High repetition rate temperature and velocity Imaging in turbulent flows using thermographic phosphors

Turbulent flows involving heat transfer and chemical reactions are prevalent in a huge range of applications such as combustors and engines, boilers, and heating and cooling devices. Directly measuring important variables using laser-based techniques has significantly contributed to our understanding of the underlying flow physics. However, many flows of interest exhibit infrequent or oscillatory behaviour, such as flame extinction or instabilities in thermal boundary layers. Capturing the flow dynamics requires simultaneous, two-dimensional temperature and velocity measurements at sampling rates commensurate with turbulent timescales. Typically this means measuring many thousands of temperature and velocity fields per second, yet there are no high repetition rate diagnostics for temperature imaging in practical, oxygen-containing systems, with the essential capability of simultaneous velocity measurements. This thesis presents a novel laser-based imaging technique based on thermographic phosphor particles. There are a huge variety of thermographic phosphors, which are solid materials with luminescence properties that can be exploited for remote thermometry. Here, phosphor particles are seeded into the flow as a tracer. An appropriate phosphor must be selected, and the particle size chosen so that the particle temperature and velocity rapidly assume that of the surrounding fluid. The particles are probed using high-speed lasers and their luminescence and scattering signals are detected using high-speed cameras to measure the flow temperature and velocity at kHz repetition rates. The development of this method is described in detail. Using the thermographic phosphor BAM:Eu, examples of simultaneous time-resolved measurements are presented in turbulent air flows between 300 and 500 K, consisting of a heated jet (Re = 10,000) and also a flow behind a heated cylinder (Re = 700). The technique permits kHz-rate temperature imaging in oxygen-containing environments. These combined diagnostics currently provide a unique capability for the investigation of transient, coupled heat and mass transfer phenomena in turbulent flows of practical engineering importance. A second objective of this work is to improve the precision of the temperature measurement. The characterisation of a different thermographic phosphor with a high temperature sensitivity, zinc oxide (ZnO), is also reported. Temperature imaging using these tracer particles is demonstrated in a jet (Re = 2,000) heated to 363 K, with a temperature precision of 1%. This extends the capabilities of this versatile technique toward the study of flows with small temperature variations. Also, unlike the majority of phosphors previously investigated for thermometry, this phosphor is a semiconductor. Exploiting the temperature-dependent luminescence of this class of materials presents interesting new opportunities for remote temperature sensing.Open Acces

Simultaneous kHz-rate temperature and velocity field measurements in the flow emanating from angled and trenched film cooling holes

AbstractTo design more efficient film cooling geometries for gas turbines, non-intrusive measurements of the flow temperature, velocity and derived quantities like the turbulent heat flux are needed in well-defined, generic flow configurations. With this aim we have applied thermographic particle image velocimetry (thermographic PIV) to investigate the flow emanating from angled and trenched cooling holes in a closed-loop optically-accessible wind tunnel facility. BAM:Eu2+ thermographic phosphor particles were seeded into the flow as a tracer. A pulsed high-speed UV laser was used to excite the particles and the luminescence was detected using two high-speed cameras to determine the temperature field by a two-colour ratiometric approach. The velocity field was measured using ordinary high-speed PIV. The simultaneously measured fields were sampled at a rate of 6kHz in a vertical plane through the centreline of the symmetrical single-row cooling holes. The flowrate and temperature of the cooling air and heated main flow were chosen to achieve density and momentum flux ratios of 1.6 and 8 respectively. For these conditions the average and RMS temperature fields show that for ordinary angled holes the jet is detached from the surface. In contrast, the trenched geometry leads to a cooling film attached to the surface. However, time-resolved image sequences show instances where hot air breaks through the cooling film and almost reaches the surface. Similar image sequences for the angled holes show that the detached coolant jet becomes unstable downstream and pockets of cold air are ejected into the main flow. This intermittency may in part explain the observation that the measured turbulent heat flux is oriented towards the cold core, but deviates from the direction of the mean temperature gradient, thereby contradicting the simple gradient diffusion hypothesis commonly used in RANS simulations

