Hugo Ajrouche

Overview:

      Our dependence on the combustion process is significant, such as in power generation, heating, and transport. The development of the combustion devices (gas turbines, piston engines, or industrial burners) rely heavily on computational fluid dynamics, which make use of the established numerical models. These models have been continuously enhanced using the inputs provided by the experiments. The understanding and outputs derived from the experiments will eventually enhance the prediction capabilities and thus enable us to develop fuel-efficient and less-polluting devices that can meet ever-stringent regulatory norms in the future.

       In the experimental combustion research, the laser diagnostics play an indispensable role, as they allow non-intrusive instantaneous measurement in harsh environments. In general, the nanoscale heat transfer is analyzed using Picosecond thermoreflectance, the velocity field is evaluated using particle image velocimetry (PIV), the temperature and the major species concentration by Spontaneous Raman Scattering (SRS), the flame location by planar laser induced florescence (PLIF) measurement of a radical, the air/fuel mixing information through tracer based PLIF, the soot formation and emissions through Laser Induced Incandescence (LII) or Two-Color Diffused Back-Illumination Imaging (DBI), the lift-off length and the ignition delay by OH* chemiluminescence, the vapor spray penetration by Schlieren. My research objectives are to invent novel techniques that can eventually lead to the development of fuel-efficient and low-polluting combustion devices, and to generate the experimental data that is so-far rarely available for model validations.

My interest in the characterization of nanomaterials and laser diagnostics for combustion process dates back to my first years of graduate studies where it was question of probing the energy levels of an atom or a molecule with lasers. Over the last decade, the potential of lasers for the characterization of the physical properties of nanomaterials and the analysis of reactive medium seemed to me so interesting and concrete that motivate me to candidate for the master degree in energy, laser and nanotechnology engineering.

 

Past works (Master in Energy, Laser & Nanotechnology Engineering): Instrumental development to study spectroscopic properties of polarized light in innovative laser

 

       Borates are interesting materials for laser applications thanks to their high damage threshold and their high transparency in the UV range. For spectroscopic aspects, ytterbium ions show advantages like a simple energy level diagram with no cross-relaxation process or excited-state absorption, a long lifetime and a broad emission compared to neodymium or thulium. Moreover, the small quantum defect between the pumping and lasing wavelengths makes this material attractive for its high efficiency and limited thermal effects. Thus, laser systems based on ytterbium-doped materials have raised much interest since the development of powerful InGaAs diodes, for compact, high-power all-solid-state lasers.

       In this context, the LYB:Yb (Li6Y(BO3)3:Yb3+) crystal has already proven interesting results for laser applications, as well as the whole solid solution LYB:Yb-LGB:Yb (Li6Y(BO3)3:Yb3+-Li6Gd(BO3)3:Yb3+), depicting the crystal growth, and thermal, mechanical and optical properties. No polarized light study had been done, since measurements had been performed on powders (crushed single crystals). Since recent studies had reported monoclinic specificities of spectroscopic properties, it was of interest to study our oriented LiLnB:Yb crystals in polarized light. In this master research, some absorption and emission cross-sections were recorded in polarized light in Yb(22%)-doped LLnB oriented cubes (chemical composition Li6Ln(BO3)3, with Ln = Y, Gd). Related laser gain cross-sections were calculated and lifetime was measured. We also determined values of the three principal refractive indexes of these biaxial crystals. These results emphasize the potential of Yb-doped LYB-LGdB monoclinic crystals as candidates for laser applications.This work is based on the crystal orientation with respect to the direction of propagation and the polarization orientation with respect to the three main axes of the dielectric frame. As expected for a monoclinic crystal, it clearly shows a significant anisotropy of absorption and emission properties. The absorption cross-section of the 1→5 transition seems to be equivalent whatever the polarization whereas the X- and Z-polarization show a significant difference for the 1→6 and 1→7 lines, which should be considered when choosing the pumping wavelength for future laser tests. For emission, a maximum is observed for the X-polarization. Additionally, these highly-doped materials evidenced strong re-absorption effects, in relation to their geometrical aspects, requiring caution for interpretation. Finally, we expect that interesting laser performances of these borate compounds can be obtained and improved, especially by an optimized crystal orientation in the monoclinic plane. Still, the broadband fluorescence emission makes these Yb-doped LY1-x GdxB (0<x<1) crystals as good candidates for ultrafast laser applications.

