Phosphor Thermometry Research Articles

Sub-∘C-precision temperature imaging using phase-shift luminescence thermometry

Abstract This work explores the potential of luminescence thermometry to provide temperature imaging with high temperature resolution using the phase-shift method. Phosphor thermometry has been widely used for temperature imaging, especially with the decay-time method. However, the phase-shift method was rarely used. In this approach, excitation is modulated in time, and due to the temperature-dependent luminescence dynamics of the painted luminescent probe, the emission waveform is phase-shifted with respect to the excitation waveform. Initially, a parametric study was performed on the effect of excitation waveform frequency and shape, based on a millisecond-decay time phosphor. The study found that the best precision is achieved with a square waveform at a frequency such that the phase shift is π 4 at the target temperature. Then, this method was compared with the well-established decay-time method. Phase-shift method offered a three-fold improvement in temperature precision under identical peak excitation power. Finally, two-dimensional measurements were demonstrated on a temperature-controlled plate using a high speed CMOS camera, and an LED illumination. A temperature precision of 0.13 ∘C was obtained at a temporal resolution of 0.25 s and a spatial resolution of around 1 mm, due to a pre-processing binning of 7 by 7 pixels. This technique can be used as a robust alternative to infrared thermometry to probe the thermal processes with small temperature differences.

Defect-Enabled Superior Negative Thermal Quenching in Palmierite Ba9La(VO4)7:Eu3+ for WLED In Situ Temperature Measuring.

Negative thermal quenching (NTQ) of the phosphors is critically important for both scientific research and practical applications, but the design of efficient NTQ phosphors is still a challenging task. Herein, we report a new strategy for developing NTQ materials by cation-vacancy engineering. Specifically, a new color-tunable Ba9La1-x(VO4):xEu3+ (BLVO:xEu3+) phosphor with abundant intrinsic cation vacancy was developed, exhibiting superior NTQ behavior under 365 nm excitation. The NTQ performance can be modulated via adjusting Eu3+ doping levels, and the emission intensity of Eu3+ ions in the BLVO:0.20Eu3+ phosphor increased by 275% at 473 K compared to room temperature. Furthermore, the reported material emitting bright white light under 365 nm excitation was well-suited for use in white light-emitting diode (WLED) phosphor and fluorescent temperature sensors, exhibiting outstanding color-rendering index (90.1) in lighting and high sensitivity (Sa = 11.83% K-1, Sr = 2.33% K-1) in temperature detecting. Lastly, the operating temperature of WLED at different currents can be monitored and displayed in real time through emission spectroscopy. All of the results demonstrated that the designed NTQ BLVO:xEu3+ can be used as a single-phase white phosphor and optical thermometry. This work provides a fresh perspective for designing high-efficient NTQ phosphors and expands the application of phosphors in WLED in situ temperature detection.

Implementation of a high-frequency phosphor thermometry technique to study the heat transfer of a single droplet impingement

Contributing to a better understanding of spray cooling systems, the heat transfer process underlying the event of a droplet impinging onto a uniformly heated stainless steel surface (SS304) was experimentally investigated. Since the heat transfer process is linked to the droplet’s hydrodynamics, high-speed videos were recorded to measure the deformation of the droplet. A series of isothermal and non-isothermal impacts were performed for Weber numbers (We) within the range 17.7≤We≤58.2. A strong relationship between the maximum spreading ratio reached by the droplet and its initial kinetic energy was found. The surface temperature directly affects the droplet hydrodynamic during the impact by promoting an oscillatory behavior of the droplet after the maximum spreading is reached. Given the spatial–temporal resolution of the heat transfer process, a high-frequency phosphor thermometry technique was implemented, finding that the temperature drop upon droplet impact was independent of impact velocity. The sharp temperature drop results in an intense thermal interaction that occurred during the first 10 ms of the impact. The maximum average heat flux registered was 98.56 W/cm2 with a cooling effectiveness of 3.5%.

