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Numerical simulations were performed to model the non-reacting and reacting flow behind a rearward step flameholder in Mach 1.6 supersonic flow with fuel injection at the step base. The combustor geometry was based on the University of Florida scramjet experimental facility. Turbulence was modeled using k-ω shear stress transport (SST), laminar flamelet was used for combustion modeling. Wall static pressure showed good agreement with experimental data for non-reacting and reacting flow. For non-reacting flow, dummy fuel helium mole fraction distribution in the recirculation region behind the step was validated with planar laser induced fluorescence (PLIF) images in experiments. To improve the combustion characteristics, air was injected in tandem with hydrogen at step base using various configurations. With all fuel injection as baseline, the case with 2 air jets around each fuel jet and air injected at 2 times the stagnation pressure of fuel showed the most improvement compared to other cases. It was most effective in reducing the local fuel richness, shortening the flame length and increasing combustion efficiency.

Rotating detonation combustors (RDCs) are studied with significant interest in recent years, due to their promulgated benefits over other pressure gain combustion systems. Although notable strides ...

This manuscript presents a study of hydrogen-enhanced methane flames in a compact mesoscale burner array, where even a minute addition of hydrogen can significantly improve flame stability with reduced combustion length scale thereby enabling volumetric flexibility in small-scale combustion systems. The effect of hydrogen enhancement is demonstrated through combustion reaction length scale and operation limit analysis using methane fuel. The added hydrogen lowered lean blow off equivalence ratios, increased flame temperatures, and shorten the flame length scale, ultimately producing a denser flame array. 10 kHz OH∗ chemiluminescence and OH planar laser induced fluorescence imaging techniques were used to estimate heat release rate and visualize flame structures. Hydrogen addition increased the OH intensity and decreased global heat release rate fluctuation, and also showed more stable operation under acoustic perturbations. Hydrogen enhancement can be a promising solution for reducing geometric constraints and improving operating capabilities for compact propulsion and power generation systems.

Fundamental understanding of chemical kinetics pathways involved in ignition and flame propagation in hydrocarbon flames is essential for designing efficient and clean combustion devices for aerosp...

This work demonstrates single-shot, two-dimensional temperature measurements in a premixed linear detonation channel using two-color OH planar laser-induced fluorescence thermometry. Detonation environments result in extreme thermodynamic conditions which create challenges in the resulting spectroscopic behavior. However, thermometry based on the ratio of the P1(9)/Q1(14) transitions within the A2Σ+←X2Π(1,0) band of OH was determined to be well-suited for detonation environments based on the spectral characteristics over a wide range of conditions. The technique is demonstrated in the post-induction region, focusing on the nominal equilibrium state of the Chapman-Jouguet conditions, on mixtures of stoichiometric H2 and O2 diluted with either N2 or Ar. High-speed chemiluminescence imaging at 2 MHz was used to assess the qualitative dynamics of the detonation structure. The measured temperature fields for the Ar dilution case are relatively spatially uniform, with the distributions mean agreeing well with the theoretical calculation of the Chapman-Jouguet temperature, consistent with preliminary work [Grib et al., AIAA SciTech Forum, (2021). 2021-0421]. Conversely, the temperature fields for a 50% N2 dilution case are highly irregular, showing a larger dynamic range in temperature as well as pockets of unburned reactants, which emphasized the significance of this measurement by highlighting various reaction scenarios dictated by detonation waves. The irregularity of the N2 dilution cases is explained in terms of the scale similarity between the channel size and the detonation cell sizes for the N2 dilution mixtures, leading to weak or failing detonation modes. Overall, the demonstrated thermometry technique produced a precision (1-σ) of approximately 4% in atmospheric conditions and approximately 7.7% in strong detonation environments. In addition, the accuracy, defined as the percent difference between the mean and CJ temperature, was approximately 1% in the Ar diluted case. The ability to distinguish and quantify detonation burning behavior is promising for employing this approach for application in pressure-gain combustion facilities such as rotating detonation combustors, however care needs to be taken when interpreting the resulting temperature field, particularly near the wave front, due to the precision and validation range of the present line pair.

Ammonia, made up of 17.8% hydrogen, has attracted a lot of attention in combustion community due to its zero carbon emission as a fuel in gas turbines. However, ammonia combustion still faces some challenges including the weak combustion and sharp NOx emissions which discourage its application. It was demonstrated that the combustion intensity of ammonia/air flame can be enhanced through adding active fuels like methane and hydrogen, while the NOx emission issue will emerge in the meantime. This study investigates regulation effect of methane and hydrogen on the emission characteristics of ammonia/air flame in a gas turbine combustor. The instantaneous OH profile and global emissions at the combustion chamber outlet are measured with Planar Laser Induced Fluorescence (PLIF) technique and the Fourier Transform Infrared (FTIR), respectively. The flames are also simulated by large eddy simulation to further reveal physical and chemical processes of the emissions formation. Results show that for NH3/air flames, the emissions behavior of the gas turbine combustor is similar to the calculated one-dimensional flames. Moreover, the NOx emissions and the unburned NH3 can be simultaneously controlled to a proper value at the equivalence ratio (φ) of approximate 1.1. The variation of NO and NO2 with φ for NH3/H2/air flames and NH3/CH4/air flames at blending ratio (Zf) of 0.1 are similar to the NH3/air flames, with the peak moving towards rich condition. This indicates that the NH3/air flame can be regulated through adding a small amount of active fuels without increasing the NOx emission level. However, when Zf = 0.3, we observe a clear large NOx emission and CO for NH3/CH4/air flames, indicating H2 is a better choice on the emission control. The LES results show that NO and OH radicals exhibit a general positive correlation. And the temperature plays a secondary role in promoting NOx formation comparing with CH4/air flame.

