Non-intrusive Optical Diagnostics Research Articles

Sooting propensity and maturity of gasoline/anisole blends in a laminar coflow diffusion flame

Liquid oxygenated biofuels such as anisole have been used as additives, since they increase the octane number and improve spark-ignition engines’ performance. In this study, the sooting propensity of a gasoline surrogate, anisole, and gasoline/anisole mixture is quantified by measuring bidimensional fields of soot volume fraction, temperature and radiative heat flux in a coflow laminar flame using well-established non-intrusive optical diagnostics. The maturity of soot particles is also measured using innovative multiwavelength line-of-sight attenuation and emission measurements. Experimental measurements shows that increasing anisole content in fuel mixture results in a greatly enhanced soot volume fraction, while overall temperature is slightly decreased. In addition, maps of soot maturity index are reported based on the spectral dependence of soot optical properties with the amount of immature soot. Mature soot is found near the outer edge of the flame for the three flames, and mature soot content was found to decrease with increased anisole addition to the fuel mixture. In addition, high temperature zones were found to correlate with the presence of more graphitic compounds within the soot particles for the investigated fuels. The comprehensive dataset reported in this study provides rich information for a better understanding of soot formation processes, and for the development and validation of soot models in coflow laminar diffusion flames using complex fuels of interest for practical applications.

CFD modeling of pyrolysis oil combustion using finite rate chemistry

This paper reports the first Computational Fluid Dynamics (CFD) model developed for biomass pyrolysis oil spray combustion using Finite-Rate Chemistry (FRC) approach. To make the CFD calculations feasible, a reduced mechanism for modeling the combustion of biomass Fast Pyrolysis Oil (FPO) based on the POLIMI 1412 mechanism and a model for eugenol oxidation was developed. The reduced mechanism consisted of 200 reactions and 71 species. This level of complexity was found to be a good tradeoff between predictive power and computational cost such that the reduced model could be used in CFD modeling. The predictive power of the reduced mechanism was demonstrated via 0D (adiabatic, premixed, constant pressure reactor), 1D (laminar counterflow flame) and 3D (CFD of a methane-air flat-flame piloted FPO spray flame) calculations. Results from CFD were compared against experimental data from non-intrusive optical diagnostics. The reduced model was successfully used in CFD calculations—the computational cost was approximately 2 orders of magnitude higher than that of a simplified model. Using the reduced mechanism, the concentration of pollutants, minor combustion products, and flame radicals could be predicted—this is added capability compared to already existing models. The CFD model using the reduced mechanism showed quantitative predictive power for major combustion products, flame temperature, some pollutants and temperature, and qualitative predictive power for flame radicals and soot.

Non-intrusive tools for electric propulsion diagnostics

This paper reports about the advances in non-intrusive diagnostic systems for ion thruster engines undergoing at Aerospazio Tecnologie. Among the various optical diagnostics developed and applied in electric propulsion, laser-induced fluorescence (LIF) is a powerful, non-intrusive and species selective technique. It is capable to bring knowledge, where other techniques cannot be applied, as for example in the ionization region. Thanks to its selectivity to a given species, LIF can provide valuable information also about the neutrals. Another important topic is the occurrence of regular plasma oscillation in Hall thrusters channel, being correlated in different complex ways with performance of the thruster. In particular, it has been demonstrated that rotating inhomogeneity in the plasma ring (also termed rotating spokes) can be one of the principal mechanisms explaining quantitatively the anomalous electron transport in the discharge channel. Since the electron transport affects directly the efficiency in Hall thrusters, the characterization of plasma oscillation results in a valuable tool to assess the conditions for optimal thruster operation. The ultra-fast imaging technique offers the chance for a non-intrusive optical diagnostics of the plasma oscillation with high spatial and temporal resolution. In this paper, we propose a new approach to this technique to obtain higher frequency resolution and higher contrast by optical filtering. Both techniques are successfully applied as diagnostic tools of Aerospazio 200 W Xenon fed Hall effect thruster.

