Constant Fuel Flow Rate Research Articles

The influence of radiation modeling on flame characteristics and emissions prediction in microcombustors with Ammonia/Hydrogen blends

This study advances numerical methodologies for designing microcombustor-based thermophotovoltaic systems by incorporating comprehensive analyses of radiation models and absorption coefficients. It examines the impact of non-premixed hydrogen-ammonia-air fuel blends on the micro combustion process using three-dimensional CFD simulations on a Y-shaped microcombustor under atmospheric conditions.Key to this research is modeling radiative heat transfer from multiple species, including ammonia (NH3), nitric oxide (NO), and water vapor, ensuring accurate predictions and optimization of thermal behavior and efficiency. Three modeling approaches were compared: a multispecies Planck-mean approximation radiation model (PMAC), a narrowband model for water vapor, and a scenario excluding radiation effects.The PMAC model, incorporating the absorption coefficients of ammonia and its products, consistently produced lower errors in flame length compared to the NB method across all equivalence ratios, with errors ranging from 0 % to 7 % for PMAC and 10 % to 30 % for NB, with the lowest error observed at Φ = 1.0. Additionally, the prediction of laminar burning velocity (LBV) differed by approximately 4 % between the two models.Considering different fuel blends, two scenarios were compared: constant input thermal power and constant fuel flow rate. Efficiency in the constant input thermal power scenario maintained higher radiation efficiency, in the range of 32–38%. Additionally, radiation efficiency initially increased with increasing H2 content but subsequently decreased for high H2 values due to effects on flame area, flame position, and temperature.

Oscillation frequency measurement of gaseous diffusion flames using electrostatic sensing techniques

The oscillation frequency of a burner flame is closely related to the combustion conditions and flame stability. Oscillation frequency measurement is essential for optimized operation of the combustion process. In this paper, the novel use of electrostatic sensors in conjunction with power spectral analysis is presented for the oscillation frequency measurement of a gaseous laminar diffusion flame. Experimental tests carried out on a combustion rig demonstrate that the developed system realises oscillation frequency measurement with a relative deviation from the reference value from an imaging system within ± 6% over a constant fuel flow rate from 0.60 L/min to 0.80 L/min. The oscillation frequency increases with the fuel flow rate and the oscillation frequencies in different regions of the diffusion flame are similar for each fuel flow rate. Under varying fuel flow rate conditions, the system can measure the instantaneous oscillation frequency with a relative deviation from the reference value from an imaging system mostly between −10% and 0%.

Influence of unsaturation of hydrocarbons on the characteristics and carcinogenicity of soot particles

This paper concerns the effect of unsaturation of hydrocarbons (single, double, and triple bonds) on soot particle characteristics (mass, number, and size) and on the carcinogenicity of soot particles. The soot particles were produced from oxygen-free pyrolysis of five hydrocarbons, namely: propane, propylene, ethane, ethylene, and acetylene. The characteristics of soot particles were measured with the aid of a differential mobility spectrometer (Cambustion-DMS-500) and measurement of soot mass concentration was confirmed using gravimetric filter measurements. The soot particle carcinogenicity was estimated from the emission quantities of total polyaromatic hydrocarbons (PAHs) and the toxicity equivalent factor (TEF) of each PAH. Oxygen-free pyrolysis of the hydrocarbon fuels was conducted in a laminar tube reactor within the temperature range of 1050 –1350oC at a constant nitrogen flow rate of 20 L/min and constant fuel flow rate of 1% (vol) on carbon-1 basis. The experimental results showed that increasing unsaturation of fuels from single to double and to triple bonds increased the mass concentration, particle size, number concentration, and carcinogenicity of soot particle notably at the initial temperature of 1050 oC. Increase in the pyrolysis temperature of the tube reactor from 1050 – 1350oC, increased the mass concentration and sizes of the soot particle while the number concentration and carcinogenicity of the soot particle decreased. There was a positive correlation between the soot particle number and the corresponding soot particle carcinogenicity, while a negative correlation was observed between the soot particle mass and size with soot particle carcinogenicity regardless of the pyrolysis temperature examined. The potential implication of these observations is that, low-temperature combustion (LTC) applications, aimed at reducing emissions of soot and NOx, could produce higher soot particle number concentration of higher carcinogenicity.

