Unstable Combustion Research Articles

Experimental study of equivalence ratio and fuel flow rate effects on nonlinear thermoacoustic instability in a swirl combustor

Industrial combustion systems such as power generation gas turbines, rocket motors, furnaces and boilers often face the problem of large-amplitude self-excited pressure oscillations that occur due to the onset of thermoacoustic instability. To prevent the onset of such instability, understanding the effects of fuel–air equivalence ratio ϕ, and fuel flow rate Vf on self-excited nonlinear thermoacoustic oscillations is of fundamental and practical importance. Experimental investigation of the roles of these parameters on triggering thermoacoustic instability in a swirl combustor has received very little attention. In this work, we design a swirling thermocoustic combustor and conduct a series of experimental tests. Autocorrelation and recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace are performed. These experimental tests allow us to study the effect of the fuel–air equivalence ratio ϕ on the onset of thermoacoustic instability by varying the fuel volume flow rate Vf. We demonstrate that the fuel volume flow rate and the equivalence ratio play different but critical roles on generating thermoacoustic instability at different frequencies and amplitudes. Maximum sound pressure level can be as high as 135 dB. In addition, mode switching, (i.e. frequency swap) is found to occur between approximately ω3≈510 Hz and ω1≈170 Hz, depending on the equivalence ratio ϕ. Furthermore, the dominant frequency corresponding to the maximum amplitude is shown to be shifted by approximately 20%, as the fuel flow rate Vf is increased and the combustion condition is changed from lean to rich. These findings are quite useful for designing a feedback control strategy to stabilize an unstable combustor. The present work opens up an applicable means to design a stable swirling combustor.

INTENSIFICATION OF HEAT TRANSFER IN A HIGH-TEMPERATURED FLUIDIZED BED

Purpose. This paper highlights experimental research of heat exchange in coarse particles to ensure the performance of ecological characteristics of Heat supply system. Methodology. The test stand has been developed to solve the defined task. It helps to do the research at the temperature of fluidizing bed and pulsating fluidizing bed at the range 800–1000°С. The temperature of the fluidized bed was provided by burning natural gas and wood waste. Sand and chamotte with a particle size of 1.0 to 5.0 mm were used as the layer material. The heat-transfer coefficient from the layer to the surface, immersed in the layer, and the density of the heat flux were measured by a calorimetric method under steady-state conditions. Smooth tubes and transversely finned with different height and rib spacing were investigated. Experiments in a high-temperature pulsating fluidized bed were carried out with pulsating combustion of natural gas in a layer or in a sublattice chamber into which natural gas and air were separately supplied. The frequency of combustion pulsations was provided by the automation system. The flash frequency was regulated in the range from 0.14 to 5 Hz. Findings. It is presented the results of physical modeling to find out the heat-transfer coefficients of smooth and finned tubes in fluidizing bed of coarse particles in the process of wood waste and gaseous fuel combusting. It is proved that the coefficient of heat transfer increases with increasing temperature by 2–2,5 times in the bed which contains particles diameter of 2.5–5 mm is 300–350Vt/(m2 К) that is much higher than for layer furnaces. The results of the experiments are presented in the form of a generalized relationship that takes into account the diameter of the particles and the value of the finning coefficient. Heat transfer of finned tubes is 15–20% less then smooth tubes but the density of heat flow referred to the area of a finned tube is 0,12–0,20 МVt/m2 that is higher than for smooth ones. At pulsating fuel supply with frequency 1-2 Hz the heat-transfer coefficient gets higher from 10–15% till 25–30%. At a ripple frequency of 2 to 5 Hz, the heat-transfer coefficient varies from 510 to 570W/(m2K). With a further increase in the pulsation frequency, the heat-transfer coefficient decreases and amounts to 515–520 W/(m2K). A significant effect of the ripple pulsation from 0.8 to 0.3 is shown: the heat-transfer coefficient is increased by 30–35%. However, an unstable combustion regime of the gas-air mixture and the ejection of material particles from the layer are observed. The results of the experiments are presented in the form of a generalized relationship. Originality. The authors have presented the obtained experimental data about heat-transfer coefficients of finned tubes in fluidizing bed and pulsing fluidizing bed of coarse particles at the temperature of layer 800–1100оС. Practical value. The results obtained at the research help to develop furnace devices with low-temperature fluidizing bed in the process of immersion in a layer of finned tubes layers. The research data is presented in criterial view which is convenient for practical using.