The flame front of a premixed ethylene–air turbulent flame in a high-speed flowfield was imaged using hydroxyl radical (OH) planar laser-induced fluorescence (PLIF). An electrically heated continuo...

The planar laser induced fluorescence (PLIF) technique is applied to investigate the flow and liquid mixing characteristics in submerged top-blow process. The liquid mixing effects are analyzed in terms of plane mixing uniformity U(t) and mixing time t95 at different air flow rate and lance submergence depth. Based on experimental data, the liquid mixing process has been divided into three stages by mixing uniformity U, the beginning stage (0.9 < U < 1) , the interim stage (0.1 < U < 0.9) and the end stage (0 < U < 0.1), the value of U drops slowly in the beginning stage and the end stage, but drops rapidly in the interim stage. The mixing time t95 is decreased obviously when increasing the air flow rate and lance submergence depth. There are significant local mixing differences in the liquid mixing process when the lance submergence depth is small. The local difference decreases evidently as the air flow rate and the lance submergence depth increase. The local differences are gradually eliminated when the lance approaches the container bottom.

The use of duckbill valves in submerged outfalls for dense effluents is now prevalent to prevent the intrusion of ambient water into the outfall pipe during periods of low production of the facility. In the present study, an experimental investigation was conducted using the technique of Planar Laser Induced Fluorescence (PLIF) to examine the mixing characteristics of 45° inclined dense jets from a duckbill shape nozzle in co-flowing currents. The measured characteristics included the jet trajectory, concentration decay, centreline peak dilution and cross-sectional concentration distribution. The Froude number ranged from 10 to 25, and the nominal Froude number from 0.2 to 3.0. Two mounting orientations of the duckbill nozzle, namely vertical and horizontal, were also investigated. The experimental results showed that two regimes, namely dense-jet- and co-flow-dominated, can be clearly observed. When the co-flow velocity increases, the centreline peak height and dilution of the inclined dense jet decrease. Comparing to the available results in the literature for inclined circular dense jets, the centreline peak dilution for inclined duckbill dense jets is generally higher. In other words, the installation of the duckbill valve also contributes to the mixing of the dense effluent with surrounding ambient waters, in addition to preventing the ambient intrusion. With respect to the mounting orientation, the horizontal induces a relatively higher dilution but also a larger centreline peak height. Thus, the horizontal orientation would be preferable when the cover water depth is sufficient, while the vertical orientation can be considered in shallow waters for full submergence. Nanyang Technological University The authors would like to thank the Nanyang Environment and Water Research Institute, Nanyang Technological University, as well as the National Natural Science Foundation, China (42006143) for the financial support.

Mixing is an inseparable part of many industries. Static mixers are motionless inserts that are widely used for mixing. In this study, planar laser-induced fluorescence (PLIF) was used to study the mixing characteristics of a multilayer static mixer. The effects of the arrangement angle of layer elements of the static mixer were examined. Arrangement angles of 0°, 45°, and 90° were investigated at four different flow-rate ratios of two mixing fluids. The mixing performance of each angle was evaluated with both qualitative and quantitative approaches. The volume fractions of the flows were obtained from the PLIF results, and the mixing index was calculated. The friction factor was measured to assess the pressure loss of each arrangement angle. The results show that the arrangement angles do not affect the mixing index for higher flow ratios. However, at the lowest flow ratio, the angle of 45° resulted in the highest mixing index of 74.9%, while at 0° and 90°, the values reached 66.4% and 69.8%, respectively. In regard to pressure loss, 45° also resulted in the lowest friction factor.

This work uses an annular co-flow spray burner as a laboratory test method for evaluating one of the important jet fuel figures of merit, namely lean blowout (LBO), and investigates the coupling of physical and chemical properties affecting this phenomenon. Previous work has shown that relative trends in LBO from different fuels in a real gas turbine are well reflected in this burner setup. Representative characteristics for fuel heat of combustion, volatility, atomization, and pressure are used to produce correlations predicting the LBO for this spray burner, thus these characteristics are considered important toward the prediction of LBO in real gas turbine combustors. The results of this testing are compared to general relations for spray flames as published in prior literature; it is found that equivalence ratio at LBO is positively correlated to fuel flow rates and effective evaporation, and inversely correlated to combustion pressure, heat of combustion, and spray droplet diameter. Using simultaneous OH Planar Laser Induced Fluorescence and Mie scattering imaging of the spray and flame structure, the liquid loading in the flame region is also analyzed for surrogate jet fuels which have comparable derived cetane number (DCN) but have very different compositions. Characteristics that can affect the amount of liquid fuel entering the flame region in near-LBO conditions are discussed. The use of surrogates allows for discussion of the competing effects of volatility and reactivity of fuel components. It is found that the most stable flames occur when the interplay of fuel atomization/volatility and reactivity result in substantial penetration of fuel droplets containing components with high DCN.