Thermographic particle image velocimetry: from phosphorescence to incandescence

Thermographic PIV is a promising technique that enables simultaneous temperature and velocity imaging in flows with intentional seeded phosphor particles. This is highly attractive to researchers in fluid mechanics and combustion, as it directly visualizes the heat and mass transfer process in turbulent flows/flames. However, three problems for this technique are: (a) it requires two lasers and three cameras running simultaneously, making it a high-cost technique; (b) several recent studies reported the multiple scattering effects for gas-phase phosphor thermometry, which may severely bias the temperature measurement for certain flow configurations; and (c) the phosphorescent emission disappears at high temperature due to thermal quenching, which limits the temperature measurements to mostly non-reacting cases below 1100 K. This dissertation is aimed at providing solutions to the issues described above. To reduce the cost of the current thermographic PIV setup, a simplified version is proposed which uses a double-pulsed laser with UV capacity and two CCD cameras operating in the double-frame mode. This experiment proves that, apart from Mie scattering, phosphorescence image pairs can also be used to perform cross-correlation and calculate the vector field. Therefore both velocity and temperature field can be extracted from phosphorescence emissions excited by a single laser (UV-PIV). Thermographic PIV with this simplified setup is demonstrated on an electrically heated air jet, and 3 K accuracy is achieved in the core region of the jet, by comparing with a thermocouple scan. A novel calibration process is also proposed to eliminate the influence of non-uniform laser profile on the temperature measurements. The same technique is also applied to visualize heat transfer in an impinging jet. By correlating the instantaneous gaseous temperature fields with the averaged \textit{Nu} profiles derived from the wall temperature, the role of vortical structures in heat transfer is investigated and discussed. During the application of thermographic PIV, the problem of multiple scattering emerged and has been reported by several studies, especially for cases where an excessive seeding is used. Multiple scattering was found to reduce the spatial resolution and bias the temperature measurements. A recent study demonstrated that the Structured Laser Illumination Planar Imaging (SLIPI) technique could effectively remove multiple scattering and near-wall effects from the LIP image. However, it is well known that the emission spectrum of some most commonly used thermographic phosphors is sensitive to the changes in laser fluence, whilst SLIPI intentionally modulates the laser profiles and thus may bring in uncertainty into the temperature retrieval. This has yet not been discussed in the literature. In this dissertation, a numerical analysis is conducted, by generating artificial laser induced phosphorescence images, to investigate the effects that SLIPI may have on the temperature measurements. To implement simultaneous temperature and velocity measurements in flames, an entirely new approach of thermographic PIV is proposed in this dissertation. This new version is based on laser-induced incandescence (LII), rather than phosphorescence. Submicron black particles are seeded into a flame, and further heated by a high-energy top-hat laser sheet to several thousands kelvin. The particle temperature $T_\mathrm{p}$ can be measured by two-color pyrometry, where the temperature increase $\Delta T$ due to laser absorption can be determined by conducting an {\em in-situ} calibration. Thus the local temperature $T_0$ can be indirectly determined by subtracting $\Delta T$ from $T_\mathrm{p}$. The same particle can also be used as PIV tracers. The concept and fundamentals of this new thermographic PIV approach are described in this thesis. The combination of LII and PIV is also applied as a tool to measure the gas-phase velocity in a two-phase flow, which is a canonical problem for multiphase flow studies.Part of the research presented in this thesis was funded by University Technology Malaysia (UTM) under grant number RG8426

Simultaneous, two-camera, 2D gas-phase temperature and velocity measurements by thermographic particle image velocimetry with ZnO tracers

This work presents simultaneous 2D temperature and velocity measurements on a heated jet using two non-intensified cameras to realize thermographic PIV. In contrast to previous studies which use separate PIV cameras and LIP cameras, the present experiment uses only a double-pulsed UV laser and two low-speed CCD cameras running in double-frame mode, greatly simplifying the setup. The intensity ratio is calculated based on the image pair recorded at two spectral lines for thermography, while the cross-correlation is performed over the two consecutive frames for PIV. A method is proposed to correct for the effects of non-uniform spatial distribution of laser fluence on the intensity ratio, by including the laser fluence into the calibration function. The laser sheet energy profiles are measured and the intensity ratio is translated into temperature according to the local laser fluence and two-colour ratio. The temperature accuracy using this technique is estimated as 3 K at 410 K, by comparing the mean temperature field with the result provided by a thermocouple. Simultaneous 2D temperature and velocity fields are presented for a simple heated jet, demonstrating the expected similarities. The demonstration shows the potential of thermographic PIV in the investigation fundamental problems on turbulent flows, and shows that the technique can be improved through the availability of a dual-cavity UV laser.The third author (A.H.) acknowledges the fnancial support from the “Young Researcher Overseas Visits Program for Vitalizing Brain Circulation” from JSPS (Japan Society for the Promotion of Science). Y. Gao was funded by EPSRC UK grants EP/K02924X/1 and EP/M015211/1

Two dimensional gas temperature measurements of fuel sprays in a high pressure cell