Past works (Research engineer in Optical diagnostics and Nanotechnology): Development of Picosecond Pump-Probe Time Domain Thermoreflectance technique that can characterize and investigate acoustic and thermal properties at very short time & length scales.

 

      The characterization of the physical properties, in particular the thermal properties, of the new materials with low dimensionality is today a major stake. For example, high-density storage technologies based on the use of heat spikes at the end of a near-field microscope are under development. Storage is done by heating locally and very quickly a substrate to write a bit. This type of technology requires the spatial and temporal evolution of the heat flow. However, the main obstacle to the miniaturization of processors and other electronic devices results from the lack of control of the heat dissipation. In this work, we are interested in the study of phonons, and in the existing competition between ballistic phonons, related to non-Fourier, and diffusive effects.

       A study of phonon transport in semiconductor alloys such as SiGe, GaAs, InGaAs has been carried out. The links between optical response and thermal response in these materials can be revealed today through the development of ultrafast optical techniques. The objective is, firstly, to verify experimentally the frequency dependence of the thermal conductivity. Thus, we discussed the frequency dependence of thermal conductivity from a theoretical point of view. Then we developed the Picosecond thermoreflectance technique. This diagnostic and scanning thermal microscopy technique have allowed the exploration of a new area of research, nanothermal, and nano-acoustic physics. It is mainly used to determine thermal and acoustic properties of thin films and multilayers. Picosecond thermoreflectance is also an unprecedented powerful technique for nanoscale heat transfer analysis and metrology, but different sources of artifacts were reported in the literature making this technique difficult to use for long delay (several ns) thermal analysis. During my work, I developed a new heterodyne picosecond thermoreflectance technique. As it uses two slightly frequency shifted lasers instead of a mechanical translation stage, it is possible to avoid all artifacts leading to erroneous thermal parameter identifications. We demonstrate the accuracy of the technique in the identification of the thermal conductivity of a 50 nm thick SiO2 layer. Then, we discuss the role of the modulation frequency for nanoscale heat transfer analysis.

For thermal measurements, time ranges up to ten nano-seconds and more are needed allowing the heat flux to propagate deeply through the sample. Unfortunately, classical optical sampling based on mechanical translation stages are very sensitive to residual heating and misalignment that can lead to erroneous analysis. We have shown a specific thermal model applicable to ultrashort-pulsed laser heating nanolayers. Details of the data acquisition and interpretation of the experimental results have been presented, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature.

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Past works (Doctoral): Thermography and multi-species concentrations measurements by SRS for turbulent combustion

 

Laser diagnostics have been proven to be an indispensable tool to analyze the flow and combustion phenomena by allowing non-intrusive measurements of the velocity field, concentration and temperature. Spontaneous Raman Scattering (SRS) is one of the few methods providing simultaneously in-situ temperature and multi-species concentrations. Measurement in turbulent flames by SRS is still challenging due to the emission background and the requirement of single-shot measurements with high spatial and temporal resolutions.