The catalytic and gas-phase combustion of fuel-lean H2/CO/O2/N2 mixtures over palladium was investigated experimentally and numerically at a global equivalence ratio φ = 0.285, H2:CO volumetric ratios 1-4, pressures 1-8 bar and catalyst surface temperatures 950-1200 K. In situ planar laser induced fluorescence (PLIF) of the OH radical monitored homogeneous combustion inside a channel-flow catalytic reactor, while 1-D Raman measurements of main gas-phase species concentrations across the channel boundary layer assessed the heterogeneous processes. Simulations were carried out with a 2-D numerical code using detailed heterogeneous and homogeneous chemical reaction mechanisms and realistic transport. The simulated and measured transverse species profiles attested to a transport-limited catalytic conversion of H2 and CO at all operating conditions. The OH-PLIF measurements and the simulations confirmed the establishment of appreciable homogeneous combustion only for p

This paper reports on an experimental investigation of the chemical impact of nanosecond repetitively pulsed (NRP) glow discharges on a laminar methane-air flame. The chosen configuration was a lean wall stabilized flame where NRP discharges were generated across the flame front. After careful selection of the excitation lines, planar laser induced fluorescence of OH and CH was conducted. Comparisons between the OH and CH fluorescence of a base flame (without plasma actuation), and those obtained during the steady state and the transient regimes of plasma actuation, were performed. First it is shown that during the steady state regime, the intensity of OH and CH fluorescence in the flame could be increased by up to 40% and 10%, respectively. In addition, the life time of OH fluorescence in the discharge channel was estimated to be between 3 and 4.5 µs. The transient regime at the beginning of plasma actuation showed that the flame began to be affected by the discharges long before OH fluorescence could be detected in the discharge channel, upstream of the flame. After 40 ms of plasma actuation, OH intensity began to increase simultaneously in both the flame and the discharge area. Based on current knowledge of nanosecond discharge chemistry, explanations for these results are proposed.

A premixed H2/air flame with N2 dilution (Uin = 533 cm/s, ϕ = 1) was formed in a quartz micro flow reactor with/without a 100-nm thick Inconel coating for the investigation of wall chemical effect of the metal surface. Two-dimensional distributions of OH radical, O atom and H atom in the hydrogen flame were measured via the planar laser induced fluorescence (PLIF) and two-photon absorption laser induced fluorescence (TALIF), respectively. It is found that the distributions of all these three main species in the hydrogen flame are significantly affected by the wall chemical effect. OH, O and H shift downstream in the Inconel-coated channel, and also their concentration becomes lower than those in the less-reactive quartz channel. Based on the measured distributions of OH, O and H over the Inconel surface, the initial sticking coefficients (S0) of the radical quenching model are optimized. It is found that S0 for Inconel are 0.4–0.5, 0.1–0.2 and

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH2O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – “stage 1” characterized by local extinction, where the overall flame and flow field remain largely unchanged, and “stage 2”, characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH2O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH2O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame.

Pyrolysis provides a route for the conversion of lignocellulosic biomass into solid, liquid and gaseous energy vectors or platform chemicals. Polycyclic aromatic hydrocarbons (PAHs) generated in the vapour phase of the biomass pyrolytic reaction may condense to form tars, which are difficult to further upgrade and cause process inefficiency. Control of tar production requires optimization of reactor design and careful control of reactor operating conditions. In this study, a vertical resistively-heated fixed-bed reactor is used to study the effect of pyrolysis peak temperatures and holding period (at peak temperature) on the formation of PAHs during pyrolysis of two lignocellulosic biomass samples, walnut shells (WS) and almond shells (AS). ‘In situ’ planar laser induced fluorescence (PLIF) is used to optically detect 3-to-5 ring PAHs in the vapour phase immediately above the reactor bed. Results show that the PAH PLIF signal appeared at ∼275 °C for the biomass samples and peaked at ∼400 °C for WS and ∼375 °C for AS, which is in agreement with previous ‘off situ’ analysis conducted by the authors. Beyond 400 °C, the PLIF signal was observed to reduce significantly and almost disappear at 550 °C. Initial PAH formation was attributed to condensation reactions occurring due to the drop in temperature along the sample bed. The detection of the PAH PLIF signal itself and its changing intensities, close to the bed, signified the rapid changes the products released from biomass undergo and emphasised the importance of using online techniques for pyrolysis studies. The detailed understanding of the temperature dependent characteristics of PAH formation from this study could help improve reactor design.

