Line Raman, Rayleigh, and laser-induced predissociation fluorescence technique for combustion with a tunable KrF excimer laser

We have applied a line UV Raman, Rayleigh, and laser-induced predissociation fluorescence technique for measurement of turbulent hydrocarbon flames. The species concentration of CO(2), O(2), CO, N(2), CH(4), H(2)O, OH, and H(2) and the temperature are measured instantaneously and simultaneously along a line of 11.4 mm, from which the gradients with respect to mixture fraction and spatial direction are obtained. The technique has been successfully tested in a laminar premixed stoichiometric methane flame and a laminar hydrogen diffusion flame. In addition the technique has been tested in a highly turbulent rich premixed methane flame. The data show that the technique can be used to provide instantaneous measurements of local profiles that describe the local flame structure in highly turbulent flames.     

Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition

This study quantifies the range of premixed flame conditions for which CH fluorescece diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios (phi) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at phi = 1.25 and phi = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as afactor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-MECH 3.0 chemistry. Sources experimental uncertainties are discussed.     

Phase-locked two-line OH planar laser-induced fluorescence thermometry in a pulsating gas turbine model combustor at atmospheric pressure

Two-line OH planar laser-induced fluorescence (PLIF) thermometry was applied to a swirling CH4/air flame in a gas turbine (GT) model combustor at atmospheric pressure, which exhibited self-excited combustion instability. The potential and limitations of the method are discussed with respect to applications in GT-like flames. A major drawback of using OH as a temperature indicator is that no temperature information can be obtained from regions where OH radicals are missing or present in insufficient concentration. The resulting bias in the average temperature is addressed and quantified for one operating condition by a comparison with results from laser Raman measurements applied in the same flame. Care was taken to minimize saturation effects by decreasing the spectral laser power density to a minimum while keeping an acceptable spatial resolution and signal-to-noise ratio. In order to correct for the influence of laser light attenuation, absorption measurements were performed on a single-shot basis and a correction procedure was applied. The accuracy was determined to 4%-7% depending on the location within the flame and on the temperature level. A GT model combustor with an optical combustion chamber is described, and phase-locked 2D temperature distributions from a pulsating flame are presented. The temperature variations during an oscillation cycle are specified, and the general flame behavior is described. Our main goals are the evaluation of the OH PLIF thermometry and the characterization of a pulsating GT-like flame.     

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.     

无机填料对环氧模塑料导热和阻燃性能的影响

采用两种无机填料Si₃N₄和Al(OH)₃ 复合填充环氧树脂制备了环氧模塑料(EMCs), 研究了两种填料用量及单独添加和复合添加对环氧模塑料导热性能和阻燃性能的影响。研究结果表明, 单独添加Si₃N₄或Al(OH)₃对环氧模塑料导热性能和阻燃性能的影响规律基本一致, 即随着填料含量的增加, 环氧模塑料的导热性能和阻燃性能均有不同程度的提高; 复合添加Si₃N₄和Al(OH)₃对环氧模塑料的导热性能和阻燃性能均起到积极作用, 但是随着填料中Si₃N₄与Al(OH)₃体积比的变化, 材料导热性能与阻燃性能会产生交叉耦合作用。当 填料中Si₃N₄与Al(OH)₃体积比为3∶2, 总体积分数为60%时, 环氧模塑料的导热率可以达到2.15 W/(m·K), 氧指数为53.5%, 垂直燃烧达到UL-94 V-0级。      

Two-dimensional imaging of soot volume fraction by the use of laser-induced incandescence

A recently developed laser-induced incandescence technique is used to make novel planar measurements of soot volume fraction within turbulent diffusion flames and droplet flames. The two-dimensional imaging technique is developed and assessed by systematic experiments in a coannular laminar diffusion flame, in which the soot characteristics have been well established. With a single point calibration procedure, agreement to within 10% was found between the values of soot volume fraction measured by this technique and those determined by conventional laser scattering-extinction methods in the flame. As a demonstration of the wide range of applicability of the technique, soot volume fraction images are also obtained from both turbulent ethene diffusion flames and from a freely falling droplet flame that burns the mixture of 75% benzene and 25% methanol. For the turbulent diffusion flames, approximately an 80% reduction in soot volume fraction was found when the Reynolds number of the fuel jet increased from 4000 to 8000. In the droplet flame case, the distribution of soot field was found to be similar to that observed in coannular laminar diffusion flames.     