Numerical characterization of 3D nonreacting supersonic cavity combustor with inlet Mach number variation

A numerical investigation of a cavity-based supersonic combustor with non-reacting upstream hydrogen fuel injection is conducted to study the effects of inlet Mach number (Ma) on flow structure and fuel-air mixing. Three different freestream Mach number cases (1.5, 2.5 and 3.5) are investigated at a constant fuel flow rate, injected at the sonic condition by considering governing equations for compressible, turbulent flow using Shear Stress Transport (SST) k-ω model. The complex flow structure is investigated by identifying various flow features namely, upstream three-dimensional bow shock, compression waves, Mach reflection, vortex in the separated boundary layer and horseshoe vortices at the downstream of the injection port. Besides this, the flow physics involved in these complex flow features are unravelled. Moreover, the performance of the combustor is characterized quantitatively in terms of mixing efficiency, total pressure loss and coefficient of pressure. However, the mixing efficiency and total pressure loss for the operating condition of Ma = 1.5, exhibits better performance than that of the other Mach number cases (2.5, 3.5) due to decrease in inclination angle of reattachment shock from 47.6° to 29.9°. The present numerical investigation also demonstrates that the three-dimensional simulation is essential in the characterization of fuel-air mixing in supersonic cavity-based combustors.

Counter-rotating dual-stage swirling combustion characteristics of hydrogen and carbon monoxide at constant fuel flow rate

In this paper, experimental and numerical methods were used to study the combustion characteristics of a counter-rotating double-stage swirling syngas combustor at constant fuel flow rate, and the effect on it of hydrogen content of syngas. In the experiment, the speed and temperature in the combustor were respectively obtained with PIV and temperature rake, while Reynolds stress equation model and the detailed chemical reaction mechanism of syngas were adopted in the numerical method. The calculation results were in good agreement with the experimental data. Research results indicated that in the working conditions of different hydrogen contents, the flow field structures in the combustor are almost the same, and the maximum temperatures at the outlet remain almost the same. However, as hydrogen content in the fuel increases, the axial velocity in the central area of flow field is increasing, and the outlet temperature distribution coefficient decreases first and then increases. In addition, it was also found in the study that the distribution structure of temperature on the central section of the combustor is almost impervious to the changes in hydrogen content, but with numerical differences, i.e. the higher hydrogen content in the fuel, the farther the stabilization position of flames in the central area is away from the head. It was also indicated in the study that the conventional combustor is no longer applicable to the combustion of syngas, especially the hydrogen-rich fuel. And the work provided the improvement scheme of hydrogen-containing fuel for gas turbine combustor.

Effect of Airflow Temperature on the Formation of Initial Flame Kernel and the Propagation Characteristics of Flame

Using liquid RP-3 aviation kerosene as the fuel to study, the effect of airflow temperature on the formation of initial flame kernel during the ignition of spray combustion and on the propagation characteristics of flame was investigated. Combining high-speed camera and dynamic temperature acquisitions at the outlet of combustor, the internal triggering mode was used under a constant fuel flow rate and airflow velocity. This combined system simultaneously recorded the formation of initial flame kernel, flame propagation, and outlet temperature variation of combustor under different airflow temperatures. MATLAB software was used to obtain the reaction zones at different moments and to analyze the effects of airflow temperature on morphological characteristics such as flame area, perimeter-to-area ratio, maximum length-to-height ratio, equivalent mean length-to-height ratio, mass center, and centroid. According to the growth rate in flame area, the ignition process can be divided into three stages: formation of flame kernel, rapid development of flame, and stable development of flame. Airflow temperature not only affects the formation time of flame kernel but also affects the growth rate of flame area. During the development of flame, the movements of mass center and centroid are irregular, and their positions do not coincide with each other. However, the overall moving trends are consistent. With the increase of the airflow temperature, the position, where the flame kernel is gradually formed, moves closer to the center of the end face of spark plug. The force of airflow on flame is the main factor that increases the flame area and heat-release rate. Therefore, the folds around the flame edge mainly result from the stretching under the action of airflow. With the increase in airflow temperature, the heat release of the initial flame kernel increases, and the ratio of perimeter to area as a characterization parameter increases by 8%, 86%, and 33%, respectively. In addition, the maximum outlet temperature rise increased by about 53%, 73.5%, and 0.65%, respectively. Meanwhile, the maximum rate of temperature rise increased by about 42.8%, 57%, and 5.1%, respectively.

