Scaling of heat transfer in gas-gas injector combustor

The scaling of heat transfer in gas-gas injector combustor is investigated theoretically, numerically and experimentally based on the previous study on the scaling of gas-gas combustion flowfield. The similarity condition of the gas-gas injector combustor heat transfer is obtained by conducting a formulation analysis of the boundary layer Navier-Stokes equations and a dimensional analysis of the corresponding heat transfer phenomenon. Then, a practicable engineering scaling criterion of the gas-gas injector combustor heat transfer is put forward. The criterion implies that when the similarity conditions of inner flowfield are satisfied, the size and the pressure of gas-gas combustion chamber can be changed, while the heat transfer can still be qualitatively similar to the distribution trend and quantitatively correlates well with the size and pressure as q ∝ pc0 .8d t-0.2. Based on the criterion, single-element injector chambers with different geometric sizes and at different chamber pressures ranging from 1 MPa to 20 MPa are numerically simulated. A single-element injector chamber is designed and hot-fire tested at seven chamber pressures from 0.92 MPa to 6.1 MPa. The inner wall heat flux are obtained and analysed. The numerical and experimental results both verified the scaling criterion in gas-gas injector combustion chambers under different chamber pressures and geometries.     

Scaling of the flow field in a combustion chamber with agas——gas injector

The scaling of the flowfield in a gas--gas combustion chamber is investigated theoretically, numerically and experimentally. To obtain the scaling criterion of the gas--gas combustion flowfield, formulation analysis of the three-dimensional (3D) Navier--Stokes equations for a gaseous multi-component mixing reaction flow is conducted and dimensional analysis on the gas--gas combustion phenomena is also carried out. The criterion implies that the size and the pressure of the gas--gas combustion chamber can be changed. Based on the criterion, multi-element injector chambers with different geometric sizes and at different chamber pressures ranging from 3~MPa to 20~MPa are numerically simulated. A multi-element injector chamber is designed and hot-fire tested at five chamber pressures from 1.64~MPa to 3.68~MPa. Wall temperature measurements are used to understand the similarity of combustion flowfields in the tests. The results have verified the similarities between combustion flowfields under different chamber pressures and geometries, with the criterion applied.     

Numerical and experimental study on shear coaxial injectors with hot hydrogen-rich gas/oxygen-rich gas and GH2 /GO2

The influences of the shear coaxial injector parameters on the combustion performance and the heat load of a combustor are studied numerically and experimentally. The injector parameters, including the ratio of the oxidizer pressure drop to the combustor pressure (DP ), the velocity ratio of fuel to oxidizer (R V ), the thickness (WO ), and the recess (HO ) of the oxidizer injector post tip, the temperature of the hydrogen-rich gas (TH ) and the oxygen-rich gas (TO ), are integrated by the orthogonal experimental design method to investigate the performance of the shear coaxial injector. The gaseous hydrogen/oxygen at ambient temperature (GH2 /GO2 ), and the hot hydrogen-rich gas/oxygen-rich gas are used here. The length of the combustion (LC ), the average temperatures of the combustor wall (TW ), and the faceplate (TF ) are selected as the indicators. The tendencies of the influences of injector parameters on the combustion performance and the heat load of the combustor for the GH2 /GO2 case are similar to those in the hot propellants case. However, the combustion performance in the hot propellant case is better than that in the GH2/GO2 case, and the heat load of the combustor is also larger than that in the latter case.     

Modeling of a self-excited pulse combustor and stability analysis

The major bottleneck for popularization and utilization of the conventional mechanical valve pulse combustors is the self-priming mode of gas supply. An aerodynamic valve (as against mechanical valve) self-excited pulse combustor of the Helmholtz-type with continuous supply of gas and air was designed and a mathematical model was established in this paper. The theoretical model employed well-stirred reactor model and a single step Arrhenius chemistry, and took those factors which might affect the combustion stability into account. The factors include the variation of the mass rate of the reactants affected by the pressure in the combustion chamber, the convective and radiation heat loss in the combustion chamber, and the heat transfer and wall friction in the tailpipe. The effect of wall temperature of combustion chamber, wall heat transfer coefficient, tailpipe length and friction coefficient on combustionstability were analyzed. The range of combustion oscillations can be predicted. It is theoretically and experimentally shown that combustion oscillations can be produced with a continuous supply of fuel and air without mechanical valves. The experimental data show qualitative agreement with predictions from the theoretical model. The theoretical model could be a tool for designing and optimizing the self-excited pulse combustor.     

