Combustor Pressure Oscillations Research Articles

Transient spray combustion characteristics in a gas–liquid pintle rocket engine under acoustic excitation

In order to investigate the acoustic oscillation characteristics of gas–liquid pintle rocket engines and elucidate the path by which spray combustion process provides energy to the combustor pressure oscillation, a LOX/GCH4 pintle engine with rectangular combustor was designed. By adding transverse velocity disturbance for the first time, the acoustic response of spray combustion process was simulated, and the effect of excitation amplitude on acoustic response was researched. Numerical results show that the adopted transverse velocity disturbance can excite the first-order transverse acoustic oscillation with same excitation frequency in the engine combustor. The acoustic response maintenance mechanism under extrinsic excitation is summarized for pintle engines. Besides, the temperature distribution inside the engine combustor tends to be uniform, and the low-frequency oscillation caused by the flame transverse swing gradually disappears. The amplitude of combustor pressure oscillation is dominated by excitement amplitude and phase difference between the pressure and heat release in combustion reaction region. In addition, the time-averaged combustor pressure can be amplified mainly by transverse velocity disturbance. The research work can provide a reference for related fire tests on the acoustic response of a subscale gas–liquid pintle engine.

Numerical analysis of combustion process and pressure oscillation phenomena in low-pressure injection natural gas/diesel dual fuel low speed marine engine

Dual fuel strategy is a promising way to achieve low-carbon emission for marine engines, but engine knock restricts the compression ratio and engine thermal efficiency by adopting premixed combustion mode. Engine knock of natural gas/diesel dual fuel low speed marine engine is not well understood under high compression ratio. The characteristics of combustion process and pressure oscillation phenomena were investigated by varying compression ratio, pre-chamber number, energy proportion of pilot diesel with a single-cylinder engine model with numerical simulation. The results show that, with increase in pre-chamber number using the same energy proportion of pilot diesel under compression ratio of 16, the maximum amplitude of pressure oscillation can be significantly reduced from 0.62 MPa to 0.23 MPa in main chamber, while both resonance modes follow (1,0) mode. With double pre-chamber, when the energy proportion of pilot diesel increases from 0.6% to 1.2%, due to the stronger intensity of jet flame, the heat release rate is accelerated as well as the accumulation of H2O2 radical become less before high speed spontaneous combustion, resulting in lower amplitude of pressure oscillation. When the energy proportion of pilot diesel increases from 1.2% to 3.0%, excessive amount of pilot diesel cannot strengthen the jet flame obviously while the amplitude of pressure oscillation increases slightly. These results suggest that about 1.0% in energy proportion of pilot diesel should be optimal by considering the thermal efficiency, pressure oscillation and NOx emission together.

Effects of Orifice Diameter of Pre-Chamber Jet Ignition on the Combustion Characteristics and Pressure Oscillations in a Kerosene-Fueled Engine

Abstract Jet orifice diameter directly impacts the combustion process of the pre-chamber jet ignition (PJI) engine and the optimized diameter is varied with the fuel properties. However, research on the optimization of the jet orifice diameter based on aviation kerosene fuel has not been reported. So, this paper investigates the effect of orifice diameter on combustion, pressure oscillation, and performance based on a kerosene-fueled single-cylinder test engine. Two pressure sensors are respectively fitted in the main combustion chamber and the pre-chamber, which can capture the pressure change process and pressure oscillations phenomenon at the two positions, respectively. The result demonstrates that the throttling of the jet orifice leads to a significant three-stage pressure imbalance between the combustion chambers. With the reduction of the orifice diameter, the combustion acceleration of PJI is enhanced, resulting in an advanced combustion phase, improved combustion stability, and enhanced knock. The time-frequency analysis proves that the pressure oscillation propagation to the pre-chamber is frequency-selective and related to the orifice diameter. By matching the pre-chamber Helmholtz resonance frequency with the main chamber resonance frequency, strong pressure oscillations can be excited in the pre-chamber. Meanwhile, the pressure oscillation energy can be absorbed by the pre-chamber, which may help reduce the engine's combustion noise. Moreover, the PJI with an orifice diameter between 2 mm and 4 mm can improve the combustion stability with the ISFC reduced by 4.7–5.6%, and the IMEP increased by 1.2–2.6%.