Characterization of PCDD/Fs and dioxin-like PCBs emitted from two woodchip boilers in Taiwan

This study investigates the formation and removal of PCDD/Fs and dl-PCBs in two woodchips boilers during different operating periods. Results indicate that combustion condition affects PCDD/F and dl-PCB formation within the woodchip combustion process. PCDD/F and dl-PCB concentrations during the start-up period are much higher than those measured during normal operation and shut-down periods due to unstable combustion. PCDD/F and dl-PCB concentrations at APCDs inlet of Plant A are significantly higher than that of Plant B due to the lower combustion temperature (500–850 °C) compared with Plant B (850–925 °C). Major PCDD/F congeners at APCDs inlet of both plants during normal operation are O8CDD, 1,2,3,4,6,7,8-H7CDD and 1,2,3,4,6,7,8-H7CDF, while major dl-PCBs are TeCB-77, PeCB-118 and PeCB-126. The removal efficiencies of PCDD/F and PCBs achieved with the APCDs of Plant A are 95.6% and 88.6%, respectively, while those of Plant B are 99.3% and 94.9%. Possibly, the AC concentration of Plant A exceeds the optimal AC concentration and, PCDD/Fs and dl-PCBs might be formed because the AC injected can supply additional reaction area and carbon source. Also, this may be due to different operating temperatures of APCDs, which affects removal efficiency of PCDD/F and dl-PCB congeners. The emission factors (PCDD/Fs + dl-PCBs) of Plants A and B are calculated as 17.86 and 1.25 μg I-TEQ/ton, respectively. Concentrations of PCDD/Fs in the BF ash of Plants A and B during normal operation are measured as 98.57 and 38.06 ng I-TEQ/g, which are significantly higher than the standard limit (1.0 ng I-TEQ/g) promulgated by Taiwan EPA.

Acoustic damping optimization of the injector based on dynamic mode decomposition in a subscale combustor

The acoustic tuning of a swirl coaxial jet injector mounted upstream a combustion chamber is investigated in the aspect of stabilization of unstable combustion. The injector, which has a primary function to mix the oxidizer injected through the central passage and the fuel injected through the peripheral holes, is designed to be used as a half-wave resonator. The least damped modes are extracted by applying Dynamic mode decomposition (DMD) and the injector length is tuned to damp the second longitudinal mode. Then, the sensitivity of the heat release perturbation to the velocity perturbation at the frequency of the second longitudinal mode is evaluated by flame transfer function calculated by DMD. The relation between these two quantities is achieved by combining two equations, that is, the Crocco’s model and the inhomogeneous wave equation, and enables us to compute Flame transfer function (FTF) at a specific mode. The gain field of FTF shows that the sensitivity of the heat release fluctuation to the inlet velocity fluctuation is minimal with the optimal injector length. The proposed approach for application of dynamic mode decomposition would be a valuable tool in tuning a resonator acoustically, computing efficiently flame transfer function, and quantifying the relative stability of a combustion chamber.

Deep learning based monitoring of furnace combustion state and measurement of heat release rate

Effective and efficient monitoring of furnace combustion state and measurement of heat release rate are important and pressing problems in the power industry. However, traditional methods including image segmentation based methods, feature based methods and shallow classifier based methods cannot meet the requirements of highly accurate. These methods are composed with several separating steps, i.e. feature selection and recognition. This paper proposes a novel deep learning based method to identify furnace combustion state and measure heat release rate. With an end-to-end network, feature extraction and classification are integrated into one framework. The deep learning model takes flame images into a multi-layer DNN (Deep Neural Network) or CNN (Convolutional Neural Network) to predict combustion state and heat release rate simultaneously. We also implement smooth and adjustment techniques which can get a trade-off between stability and sensitivity, ensuring both accurate prediction of burner state and fast detection of unstable combustion. The proposed system achieved state-of-the-art 99.9% accuracy in predicting combustion state with a speed of 1 ms per image. Experimental results show that this method has great potential for practical applications on power plants.