Part-load performance of direct-firing and co-firing of coal and biomass in a power generation system integrated with a CO2 capture and compression system

Bioenergy with Carbon Capture and Storage (BECCS) is recognised as a key technology to mitigate CO2 emissions and achieve stringent climate targets due to its potential for negative emissions. However, the cost for its deployment is expected to be higher than for fossil-based power plants with CCS. To help in the transition to fully replace fossil fuels, co-firing of coal and biomass provide a less expensive means. Therefore, this work examines the co-firing at various levels in a pulverised supercritical power plant with post-combustion CO2 capture, using a fully integrated model developed in Aspen Plus. Co-firing offers flexibility in terms of the biomass resources needed. This work also investigates flexibility within operation. As a result, the performance of the power plant at various part-loads (40%, 60% and 80%) is studied and compared to the baseline at 100%, using a constant fuel flowrate. It was found that the net power output and net efficiency decrease when the biomass fraction increases for constant heat input and constant fuel flow rate cases. At constant heat input, more fuel is required as the biomass fraction is increased; whilst at constant fuel input, derating occurs, e.g. 30% derating of the power output capacity at firing 100% biomass compared to 100% coal. Co-firing of coal and biomass resulted in substantial power derating at each part-load operation.

Dynamic data-driven prediction of instability in a swirl-stabilized combustor

Combustion instability poses a negative impact on the performance and structural durability of both land-based and aircraft gas turbine engines, and early detection of combustion instabilities is of paramount importance not only for performance monitoring and fault diagnosis, but also for initiating efficient decision and control of such engines. Combustion instability is, in general, characterized by self-sustained growth of large-amplitude pressure tones that are caused by a positive feedback arising from complex coupling of localized hydrodynamic perturbations, heat energy release, and acoustics of the combustor. This paper proposes a fast dynamic data-driven method for detecting early onsets of thermo-acoustic instabilities, where the underlying algorithms are built upon the concepts of symbolic time series analysis (STSA) via generalization of D-Markov machine construction. The proposed method captures the spatiotemporal co-dependence among time series from heterogeneous sensors (e.g. pressure and chemiluminescence) to generate an information-theoretic precursor, which is uniformly applicable across multiple operating regimes of the combustion process. The proposed method is experimentally validated on the time-series data, generated from a laboratory-scale swirl-stabilized combustor, while inducing thermo-acoustic instabilities for various protocols (e.g. increasing Reynolds number ( Re) at a constant fuel flow rate and reducing equivalence ratio at a constant air flow rate) at varying air-fuel premixing levels. The underlying algorithms are developed based on D-Markov entropy rates, and the resulting instability precursor measure is rigorously compared with the state-of-the-art techniques in terms of its performance of instability prediction, computational complexity, and robustness to sensor noise.

Effects of Fuel Impingement-Cooling on the Combustion Flow in a Small Bipropellant Liquid Rocket Thruster

ABSTRACTIn the present study, a three-dimensional computer code, based on the computer software KIVA-3, was developed for the combustion-flow simulation of a bipropellant liquid rocket thruster. A jets-impingement model is proposed for the unlike-doublet jet impingement issue. The computer code is employed to simulate a small bipropellant liquid rocket engine installed with three unlike-doublet injectors of NTO and MMH as well as six fuel injectors injecting MMH toward the combustor wall for cooling. Effects of the fuel-injection cooling on the combustion flow, combustion efficiency, and wall temperature were investigated. The results obtained from the present study show that under the present injector configuration and a constant total fuel-flow rate, higher cooling-fuel ratios make the atomized mixing flow of NTO and MMH shift toward the combustor wall, resulting in lower combustion efficiency and chamber pressure; however, low cooling-fuel ratios are unable to keep the wall-temperature sufficiently low. To overcome this issue, the proposed three-dimensional computer code calculates/locates the optimal cooling-fuel ratio that bears high combustion efficiency for a bipropellant liquid rocket combustor while keeping the chamber wall sufficiently cool.