Experimental investigations of single particle and particle group combustion in a laminar flow reactor using simultaneous volumetric OH-LIF imaging and diffuse backlight-illumination

The volatile combustion of Colombian high-volatile bituminous coal was experimentally studied in a laboratory laminar flow reactor. The volatile flames corresponding to the single particle and particle group combustion were visualized using non-intrusive multi-parameter optical diagnostics. In the present study, high-speed laser-induced fluorescence of OH radicals (OH-LIF) was applied to study igniting particles by temporally tracking OH-LIF signals in the gas-phase flame. A novel acousto-optic deflector combined with a 10 kHz dye laser was employed for laser scanning through a probe volume with a thickness of a few millimeters. The three-dimensional OH-LIF signals were used to reconstruct the volatile flame structures of burning particles. Simultaneously, diffuse backlight-illumination (DBI) is implemented to measure the size and the spatial distribution of particles to distinguish between single and group particle combustion. For single particles, starting from the onset of ignition, the OH-LIF intensity reaches its maximum within several milliseconds, which is temporally resolved by employing a laser scanning system. The gas-phase ignition starts downstream of the particles. As the particle size increases, the flame stand-off distance increases, whereas the ratio of the flame stand-off distance and the particle diameter decreases, which ranges from 2 to 4 for the coal particles investigated. For particle groups, the flame topology is evaluated for individual reconstructions with different particle number densities (PND). As the PND increases, the volatile flames are pushed outwards to the boundary of particle clouds and a non-flammable region emerges in the center of volatile flames. Soot formation is observed and becomes increasingly intensive as the PND increases.

Numerical and experimental investigations of geometrical parameters on GH2/GO2 injector

To study the influence of injector geometrical parameters on combustor flow fields, detailed numerical and experimental investigations of a single-element injector with gaseous hydrogen and gaseous oxygen (GH2/GO2) propellants were performed at atmospheric pressure. The flow field characteristics of three different injector forms: shear-coaxial, swirl-coaxial and dual-swirl were discussed. The simulations of the combustion process with the use of steady Reynolds-averaged Navier-Stokes (RANS) techniques on a base of the ANSYS software were conducted. The Realizable k-ε turbulence model and eddy-dissipation concept combustion model were employed taking into account reactions of chemical kinetics. Results from the simulations demonstrated the detailed flame structures of the combustion flow fields. Subsequently, an advanced, non-intrusive optical diagnostics approach was adopted, including high-speed imaging and OH* chemiluminescence. The results show that the swirl flow injector approach elicits a distinctly different ignition process, in which the heat load from the swirl flow results in substantial damage to the chamber wall. Finally, with the help of advanced imaging processing method, it verified the validation of the numerical simulated model and acquired a specific linear relationship between the optical data and numerical calculations.

Investigation on the conditions leading to the micro-explosion of emulsified fuel droplet using two colors LIF method

Biofuels in the form of water-in-oil (W/O) emulsions are a promising substitution for hydrocarbon fuels, mainly thanks to the micro-explosion phenomenon that occurs under strong heating. Micro-explosion is the atomization of a W/O emulsion drop into daughter droplets because of the water embedded droplets phase change. This atomization leads to the reduction of pollutants and unburnt hydrocarbons emissions. However, micro-explosion is not a systematic phenomenon: indeed, for some cases the water phase change is weak and leads to a partial atomization, called “Puffing”. This paper relates a novel approach to better understand the occurrence of micro-explosion. It consists on the identification of the drop triggering the atomization, called the trigger drop, and measure its properties (temperature, size and position within the emulsion drop) thanks to non-intrusive optical diagnostics. Two colors laser induced fluorescence (LIF2C) is the main method used in this work for temperatures. These temperature measurements showed promising results of both quantitative and qualitative aspects. After the calibration steps and the method validation, it is applied for the temperature measurements of water droplets embedded in oil with the maximum temperature recorded around 130 °C. Using these measurements methods, a correlation is found between quality of atomization (either micro-explosion or puffing) and the properties of the trigger drop (temperature, size and position). When one of these parameters is not optimal, it can indeed be compensated by the others.