Passive instability control by a heat exchanger in a combustor with nonuniform temperature

Thermoacoustic instabilities, caused by the feedback between unsteady heat release and acoustic pressure perturbations, are characterised by large-amplitude pressure oscillations. These oscillations, if uncontrolled, pose a threat to the integrity of combustion systems. One strategy to mitigate them is by installing cavity-backed perforated plates with bias flow into the combustion chamber. In this study, we consider a generic combustor configuration: a one-dimentional tube (with open and/or closed ends) containing a compact heat source and a heat exchanger tube row. The idea is to use the heat exchanger tube row as a device (analogously to a cavity-backed perforated plate) to manipulate the downstream end condition. We simulate the row of heat exchanger tubes by a slit-plate with bias flow. We derive the characteristic equation for the complex eigenfrequencies of this set-up. From the growth rates (imaginary parts of the eigenfrequencies), we construct stability maps for various system parameter combinations. The results, obtained for the first two modes of the system, show that by varying the cavity length or the bias flow velocity through the slits, we can stabilise a previously unstable combustion system.

Characterization of Combustion Dynamics, Detection, and Prevention of an Unstable Combustion State Based on a Complex-Network Theory

Thermoacoustic instability is of interest to applied physicists and engineers as a significant, widely observed phenomenon in various types of engines. This study shows how to use a pragmatic, online detection methodology, based on the theory of complex networks, to control unstable combustion in a turbulent thermoacoustic system. This combination of network physics, nonlinear dynamics, and engineering helps to avoid blowout conditions and engine shutdown.

Applications of Dynamic Mode Decomposition to Unstable Shock-Induced Combustion

Dynamic mode decomposition (DMD) method was applied for the further study of periodical characteristics of the unsteady shock-induced combustion. The case of Lehr’s experiments was numerically simulated using 4 levels of grids. FFT result reveals that almost all the grid systems oscillate at frequencies around 430-435 kHz and the measureed one is around 425 kHz. To identify more resonant modes with low frequencies, DMD method is adopted for 4 grid systems. Several major frequencies are extracted and their damping coefficients are calculated at the same time, which is a quantification parameter for combustion stabilization.

Unsteady gasless combustion of planar symmetrical samples

Influence of ignition conditions on the modes of unstable combustion of planar symmetrical green samples was numerically explored in terms of 2D theoretical model. Within the range of instability, ignition conditions were found to define the number of self-propagating hot spots and their trajectories. In case of uniform distribution of ignition temperature over the inner boundary of annular sample, the combustion front propagates in the form of expanding ring.

Dynamics of robust structures in turbulent swirling reacting flows

High-speed synchronized stereo particle-imaging velocimetry and OH planar laser-induced fluorescence (PIV/OH-PLIF) measurements are performed on multiple $R{-}\unicode[STIX]{x1D703}$ planes downstream of a high-Reynolds-number swirling jet. Dynamic-mode decomposition (DMD) – a frequency-resolved data-reduction technique – is used to identify and characterize recurrent flow structures. Illustrative results are presented in a swirling flow field for two cases – the nominal flow dynamics and where self-excited combustion driven oscillations provide strong axisymmetric narrowband forcing of the flow. The robust constituent of the nominal reacting swirl flow corresponds to a helical shear-layer disturbance at a Strouhal number ($St$) of ${\sim}0.30$, $St=fD/U_{0}$, where $f$, $D$ and $U_{0}$ denote the precessing vortex core (PVC) frequency (${\sim}800~\text{Hz}$), the swirler exit diameter (19 mm) and the bulk velocity at the swirler exit ($50~\text{m}~\text{s}^{-1}$) respectively. Planar projections of the PVC reveal a pair of oscillating skew-symmetric regions of velocity, vorticity and OH-PLIF intensity that rotate in the same direction as the mean tangential flow. During combustion instabilities, the large-amplitude acoustics-induced axisymmetric forcing of the flow results in a fundamentally different flow response dominated by a nearly axisymmetric disturbance and almost complete suppression of the large-scale helical shear-layer disturbances dominating the nominal flow. In addition, reverse axial flows around the centreline are significantly reduced. Time traces of the robust constituent show reverse axial flows around the centreline and negative axial vorticity along the inner swirling shear layer when the planar velocity is in the same direction as the mean tangential flow. For both stable and unstable combustion, recurrent flow structures decay rapidly downstream of the air swirler, as revealed by the decreasing amplitude of the velocity, axial vorticity and OH-PLIF intensity.

