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 degrees 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 Juleff 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 thestrong 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 modelling. Fluent software 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.
A typical furnace was modelled in Fluent, with the bed being represented as a semi-porous region at a fixed temperature (1200 degrees 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 results 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.
Dr. Gavin Tabor is from the School of Engineering at the University of Exeter.
1. 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.