Toowoomba Foundry is seeking help with the design of tall tapered feeders, which supply extra molten iron to poured castings as they cool. There is a problem with the reliability of the feeders, particularly for thicker castings of the newer spheroidal graphite irons. An effective feeder remains molten until the casting has set. The setting of the melt in the feeder is delayed by making it large enough to retain its heat for longer than the casting, and by placing it close to the thermal centre of the casting. So a larger feeder is more reliable. But any metal remaining in the feeder is recycled, and has an associated energy cost. If a feeder is too small, it will set too soon, and the casting will have unwanted holes in it that may require the entire casting to be recycled. Thus there is a tension between making the feeder smaller so as to minimise recycled metal, and having the feeder large enough that the casting is good.
Existing design methods use purely conductive models of heat transport. We investigate the relevance of convection in the cooling feeder, and set up a boundary-layer model of flow driven by density differences. We find that convection is a significant factor in the design of a feeder, effectively maintaining constant temperature across it. The height of the feeder is important mainly in providing the driving force for this flow
