Bertrand Garnier

A new method for simultaneous measurement of convective and radiative heat flux in car underhood applications

A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2,

Implementation of an inverse method for identification of reticulation kinetics from temperature measurements on a thick sample

Abstract The aim of this article is to identify a reaction function involved in a model of vulcanization by using experimental study on thick pieces of rubber. The methods of determination of the parameters of the model are described, the inverse method is explained, results are given and commented on.

Numerical and experimental hydrodynamic study of a coolant distributor for grinding applications

ABSTRACT In grinding, the high frictional energy is converted into heat, which may cause thermal damage and degradation of the wheel and the workpiece. Unwanted thermal effects must thus be reduced, often by external cooling using a curved-duct coolant distributor to match the wheel geometry. The performance of such a system depends strongly on the impinging jet flow properties to ensure efficient sprinkling of the hot spots. The fluid distributor, placed above the workpiece, is pierced with a certain number of identical nozzle fittings, providing multiple jets at the outlet of the nozzles. These jets sprinkle the solids over a given zone and remove the heat by convective transfer. The cooling is hence dependent on the flow structure, meaning the jet diameters, trajectories and velocities, determined up-flow by the distributor design. The present study is devoted to the hydrodynamics aspects of the fluid distributor, aiming to determine the flow-rate distribution at the different orifices and the flow-rate–pressure relationship, for a variety of nozzle diameters and feeding flow rates, under isothermal conditions. A simple hydraulic balance in the device was not able to predict with sufficient accuracy the actual measurements, even when the Venturi effect was accounted for. This discrepancy is due to the curvature of the distributor, inducing secondary flows in interaction with the nozzle outlets, which leads to a rather complex flow pattern. To overcome this issue, a computational fluid dynamics (CFD) tool was used and compared with in situ experiments – global flow rate and pressure measurements were additionally taken with particle image velocimetry (PIV) to gain insight into the local structure. Simulations were performed with a 3D turbulence model for Reynolds numbers up to 100,000. This model provides an efficient tool for coupling with the thermal study at a later step, allowing global sizing and energetic optimization of the grinding process.

Comparison of experimental and simulated effective thermal conductivity of polymer matrix filled with metallic spheres: Thermal contact resistance and particle size effect

In this paper, we present a numerical and experimental study of a composite material with conducting spheres embedded in a polymer matrix. Our main objective is to study how the particle size and thermal contact affect the overall thermal properties of composites. In the numerical study, finite elements method is used to model heat transfer and to calculate the effective thermal conductivity. A periodical method was used to measure simultaneously thermal conductivity and diffusivity of the composite consisting of an epoxy resin matrix filled with brass spheres of different sizes. A comparison between the numerically calculated, measured and analytical thermal conductivity values for various samples is performed and the significance of the findings is discussed in the paper.

Experimental and Numerical Study of the Coolants Distributor for Machining Process

Grinding is a machining process which may encounter excessive heat generated by the friction between the wheel and the material, and therefore degrade the tool as well as the material. The heat has hence, to be removed as efficiently as possible, most often by external cooling. The fluid is projected on the hot interface between the tool and the material through a curved duct coolant distributor. The performance of such a system is strongly dependent on the fluid flow in the curved duct and on the impinging jet flow properties. To optimize cooling setup, CFD simulations and in-situ experiments using particle image velocimetry (PIV) have been made, as well as global flow rate and pressure measurements in the curved duct. A three-dimensional model of a curved duct with 25 outlet nozzles has been studied for duct Reynolds number up to 100,000. Different geometrical configurations for various nozzle diameters have been studied. Due to the complexity of the distributor geometry, it is shown that the global hydraulic balance is not appropriate for sizing the industrial process. On the contrary, satisfactory trend matching in fluid flow streamline behavior is between numerical and experimental results, and an accurate prediction of the pressure drop both show that the numerical simulation is reliable to capture the flow pattern within the curved channel distributor.Copyright