Background Oriented Schlieren (BOS) imaging of condensation from humid air on wettability-engineered surfaces

Condensation of water vapor in presence of non-condensable gas is observed in several engineering applications ranging from power generation to water harvesting. Non-intrusive optical diagnostics of the thermal and vapor concentration boundary layers on a condensing surface can offer a veritable means of predicting the condensation heat transfer coefficient (CHTC), but the literature in this field is far from being well-developed. Here we perform a Schlieren imaging-based mapping of the thermogravitational density boundary layer during condensation from humid air on vertical surfaces of three different wettability, i.e., superhydrophilic, hydrophilic (control) and superhydrophobic. A lens-free, Background Oriented Schlieren (BOS) technique is used to cross-correlate the refraction of light and quantify the density gradient fields near the condenser plates for three different humidity ratios (13.5, 15.5 and 17.5 g/kg dry air) at a constant degree of subcooling (19.5 °C). CHTC measurement from condensate collection data is used to validate the non-intrusive data. The mean refraction angle and the mean density gradient near the plate are found to scale linearly with the CHTC and the condensate mass flux, respectively. The study clearly shows the potential of BOS technique in predicting the condensation heat transfer rates from humid air.

Experimental and numerical study of biomass fast pyrolysis oil spray combustion: Advanced laser diagnostics and emission spectrometry

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, CH4/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.

Numerical and experimental investigations of single-element and double-element injectors using gaseous oxygen/gaseous methane

The objective of this study is to investigate the effects of fuel/oxidizer (F/O) momentum ratio on the mechanism of mixing and combustion of single-element and double-element shear coaxial injectors numerically and experimentally using gaseous oxygen/gaseous methane (GO2/GCH4) and non-intrusive optical diagnostics technique based on planar laser induced fluorescence (PLIF). Instantaneous OH-PLIF images demonstrate that the flame with higher F/O momentum ratio exhibits appearance characteristics with more obvious wrinkled shear layers, extinction and kernels events. Simulation and time-averaged results show that with the increase of F/O momentum ratio at the condition of fixed total mass flow rate, the initial combustion position becomes closer to the front of the chamber. The large mass flow rate of the double-element combustor is beneficial to the thermal protection of the chamber wall. Based on the high-speed images and velocity distribution obtained from numerical data, it is detrimental for the propagation of the flame front at the injection condition of high F/O momentum ratio. In addition, according to the infrared thermal pictures, the flame outside the chamber becomes broader with the increase of F/O momentum ratio, and the plume of double-element combustor will challenge the thermal protection around the equipment. Moreover, new methods are applied in this paper to extract the flame boundary of the OH-PLIF diagrams and to calculate the number of the pixels of the plume from infrared thermal imager results.

Fuel Assembly Oscillation-Induced Flow in Initially Stagnant Fluid

The effect of water on the dynamics response of fuel bundles in pressurized water reactors during external forcing is studied experimentally inside a large facility that houses a full-height bundle and is operated on an earthquake shake table. This configuration is directly relevant to earthquakes and loss-of-coolant accidents. Most data to date have been focused on structural response and some pointwise measurements of liquid velocity. Here, structure displacement coupled with velocity field are investigated with nonintrusive optical diagnostics in initially stagnant water. Data indicate that a flow develops as the structure oscillates: both a cross flow through the bundle and an axial pulsatile flow that was not anticipated. A physical mechanism is proposed as a source of this structure-induced flow that is driven by pressure gradients around the fuel bundle.