YAG:Dy co-doped with Tb for lifetime-based phosphor thermometry from room temperature to 1600 °C

Phosphor thermometry has demonstrated significant potential for nondestructive high-temperature measurements in gas turbines. To expand the measurement range through lifetime-based phosphor thermometry, we developed a novel phosphor, YAG:Dy co-doped with Tb (YAG:Dy,Tb). Three YAG:Dy,Tb samples with varying Tb concentrations were synthesized through the sol–gel method. A fiber-optic-coupled measurement system was established to capture multiple emission peaks of YAG co-doped with Dy3+ and Tb3+ at 544 nm, 484 nm, and 458 nm. Efficient energy transfer from Dy3+ to Tb3+ resulted in a substantial enhancement of Tb3+ emission at 544 nm under 355 nm excitation. Owing to the energy transfer, the temperature measurement range under the lifetime method was extended from room temperature to 1600 °C using the combination of Tb3+ emission at 544 nm and Dy3+ emission at 458 nm. YAG:Dy,Tb samples with higher concentrations of Tb3+ exhibited superior temperature measurement performance, mainly owing to their stronger signal-to-noise ratio at >1000 °C. The performances of different emission peaks were also compared according to temperature uncertainty, which generally ranged from 0.1 °C to 2.7 °C across the entire measurement range.

On the intrusiveness of phosphor thermometry for metal surface temperature measurements in reciprocating engines: an experimental and heat conduction modeling study

Abstract Lifetime-based phosphor thermometry has been applied in a wide variety of surface temperature measurement applications due to its relative ease of implementation and robustness to background interference when compared to other optical temperature measurement methods. It is often assumed that the technique is minimally intrusive if the thickness of the applied phosphor coating is < 20 µm. To evaluate this assumption, high-speed phosphor surface temperature and thermocouple measurements were performed on 4140 steel substrates installed in the cylinder head of an optically-accessible internal-combustion engine for four operating conditions. For phosphor thermometry measurements, four substrates were studied, each coated with a phosphor layer of different thickness ranging between 6 µm and 47 µm. The phosphor thermometry measured temperature swings during combustion were shown to be heavily impacted by the presence of the phosphor coatings, increasing by roughly a factor of 2 - 2.5 when increasing the thickness from 6 µm to 30 µm. A technique was implemented which utilizes the temperature data in combination with a heat conduction model to provide estimates of the temperature swing and heat transfer flux in the absence of the phosphor coating. It was shown that even the 6 µm phosphor coating could lead to an order of magnitude increase in the temperature swing relative to the uncoated 4140 steel substrate. Despite the intrusiveness of phosphor thermometry for the surface temperature measurements, reasonable agreement was demonstrated between heat flux estimates determined with the heat transfer modeling technique and those deduced from temperature-swing measurements using two different high-speed thermocouples. The results indicate that phosphor surface thermometry can be a reliable surface temperature and heat flux diagnostic for transient high heat flux environments, as long as proper care is taken to account for the impact of the phosphor layer on the measurement.

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Temperature imaging during the hydrogen release reaction from a liquid organic hydrogen carrier (LOHC) system using phosphor thermometry

Liquid organic hydrogen carrier (LOHC) systems offer a particularly interesting option for chemical hydrogen storage. In order to characterize and understand the endothermal hydrogen release from the carrier liquid and to evaluate suitable catalyst materials, knowledge of the temperature fields in the dehydrogenation reactor is important. One suitable technique for planar temperature sensing in reacting systems is phosphor thermometry. It is based on the excitation of a luminescent material by a laser pulse and detection of the subsequent phosphorescence signal. We investigated the luminescence of the thermographic phosphor (Sr,Ca)SiAIN3:Eu2+ (“SCASN:Eu2+”) dispersed in the H0-DBT / H18-DBT LOHC system in a temperature range from 400 to 600 K. A measurement cell enables repeatable and homogeneous measurement conditions of the hydrogen release reaction. A catalytic plate was put inside the heated LOHC. Temperature fields during the hydrogen release reaction were measured for the first time using the phosphorescence decay time (PDT) and the phosphorescence intensity ratio method (PIR). As expected, a strong cooling at the catalyst surface during the endothermal hydrogen release reaction could be observed, which was quantified to be in the range of 40 K.Graphical abstract

Three-dimensional surface temperature measurement using lifetime-based phosphor thermometry