Experimental study of gasoline engine with EGR dilution based on reactivity controlled turbulent jet ignition (RCTJI)

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines, but it also deteriorates the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition way for improving ignition stability and burning rate. However, the EGR in the pre-chamber will worsen the reactivity of the fuel-air mixture in the pre-chamber and subsequently lower the intensity of the TJI. To this end, the reactivity controlled turbulent jet ignition (RCTJI) based on a newly air-assisted pre-chamber turbulent jet ignition with a scavenging function has been proposed to improve the reactivity in the pre-chamber for the first time. The combustion characteristics, pressure oscillation, and combustion stability of a single-cylinder spark-ignition engine under four ignition modes of spark ignition (SI), passive TJI, active TJI, and scavenged TJI have been investigated. The results show that three TJI modes can obviously shorten the combustion duration due to the fast jet flame. Compared to the SI mode, the spark timing of TJI is not sensitive to the change in EGR rate. In particular, the scavenged TJI could effectively promote the chemical reactivity in the pre-chamber, and subsequently improves the intensity of the turbulent jet flame and shortens the ignition delay and combustion duration. As a result, using the scavenged TJI can effectively reduce cycle variation and achieve the lowest indicated specific fuel consumption (ISFC) at high EGR rates. However, with the increase in EGR rate, the SI combustion gradually becomes unstable. In contrast, the scavenged TJI can effectively extend the EGR tolerance. In addition, the present work further has confirmed the three main combustion stages of jet ignition in terms of the heat release rate.

The effect of ignition intensity and in-cylinder pressure on the knock intensity and detonation formation in internal combustion engines

Engine downsizing with a higher compression ratio and a higher boosting pressure improves the thermal efficiency of engines. However, with further downsizing of engines, knock intensity is aggravated until super knock occurs which is accompanied by a detonation wave that potentially can destroy engines rapidly. Therefore, it is essential to reveal the parameters that influence the knock intensity and the formation of a super knock. Self-designed detonation bomb experiments and corresponding numerical simulations were conducted to explore the end-gas combustion mode as well as the knock intensity problem. The experiment was conducted at three typical in-cylinder pressures for representing different mixture energy densities, which classify the knock phenomenon into subcritical (lower pressure), critical (medium pressure) and supercritical (higher pressure) conditions. Spark ignitions with low and high intensity realized by adjusting the spark ignition energy were given in each condition, respectively. The in-cylinder combustion process and pressure oscillation process were monitored by the synchronous acquisition of three pressure sensors that were installed in different positions of the chamber. It is found that irrespective of the low or high ignition intensity, the super knock as well as the detonation would not occur at the lower pressure, and only mild knock occurs. At the medium pressure, the high ignition intensity would result in a detonation wave, while the low ignition intensity would not. At the higher pressure, despite the intensity of ignition, a detonation wave as well as the super knock would always occur. Therefore, the ignition intensity and the in-cylinder pressure are two essential factors that affect the knock intensity of engines.

Experimental Study on Combustion Characteristics and Pressure Oscillations of Turbulent Jet Ignition in a Confined Space

ABSTRACT Flame shapes, flame tip velocities and pressure oscillations with turbulent jet ignition (TJI) combustion were studied comprehensively in a newly designed constant-volume combustion bomb (CVCB) by employing high-speed Schlieren photography. In the CVCB, the pre-chamber was effective at accelerating the flame and promoting turbulent flame formation. The results show that when the jet flame departs from the jet orifice, the flame tip velocity naturally decreases, mainly due to the fluid dynamic mechanism. And a temporal acceleration due to the emergence of additional flame wrinkles is observed at high pressures. When the initial pressure is sufficiently high, pressure waves can be distinguished clearly. The continuous wavelet transform suggests that a 1–3 kHz bandpass filter can be used to filter the pressure oscillation. Comparing pre-chambers with different holes reveals that intense pressure waves are formed with an increase in the number of jet orifices, and the propagating flame can be pushed backward during the interaction between the forward flame and the reflected pressure wave due to the reduced flame tip velocity and increased pressure wave intensity.

