Laser-induced Phosphorescence Research Articles

Simultaneous investigation of flow and thermal fields inside a square duct with a built-in heated circular cylinder

Air-based cooling methods are widely used for heat exchange and thermal management, due to their potential for heat removal with minimal complexity and operating costs. However, the physical mechanisms involved in these processes are not yet fully understood, due to the complexities involved in the many interacting underlying sub-processes pertaining to the energy transfer from the surface to the bulk fluid. When dealing with turbulent flows involving heat transfer, one needs to delineate the coupled effects of velocity and temperature to ascertain the heat transfer dynamics. Here, we present the first-ever application of an advanced methodology based on the concept of two-color laser-induced phosphorescence (2cLIP) and particle image velocimetry (PIV), elucidating the mechanisms involved in the heat transfer from the surface of a cylinder placed inside a confined square duct. The potential advantage of the technique is in measuring the whole-field velocity and thermal fields simultaneously by incorporating modifications to the conventional PIV systems through the usage of a single tracer (BAM:Eu+2) for measuring the velocity and temperature fields. This aids in reducing the costs and complexities involved in setting up separate systems for measuring the temperature field distributions. Measurements reported here, were performed for the Reynolds numbers of 270 and 360, and for the heat flux levels of 10, 12 and 15 W. Experimental observations revealed that, with an increase in the mass flow rate from 3.06×10-4 (kg/s) to 4.08×10-4 (kg/s), the heat removal rate from the surface of the cylinder was enhanced. Also, the heat transfer was witnessed to enhance in the rear portion of the cylinder, in the case of increased cylinder heat input at constant mass flow rate. Also, a local peak in the heat transfer coefficient was indicative of the flow separation from the cylinder surface. The spatio-temporally averaged Nusselt number (Nu¯D) was witnessed to be higher for higher mass flow rate and was seen to increase with increasing cylinder heat input at constant mass flow rate. To the best of the knowledge of the authors, the work reported is one of the first attempts to carry out the simultaneous, whole-field, non-intrusive measurements of flow and heat transfer in a confined channel fitted with a heated cylinder using Thermographic-PIV technique.

Phosphor thermometry in heat transfer fluids and liquid organic hydrogen carrier systems using (Sr,Ca)SiAlN3:Eu2+

Temperature control in heat exchangers in reacting and non-reacting flows is of great importance for process optimization. In this context, phosphor thermometry is a promising technique for remote planar temperature sensing. The thermometry technique is based on exciting a luminescent material by a laser pulse and analyzing the subsequent phosphorescence signal. A particular interesting application is chemical hydrogen storage using liquid organic hydrogen carrier (LOHC) systems. The knowledge of temperature fields is of special interest for the characterization and understanding of hydrogen release from the carrier liquid. We investigated the luminescence properties of the thermographic phosphor (Sr,Ca)SiAIN3:Eu2+ (‘SCASN:Eu2+’) dispersed in different heat transfer fluids, in particular LOHC systems, using a newly developed calibration cell. As heat transfer fluids may be excited to fluorescence by the laser as well, their absorption and florescence behavior is studied to develop an excitation and detection concept for thermometry. We found strong absorption of the heat transfer fluids from the UV range to a wavelength of about 400 nm. In addition, fluorescence signals were found in the visible wavelength range, which can interfere with the phosphor emissions. These fluorescence signals should therefore be circumvented by utilizing the different luminescence decay times in the chosen detection strategy. For thermometry, the SCASN:Eu2+ particles were excited by a laser sheet of a 532 nm Nd:YAG laser. A spectrometer and photomultiplier tube (PMT) were used to detect the emission spectrum and phosphorescence decay time (PDT). Two temperature evaluation strategies were applied, which are based on either the intensity ratio of two spectral emission regions (two-color laser-induced phosphorescence) or the PDT. The results obtained show an applicable measurement range between 293 K and 598 K for the intensity ratio method with a maximum relative sensitivity of 0.12% K−1 at 293 K. For the PDT method, the phosphor allows measurements between 423 K and 598 K with a maximum relative sensitivity of 0.56% K−1 at 598 K.