烧蚀支板对超燃冲压发动机燃烧室性能的影响

为了提高超燃冲压发动机燃烧室的性能,本文提出了燃料喷注支板与烧蚀支板组合的燃烧室新方案,并研究了新方案对超燃冲压发动机燃烧室性能的影响。相比于单燃料喷注支板方式而言,加入烧蚀支板后,虽然燃烧室内的总压恢复系数有所下降,但燃烧室内燃料与空气的混合效率、燃烧效率均有显著提高,燃烧效率的提高弥补了燃烧室内总压损失所带来的机械能损失,使得燃料喷注支板和烧蚀支板组合方式下的燃烧室比冲高于单燃料喷注支板时的比冲。     

Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder

Combustion characteristics of a laboratory dual-mode ramjet/scramjet combustor were studied experimentally. The combustor consists of a sonic fuel jet injected into a supersonic crossflow upstream of a wall cavity pilot flame. These fundamental components are contained in many dual-mode combustor designs. Experiments were performed with an isolator entrance Mach number of 2.2. Air stagnation temperatures were varied from 1040 to 1490 K, which correspond to flight Mach numbers of 4.3–5.4. Both pure hydrogen and a mixture of hydrogen and ethylene fuels were used. High speed imaging of the flame luminosity was performed along with measurements of the isolator and combustor wall pressures. For ramjet mode operation, two distinct combustion stabilization locations were found for fuel injection a sufficient distance upstream of the cavity. At low T₀, the combustion was anchored at the leading edge of the cavity by heat release in the cavity shear layer. At high T₀, the combustion was stabilized a short distance downstream of the fuel injection jet in the jet-wake. For an intermediate range of T₀, the reaction zone oscillated between the jet-wake and cavity stabilization locations. Wall pressure measurements showed that cavity stabilized combustion was the steadiest, followed by jet-wake stabilized, and the oscillatory case. For fuel injection close to the cavity, a hybrid stabilization mode was found in which the reaction zone locations for the two stabilization modes overlapped. For this hybrid stabilization, cavity fueling rate was an important factor in the steadiness of the flow field. Scramjet mode combustion was found to only exist in the cavity stabilized location for the conditions studied.     

Examination of mode shapes in an unstable model combustor

The coupling between the fluid dynamics, heat addition, and the acoustics of a combustor system determine whether it is prone toward combustion instability. This paper presents results from a benchmark study of the eigenmodes in an unstable experimental combustor. The axisymmetric combustor configuration is representative of a number of practical systems and comprises an injector tube, geometric expansion into a combustion chamber, and a short converging nozzle. Instability limit cycle amplitudes ranged from 5% to nearly 50% of the mean 2.2 MPa pressure. Multiple harmonics were measured for the highly unstable cases. The model combustor was designed to provide a fairly comprehensive set of tested effects: sonic vs subsonic inlets; oxidizer tube lengths that were either quarter-wave, half-wave, or off-resonant acoustic equivalents to the combustion chamber; a significant injector mean flow with Ma∼0.4; and a varied combustion chamber length. The measured mode shape data were analyzed and reduced to provide comparison with results from a linearized one-dimensional Euler model, which included the effects of real boundary conditions, entropy generation, area change, and heat and mass addition, but did not include a model for unsteady heat addition. For low-amplitude instabilities, the measured resonance frequencies agreed with those calculated by the model for the injector tube-combustion chamber system. Resonance frequencies for the high-amplitude oscillation cases corresponded to the first longitudinal frequency of the combustion chamber and its integer multiples. Good quantitative agreement was obtained between computed and measured phase difference profiles, and mode envelopes agreed qualitatively. These results provide a basis for subsequent combustion response studies on the effects of unsteady heat addition.     

Prediction of soot and thermal radiation in a model gas turbine combustor burning kerosene fuel spray at different swirl levels

