Didier Saury

Experimental study of flash evaporation of a water film

Abstract This experimental study of the flash evaporation phenomenon of a water film was carried out with an initial water height of 15 mm, superheats ranging from 1 to 35 K and initial temperatures from 30 to 75 °C. During a sudden pressure drop, temperature measurements of the water film allowed us to determine the water mass evaporated by this phenomenon as well as the mass flow rates. A correlation between the water mass evaporated by flashing and the superheat was then obtained. Evolution of the flash evaporation rate coefficient let us estimate the duration of the flash evaporation phenomenon.

Interaction of radiation with double-diffusive natural convection in a three-dimensional cubic cavity filled with a non-gray gas mixture in cooperating cases

ABSTRACT A three-dimensional (3D) numerical study has been performed to investigate the effects of non-gray gas radiation on double-diffusive natural convection in a cubic enclosure filled with either air–H2O or air–CO2 mixtures in cooperating situations. Gas radiation was taken into account by the discrete ordinates method (DOM) associated with the spectral line weighted-sum-of-gray-gases (SLW) spectral model. Results obtained for two average concentrations of H2O and CO2 (10% and 20%) show that radiation modifies the temperature and concentration structures by creating oblique stratifications. The heat transfer rate is decreased, whereas mass transfer is not much modified. In addition, a comparison between 2D and 3D results is presented.

Experimental investigations in an air-filled differentially-heated cavity at large Rayleigh Numbers

A large-scale experimental differentially heated cavity was built and instrumented. Rayleigh numbers up to 1.2×1011 can be obtained with a temperature difference, ΔT = 20°C, between the hot and cold walls leaning in the range of validity of the Boussinesq approximation. Previous data obtained locally for mean velocity by 2D LDV in the range give rise to questions regarding the general air flow circulation in the cavity. Particularly, a downstream flow along the vertical boundary layer was observed. This reverse flow caused by the temperature stratification outside this layer is not present in the upstream parts and was not previously observed in smaller cavities. The question of the global circulation in this cavity is thus posed. Evolution laws providing Nusselt numbers are also given and when possible compared to the literature for a large range of Rayleigh numbers.

Coupled temperature and velocity measurements in turbulent natural convection flows

The goal of this paper is to develop simultaneous measurements of the temperature and velocity in order to contribute to the development of models adapted to natural convection flows and better apprehend their turbulent characteristics. The experimental setup consists in a vertical open channel whose sidewalls are kept at ambient temperature. A heated square bar is placed into the lower part of this channel close from one of its wall. It drives air in the whole channel. The temperature of the bar is chosen to obtain a turbulent air flow. This study focuses on the measurement technique developed to realize synchronized temperature and velocity measurements in a turbulent natural convection flow. This technique permit to measure turbulent heat fluxes ( or ). Measurement strategies will be presented and discussed in the paper. Some experimental care is needed to avoid disturbing the airflow. The challenge is to choose two complementary measurement techniques which have to be synchronized but which cannot be carried out at the same location when a laser is used. In this study, the velocity and the temperature measurements are respectively carried out using PIV technique and a specific K type micro-thermocouple (12.7 ?m in diameter). The location of the thermocouple with respect to the laser sheet has been investigated as well as the influence of the laser on the temperature measurements. The criterion used for finding the best location is the correlation coefficient between the temperature and the velocity data. Some preliminary results of coupled velocity-temperature measurements are provided showing the feasibility of this kind of measurements.

Numerical investigation of coupled natural convection and radiation in a differentially heated cubic cavity filled with humid air. Effects of the cavity size

ABSTRACT A numerical study of natural convection with surface and air/H2O mixture radiation in a differentially heated cubic square cavity is presented. The coupled flow and heat transfers in the cavity are predicted by coupling a finite volume method with a spectral line weighted sum of gray gase model to describe gas radiative properties. The radiative transfer equation is solved by means of the discrete ordinate method. Simulations are performed at Ra = 106, considering different combinations of passive wall and/or gas radiation properties and different cavity length. It was found that in presence of a participative medium representative of building, cavity length has a strong influence on temperature and velocity fields which affect the global circulation and heat transfers in the cavity. For each steady-state solution, the convective and radiative contributions to the global heat transfer are discussed. More specifically, boundary layer thickness, thermal stratification parameter, and three-dimensional effects are compared to pure convective case results. The results suggest that radiative effects, often considered as negligible in view of the relatively low optical thickness, may not be neglected when trying to predict regime transitions.