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NOx emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.

Experimental Characterization of Methane Inverse Diffusion Flame

This article presents 10-kHz images of OH-PLIF simultaneously with 2-D PIV measurements in an inverse methane diffusion flame. Under a constant fuel flow rate, the central air jet Re was varied, leading to air to fuel velocity ratio, Vr, to vary from 8.3 to 66.5. Starting from Vr = 20.7, the flame is commonly characterized by three distinct zones. The length of the lower fuel entrainment region is inversely proportional to Vr. The flames investigated resemble a string shear layer confining this zone, and converging into the second distinct region, the flame neck zone. The third region is the rest of the flame, which spreads in a jet-like manner. The inverse diffusion flames exhibit varying degrees of partial premixing, depending upon on the velocity ratio Vr, and this region of partial premixing evolves into a well-mixed reaction zone along the flame centerline. The OH distribution correlated with the changes in the mean characteristics of the flow through reduction in the local Reynolds number due to heat release. The existence of a flame suppresses or laminarizes the turbulence at early axial locations and promotes fluctuations at the flame tip for flames with Vr < 49.8. In addition, the flame jet width can be correlated to the OH distribution. In upstream regions of the flames, the breaks in OH are counterbalanced by flame closures and are governed by edge flame propagation. These local extinctions were found to occur at locations where large flow structures were impinging on the flame and are associated with a locally higher strain rate or correlated to the local high strain rates at the flame hole edges without this flow impinging. Another contributor to re-ignition was found to be growing flame kernels. As the flames approach global blow-off, these kernels become the main mechanism for re-ignition further downstream of the flames. At low Vr, laminarization within the early regions of the flame provides an effective shield, preventing the jet flow from penetrating through the flame.

Pumpless Fuel Supply Using Pressurized Fuel Regulated by Autonomous Flow-Rate Regulation Valves

A pumpless fuel supply using pressurized fuel with autonomous flow regulation valves is presented. Since micropumps and their control circuitry consume a portion of the electrical power generated in fuel cells, fuel supply without micropumps makes it possible to provide more efficient and inexpensive fuel cells than conventional ones. The flow regulation valves in the present system maintain the constant fuel flow rate from the pressurized fuel chamber even though the fuel pressure decreases. They autonomously adjust fluidic resistance of the channel according to fuel pressure so as to maintain constant flow rate. Compared to previous pumpless fuel supply methods, the present method offers more uniform fuel flow without any fluctuation using a simple structure. The prototypes were fabricated by a polymer micromolding process. In the experimental study using the pressurized deionized water, prototypes with pressure regulation valves showed constant flow rate of 5.38 ± 0.52 μl/s over 80 min and 5.89 ± 0.62 μl/s over 134 min, for the initial pressure in the fuel chamber of 50 and 100 kPa, respectively, while the other prototypes having the same fluidic geometry without flow regulation valves showed higher and gradually decreasing flow rate. The present pumpless fuel supply method providing constant flow rate with autonomous valve operation will be beneficial for the development of next-generation fuel cells.

Response of a planar solid oxide fuel cell to step load and inlet flow temperature changes

To explore the dynamic characteristics of the SOFC systems and to develop relevant control strategies, a previously developed steady state SOFC model is converted to a dynamic model. The model includes mass, momentum, thermal and electrochemical analysis, as well as the kinetic model of hydrocarbon reactions. Applying two control strategies i.e., cell constant fuel flow rate and constant fuel utilization during the transient time, the model is implemented to analyse the dynamic behaviour of a planar direct internal reforming (DIR) SOFC cell under several step-load changes. Transient response, resulting from an inlet temperature variation, is also investigated. The results show that the relaxation time is strongly related to the thermal behaviour of the cell and the applied control strategy. However, it is almost independent of the load variation magnitude.