Combustion of kerosene fuel spray has been numerically simulated in a laboratory scale combustor geometry to predict soot and the effects of thermal radiation at different swirl levels of primary air flow. The two-phase motion in the combustor is simulated using an Eulerian–Lagragian formulation considering the stochastic separated flow model. The Favre-averaged governing equations are solved for the gas phase with the turbulent quantities simulated by realisable k–? model. The injection of the fuel is considered through a pressure swirl atomiser and the combustion is simulated by a laminar flamelet model with detailed kinetics of kerosene combustion. Soot formation in the flame is predicted using an empirical model with the model parameters adjusted for kerosene fuel. Contributions of gas phase and soot towards thermal radiation have been considered to predict the incident heat flux on the combustor wall and fuel injector. Swirl in the primary flow significantly influences the flow and flame structures in the combustor. The stronger recirculation at high swirl draws more air into the flame region, reduces the flame length and peak flame temperature and also brings the soot laden zone closer to the inlet plane. As a result, the radiative heat flux on the peripheral wall decreases at high swirl and also shifts closer to the inlet plane. However, increased swirl increases the combustor wall temperature due to radial spreading of the flame. The high incident radiative heat flux and the high surface temperature make the fuel injector a critical item in the combustor. The injector peak temperature increases with the increase in swirl flow mainly because the flame is located closer to the inlet plane. On the other hand, a more uniform temperature distribution in the exhaust gas can be attained at the combustor exit at high swirl condition.     

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm³ was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.     

Flamelet structure in turbulent premixed swirling oxy-combustion of methane

Two key flame macrostructures in swirling flows have been observed in experiments of oxy-combustion (as well as air-combustion); as the equivalence ratio is raised, the flame moves from being stabilized on just the inner shear layer (Flame III) to getting stabilized on both the inner and outer shear layers (Flame IV). We report results of an LES investigation of two different inlet oxy-fuel mixtures, in a turbulent swirling flow at $\text{Re}=20,000,$ that capture these two macrostructures. Previous work on the effects of heat loss have mostly focused on its impact on macro-scale observations. In this paper, we examine how heat loss impacts the flame microstructures as well for these two macrostructures. For both flames, the flamelet structure, as represented by a scatter plot of the normalized fuel concentration against the normalized temperature, depends on whether the combustor walls are adiabatic or non-adiabatic. For the adiabatic case, the flamelets of both macrostructures behave like strained flames. When wall heat transfer is included, Flame III microstructure is more bimodal. Since this flame extends farther downstream and part of it propagates along the walls, heat transfer has a greater impact on it’s microstructure. These results show that heat loss impacts not just the macro properties of the flame such as its shape or interactions with the wall, but also fundamentally changes its internal structure. Scatter plots of the turbulent flames are constructed and compared to different 1D laminar flame profiles (e.g., strained or with heat loss), and comparisons suggest the important role of the wall thermal boundary conditions in the accurate simulations of combustion dynamics and interpretations of experimental data, including data reduction and scaling.     

Control of oscillating combustion and noise based on local flame structure

To control combustion oscillations, the characteristics of an oscillating swirl injection premixed flame have been investigated, and control of oscillating combustion and noise based on local flame structure has been conducted. The r.m.s. value of pressure fluctuations and noise level show significantly large values between = 0.8 and 1.1. The beating of pressure fluctuations is observed for the large oscillating flame conditions in this combustor. Relationship between beating of pressure fluctuations and local flame structure was observed by the simultaneous measurement of CH/OH planar laser induced fluorescence and pressure fluctuations. The local flame structure and beating of pressure fluctuations are related and the most complicated flame is formed in the middle pressure fluctuating region of beating. The beating of pressure fluctuations, which plays important roles in noise generation and nitric oxide emission in this combustor, could be controlled by injecting secondary fuel into the recirculating region of oscillating flames. Injecting secondary fuel prevented lean blowout, and low NO_x combustion was also achieved even for the case of pure methane injection as a secondary fuel. By injecting secondary fuel into the recirculating region near the swirl injector, the flame lifted from the swirl injector and its reaction region became uniform and widespread, hence resulting in low nitric oxide emission. Secondary mixture injection, fuel diluted with air, is not effective for control of combustion oscillations suppression and lean blowout prevention.     

A comparative study of H2-air premixed flame in micro combustors with different physical and boundary conditions

A numerical study of H₂-air premixed combustion in the micro channels with a detailed chemical reaction mechanism is performed by solving the two-dimensional fully elliptic governing equations of continuity, momentum, energy and species, coupled with the energy equation in the solid wall. A reference case is defined as the combustion in a cylindrical tube with 0.8 mm inner diameter and 8 mm length with a non-slip wall and a uniform velocity profile at the inlet plane. Different physical and boundary conditions have been applied in order to investigate their respective effects on the flame temperature. The conditions studied in the current paper include the combustor size and geometry, inlet velocity profile, axial heat conduction in the solid wall and slip-wall and temperature jump at the gas–solid interface. It is noted that effects of Knudsen number (slip-wall and temperature jump) on the thermal and fluid field are not very significant in a d = 0.4 mm micro combustor. Furthermore, the qualitative effects of Knudsen number on the flame temperature are analysed. The results of this paper indicate that these various boundary and physical conditions have effects on the flame temperature to different extent and should be carefully monitored when applied for different applications.     