Effects of LOX Particle Diameter on Combustion Characteristics of a Gas-Liquid Pintle Rocket Engine

LOX/GCH4pintle injector is suitable for variable-thrust liquid rocket engines. In order to provide a reference for the later design and experiments, three-dimensional numerical simulations with the Euler-Lagrange method were performed to study the effect of the initial particle diameter on the combustion characteristics of a LOX/GCH4pintle rocket engine. Numerical results show that, as the momentum ratio between the radial LOX jet and the axial gas jet is 0.033, the angle between the LOX particle trace and the combustor axial is very small. Due to the large recirculation zones, premixed combustion mainly occurs in the injector wake region. As the initial LOX particle diameter increases, the LOX evaporation rate and the combustion efficiency decrease until the combustion terminates with the initial LOX particle diameter greater than 110 μm. The oscillation amplitude of the combustor pressure increases significantly along with the increase of the initial LOX particle diameter, and the low-frequency unstable combustion occurs when the initial LOX particle diameter exceeds 60 μm. The combustor pressure oscillation at about 40 Hz couples with the swinging process of spray and flame, while the unsteady LOX evaporation amplifies the combustor pressure oscillations at 80 Hz and its harmonic frequency.

Phase transition between pressure and heat release oscillations in hybrid rocket combustion

To understand the initiating mechanism of low frequency instability (LFI) in hybrid rocket combustion, numerical calculation was performed using a direct numerical simulation (DNS). Special attention was paid to the effect of the equivalence ratio on the shift in the phase difference between pressure (p’) and heat release oscillations (q’). A change in the equivalence ratio was considered by varying the temperature and composition of the combustion gas that entered into the post combustion chamber. In the obtained results, additional combustion was successfully simulated along with a vortex generation over the backward step, and combustion pressure and heat release oscillations were clearly reproduced as small vortices that move downward. In addition, the obtained results confirmed that the phase difference between pressure and heat release oscillations shifted because of the changes in the propagation speed of the pressure wave when the temperature of the combustion gas changes.

밀폐용기 연소실험 시 센서위치에 따라 변화하는 압력 진동에 대한 수치적 연구

밀폐용기 연소실험 시 센서위치에 따라 변화하는 압력 진동에 대한 수치적 연구

Experimental study on combustion pressure oscillation of soybean bio diesel oil

Abstract: The principal objectives of this study were to examine in-cylinder combustion pressure oscillation characteristics of soybean biodiesel in time domain and time-frequency domain, and their influences on the control and operational parameters, such as injection timing, exhaust gas recirculation (EGR) ratio, engine load and engine speed. In this study, the combustion pressure oscillation characteristics of biodiesel engine for various injection timing, EGR ratio and engine speed were investigated. The corresponding relation of pressure characteristics in the time domain and frequency domain were obtained. The results showed that the pressure oscillation and peak pressure rise acceleration occurred mainly in the diffusion combustion, and the peak pressure rise rate located in the premixed combustion. The in-cylinder pressure level curve can be divided into three stages. The pressure levels of stage 1, stage 2 and stage 3 represent the peak in-cylinder pressure, the maximum amplitude of pressure rise rate and pressure rise acceleration, respectively. As the injection timing retards, the pressure levels of stage 1 and stage 3 decrease gradually. The pressure level curve of stage 3 with 25o before top dead center (BTDC) is the highest and the oscillation is the most significant. It is worth noting that the location of each stage with various operate conditions is not fixed. At 0.41 MPa indicated mean effective pressure (IMEP), with the increase of EGR rate, the pressure levels of stage 1 and stage 2 decrease gradually. The pressure level curve of stage 3 and the maximum amplitude of pressure rise acceleration with 0% EGR rate are the highest. The oscillation with 0% EGR rate is the most significant at 0.41 MPa IMEP. Compared to 0.41 MPa IMEP, the frequency bands of stage 1 and stage 2 at 1.1 MPa IMEP are relatively low due to the soft combustion in the cylinder. As EGR rate increases, the pressure level of stage 1 decreases, and those of stage 2 and stage 3 increase gradually. The oscillation with 30% EGR rate is the most significant. With the increase of engine speed, the pressure levels of stage 1 and stage 2 decrease, and move to the low frequency. The pressure level in the high frequency domain at 1600 r/min is less than that at 1100 r/min, and the combustion process is smooth. Keywords: combustion pressure oscillation, soybean biodiesel, injection timing, exhaust gas recirculation, engine speed DOI: 10.3965/j.ijabe.20160906.1680 Citation: Ma P, Liu H L, Liu S Y. Experimental study on combustion pressure oscillation of soybean bio diesel oil. Int J Agric & Biol Eng, 2016; 9(6): 156－166.