Thermal Properties of Sintered Diamond Composites Used in Grinding

Sintered diamonds are used in grinding because they offer better mechanical properties than conventional materials (mineral or silicon carbide abrasives) and yield high grinding speed and long life. In addition, because of their thermal performance, they contribute to cooling the workpiece, avoiding excessive temperatures. Thus in order to choose the best material for the worktool, one often must know the thermal conductivity of sintered diamond. In this work, the thermal conductivity of sintered diamond is evaluated as a function of the volume fraction of diamond in the composite and for two types of metallic binders: hard and soft. The measurement technique is based on the flash method that associates heating and measurement devices without sample contact and on parameter estimation using a three-layer thermal model. With a hard metallic binder, the thermal conductivity of sintered diamond was found to increase up to 64% for diamond volume fraction increasing from 0 to 25%. The increase is much smaller for the soft binder: 35% for diamond volume reaching 25%. In addition, experimental data were found far below the value predicted by conventional analytical models for effective thermal conductivity. A possible explanation is that the thermal conductivity of such composites is affected by poor heat transfer at the diamond/binder interface, the thermal contact resistance between matrix and diamond particles being estimated at between 0.75 and 1.25 10−6 m2K.W−1.Copyright

Mixing, Reaction and Heat Transfer in a Pulsatile Flow Microreactor: Infrared Measurements

Single-phase microreactors and micro-heat-exchangers have been widely studied over the last decade. Greater safety, better temperature control and hence better reaction products are provided by the high surface-to-volume ratios and compactness of microscale devices, making them more attractive than conventional systems for future industrial applications. Since the flow in microfluidic devices is predominantly laminar long mixing channels or complex geometries are needed for molecular diffusion and completion of the reaction. Hoping to remove this drawback, we demonstrate experimentally the merits of flow pulsation in enhancing mixing efficiency in a simple T channel (50 μm high, 500 μm wide and 40 mm long). For this purpose, a U-shaped PDMS microchannel is enclosed by a 0.1 mm glass plate coated with an opaque layer that constitutes the fourth wall of the microchannel. We investigate experimentally the effect of flow pulsation on mixing of an acid-base neutralization reaction. The mixing of HCl and NaOH reactants, both of 0.8 mol/L concentration, in a T-shaped microchannel is assured by time-pulsed flows at small Reynolds numbers. The effect of mixing is observed at the location along the microchannel at which the temperature reaches its maximum value. For this purpose, infrared thermography, a non-intrusive temperature measurement technique with spatial resolution of a few tens of microns, is used. The mixing efficiency is shown to depend strongly on the ratio between the peak amplitude and the mean flow rate (between 0.4 and 1 mL/h). Saturation is observed for values of this ratio greater than 2.5. Mixing also appears to be enhanced for frequencies of the periodic inlet flows increasing from 1.25 Hz to 5 Hz, i.e. for increasing Strouhal number.© 2012 ASME

Thin-Film Heat-Flux Microsensor for Heat-Transfer Measurement in Micro-Heat Exchangers/Microreactors

Heat-transfer analysis in microfluidic devices is of great importance in applications such as micro-heat exchangers and microreactors. This work reports on improvements in temperature measurement techniques, which can be the source of large errors due to their intrusiveness and the unreliability of conventional thermal sensors. Gold thin films were deposited on a borosilicate substrate to realize a 2D heat flux sensor for heat-transfer measurement along the main flow within microchannels. Two applications are shown, one related to micro-heat exchangers and the other to microreactors. For the micro-heat exchanger, the effect of length scale on heat transfer in a straight microchannel was investigated and the validity of macroscale correlations for convective heat transfer was checked for deionized water flowing in microchannels of heights 12 to 52 μm. For the microreactor, the reaction enthalpy of an acid–base reaction measured using the new heat-flux sensor had only a 5% discrepancy from the standard value, showing the efficiency of the new thin-film device.Copyright