A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nu number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.© 2010 ASME

Numerical Simulation of Thermofluid Characteristics of Two-Phase Slug Flow in Microchannels

The blade tip region encounters high thermal loads because of the hot gas leakage flows, and it must therefore be cooled to ensure a long durability and safe operation. A common way to cool a blade tip is to design serpentine passages with 180° turn under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase blade tip lifetime. Pins, dimples and protrusions are well recognized as effective devices to augment heat transfer in various applications. In this paper, enhanced heat transfer of an internal blade tip-wall has been predicted numerically. The computational models consist of a two-pass channel with 180° turn and arrays of circular pins or hemispherical dimples or protrusions internally mounted on the tip-wall. Inlet Reynolds numbers are ranging from 100,000 to 600,000. The overall performance of the two-pass channels is evaluated. Numerical results show that the heat transfer enhancement of the pinned tip is up to a factor of 3.0 higher than that of a smooth tip while the dimpled-tip and protruded-tip provide about 2.0 times higher heat transfer. These augmentations are achieved at the cost of an increase of pressure drop by less than 10%. By comparing the present cooling concepts with pins, dimples and protrusions, it is shown that the pinned-tip exhibit best performance to improve the blade tip cooling. However, when disregarding the added active area and considering the added mechanical stress, it is suggested that the usage of dimples is more suitable to enhance blade tip cooling, especially at low Reynolds numbers.© 2010 ASME

Comparisons of Pins/Dimples/Protrusions Cooling Concepts for a Turbine Blade Tip-Wall at High Reynolds Numbers

One of the approaches for microlnanoscale heat transfer in semiconductors and dielectric materials is to use the Boltzmann transport equation, which reduces to the equation of phonon radiative transfer under the relaxation time approximation. Transfer and generation of entropy are processes inherently associated with thermal energy transport, yet little has been done to analyze entropy generation in solids at length scales comparable with or smaller than the mean free path of heat carriers. This work extends the concept of radiation entropy in a participating medium to phonon radiation, thus, providing a method to evaluate entropy generation at both large and small length scales. The conventional formula for entropy generation in heat diffusion can be derived under the local equilibrium assumption. Furthermore, the phonon brightness temperature is introduced to describe the nature of nonequilibrium heat conduction. A diamond film is used as a numerical example to illustrate the distribution of entropy generation at the walls and inside the film at low temperatures. A fundamental knowledge of the entropy generation processes provides a thermodynamic understanding of heat transport in solid microstructures ; this is particularly important for the performance evaluation of thermal systems and microdevices.

Entropy Generation in Thin Films Evaluated From Phonon Radiative Transport

Thermal rectification is a phenomenon in which thermal transport along a specific axis is dependent upon the sign of the temperature gradient or heat current. This phenomenon offers improved thermal management of electronics as size scales continue to decrease and new technologies emerge by having directions of preferred thermal transport. For most applications where thermally rectifying materials could be of use they would need to exhibit one direction with high thermal conductivity to allow for efficient transport of heat from heat generating components to a sink and one direction with low conductivity to insulate the temperature and heat flux sensitive components. In the process of understanding and developing these materials multiple mechanisms have been found which produce thermally rectifying behavior and much work has been and is being done to improve our understanding of the mechanisms and how these mechanisms can be used with our improved ability to fabricate at the nanoscale to produce efficient materials which have high levels of thermal rectification.

A Review of Thermal Rectification Observations and Models in SolidMaterials

Two mechanisms that increase heat dissipation at solid-liquid interfaces are investigated from the atomistic point of view using nonequilibrium molecular dynamics (NEMD) simulation. The mechanisms include surface functionalization, where −OH terminated headgroups and self-assembled monolayers (SAMs) with different chain lengths are used to recondition and modify the hydrophilicity of silica surface, and vibrational matching between crystalline silica and liquid water, where three-dimensional quartz nanopillars are grown at the interface in the direction of the heat flux with different lengths to rectify the vibrational frequencies of quartz surface atoms. The heat dissipation is measured in terms of the interfacial thermal conductance at the solid-liquid interface, whereas the thermal conductance is obtained by imposing a one-dimensional heat flux across the simulation domain. The heat dissipation is enhanced by a factor of 2 to 3 for both fully hydroxylated and pillar modified surfaces. The SAMs enhance the overall thermal conductance between silica and water further (20% higher thermal conductance compared to the fully hydroxylated silica surface). Moreover, the modification of the vibrational states at the silica surface provides a tunable path to enhance the heat dissipation, which can also be easily applied to other fluids.Copyright © 2010 by ASME

Surface Functionalization Mechanisms of Enhancing Heat Transfer at Solid-Liquid Interfaces

Here a comprehensive survey of the literature in the area of numerical heat transfer (NHT) published in 2010 and 2011 has been conducted. Due to the immenseness of the literature volume, journals s...

Yingbin Zheng

Papers

Literature Survey of Numerical Heat Transfer (2010-2011)