A. Mosyak

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.

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.

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...

Low-speed streaks in drag-reduced turbulent flow

The effect of a surfactant drag-reducing additive (530 ppm Habon G solution) on the structure of wall turbulence, both in a flume and in pipe flow, was investigated experimentally. Real-time infrared thermography was used for flow visualization and measurements of the spanwise spacing between the thermal streaks. The experiments were carried out over a broad range of friction velocities, i.e., up*=0.51–3.27 cm/s. With wall shear velocities 1.54⩽up*⩽3.27 cm/s drag reduction of 82%–85% was achieved in a tube flow, well below the predictions of the Virk maximum drag reduction asymptote proposed for high polymers. The results of spanwise streak spacing indicate that wall shear velocity may be an appropriate parameter for describing nondimensional streak spacing behavior in drag reducing flows. A hypothesis, based on the average spanwise streak spacing λ+, can be applied to describe the mean velocity profiles of Habon G solutions. The ratio (λp+−100)/100 was applied to describe mean velocity profiles with 530 ...

Convective boiling in parallel microchannels

Experiments were performed with clear water and with surfactant flowing in parallel triangular microchannels. The study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. Different flow patterns were observed simultaneously in various microchannels at a fixed value of water or surfactant flow rates. Depending on flow and heat flux, pressure and temperature instabilities in the heated microchannels were studied. This work develops a practical modeling approach for two-phase microchannel heat sinks and also considers effect of surfactant on convective boiling in microchannels.

Surface temperature measurement of a heated capillary tube by means of an infrared technique

We describe a method to measure the surface temperature of small-size devices by means of an infrared technique. The method is based on adjusting the temperature of the background to a level equal to the measured temperature of the object surface. The proposed method of infrared measurement was applied to investigate the average and local heat transfer coefficient in a small tube of inner diameter of 1.07 mm in laminar flow, in the range of Reynolds numbers 10 < Re < 400. It was shown that the heat transfer coefficient is much lower in the region of Reynolds numbers 10 < Re < 100 than that predicted theoretically for laminar flow in tubes of larger diameter.

Heat transfer to two-phase flow in inclined tubes

Abstract Experiments were performed to study the flow regimes and heat transfer in air–water flow in 8° inclined tubes of inner diameter of 49.2 and 25 mm. The flow regimes were investigated by using high-speed video technique and conductive tomography. The thermal patterns on the heated wall and local heat transfer coefficients were obtained by infrared thermography. Under the conditions studied, disturbance waves of different forms were observed. The analysis of the behavior of the heat transfer coefficients, together with flow visualization and conductive tomography showed that dryout took place in the open annular flow regimes with motionless or slowly moving droplets. Even under these conditions, the heat transfer coefficient is about 10 times higher than that for single-phase airflow.

Effect of Constant Heat Flux Boundary Condition on Wall Temperature Fluctuations

An experimental study of the wall temperature fluctuations under different thermal-wall boundary conditions was carried out. Statistics obtained from the experiments are compared with existing experimental and numerical data. The wall temperature fields are also examined in terms of the coherent thermal structures. In addition the effect of the thermal entrance region on the wall temperature distribution is also studied. For water flow in a flume and in a rectangular channel, the mean spacing of the thermal streaks does not depend on the thermal entrance length and on the type of thermal-wall boundary conditions. The wall temperature fluctuations depend strongly on the type of wall thermal boundary conditions. Overall, the picture that emerges from this investigation confirms the hypothesis that moderate-Prandtl-number heat transfer at a solid wall is governed by the large-scale coherent flow structures