Premixed charge compression ignition (PCCI) is a promising low-emission combustion concept. By partially mixing the fuel, air and exhaust gas before auto-ignition, the soot and NOx emissions are lower than for conventional diesel combustion. However, the fundamental aspects of the mixing process of the fuel spray with the ambient air are still not well understood, especially not in terms of the temperature distribution of the fuel/air mixture. This thesis focuses on the 2D temperature distribution measurement of fuel sprays under conditions relevant to PCCI-mode combustion in heavy-duty Diesel engines. Experiments are performed in the High Pressure Cell (HPC), which simulates engine conditions while providing much better optical accessibility than a real engine. The temperature field which is produced by the pre-combustion technique is also measured, to characterize the ambient condition which the sprays are injected in. Advanced diagnostics are applied to provide detailed information on fuel sprays and the ambient condition, to improve the understanding of the mixing mechanism and its consequences for the combustion process. Chapter 2 describes the pre-combustion technology of the HPC and the modeling of the cooling process. The analytical modeling is based on experimental observations, assuming that turbulent convection is dominating the cooling process. A natural and a forced convection model are used to estimate the thermal boundary layer thickness and the core temperature in the scenarios without and with fan-mixing, respectively. Without fan-mixing the heat transfer rate decays together with the turbulent kinetic energy. If fan mixing is added, a constant turbulent flow is maintained, that is, the turbulent kinetic energy is constant during the cooling process. The modeling results match experimental results very well. In the first scenario, the core temperature is about 5 – 7% higher than the bulk temperature. In the second scenario the difference is 2 – 6%. These results are reasonable when compared with experiments. The model provides a good estimation for the ambient condition during fuel spray measurements. In Chapter 5, Laser Induced Phosphorescence (LIP) is chosen to measure the temperature field of the ambient gas prior to fuel injection. The BAM (BaMgAl10O17:Eu) is chosen as a tracer phosphor for its high signal-to-noise ratio and its capability to survive the pre-combustion. A seeding device to seed the 3 µm solid particles into gaseous flow was designed and implemented, which performed beyond expectation. Particle agglomeration was not observed, probably due to the high shear forces induced. Particle sticking is not a major concern as long as stainless steel tubing is used and Teflon material is avoided. BAM-LIP is excited by a 355 nm YAG laser. Results show that BAM-LIP can be used to measure the temperature field of the residual gas in the HPC below 650 K. The precision of the experiments is better than 30 K at 400 K and 60 K at 650 K. The spatial resolution was estimated to be 3 mm in the plane of the laser sheet and 10 mm along the line of sight, primarily determined by multiple scattering present in the experiments. Temperature field results show that there is a significant temperature gradient in the vertical direction present during the cooling phase in the HPC when the mixing fan is not used. This finding supports the interpretation of the analytical model, which overpredicts the temperature for neglecting the buoyancy effect. However the BAM-LIP method is currently not able to provide 2D temperature distribution prior to fuel injection, due to the lack of signal due to particle falling. Possible improvements have been recommended. In Chapter 3, the physical processes in a fuel spray, as it is injected into stagnant ambient gas, are explained and two phenomenological spray models are compared in their prediction of temperature distribution in a fuel spray. The major difference between the Versaevel and the Valencia model exists in their assumptions for radial profiles of fuel concentration and velocity. The Versaevel model assumes a top-hat profile while the Valencia model assumes a Gaussian profile, which is observed by averaging multiple injections. Both models predict spray penetration very well, however, they differ considerably in their prediction of the temperature distribution. The Valencia model predicts lower central line temperatures than the Versaevel model. Laser Induced Fluorescence (LIF) using 10% toluene as a tracer, as described in Chapter 5, is used as a tool to measure 2D temperatures during and after injection of fuel inside the HPC and an optical engine. The error analysis and evaluation of the toluene LIF method was performed on the HPC, while the calibration was performed in the optical engine. The toluene LIF method is capable of measuring temperatures up to 700 K; above that the signal becomes too weak. The precision of the spray temperature measurements is 4% and the spatial resolution is 1.3 mm. Experimental results from the HPC reveal a hot zone in the fully developed spray. Two camera configurations are compared. An opposite side camera setup seems to be beneficial over a one-side setup because it avoids the dichroic beam splitter requirement, but the precision is lower because of different light paths. The toluene LIF method offers a relatively simple and precise way to measure the 2D temperature distribution in fuel sprays. However, several improvements can be done to improve the absolute accuracy, For example using more sensitive camera and applying flat field correction with a light source of the relevant wavelength. In general, the toluene LIF method is capable of providing 2D temperature information in a fuel spray with 4% precision, which makes it possible to detect the temperature gradients in sprays. The BAM-LIP method could be used in measuring the temperature distribution in a gas-phase environment, where combustible tracers, such as toluene, are not applicable. Both methods might be applied in more applications such as in burners and internal combustion engines

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.The project has been partly funded by the Universiti Teknologi Malaysia

Planar measurements of spray-induced wall cooling using phosphor thermometry

The wall cooling induced by spray impingement is investigated using phosphor thermometry. Thin coatings of zinc oxide (ZnO) phosphor were applied with a transparent chemical binder onto a steel surface. Instantaneous spatially resolved temperatures were determined using the spectral intensity ratio method directly after the injection of UV-grade hexane onto the surface using a commercial gasoline injector. The investigations showed that 2D temperature measurements with high spatial and shot-to-shot precision of, respectively, 0.5 and 0.6 K can be achieved, allowing the accurate resolution of the cooling induced by the spray. The presence of a liquid film over the phosphor coating during measurements showed no noticeable influence on the measured temperatures. However, in some cases a change in the intensity ratio at the spray impingement area, in the form of a permanent “stain”, could be observed after multiple injections. The formation of this stain was less likely with increasing annealing time of the coating as well as lower plate operating temperatures during the injection experiments. Finally, the experimental results indicate a noticeable influence of the thickness of the phosphor coating on the measured spray-induced wall cooling history. Hence, for quantitative analysis, a compromise between coating thickness and measurement accuracy needs to be considered for similar applications where the heat transfer rates are very high