Review of Development and Comparison of Surface Thermometry Methods in Combustion Environments: Principles, Current State of the Art, and Applications

Temperature is one of the most important parameters in the combustion processes. Accurate surface temperature can help to gain insight into the combustion characteristics of various solid or liquid fuels, as well as to evaluate the operating status of combustion power facilities such as internal combustion engines and gas turbines. This paper mainly summarizes and compares the main surface thermometry techniques, from the aspects of their principles, current state of development, and specific applications. These techniques are divided into two categories: contact-based thermometry and non-intrusive thermometry. In contact-based thermometry, conventional thermocouples as well as thin-film thermocouples are introduced. These methods have been developed for a long time and are simple and economical. However, such methods have disadvantages such as interference to flow and temperature field and poor dynamic performance. Furthermore, this paper reviews the latest non-intrusive thermometry methods, which have gained more interest in recent years, including radiation thermometry, laser-induced phosphorescence, liquid crystal thermography, the temperature-sensitive paint technique, and the temperature-indicating paint technique. Among them, we highlighted radiation thermometry, which has the widest measurement ranges and is easy to acquire results with spatial resolution, as well as laser-induced phosphorescence thermometry, which is not interfered with by the emissivity and surrounding environment, and has the advantages of fast response, high sensitivity, and small errors. Particularly, laser-induced phosphoresce has attracted a great deal of attention, as it gets rid of the influence of emissivity. In recent years, it has been widely used in the thermometry of various combustion devices and fuels. At the end of this paper, the research progress of the above-mentioned laser-induced phosphorescence and other techniques in recent years for the surface thermometry of various solid or liquid fuels is summarized, as well as applications of combustion facilities such as internal combustion engines, gas turbines, and aero engines, which reveal the great development potential of laser-induced phosphorescence technology in the field of surface thermometry.

Dynamic characterization of wall temperature in LOX/CH[formula omitted] rocket engine operating conditions using phosphor thermometry

Accurate surface temperature prediction is essential for the lifetime and efficiency of a rocket engine. The new ambition of future reusable engines requires perfect knowledge of the thermal environment encountered. In the design phase of such systems and to improve numerical simulations, more resolved experimental data are needed. Experiments are thus conducted by Lab. EM2C, ONERA, CNES, and ArianeGroup on the cryotechnic test bench MASCOTTE operated by ONERA. Conditions representative of those encountered in the gas generator of a rocket engine are investigated. Among them is the cryogenic injection of transcritical methane and oxygen at a very low mixture ratio (0.3) and high pressure (up to 60 bar) through a single coaxial injection element. The present work aims at characterizing the temperature of several surfaces of the combustion chamber in a series of hot fire tests performed in the transient thermal regime and in a short period of time (∼30 s). For this purpose, dynamic Laser Induced Phosphorescence (LIP) is implemented with the Full Spectrum Fitting method (FSF method), a dedicated approach developed by Lab. EM2C. Single-shot measurements are performed to simultaneously track the temperature of five surfaces at 10 Hz located near and downstream of the flame with a 30 K uncertainty. Data indicate that the temperature peak at ignition can be recorded with LIP measurements in contrast to conventional sensors. The analysis of gas and surface temperatures shows that convection is the predominant heat transfer mode and drives the surface temperature. Thanks to LIP thermometry on the internal and external faces of one of the silica windows, a 1D semi-infinite medium model is applied, showing excellent agreement with the measurements. Fitting this model on the measurements allows the estimation of the local convection coefficient and the prediction of the temperature at any depth inside the walls.

Observations and modelling of glyoxal in the tropical Atlantic marine boundary layer