Non-intrusive optical diagnostics of co- and counter-swirling flames in a dual swirl pulverized coal combustion burner

Optical non-intrusive measurements are performed to a 10kWth laboratory-scale dual swirl pulverized coal combustion burner in order to elucidate the behaviors of internal recirculation zone (IRZ) and heat release region. The pulverized coal flame is operated with four different swirl combinations: co-swirling (low- and high-swirl) and counter-swirling (low- and high-swirl). The flow field is measured using particle image velocimetry (PIV). The IRZ area increases for the high-swirl conditions than that in the low-swirl conditions. With changing from co- to counter-swirl combination, the IRZ appearance changes from a heart- to an elongate-shape IRZ because double stagnation points for the co-swirling flames merges to one stagnation point for the counter-swirling flames. For the co-swirling conditions, a tube-type vortex is detected at the end of the IRZ near the double stagnation points. However, it is not present in the counter-swirling conditions. From two-color pyrometry, the overall temperature in the high-swirl flames is lower than the low-swirl flames. For the counter-swirling flames, higher temperature is observed due to the better mixing, which allows for a more intense combustion reaction of pulverized coal particles. Flame color measurements (OH and CH chemiluminescence band light emission) shows that the devolatilization of coal particles is observed at more upstream region comparing with the heat release reaction for volatile gas combustion. For the high-swirl flames, the CH∗ band intensity for heat release decreases, however its overall dimension increases. For the counter-swirling flames, the higher heat release is observed.

Investigation of non-premixed flame combustion characters in GO2/GH2 shear coaxial injectors using non-intrusive optical diagnostics

Single-element combustor experiments are conducted for three shear coaxial geometry configuration injectors by using gaseous oxygen and gaseous hydrogen (GO2/GH2) as propellants. During the combustion process, several spatially and timeresolved non-intrusive optical techniques, such as OH planar laser induced fluorescence (PLIF), high speed imaging, and infrared imaging, are simultaneously employed to observe the OH radical concentration distribution, flame fluctuations, and temperature fields. The results demonstrate that the turbulent flow phenomenon of non-premixed flame exhibits a remarkable periodicity, and the mixing ratio becomes a crucial factor to influence the combustion flame length. The high speed and infrared images have a consistent temperature field trend. As for the OH-PLIF images, an intuitionistic local flame structure is revealed by single-shot instantaneous images. Furthermore, the means and standard deviations of OH radical intensity are acquired to provide statistical information regarding the flame, which may be helpful for validation of numerical simulations in future. Parameters of structure configurations, such as impinging angle and oxygen post thickness, play an important role in the reaction zone distribution. Based on a successful flame contour extraction method assembled with non-linear anisotropic diffusive filtering and variational level-set, it is possible to implement a fractal analysis to describe the fractal characteristics of the non-premixed flame contour. As a result, the flame front cannot be regarded as a fractal object. However, this turbulent process presents a self-similarity characteristic.

Validation Facility and Model Development for Nuclear Fuel Assembly Response to Seismic Loading

A joint experimental and numerical campaign is conducted to provide validation dataset of high-fidelity fluid–structure interaction (FSI) models of nuclear fuel assemblies during seismic loading. A refractive index-matched (RIM) flow loop is operated on a six-degree-of-freedom shake table and instrumented with nonintrusive optical diagnostics. The test section can house up to three full-height fuel assemblies. To guarantee reproducible and controlled initial conditions, special care is given to the test section inlet plenum; in particular, it is designed to minimize secondary pulsatile flow that may arise due to ground acceleration. A single transparent surrogate 6×6 fuel subassembly is used near prototypical Reynolds number, Re=105 based on hydraulic diameter. To preserve dynamic similarity of the model with prototype, the main dimensionless parameters are matched and custom spacer grids are designed. Special instruments are developed to characterize fluid and structure response and to operate in this challenging shaking environment. In parallel to the earlier experiments, we also conducted fully coupled direct numerical simulations, where the equations for the fluid and the structure are simultaneously advanced in time using a partitioned scheme. To deal with the highly complex geometrical configuration, which also involves large displacements and deformations, we utilize a second-order accurate, immersed boundary (IB) formulation, where the geometry is immersed in a block-structured grid with adaptive mesh refinement (AMR). To explore a wide parametric range, we will consider several subsets of the experimental configuration. A typical computation involves 60,000 cores, on leadership high-performance computing facilities (i.e., IBM Blue-Gene Q–MIRA).

