Precise surface temperature measurements at kHz-rates using phosphor thermometry to study flame-wall interactions in narrow passages

The thermographic phosphor ScVO4:Bi3+ is used to obtain time-resolved surface temperature measurements with sub-oC precision at 5 kHz. Measurements are used to study transient heat loss and flame-wall interactions (FWI) within a dedicated narrow two-wall passage (crevice) in an optically accessible fixed volume chamber. This passage emulates a crevice relevant in many technical environments, where FWI is less understood due to lack of detailed measurements. Chemiluminescence (CH*) imaging is performed simultaneously with phosphor thermometry to resolve how the spatiotemporal flame features influence the local surface temperature. ScVO4:Bi3+ is benchmarked against Gd3Ga5O12:Cr,Ce, a common phosphor used at low-kHz rates in FWI environments. ScVO4:Bi3+ is shown to offer higher luminescence signal levels and temperature sensitivity as well as negligible cross dependence on the excitation laser fluence, improving the precision and repeatability of the wall temperature measurement. ScVO4:Bi3+ is further used to resolve transient heat loss for variations in crevice spacing and uniquely capture temperature transients associated with flame dynamics. Taking advantage of these precise surface temperature measurements the wall heat flux is calculated with crevice spacing of 1.2 mm, where flame extinction is prevalent. Wall heat flux and estimated quenching distance are reported for flames that actively burn or extinguish at the measurement location.Comment: This manuscript was recently accepted to Combustion and Flame. The article does not yet have the associated volume and page numbers, as it is still in the proofing stage

A self-affine geometrical model of dynamic RT-PMMA fractures: implications for fracture energy measurements

Profilometric imaging of fracture surfaces of rubber toughened polymer has been performed at two different resolutions (a) at large scales [10 μ\upmu m–25 mm] using an opto-mechanical profilometer and (b) at small scales [0.195 μ\upmu m–0.48 mm] using an interferometric optical microscope. We introduced a self-affine geometrical model using two parameters: the Hurst exponent and the topothesy. We showed that for rubber toughened materials the approximation of the created surface by a mean flat plane leads to a poor estimation of the dynamic fracture energy GIdcG_{Idc}. The description of the created rough fracture surface by a self-affine model is shown to provide a significantly better approximation. A new and original geometrical method is introduced to estimate self-affine parameters: the 3D surface scaling method. Hurst exponents are shown to be unique, χ=0.6±0.1\chi =0.6\pm 0.1 for the different fracture zones and measurement scales. Topothesy ratios indicate a significant difference of fracture surface roughness amplitude depending on the observation resolution when the detrending technique is not correctly introduced.ANR Carenc

Simultaneous temperature and velocity imaging in turbulent flows using thermographic phosphor tracer particles

Combined measurements of velocity and temperature are essential to improve our understanding of turbulent flows involving heat transfer or chemical reactions. However, performing such measurements is a very difficult task. The presence of particles, which are seeded into the flow as tracers for the flow velocity, strongly interferes with classic optical thermometry techniques such as Rayleigh scattering. A review of the current approaches shows that a technique that can measure both quantities simultaneously, in two dimensions and over a wide range of flow conditions is yet to be found. An alternative approach to this problem, presented in this dissertation, uses tracer particles made of temperature-sensitive luminescent material, which are capable of also indicating the gas temperature. Thermographic phosphors are shown to be clear candidates for this concept. Made of ceramic material, they are chemically inert and survive low and high temperature environments. The temperature has a strong influence on the luminescence process allowing various ways to perform thermometry Currently, phosphors are used for surface temperature measurements, but a phosphor suitable for two-dimensional measurements in turbulent flows must meet stringent requirements in terms of luminescence properties. In this respect, the temperature dependence of the emission spectrum, a high quantum efficiency and a short lifetime are essential. Micrometre-size refractory particles are widely used for PIV and are able to follow the fluid motion without slip for a wide range of fluid velocities and turbulence intensities However, for the concept to be valid, the ability of phosphor particles to follow fluctuations in the gas temperature must be demonstrated. Using theoretical heat transfer models, it is shown that the temperature response of a particle is faster than its velocity response irrespective of the gas temperature. These response times have a quadratic dependence on the particle diameter so only small particles can be used. Various aspects of the practical implementation of the flow measurement concept, such as the excitation, particle seeding, detection, image processing and calibration, are considered, tested and developed, with the objective of providing high signal levels and to permit precise, accurate, and highly resolved measurements. In order to determine whether a sufficient signal level can be obtained for a reasonable particle seeding density, i.e. that does not have any effect on the gas properties, a particle counting tool is implemented. This system is used to characterise the phosphorescence intensity of 2 μm diameter particles made of BAM:Eu2+, a phosphor with very advantageous properties for flow measurements. It is shown that a seeding density comparable to that of conventional PIV and relatively small laser fluence provide sufficient signal levels for precise single shot measurements. The technique is demonstrated in a turbulent heated jet from 300 K to 700 K. Single shot measurements of temperature and velocity are presented with a single-shot, single-pixel temperature precision of 2-5 %, a temperature accuracy of 2%, and a spatial resolution of 400 μm. An additional concept is explored. By seeding two streams with different materials, the phosphorescence signal can be used to visualise the turbulent mixing between the streams. This concept is demonstrated in the same turbulent heated jet. Future developments and applications of the thermographic phosphor tracer particle concept are discussed. Owing to the very wide variety of thermographic phosphors, the results presented in this dissertation constitute a solid foundation for the expansion of this promising technique.Open Acces

Rapid crack propagation in PA11: An application to pipe structure

Dynamic fracture mechanism in Polyamide 11 (PA11) material has been described at laboratory scale to access to an intrinsic material parameter. A liquid transportation application is considered with polymer pipes. A preliminary numerical analysis of the rapid crack propagation (RCP) in polymer pipe is firstly realised. Two boundary conditions, imposed displacement or pressure, are numerically investigated. The work of external forces is not negligible for pressurized polymer pipe. A reliable estimate of the dynamic energy release rate GId is in this last case not guaranteed. To limit unwanted structural effects a specific experimental device has been used to ensure a permanent regime of RCP in Pre-Stressed Pipe Specimen (PS2). Experimental dynamic fracture tests are realised with Polyamide 11 PS2. Dynamic instabilities inducing “ring-off” and “snake” mechanisms which could appear during full-scale test are not observed with this new test. A finite element procedure is used to estimate the material toughness GID of PA11. Knowing the crack tip location during RCP inertia effects (i.e. kinetic energy) are quantified. The mean crack tip velocity is observed not to change in PA11 whatever the crack configuration (branching or not). This velocity is known to be the crack branching velocity (≈0.6cR). The average dynamic energy release rate 〈GID〉 is equal to 1.5± 0.1 kJm−2 at the crack branching velocity. The nontrivial fracture surface roughness is observed with a scanning electron microscope

Dynamic fracture in rubber toughened polymers: the influence of the fracture surface roughness

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