L. Tadrist

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

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.

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

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

Flow Boiling in Minichannels Under Normal, Hyper and Microgravity: Frictional Pressure Loss and Flow Patterns

We present in this paper, flow boiling results obtained during parabolic flights campaigns. The experimental aim is to obtain the local heat transfer coefficient and the influence of gravity on HFE-7100 flow boiling in minichannels. The hydraulic diameter investigated is: 0.84 mm. The influence of hypergravity and microgravity solely on the frictional pressure loss is evidenced in this paper, and explained using the flow patterns.Copyright

Ionic and Non-Ionic Laminar Liquid Flows in Fused Silica Microtubes

We previously presented results obtained for liquid flow of tap water flow in 530 to 50 μm-diameters microtubes and evidenced a clear deviation from the classical Stokes flow theory which predict a constant Poiseuille number of 64 [1]. We assumed that using the EDL theory and a constant ζ potential approach may explain our results [2]. To confirm the previous obtained results, we used our dedicated experimental set-up [3] to study other fluids: ionic (distilled water, KCl solutions) and nonionic (n-dodecane). Here, for distilled water and n-dodecane, the same range of microtubes diameters have been investigated. For KCl solutions, the experiments were realized at constant microtube diameter (152 μm) for 5 different solutions concentrations. For a given diameter, the experiments are performed using 4 microtubes’ lengths then to ensure an accurate calculation of the Poiseuille number. We evidence different behaviours for given experimental conditions (that is microtubes inner diameters and surface roughness) depending of the fluid ionic characteristics. Here, we mainly present new experimental.Copyright

Gravity Influence of Convective Boiling Stability in a Minichannel

We previously evidenced the influence of confinement and inlet conditions on convective boiling stability in a minichannel. The experiments were realized based on an upward n-pentane two-phase flow. Here, we present results of convective boiling in a minichannel for several minichannel orientations which can be modified from the horizontal (heating surface on the top) through the vertical (previous situation already studied of upward flow) to the horizontal (heating surface on the bottom). We present the results obtained for the same heat flux provided to the minichannel (Q W = 92 kW.m−2 ), the same range of inlet mass velocity (73 to 2300 Kg.m−2 .s−1 ) for 5 different minichannel’s orientations: −90°, −45°, 0°, 45° and +90°. The consequences on the minichannel total pressure drop, fluid-wall temperature, and two-phase flow stability are discussed.Copyright

Modeling of Surface-Fluid Electrokinetic Coupling on the Laminar Flow Friction Factor in Microtubes

The present work’s originality lies in the evidence of a non negligible effect of the fluid ions’ and co-ions’ interaction with the wall surface in a microtube. This study is based on the EDL theory (Electrical Double Layer) which is developed here for a circular geometry. High electrical surface potentials are taken into account for the present study; they induce the nonlinearity of the problem’s main equation (Poisson-Boltzmann equation). The electrical field is determined, then the velocity profile and finally the Poiseuille number. We show that even with the EDL effect taken into account, the Poiseuille number does not depend on the mean velocity. Our model agrees with the experimental results for high surface potentials (> 25 mV). This is found by comparing with experiments previously carried out with microtubes ranging from 530 to 50 μm.Copyright

Boiling Flow Pressure Drop Modeling in a Minichannel

The present paper deals with two-phase flow pressure drop modeling. This is based on solving the mass, momentum and energy balance equations in steady state conditions. In the two-phase zone, the liquid-vapor is assumed to be a homogeneous fluid. The calculated pressure drop variation is found to be similar to the experimental one when the two–phase flow is steady. In the unsteady state conditions characterized by high amplitude of the pressure fluctuations, the computed pressure drop is found to be different from the experimental mean pressure drop. This difference is all the higher as the pressure fluctuation is high.Copyright