Laser-induced predissociative fluorescence: dynamics and polarization and the effect of lower-state rotational energy transfer on quantitative diagnostics

Laser-induced predissociative fluorescence is often used for diagnostics because its short-lived upper states are minimally disturbed by collisions. We discuss the effects of lower-state collisions with parameters relevant to our atmospheric H(2)-O(2) flame. A pulse of tunable KrF excimer-laser light induces the A ? X, Q(1)(11), 3 ? 0 transition in OH. We measure the intensity and the polarization of the resulting A ? X, Q(1)(11), 3 ? 2 fluorescence as a function of laser brightness. A simple model that uses no adjustable parameters produces a reasonable fit to the data. It predicts that, even at very modest laser energies, the fluorescence intensity is almost directly proportional to the rate constant for rotational energy transfer (RET) within the lower vibrational state. That rate constant can be a strong function of local conditions. Furthermore, under typical operating conditions the excimer will pump an amount of OH out of the lower state that is many times as large as that originally present. This occurs because RET within the X-state continuously replenishes the lower state during the laser pulse. Even when this occurs, the signal may still vary linearly with laser intensity, and the polarization may be nearly that expected for weak pumping. At the higher laser intensities, a significant fraction of the measured OH arises from two-photon photodissociation of the water from the flame reaction.     

Measurements of OH mole fraction and temperature up to 20 kHz by using a diode-laser-based UV absorption sensor

Diode-laser-based sum-frequency generation of ultraviolet (UV) radiation at 313.5 nm was utilized for high-speed absorption measurements of OH mole fraction and temperature at rates up to 20 kHz. Sensor performance was characterized over a wide range of operating conditions in a 25.4 mm path-length, steady, C2H4-air diffusion flame through comparisons with coherent anti-Stokes Raman spectroscopy (CARS), planar laser-induced fluorescence (PLIF), and a two-dimensional numerical simulation with detailed chemical kinetics. Experimental uncertainties of 5% and 11% were achieved for measured temperatures and OH mole fractions, respectively, with standard deviations of < 3% at 20 kHz and an OH detection limit of < 1 part per million in a 1 m path length. After validation in a steady flame, high-speed diode-laser-based measurements of OH mole fraction and temperature were demonstrated for the first time in the unsteady exhaust of a liquid-fueled, swirl-stabilized combustor. Typical agreement of approximately 5% was achieved with CARS temperature measurements at various fuel/air ratios, and sensor precision was sufficient to capture oscillations of temperature and OH mole fraction for potential use with multiparameter control strategies in combustors of practical interest.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. I. A-X(0,0) excitation

Three different high-pressure flame measurement strategies for NO laser-induced fluorescence (LIF) with A-X(0,0) excitation have been studied previously with computational simulations and experiments in flames up to 15 bars. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. We investigate the NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three previous schemes and for two newly chosen excitation schemes with wavelength-resolved LIF measurements in premixed methane and air flames at pressures between 1 and 60 bars and a range of fuel/air ratios. In slightly lean flames with an equivalence ratio of 0.83 at 60 bars, the contribution of O2 LIF to the NO LIF signal varies between 8% and 29% for the previous schemes. The O2 interference is best suppressed with excitation at 226.03 nm.     

Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.     

Effects of Gravity on Soot Formation in a Coflow Laminar Methane/Air Diffusion Flame

Simulations of a laminar coflow methane/air diffusion flame at atmospheric pressure are conducted to gain better understanding of the effects of gravity on soot formation by using detailed gas-phase chemistry, complex thermal and transport properties coupled with a semiempirical two-equation soot model and a nongray radiation model. Soot oxidation by O₂, OH and O was considered. Thermal radiation was calculated using the discrete ordinate method coupled with a statistical narrow-band correlated-K model. The spectral absorption coefficient of soot was obtained by Rayleigh’s theory for small particles. The results show that the peak temperature decreases with the decrease of the gravity level. The peak soot volume fraction in microgravity is about twice of that in normal gravity under the present conditions. The numerical results agree very well with available experimental results. The predicted results also show that gravity affects the location and intensity for soot nucleation and surface growth.     

玉米淀粉粉尘爆炸特性及火焰传播过程的试验研究?

