M. A. Ebadian

In recent years, high-heat-flux cooling techniques have received great attention from researchers around the world due to its importance in thermal management of both commercial and defense high-power electronic devices. Although impressive progress has been made during the last few decades, high-heat-flux removal still largely remains as a challenging subject that needs further exploration and study. In this paper, we have reviewed recent developments in several high-heat-flux heat removal techniques, including microchannels, jet impingements, sprays, wettability effects, and piezoelectrically driven droplets. High-heat-flux removal can be achieved effectively by either single-phase flow or two-phase flow boiling heat transfer. Better understandings of the underlying heat transfer mechanisms for performance improvement are discussed.

Laser-Induced Incandescence (LII) is used in this study to measure the soot volume fraction for steady and unsteady laminar ethylene diffusion flames. For the steady flame the soot profiles obtained in this study using LII showed good agreement with those obtained previously using scattering/extinction technique. For the unsteady or flickering flames, we generated very repeatable time-varying diffusion flames by forcing the fuel flow at frequencies between 1–10 Hz. Phase lock images of the soot volume fractions were obtained for different phases between 0° and 360°. The sequential images showed the dynamics of the interactions between the generated vortices in the fuel and the flame. The phase-locked soot images revealed the entire motion process of the soot field during each pulsation period. The results obtained in the course of this study show that the soot emission decreased by lowering the oscillation frequency of the flame.

This paper presents an experimental investigation of condensation heat transfer and pressure drop characteristics of refrigerant R-134a flowing through an annular helicoidal passage with the hydraulic diameter of 8.5 mm. The angles of helix axis are oriented at 0, 45, 90 degrees to gravity. The overall and refrigerant-side heat transfer coefficients and pressure drops are experimentally determined at saturation temperature 35°C, refrigerant mass flux 35–180 kg/s·m2, and cooling water temperature 27°C. The results show that orientation has significant influence on the thermal and hydraulic behaviors of the helical pipe. The results can be employed for reference in the effective design of annular helicoidal heat exchangers with R-134a as the working fluid.

Because of the complexity of multiphase slurry flow, most experiments and simulations described in the open literature are focused on single-species solid-liquid flow. In this paper, a Eulerian granular multiphase (EGM) model is introduced to make the numerical investigations in the entrance region of a straight pipe for a double-species slurry flow. In order for the study to obtain the numerical solution in fully developed turbulent flow, the k–ε turbulent model was used with the Eulerian granular multiphase model. An O-type structured grid was chosen to discretize the entire computation domain, and a control volume finite difference method (CVFDM) was applied in the governing equations. Some simulation results are compared with the authors’ experimental data to validate the numerical investigation. These numerical results for the pressure gradients are found to be in good agreement with the experimental data. It is very difficult to obtain some important flow characteristics in the double-species slurry flow by experiments; therefore, the volume fractions (or concentrations) of solids, the average velocity distributions, and the turbulent kinetic energy and dissipation rates of water in the pipe entrance will be displayed and analyzed in the numerical investigations, which have seldom been reported in the open literature.