An experimental study into the effect of injector pressure loss on self-sustained combustion instabilities in a swirled spray burner

Combustion instabilities depend on a variety of parameters and operating conditions. It is known, especially in the field of liquid rocket propulsion, that the pressure loss of an injector has an effect on its dynamics and on the coupling between the combustion chamber and the fuel manifold. However, its influence is not well documented in the technical literature dealing with gas turbine combustion dynamics. Effects of changes in this key design parameter are investigated in the present article by testing different swirlers at constant thermal power on a broad range of injection velocities in a well controlled laboratory scale single injector swirled combustor using liquid fuel. The objective is to study the impact of injection pressure losses on the occurrence and level of combustion instabilities by making use of a set of injectors having nearly the same outlet velocity profiles, the same swirl number and that establish flames that are essentially identical in shape. It is found that combustion oscillations appear on a wider range of operating conditions for injectors with the highest pressure loss, but that the pressure fluctuations caused by thermoacoustic oscillations are greatest when the injector head loss is low. Four types of instabilities coupled by two modes may be distinguished: the first group features a lower frequency, arises when the injector pressure loss is low and corresponds to a weakly coupled chamber-plenum mode. The second group appears in the form of a constant amplitude limit cycle, or as bursts at a slightly higher frequency and is coupled by a chamber mode. Spontaneous switching between these two types of instabilities is also observed in a narrow domain.     

An improved model to calculate radiative heat transfer in hot combustion gases

Radiative heat transfer plays an important role in the chemical reactions in the combustor. The widely used WSGG model proposed by Smith is established for normal pressure, which shows inevitable computational errors when dealing with radiative heat transfer problems at reduced or elevated pressures. In this paper, an improved global model is established to calculate the radiant energy exchanges between combustion gases and combustor chamber walls. Compared with the Smith model, the new model shows better performance in a wide range of pressure regions. The model accuracy is examined by computing the emissivity, radiative heat flux as well as the radiative source of H₂O–CO₂ gas mixtures at different pressure values. Finally, the radiative heat transfer inside a 3D TBCC(turbine-based combined cycle) engine exhaust system where strong gradients of pressure and temperature exist, is also addressed. The computational results show that the developed model provides approximate results at much less computational costs than the high-precision MSMGFSK-c8 model, which makes it competitive in complicated combustion systems.     

Experimental investigation of combustion instabilities of a mesoscale multinozzle array in a lean-premixed combustor

Self-excited combustion instabilities in a mesoscale multinozzle array, also referred to as a micromixer-type injector, have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. The injector assembly consists of sixty identical swirl injectors of 6.5 mm inner diameter, which are evenly distributed across the combustor dump plane. Their flow paths are divided into two groups – inner and outer stages – to form radially stratified reactant stoichiometry for the control of self-excited instabilities. OH PLIF measurements of stable flames reveal that the presence of radial staging has a remarkable influence on stabilization mechanisms, reactant jet penetration/merging, and interactions between adjacent flame fronts. In an inner enrichment case, two outer (leaner) streams merge into a single jet structure, whereas the inner (richer) reactant jets penetrate far downstream without noticeable interactions between neighboring flames. The constructed stability map in the 〈?_i, ?_o〉 domain indicates that strong self-excited instabilities occur under even split and outer enrichment conditions at relatively high global equivalence ratios. This is attributed to large-scale flame surface deformation in the streamwise direction, as manifested by vigorous detachment/attachment movements. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations, because the center flames are securely anchored during the whole period of oscillation, giving rise to a moderate lateral motion. We demonstrate that the collective motion of sixty flames – rather than the individual local flame dynamics – play a central role in the development of limit cycle oscillations. This suggests that the distribution pattern of the injector array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.     