为了全面地认识玉米淀粉粉尘爆炸的敏感性和爆炸破坏效应,分别采用粉尘云着火温度装置、20 L球粉尘爆炸装置和粉尘云火焰传播装置对玉米淀粉的粉尘云着火温度、爆炸下限质量浓度、爆炸压力、爆炸氧极限浓度以及粉尘云火焰传播过程进行了研究。结果表明:玉米淀粉粉尘云最低着火温度在380~390℃之间;粉尘云爆炸氧极限浓度(体积分数)在10%~11%之间;爆炸下限质量浓度和最大爆炸压力随着化学点火具质量的增加而呈现出不同的变化特征,随着化学点火具质量的增加,玉米淀粉的爆炸下限质量浓度逐渐降低,而玉米淀粉爆炸压力逐渐升高。在不同的粉尘质量浓度条件下,粉尘云火焰传播速度和火焰温度有一定的变化,在粉尘质量浓度为500 g/m3时,火焰传播速度和火焰温度均达到最大值,分别为13.81 m/s和1 107℃。     

Two-point time-series measurementsof hydroxyl concentration in a turbulent nonpremixed flame

Quantitative two-point hydroxyl time-series measurements have been performed in a turbulent nonpremixed flame by using two-point picosecond time-resolved laser-induced fluorescence. The current diagnostic system has been improved from its preliminary version to address optical aberrations and fluorescence lifetime fluctuations. In particular, with a newly designed collection system, the aberration-limited blur spot is reduced from 6 mm to 180 microm. Additional photon-counting channels enable the recovery of absolute OH concentrations through a triple-bin integration method. The present research represents what we believe to be the first application of this two-point technique to turbulent flames. Two independent schemes have been applied to remove uncorrelated noise in the derived two-point statistics. We show that optical aberrations can have a significant effect on space-time correlations. However, the sampling rate and fluctuations in the fluorescence lifetime barely affect the spatial autocorrelation function and thus the integral length scale. Such length scales for hydroxyl are found to rise linearly with increasing axial distance at peak [OH] locations. Along the jet centerline, the integral length scale varies slightly below the flame tip but increases rapidly above the flame tip. In addition, the OH length scale demonstrates the same trend as the OH time scale along the jet centerline, but the opposite trend at peak [OH] locations.     

聚甲醛阻燃的研究

选用氢氧化镁(Mg(OH)2)、聚磷酸铵(APP)作为聚甲醛(POM)的阻燃剂,研究了它们对POM阻燃效果及力学性能的影响。结果表明,加入Mg(OH)2阻燃剂后,POM的阻燃性能有较大的提高,当Mg(OH)2的加入量为60％时．极限氧指数由15提高到40．水平燃烧速度由0．33mm／s降至0．31mm／s。APP阻燃POM的效果优于Mg(OH)2,在APP加入量达到25％时即制得自熄的POM。两种阻燃剂的加入都使POM的力学性能下降。     

Background corrections for laser-induced-fluorescence measurements of nitric oxide in lean, high-pressure, premixed methane flames

An experimental technique is presented that both minimizes and accounts for the interference background when laser-induced-fluorescence (LIF) measurements are made of NO in lean, high-pressure, premixed, CH(4)/O(2)/N(2) flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. O(2) fluorescence interferences are particularly troublesome in lean premixed flames. An excitation-detection scheme that minimizes the effects of these interferences is identified. A procedure that corrects the resulting LIF signal so as to account for any remaining interference signal is then developed. This correction is found to vary from less than 10% of the overall NO signal at atmospheric pressure to over 40% of the overall signal at 14.6 atm for LIF measurements of NO in a series of worst-case flames (phi = 0.6, dilution ratio 2.2). The correction technique is further demonstrated to be portable over a useful range of flame conditions at each pressure.     

Analysis of laser-induced-fluorescence carbon monoxide measurements in turbulent nonpremixed flames

The influence of fluctuating concentrations and temperature on the laser-induced-fluorescence (LIF) measurement of CO in turbulent flames is described, under conditions in which the fluorescence and the temperature are measured independently. The analysis shows that correlations between CO concentration and temperature can bias the averaged mole fraction extracted from LIF measurements. The magnitude of the bias can exceed the order of the average CO mole fraction. Further, LIF measurements of CO concentrations in a turbulent, nonpremixed, natural gas flame are described. The averaged CO mole fractions are derived from the fluorescence measurements by the use of flame temperatures independently measured by coherent anti-Stokes Raman spectroscopy. Analysis of the fluctuations in measured temperature and fluorescence indicates that temperature and CO concentrations in flame regions with intensive mixing are indeed correlated. In the flame regions where burnout of CO has ceased, the LIF measurements of the CO mole fraction correspond to the probe measurements in exhaust.     

