conditions. Intergranular corrosion attack was evidenced for AISI310SS, limiting the choice of materials to 9Cr1Mo and Sanicro 28, depending upon the lifetime expected at the design stage.

Modeling studies of baffle-type combustors

Geometrically similar ramjet-type combustion chambers of 1.61, 4.026, and 6.065-inch diameter and lengths of 5.25 D and 4.25 D were operated at Reynolds numbers ranging from 24,000 to 230,000 and ambient pressure levels from 0.2 to 1.5 atmospheres, with propane fuel. Flameholders were annular V-gutters having a leading-edge included angle of 30° and producing 44% blockage of the combustor cross section. Excepting the wall thickness of some burners, all dimensions of burners, flameholders, upstream piping and mixing section were scaled in proportion to burner inside diameters. Analysis indicated the importance of keeping the following parameters constant if the model experiment was to reproduce prototype performance: o a. Reynolds number, DU ϱ/μ . b. Chemical loading parameter, kϱ n−1 D/U . c. Mach number, M U/C . d. External heat-loss group, λ G / D [( L w /λ w )+(1/ h c+r,0 )]. e. Helmholtz resonator group (D 3 /V) 1/2 /M . Analysis indicated the probable importance of group d above—the external heat-loss group—and and a poorer performance to be expected of larger burners modeled by use of groups a and b alone, because of abnormally high external loss from large burners. An experimental study of the effect of wall losses, using emissivity control or radiation shielding, confirmed the analysis and explained some of the deviations in burner efficiency results of modeling based on groups a and b alone. This indicated the importance of including group d if the prototype walls run hot. Longitudinal, tangential, or radial oscillations that may occur in modeled combustors are scaled only if the Mach number is kept constant. As indicated above, this is automatic with modeling based on Reynolds number and the chemical loading parameter if, but only if, the pressure exponent n is 2.