The heterogeneous and homogeneous combustion of C3H8/O2/N2 mixtures over Rh was investigated at pressures 1-6 bar, catalyst surface temperatures 680-1100 K and C3H8-to-O2 equivalence ratios 0.25-0.52. Non-intrusive laser-based measurements were applied in a channel-flow catalytic reactor and involved 1-D Raman spectroscopy of major gas-phase species across the channel boundary layer for assessing the catalytic reactivity and planar laser induced fluorescence (PLIF) of the OH radical for monitoring homogeneous combustion. Simulations were carried out with a 2-D numerical code that included detailed hetero-/homogeneous chemical reaction mechanisms. By comparing the Raman-measured and predicted transverse profiles of the limiting C3H8 reactant, the suitability of a detailed surface reaction mechanism was initially evaluated and subsequently a one-step reaction was constructed, which was applicable for the C3H8 total oxidation over Rh at pressures 1 to 6 bar. The catalytic reactivity of C3H8 over Rh displayed a ∼p0.14 pressure dependence, which was substantially lower than a previously reported ∼p0.70 dependence over Pt. The weak pressure dependence of the C3H8 reactivity on Rh suggested caution when selecting catalysts for high-pressure power systems (recuperative microreactors, small-scale turbines) fueled with C3H8 or LPG (liquefied petroleum gas). Comparisons of PLIF-measured and predicted distributions of the OH radical indicated that the employed gas-phase reaction mechanism captured the onset of homogeneous ignition at pressures greater than or equal to 3 bar as well as the ensuing flame shapes. Predicted and measured homogeneous ignition distances agreed within 2.5% at 6 bar. With decreasing pressure, the predictions yielded gradually increasing but still modest underpredictions (up to 11.2% at 3 bar) of the homogeneous ignition distances. The key gas-phase reactions affecting homogeneous combustion at various pressures were finally identified.

This paper examines the scram/dual-mode combustion limits of hydrocabon fuels within a Mach 8, scramjet combustor. Flight-equivalent flows were delivered to the axisymmetric, cavity combustor via a reflected shock tunnel. Two scramjet fuels were examined: ethylene and a surrogate mixture representing endothermically cracked n-dodecane. Combustion modes were examined via static pressure sensors and through both chemiluminescence imaging, and planar laser induced fluorescence (PLIF) of the OH combustion radical in the combustor exhaust plume. Ethylene-fuelled experiments developed scram-mode combustion under reduced fuelling conditions, experiencing shock wave dominated flowfields. OH PLIF diagnostics indicated such combustion modes developed a ring-like structure of combustion products, primarily axisymmetrically adjacent to the combustor wall. Increased fuelling anchored combustion downstream of the fuel injector, while further increases instigated dual-mode combustion. In this mode, subsonic combustion regions combine with the supersonic coreflow to permit the transfer of information upstream with substantially increased pressure encountered. Optical diagnostics indicate broadly asymmetric, unsteady combustion features. The surrogate mixture representing endothermically cracked n-dodecane experienced rapid onset from no-combustion (optically confirmed) to fully developed dual-mode combustion at critical fuelling rates. OH PLIF signals and chemiluminescence of this fuel were weaker than comparable ethylene cases, indicating potential differences in combustion pathways.

Shock tunnel experiments are presented that examine combustion modes of ethylene and a 64/36% ethylene/methane mixture under Mach 8 flight-representative conditions within an axisymmetric scramjet ...

The paper presents the results of an experimental and numerical study on the combustion characteristics of a supersonic combustor operation enhanced by pilot hydrogen and air throttling, including ...

Flame behavior at conditions approaching boundary layer flashback was studied in fuel-lean premixed swirl flames with a central bluff-body flame holder. High-speed OH* chemiluminescence images were...

This experimental study explored the effects of axisymmetric sinusoidal excitation on structural and mixing characteristics in the equidensity jet in crossflow (JICF). Planar laser-induced fluoresc...

A 1/17th water physical model of a 200-ton steel ladle furnace with a single gas injection was used to simulate bath heating using a single burner to mimic the heat flux due to electric arcs in the industrial steel ladle. Two phases were considered, using water to simulate the molten steel and air to simulate the argon injection at a flow rate of 1.54 NL min−1. The planar Laser-Induced Fluorescence (PLIF) technique was for the first time experimentally implemented to measure temperature fields in a longitudinal plane of the gas-stirred ladle model. PLIF employs a laser source of 532-nm wavelength to light water seeded with rhodamine B, which emits fluorescence depending on its temperature, after a complex calibration is made. Next, the fluorescence is captured by a camera with a 550-nm wavelength filter. The PLIF measurements were validated by local thermocouple measurements at five different locations in the measurement plane. Temperature fields measured by PLIF are in good agreement with those obtained locally by thermocouples, so the PLIF technique can be used to measure temperature fields with the advantage of getting a complete temperature contour field, in contrast to point values of temperatures with thermocouples. Experiments were carried out to study the thermal mixing for two common tuyere positions, i.e., axisymmetric and eccentric (mid-radius) positions. Results on the injection mode show that axisymmetric gas injection is a more efficient heat transfer configuration between the burner and the liquid phase than is the symmetric injection mode for the particular heating configuration studied in this work.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2020-04-17, 資料番号: PA2120004000