Experimental study on multiple-pulse performance characteristics of ammonium perchlorate/aluminum powder rocket motor

The performance characteristics of ammonium perchlorate/aluminum powder rocket motor were investigated experimentally based on a powder rocket testing system. Three-pulse experiment with one hour interval and four-pulse experiment with 15s interval were conducted with a bi-propellant powder feed system. The experiments demonstrate the feasibility of the powder rocket for the multiple-pulse operation and the synchronization of powder feed method. The multiple-pulse performance characteristics were analyzed accordingly. It shows that the motor initiates steadily with the mass flow rate of 2.5g/s for oxidizer powder fluidization gas and 2.0g/s for fuel powder fluidization gas. The relative standard deviation was adopted to describe the repeatability characteristics of the pulses. The relative standard deviation of ammonium perchlorate and aluminum pistons velocity is 0.072 and 0.007, and that of oxidizer-fuel mass feed ratio is 0.052. The motor performed well with good repeatability of combustor pressure, start-up response and combustion efficiency during the multiple-pulse tests. Low frequency combustion pressure oscillations，3.6–4.2Hz with amplitudes up to 18.7% of mean combustor pressure, were encountered. The phenomenon appeared seriously at first pulse, then alleviated at the following pulses. Further analysis of test results showed that increasing chamber pressure could ameliorate the oscillation.

Design and preliminary results of an atmospheric-pressure model gas turbine combustor utilizing varying CO2 doping concentration in CH4 to emulate biogas combustion

Utilization of biogas can provide a source of renewable energy in both heat and power generation. Combustion of biogas in land-based gas turbines for power generation is a promising approach to reducing greenhouse gases and US dependence on foreign–source fossil fuels. Biogas is a byproduct from the decomposition of organic matter and consists primarily of CH4 and large amounts of CO2. The focus of this research was to design a combustion device and investigate the effects of increasing levels of CO2 addition to the combustion of pure CH4 with air. Using an atmospheric-pressure, swirl-stabilized dump combustor, emissions data and flame stability limitations were measured and analyzed. In particular, CO2, CO, and NOx emissions were the main focus of the combustion products. Additionally, the occurrence of lean blowout and combustion pressure oscillations, which impose significant limitations in operation ranges for actual gas turbines, was observed. Preliminary kinetic and equilibrium modeling was performed using Cantera and CEA for the CH4/CO2/Air combustion systems to analyze the effect of CO2 upon adiabatic flame temperature and emission levels. The numerical and experimental results show similar dependence of emissions on equivalence ratio, CO2 addition, inlet air temperature, and combustor residence time.

An experimental study of fuel–air mixing section on unstable combustion in a dump combustor

The main objective of this study is effect of the various fuel–air mixing section geometries on the unstable combustion. For the purpose of observing the combustion pressure oscillation and phase difference at each of the dynamic pressure results, the multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section. By using an optically accessible quartz-type combustor, we were able to OH* measurements to characterize the flame structure and heat release oscillation with the use of a high-speed ICCD camera. In this study, we observed two dominant instability frequencies. Lower frequencies were measured around 240–380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were measured around 410–830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter-wave mode in the inlet mixing section. These second instability mode characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling of the combustor and inlet mixing section geometry.

The role of charge stratification for reducing ringing in gasoline engine homogeneous charge compression ignition combustion investigated by optical imaging

Homogeneous charge compression ignition offers the possibility to reduce the fuel consumption of gasoline passenger car engines. However, the combustion strategy is limited to low loads due to pressure oscillations at higher loads. A strategy for extending the homogeneous charge compression ignition load range is charge stratification, using, for example, late direct injection to prolong the combustion duration and reduce the rate of pressure rises and pressure oscillations. In this study, local temperatures and fuel concentrations near top dead centre in a gasoline engine operating in homogeneous charge compression ignition mode were measured using two-wavelength planar laser-induced fluorescence, and the following combustion was analysed using high-speed video to investigate the effects of fuel and temperature stratification on combustion in order to explain the ringing inhibiting effect of charge stratification for fuels displaying single-stage ignition. The extent of spatial distribution of combustion timing correlated well with the extent of fuel and temperature stratification. Furthermore, the gas was leaner and hotter in early igniting regions, while it was richer and colder in late igniting regions. The dampening effects of charge stratification on the combustion speed and pressure oscillations are probably due to rich conditions in the latest burning regions (where combustion is usually most intense) slowing down combustion, which explains why the strategy only works when the global air-to-fuel ratio is not excessively lean.