Non-premixed acoustically perturbed swirling flame dynamics

An investigation into the response of non-premixed swirling flames to acoustic perturbations at various frequencies ( f p = 0–315 Hz) and swirl intensities ( S = 0.09 and 0.34) is carried out. Perturbations are generated using a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes | u′/U avg | in the 0.03–0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. Flame heat release rate is quantitatively measured using a photomultiplier with a 430 nm bandpass filter for observing CH∗ chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. The flame response is observed to exhibit a low-pass filter characteristic with minimal flame response beyond pulsing frequencies of 200 Hz. Flames at lower fuel flow rates are observed to remain attached to the central fuel pipe at all acoustic pulsing frequencies. PIV imaging of the associated isothermal fields show the amplification in flame aspect ratio is caused by the narrowing of the inner recirculation zone (IRZ). Good correlation is observed between the estimated flame surface area and the heat release rate signature at higher swirl intensity flame configurations. A flame response index analogous to the Rayleigh criterion in non-forced flames is used to assess the potential for a strong flame response at specific perturbation configurations and is found to be a good predictor of highly responsive modes. Phase conditioned analysis of the flame dynamics yield additional criteria in highly responsive modes to include the effective amplitude of velocity oscillations induced by the acoustic pulsing. In addition, highly responsive modes were characterized by velocity to heat release rate phase differences in the ± π/2 range. A final observed characteristic in highly responsive flames is a Strouhal number between 1 and 3.5 based on the burner co-flow annulus diameter ( St = f pU avg/d m ). Finally, wavelet analyses of heat release rate perturbations indicate highly responsive modes are characterized by sustained low frequency oscillations which accompany the high amplitude velocity perturbations at these modes. Higher intensity low frequency heat release rate oscillations are observed for lean flame/low pulsing frequency conditions.

Combustion and heat transfer at meso-scale with thermal recuperation

Abstract Results are presented from successfully designed and fabricated meso-scale ceramic combustors that incorporate internal thermal energy recirculation. The combustor provided sustained operation using propane and air as the reactants. Flames could be obtained well below the normal quenching distance. The development required examination of several different combustor designs and materials. Flammability limits of these combustors have been determined experimentally. Experimental investigations have been performed on the effects of flame holder geometry, material conductivity, equivalence ratio, and inlet Reynolds number on the combustor performance. Measurement of the reactant preheating and product exhaust temperatures was performed using K-type thermocouples which were installed with minimal intrusion to the flow. The reactant preheating temperatures were observed to be in the range 700 K–1000 K. However, the combustor suffered significant overall heat loss (50–85%) which was implied by the low exhaust temperatures (500 K–750 K). For a constant fuel flow rate, the exhaust temperature increased monotonously with decrease in equivalence ratio until the blow-off condition implying that the combustor’s maximum thermal efficiency occurs at its lean blow-off limit. Thermal imaging of the combustor walls was performed using infrared camera to obtain the temperature distribution within the combustor. Numerical simulations were performed with the aid of CFD software using a heat loss coefficient chosen so as to give best correlation with experimental results. These CFD simulations helped to obtain better insight of the dependence of combustor performance on thermal conductivity of the material and heat load.

Energy-Savings Modeling of Oil Pipelines That Use Drag-Reducing Additives

New data are presented for drag reduction in a 307 mm in inner diameter × 84 km in length commercial pipeline using 12−25 vppm of a drag-reducing additive (DRA) gel that induced 36−45% drag reduction (DR) in diesel fuel flowing at Reynolds numbers from 60 000 to 160 000. These data were correlated by Hinkebein’s method for scaling drag reduction onset and slope parameters on Prandtl−Karman coordinates to form a model that simulates oil flow with DRA in pipelines. Also, energy savings from DRA use were obtained in two ways: first by the power reduction at constant fuel flow rate and second from the increase in fuel flow rate at constant power. At conditions corresponding to a fuel flow rate of 320 m3/h, the use of 20 vppm DRA can provide either a 42.6% reduction in energy or a 43.0% increase in flow rate.