This paper investigated the hydrogen enriched methane/air flames diluted with CO2. The turbulent premixed flame was stabilized on a Bunsen type burner and the two dimensional instantaneous OH profile was measured by Planar Laser Induced Fluorescence (PLIF). The flame front structure characteristics were obtained by extracting the flame front from OH-PLIF images. And the turbulence-flame interaction was analyzed through the statistic parameters. The role of hydrogen addition as well as CO2 dilution on the features of turbulent flame were revealed by those parameters. In this work, hydrogen fractions of 0, 0.2 and CO2 dilution ratios of 0, 0.05 and 0.1 were studied. Results showed that hydrogen addition can enhance turbulent burning velocity ST/SL through decreasing the scale of the finer structure of the wrinkled flame front, caused by the smaller flame instability scale. In contrast, CO2 dilution decreased turbulent burning velocity ST/SL due to its inactive response to turbulence perturbation and larger flame wrinkles. For all flames, the probability density function (PDF) profile of the local curvature radius R shows a bias to positive value, resulted from the flame intrinsic instability. The PDF profile of R decreases with CO2 dilution, while the value of local curvature radius corresponding to the peak PDF is larger. This indicates that larger wrinkles structure was generated due to CO2 dilution, which leads to the decrease in ST/SL as a consequence. Hydrogen addition increases the flame volume and results in more intense combustion. CO2 dilution has a decrease effect on flame volume for both XH2 = 0 and XH2 = 0.2 while the decrease is obvious at XH2 = 0.2, ZCO2 = 0.1. In all, hydrogen enrichment improves the combustion while CO2 can moderate combustion. Therefore, adding hydrogen and CO2 in natural gas can be a potential method for adjusting the combustion intensity in combustion chamber during the combustor design.

In the near future, lean-premixed hydrogen combustion technology for low-emission gas turbine engines is expected to be crucial in the effort to mitigate greenhouse gas emissions, in association with energy system decarbonization. In this study, we examine combustion instabilities in lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array; little is currently known about how these flames respond to acoustic perturbations. Several measurement techniques, including phase-synchronized OH* chemiluminescence imaging, OH Planar Laser Induced Fluorescence, acoustic pressure, and the two microphone method, are used, together with reduced-order acoustic modeling, to identify the key physical properties of lean-premixed hydrogen-air flames, in comparison with lean-premixed methane-air flames. We show that extremely compact lean-premixed hydrogen flames are preferentially coupled to higher eigenmodes of a given system, L3 – L6, including approximately 1 kHz high-frequency instabilities, while the instabilities of methane-air flames are predominantly limited to the first longitudinal mode under the same range of operating conditions. This is attributed to the fact that the dynamics of the methane flames are governed by the collective motion of constituent flames, involving a complex process of the emergence, convection, and interaction of large-scale structures. By contrast, the hydrogen flames in the multinozzle configuration oscillate in isolation within a very short distance and without strong flame-to-flame interactions. This is particularly suitable for accommodating high-frequency heat release modulations. The triggering of intense sound generation from lean-premixed mesoscale hydrogen flames is correlated strongly with a combination of flame surface destruction due to front merging and the flames’ close proximity to a pressure antinode. These results, for the first time, highlight the key features of self-excited combustion instabilities of mesoscale multinozzle hydrogen-air flames in a well-controlled laboratory-scale experiment, and could pave the way for future carbon-neutral gas turbine combustion technology.

This paper studies the effect of internozzle spacing on the lean blow-off of a linear combustor array. The combustor’s lean operability limit did not have a monotonic relationship with internozzle ...

Turbulent combustion remains an unsolved problem, and the simulation models that have been developed, although accurate for subsonic turbulent combustion, have limited accuracy when applied to supe...

The paper present experimental results for temperature fields in heterogeneous droplets of liquids, freely falling in a preheated gaseous medium, before their micro-explosive fragmentation. The optical technique of Planar Laser Induced Fluorescence was applied. Unlike the known results of other authors, such fields were obtained for the first time without using holders for droplets, the latter heated in the temperature range of 900 $$^{\circ}$$ C to 1100 $$^{\circ}$$ C. The aim was to approach promising gas-vapor-droplet technologies. It is shown that for micro-explosive disintegration of a droplet containing a flammable component (by the example of Diesel and oils of two grades) and water, the necessary and sufficient condition is that the water reaches the boiling point. In this case, local superheating of the inter-component interface is sufficient. Motion intensifies the transformation of droplet and its subsequent destruction with appearance of polydisperse cloud.

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities Uj and co-flow velocities Uc were examined. Data show that flames with Uc = 0.6 m/s stabilize much further away from the nozzle than those with Uc = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for Uc = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with Uc = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For Uc = 0.6 m/s and relatively large bulk jet velocities (Uj = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of Uj = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and Uc = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.

