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By Gavin Tabor, School of Engineering, Computer Science and Mathematics, University of Exeter, Exeter, Devon, UK
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Archaeological reconstruction of a typical furnace (insert shows the front wall face on)
Courtesy Dr. Gillian Juleff
The manufacture of iron and steel requires high
temperatures and precise control of chemical
conditions. Today’s sophisticated blast
furnaces are the heirs to a long period of technological
development and innovation stretching
back into prehistory. One ancient technique for
smelting iron is the ‘bloomery process’, in which
iron ore and charcoal are heated to temperatures
in excess of 1100°C in order to initiate a reduction
process that results in a mixture of lumps of
iron metal and lumps of slag being produced. For
a long time, archaeometallurgists believed that
the high temperatures necessary for this process
could only be generated by means of bellows-driven
furnaces.
Archaeological excavations carried out in the 1990s
in Sri Lanka by Dr. Gillian Juleff1 (also from the University
of Exeter) have uncovered an extensive, yet previously
unknown, iron-producing industry dating
to the 1st Millennium AD. The industry displayed
a high degree of organization and produced high
quality iron and steel, which was traded throughout
the region, reaching as far as the Islamic world.
Confounding expectations, no evidence of bellows
was found. On the contrary, it was suggested that
the furnaces may have been shaped so that the
strong winds in the area acted to draw air through
the furnace. Having suggested this mechanism, it
was important to demonstrate its validity, both through
experimentation (building a replica furnace) and
through detailed modeling. FLUENT was an ideal
choice to model the air flows around and through
the furnace and the heat transfer from the furnace
bed to the surrounding environment.
All the furnaces excavated at the Sri Lanka site
(around 80 in all) consist of long trenches dug
into the crests of hills, lined with clay, and each
with a clay front wall punctuated by air vents. As
the wind blows up the hillside, it is diverted over
the top of the furnace, creating a high pressure
region in front of the furnace wall and a low pressure
region above the furnace bed. This pressure
differential draws air into the bed through the vents
in the front wall.
Pathlines colored by temperature
Air flow through the furnace as computed by FLUENT
A typical furnace was modeled in FLUENT, with
the bed being represented as a semi-porous region
at a fixed temperature (1200°C or 1500°C). The
furnace walls were assigned the material properties
of clay brick, and sandstone was used as the material
for the hillside. Full 3D calculations were performed
on a mesh of 1.1 million cells, taking into
account heat transfer and buoyancy effects. The
results2 provided great insight into the flow patterns
through and around the furnace, and proved
conclusively that these wind-powered furnaces easily
provide sufficient air flow through the bed to
create the high temperatures necessary to smelt
iron. In fact, these results suggest that the output
from the furnaces could have been even higher
quality carbon steel.
References:
- G. Juleff, An ancient wind-powered iron-smelting
technology in Sri Lanka, Nature, 379, p.60, 1996.
2 G. Tabor, D. Molinari and G. Juleff, Computational
- simulation of air flows through a Sri Lankan wind driven
furnace, submitted to J. Arch. Sci, 2003.
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