Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406 × 2.032 mm cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal that the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Annual flow is identified as the dominant flow pattern for conditions relevant to two-phase micro-channel heat sinks, and forms the basis for development of a theoretical model for both pressure drop and heat transfer in micro-channels. Features unique to two-phase micro-channel flow, such as laminar liquid and gas flows, smooth liquid-gas interface, and strong entrainment and deposition effects are incorporated into the model. The model shows good agreement with experimental data for water-cooled heat sinks.

Natural Convection From Protruding Heat Sources in a Channel

Natural convection has important implications in many applications like cooling of electronic equipment due to its low cost and easy maintenance. In the present study, two-dimensional natural convection heat transfer to air from multiple identical protruding heat sources, which simulate electronic components, located in a horizontal channel has been studied numerically. The fluid flow and temperature profiles, above the heating elements placed between an adiabatic lower plate and an isothermal upper plate, are obtained using numerical simulation. The effects of source temperatures, channel dimensions, openings, boundary conditions, and source locations on the heat transfer from and flow above the protruding sources are investigated. Different configurations of channel dimensions and separation distances of heat sources are considered and their effects on natural convection heat transfer characteristics are studied. The results show that the channel dimensions have a significant effect on fluid flow. However, their effects on heat transfer are found to be small. The separation distance is found to be an important parameter affecting the heat transfer rate. The numerical results of temperature profiles are compared with the experimental measurements performed using Filtered Rayleigh Scattering (FRS) technique in an earlier study, indicating good agreement. It is observed that adiabatic upper plate assumption leads to better temperature predictions than isothermal plate assumption.

Numerical Study of the Effect of Magnetic Fields on Melt-Crystal Interface-Deflection in Czochralski Crystal Growth

In order to control the impurity distribution and remove defects in a crystal grown in Czochralski growth for high quality crystals of silicon, it is necessary to study and control the melt-crystal interface shape, which plays an important role in control of the crystal quality. The melt-crystal interface interacts with and is determined by the convective thermal flow of the melt in the crucible. Application of magnetic field in the Czochralski system is an effective tool to control the convective thermal flow in the crucible. Therefore, the shape of the melt-crystal interface can be modified accordingly. Numerical study is performed in this paper to understand the effect of magnetic field on the interface deflection in Czochralski system. Comparisons have been carried out by computations for four arrangements of the magnetic field: without magnetic field, a vertical magnetic field and two types of cusp-shaped magnetic field. The velocity, pressure, thermal and electromagnetic fields are solved with adaptation of the mesh to the iteratively modified interface shape. The multi-block technique is applied to discretize the melt field in the crucible and the solid field of silicon crystal. The unknown shape of the melt-crystal interface is achieved by an iterative procedure. The computation results show that the magnetic fields have obvious effects on both the pattern and strength of the convective flow and the interface shape. Applying magnetic field in the Czochralski system, therefore, is an effective tool to control the quality of bulk crystal in Czochralski growth process.

Thermomechanical Model of Spot Welding for Calculating Residual Stresses

A comprehensive axisymmetric model of the coupled thermal-electrical-mechanical analysis predicting weld nugget development and residual stresses for the resistance spot welding process of Al-alloys is developed. The model estimates the heat generation at the faying surface, the workpiece-electrode interface, and the Joule heating of the workpiece and electrode. The phase change due to melting in the weld pool is considered. The contact area and its pressure distribution at both the faying surface and the electrode-workpiece interface are determined from a coupled thermal-mechanical model using a finite element method. The knowledge of the interface pressure provides accurate prediction of the interfacial heat generation. For the numerical model, temperature dependent thermal, electrical and mechanical properties are used. The proposed model can successfidly calculate the nugget diameter and thickness, and predict the residual stresses and the elastic-plastic deformation history. The calculated nugget shape and the deformation of sheets based on the model are compared with the experimental data. The computed residual stresses approach the distribution of experimental measurement of the residual stress.

