Hot Cap Material Choice
Our first major setback with running the engine was encountered as soon as heat was applied. One aspect of the previous design which had been considered for improvement was the thermal conductivity of the hot cap. Steel had been previously used which combines a high melting point temperature (1350 degrees for stainless) with a low cost solution. One disadvantage of using steel however is that its thermal conductivity is comparatively low with other alloys at 12 W/mK (see materials section). With this in mind aluminium was selected for the hot cap.
Although an exact operating temperature had not been specified for the engine, an estimation of 350 degrees was made based on the use of a methane Bunsen burner. An important lesson was soon to be learned regarding assumptions in engineering. When the gas torch was applied to the aluminium hot cap it did not take long before a hole appeared in the component where the heat was being applied. The conclusion was reached that the gas was burning at a far higher heat than we had at first anticipated. We were very fortunate in that no collateral damage was done to any of the components within the hot cap. It was a minor miracle that the regenerator tube of 0.5mm wall thickness did not melt.
Fortunately for the group, the workshop technicians were able to quickly machine the component again, this time out of mild steel and with an extended 1.5mm wall thickness. Although this would seriously affect the thermal conductivity of the hot cap, we were more concerned that there would be no repeat of the previous incident. Steven Black, the welding technician, assured us that the previous design with a mild steel hot cap had not encountered this problem whilst using the same heat source.
Although an exact operating temperature had not been specified for the engine, an estimation of 350 degrees was made based on the use of a methane Bunsen burner. An important lesson was soon to be learned regarding assumptions in engineering. When the gas torch was applied to the aluminium hot cap it did not take long before a hole appeared in the component where the heat was being applied. The conclusion was reached that the gas was burning at a far higher heat than we had at first anticipated. We were very fortunate in that no collateral damage was done to any of the components within the hot cap. It was a minor miracle that the regenerator tube of 0.5mm wall thickness did not melt.
Fortunately for the group, the workshop technicians were able to quickly machine the component again, this time out of mild steel and with an extended 1.5mm wall thickness. Although this would seriously affect the thermal conductivity of the hot cap, we were more concerned that there would be no repeat of the previous incident. Steven Black, the welding technician, assured us that the previous design with a mild steel hot cap had not encountered this problem whilst using the same heat source.