Crank-angle-resolved laser-induced fluorescence imaging of NO in a spark-ignition engine at 248 nm and correlations to flame front propagation and pressure release

Inside the combustion chamber of a spark-ignition engine, NO fluorescence is excited with a narrow-band tunable KrF excimer laser. The fluorescence light is detected by an intensified CCD camera that yields images of the NO distributions. Rotational-vibrational transitions of NO are excited by the A(2)Σ+ ? X(2)Π (0, 2) band system around 248 nm. Single laser shot planar NO distributions are obtained with good signal-to-noise ratio at all crank angles and allow us to locate areas of NO formation during combustion. The pressure within the combustion chamber is measured simultaneously with the NO distributions, which allows the evaluation of correlations between indicated work and NO formation. The crank-angle-resolved sequences of two-dimensional NO distributions and averaged pressure traces are presented for different engine-operating conditions. In addition, laser-induced predissociation fluorescence of OH excited by the same laser source is measured in order to visualize the corresponding flame front propagation and to compare the time of formation of NO relative to that of OH.     

Mechanical properties and flame-retardant of PP/MRP/Mg(OH)2/Al(OH)3 composites

The flame retardant and mechanical properties of polypropylene (PP) composites filled with microencapsulated red phosphorus (MRP) and magnesium hydrate (Mg(OH)₂)/aluminum hydrate (Al(OH)₃) were measured. It was found that the synergistic effects between the MRP and Mg(OH)₂/Al(OH)₃ on the flame retardant and tensile properties of the composites were significant. The limit oxygen index and smoke density rank of the composites increased nonlinearly while the horizontal combustibility rate decreased nonlinearly with increasing the MRP weight fraction. The Young modulus and the tensile elongation at break increased while the tensile yield strength and tensile fracture strength decreased slightly with increasing the MRP weight fraction. Both the V-notched Izod and Charpy impact strength increased with increasing the MRP weight fraction. Moreover, the tensile yield strength of the composites estimated using an equation published previously was roughly close to the measured data.     

Thermally assisted fluorescence of laser-excited OH A(2)Σ(+) as a flame diagnostic tool

A simple laser-induced-fluorescence measurement technique for turbulent flame temperature and OH concentration measurement is proposed and successfully tested. The main idea is that a narrow-band tunable excimer laser beam (λ = 308 nm) is focused into a turbulent atmospheric-pressure nonpremixed flame. The OH molecule Q(1)(3) (X(2)II υ″ = 0, A(2)Σ+υ″ = 0) transition is excited. By subsequent vibrational energy transfer, the distribution in the molecules' upper electronic energy level (excited Σ(+) state) is redistributed. By evaluating the spectrum of the broadband emission from the υ' = 1 ? υ″ = 0 and υ' = 2 ? υ″ = 1 bands with a full spectral fit, we were able to evaluate the temperature and the OH molecule density. The concurrent processes of quenching, vibrational energy transfer, and rotational energy transfer were taken into account in the evaluation process. The results were compared with numerical flame calculations and revealed good agreement. One problem with this new proposed application of laser-induced fluorescence is the self-absorption of the emitted light. This, however, is shown not to be serious, but it has to be checked carefully. The main advantages are a simple experimental setup and procedure, high signal intensity, and a simple and straightforward data evaluation method. Thus this measurement technique is suitable for turbulent flame temperature and OH concentration measurement, and it is an alternative to other well-established techniques that are much more complicated.     

初始湍流对粉尘爆炸影响的实验研究

采用大型实验装置对管道相连的粉尘操作设备中粉尘爆炸的火焰和压力传播过程及其影响因素进行实验研究。实验装置由两个通过管道连接的不同体积容器构成,爆炸从一个容器中通过管道传播到另一个容器中。在不同的初始湍流度条件下进行粉尘爆炸实验,测试不同位置的火焰和压力信号。结果表明:随着初始湍流度的增大,爆炸的猛烈程度增强,火焰和冲击波的传播速度加快,初始湍流作为影响粉尘爆炸发展过程的重要因素之一,在进行粉尘爆炸的安全防护和设计时必须考虑其作用。