Experimental Study on Instability Characteristics of Low-Swirl Flames in a Multinozzle Combustor With Different Swirling Arrays

This paper presents experimental study on self-excited combustion instability characteristics of premixed low-swirl flames in a multinozzle can combustor with counterswirl and coswirl arrays. Experiments were carried out over a wide range of inlet velocity from 4 m/s to 15.5 m/s and equivalence ratio from 0.5 to 0.85. Phase-locked OH planar laser-induced fluorescence was employed to measure flame shape and identify heat release rate. Four operation regions: stable combustion region, unstable combustion region, flashback region, and extinguish region are observed for both array burners. The amplitude of pressure fluctuation for counterswirl arrangement is less than the coswirl array, and the stable operating window of the counterswirl array is wider. In the unstable combustion region, the counterswirl flame triggers the 2L mode of the combustion system, while the coswirl flame incites three longitudinal modes with the highest amplitude near 3L. Rayleigh index distribution reveals neighboring flame interaction results in thermoacoustic coupling for multinozzle flames. Additionally, for the counterswirl array, thermoacoustic couplings also exit in the flame base region and shear region while, for the coswirl array, the instability driving zones also locate at the lip region and the tail of center flame which is totally different with counterswirl flame.

Development of Low Emission Combustor for 110 MWe Gas Turbine

Background: This is a review article. The stages of development and finishing of Low-Emission Combustor (LEC) for GTE- 110 gas turbine with capacity of 110 MW are presented. The main problem referred to in the article is pressure pulsations arising in certain modes of LEC operation. Methods: A detailed account of numerical and experimental research is given for detection of contributing factors on arising of unstable combustion, leading to pressure pulsations with high magnitudes (more than 2% of air pressure at the inlet of LEC). A computational and experimental approach to research of processes in LEC is described. Method of influence upon the border of steady combustion area is shown. Findings: Relationship between position and shape of flame in the volume of flame tube and stability of combustion process is described. Design criterion of stable combustion, allowing comparison of modifications of LEC structures and operation modes is presented. Technical solution developed on the basis of calculation and experimental studies, allowing implementation of stable and low-emission operation of LEC in all design operation modes of Gas Turbine (GT), is presented. The results of tests at pressures up to 450 kPa in the form of diagrams of combustion efficiency and NOx emissions over the entire load range of GT are presented. Improvements: Control method of combustor to achieve these characteristics was developed.

Ignition and unstable regimes of gasless combustion of a disk-shaped sample

The effect of ignition conditions and the parameters of the melting of the inert component on unstable combustion modes is studied numerically using a model of solid-flame combustion of a disk. It is shown that the shape of the heated region initiating combustion of the disk determines the number and trajectories of hotspots of the self-propagating combustion zone. The effect of the phase transition on the combustion characteristics is the more pronounced the closer the phase transition temperature to the combustion temperature. In this case, the combustion front takes the shape of a ring.

Combustion Dynamics in Postchamber of Hybrid Rocket Using Radical Chemiluminescence Images

A series of tests are conducted to understand the basic features of the coupling mechanism of combustion pressure with the local heat release fluctuation upon the occurrence of low-frequency instability in a hybrid rocket engine. In stable combustion, combustion pressure fluctuates at the frequency of about 20 Hz because of the thermal lag of solid fuel burning. Also, there exist steady spatial distributions of heat release, which are similar to a shear-layer structure. However, when low-frequency instability occurs, the combustion pressure is amplified, and the luminosity starts to fluctuate at a frequency of about 20 Hz. More interestingly, a positive coupling is observed between two fluctuations. The radical luminosity images and proper orthogonal decomposition analysis show the lumped flow structures with time-varying intensity in unstable combustion. A periodic change in the local heat release appears to generate large-scale flow rotations, and the large-scale flow rotations are likely to be associated with the periodic flame extinction at a frequency of about 20 Hz. Also, the flame extinction seems to take a crucial role in the occurrence of low-frequency instability, although it is not clearly understood why the flame extinction occurs in the specific geometric configuration.

