G. Hetsroni

Particles-turbulence interaction

Abstract The interaction between solid particles and the turbulence of the carrier fluid is considered. Theoretical considerations suggest that particles with low Reynolds number cause suppression of the turbulence, while particles with higher Reynolds number cause enhancement of turbulence due to wake shedding. A review of the available experimental data supports this observation.

A uniform temperature heat sink for cooling of electronic devices

Experimental investigation of a heat sink for cooling of electronic devices is performed. The objective is to keep the operating temperature at a relatively low level of about 323–333 K, using a dielectric liquid that boils at a lower temperature, while reducing the undesired temperature variation in the both streamwise and transverse directions. The experimental study is based on systematic measurements of temperature, flow and pressure, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250 lm. Experiments on flow boiling of Vertrel XF in the microchannel heat sink are performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the two-phase results to a heat sink cooled by single-phase water flow. 2002 Elsevier Science Ltd. All rights reserved.

Nonuniform temperature distribution in electronic devices cooled by flow in parallel microchannels

We fabricated a novel thermal microsystem (simulating a computer chip) consisting of a heater, microchannels, inlet and outlet plena and we studied the effect of the geometry on the flow and heat transfer. The vapor-water two-phase flow patterns were observed in the parallel microchannels through a microscope and high-speed video camera. It was observed that hydraulic instabilities occur. Existence of a periodic annular flow was also observed, which consists of a symmetrically distributed liquid ring surrounding the vapor core. Along the microchannel axis, the periodic dry zone appears and develops. The thermal visualization and temperature measurements of the heated device were carried out using infrared thermography. As long as the flow was single phase liquid, the forced convection heat transfer resulted in a moderate irregularity on the heated chip. These temperature differences do not cause damage to a real electronic device. The steady-state heat transfer for different types of microchannels has been studied also at the range of heat flux where phase change of the working fluid from liquid to vapor took place. Under conditions of flow boiling in microchannels, a significant enhancement of heat transfer was established. In the case of uniform heat flux the hydraulic instabilities lead to irregularity of temperature distribution on the heated chip. In the case of nonuniform heat flux the irregularity increased drastically.

Two-phase flow patterns in parallel micro-channels

Abstract Micro-channel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro-electronic devices. Some of the important aspects associated with two-phase flows in micro-channels, is to study the bubble behavior and flow regimes in diabatic, parallel micro-channels. Most of the reports in the literature present data of only a single channel and mostly adiabatic. This does not account for flow mixing and hydrodynamic instability that occurs in parallel micro-channels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air–water and steam–water flow in parallel triangular micro-channels. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air–water flow different flow patterns were observed simultaneously in the various micro-channels at a fixed values of water and gas flow rates. In steam–water flow, instability in uniformly heated micro-channels was observed. This work develops a practical modeling approach for two-phase micro-channel heat sinks and considers the discrepancy between flow patterns of air–water and steam–water flow in parallel micro-channels.

Heat transfer to a liquid—solid mixture in a flume

Infrared thermography was used to study the thermal interaction between the particle-laden turbulent flow and a heated plate. Thermal streaks were observed on the heated plate, and the dimensionless mean distance A+ between the streaks was found to increase with the Reynolds number. Introduction of polystyrene particles, within the uncertainty limits, did not change the value of A+ in the flows with different particle volumetric loading Qp/Qo. Particles generally accumulated in the warmer regions of the heated plate, i.e. in the low velocity streaks. The maximum amplitude of wall temperature fluctuations reached ±40% of the difference between the average wall temperature and the fluid bulk temperature. Consequently, there are variations in the heat transfer coefficient of the same order. The addition of particles changed the temperature fluctuations level. The location of the maximum of the dimensionless average heat transfer coefficient αp/αo = f(Qp/Qo) depended on the Reynolds number and coincided with the location of the minimum non-dimensional RMS temperature fluctuation ratio, at the same Reynolds numbers. Many of the experimental observations are presented here on an ad hoc basis, since we do not have, at this stage, a theory which can adequately describe the wealth of experimental observations.