Abstract In this study, we propose a three-dimensional (3D) surface temperature measurement method based on the principle of stereoscopic 3D reconstruction and the dependence of phosphorescence lifetime on temperature. A 385-nm UV(Ultraviolet) light was used as the excitation light, and two high-speed cameras were used as the detectors. The phosphor MFG (Mg4FGeO6: Mn4+) was mixed with the binder HPC and sprayed onto the tested 3D surface. The natural texture generated by the surface roughness of the phosphor coating was used as a feature for cross-correlation calculations. The digital image correlation (DIC) algorithm was used to match these feature positions in the phosphorescent images from the two cameras. The effects of the excitation angle and detecting angle were analyzed. The results indicate that the temperature measurement based on phosphorescent lifetime was not affected by the excitation and detecting angle. The method was validated on a turbine blade as an example of a 3D surface to demonstrate the capability. A comparison of the measurement results with the thermocouples proves that the current method can successfully measure the temperature on 3D surfaces with a maximum difference of 1.63°C. The spatial accuracy of the method was obtained by comparing with the measurement results of a 3D scanner, which shows that the maximum absolute error of the 3D reconstruction was 0.350 mm. The current study proposes a promising 3D surface temperature measurement method, which is expected to be widely used in gas turbine blades, Internal Combustion (IC) engine cylinders, complex curved heat exchangers, and other fields due to its non-contact measurement, low susceptibility to infrared radiation interference, high measurement accuracy, and ability to withstand harsh environments.

The relative position of pyrolysis onset and flame front location for downward flame spread

The interaction of solid and gas phase phenomena in downward flame spread is studied under buoyant conditions. Phosphor thermometry (PT) was used to determine the spatiotemporally resolved surface temperatures both ahead and beneath the flame while CH* chemiluminescence allowed for the quantification of the flame location relative to the fuel surface. The combination of these measurements allowed for the superposition of the flame front location and the surface temperature distribution. The standoff distance (∼1.1 mm) and preheated distance along the surface (∼3.5 mm) were quantified. The application of PT allowed for the continued measurement of surface temperatures in the pyrolyzing region beneath the flame, where the surface temperature was found to remain relatively constant between 300 and 350 °C. The combination of measurements allowed for the calculation of the gas-phase heat flux distribution ahead of the flame front using a point-source approximation, which compared well with previous studies. PT and CH* measurements were used to expand previous work to quantify the thermal gradients through the solid both ahead of and behind the flame front. Surface temperature measurements were coupled with thermogravimetric analysis (TGA) and a simple pyrolysis model to determine that the onset of pyrolysis occurs approximately 1 mm ahead of the flame front. This approach was also used to illustrate that pyrolysis gases were generated in sufficient volume to result in a flammable mixture at the leading edge of the flame. This study highlights the benefit of optical diagnostic techniques in exploring the controlling mechanisms of flame spread, and provides insight into the interaction of solid and gas-phase phenomena in downward flame spread.

Development of fully fiber-coupled phosphor thermometry imaging for combustion applications

Fully fiber-coupled phosphor thermometry imaging was demonstrated in an atmospheric fuel-flexible jet-stabilized combustor operated with methane–hydrogen–air flames. Different fuel-air mixedness and operating conditions ranging from 100% methane to 100% hydrogen were investigated. The phosphor GdAlO3:Cr was excited at 532 nm using a fiber bundle, while an imaging fiber bundle was used for signal detection. The presented setup allowed to cover a temperature range from 400–1100 °C with a precision of 0.2–2%. Temperature maps of the combustor walls for six operating conditions will be presented and the influence of the different flame conditions on the wall temperatures will be discussed.

Non-intrusive thermometry in water: a novel temporal method using rise time-decay time ratio of ZnO phosphorescence in comparison with RhB PLIF and ZnO spectral imagery

The temporal characteristics of ZnO phosphor emission and the possibility to obtain instantaneous and mean 2D temperature fields in fluids have been analysed in this article. Spectrometer tests have been performed by mixing ZnO micro-particles in liquid (water) to analyse the emission characteristics of the phosphor upon excitation by a 266 nm laser. Using a fast gating camera, it has been shown that 2D phosphor thermometry can be performed through temporal methods using ZnO phosphors, which were thought previously to be unachievable. Additionally, a two-camera temporal method is proposed by exploiting the rise and decay times of the ZnO phosphor emission upon excitation. Sensitivity of the metrology is proposed and compared with commonly used Rhodamine B fluorescence and ZnO spectral methods.