This paper reports on the effects of fuel and equivalence ratio on the response of lean premixed swirl flames to acoustic perturbations of the flow, at atmospheric pressure. The response is analyzed using flame transfer functions, which relate the relative heat release rate fluctuations from the flame to the relative velocity fluctuations of the incoming flow. Two fuels, propane and methane, and five equivalence ratios are considered. The 10 flames investigated are selected to exhibit the local maximum of the transfer function gain around the same frequency, 176 Hz. The results show that changing fuel and equivalence ratio influences both the gain and the phase of the transfer function. The changes observed at 176 Hz, where the dynamics of the flame is mainly controlled by the flame vortex roll-up mechanism, are discussed. Based on the analysis of the flowfields and the flame wrinkling, the laminar burning velocity and the flame temperature are identified as the main parameters controlling the gain. They have two competing effects: first, by enhancing the flame vortex roll-up and, second, by affecting the strength of the vortex generated by the acoustic forcing due to changes in the height of the flame stabilization location. Copyright © 2019 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

The present paper reports on the combined stereoscopic particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements of turbulent transport for model swirl burners without combustion. Two flow types were considered, namely the mixing of a free jet with surrounding air for different swirl rates of the jet (Re = 5 × 103) and the mixing of a pilot jet (Re = 2 × 104) with a high-swirl co-flow of a generic gas turbine burner (Re = 3 × 104). The measured spatial distributions of the turbulent Reynolds stresses and fluxes were compared with their predictions by gradient turbulent transport models. The local values of the turbulent viscosity and turbulent diffusivity coefficients were evaluated based on Boussinesq’s and gradient diffusion hypotheses. The studied flows with high swirl were characterized by a vortex core breakdown and intensive coherent flow fluctuations associated with large-scale vortex structures. Therefore, the contribution of the coherent flow fluctuations to the turbulent transport was evaluated based on proper orthogonal decomposition (POD). The turbulent viscosity and diffusion coefficients were also evaluated for the stochastic (residual) component of the velocity fluctuations. The high-swirl flows with vortex breakdown for the free jet and for the combustion chamber were characterized by intensive turbulent fluctuations, which contributed substantially to the local turbulent transport of mass and momentum. Moreover, the high-swirl flows were characterized by counter-gradient transport for one Reynolds shear stress component near the jet axis and in the outer region of the mixing layer.

The in-gas-jet laser spectroscopy method relies on the production of uniform and low-temperature gas jets to fully resolve the atomic hyperfine structure and efficiently determine fundamental nuclear properties of short-lived isotopes from, e.g., the hardly accessible actinide and transactinide elements. In this article we present the studies devoted to designing, producing, and characterizing the flow properties of a convergent-divergent (de Laval) hypersonic nozzle with a superior performance for laser spectroscopy applications. A novel flow mapping technique, based on resonance ionization spectroscopy (RIS), has been employed to characterize the local flow properties of an argon gas jet formed by this nozzle, revealing a 61.5-mm long, highly collimated atomic jet at a uniform low temperature of 16.6(5) K [Mach 8.11(12)] that will enable laser spectroscopy experiments on heavy-exotic nuclei with an unprecedented spectral resolution and a high efficiency. These results have been compared with those obtained by planar laser induced fluorescence spectroscopy (PLIFS) studies and show a good agreement between the two techniques and a significant improvement in efficiency of the RIS mapping method with respect to PLIFS. The data are compared to state-of-the-art fluid-dynamics calculations that were carried out to obtain the nozzle contour and simulate its performance, as well as to explain the observation of a possible onset of argon nucleation.

Syngas produced by gasification, which contains a high hydrogen content, has significant potential. The variation in the hydrogen content and dilution combustion are effective means to improve the steady combustion of syngas and reduce NOx emissions. OH planar laser-induced fluorescence technology (OH-PLIF) was applied in the present investigation of the turbulence of a premixed flame of syngas with varied compositions of H2/CO. The flame front structure and turbulent flame velocities of syngas with varied compositions and turbulent intensities were analyzed and calculated. Results showed that the trend in the turbulent flame speed with different hydrogen proportions and dilutions was similar to that of the laminar flame speed of the corresponding syngas. A higher hydrogen proportion induced a higher turbulent flame speed, higher OH concentration, and a smaller flame. Dilution had the opposite effect. Increasing the Reynolds number also increased the turbulent flame speed and OH concentration. In addition, the effect of the turbulence on the combustion of syngas was independent of the composition of syngas after the analysis of the ratio between the turbulent flame speed and the corresponding laminar flame speed, for the turbulent flames under low turbulent intensity. These research results provide a theoretical basis for the practical application of syngas with a complex composition in gas turbine power generation.

In this paper, the spray characteristics of a double-swirl low-emission combustor are analyzed by using Particle Imaging Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) technologies in an optical three-sector combustor test rig. Interactions between sectors and the influence of main stage swirl intensity on spray structure are explained. The results illustrate that the swirl intensity has great effect on the flow field and spray structure. The spray cone angle is bigger when the swirl number is 0.7, 0.9 than that when the swirl number is 0.5. The fuel distribution zone is larger and the distribution is more uniform when the swirl number is 0.5. The fuel concentration in the center area of the center plane of side sector (Plane 5) is larger than that of the center plane of middle sector (Plane 1). The spray cone angle in Plane 5 is larger than that in Plane 1. The width of spray cone becomes larger with the increase of Fuel–Air Ratio (FAR), whereas the spray cone angle under different fuel–air ratios are absolutely the same. The results of the mechanism of spray organization in this study can be used to support the design of new low-emission combustor. Keywords: Combustor, Fuel–air ratio, PIV, PLIF, Pressure drop, Swirl intensity

Thermometry measurements were performed in a scramjet combustor using thermally-assisted laser-induced fluorescence. Experimental data were obtained through laser-induced fluorescence where a laser...