Assessment of Thermal Cycling in a Rectifier For Wave Power Generation

Natural convection allows for passive cooling which isused in many engineering applications. Placing dissipatingcomponents on a common vertical heatsink can be opti-mized to give the best possible ...

Comparative Study of Radiative Effects on Double Diffusive Convection in Nongray Air-CO2 Mixtures in Cooperating and Opposing Flow

This study analyses the effects of nongray gas radiation on double diffusive convection, in a square differentially heated cavity filled with air-CO2 mixtures, when the buoyancy forces (thermal and mass) are cooperating or opposing. The radiative source term in the energy equation is evaluated by the discrete ordinate method (solving the radiative transfer equation) and the SLW spectral model (accounting for real radiative properties of absorbing species). Here, gas absorption varies with the local temperature and concentration of pollutant, which induces a strong direct coupling between the concentration and thermal fields that would not exist with gray gas. Simulations are performed at different concentrations of CO2 corresponding to different flow regimes (thermal, transitional, and mass). Results show the following: (i) in cooperating flow, radiation modifies essentially the heat transfer and the characteristics of temperature and concentration fields; (ii) in opposing flow, radiation effects are more important and depend on the nature of the flow regime.

Experimental and Numerical Investigation of Turbulent Natural Convection Flow in a Vertical Channel With a Heated Obstacle

In the present study, experiments were carried out for natural turbulent convection induced by a heated square bar in a two-dimensional (2D) open vertical channel for different Rayleigh numbers and bar positions. For this purpose, particle image velocimetry (PIV) system has been employed to investigate the velocity field in the vertical center channel plane. The present work is also concerned with computational fluid dynamics (CFD) simulation by employing large Eddy simulation (LES) turbulence model, used in fire dynamic simulation (FDS) code. Calculations were performed for different chimney aspect ratios A* (height Lb over width d) and modified Rayleigh numbers ranging between 4 × 107 and 108. Experimental and numerical results included mean velocity profiles; flow structure and Nusselt number were presented and discussed. To validate CFD code, velocity profiles along channel elevation were compared with our experimental measurements, and a good agreement was observed. Therefore, FDS code is a useful tool to simulate natural turbulent convection dynamic field, and consequently the thermal field in such situation. CFD code has been used to study the best heated bar location (corresponding to the best cooling effect) in the channel as well as the best airflow rate. This best location and its explanation are discussed in this paper.

Numerical Study of Fluid Flow in an Enclosed Electrical Motor

Nowadays, design of electrical motors runs up more and more against the problems of heatings during operation. These excessive heatings result from a weak heat dissipation. The improvement of cooling system passes thought the improvement of convective heat transfers within the motors. However, these heat transfers strongly depend on the flow structure. This structure is particularly significant for the enclosed motors. The air is completely enclosed inside the machine and consequently, it is never renewed. Indeed, for this kind of configuration, the flow structure is only regulated by the geometry. It is thus not possible to modify it from the outside as for the open motors. That is why, the numerical study presented below analyzes the flow structure in an enclosed induction motor of railway traction. The sliding mesh method has been carried out in order to keep a significant accuracy on the flow nature. Then, some air spaces in the motor have been proved to be particularly sensitive for this kind of enclosed motor.Copyright

Study of the Local Convective Heat Transfer Downstream a Backward Facing Step for Various Upstream Airflow Conditions and Enlargement Factors

This paper presents an analyse of the heat transfer coefficient downstream a backward facing step with various upstream airflow conditions: uniform flow outside a laminar boundary layer, uniform flow outside a turbulent boundary layer, and a fully developed flow. The local heat fluxes are obtained from temperature determination by infrared thermography on an assumed thermally thick wall used as boundary conditions of a numerical model. In this article we mainly focus on the maximum heat flux point position determined experimentally and numerically, and also on the influence of the expansion ration on the value of the maximum Nusselt number.Copyright