Abstract. In situ field measurements of glyoxal at the surface in the tropical marine boundary layer have been made with a temporal resolution of a few minutes during two 4-week campaigns in June–July and August–September 2014 at the Cape Verde Atmospheric Observatory (CVAO; 16∘52′ N, 24∘52′ W). Using laser-induced phosphorescence spectroscopy with an instrumental detection limit of ∼1 pptv (1 h averaging), volume mixing ratios up to ∼10 pptv were observed, with 24 h averaged mixing ratios of 4.9 and 6.3 pptv observed during the first and second campaigns, respectively. Some diel behaviour was observed, but this was not marked. A box model using the detailed Master Chemical Mechanism (version 3.2) and constrained with detailed observations of a suite of species co-measured at the observatory was used to calculate glyoxal mixing ratios. There is a general model underestimation of the glyoxal observations during both campaigns, with mean midday (11:00–13:00) observed-to-modelled ratios for glyoxal of 3.2 and 4.2 for the two campaigns, respectively, and higher ratios at night. A rate of production analysis shows the dominant sources of glyoxal in this environment to be the reactions of OH with glycolaldehyde and acetylene, with a significant contribution from the reaction of OH with the peroxide HC(O)CH2OOH, which itself derives from OH oxidation of acetaldehyde. Increased mixing ratios of acetaldehyde, which is unconstrained and potentially underestimated in the base model, can significantly improve the agreement between the observed and modelled glyoxal during the day. Mean midday observed-to-modelled glyoxal ratios decreased to 1.3 and 1.8 for campaigns 1 and 2, respectively, on constraint to a fixed acetaldehyde mixing ratio of 200 pptv, which is consistent with recent airborne measurements near CVAO. However, a significant model under-prediction remains at night. The model showed limited sensitivity to changes in deposition rates of model intermediates and the uptake of glyoxal onto aerosol compared with sensitivity to uncertainties in chemical precursors. The midday (11:00–13:00) mean modelled glyoxal mixing ratio decreased by factors of 0.87 and 0.90 on doubling the deposition rates of model intermediates and aerosol uptake of glyoxal, respectively, and increased by factors of 1.10 and 1.06 on halving the deposition rates of model intermediates and aerosol uptake of glyoxal, respectively. Although measured levels of monoterpenes at the site (total of ∼1 pptv) do not significantly influence the model calculated levels of glyoxal, transport of air from a source region with high monoterpene emissions to the site has the potential to give elevated mixing ratios of glyoxal from monoterpene oxidation products, but the values are highly sensitive to the deposition rates of these oxidised intermediates. A source of glyoxal derived from production in the ocean surface organic microlayer cannot be ruled out on the basis of this work and may be significant at night.

Simultaneous imaging of two-phase velocities in particle-laden flows by two-color optical phase discrimination.

In this Letter, we present a particle image/tracking velocimetry (PIV/PTV) technique for simultaneous velocity measurement of both fluid and particle phases, adopting newly developed optical phase discrimination methods and novel optical particles. Spherical acrylic (PMMA) particles of diameter ∼O(100µm) were used as the particle phase, while fine BAM:Eu2+ phosphors of diameter ∼O(1µm) were used as the fluid tracer. Under Nd:YAG 355 nm laser excitation, both the laser-induced fluorescence (LIF) from PMMA and laser-induced phosphorescence (LIP) from BAM:Eu2+ provided sufficiently strong signals for PIV imaging with two non-intensified cameras and were clearly separable for phase discrimination using spectral filters and temporal profiles. The advantages of the PIV/PTV method include the relatively low cost of BAM:Eu2+ phosphors, high sphericity and narrow size distribution of PMMA particles with LIF emission, and direct optical discrimination eliminating artifacts, while requiring much less computational capacity for PIV/PTV processing of complex particle-laden flows.

Temperature imaging of mobile BaMgAl10O17:Eu phosphor aggregates under high radiation flux

Planar laser-induced phosphorescence (PLIP) has been used for the temperature measurement of suspended phosphor aggregates. Two-dimensional surface temperature of fluidized phosphor aggregates was measured through a pair of spatial and wavelength resolved images in an environment where phosphors are heated by high-flux radiation. The heating was supplied by a multi-diode laser system which provides a well-characterized high-flux radiation up to 28.87 MW/m2. Phosphors made of BaMgAl10O17:Eu (BAM) were selected as the material and suspended in a fluidized bed. Single-shot temperature imaging of BAM aggregates were inferred and compared at several heat fluxes. With the increasing heat flux up to 28.87 MW/m2, the BAM aggregates were found to exhibit a wider range of temperature distribution, and the maximum average aggregate temperature achieved 723 K, while the maximum temperature of a single aggregate could reach up to 1063 K. The wider temperature distributions that observed under higher radiation fluxes were caused by the elevated temperature of cooling air and the non-uniform aggregate surfaces. This non-intrusive method of measuring temperature offers advantages over other available methods in the study of heat transfer processes involving high-temperature reactions.