Effect of fuel injection location on a plasma jet assisted combustion with a backward-facing step

Non-reacting and reacting experiments on the ignition by a plasma jet (PJ) torch were performed to understand the correlation between fuel injection location and combustion characteristics in unheated Mach 2 airflow. Fuel was injected through three sonic injectors in the recirculation region behind a backward-facing step: a parallel injector at 2 mm from the bottom wall and two normal injectors at 2 and 9 mm from the step wall. In order to mitigate the combustion pressure interaction with nozzle, an isolator was installed between the nozzle and combustor. The combustion performance of normal injection was little affected by the difference of fuel injection locations. Moreover, normally injected fuel was escaped not to be held in the recirculation region despite of low fuel injection rates. This led to lower combustion performance relative to the parallel injection which provided fuel not to leave the recirculation region. In this case, the role of the recirculation region was to fully hold fuel, and the PJ torch provided hot gases as a heat source and acted as a flame-holder to ignite fuel–air mixtures. In a low temperature inflow condition, combustible regions were constrained around the bottom wall where embedded with the PJ torch. When thermal choking occurred in the combustor, it induced shock train both in the combustor and isolator. Under this unstable condition, the combustion performance of the normal injection was lower than that of the parallel injection. This is because the normal injection led most fuel into low temperature incoming air-stream.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

Effect of injection dynamics on detonation wave propagation in a linear detonation combustor

The effect of reactant injection and mixing on detonation wave propagation is studied in a self-excited, optically-accessible linear detonation combustor operated with natural gas and oxygen. Fuel injection and mixing processes are captured with 100 kHz planar laser induced fluorescence (PLIF) measurements of acetone tracer injected into the fuel stream. Measurements are acquired at multiple orthogonal planes downstream of the reactant injection site to investigate the three-dimensional mixing field in the chamber. The fuel distribution field is correlated with simultaneously acquired OH${}^{*}$ chemiluminescence measurements that provide a qualitative indication of heat release in the combustor. These measurements are used to provide quantitative information of the fuel injector recovery process and its impact on detonation wave structure across a range of equivalence ratios. While significant differences in the detonation wavefront are observed with change in equivalence ratio, the characterization of the fuel refill process into the chamber after the passage of the detonation wave highlights some key generalizable features. The time available for fuel recovery is consistently between 12 – 19% of the detonation wave period across an equivalence ratio range of 0.83 – 1.48. A linear correlation between injector recovery times and the ratio of the average detonation wave pressure amplitude relative to the pressure drop across the fuel injector is observed. Instantaneous and phase-averaged measurements of acetone-PLIF with the time-coincident OH${}^{*}$ chemiluminescence images provide qualitative information of wave structure and injection dynamics along with quantification of fuel injector recovery, a key metric that drives combustor operation and performance. These measurements significantly enhance the ability to obtain detailed information on the intra- and inter-cycle spatiotemporal evolution of the reactant refill process and its coupled effects on the detonation wave structure and propagation.     

微型火焰管中燃烧的研究

提出了一种新型的微动力机电系统观念,即微型热光电 TPV(thermo photovoltaic)系统。微型燃烧室是微型TPV系统中最重要的部分之一。为了获得较高的能量转换效率,需要使燃烧器壁面四周处于较高且分布均匀的温度状态。尺寸效应对微型燃烧室中的持续燃烧带来了很大的影响。为了分析微型燃烧器中燃烧的可行性和有关影响因素,在不同工况下进行实验。结果表明,在一定的流量和混合比范围内,可以在微型火焰管内维持稳定的燃烧,高温能够在燃烧室四周均匀分布。     

“Slow” active control of combustion instabilities by modification of liquid fuel spray properties

This paper describes an experimental investigation of the feasibility of using “slow” active control approaches, which “instantaneously” change liquid fuel spray properties, to suppress combustion instabilities. The objective of this control approach was to break up the feedback between the combustion process heat release and combustor pressure oscillations that drive the instability by changing the characteristics of the combustion process (e.g., the characteristic combustion time). To demonstrate the feasibility of such control, this study used a proprietary fuel injector (Nanomiser^TM), which can vary its fuel spray properties, to investigate the dependence of acoustics–combustion process coupling, i.e., the driving of combustion instabilities, upon the fuel spray properties. This study showed that by changing the spray characteristics it is possible to significantly damp combustion instabilities. Furthermore, using combustion zone chemiluminescence distributions, which were obtained by Abel’s deconvolution synchronized with measured acoustic data, it has been shown that the instabilities were mostly driven midway between the combustor centerline and wall, a short distance downstream from the flame holder, where the mean axial flow velocity is approximately zero in the vortex near the flame holder. The results of this study strongly suggest that a “slow” active control system that employs controllable fuel injectors could be effectively used to prevent the onset of detrimental combustion instabilities.