SESSION 1
RADIATION TRANSFER IN MATERIALS PROCESSING AND MANUFACTURING
RADIATION HEAT TRANSFER IN MANUFACTURING AND MATERIALS
PROCESSING
T. L. Bergman*, R. Viskanta**
*Department of Mechanical Engineering
The University of Texas at Austin, Austin, USA
**School of Mechanical Engineering
Purdue University, West Lafayette, USA
This review discusses radiation heat transfer which occurs in conjunction with
a variety of manufacturing and materials processing applications. Practical
needs in manufacturing and materials processing thermal analysis are noted, and
the role of radiation heat transfer in meeting these requirements is
discussed. Specifically, different types of radiative heating strategies are
categorized, radiation sources commonly used are described, and issues which
are somewhat unique to radiation heat transfer in manufacturing and materials
processing operations, such as matching the spectral characteristics of the
source and load, are identified. The need for development of robust inverse
analysis tools for process and equipment design, as well as thermal process
control is noted throughout the review.
Surface radiation heat transfer, although fairly well understood in principle,
is used in many manufacturing operations (such as in electronics
manufacturing) and needs for improved understanding and development of new
analysis techniques are discussed. Specific topics include i)the practical need
for improved surface exchange analysis techniques in complex geometries and/or
in systems involving moving materials, ii) coupled macro- and microscale
radiation heat transfer for process analysis, thermal control and inspection,
and iii) forward analysis and identification of dimensionless parameters
describing highly coupled and multiple mode heat transfer operations.
Volumetric radiative transfer in semitransparent materials at high temperature, important in a number of specific operations, such as fabrication of composite
epoxy-fiber structures, crystal growth and glass manufacturing, is described.
Here, the examples are selected to illustrate i) the importance of matching
source and load spectral characteristics, ii) combined volumetric radiative
and surface convective heating utilizing flames and other high temperature
sources, and iii) the relevance and impact of dependent scattering
phenomena.
Finally, needs and challenges in radiation thermometry in practical systems
involving, for example, i) moving materials, ii) materials of high purity, or
iii) radiatively participating process and/or plant gases are discussed, and
recent advances in radiation thermometry theory and applications are
reviewed.
RADIATION-CONVECTION INTERACTIONS IN SOLIDIFICATION OF
SEMI-TRANSPARENT CRYSTALS
M. Kassemi
Processing Science &Technology Branch
NASA Lewis Research Center
Cleveland, Ohio
M.H.N. Naraghi
Mechanical Engineering Department
Manhattan College
This paper studies the interaction of radiation heat transfer with conduction
and convection during solidification of semitransparent oxide crystals. A
comprehensive numerical model is presented for solidification of two important
oxide crystals, BSO and YAG, by the vertical Bridgman technique. Bismuth
Silicon Oxide (BSO) is an optically active semi-insulating material that is
photoconductive and has widespread applications in optical information
processing and computing components, such as spatial light modulators and
volume holographic optical elements and filters. Yttrium Aluminum Oxide Garnet
(YAG) is another important optically active oxide crystal which is used in many
laser devices. These two materials were chosen because they have well-defined
experimental counterparts and their thermophysical and radiative properties are
relatively wellknown.
A schematic of the Bridgman crystal growth configuration is shown in Fig. 1. In
solidification of semiconductors, usually both the crystal and the melt are
opaque to thermal radiation. Furthermore, both phases typically have relatively
high thermal conductivities. Therefore, conduction is the dominant mode of heat
transfer in both the solid and the melt. Oxide crystals, on the other hand, are
usually semi-transparent to thermal radiation in the solid phase and almost
opaque in the melt. They also have relatively low thermal conductivities in
both phases. Therefore, heat transfer during the solidification of oxide
crystals is governed by an intricate balance between convection and conduction
in the melt and conduction and radiation in the solid. In a sense, the solid
acts as a light pipe through which the interface loses a considerable amount of
heat (by emission) to the cold sections of the crucible wall or directly to the
furnace (if the crucible is also transparent).
A radiation heat transfer model is developed based on exchange factors for
multi-dimensional complicated geometries encountered in crystal growth. The
radiation model takes into account the wavelength-dependant semi-transparency
of oxide crystals such as BSO and YAG which are transparent to radiation below
6 microns and opaque to radiation in the rest of the spectrum. It is shown that
the radiation model can be easily incorporated into existing finite element
codes for fluid flow and heat transfer such as FIDAP. During numerical
simulations the algorithm tracks the position and shape of the interface as the
solidification process proceeds and updates the radiation exchange factors
based on the changing geometry.
The model was applied to the processing of both YAG and BSO under realistic
experimental conditions. From the numerical simulations the following
conclusions can be drawn:
- Under the experimental conditions considered in this paper, radiation is
the dominant heat transfer mode at the interface for solidification of BSO and
YAG.
- For both BSO and YAG, the interface attains a highly stretched parabolic
shape largely because of the nonuniform radiative loss from the interface. In
both cases, the interface is convex into the melt and there are two vortices
rotating near the interface with the flow rising from the region near the wall
as shown in Fig. 2a.
- If radiation is neglected or the crystal is treated as opaque, the
interface is only very mildly curved due to the mismatch among the thermal
conductivities as shown in Fig 2b. The interface is usually convex into the
phase with the lower conductivity. In the absence of radiation effects, the
flow structure indicates two large vortices in the upper portion of the melt
and two smaller vortices near the interface. The direction of the rotation of
the smaller vortices depends on the shape of the interface.
- For YAG, because of its higher conductivity, conduction and radiation are
the dominant heat transfer mechanisms. For BSO, convection plays a more
important role and the recirculating flow can compensate for a significant
amount of the radiant heat loss from the center of the interface rendering a
much flatter interface in comparison to YAG.
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| Figure 1: Cross Sectional View of the Crucible in a Vertical Bridgman
Furnace. | Figure 2: Solidification of YAG for Gr=2053: a) Radiation Effects
(Semitransparent Crystal), b) No-Radiation (Opaque Crystal).
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THERMAL TRANSPORT IN OPTICAL FIBER MANUFACTURING
Deborah A. KAMINSKI
Dept. of Mechanical Eng., Aeronautical Eng., and Mechanics
Rensselaer Polytechnic Institute
Troy, NY, USA
A numerical model of combined radiative and convective heat transfer in a fiber
draw furnace was formulated and solved. The model was used to predict glass
temperatures and identify important heat transfer modes. The energy equation,
which included conductive, convective, and radiative terms, was discretized
using a control-volume-based finite element technique. Thermal radiation within
the glass was approximated by the P1 method using a two-band
spectral absorption coefficient. Surface-to-surface radiation from the muffle
wall to the outer surface of the class was computed by a full enclosure
analysis. A cosinusoidal glass profile was assumed and a continuity-satisfying,
velocity field was specified.
The results of the calculation showed that radiation was an important mode for
air, arson and carbon dioxide purge gases, but that conduction was dominant for
the case of a helium purge gas. The glass preform attains its asymtotic
temperature higher in the furnace with helium than with any of the other gases
studied. Temperatures are relatively insensitive to final fiber velocity and to
the spacing, between the glass and the furnace wall.
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