Over the last decade the need for reliable data to validate combustion models has spurred the development of spontaneous Raman scattering (SRS) as a multispecies diagnostics tool [1, 2]. Due to its poor efficiency, gas analysis by SRS was for a long time limited to large control volumes and long exposure times. These constraints are unsuitable for the analysis of turbulent flames which requires single-shot measurements with high spatial and temporal resolutions, and only a few works on the subject were proposed in the 1980s, whereas laser diagnostics underwent considerable development at that time. Probe size still remained significantly larger than the smallest scales of turbulence, and thus errors in temperature and density measurements could be expected. Therefore, efforts to develop single-shot SRS measurements with high spatial resolution have been made over the past few years. Such measurements require laser energies greater than 1 J per pulse associated with a long pulse duration to avoid optical breakdown, and a very sensitive detector. Furthermore, flame emission and soot radiation can produce strong background emissions that make Raman detection very difficult and even impossible in certain spectral ranges. Continuous flame luminosity prevents the accurate determination of temperature and species concentration in combustion environments. Thus fast temporal gating is required to remove this interference, increase signal-to-noise ratio and provide quantitative data. For single-shot SRS measurements, full-frame back-illuminated CCD cameras are remarkable for their high quantum efficiency, wide dynamic range, good spatial resolution and low noise. However, due to their full-frame architecture, these cameras require rapid shutters, otherwise the pixels may be exposed during the readout time, causing smearing, especially when the camera is in front of a luminous medium like flames. Thus several works have been performed with intensified detectors (ICCD) due to their fast electronic gating (<10 ns), but in spite of their lower quantum efficiency and higher shot-noise levels which lead to additional uncertainties on single-shot Raman measurements. Various devices have also been suggested to solve the gating issue of full-frame back-illuminated CCD cameras. Commercial mechanical shutters offer minimum opening times of several milliseconds, which is too long to efficiently remove flame emission and avoid the camera saturation. Currently, mechanical shutters based on rotary chopper wheels are the best technical solution for line-imaging Raman measurements in flames, allowing gating as fast as 3.9 µs with a 500 µm slit. Nguyen et al also proposed a high-speed mechanical shutter using a rotary optical chopper with a gating time of less than 10 µs at 30 Hz. More recently, Kojima et al described a time-gated detection method limited to point-wise measurements using the frame-transfer of a single CCD detector providing a gate duration of 5 µs. These gate widths are attractive when taking account of turbulence time scales and help to reduce flame emission. However, they are much longer than the laser pulse duration, and could be further reduced to investigate intense luminous flames.

Based on the conservation of polarization by SRS, an electro-optical shutter using the fast electronic gating of a Pockels cell can be used as an active polarization-rotating element, decreasing on–off time to 100 ns. My doctoral research at CORIA Laboratory, explains the development of new Raman setup based on this ultra-fast shutter to perform Thermography and multi-species concentrations measurements by spontaneous Raman scattering for turbulent combustion.

 

The originality of my doctoral research consists in use of a large aperture Pockels cell based electro-optical shutter (PCS), that allows removing unpolarised background flame emission and compatible with a 1D measurement. A significant reduction of flame emission was observed and consequently signal to noise ratio was enhanced. The ability of SRS in terms of thermometric single-shot method was demonstrated successfully in premixed laminar flames and sooty laminar diffusion flames. The measured temperature in burnt gases and those calculated by adiabatic flame modelling was within 1 %. Thermometric Raman analysis for low temperatures demonstrates the reliability of measurements, with a better accuracy for O2 compared to N2. Subsequently, the ability of SRS technique to simultaneously measure instantaneous concentrations of N2, O2 and CO was demonstrated. The ability to measure single-shot scalar values accurately is assessed by comparing different CCD detectors with the PCS. The results obtained from the BI-CCD and the BI-EMCCD concerning temperature, temperature gradient and high density were in good agreement with the COSILAB calculation for 1D laminar adiabatic flame. The BI-EMCCD observed to be the most sensitive in detecting low concentration elements like CO. Finally, SRS technique was applied to a turbulent sooting jet flame, illustrating the potential of this technique to build an important database for flame modelling.

 

Past works (Senior Lecture and Researcher): modeling CO2 spectrum and spontaneous Raman scattering (SRS) measurements of this molecule in complex reactive flows: turbulent flames & plasmas.

 

CO2 is a major product of hydrocarbons combustion. Consequently, its in-situ quantification is essential to characterize combustion process. However, CO2 SRS is ruled by a more complex spectroscopy than for diatomic molecules. Even if a lot of works already described deeply CO2 energy states, databases for CO2 at high temperature such as HITEMP or CDSD are unfortunately designed for absorption or emission experiments. Therefore these common spectroscopic databases do not include Raman transition moments, which are however essential to simulate correctly CO2 Raman spectra. Nevertheless, a new table with a set of CO2 Raman shifts associated to their polarizability transition moment has been recently released by Lemus et al. These data have been computed for combustion optical diagnostic purposes. The large number of vibrational transitions involved in this model should be acceptable to build quite reliable CO2 spectra at high temperature. By harnessing this recent table, instantaneous CO2 density number measurements should be carried out by the spectral fitting method. This work presents the first steps to validate CO2 measurements by SRS using a spectral fitting method. Average and instantaneous Raman CO2 spectra have been processed for number density measurements thanks to a new model for CO2 SRS simulation at high temperature. Accuracy and uncertainties of CO2 density number in simplified environment are analyzed. These initial results highlight the potential of the SRS simulation fitting based diagnostic for CO2 measurements in turbulent flames.