Numerical simulation of laser heating effect in optical temperature measurement based on Kubelka-Munk theory

The fluorescence intensity ratio (FIR) technique is widely used in many fields as one of the sensing methods of phosphor thermometry. In the gas turbine field, the rare earth ions are often doped to thermal barrier coating (TBC). When excited by laser pulses TBC glows phosphorescent, and it can be utilized for thermometry. The laser energy absorbed by the TBC is not only used to generate the luminescence but also introduces heat into the TBC and causes a temperature rise further increasing optical temperature measurement error. Therefore, it’s essential to predict the temperature rise and the temperature measurement error, and take measures to reduce the error due to the laser heating effect. However, there are few researches concerning this problem, and most of them focus on the relationship between the laser energy and the point temperature measured by the FIR technique. Here we show a method for evaluating the effect of laser heating on temperature fields and optical temperature measurement errors by establishing thermal equivalent model and discrete model of body luminescence, considering laser and phosphorescence scattering in coatings made of YSZ based on Kubelka-Munk theory. We found that continuous pulsed laser heating caused severe temperature rise and significant temperature measurement error. Furthermore, the temperature rise can be decreased to the level of a single pulsed laser heating by decreasing the frequency of laser. In addition, the main influencing factors of the temperature measurement error are the density of laser energy and coating thickness.

Phosphor thermometry evaluation of heat transfer enhancement on a hot plate achieved by a vortex-based fluidic oscillator

Recently, a vortex-based feedback fluidic oscillator has been proposed as an alternative to conventional feedback fluidic oscillators. This study aimed to compare the heat transfer performance of a newly designed fluidic oscillator with that of conventional feedback fluidic oscillators with and without an output diffuser using phosphor thermometry. To obtain the instantaneous temperature field in a wide range of high temperatures, a lifetime-based method was employed in two-dimensional phosphor thermography, with Mg4FGeO6: Mn serving as the sensor material and being excited by a UV-LED. A high-speed camera was used to capture images of the hot plate at 4,000 fps. The calibration test was conducted in the temperature range of 25–325 ˚C, and the results were compared with data from thermocouples. The study investigated the impact of the mass flow rate and throat-to-plate distance ratio of three fluidic oscillators to obtain the optimum Nusselt number. The results demonstrated that the newly designed fluidic oscillator exhibited significantly superior cooling effects in comparison to other types of fluidic oscillators. At the lowest ratio of the throat-to-wall distance to the outlet throat height (Z/D = 4.5) and a flow rate of 100 Lpm, the vortex-based fluidic oscillator demonstrated 78 % and 45 % higher heat transfer rates compared to the conventional oscillator with and without an external diffuser, respectively. Furthermore, the newly designed oscillator was more sensitive to the mass flow rate and throat-to-plate distance ratio than the other oscillators.

Parametric study of intensity-ratio-based phosphor thermometry using Mg4FGeO6:Mn4+ for instantaneous temperature measurement

Intensity-ratio-based phosphor thermometry is advantageous for instantaneous temperature measurement because it avoids interframe displacement due to moving phosphor coated samples that is encountered with lifetime-based methods. This study was aimed at investigating the effects of exposure time, excitation energy, and wavelength on intensity-ratio-based phosphor thermometry using Mg4FGeO6:Mn4+ (MFG). First, the temperature-dependent emission spectra of MFG were obtained under 355 nm and 405 nm excitations. The spectra exhibited different trends in the two cases, potentially because of thermally enhanced phonon-assisted absorption. Next, the shot-to-shot calibration uncertainties were evaluated from the temperature calibration results under different exposure time and excitation irradiance/fluence values for light-emitting diode/laser. The uncertainties under 355 nm and 405 nm excitations exhibited similar trends, i.e. they increased and decreased with increasing temperature and exposure time, respectively. For the minimum exposure time of 4 μs under 355 nm excitation, the uncertainty increased from 1.5 K at 323 K to 3.3 K at 673 K at a constant excitation fluence of 3.14 mJ cm−2. The uncertainties under 405 nm excitation were higher than those under 355 nm excitation. In addition, the inaccuracies due to excitation non-uniformity were evaluated from the calibration results under different excitation irradiance/fluence. The calibration curves were more sensitive to excitation irradiance at higher temperatures under 405 nm excitation. Consequently, the excitation non-uniformity induced inaccuracies under 405 nm excitation and different exposure time values were small at low temperatures (0.4–1.1 K at 323 K) and increased rapidly with temperature (2.6–14.4 K at 673 K). In the case of 355 nm excitation, the calibration curves exhibited small sensitivity to excitation fluence only at low temperatures. As the temperature increased, the inaccuracies under 355 nm excitation decreased and then increased, and exhibited values between 0.5 K and 2.5 K.