In the present study, we performed a laboratory investigation to examine the mixing characteristics of 45° inclined dense jets from different nozzle geometries using the technique of Planar Laser Induced Fluorescence (PLIF). The geometries included the circular, square and diamond shapes, as well as the duckbill shape of non-return duckbill valves that are now commonly used for brine outfalls and the star shape of non-return star valves that are available commercially but have not been adopted so far. The concentration centrelines, cross-sectional profiles and spread widths were quantified in the experiments. The results showed that the circular, square and diamond nozzle geometries have similar behavior, implying that the differences of their discharge length scale are not sufficiently large to induce a significant effect. On the other hand, the duckbill and star nozzle geometries have relatively higher dilutions both at the centreline peak and return points. Interestingly, the duckbill nozzle has a relatively lower rise height compared to the others, potentially due to the strong influence of axis-switching effect. In addition, we also performed numerical simulations using the Large Eddy Simulations (LES) approach with the Dynamic Smagorinsky sub-grid model for the diamond, duckbill and star shape nozzle geometries in the experiments. The comparison showed that the time-averaged geometrical characteristics from the three different nozzle geometries can be simulated reasonably well before the centreline peak but with slight over-predictions after that. Meanwhile, the dilution characteristics are underestimated by ~25% generally, which are similar to previous LES results with the reference circular nozzle. The spectral density distribution of the concentration fluctuations clearly demonstrated that the production of turbulence energy in the larger eddies is enhanced by the non-circular nozzles, which is also consistent with the increase in dilutions with these nozzles.

Incipient choking and unstart initiation mechanisms are investigated experimentally in an ethylene-fueled constant-area direct-connect supersonic combustor with a circular cross section (diameter o...

Planar laser induced fluorescence (PLIF) technique was used to investigate concentration profiles and reaction progress of a turbulent reactive mixing process in a baffled stirred tank reactor. Rushton turbine, up-pumping pitched blade turbine (PBTU), and down-pumping pitched blade turbine (PBTD) were used to study the effect of mixing on the fast Fenton reaction progress. For determining the reaction progress, a criterion was developed, which was based on obtaining the number of mixed pixels with a defined uniformity percentage for the entire reactor area. It was found that impeller type, impeller clearance, and reactant injection location significantly altered concentration profiles and reaction progress. The reaction time scale along with the micro- and macromixing time scales were determined and it was concluded that the system performance was controlled by macromixing in the tank. Mean velocity field of the impellers was obtained by particle image velocimetry (PIV) technique and reasonably validated the concentration distribution results of PLIF technique.

The relationships between flow field, flame structure, and soot distribution were investigated experimentally in turbulent 4 kW non-premixed swirl flames of methane and air at three different air supply rates. Auto-compensating laser-induced incandescence, particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence techniques were used to measure soot volume fractions and primary soot particle size, velocity fields, OH, and OH* fields, respectively. While the instantaneous values of soot volume fractions were similar among the flames, the time-averaged soot volume fraction decreased by nearly a factor of three for a 7% increase in the air flow rate due to increased soot intermittency. Peak soot volume fractions occurred between two shear layers caused by the central fuel jet injection, recirculation zone, and swirling air jet interfaces. The decrease in soot towards the air side corresponded to locations with elevated OH, demonstrating its role in soot oxidation. The velocity measurements showed that engulfing the jet through a stronger recirculation rate induced by the higher air velocity enhanced the turbulent mixing between the recirculating fluid and fuel jet, and led to a shorter fuel jet penetration length. The reduction in soot volume fraction with air flow rate therefore is associated with greater turbulent mixing and higher oxidation rate in the recirculation zone. In contrast, the primary soot particle size, whose range was between 30 and 60 nm, did not show a dependence on the air flow rate. These results highlight the sensitivity of soot processes to fluid mechanics under turbulent conditions.

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, Ka = 180 , and Prandtl number, Pr = 77 . By employing this technique, falling-film flows are investigated with Reynolds numbers in the range Re = 18–66, wave frequencies set to f w = 7 , 12 and 17 Hz, and a wall heat flux set to q = 2.5 W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with q = 0 W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2–3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux ( q = 2.5 W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between the HTC and the film height in thinner film regions. When the imposed wave frequency is increased, a narrower range of HTCs is observed, which we link to the evolution of the film topology and the associated redistribution of the fluid flow upstream of the imaging location, as the liquid viscosity decreases. The local and instantaneous Nu is strongly coupled to the film height and experiences variations that increase as f w is reduced.

