Microscopic Characterization of Frost Surface During Liquid-Ice Phase Change Period

In response to the need for developing a better model to predict frost formation and defrosting processes in freezer and evaporator applications, a microscopic analysis of frost growth on a flat surface is conducted to determine the microscopic characteristics of a frost layer during the early growth period when sub-cooled droplets are formed and changed to the ice. The surface characterization is performed by employing the holographic interferometry technique to determine the air-frost interface temperature, and the video microscope to determine the mean droplet size and ice particle fractions. Typical experimental results are presented to demonstrate the test technique. Preliminary experimentally determined frost thickness and air-frost interface temperature are compared with simulation results.

An Experimental Investigation of Flow Past a Smooth Circular Cylinder at the Sub-Critical Region

An experimental investigation for the incompressible flow past a smooth circular cylinder at the sub-critical region is presented in detail. A smooth circular cylinder is placed in a wind tunnel and the local pressure distribution on the cylinder surface is measured subtly. The Reynolds Number ranges from 104 to 8 × 104. The experimental data show that there exists a nadir point of the surface pressure in the front the across section of the cylinder and the pressure nadir position varies with the Reynolds number. It is found that this point tends to move forward of the cylinder as Reynolds number increases. Based on the present experimental findings, a simple algebraic expression describing the relationship between the location of the pressure’s nadir and Reynolds number is proposed.

A PIV Study of Natural Convection Induced by Oscillating Thermal Gradients

Microscale fluid flow and mass transfer are of both fundamental and practical importance to the design of solidification systems for melt processing of materials. These microscale fluid flow phenomena are affected by the macroscopic bulk flow motion and heat transfer away from the solidification front. As a first step towards a systematic understanding of the interactions of the micro- and macro-scale phenomena, a miniature cavity of a few millimeters in size is considered, where an oscillating temperature gradient is established to simulate the driving force under perturbed solidification conditions typical of microgravity environments. Flow visualization and velocity measurements of the transient oscillating fluid motion under two sets of thermal conditions are conducted using the Particle Image Velocimetry (PIV) technique. These experimental results are used to validate numerical simulations carried out using a finite element based model, developed by the authors for the prediction of flows in microgravity environments. The visualized flow pattern and velocity measurements in the two test cases compare very well with the numerical simulations. The numerical model is now ready to be used as a reliable tool for understanding and predicting the structure of fluid flow and heat transfer in microgravity environments.

Computational Modeling of Enhanced Laminar Flow Heat Transfer in Viscoplastic Fluids in Corrugated-Plate Channels

The enhanced heat transfer in laminar viscoplastic, shear thinning, Herschel-Bulkley fluid flows in sinusoidal corrugated-plate channels is investigated. With uniform-temperature plate walls, periodically developed flows are considered for a wide range of flow rates (10 ≤ Reg ≤ 700) and pseudoplastic flow behavior indices (n = 0.54, 0.8, and 1.0; the latter representing a Bingham plastic). The effects of fluid yield stress are simulated for the case where τy = 1.59 N/m2, representing a 0.5% xantham gum aqueous solution. Typical velocity and temperature distributions, along with extended results for isothermal friction factor ƒ and Colburn factor j are presented. The effect of the yield stress is found to be most dominant at low Reg regardless of the power law index n, and the recirculation or swirl in the wall trough regions is weaker than in the cases of Newtonian and power-law liquids. At higher Reg, the performance of the Herschel-Bulkley fluid asymptotically approaches that of the non-yield-stress power-law fluid. At low Reg, the yield stress increases ƒ by an order of magnitude and j is enhanced because of the higher wall gradients imposed by the plug-like flow field. The relative heat transfer enhancement, represented by the ratio (j/ƒ), and the role of the fluid yield stress and shear-thinning (or pseudoplastic) behaviors are also discussed.

Numerical Prediction of Enhanced Heat Transfer Using Oval Tubes and Delta Winglets

Unsteady three-dimensional laminar flow and heat transfer in a channel with a built-in oval tube and delta winglets have been obtained through the solution of the complete Navier-Stokes and energy equations using a body-fitted grid and a finite-volume method. The geometrical configuration represents an element of a gas-liquid fin-tube cross flow heat exchanger. The air-cooled condensers of the geothermal power plants also use fin-tube heat exchangers. The size of such heat exchangers can be reduced through enhancement in transport coefficients on the air (gas) side, which are usually small compared to the liquid side. In a suggested strategy, oval tubes are used in place of circular tubes, and delta winglet type vortex generators in common-flow-down configuration are mounted on the fin-surface in front of the tubes, while another delta winglet pair in common-flow-up configuration is mounted downstream of the first set of winglets. An evaluation of this augmentation strategy is attempted in this investigation. The investigation was carried out for a winglet angle of attack of 40 degrees to the incoming flow. The structures of the velocity field, and the heat transfer characteristics have been presented. The results indicate that vortex generators in conjunction with the oval tube show definite promise for the improvement of fin-tube heat exchangers.