A Hybrid Approach for the Modelling of Axial Compressor Surge

This paper discusses the results of a hybrid approach, which couples the Moore–Greitzer potential flow model and the Navier–Stokes solution to predict the viscous effects on the axial compressor surge. The viscous effects are accounted by the inclusion of a duct between the compressor exit and the plenum inlet. The unsteady mass flow solution from the Moore–Greitzer model is supplied as an inlet boundary condition, to the duct, to solve the Navier–Stokes equation. The viscosity is seen to damp the surge oscillations and alter the character of oscillations significantly. A shortcoming of the Moore–Greitzer model in predicting proper phase relationship between the compressor flow and the plenum pressure is pointed out and the proposed hybrid approach is shown to overcome this flaw to a greater extent. It is depicted that this hybrid approach captures the existence of rotating stall disturbances in the combustor, during a typical classic surge cycle, as reported in the literatures. Thus this hybrid model is more physically appropriate in describing the plenum pressure oscillations than the Moore–Greitzer model. The combustor pressure oscillations from the hybrid approach shows two distinct time scales that can influence the combustor efficiency.

Regarding the perturbed operating process of DB propellant rocket motor at extreme initial grain temperatures

Despite many decades of study, the combustion instability of several DB propellants is still of particular concern, especially at extreme grain temperature conditions of rocket motor operating. The purpose of the first part of the paper is to give an overview of our main experimental results on combustion instabilities and pressure oscillations in DB propellant segmented grain rocket motors (SPRM-01, large L/D ratio), working at extreme initial grain temperatures. Thus, we recorded some particular pressure-time traces with significant perturbed pressure signal that was FFT analysed. An updated mathematical model incorporating transient frequency-dependent combustion response, in conjunction with pressure-dependent burning, is applied to investigate and predict the DB propellant combustion instability phenomenon. The susceptibility of the tested motor SPRM-01 with DB propellant to get a perturbed working and to go unstable with pressure was evidenced and this risk has to be evaluated. In the last part of our paper we evaluated the influence of recorded perturbed thrust on the rocket behaviour on the trajectory. The study revealed that at firing-table initial conditions, this kind of perturbed motor operating may not lead to an unstable rocket flight, but the ballistic parameters would be influenced in an unacceptable manner.

Calculation on cylinder pressure fluctuation by using the wave equation in KIVA program

Cylinder pressure fluctuation during combustion process of internal combustion engine is closely related to combustion noise and knock. The current studies are based on cylinder pressure test to obtain information on combustion noise and knock, but there is little for simulation of combustion pressure fluctuation. Based on effects of combustion process in the combustion chamber on cylinder pressure by using wave equation, the mechanism of pressure fluctuation during combustion is researched with three-dimensional acoustic wave equation and flow field model of KIVA program. The cylinder pressure fluctuation curve, temperature field and acoustic field are obtained from the coupled calculation of the wave equation and KIVA program. The frequency spectrum analysis is taken with the cylinder pressure oscillation of cylinder pressure measured and calculated. The calculation result is consistent with the experimental result. This indicates that the cylinder pressure fluctuation can be correctly calculated with the wave equation. Analysis of calculation results of temperature field and acoustic field shows that sound field changes faster than flame propagates, and distribution of sound field is more complicated. Combustion pressure oscillation in the diesel engine is under highly unstable condition. This indicates that the combination of cylinder pressure fluctuation model and combustion model is an effective method to study the pressure oscillations and a new method to study the combustion noise and knock.

Investigation of pressure pulsations in the furnace and flue gas tract of the pulverized coal combustion utility boiler

The paper presents new experimental method developed and new measuring system developed and installed into flame-gas tract of utility boiler. Investigations have been performed at the steam boiler of unit 2 at TPP 'Ptolemais', Ptolemais, Greece, which suffered from very unstable combustion and great pressure oscillations. Experimental method, based on high-speed acquisition system, was developed and used for detection of pressure oscillations and determination of the origin of boiler unstable operation. Signals were obtained from piezoelectric pressure sensors located along the flame-gas tract of the steam boiler and time and frequency domain analysis were used for post processing of collected data. Investigations of the pressure oscillations in boiler gas tract have contributed to reveal origin of the boiler unstable operation, and have been of the great help in establishing proper boiler operation.

Active Control of Pressure Oscillations in a Liquid-Fueled Sector Combustor

A system for the active control of combustor pressure oscillations in liquid-fueled, lean, premixed combustors was demonstrated in a three-nozzle sector combustor, using full-scale engine hardware. Modulation of a portion of the premixed fuel flow led to a reduction of 6.5 dB (2.1 times) in the amplitude of the dominant pressure oscillations mode. Combustor emissions were not adversely affected by the control.

“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.