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THERMAL ISSUES IN EMERGING TECHNOLOGIES

A Review of Microscale Transport in the Thermal Processing of New and Emerging Advanced Materials

[+] Author and Article Information
Yogesh Jaluria

Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ 08854jaluria@jove.rutgers.edu

Jing Yang

Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ 08854

J. Heat Transfer 133(6), 060906 (Mar 07, 2011) (14 pages) doi:10.1115/1.4003512 History: Received January 13, 2010; Revised July 06, 2010; Published March 07, 2011; Online March 07, 2011

This paper reviews the microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated are in the micrometer length scale and, in others, underlying transformations that determine product quality and characteristics are at micro- or nanoscale levels. The basic considerations in these transport phenomena are outlined. A few important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where micro- and nanoscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition, and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation and experimentation, as compared with those for engineering or macroscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscous dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro- and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved than those over macro- or industrial scales because of difficult access to the regions of interest, relatively small effects such as tension, buoyancy effects, viscous rupture, bubble entrapment, and other mechanisms that are difficult to measure and that can make the process infeasible. It thus becomes difficult to achieve desired accuracy for validating the mathematical and numerical models. This paper reviews some of the approaches that have been adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Structure of a typical optical fiber and the transmission of light in a fiber with a step change in the refractive index between the core and the cladding

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Figure 2

Sketches of a microstructured optical fiber and of a hollow fiber with a single core

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Figure 3

Sketch of the drawing process for a hollow fiber

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Figure 4

Influence of the absorption coefficient on the neck-down profiles for a double-layer optical fiber

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Figure 5

Neck-down profiles for hollow fiber drawing, generated by using the zonal method and the optically thick method

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Figure 6

Streamlines and isotherms in the furnace and in the fiber for a typical case of hollow fiber drawing with a parabolic furnace temperature distribution at a drawing speed of 10 m/s

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Figure 7

Variation of the collapse ratio along the axis with different pressurizations in the core

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Figure 8

Variation of the draw tension with the drawing temperature for different drawing speeds

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Figure 9

Effect of operating conditions on the porosity of a microstructured polymer optical fiber. (a) Effect of furnace temperature, with E2 being at higher temperature than E1. (b) Effect of feeding speed Vo in μm/s and drawing speed Vf in cm/s. (c) Effect of number of holes in the microstructured polymer fiber.

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Figure 10

A typical optical fiber coating applicator

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Figure 11

Entrance and exit menisci obtained due to the optical fiber moving in microchannels

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Figure 12

Experimental coating applicator and die system

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Figure 13

(a) Entrance flow in a microchannel with an annular gap thickness of 103 μm at different pressures. (b) Entrance flow in a microchannel of annular gap thickness 103 μm at different pressures and into an open reservoir.

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Figure 14

Dependence of fiber speed, for meniscus break down for a fiber entering a microchannel, on the imposed pressure

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Figure 15

Location of the meniscus in the inlet microchannel as function of the pressure

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Figure 16

Calculated flow field and temperature, with a prescribed meniscus of height around 100 μm obtained experimentally

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Figure 17

Calculated and measured velocity distributions in the coating applicator

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Figure 18

Pressure distribution in the chamber and the exit die, which consists of a converging microchannel, for polymer coating of a moving wire or fiber under isothermal conditions

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Figure 19

Calculated pressure distribution in the applicator/die system for the coating process when thermal effects due to heat transfer and viscous dissipation are included

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Figure 20

Calculated meniscus at the exit of the microchannel in the coating process, along with experimental measurements of the profile, for glycerin at fiber speeds of (a) 20 m/min and (b) 75 m/min

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Figure 21

A sketch of an impingement type CVD reactor and the corresponding system for silcon deposition

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Figure 22

Comparison between numerical predictions and experiments for chemical vapor deposition of silicon in a horizontal reactor

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Figure 23

Dependence of average concentration of E′ defects on furnace wall temperature

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Figure 24

Concentration of E′ defects along the centerline for various B2O3 concentrations

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Figure 25

Response surface for uniformity of deposition (Up) for an impingement CVD reactor for depositing silicon in terms of inlet velocities and susceptor temperatures

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Figure 26

(a) Degree of conversion of pure starch as a function of axial location and barrel temperature in a single-screw extruder. (b) Comparisons between the predicted and experimental values on degree of conversion at the die exit.

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