Convective heat transfer in droplets of fuel microemulsions during conductive heating

The experimental findings of the convective heat transfer in drops and films of fuel microemulsions heated on hot surfaces are presented. The study examines the Rayleigh-Benard-Marangoni convection in the films of fuel microemulsions heated from below under the conditions close to saturation and the forced convection in the drops of the same liquids in the Leidenfrost state. Laser-induced phosphorescence method is used to record the temperature of drops. Features and characteristics of vaporization and coalescence of the dispersed phase of microemulsions are analyzed by optical microscopy. The experiments are performed with the heated substrates made of sapphire and stainless steel. The fuel microemulsions are manufactured from diesel fuel, fatty acids methyl esters of rapeseed and sunflower oils, distilled water, surfactant – polyethylene glycol ether of isononylphenol, and co-surfactant – 2-ethylhexanol. The results show that the evaporation rate of the dispersed phase of microemulsions largely depends on the total concentration of emulsifier and distilled water. At surface temperatures below 80 °C, the mean diameter of droplets of the dispersed phase decreases linearly with increasing temperature; when the temperature is above 80 °C, the droplets are enlarged about two times, initiating the bubble boiling. The number of the large coalesced droplets of the dispersed phase decreases linearly with time; the process occurs more intensively at surface temperatures above 80 °C. At a local liquid temperature of about 120–150 °C, an intensely heated microemulsion drop partially disintegrates (puffing). With an increase in the surface temperature of the substrate from 360 °C to 425 °C, the evaporation rate of microemulsion drops practically does not change, i.e., it reaches certain asymptotic values. The significant heterogeneity of temperature fields of microemulsion drops in the Leidenfrost state is shown.

Application of the laser induced phosphorescence method to the analysis of temperature distribution in heated and evaporating droplets

Results of detailed analysis of temperature fields in droplets of four widely used liquids (water, kerosene, Diesel and gasoline (petroleum oil) fuels) are presented. Single droplets suspended on a wire were heated in a flow of hot air. The initial droplet radii were in the range 1 to 2 mm, air temperature was in the range 20 ∘C to 500 ∘C, air flow velocity was 3-3.5 m/s. The droplet temperature was measured based on Laser Induced Phosphorescence (LIP). BAM:Eu (BaMgAl10O17:Eu2+) microparticles were introduced into the droplets for the emission of a temperature-sensitive phosphorescent signal. Optical sectioning inside the droplet was performed using a thin laser sheet, while two cameras detected the phosphorescence signal in two spectral bands. A ratiometric approach using the pixel-to-pixel ratio of the images recorded by the two cameras allowed us to determine the local temperature within the heated and evaporating droplet. The range of applicability and the advantages/shortcomings of the method are established alongside the sources of errors. The experimentally observed droplet surface temperatures are compared with the predictions of the customised version of ANSYS Fluent with the Effective Thermal Conductivity (ETC) model implemented into it via User Defined Functions (UDF). It is shown that ANSYS Fluent can correctly predict the trend of the time evolution of these temperatures.

Non-invasive, spatially averaged temperature measurements of falling acetone droplets in nitrogen atmosphere at elevated pressures and temperatures

Mean temperatures of acetone, millimeter-sized, freely falling droplets in a nitrogen atmosphere are studied at two different locations downstream a capillary-type injector. Injection and ambient temperatures have been varied independently between 433 and 513K and pressures from 2 to 6MPa. The ratio of simultaneously measured laser-induced phosphorescence (LIP) and fluorescence (LIF) is shown to be a suitable approach for measuring temperatures in the given conditions. Cumulated uncertainties (2σ) of mean droplet temperatures of ±3K are estimated. The temperatures measured in this study are partly compared to results of previous investigations where droplet temperatures have been estimated by an extrapolation of temperature mixture fraction data in the gas phase obtained from spontaneous Raman scattering. Overall a reasonable agreement has been observed which indicates the applicability of the previous approach for estimations based on the adiabatic mixing assumption and the extrapolation of the Raman-measured data.

Characterization of the phosphor (Sr,Ca)SiAlN3: Eu2+ for temperature sensing

Laser-induced phosphorescence based on luminescent particles is a suitable technique for remote temperature sensing. The luminescence characteristics of the phosphor (Sr,Ca)SiAlN3:Eu2+ (”SCASN”) were investigated regarding the suitability of the material for phosphor thermometry. The phosphor has a high quantum efficiency, a very broad excitation band, an emission peak centered at about 620 nm and a short lifetime of less than 1μs, which makes it promising for thermometry measurements. After excitation at 532 nm, the luminescence emission reveals a blue shift with increasing temperature, which can be used for temperature calibration based on the two-color intensity ratio method. The excitation and emission characteristics are favorable for measurements in environments containing fuels and oils, which tend to fluorescence after UV-excitation. The phosphor allows for measurements up to 800 K, with a quenching temperature of 530 K. Spectral characteristics are presented for both bulk powder and a surface coating. For the former, a significant shift in the position of the emission peak is observed, which is most probably due to distinct re-absorption. A change in excitation laser fluence slightly affected the luminescence emission spectrum of both bulk material and coating and thus requires consideration in temperature determination.

