Thermal Imaging Metrology with a Smartphone Sensor

Thermal imaging cameras are expensive, particularly those designed for measuring high temperature objects with low measurement uncertainty. A wide range of research and industrial applications would benefit from lower cost temperature imaging sensors with improved metrology. To address this problem, we present the first ever quantification methodology for the temperature measurement performance of an ultra-low cost thermal imaging system based on a smartphone sensor. The camera was formed from a back illuminated silicon Complementary Metal Oxide Semiconductor (CMOS) sensor, developed for the smartphone camera market. It was packaged for use with a Raspberry Pi computer. We designed and fitted a custom-made triplet lens assembly. The system performance was characterised with a range of state-of-the-art techniques and metrics: establishing a temperature resolution of below 10 °C in the range 600–1000 °C. Furthermore, the scene dependent aspects of combined uncertainty were considered. The minimum angular subtense for which an accurate thermal measurement could be made was determined to be 1.35°, which corresponds to a 23 mm bar at a distance of 1 m, or 45:1 field-of-view in radiation thermometer nomenclature

Development steps of 2-color laser-induced fluorescence with MDR-enhanced energy transfer for instantaneous planar temperature measurement of micro-droplets and sprays

[EN] A new method for instantaneous measurement of temperature, size and velocity of micro-droplets has been developed. The method is based on the well-known 2-color laser-induced fluorescence (2cLIF) technique, but uses a pulsed laser for 2-dimensional imaging without motion blur and an adjusted dye mixture for suppression of LIFMDRs by utilizing the MDR-enhanced energy transfer effect. This work presents the development steps that are necessary to verify feasibility of pulsed 2D-2cLIF-EET for micro droplet and hollow-cone spray applications.This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative of the German federal and state governments to promote science and research at German universities.Palmer, J.; Reddemann, M.; Kirsch, V.; Kneer, R. (2017). Development steps of 2-color laser-induced fluorescence with MDR-enhanced energy transfer for instantaneous planar temperature measurement of micro-droplets and sprays. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 661-668. https://doi.org/10.4995/ILASS2017.2017.4591OCS66166

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

Quantitative Thermography and Image Quality in Additive Manufacturing of Metal

This thesis presents work on quantitative thermography in the additive manufacturing of metals process. The work is motivated by a need for accurate, spatiotemporally resolved measurements of the thermal fields near the heat source, which is usually 50-500 µm in size. This level of detail requires a high spatial sampling rate, which can be provided by near infrared sensitive silicon-based instruments. The high spatial sampling rate means that the resolution of the instruments is limited by the imaging components. The imaging performance is characterised by the spatial transfer function. In this work three distinct silicon based thermographic instruments were designed and constructed. The three instruments were trialled in additive manufacturing of metals applications. The three trials were: a low-cost smart-phone-sensor system used on a commercial direct energy deposition machine; a high-performance sensor system with a telephoto lens used on a modified commercial machine; and a high performance, high magnification system used on a custom built process replicator. The performance of the three systems for their applications was assessed. The three instruments have provided valid research data which paves the way for future studies using these technologies. The instrument used for thermography on the process replicator could resolve previously unseen levels of thermal detail in the process, having an instantaneous field of view of 3 µm. The measurement field of view of this instrument was found to be a circle of 130 µm diameter. The cooling rates in the process replicator for the alloy (Ti-6-4). were measured to be 0.06- 0.14 °C µs -1 , which is consistent with literature for this material. The spatial transfer function of the instruments was calculated using methods developed for this thesis. Measurements of the spatial transfer function were used to reconstruct the thermal fields and a method for validating the reconstruction was devised. A reconstruction method devised for this work was found to outperform the standard reconstruction methods used in literature, for scenes similar to those found in the additive manufacture of metals