Image processing is of primary importance in the laser diagnostic on turbulent combustion, which can provide quantitative information, such as velocity field, intermediate species profile. Turbulent premixed flame front indicates the flame-turbulence interaction, can be decided from the OH-PLIF (Planar Laser Induced Fluorescence) technique due to the sharp increase of the OH distribution from unburned to burned region. In this paper, an adaptive threshold binarization method was proposed based on the local gray histogram of the OH-PLIF images when the signal-noise ratio is relatively low, i.e. turbulent flames at intensive turbulence and high pressure. The noise was eliminated by seed-mediated growth to obtain the legible flame front. Effect of different flame front identification methods, namely global binarization method, the canny operator of Matlab and the proposed adaptive threshold binarization method, on the flame structure parameters at various conditions was investigated. Results show that the flame fronts are seriously vague using the global binarization and the canny operator when the signal-noise ratio is low at intensive turbulence and high pressure, while continuous flame front can still be attracted by the proposed adaptive threshold binarization method. The turbulent flame front parameters from different methods are almost identical at weak turbulence and atmospheric pressure. While due to the noise introduced, the value of flame brush thickness, flame height and flame surface density from the proposed method is smaller than that of other two methods at high pressure. The adaptive threshold binarization method can obtain a better flame front identification at low signal-noise ratio conditions, results in reasonable premixed turbulent flame front parameters at intensive turbulence and high pressure.

Aircraft icing poses a serious threat to flight safety. Unfrozen impinging water on the surface of the aircraft will run back under the effect of high-speed airflow, altering liquid distribution and heat transfer characteristics. In this paper, a series of experiments were conducted over a wide range of air speed (Ua = 20.8–56.6 m/s), film Reynolds number (Ref = 26–128) to investigate the dynamics of ultrathin water film on an Aluminum substrate. The analyses of water film thickness and wavy characteristics were conducted through the planar laser-induced fluorescence method, in which the variations of transient characteristics of water film thickness with both the air speed and flow rate were analyzed. The statistics of water film thickness characteristics indicates that, the ratio of surface roughness of water film to its mean thickness is 0.4–0.8. The variation of the wave parameters of gas-liquid interface with both the air speed and the Reynolds number of the water film are also presented and analyzed in detail, including the main frequency of wave, interface wave height, surface wave velocity, interface wave curvature, and wave length parameters. This work is helpful to fundamentally understanding the wavy characteristics of shear-driven water film, as well as optimize the de-icing system for aircraft.

The objective of this work was to move towards developing a comprehensible Computational Fluid Dynamics (CFD) model to facilitate the predictive modeling of Fast Pyrolysis Oil (FPO) spray combustion. A CFD model was implemented from the literature and results were compared to 2D data from non-intrusive optical diagnostics involving Planar Laser Induced Fluorescence of the OH radical, Mie scattering imaging and two-color pyrometry using a laboratory-scale, CH 4 /air flat-flame with an air-assist atomizer. Furthermore, flame radiation and contributions from graybody sources, chemiluminescence and soot were studied experimentally using emission spectroscopy and Laser Induced Incandescence (LII). Reasonable qualitative agreement was found between experimental and model results in terms of flame structure and temperature. Emission spectroscopy and LII results revealed and confirmed earlier observations regarding the low soot concentration of FPO spray flames; furthermore, it was shown that a significant portion of flame radiation originated from graybody char radiation and chemiluminescence from the Na-content of the FPO. These suggest that the treatment of soot formation might not be important in future computational models; however, the description of char formation and Na chemiluminescence will be important for accurately predicting temperature and radiation profiles, important from the point of e.g., large-scale power applications. Confirmed low soot concentrations are promising from an environmental point of view.

Combustion of ethylene was examined experimentally via free-jet testing of a fundamental, axisymmetric combustor model at conditions representative of Mach 7-8 flight at 50 kPa dynamic pressure. The model consisted of diffusor, constant area isolator, cavity and diverging combustor sections. Pressure data were measured along the wall of the model, and were complemented with OH PLIF visualisation at the exit of the diverging section. Robust combustion of ethylene was observed for two stagnation enthalpy cases for equivalence ratios of 0.3 or greater. A low enthalpy, low equivalence ratio case showed significant, but structurally different, OH PLIF signals even though there was little combustion-induced pressure rise. Edge-detection techniques were applied to the PLIF images, and the resulting azimuthal distributions analysed. Mean radii and standard deviations are presented for the inner and outer edges of the OH rich band for four tunnel/fuelling conditions. Finally, amplitude spectra of fluctuations in the edge radii are shown, indicating that low wavenumber oscillations are dominant. These experimental results provide useful data for comparison with numerical simulation in future studies.

Cavity-enhanced combustion in an optical axisymmetric scramjet with a diverging combustor is experimentally investigated in Mach 4.5 flows. The axisymmetric scramjet emphasizes generic combustion a...

The planar laser-induced fluorescence (PLIF) technique was implemented to measure mixing time in a 1/17 water model of a 200-ton ladle furnace. The results were compared to those obtained using the conventional method of pH probes. PLIF determinations were done at two different planes, and pH probe determinations were performed at two different locations. The results suggest that mixing times measured by PLIF are similar to those obtained under optimal conditions by the pH probe and that PLIF technique is more accurate and less sensitive to the location of the measurement than the pH probe method. In addition, the particle image velocimetry (PIV) technique was used to measure the effect of the immersed probe on the fluid-dynamic structure of the system. The presence of the probe affects greatly fluid dynamics and consequently the mixing behavior, which could explain the differences found in its mixing time measurements at different probe locations. This study shows the feasibility of the PLIF technique used to measure mixing time in physical models of gas-stirred ladles; it is not intrusive and allows the visualization of the mixing phenomena in a complete plane of the system.