Thermal response of a turbulent premixed flame to the imposed inlet oscillating velocity

Thermal response is known as thermal behavior of an unstable combustor. Such investigation, which has not been found in the literature, is important in terms of safety and prevention of the structural failure. In this study, the thermal response of a combustor with an inlet excitation is numerically investigated. Due to the geometry shape, two recirculating zones are found. Any change in the amplitude and frequency can affect these recirculation zones. At low fixed frequencies (below 50 Hz) and with a change in the amplitude, these two recirculation zones have no important influence on the heat release. Thus, at the low frequencies, excitation amplitude has no considerable effect on flame transfer function. For both adiabatic and convective cases, at fixed frequency, when amplitude increases, mass flow rate from cold to hot gases increases and this makes a reduction in the maximum temperature. Further, at a contestant amplitude, with increasing the frequency, the maximum temperature reduces, with a higher reduction for convective case. The physical interpretation of observed changes is sought in the relation between hydrodynamic and thermal field, relative length of combustor respect to the acoustic wavelength and so on.

Synthesis of ethylene diamine-based ferrocene terminated dendrimers and their application as burning rate catalysts

Ferrocene-based derivatives are widely used as ferrocene-based burning rate catalysts (BRCs) for ammonium perchlorate (AP)-based propellant. However, in long storage, small ferrocene-based derivatives migrate to the surface of the propellant, which results in changes in the designed burning parameters and finally causes unstable combustion. To retard the migration of ferrocene-based BRCs in the propellant and to increase the combustion of the solid propellant, zero to third generation ethylene diamine-based ferrocene terminated dendrimers (0G, 1G, 2G and 3G) were synthesized. The synthesis of these dendrimers was confirmed by 1H NMR and FT-IR spectroscopy. The electrochemical behavior of 0G, 1G, 2G and 3G was investigated by cyclic voltammetry (CV) and the burning rate catalytic activity of 0G, 1G, 2G and 3G on thermal disintegration of AP was examined by thermogravimetry (TG) and differential thermogravimetry (DTG) techniques. Anti-migration studies show that 1G, 2G and 3G exhibit improved anti-migration behavior in the AP-based propellant.

Investigation of reactivity-controlled compression ignition combustion in a two-stroke engine

In this study, a two-stroke outboard engine was modified to accommodate two direct fuel injectors for reactivity-controlled compression ignition combustion in one cylinder, while the production direct-injection spark ignition combustion system was maintained in the other cylinder. This setup enabled direct comparisons of the performances at equivalent operating conditions. The engine’s octane requirement for homogeneous charge compression ignition combustion was studied with primary reference fuels for ranges of engine speed, load, and delivery ratio. The resulting primary reference fuel requirement was used to determine the baseline ratio of low-reactivity fuel to high-reactivity fuel. Experiments using gasoline and diesel fuel were unsuccessful as they resulted in unstable combustion and rapid accumulation of particulate matter in the emissions-sampling equipment. Reactivity-controlled compression ignition experiments with gasoline and n-heptane (higher volatility) proved to be successful. The low-reactivity fuel fraction and the high-reactivity start-of-injection timing were found to be independent combustion-control levers. At 1500 r/min, an indicated mean effective pressure of 2.5 bar and nitrogen oxides emissions of 1.25 g/kW h, reactivity-controlled compression ignition resulted in a lower coefficient of variation in the indicated mean effective pressure, lower carbon monoxide emissions and a significantly higher gross indicated efficiency than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (36.3% versus 27.0% and 25.7% respectively); at 1200 r/min, an indicated mean effective pressure of 2.0 bar and hydrocarbon + nitrogen oxide emissions of about 16.5 g/kW h, the reactivity-controlled compression ignition efficiency was still significantly better than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (32.9% versus 25.2% and 26.6% respectively). Overall, the viability of reactivity-controlled compression ignition combustion in a two-stroke engine was demonstrated; with further design optimization, it will probably be possible to use a standard diesel fuel instead of n-heptane.