 

The second part of the BioEngine project demonstrated the significance of one of the theories (triple flame), which was not conclusive before this work. The reported findings will help modelers to improve predictions in practical devices, such as gas-turbine combustor. In most devices combustion process occurs in turbulent partially premixed mode, e.g. in industrial and domestic burners, swirl-stabilized gas turbine combustors, and piston engines.

 

The flame speed was measured using PIV and the flame-front was imaged through OH-PLIF. The flame speed in a stratified mixture was shown to exceed the stoichiometric adiabatic value and at certain stratification level it reaches a peak. This behavior was explained as the balance between the availability of excess reactants from the premixed branches and their temperature which is affected by the flame stretch. The laminar triple flame work was extended to turbulent flow conditions. This work conclusively demonstrates that the flame stabilization in turbulent PPFs occur through a triple flame structure. A schematic of the turbulent triple flame structure was constructed based on the velocity field (PIV), flame-front (OH-PLIF) and the mixture fraction field (acetone-PLIF) information. The laminar and turbulent work contributes to our understanding of the flame structure and the stabilization mechanism of turbulent PPFs.

Additionally, I had implemented an active turbulence grid which can provide control over the turbulence parameters, independent of bulk velocity which is necessary to maintain the same residence time. This work provides an insight into flow/flame interaction at different scales, and highlights role of the smaller scales. The interaction of active grid generated turbulence with a premixed flame was investigated through simultaneous OH-PLIF/PIV measurements and the proper orthogonal decomposition processing technique.

 

Recent works (Postdoctoral):

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1. Validation of the New One Shot Engine (NOSE) as Set up to Characterize ECN Spray A

For the last 20 years, to comply with increasingly drastic emission standards, many studies have focused on spray and combustion for Internal Combustion Engines in order to improve efficiency and reduce pollutant emissions. Engine Combustion Network (ECN), initiated by Sandia National Lab is an international network about experimental and simulation analysis of combustion phenomena for diesel and gasoline engines to develop and validate Computational Fluid Dynamics (CFD) model. CFD simulation has made it possible to compute a wide variety of chamber geometries and operating conditions for optimization at a substantially lower cost than with experimental methods. However, the predictability of CFD depends on the degree of understanding of the physical phenomena of spray and combustion in the chamber. This can be achieved thanks to optical techniques. In order to provide accurate data about diesel spray and combustion processes in Diesel engines, several combustion chambers were developed to reach High Pressure-High Temperature (HPHT) thermodynamic conditions, representing current common-rail diesel engine operating modes. For example, Constant-Volume Preburn (CVP) chamber, Constant-Pressure Flow (CPF) chamber, Rapid Cycling Machine (RCYM) are frequently used to investigate basic phenomena of diesel spray and combustion by using optical set-ups. RCYM seems to be an interesting option to study spray in HPHT conditions, due to no presence of combustion product like in the CVP and the high pressure possibility to cover a full range of diesel engine working conditions (up to 100 bar).

Thus, a “New One Shot Engine” (NOSE) was designed to meet the Engine Combustion Network (ECN, https://ecn.sandia.gov/) objective of providing highly accurate experimental results in order to validate models and enhance our scientific understanding of spray combustion in conditions specific to engines. NOSE is able to produce HPHT environments representative of diesel engine cylinders. The advantage of this kind of set-up in comparison to pre-burn or flue chambers is that the initial gas mixture can be well controlled in terms of species and mole fraction. First, the details of the NOSE design and operating conditions to meet the ECN Spray A standard condition are fully described to validate this set-up as an ECN set-up. Non-reactive conditions (pure nitrogen) were reached: ambient pressure (near 60 bar), ambient density (22.8 kg/m3) and various ambient temperatures (800, 850, and 900 K). PIV measurements and simulation showed that a near-quiescent atmosphere was achieved with a mean gas velocity below 1 m/s. For the experimental result, the liquid and vapor spray parameters were characterized by Diffused-Back Illumination and the Schlieren technique. All experimental results are compared with the ECN database for Spray A. A good agreement was found, which confirms the accurate control of NOSE to reach Spray A condition requirements.