Temperature Fields of the Droplets and Gases Mixture

In this research, we obtain gas–vapor mixture temperature fields generated by blending droplets and high-temperature combustion products. Similar experiments are conducted for droplet injection into heated air flow. This kind of measurement is essential for high-temperature and high-speed processes in contact heat exchangers or in liquid treatment chambers, as well as in firefighting systems. Experiments are conducted using an optical system based on Laser-Induced Phosphorescence as well as two types of thermocouples with a similar measurement range but different response times (0.1–3 s) and accuracy (1–5 °C). In our experiments, we inject droplets into the heated air flow (first scheme) and into the flow of high-temperature combustion products (second scheme). We concentrate on the unsteady inhomogeneous temperature fields of the gas–vapor mixture produced by blending the above-mentioned flows and monitoring the lifetime of the relatively low gas temperature after droplets passes through the observation area. The scientific novelty of this research comes from the first ever comparison of the temperature measurements of a gas–vapor–droplet mixture obtained by contact and non-contact systems. The advantages and limitations of the contact and non-contact techniques are defined for the measurement of gas–vapor mixture temperature.

Projection of holographic images in volumetric fluorescent fluids for near-eye displays

The optical setup for holographic projection on the scatterings in fluorescent liquids is presented. Such media can be used as volumetric screens for near-eye holographic displays, solving the problem of speckle noise and very small exit pupils in existing setups. Three different oils (canola, olive and engine oil) with 532 nm laser and tonic water with 405 nm laser are used for projecting holographic fields, the quality of such images is investigated. The laser wavelength is cut out from acquisition on a camera and only filtered fluorescent light is observed. The best and brightest results are obtained with engine oil. Full Text: PDF ReferencesX. Li, C. P. Chen, H. Gao, et al. "Video-Rate Holographic Display Using Azo-Dye-Doped Liquid Crystal", Journal of display technology 10(6), 438-443 (2014). CrossRef X. Li, Z. Song, F. Li, X. Dong, W. Liu, "79‐3: Video‐rate Holographic Display in ZnSe layer‐assisted Quantum Dot Doped Liquid Crystal with High‐photorefractive Sensitivity", SID Symposium Digest of Technical Papers. Vol. 48. No. 1. 2017, CrossRef Sasaki, Takeo, et al. "Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm–1 and response time of 8 ms", Applied Physics Letters 6(2) (2013) CrossRef N. Tsutsumi, K. Kinashi, A. Nomura, W. Sasaki, "Quickly Updatable Hologram Images Using Poly(N-vinyl Carbazole) (PVCz) Photorefractive Polymer Composite", Materials 5.8: 1477-1486 (2012) CrossRef M. Makowski, "Simple holographic projection in color", et al. Optics express 20.22: 25130-25136 (2012) CrossRef A. Yagi, M. Imura, Y, Kuroda, O. Oshiro, "360-degree fog projection interactive display", SIGGRAPH Asia 2011 Emerging Technologies. ACM, (2011) CrossRef C.H. Hsu, K. L. Hua, W. H. Cheng. "Omni-Tube: a low-cost portable omnidirectional interactive 3D display", SIGGRAPH Asia 2012 Posters. ACM, (2012) CrossRef Z. Zeng, H. Zheng, X. Lu, H. Gao, Y. Yu, "Dynamic holographic three-dimensional projection based on liquid crystal spatial light modulator and cylindrical fog screen", Opt Rev (2015) 22: 853 CrossRef I. Rakkolainen, "Feasible mid-air virtual reality with the immaterial projection screen technology", 3DTV-Conference, Tampere (2010) CrossRef S. Yanfeng, et al. "A multi-plane optical see-through holographic three-dimensional display for augmented reality applications", Optik 157: 190-196 (2018) CrossRef G. Li, D. Lee, Y. Jeong, J. Cho, B. Lee, "Holographic display for see-through augmented reality using mirror-lens holographic optical element", Opt. Lett. 41(11), 2486-2489 (2016) CrossRef C. L. Lin, Y. Z. Su, M. W. Hung, K. C. Huang "Augmented reality system", Proc. SPIE 7798, Applications of Digital Image Processing XXXIII, 779826 (2010) CrossRef A. Maimone, A. Georgiou, J. S. Kollin, "Holographic near-eye displays for virtual and augmented reality", ACM Trans. Graph. 36, 4, 1-16 (2017) CrossRef M. Quinten, Optical properties of nanoparticle systems: Mie and beyond (John Wiley & Sons 2010). CrossRef J.-W. Liaw, S.-W. Tsai, H.-H. Lin, T.-C. Yen, B.-R. Chen, "Wavelength-dependent Faraday–Tyndall effect on laser-induced microbubble in gold colloid", Journal of Quantitative Spectroscopy and Radiative Transfer 113(17), 2234-2242 (2012), CrossRef T. Mu et al. "Classification of edible oils using 532 nm laser-induced fluorescence combined with support vector machine", Anal. Methods 5, 6960 (2013) CrossRef T. Mu et al. "Classification of Motor Oil Using Laser-Induced Fluorescence and Phosphorescence", Analytical Letters 49:8, 1233-1239 (2015) CrossRef V. Rostampour, M. J. Lynch, "Quantitative Techniques To Discriminate Petroleum Oils Using LED-induced Fluorescence", WIT Transactions on Ecology and the Environment 95, 265 262 (2006) CrossRef F. Wyrowski and O. Bryngdahl, "Iterative Fourier-transform algorithm applied to computer holography", Opt. Soc. Am. A 5(7), 1058-1065 (1988) CrossRef

