Christophe Le Niliot

Flow Boiling in Minichannels Under Normal, Hyper-, and Microgravity: Local Heat Transfer Analysis Using Inverse Methods

Boiling in microchannels is a very efficient mode of heat transfer since high heat and mass transfer coefficients are achieved. Here, the objective is to provide basic knowledge on the systems of biphasic cooling in mini- and microchannels during hyper- and microgravity. The experimental activities are performed in the frame of the MAP Boiling project founded by ESA. Analysis using inverse methods allows us to estimate local flow boiling heat transfers in the minichannels. To observe the influence of gravity level on the fluid flow and to take data measurements, an experimental setup is designed with two identical channels: one for the visualization and the other one for the data acquisition. These two devices enable us to study the influence of gravity on the temperature and pressure measurements. The two minichannels are modeled as a rectangular rod made up of three materials: a layer of polycarbonate (λ =0.2 W m -1 K -1 ) used as an insulator, a cement rod (λ=0.83 W m -1 K -1 ) instrumented with 21 K-type thermocouples, and in the middle a layer of Inconel® (λ=10.8 W m -1 K -1 ) in which the minichannel is engraved. Pressure and temperature measurements are carried out simultaneously at various levels of the minichannel. Above the channel, we have a set of temperature and pressure gauges and inside the cement rods, five heating wires provide a power of 11 W. The K-type thermocouple sensors enable us to acquire the temperature in various locations (x, y, and z) of the device. With these temperatures and the knowledge of the boundary conditions, we are able to solve the problem using inverse methods and obtain local heat fluxes and local surface temperatures on several locations. The experiments are conducted with HFE-7100 as this fluid has a low boiling temperature at the cabin pressure on Board A300. We applied for each experiment a constant heat flux (Qw =33 kW m -2 ) for the PF52 campaigns (Parabolic Flights). The mass flow rate varies in the range of 1

Identification of space and time varying thermal resistance: Numerical feasibility for plasma facing materials

Abstract The present paper deals with a non-linear unsteady calculation combined with the conjugate gradient method (CGM) and the adjoint state, in order to characterize in-situ the spatial and time variation of the thermal resistance of a surface layer. This paper presents the numerical feasibility of this method for the plasma-facing components (PFC), and precisely on the surface carbon layer (SCL), usually poorly attached to the PFC in the fusion machines, a realistic experiment design was used. The accuracy of the method is examined by using simulated inexact infrared measurements obtained on the SCL surface. The advantages of applying the CGM with the adjoint state in the present study, are that no prior information is needed on the time variation and for the initial guesses of the unknown thermal resistance.

Prediction of spatial resolutions of future IR cameras at ITER

Here are presented the predictions of the spatial resolutions of one of the future IR camera that will survey the divertor of the Tokamak ITER. The objective is to associate, in Fourier space, the optical transfer function and the detector transfer function to calculate the total transfer function (TTF) of the virtual IR camera. The modulation transfer function (modulus of TTF) quantifies the ‘imaging’ performances of the virtual camera. Its ‘measuring’ performances are estimated by the simulation of the slit response function experiment. Finally, some results of sharp temperature profile measurements in realistic plasma situations are presented.

Flow Boiling in Minichannels Under Normal, Hyper and Microgravity: Local Heat Transfer Analysis Using Inverse Methods

The objective presented in this paper is here to provide basic knowledge on the systems of biphasic cooling in mini and microchannels during hyper and microgravity. The experimental activities are performed in the frame of the MAP Boiling project founded by ESA. The main aspect of this paper is to present the use of inverse methods to estimate local flow boiling heat transfers coefficient in minichannels. To observe the influence of gravity level on the fluid flow and to take data measurements, an experimental setup is designed with two identical channels; one for the visualization and the other one for the acquisition of data. These two devices enable us to study the influence of gravity on the temperatures and pressures measurements. The two minichannels are modeled as a rectangular rod made up of three materials; a layer of polycarbonate® (λ = 0,2 W.m−1 .K−1 ) used as insulator, a cement rod (λ = 0,83 W.m−1 .K−1 ) instrumented with 21 K-type thermocouples and in the middle a layer of incone® (λ = 10,8 W.m−1 .K−1 ) in which the minichannel is engraved. Pressures and temperatures measurements are carried out simultaneously at various levels of the minichannel. Above the channel, we have a set of temperatures and pressures gauges and inside the cement rod, 5 heating wires providing a power of 11 W. The K-type thermocouples sensors enable us to acquire the temperature in various locations (x, y and z) of the device. With these temperatures and the knowledge of the boundary conditions, we are able to solve the problem using inverse methods and to obtain local heat flux and local surface temperatures on several locations. All the results on hydrodynamics and pressure drop will be provided in a second paper in the same congress.Copyright

Development and validation of a numerical tool for simulating the surface temperature field and the infrared radiance rendering in an urban scene

A new simulator devoted to urban environment is presented. Its aims at generating the synthetic scene viewed by an infrared sensor after solving the direct heat transfer problem at the surface level. The software SOLENE (CERMA, Nantes) was coupled with two tools for realising this task: SUSHI (Simulation in Urban Scene of Heat dIffusion) and MOHICANS. SUSHI purpose is to compute the external surface temperature of a building based on a 1D or a 2D heat transfer model. The 2D model is used in specific parts of the walls for simulating the impact of the thermal bridges on the façade temperature. Then, MOHICANS yields the infrared at-sensor radiance taking account reflections and atmosphere radiative contributions. A joined ground and airborne experiment has been done to validate this simulator. The results of this validation are presented showing a good adequacy between simulated and measured values for both temperature and infrared radiance.

Two-Phase Flow Experimental Study: Influence of Gravity Level on Local Boiling Heat Transfer Inside a Minichannel

Flow boiling in minichannels is the most complex convective phase change process. Indeed, there are a lot of physical parameters that influence the two-phase flow during boiling. Here, we will focus on the influence of one of this factor: the gravity level. Actually, there are not many mechanisms that have been proposed for the role of this bound on boiling phenomena. In fact, there is not complete agreement on the importance of gravity on heat and mass transfers with phase change because there is a lack of experimental data at this small scale and because reproducing different gravity levels during parabolic flights has a cost. In this line, the goal of this work is to obtain benchmark data on the local heat transfer coefficient in a minichannel during hyper and microgravity. We want to acquire a better knowledge of the elementary phenomena which control the heat and mass transfers during convective boiling. Indeed, boiling in microscale geometry is a very efficient mode of heat transfer since high heat and mass transfer coefficients are achieved. Actually, minichannels and microchannels are widely used in industry and they are already attractive in many domains such as design of compact evaporators and heat exchangers. They provide an effective method of fluid movement and they have large heat dissipation capabilities. In these situations, their compact size and heat transfer abilities are unrivalled. In this communication, the objective is to acquire better knowledge of the conditions that influence the two-phase flow under microgravity. The expected results will contribute to the development of microgravity models. To perform these investigations, we used an experimental data coupling with an inverse method based on BEM (Boundary Element Method). This non intrusive approach allows us to solve a 3D multi domain IHCP (Inverse Heat Conduction Problem). With this analysis, we are able to quantify the local heat flux, the local temperature and the local heat transfer coefficient in a minichannel (254 μm) by inversing thermocouples data without disturbing the established flow.Copyright