Effect of Optimization Criteria on Direct-Injection Homegeneous Charge Compression Ignition Gasoline Engine Performance and Emissions Using Fully Automated Experiments and Microgenetic Algorithms

Homogeneous charge compression ignition (HCCI) is a new low-emission engine concept. Combustion under homogeneous, low equivalence ratio conditions results in modest temperature combustion products, containing very low concentrations of NOx and PM as well as providing high thermal efficiency. However, this combustion mode can produce higher HC and CO emissions than those of conventional engines. Control of the start of combustion timing is difficult with pre-mixed charge HCCI. Accordingly, in the present study charge preparation and combustion phasing control is achieved with direct injection. An electronically controlled Caterpillar single-cylinder oil test engine (SCOTE), originally designed for heavy-duty diesel applications, was converted to a direct-injection gasoline engine. The engine features an electronically controlled low-pressure direct injection-gasoline (DI-G) injector with a 60 deg spray angle that is capable of multiple injections. The use of double injection was explored for emission control, and the engine was optimized using fully automated experiments and a microgenetic algorithm optimization code. The variables changed during the optimization include the intake air temperature, start of injection timing, and the split injection parameters (percent mass of fuel in each injection, dwell between the pulses) using three different objective (merit) functions. The engine performance and emissions were determined at 700 rev/min with a constant fuel flow rate at 10 MPa fuel injection pressure. The results show the choice of merit or objective function (optimization goal) determines the engine performance, and that significant emission reductions can be achieved with optimal injection strategies. Merit function formulations are presented that minimized PM, HC, and NOx emissions, respectively.

부분적 예혼합화염제어에 의한 연소 라디칼 및 NOx 배출물 특성

This paper presents an investigation on , CH, OH radicals and NOx emissions in partially premixed flames with acoustic excitation. The radicals are visualized by the digital image technique with optical filters and ICCD camera while NOx emissions are determined by a chemiluminescent detection(NOx analyser). The measurements are made in flames with an overall equivalence ratio 0.5 and a center tube equivalence ratio varing from 1.1 to 5.0 for a constant fuel flow rate. In the case of excitation, the visual shape of the flame is changed from laminar to turbulent-like flames. Images of , CH, and OH radicals resemble those of the flame appearances as the excitation phase is varied, and the radicals generated at the upstream are convected toward the downstream. It is inferred that the flame characteristics is affected by the flow characteristics of air-fuel mixture. In the case of acoustic excitation, OH radicals are much increased relative to unexcitation. From the radicals and flame visualization under acoustic excitation, the reduction of flame length affects the shorter residence time of center tube mixture, and significantly influences the NOx reduction.

Extinction Characteristics of Counterflow Diffusion Flame of n-Heptane Spray.

An experimental study was performed of the extinction characteristics of counterflow diffusion flames in which polydisperse n-heptane sprays carried by a nitrogen stream from the lower duct, burnt in oxidizer streams from the upper duct. The distance between the ducts was varied to vary the strain rate with constant fuel flow rate, so that the upstream spray characteristics were independent of the strain rate. The burning behavior of the spray is observed and the oxygen concentration at extinction was determined as a function of the strain rate. The flat blue diffusion flame was observed between two ducts, which is supported by vaporization of small droplets. Relatively large droplets penetrate the flat blue flame and continue to burn on the oxidizer side with an envelope flame surrounding each single droplet. The burning behaviors and extinction characteristics were discussed based on the characteristic time scales such as the droplet response time to the flow and the vaporization time.

Exhaust and In-Situ Measurements of Nitric Oxide for Laminar Partially Premixed C2H6-Air Flames: Effect of Premixing Level at Constant Fuel Flowrate

Exhaust and In-Situ Measurements of Nitric Oxide for Laminar Partially Premixed C₂H₆-Air Flames: Effect of Premixing Level at Constant Fuel Flowrate