The Effects of Dimpled Surface Geometry on Heat Transfer in an Impinging Jet Flow

The objective of this study was to characterize the heat transfer performance of a dimpled surface in an impinging jet flow field. Using a statistical design of experiments approach we designed 8 (23) test plates to study the effects of dimple spacing, dimple depth and dimple diameter and compared them to smooth plate heat transfer. The plates were placed opposite a square jet and tests were run for Reynolds numbers based on jet hydraulic diameter of 10,000 to 30,000 at a range of jet to plate spacings. Plate averaged heat transfer coefficients, based on actual surface area (including dimple area) were measured under steady state conditions. The results show that the dimple spacing to diameter ratio has the most significant effect on heat transfer performance at high velocities, while the dimple depth to diameter ratio is more significant at lower velocities. The effect of dimple diameter was found to be significant only under poor heat transfer conditions. Particle Image Velocimetry images of the dimple surface flow field showed enhanced entrainment at high velocities which may explain why the dimple spacing to diameter effect is more significant at high velocities.

Fundamental Effects of Boundary Structure on Nano-Scale Conductive Heat Transfer

Modeling of energy transport in nanostructures is very different from that in larger scaled materials. Researchers have reported considerably lower “thermal conductivity” in nano-scaled materials compared to the corresponding bulk material property. This observation is most often attributed to phonon-boundary interactions. However, in-depth explanations of this phenomenon are lacking. This study defines the fundamentals of low-dimensional phonon transport by relating the structural boundary characteristics of a material with the mechanisms of phonon transport. The structural boundary of a material controls phonon transport, creating a “boundary region” rather than a boundary surface, resulting in apparent differences in energy transport efficiency between the bulk material and the nano thin materials.

Thermoelectric Mapping of Nanostructures

There is intense interest to develop nanowires [1] and superlattices [2] that may offer superior thermoelectric figure of merit for efficient energy conversion. Meanwhile, the advance of semiconductor processing techniques has yielded impurity-doped semiconductor nanostructures with a doped region as small as a few nanometers. These include shallow junction Si field-effect transistors, strained Si/SiGe/Ge heterostructures and quantum dots, III-V heterostructures, and doped nanowires and nanotubes. Due to various size confinement effects, these doped semiconductor nanostructures often have unique electrical, optoelectronic, or thermoelectric properties that may lead to a wide range of applications. In contrast to the progress made in synthesizing thermoelectric nanostructures and in fabricating doped semiconductor nanostructures, the ability to quantify thermoelectric property and carrier concentration in comparable length scale has been lagging behind. For example, the 1997 U.S. Roadmap of Semiconductors from the Semiconductor Industry Association (SIA) defines the need for nanometer-scale measurements of carrier concentration profiles [3]. Though progress has been made, currently no technique can satisfy the requirements posted by the SIA roadmap due to the lack of either spatial resolution or accuracy.

High-Performance Computing for Fluid Flow and Heat Transfer

The use of computers in heat transfer and fluid flow has become so commonplace today that no one would consider working in either field without some knowledge of computing. Problems are now being solved on a daily basis that even a few years ago were considered intractable. While we once thought that a problem with a few million nodes was huge a few years ago, researchers are now addressing problems with over 100 million nodes. At such levels of detail, one can begin to model processes at the micro level of physics. When researchers are able to quickly analyze these gigantic data sets and can generate insightful graphical displays, the understanding of fundamental processes and governing relations will escalate tremendously.

Phonon Heat Conduction in Micro- and Nano-Cylindrical and Spherical Media

Phonon heat conduction in thin films and superlattices of dielectric materials has attracted extensive attentions in the past decade. Nonetheless, a wide range of micro- and nanoscale thermal problems is associated with non-planar geometries such as spherical and cylindrical media. With the rapid growth of research and development in nano-materials and nanostructures, understanding of heat transfer in micro- and nano-cylindrical and spherical media becomes important. Examples include cladding and coating for optic fibers and nanowires.