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    2. X-ray diagnostics of dodecane jet in spray A conditions using the new one shot engine

After the validation of the new one shot engine that we can reach spray A condition, we start to implement different optical diagnostics in order to obtain different parameter such as liquid mass distribution, liquid length, vapor spray penetration, lift-off length, ignition delay. Quantifying liquid mass distribution data in the dense near nozzle area to develop and optimize diesel spray by optical diagnostic is challenging. Optical methods, while providing valuable information, have intrinsic limitations due to the strong scattering of visible light at gas-liquid boundaries. Because of the high density of the droplets near the nozzle, most optical methods are ineffective in this area and prevent the acquisition of reliable quantitative data. X-ray diagnostics offer a solution to this issue, since the main interaction between the fuel and the X-rays is absorption, rather than scattering, thus X-ray technique offers an appealing alternative to optical techniques for studying fuel sprays. Over the last decade, x-ray radiography experiments have demonstrated the ability to perform quantitative measurements in complex sprays.

In the present work, an X-ray technique based on X-ray absorption has been conducted to perform measurements in dodecane fuel spray injected from a single- hole nozzle at high injection pressure and high temperature. The working fluid has been doped with DPX 9 containing a Cerium additive, which acts as a contrast agent. The first step of this work was to address the effect of this dopant, which increases the sensitivity of X-ray diagnostics due its strong photon absorption, on the behavior and the physical characteristics of n-dodecane spray. Comparisons of the diffused back illumination images acquired from n-dodecane spray with and without DPX 9 under similar operating conditions show several significant differences. The current data show clearly that the liquid penetration length is different when DPX 9 is mixed with dodecane. To address this problem, the dodecane was doped with a several quantities of DPX containing 25% ± 0.5 of Cerium. Experiments show that 1.25% of Ce doesn’t affect the behaviour of spray. Radiography and density measurements at ambient pressure and 60 bars are presented. Spray cone angle around 5° is obtained. The obtained data shows that the result is a compromise between the concentration of dopant for which the physical characteristics of the spray do not change and the visualization of the jet by X-ray for this concentration.

 

Recent works (Research Engineer):

 

1. Spray and Combustion Characterizations of Acetone-Butanol- Ethanol (ABE) blend at High-Pressure and High-Temperature Conditions

Due to the increase of the energy demand and the limitation of the oil resources during last few decades, butanol became an alternative fuel, considered in transportation sector as a mean of stainable energy and also reduction of greenhouse gas relative to using conventional fuel. Indeed, it can be produced from renewable bioressources in the form of agricultural biomass and wastes. Moreover, butanol induces less fuel consumption as its energy content is higher than ethanol, up to 30%. The lower water solubility of butanol is consequently decreasing the tendency of microbial-induced corrosion in fuel storage and pipelines during transportation. Moreover, the cetane number (CN) of butanol (CN = 25), higher than ethanol (CN = 8) leads to easier ignition in compression ignition (CI) engine. Also it is suitable with diesel injection system by mean of high level of viscosity like diesel fuel and no water content unlike ethanol. Finally, researchers have considered butanol as one means to reduce emissions, CO, HC, NOx, and Soot and also to improve combustion efficiency especially in the advanced combustion modes.

Bio-butanol can be produced from agricultural crops and lignocellulosic biomass by using some of Clostridium bacteria, Clostridium beijerinckii or Clostridium acetobutylicum, to ferment lignocellulosic hydrolysate sugars to mixture of Acetone, Butanol and Ethanol (ABE) in volume ratio 3:6:1, and after that it is distilled to separate butanol from the ABE mixture. But, the intermediate fermentation product, ABE mixture, can also be considered as potential new alternative fuel itself for CI engines because its physical and chemical properties are quite similar to butanol. In addition, if ABE mixture can be used as fuel, the cost and energy consumption of the separate process of butanol from ABE mixture would be eliminated. 