Enhancing process efficiency through improved temperature measurement: the EMPRESS projects

EMPRESS 2 is a European project to enhance the efficiency of high value manufacturing processes by improving temperature measurement and control capability. This project seeks to address four contemporary thermometry challenges in this sector, and new developments from this and its predecessor project, EMPRESS, will be described:• Below 1000°C many industrial processes require reliable surface thermometry e.g. welding, coating, forging and forming. Conventional non-contact surface thermometry techniques e.g. thermal imaging are prone to large errors (tens of degrees) due to reflected thermal radiation and unknown emissivity. Contact thermometry approaches are prone to similarly large errors. Traceable imaging phosphor thermometry is being developed to overcome these difficulties, and is being combined with quantitative thermography to determine emissivity for thermometry over wide fields of view.• Above 1300°C sensor drift is a significant unaddressed issue for casting, forging and heat treatment, causing large errors. There is a need for more stable sensors and standardisation of at least one new thermocouple type to fill the gap from 1300°C to 1800°C. This is being addressed through improved Pt-Rh thermocouples and optimisation of double-walled mineral insulated, metal sheathed thermocouples by mitigating insulation breakdown and drift effects.• Combustion temperature measurement is very challenging and traceability is almost non-existent; for example, thermocouple measurements of flame temperatures can be in error by hundreds of degrees. A ‘standard flame’ that can be transported to users’ sites has been developed, and is being deployed in several high value manufacturing and industrial applications to a) demonstrate the possibility of reducing flame temperature uncertainties by at least an order of magnitude and b) for the first time to demonstrate in-situ traceability to the International Temperature Scale of 1990 (ITS-90).• Many processes are not amenable to any conventional thermometry techniques due to inaccessibility, ionising radiation, electromagnetic interference, and contamination; here methods based on optical fibres are ideal but there are no traceable calibration techniques for such sensors currently available. A suite of different fibre-optic thermometers and calibration techniques is being developed to address this. In some cases (ionising radiation) darkening of the fibre is a problem, and this is being overcome by the development of novel thermometry approaches based on practical ‘hollow core’ fibres.

Novel Strategy for Thermal Evaluation of Film-Cooled Blades Using Thermographic Phosphors at Simulated Engine Conditions

Abstract Practical strategy for the thermal evaluation of film-cooled blade is of great importance to the gas turbine community. Due to the physical or methodology limitations, it is difficult to evaluate the blade’s thermal performance at simulated engine conditions. The present study proposed novel focal-sweep-based phosphor thermometry for blade cooling inspection. While Mg4FGeO6:Mn (MFG) served as the temperature sensor to quantify the blade temperatures as well as simulated the thermal barrier coating (TBC) effect, the focal sweep method was adopted to overcome the optical constraints in cascade testing. The obtained MFG results of microstructures, jet impingement, and anti-erosion test demonstrated that the MFG phosphor is robust enough to simulate the thermal insulation effect of TBC and can withstand high-speed flow erosion. Furthermore, the proposed strategy clearly captured the blade temperature distributions (mainstream at T0,∞=∼850K) with high spatial resolution, which was then successfully remapped onto the three-dimensional twisted blade. Additional comparisons with the thermocouples demonstrated that the simulated TBC has a thermal insulation effect of about 68 K. This study addressed the common problems of phosphor thermometry in blade cooling evaluation, offering a practical strategy for future thermal diagnostics of the gas turbine.

High spatial resolution fluid thermometry in boundary layers by macroscopic imaging of individual phosphor tracer particles

We introduce a measurement concept using seeded thermographic phosphor particles, which achieves high spatial resolution and rejection of surface induced signals, and is thereby applicable to resolve two-dimensional temperature distribution in sub-millimeter thermal boundary layers. Unlike previous implementations of ratiometric phosphor thermometry in fluid flows, which is based on the division of two spectrally or temporally separated images of the luminescence from groups of seeded particles, here we treat the individual phosphor particles as independent temperature detectors positioned at the discrete particle locations. 2D rotated Gaussian functions are fitted to each particle image as to integrate particle signals in the two frames for ratio-based thermometry and to position the particles with sub-pixel resolution (<10μm). In addition, the fitting method allows to separate the luminescence signal of the imaged particles from interfering signals with a low spatial frequency, for example from surface reflection or re-scattering of luminescence light. After assessing the spatial resolution, and the robustness of the temperature measurements against high levels of re-scattered signals, near-wall measurements are demonstrated. The ability to finely resolve the temperature distribution within a 500μm thin thermal boundary layer is validated against the laminar Prandtl–Blasius equation. As the thermometry counterpart and complement to Particle Tracking Velocimetry, this technique allows to probe the fine details of heat transfer in boundary layers.