Nonintrusive and Multidimensional Optical Diagnostics and Their Applications in the Study of Thermal-Fluid Systems

Nonintrusive measurements are highly desirable in the study of modern thermal-fluid systems. One of the key bottlenecks of studying such systems stems from the experimental difficulty of resolving the governing processes with the necessary temporal and spatial resolution. The research and development of modern power and energy systems (e.g., aero-propulsion engine) require understanding such processes under extreme conditions, such as sub-millisecond time resolution, sub-millimeter spatial resolution, high temperature and pressure, and multiphase flows. Traditional experimental techniques are no longer capable of meeting all these requirements. Therefore, this paper discusses nonintrusive laser diagnostics and their prospective of meeting these challenges. Using laser beams as probes and being nonintrusive, laser diagnostics can be applied to any harsh environments, at least theoretically. This paper specifically focuses on the recent work from the author's group to apply tomographic laser techniques to obtain multidimensional measurements in thermal-fluid systems. Examples include two-dimensional imaging of temperature fields at 50 kHz, and three-dimensional measurements of combustion parameters using emission spectroscopy at multi-kilohertz. Lastly, as an example, this paper discusses our ongoing efforts of applying advanced laser diagnostics for the study of thermal management of batteries in a hybrid vehicle.

Carbon nanoparticles in the radiation field of the stationary arc discharge

The paper considers a simple theoretical model of heating the nanoparticles, depending on their size and the parameters of the radiating arc and the surrounding gas. This problem is of interest to diagnostics and modeling of the dynamics of the nanoparticles formation and their local size distribution. Heating of nanoparticles by the radiation can affect the process of synthesis. The degree of heating of the particle is determined by its geometry, which opens, apparently, additional possibilities for nonintrusive optical diagnostics.

Energy conversion in high enthalpy flows and non-equilibrium plasmas

Recent developments in the study of very high energy non-equilibrium fluid flows are reviewed. These are flows of molecular gases which exhibit substantial degrees of mode disequilibrium, specifically high energy in molecular vibrational and electronic modes, and high electron energies when the gases are weakly ionized. In contrast, the modes of molecular translation and rotation remain at lower energies. Attention is focused on high density, collision-dominated gases. Studies in two systems are presented: A small wind tunnel where an M=5 steady air flow over small models is produced, and a flowing carbon monoxide gas laser, exhibiting very high energy loading of the vibrational quantum states. The development of non-intrusive optical diagnostics to measure vibrational and electronic state populations and rotational/translational mode temperatures in the flows, with high spatial and temporal resolution, is presented. Kinetic modeling and experimental validation studies in these environments are also discussed.

Structural and stability characteristics of jets in crossflow

AbstractThis experimental study examines the relationship between transverse jet structural characteristics and the shear layer instabilities forming on the upstream side of the jet column. Jets composed of mixtures of helium and nitrogen were introduced perpendicularly into a low-speed wind tunnel using several alternative injectors: convergent circular nozzles mounted either flush with or elevated above the tunnel floor, and a flush-mounted circular pipe. Both non-intrusive optical diagnostics (planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV)) and intrusive probe-based (hot-wire anemometry) measurements were used to explore a range of jet-to-crossflow momentum flux ratios and density ratios for which previous studies have identified upstream shear layer transition from convective to absolute instability. Remarkable correspondences were identified between formation of the well-known counter-rotating vortex pair (CVP) associated with the jet cross-section and conditions producing strong upstream shear layer vorticity rollup, arising typically from absolute instability in the shear layer. In contrast, asymmetries in the jet mean cross-sectional shape and/or lack of a clear CVP were observed to correspond to weaker, convectively unstable jet shear layers.