Thus, after NOSE has been evaluated to investigate macroscopic spray-combustion parameters by validating Spray-A conditions of Engine Combustion Network. We try to study the spray-combustion characteristics of ABE mixture (volume ratio 3:6:1), blended with n-dodecane in proportional 20% by volume (ABE20), compared to n-dodecane as reference fuel. The macroscopic spray and combustion parameters are investigated, for non-reactive conditions, in pure Nitrogen and for reactive conditions, in 15% oxygen, at ambient pressure 60 bar, ambient density 22.8 kg/m3 and different ambient temperatures (800 K, 850 K and 900 K). In reactive conditions, the lift-off length will be measured by OH* chemiluminescence images at 310 nm. The Schlieren technique is also used to measure the ignition delay. The results about ignition delay will be discussed by comparing both fuels. In conclusion, the behavior of both fuels as function of temperature is similar even if liquid length of ABE20 is shorter than n-dodecane in all ambient temperature cases. In the other hand, no real difference for vapor spray penetration between two fuels is obtained. The vaporization properties and the lower ability of auto-ignition and of ABE20 lead longer ignition delays and lift-off length.

 

    2. Impact of nitric oxide on n-heptane and n-dodecane autoignition in a new high-pressure and high-temperature chamber

In piston engines, the reacting mixture always contains a certain amount of residual gases resulting from the incomplete combustion of species such as carbon monoxide and unburned hydrocarbons in the previous combustion cycle. One of these leftover combustion species is nitric oxide (NO) which has an impact on the autoignition kinetics. Some studies have reported that the appearance of small quantities of NO typically occurs inside the CVP at a concentration of a few dozen ppm after mixing with the fresh charge. No studies are yet available confirming the absence of NO during premixed burn in this kind of chamber. However, the existence of these burnt products has been reported in few studies to have a significant impact on autoignition in SI engines and HCCI engines. Ignition delay (ID) is a key combustion property for fuels used in compression-ignition (CI) engines, and is commonly used as the metric for the validation of combustion simulations. A promoting effect of NO was often observed, particularly at high temperatures and low NO concentrations, whereas inhibiting effects have also been reported in other conditions.

The purpose of this work was to study the influence of NO on ID for different fuels in NOSE. Pure n-heptane and pure n-dodecane were the fuels used. First, to determine the ability of fuels to auto-ignite, the ignition delays of these fuels without NO were characterized and compared. Experiments were conducted at the Spray A operating conditions of ECN in NOSE. The engine set-up was also improved to extend conditions to different ambient temperatures (800 K and 850 K). To provide a homogeneous temperature field at 800 K or 900 K two specific compression ratios were tested 12.6 and 15 respectively. In the thermodynamic conditions chosen (60 bar and over 800–900 K), the effect of NO on ID and LOL was studied by adding 0-200 ppm NO to the engine intake. To measure ID, different techniques such as Schlieren visualization and OH* chemiluminescence were used to analyze the cool flame and the high temperature ignition (hot flame). The Lift-Off Length (LOL) was measured by OH* chemiluminescence images. The observed influence of NO addition on ID and LOL was analyzed. The related chemical reasons responsible for the effects are then discussed.

In the thermodynamic conditions chosen, NO had a strong effect on ID, with increases in NO tending to reduce the ignition delay. Results showed that ID and LOL presented the same trend as a function of temperature and NO concentration. Experimentally, at 900 K the ignition of n-dodecane was promoted by NO up to 100 ppm, whilst higher NO levels did not further promote ignition and a stabilization of the value has been noticed. For n-heptane, stronger promoting effects were observed in the same temperature conditions: the ignition delays were monotonically reduced with up to 200 ppm NO addition. At a lower temperature (800 K) the inhibiting effect was observed for n-dodecane for [NO] greater than 40 ppm, whereas only a promoting effect was observed for n-heptane. The experimental results of LOL showed that NO shortened LOL in almost all cases, and this varied with both the NO concentration and the mixture temperature. Thus, fuels with shorter ignition delays produce shorter lift-off lengths.