Time-to-Market: A Practical CFD Application in the Telecom Industry

This paper summarizes the practical use of CFD (Computational Fluid Dynamics) using a commercially available package, FLOTHERM [1], in a tight and highly competitive marketplace to produce a functional pre-production piece of telecom gear with no prototyping for thermal issues. The paper highlights the direct production, noprototype, analytical thermal performance verification of a small CMTS (Cable Modem Termination System) used in telecom applications.

Thermal Process Maps for Controlling Microstructure in Laser Deposited Materials

The ability to predict and control microstructure in laser deposited materials requires an understanding of the thermal conditions at the onset of solidification. To this end, the focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (e.g., laser power and velocity). Results presented herein are based on the Rosenthal solution for a moving point heat source traversing an infinite substrate. Ongoing work includes the effect of a distributed laser power through superposition of the Rosenthal solution, while the effects of finite geometry, temperature dependent properties and latent heat of transformation are included through thermal finite element modeling of the laser deposition process.

An Experimental Study of Heat Transfer With an Impinging Circular Water Jet

Impinging jet is widely used in both traditional industrial and new high-tech fields. High efficiency heat transfer in impinging jet cooling makes it an important method for heat transfer enhancement, in particular in cooling of electronic devices with high heat density. This paper presents an experimental study of heat transfer by an impinging circular water jet. A Constantan foil with the size of 5 mm × 5 mm was used to simulate a microelectronic chip with heat generated by passing an electrical current through the foil. A high heat flux over 106 W/m2 was achieved. The surface temperature was measured by a thermocouple glued onto the back surface of the foil. Both a free surface jet and a submerged jet were investigated. Effect of the nozzle-to-surface spacing as well as the jet speed at the exit of the nozzle on cooling was examined. By positioning the jet away from the center of the heating foil surface, the radial variation of the heat transfer coefficients over the foil was also investigated. Quantitative heat transfer data have been obtained and analyzed.

Characterization of Convection Patterns During the Solidification Process Using Particle Image Velocimetry

This experimental study focused mainly on the solidification of a binary mixture of ammonium chloride and water (NH4Cl-H2O) in a differentially heated cavity. One vertical wall is cooled at temperature TC, and the opposite vertical wall is kept at constant temperature TH = +20°C. The effect on the solidification process of the initial concentration of ammonium chloride and cooling conditions is examined. Particle image velocimetry (PIV) is used for the visualization of the dynamic field during the solidification process. The temperature distribution at discrete locations in the solution and on the vertical cooling wall was monitored using thermocouples. The convection flow patterns, the ice thickness, and the temperature distribution were obtained for various initial concentrations of ammonium chloride ranging from 0wt% to 20wt% (sub-eutectic and near-eutectic growth). The convection patterns obtained for different initial concentrations showed significant differences. The results showed that the process of solidification is slower with an increase in the initial concentration levels of the binary solution. The ice growth rate was almost double at the bottom of the cavity.

LES Simulation in a Thin Liquid Film Subjected to High Thermal Stresses

Thermal transfers occurring in the vicinity of an air-liquid interface have been studied numerically through a Large Eddy Simulation technique. Results obtained clearly show the unsteady response of the liquid film submitted to such thermal stress. Structures are created at the interface in a very small layer and it has been found that turbulence acts strongly in that layer. Moreover, even if the configuration studied tends to weaken buoyancy, the dissipation was found to obey a −3 power law. This clearly indicates that the buoyancy pilots the way turbulence behaves on the flow field; the latter is mainly characterized by a strong mixing phenomenon taking place in the already identified layer at the interface.

Analysis of Melting in a Mixed Metal Powder Bed With Finite Thickness Subjected to Constant Heat Flux Heating

Melting of a subcooled powder bed with the finite thickness that contains a mixture of two metal powders with significantly different melting points is investigated analytically. Shrinkage induced by melting is taken into account in the physical model. The temperature distributions in the liquid and solid phases were obtained using an exact solution and an integral approximate solution, respectively. The effects of porosity, Stefan number, and subcooling on the surface temperature and solid-liquid interface are also investigated. The present work built solid foundation to investigate the complex three-dimensional selective laser sintering (SLS) process.