Species measurements in a nitrogen-diluted, ethylene air diffusion flame using direct sampling mass spectrometry and tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy combined with extractive sampling is used to quantify fuel and acetylene concentrations in a 32% ethylene-diluted, nitrogen diffusion flame. Additional major species concentrations are determined using quartz microprobe, direct-sampling mass spectrometry. Quantification of these mass spectra used a multilinear regression technique anchored by computed nitrogen concentration profiles in a simulation from Yale University. Because of the overlap in mass spectra features for ethylene, nitrogen, and carbon monoxide, both optical and mass spectrometric techniques are required to determine the suite of major species in this flame system. Good agreement is observed between these experimental measurements and numerical simulation of this flame system. Probe clogging in these sooting flames remains a challenge for all probe techniques and additional, non-intrusive optical diagnostics must be applied to characterize flame regions with substantial soot formation.

Assessment of subgrid-scale models with a large-eddy simulation-dedicated experimental database: The pulsatile impinging jet in turbulent cross-flow

Large-Eddy Simulation (LES) in complex geometries and industrial applications like piston engines, gas turbines, or aircraft engines requires the use of advanced subgrid-scale (SGS) models able to take into account the main flow features and the turbulence anisotropy. Keeping this goal in mind, this paper reports a LES-dedicated experiment of a pulsatile hot-jet impinging a flat-plate in the presence of a cold turbulent cross-flow. Unlike commonly used academic test cases, this configuration involves different flow features encountered in complex configurations: shear/rotating regions, stagnation point, wall-turbulence, and the propagation of a vortex ring along the wall. This experiment was also designed with the aim to use quantitative and nonintrusive optical diagnostics such as Particle Image Velocimetry, and to easily perform a LES involving a relatively simple geometry and well-controlled boundary conditions. Hence, two eddy-viscosity-based SGS models are investigated: the dynamic Smagorinsky model [M. Germano, U. Piomelli, P. Moin, and W. Cabot, “A dynamic subgrid-scale eddy viscosity model,” Phys. Fluids A 3(7), 1760–1765 (1991)] and the σ-model [F. Nicoud, H. B. Toda, O. Cabrit, S. Bose, and J. Lee, “Using singular values to build a subgrid-scale model for large eddy simulations,” Phys. Fluids 23(8), 085106 (2011)]. Both models give similar results during the first phase of the experiment. However, it was found that the dynamic Smagorinsky model could not accurately predict the vortex-ring propagation, while the σ-model provides a better agreement with the experimental measurements. Setting aside the implementation of the dynamic procedure (implemented here in its simplest form, i.e., without averaging over homogeneous directions and with clipping of negative values to ensure numerical stability), it is suggested that the mitigated predictions of the dynamic Smagorinsky model are due to the dynamic constant, which strongly depends on the mesh resolution. Indeed, the shear-stress near the wall increases during the vortex-ring impingement leading to a less refined mesh in terms of wall units, y+. This loss of resolution induces a poor damping of the dynamic constant, which is no longer able to adjust itself to ensure the expected y3-behavior near the wall. It is shown that the dynamic constant is never small enough to properly balance the large values of the squared magnitude of the strain-rate tensor, 2SijSij. The experimental database is made available to the community upon request to the authors.

Experimental and numerical approaches of aerosol removal in spray conditions for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear pressurized water reactor (PWR) containment. The TOSQAN facility, which is highly instrumented with non-intrusive optical diagnostics, is particularly adapted to nuclear safety code validation. The present work is devoted to study a water spray injection used as a mitigation means in order to washout aerosol fission products.