Thermographic 3D particle tracking velocimetry for turbulent gas flows

Turbulent flows are characterized by diverse and unsteady three-dimensional (3D) features that require 3D measurements to study. In case of non-isothermal flows, combined 3D measurements of temperature and velocity are necessary. In this paper, a thermographic 3D particle tracking velocimetry (thermographic 3D-PTV) concept is introduced for simultaneous 3D temperature and velocity measurements in turbulent gas flows. It is based on sub-micron thermographic phosphor particles seeded into the flow as flow tracers with low response times of a few microseconds. To obtain each tracer’s position and velocity, the measurement region is illuminated volumetrically using a double-pulse green laser and Mie-scattered light is imaged by four double-frame cameras. Following the pinhole model-based calibration of all cameras, 3D particle positions are computed for both laser pulse-times using a fast minimum line of sight reconstruction code. Three-component velocities are derived from tracking individual particles between these time steps. For simultaneous 3D thermometry, temperature dependent luminescence emissions from the same phosphor particles are exploited. These emissions are excited using a UV laser synchronized with the first green laser pulse and imaged using two cameras equipped with spectral filters for ratiometric phosphor thermometry. As a result, instantaneous 3D fields of discrete temperature and velocity measurements are obtained throughout the volume. The concept is demonstrated in a turbulent heated gas jet emerging from a circular nozzle at a particle image concentration of 0.005 particle per pixel, where the symmetry of the velocity and temperature distributions about the jet axis is successfully reconstructed.

Phosphor thermometry for surface temperature measurements of composite materials during fire test

This study presents the implementation of phosphor thermometry method to obtain pointwise surface temperature measurements of a carbon fibers reinforced polymers composite exposed to a flame. Phosphor thermometry has previously been used to measure surface temperature in relevant applications, mainly on metallic materials. In the present study, it has been applied to measure the backside temperature of a carbon fiber Poly Phenylene Sulfide (PPS) polymer composite exposed to a flame, in order to analyze the feasibility, the limits and the advantages of the method compared to infrared thermography. Mg4FGeO6:Mn4+ thermographic phosphor is coated on the back side of the composite sample and excited with a 10 Hz repetition rate Nd:YAG laser at 355 nm. The time-resolved phosphorescence signals are processed with the lifetime approach to determine the temperature. A coating of small thickness allows the surface temperature to be measured up to 400 °C, and it is shown that this coating has no effect on the heat exchange at the surface. The comparison between the phosphor thermometry results with the ones obtained with an infrared camera shows an interesting methodology to determine the infrared emissivity of the sample at high temperature.

Data recovery of 2D lifetime-based phosphor thermometry using deep neural networks

Thermographic phosphor (TP) thermometry has been widely used as one of the newly developed non-contact surface temperature measurement methods. However, temperature information is frequently lost locally because chemical bonding coatings are easily damaged during the measurement. This limits its application, such as in the case of jet impact, high-speed motion, high vibration, etc. We proposed to use of deep neural networks (DNNs) as a tool for recovering lost data. In this study, we captured the dynamic two-dimensional (2D) temperature field of jet impingement cooling a high temperature plate by TP thermometry. Different parts of the temperature information, including jet impact area and non-impact area, were then removed to assume the coating was damaged. A prediction model was established by the DNNs using the 2D space and time coordinates as the input dataset and using the temperature information corresponding to the coordinates as the output dataset. The removed temperature information was then recovered using the developed regression model and compared with the raw temperature field to evaluate predicted results. The results showed that the removed data can be successfully recovered using the established DNN prediction model, in which the predicted accuracy was greater than 94.94%. While in the central and the boundary area of the jet, the model has a relatively poor performance, which is mainly due to the large temperature gradients. These results indicate that the established DNN model can be used to recover the lost temperature information but it is limited for applications in regions with drastic temperature changes.