Turbulence intensity in dilute two-phase flows—3 The particles-turbulence interaction in dilute two-phase flow

Abstract We propose a simplified theory for the particles-turbulence interaction in a dilute two-phase flow with particles of arbitrary sizes. The theory takes into account two sources of turbulence in particle-laden flows: (i) the carrier fluid velocity gradients; and (ii) turbulent wakes behind the coarse particles. The theoretical description is based on the modified mixing-length theory and turbulent kinetic energy balance method. The solution of the problem for particle-laden flow does not require any additional quantitative empirical data; only the standard semi-empirical parameters for pure carrier fluid are used. The dimensional analysis of the system of equations is used to reduce to the minimum the number of nondimensional parameters in turbulent particle-laden flows. In the limits of fine and coarse particles the asymptotic expression for turbulence intensity is found. It is shown that in the former case the carrier fluid fluctuations intensity is found. It is shown that in the former case the carrier fluid fluctuations depend only on the value of the total mass content of the admixture; whereas in the latter one they are determined by the total mass content of the admixture, the density ratio of the phase and the aerodynamic properties of the particles. The proposed theory is applied to predict the fluctuations intensity in various types of turbulent particle-laden flows. The results of the calculations are in fairly good agreement with the experimental data.

The effect of surfactants on bubble growth, wall thermal patterns and heat transfer in pool boiling

Abstract During nucleate pool boiling of pure water and water with cationic surfactant, the motion of bubbles and the temperature of the heated surface were recorded by a high-speed video camera and an infrared radiometer. All experiments were performed at saturated boiling conditions. The boiling curves for various concentrations were obtained and compared. The results show that the bubble behavior and the heat transfer mechanism for the surfactant solution are quite different from those of pure water. The heat transfer dependence on the relative changes of both the surface tension and the kinematic viscosity was discussed.

On the low reynolds number motion of two droplets

Abstract Exact solutions are derived for the quasi steady-state creeping flow internal and external to two spherical droplets moving along their line-of-centers. Numerical results are presented, which include all previous solutions as special cases.

Kinetic study of the thermal decomposition of polypropylene, oil shale, and their mixture

The thermal decomposition behavior of polypropylene, oil shale and a 1:3 mixture of the two was investigated in a thermogravimetric analyzer (TGA) reaction system in an argon atmosphere, with a view to comparing the process of the mixture with those of the individual components. Experiments were conducted at three heating rates of 5, 10, and 15 K min 21 , in the temperature range of 30‐9008C. The obtained activation energies were 250 kJ kg 21 for polypropylene, 63 kJ kg 21 for the oil shale organic matter, and 242 kJ kg 21 for the mixture. The results indicate that the characteristics of the process depend on the heating rate, and that polypropylene acts as a catalyst in the degradation of oil shale in the mixture. q 2000 Elsevier Science Ltd. All rights reserved.

Effect of wall boundary condition on scalar transfer in a fully developed turbulent flume

We performed direct numerical simulation of fully developed turbulent velocity and temperature fields in a flume, for Reynolds number, based on the wall shear velocity and the height of the flume, Re=171 and Prandtl numbers Pr=1.0 and Pr=5.4. To elucidate exactly the role of the wall boundary condition for passive scalar, the system considered was the flow at constant properties of the fluid. Two types of thermal wall boundary conditions (BCs) for the dimensionless temperature equation were studied: isothermal wall boundary condition—H1, and isoflux wall boundary condition—H2. The profile of the mean temperature was not affected by the type of BC. However, the type of BC has a profound effect on the statistics of the temperature fluctuations in the near-wall region y+<10. Comparison of near-wall statistics of temperature fluctuations shows that at Pr=1 the buffer part of the turbulent boundary layer significantly influences the scalar transfer in the conductive sublayer, whereas at Pr=5.4 the near-wall te...