A laser-induced fluorescence technique for temperature imaging of micro-droplets avoiding morphology dependent resonances

  • Eine Laser-induzierte Fluoreszenzmethode für die bildgebende Temperaturmessung von Mikrotropfen ohne Einflüsse optischer Resonanzen

Palmer, Johannes; Kneer, Reinhold (Thesis advisor); Beyrau, Frank (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019


This thesis introduces a new laser-optical method to measure temperature of micro-droplets and sprays that are affected by morphology-dependent resonances. The method is called “pulsed Laser-Induced Fluorescence thermometry with Enhanced Energy Transfer” (LIF-EET) and is based on the well-established LIF-thermometry. Different than the original approach, pulsed LIF-EET allows imaging without motion-blur and therefore a simultaneous spatial determination of temperature and size. The new method is developed step-wise in experiments on droplet streams. Key feature is a combination of two fluorophores, called Pyrromethene 597-8C9 (PM597) and Oil Blue N (OBN), that are seeded into the droplets. PM597 is utilized for the temperature measurement. After excitation by a laser, the dye emits a temperature dependent fluorescence signal. Fluorescence is recorded by two detectors operated in wavelength regions with different temperature sensitivities. The signal-ratio yields the desired temperature information without any influence of fluctuating experimental parameters. Unlike conventional LIF-thermometry, the new method applies a pulsed laser that on the one hand avoids motion-blur, but on the other hand induces unwanted optical resonances by the laser light and more importantly by the fluorescence - also known as “lasing”. Lasing distorts PM597's emission spectrum and makes LIF-thermometry unreliable. However, this work shows that lasing can be controlled by the second dye OBN, which enables a long-range energy transfer for a red-shift of the lasing wavelength. As a consequence, PM597's emission spectrum is restored for a feasible temperature measurement, while lasing occurs only for OBN now. The special feature of the approach is the spatial dependency of the energy transfer that is particularly efficient in the lasing region at the droplet surface. The mechanism is also known as “enhanced energy transfer”, which also labels the new method. The scientific achievement of this thesis goes beyond the implementation of the method, as each conducted experiment reveals physical details about the underlying mechanisms. Practical feasibility of the new pulsed LIF-EET method is also illustrated in this thesis in two realistic applications: i) a scientifically relevant droplet stream experiment under evaporating conditions and ii) the jet breakup and mixture formation of a hollow-cone spray under engine-like conditions.