Gas-Vapor Mixture Temperature in the Near-Surface Layer of a Rapidly-Evaporating Water Droplet.

Mathematical modeling of the heat and mass transfer processes in the evaporating droplet–high-temperature gas medium system is difficult due to the need to describe the dynamics of the formation of the quasi-steady temperature field of evaporating droplets, as well as of a gas-vapor buffer layer around them and in their trace during evaporation in high-temperature gas flows. We used planar laser-induced fluorescence (PLIF) and laser-induced phosphorescence (LIP). The experiments were conducted with water droplets (initial radius 1–2 mm) heated in a hot air flow (temperature 20–500 °C, velocity 0.5–6 m/s). Unsteady temperature fields of water droplets and the gas-vapor mixture around them were recorded. High inhomogeneity of temperature fields under study has been validated. To determine the temperature in the so called dead zones, we solved the problem of heat transfer, in which the temperature in boundary conditions was set on the basis of experimental values.

Uncertainty analysis in structured laser illumination planar imaging (SLIPI) applied to non-linear signals: gas-phase phosphor thermometry

Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the synthetic PIV image generator for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.

Uncertainty analysis in Structured Laser Illumination Planar Imaging (SLIPI) applied to non-linear signals: gas-phase phosphor thermometry

Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the Synthetic PIV Image Generator (SIG) for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.

Structured illumination for two-dimensional laser induced fluorescence imaging to eliminate stray light interference

<sec>Laser sheet imaging, also called planar laser imaging, is one of the most versatile optical imaging techniques and has been frequently used in several different areas. However, when applied to the limited operating space and strong light scattering media, the light originating from indirect reflections, multiple scattering and surrounding backgrounds can produce error especially in intensity-ratio based measurements.</sec><sec>This work is motivated by these challenges, with the overall aim of making laser sheet imaging technique applicable for the study of eliminating the stray light interference. Therefore a novel two-dimensional imaging technique named structured laser illumination planar imaging (SLIPI) is developed based on planar laser imaging but uses a sophisticated illumination scheme i.e. spatial intensity modulation, to differentiate between the intensity contribution arising from useful signals and that from stray light. By recording and dealing with images, the SLIPI method can suppress the diffuse light and retain the useful signals.</sec><sec>In this paper, we first use the MATLAB software to simulate the phase-shift SLIPI method, and the results show that the stray light interference can be eliminated completely. Furthermore, the phase-shift SLIPI is combined with the liquid solution (Rhodamine B solution) laser induced fluorescence (LIF) approach to imagine the concentration distribution. By recording three images, between which this encoding is changed noticeably only for the useful LIF signals, the phase-shift SLIPI method is evidenced to be able to remove the diffuse light contribution, thus improving and enhancing the visualization quality. The instantaneous SLIPI images of rapidly moving samples, a key feature to study dynamic liquid solution diffusion behavior, are also acquired. The lock-in amplifier SLIPI technique is then experimentally studied under Rhodamine B diffused solution, and the phase-shift SLIPI method can remove the unwanted background interferences and achieve the significant improvements in terms of pronounced concentration distribution within the Rhodamine B solution.</sec><sec>The SLIPI technique is relatively inexpensive: the cost does not exceed the cost of an ordinary laser sheet arrangement noticeably, and it can combine with several other linear imaging techniques, such as Rayleigh scattering, particle image velocimetry and laser-induced phosphorescence. </sec>

