C. Le Niliot

Laboratory and numerical investigations on Bénard–Marangoni convection in circular vessels

This study is concerned with supercritical Benard–Marangoni flows of a high Prandtl number (Pr≈4000) fluid in cylinders of small to medium aspect ratio, 5⩽A⩽10. Laboratory experiments and numerical simulations, handled with an accurate spectral solver of the unsteady three-dimensional Boussinesq equations, are used to get a good understanding of the flow behavior. Experimental and numerical results are successively presented and then discussed. The confinement effects are especially pointed out and carefully analyzed.

Surface temperature measurement of plasma facing components with active pyrometry

In fusion devices like ITER, plasma facing components will be in metal, (Tungsten and Beryllium), with emissivity in the range of 0.1–0.4. Therefore, surface temperature monitoring by infrared system will become more challenging due to low emissivity and consequently non negligible reflected flux. The active pyrometry method proposed in this paper allows surface temperature measurements independently of reflected and parasitic fluxes. A local increase of the surface temperature (ΔT(t)~10 °C) introduced by a transient heating source (pulsed or modulated) results in an additional component of the flux collected by the detector. A filtering of the signal allows extracting a temporal flux proportional only to the variation of the emitted flux. The ratio of simultaneous measurements at two wavelengths allows solving the unknown emissivity (same as for classical bicolour pyrometry). In this paper, it is described how the active pyrometry method is adapted to the surface temperature measurements of metallic PFCs independently of the reflected fluxes. Experimental results for carbon and tungsten samples are reported. Finally, it is shown how, by using the active pyrometry, the overall 2D standard IR perturbed by a reflected flux is corrected to recover the full 2D surface temperature close to the real surface temperature.

Local heat transfer estimation in microchannels during convective boiling under microgravity conditions: 3D inverse heat conduction problem using BEM techniques

Two-phase and boiling flow instabilities are complex, due to phase change and the existence of several interfaces. To fully understand the high heat transfer potential of boiling flows in microscales geometry, it is vital to quantify these transfers. To perform this task, an experimental device has been designed to observe flow patterns. Analysis is made up by using an inverse method which allows us to estimate the local heat transfers while boiling occurs inside a microchannel. In our configuration, the direct measurement would impair the accuracy of the searched heat transfer coefficient because thermocouples implanted on the surface minichannels would disturb the established flow. In this communication, we are solving a 3D IHCP which consists in estimating using experimental data measurements the surface temperature and the surface heat flux in a minichannel during convective boiling under several gravity levels (g, 1g, 1.8g). The considered IHCP is formulated as a mathematical optimization problem and solved using the boundary element method (BEM).

Inverse heat conduction problem using thermocouple deconvolution: application to the heat flux estimation in a tokamak

Abstract Internal components of magnetic confinement fusion machines are subjected to significant heat fluxes. A large part of this power is directed towards plasma facing components. Even if these components are designed to receive about , surface temperature and heat flux measurements are important issues to guarantee safe plasma operations. In JET tokamak, few embedded thermocouples (TC) located 1 cm below the tile surface are used to measure the bulk temperatures of the Carbon Fiber tiles (coated with about 20 m of tungsten with the ITER-like wall). We propose here to use an inverse thermal calculation based on Thermal Quadrupole method to locally deduce the deposited heat flux. The calculation requires the location of the peak and the normalized 1D-shape of the heat flux deposited on the target.

Surface temperature measurement of plasma facing components with pulsed active pyrometry

In fusion devices, like Tore Supra, the surface temperature of the carbon plasma facing components (PFC) is measured by standard infrared (IR) system in real time for control of integrity of PFCs. The emissivity of carbon is known and high (ε~0.8-0.9) and therefore the contribution of the reflected flux in the total flux collected can be neglected. For future tokamaks like WEST and ITER, PFCs will be in metal (Be/W) with low and variable emissivities (ε~0.1-0.4). Consequently, the surface temperature measurement will become challenging due to low emissivity and non-negligible reflected fluxes. The pulsed active pyrometry proposed in this paper allows a punctual surface temperature measurement independently of the emissivity and the reflected fluxes. Experimental results for aluminium and tungsten samples are reported. In addition it is shown that using the local measurement by active pyrometry, an overall 2D standard IR perturbed by a reflected flux can be corrected to recover the full 2D real surface temperature.

Successive identification of surface heat flux and thermophysical properties of plasma facing components inside the JET tokamak: numerical feasibility

We present here the numerical feasibility of using two thermal diagnostics (IR surface and embedded thermocouple temperature measurements), that outfit the same carbon tile inside the JET Fusion reactor and whose combination enables to identify, on one hand, a surface heat flux history and, on the other hand, the spatial and time variation of the thermal resistance of an unknown deposited surface carbon layer (SCL). The Conjugate Gradient Method (CGM) and the adjoint state were applied to perform these two identifications.

Concept And Development Of Instruments For ITER Thermography

We give here a short overview of the status of the development for ITER thermography as performed by the CEA‐Cadarache and some of its collaboration partners. The topics that have been included in this synthesis are the status of the optical design, the role of multi‐wavelength mesurements, multicolour pyroreflectometry, photothermal methods, and reflection simulations and measurements.

Flow boiling in microgravity condition investigation using inverse techniques

The objective is to provide a method to obtain local heat transfer coefficients in small channels when flow boiling occurs. The experimental device has been developed to perform investigations in parabolic flights campaigns on board A300-ZéroG. Simultaneously flow visualization and thermo-hydraulic measurements are carried out to investigate the two phase flow and heat transfer in minichannels. The experiments are conducted with HFE-7100 in several operating conditions for three hydraulic diameters.The investigations concern flow pattern and the associated heat transfer coefficient during convective for several gravity levels. We mainly on the thermal measurements which consists in inversing experimental temperature measurements (thermocouples) to derive the local surface temperature and heat flux. For the investigated operating conditions, the heat transfer coefficient is found to vary along the flow axis especially at the channel entrance zone.

Heat Flux estimation in WEST divertor with embedded thermocouples

The present paper deals with the surface heat flux estimation with embedded thermocouples (TC) in a Plasma Facing Component (PFC) of the WEST Tokamak. A 2D nonlinear unsteady calculation combined with the Conjugate Gradient Method (CGM) and the adjoint state is achieved in order to estimate the time evolution of the heat flux amplitude and decay length λq . The method is applied on different synthetic measurements in order to evaluate the accuracy of the method. The synthetic measurements are generated with realistic values of λq and magnitudes as those expected for ITER.

Convective Boiling Between 2D Plates: Shear Flow Influence on Bubble Growth, Detachment and Evolution

The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 vapor bubbles confined in a two-dimensional plane created on an artificial nucleation site. The experiments were performed by varying the shear flow and the bubble nucleation rate through the heat flux supplied under normal gravity and microgravity. The distance between the plates was kept constant at 1 mm. The first results of these experiments are mainly related to the formation of a bubble on an isolated site with or without convection and to the associated heat transfer when confinement is imposed on a bubble. To this effect, we implemented an experimental device allowing the observation of the flow by means of both a visible video camera and an infrared video camera. Here, we present the first results obtained from the campaign performed in December 2007 concerning the influence of gravity on the bubble detachment diameter and the first images of 2D bubbles obtained in microgravity by means of an infrared camera.Copyright