Helical smelting lance performance

Services - Case Study

Written by David Featherston

A typical lance that is commonly used in the smelting of metal ore was modelled using Computational Fluid Dynamics (CFD) methods. These lances are immersed into a furnace where ore is reduced to produce metal. The lance normally consists of a centre tube surrounded by a stainless steel hollow annulus. Varying amounts of fuel, air and oxygen are supplied through the tube and annulus. The air flow through the annulus acts as a coolant, which assists at improving the life of a lance by forming a protective slag layer on its outside. Two identical lances were modelled using CFD, one with a helical spiral and one without. All information regarding this example is available in the public domain.

The CFD lance models consisted of hexahedral and prism cells. The helix model consisted of 156k cells, while the straight lance model consisted of 74k cells. Cell wall spacing was identical in the two models. The geometry and mesh took approximately 2 hours to construct from initial concept to initial CFD simulation. A standard k-e RANS turbulence model, with viscous heating and standard wall functions was applied.
helical lance cfd

The lance had the following specifications: length: 10.8 m; outside diameter: 250mm; annulus width: 70mm; helix flow channels: 2; helix rate: 360°/3600m length. Air flow through the lance was modelled. Tube and annulus inlet flow rates of 6kg/s @ 20°C were used. 1000kW of heat was applied to the outside surface of each lance using a constant heat flux boundary condition.

It was found that the helix configuration enhanced heat transfer through the lance, resulting in twice the heat transfer to the centre flow and average outside surface temperatures of 204°C lower than without the helix. However, the pressure loss through the lance per metre increased by 34 percent.

Lance geometry	Heat transfer to centre flow	Average lance outside surface temperature	Maximum lance pressure
Straight	17.4 kW	909°C	418 Pa/m
Helix	41.8 kW	713°C	561 Pa/m

The helix was found to increase the flow velocity by approximately 6 percent and the surface area by approximately 20 percent. The helix was also found to act as a fin, promoting better heat transfer to the inner tube. A comparison of the tube surface temperatures is shown in the contour plots below. Temperatures are given in Celsius (°C).

helix flow velocity

References
Furnace image (http://www.ausmelt.com.au/non_ferrous.htm)