Temperature and Velocity of the Gas–Vapor Mixture in the Trace of Several Evaporating Water Droplets

The optical techniques (particle image velocimetry (PIV), laser-induced phosphorescence (LIP), planar laser-induced fluorescence (PLIF)) are used to study unsteady and inhomogeneous temperature and velocity fields of a gas–vapor mixture forming in the immediate vicinity of rapidly evaporating water droplets. Experiments involve various arrangements of several (two, three, and five) water droplets in a heated air flow. We establish the dependencies of the temperature and velocity of a gas–vapor mixture in the trace of each droplet on the heating time, velocity and temperature of the air flow, initial dimensions, and droplet arrangement scheme. Distinctive features of the synergistic effect of a droplet group on their temperature and aerodynamic traces are identified. Longitudinal and transversal dimensions of the aerodynamic and thermal traces of evaporating droplets are established. The length of the temperature trace of one droplet equals 10–12 of its radii, and the width of the temperature and aerodynamic trace of a droplet is no larger than its diameter.

Analysis of wall temperature and heat flux distributions in a swirled combustor powered by a methane-air and a [formula omitted]-diluted oxyflame

The behavior of technically premixed CO2-diluted methane oxyflames is compared to operation with methane-air flames in a labscale combustor equipped with an axial-plus-tangential swirler. It has been shown in a former study that the stabilization regimes and topologies of CO2-diluted oxy-flames can be deduced from methane-air flames despite large changes of the oxidizer and fuel flowrates. This work proceeds and focuses on differences in distributions of wall temperature and wall heat fluxes for a methane-air and a CO2-diluted methane oxyflame sharing the same thermal power, equivalence ratio, adiabatic flame temperature and swirl number. Laser-Induced Phosphorescence measurements are used to determine the temperature distributions along the combustor metallic back plane wall and along the internal and external surfaces of the quartz windows. These data are used to determine the local heat flux through the combustor windows. It is found that both the air and CO2-diluted oxy-flames feature approximately the same temperature distributions and total thermal loads along the combustion chamber walls. As high concentrations of carbon dioxide are known to significantly enhance radiative heat transfer, an analysis is carried out so as to account for the mechanisms that provide this similarity in the wall temperatures and heat fluxes between the two flames. The low order model allows for determining the origin of the heat flux and shows how convective heat transfer is supplemented by radiative heat transfer when switching from air to CO2-diluted oxy-combustion with a global thermal load that remains roughly unaltered for the studied combustion chamber.

Temperature and velocity fields of the gas-vapor flow near evaporating water droplets

Experimental studies of unsteady temperature and velocity fields of the gas-vapor flow in the immediate vicinity of evaporating water droplets were performed in the interests of developing high-temperature (over 300°С) gas-vapor-droplet applications. The formation time of virtually homogeneous (temperature variations of under 2–3°С) temperature fields of evaporating water droplets was established using Particle Image Velocimetry, Laser Induced Phosphorescence, Planar Laser Induced Fluorescence. We observed highly inhomogeneous and unsteady temperature and velocity fields of the gas-vapor mixture and determined the lateral and transversal dimensions of the aerodynamic and thermal traces of evaporating water droplets. The degree of impact of several parameters on the latter was evaluated: initial temperature (20–500 °C) and velocity of the incoming flow (0.5–5 m/s), as well as the initial droplet size (1–2 mm). The role of evaporation and convective heat exchange between a droplet and gases in forming of thermal and aerodynamic trace was analyzed. The heat flow density was found to vary nonlinearly with time (in the immediate vicinity of an evaporating droplet), and the impact of droplet trace temperature and velocity on the heat flow density was evaluated. Both advantages and limitations of the used techniques are established in terms of reliable plotting of unsteady temperature and velocity fields of gas-vapor mixture in the trace of evaporating water droplets, considering the heating lag of the latter.