Effect of dissimilar valve lift on a bi-fuel CNG engine operation

The combustion of spark ignition engines converted to bi-fuel CNG is unstable and proper air and fuel mixing strategy is a concern. Also, CNG fuel causes the coefficient of variation for indicated mean effective pressure (COVimep) higher than 10% which is into the region of unstable combustion. In order to create stable combustion more turbulence is required. This paper studies the valve movement with dissimilar valve lift (DVL) to increase swirl in the engine. The intake valve is the last point of airflow entry into an engine and the modification of the movement can contribute to increase turbulence. Three DVL setting simulated via computational fluid dynamics (CFD) gave improvement in peak pressure by 4% and a 32.2% improvement in flame propagation speed compared to baseline CNG. Engine testing shows that, the engine COVimep improves up to 8.7%, while efficiency improves by 5.7% and BSFC is reduced by 5.4% respectively with the 1 mm DVL at 4000 rpm compared to baseline CNG. The rate of heat release (ROHR) also shows early heat release of the fuel. The novelty is better mileage for future CNG engine design.

Pre-injection strategy for pilot diesel compression ignition natural gas engine

For pilot fuel compression ignition natural gas (CING) engine, pre-injection strategy of pilot fuel is an important ways to improve engine performance, emissions and combustion. In this study, effects of pre-injection parameters on combustion and emissions performance were experimentally studied in a pilot diesel CING engine which was modified from a turbocharged six-cylinder diesel engine. The cylinder pressure, heart release rate (HRR), start of combustion (SOC), duration of combustion (DOC) and coefficient of variation (COVIMEP), as well as NOx and HC emissions were analyzed. The results indicate that early pre-injection mode leads to lower cylinder pressure and HRR due to decrease of combustion intensity, and thereby lower NOx emission is obtained. In contrast, closely pre-injection timing leads to largely strengthening in combustion which is not beneficial for improving NOx emission performance.Furthermore, effects of pre-injection quantity ratio and fuel injection pressure on combustion and emissions of early pre-injection operation modes (on pre-injection timing of 70°CA BTDC) were analyzed. Increase of pre-injection quantity ratio leads to decreasing in ignition intensity which delayed SOC and slowed burning rate of in-cylinder mixture, and thereby leads to lower cylinder pressure and HRR. Due to slowing in in-cylinder combustion, combustion temperature decreased, and hence leads to lower NOx emission and higher HC emission. However, too high pre-injection quantity ratio leads to unstable ignition and even unstable combustion which is not beneficial for engine performance. Increasing fuel injection pressure leads to larger fraction of premixed quantity of injected fuel which increased ignition energy and leads to rapid combustion, and hence higher cylinder pressure and HRR were obtained. Due to the increase of burning rate of in-cylinder mixture, in-cylinder combustion temperature was increased, and hence leads to higher NOx emission and lower HC emission.

한국형발사체 7톤 파워팩 시스템 고주파 신호 분석

7톤 엔진 시험 전에 엔진 시동/정격구동/종료 특성을 확인하고자 나로우주센터 터보펌프 실매질 시험설비에서 파워팩 시험을 수행하였다. 가스발생기 연소실의 동압은 0.2 bar이하로 측정되어 연소 불안정을 발생하지 않았고, 터보펌프에 장착된 RPM 및 가속도 센서 신호 분석 결과, 예측 RPM 범위 안에서 파워팩 시험이 안정하게 수행되었다는 것을 확인하였다. The 7tonf-class power pack test at turbopump test facility in Naro space center was performed for confirmations of starting/running/ending operation characteristics before 7tonf rocket engine hot-firing test. The dynamic pressure mounted on a combustion chamber of gas generator is measured under 0.2 bar which does not conditioned to the unstable combustion. The analysis results of RPM and acceleration sensors mounted on the turbopump, the power pack test was performed to the estimated RPM with the stable combustion.