Thermographic Particle Velocimetry (TPV) for Simultaneous Interfacial Temperature and Velocity Measurements

AbstractWe present an experimental technique, that we refer to as ‘thermographic particle velocimetry’ (TPV), which is capable of the simultaneous measurement of two-dimensional (2-D) surface temperature and velocity at the interface of multiphase flows. The development of the technique has been motivated by the need to study gravity-driven liquid-film flows over inclined heated substrates, however, the same measurement principle can be applied for the recovery of 2-D temperature- and velocity-field information at the interface of any flow with a sufficient density gradient between two fluid phases. The proposed technique relies on a single infrared (IR) imager and is based on the employment of highly reflective (here, silver-coated) particles which, when suspended near or at the interface, can be distinguished from the surrounding fluid domain due to their different emissivity. Image processing steps used to recover the temperature and velocity distributions include the decomposition of each original raw IR image into separate thermal and particle images, the application of perspective distortion corrections and spatial calibration, and finally the implementation of standard particle velocimetry algorithms. This procedure is demonstrated by application of the technique to a heated and stirred flow in an open container. In addition, two validation experiments are presented, one dedicated to the measurement of interfacial temperature and one to the measurement of interfacial velocity. The deviations between the results generated from TPV and those from accompanying conventional techniques do not exceed the errors associated with the latter

A simultaneous planar laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry technique for the investigation of thin liquid-film flows

AbstractA simultaneous measurement technique based on planar laser-induced fluorescence imaging (PLIF) and particle image/tracking velocimetry (PIV/PTV) is described for the investigation of the hydrodynamic characteristics of harmonically excited liquid thin-film flows. The technique is applied as part of an extensive experimental campaign that covers four different Kapitza (Ka) number liquids, Reynolds (Re) numbers spanning the range 2.3–320, and inlet-forced/wave frequencies in the range 1–10Hz. Film thicknesses (from PLIF) for flat (viscous and unforced) films are compared to micrometer stage measurements and analytical predictions (Nusselt solution), with a resulting mean deviation being lower than the nominal resolution of the imaging setup (around 20μm). Relative deviations are calculated between PTV-derived interfacial and bulk velocities and analytical results, with mean values amounting to no more than 3.2% for both test cases. In addition, flow rates recovered using LIF/PTV (film thickness and velocity profile) data are compared to direct flowmeter readings. The mean relative deviation is found to be 1.6% for a total of six flat and nine wavy flows. The practice of wave/phase-locked flow-field averaging is also implemented, allowing the generation of highly localized velocity profile, bulk velocity and flow rate data along the wave topology. Based on this data, velocity profiles are extracted from 20 locations along the wave topology and compared to analytically derived ones based on local film thickness measurements and the Nusselt solution. Increasing the waviness by modulating the forcing frequency is found to result in lower absolute deviations between experiments and theoretical predictions ahead of the wave crests, and higher deviations behind the wave crests. At the wave crests, experimentally derived interfacial velocities are overestimated by nearly 100%. Finally, locally non-parabolic velocity profiles are identified ahead of the wave crests; a phenomenon potentially linked to the cross-stream velocity field

Development of a Thermographic Imaging Technique for Simultaneous Interfacial Temperature and Velocity Measurements

An experimental technique, hereby referred to as ‘thermographic particle velocimetry’ (TPV) and capable of recovering twodimensional (2-D) surface temperature and velocity measurements at the interface of multiphase flows is presented. The proposed technique employs a single infrared (IR) imager and highly reflective, silver-coated particles, which when suspended near or at the interface, can be distinguished from the surrounding fluid due to their different emissivity. The development of TPV builds upon our previous IR imaging studies of heated liquid-film flows; yet, the same measurement principle can be applied for the recovery of 2-D temperature- and velocity-field information at the interface of any flow with a significant density gradient between two fluid phases. The image processing steps used to recover the temperature and velocity distributions from raw IR frames are demonstrated by application of TPV in a heated and stirred flow in an open container, and include the decomposition of each raw frame into separate thermal and particle frames, the application of perspective distortion corrections and spatial calibration, and the implementation of standard particle image velocimetry algorithms. Validation experiments dedicated to the measurement of interfacial temperature and velocity were also conducted, with deviations between the results generated from TPV and those from accompanying conventional techniques not exceeding the errors associated with the latter. Finally, the capabilities of the proposed technique are demonstrated by conducting temperature and velocity measurements at the gas-liquid interface of a wavy film flow downstream of a localised heater

Self-similarity of solitary waves on inertia-dominated falling liquid films

We propose consistent scaling of solitary waves on inertia-dominated falling liquid films, which accurately accounts for the driving physical mechanisms and leads to a self-similar characterization of solitary waves. Direct numerical simulations of the entire two-phase system are conducted using a state-of-the-art finite volume framework for interfacial flows in an open domain that was previously validated against experimental film-flow data with excellent agreement. We present a detailed analysis of the wave shape and the dispersion of solitary waves on 34 different water films with Reynolds numbers Re=20–120 and surface tension coefficients σ=0.0512–0.072Nm−1 on substrates with inclination angles β=19◦ − 90◦. Following a detailed analysis of these cases we formulate a consistent characterization of the shape and dispersion of solitary waves, based on a newly proposed scaling derived from the Nusselt flat film solution, that unveils a self-similarity as well as the driving mechanism of solitary waves on gravity-driven liquid films. Our results demonstrate that the shape of solitary waves, i.e., height and asymmetry of the wave, is predominantly influenced by the balance of inertia and surface tension. Furthermore, we find that the dispersion of solitary waves on the inertia-dominated falling liquid films considered in this study is governed by nonlinear effects and only driven by inertia, with surface tension and gravity having a negligible influence

Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, , and Prandtl number, . By employing this technique, falling-film flows are investigated with Reynolds numbers in the range , wave frequencies set to , 12 and 17 Hz, and a wall heat flux set to  W cm−2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows with  W cm−2. Quality assurance experiments are performed that reveal deviations of up to 2-3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 °C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux ( W cm−2) results in a pronounced “thinning” of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but “fuller” compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between the HTC and the film height in thinner film regions. When the imposed wave frequency is increased, a narrower range of HTCs is observed, which we link to the evolution of the film topology and the associated redistribution of the fluid flow upstream of the imaging location, as the liquid viscosity decreases. The local and instantaneous Nu is strongly coupled to the film height and experiences variations that increase as is reduced

Tackling coolant freezing in generation-IV molten salt reactors

In this study we describe an experimental system designed to simulate the conditions of transient freezing which can occur in abnormal behaviour of molten salt reactors (MSRs). Freezing of coolant is indeed one of the main technical challenges preventing the deployment of MSR. First a novel experimental technique is presented by which it is possible to accurately track the growth of the solidified layer of fluid near a cold surface in an internal flow of liquid. This scenario simulates the possible solidification of a molten salt coolant over a cold wall inside the piping system of the MSR. Specifically, we conducted measurements using water as a simulant for the molten salt, and liquid nitrogen to achieve high heat removal rate at the wall. Particle image velocimetry and planar induced fluorescence were used as diagnostic techniques to track the growth of the solid layer. In addition this study describes a thermo-hydraulic model which has been used to characterise transient freezing in internal flow and compares the said model with the experiments. The numerical simulations were shown to be able to capture qualitatively and quantitatively all the essential processes involved in internal flow transient freezing. Accurate numerical predictive tools such the one presented in this work are essential in simulating the behaviour of MSR under accident conditions

Combined PLIF-IR thermal measurements of wavy film flows undergoing forced harmonic excitation

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.A combined PLIF/IR thermography technique was developed and employed towards the measurement of unsteady and conjugate heat transfer in thin, gravity-driven falling liquid film flows (with and without flow pulsation) over an inclined heated metal foil. Simultaneous, local film thickness, film and substrate temperature, heat flux exchanged with a heated foil and heat transfer coefficient results are reported for a range of electrically applied heat input values, flow Reynolds (Re) numbers and flow pulsation frequencies. Moreover, interfacial wave velocities were calculated from cross-correlations across successive thickness profiles. Results concerning the instantaneous and local heat transfer coefficient variation and how this is correlated with the instantaneous and local film thickness variation (waves) suggest that the heat transfer coefficient experiences an enhancement in thinner films. The particular observation is most probably attributed to a number of unsteady flow phenomena within the wavy fluid films that are not captured by the steady analysis. At low flow Re number values the mean Nusselt (Nu) was around 2.5, in agreement with laminar flow theory, while at higher Re values, higher Nu were observed. Finally, lower wave amplitude intensities were associated with higher heat transfer coefficient fluctuation intensities.cf201

Hydrodynamic characteristics of harmonically excited thin-film flows : experiments and computations

We present new results from the simultaneous application of Planar Laser-Induced Fluorescence (PLIF) and Particle Tracking Velocimetry (PTV), complemented by Direct Numerical Simula- tions (DNSs), aimed at the detailed hydrodynamic characteriza- tion of harmonically excited liquid-film flows. The experimental campaign spans the Reynolds number range Re = 8 − 320, and three Kapitza numbers Ka = 85, 350 and 1800. PLIF was em- ployed in order to generate spatiotemporally resolved film-height data, and PTV to generate two-dimensional (2D) planar velocity- vector maps of the flow-field underneath the wavy interface. By combining the two optical techniques, instantaneous and highly localised flow-rate data were retrieved, based on which the ef- fect of local film topology on the flow-field is studied in detail. Surprisingly, the instantaneous flow rate is found to vary linearly with the instantaneous film-height, while both experimental and numerical flow-rate data are closely approximated by a simple analytical relationship with only minor deviations. This relation- ship, which is reported here for the first time, includes the wave speed c and mean flow-rate Q, both of which can be obtained by simple and inexpensive methods, thus allowing for spatiotempo- rally resolved flow-rate predictions to be made without requiring any knowledge of flow-field information.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Experimental study of falling films by simultaneous laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry

measurement technique based on the simultaneous implementation of Laser-Induced Fluorescence (LIF), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) has been applied to the study of wavy liquid falling film flows characterized by low Reynolds (Re) and Kapitza (Ka) numbers. The presently examined Re number range was 2.2 – 8.2, while the Ka number range was 28.6 – 41.4. The experimental methodology was developed with the ultimate aim of allowing for the evaluation of the local and instantaneous film thickness, interfacial velocity and velocity field from within the illuminated liquid volume underneath the wavy interface. The major challenges associated with the simultaneous implementation of the two optical diagnostic techniques were, firstly, the development of a refractive index correction approach allowing for liquids of different properties (surface tension and viscosity) to be tested, secondly, the identification of the location of the two liquid boundaries (solid-liquid and gasliquid) in the LIF images, and lastly, the isolation of out-of-plane reflections from primary scattering regions in the raw PIV images. Following a detailed account of the novel practices formulated and utilized in tackling the aforementioned challenges, the efficacy of the proposed methodology is demonstrated through comparisons between laser-based measurements conducted in flat films, film thickness measurements performed with a micrometer, and the solution to the Navier-Stokes equation based on the assumptions of one-dimensional (1-D), steady and fully developed flow. In addition, sample film topology results are presented for a range of flow pulsation frequencies (1 – 8 Hz), while film thickness and interfacial velocity time traces were reconstructed and are presented along with film thickness and interfacial velocity statistical results for select flow conditions