Speaker
Description
Recent high enthalpy demise ground testing has demonstrated that heating to objects is significantly impacted by length scale and flow path. As a result, geometric features such as holes, ribs, lattices and sharp edges can significantly increase the aerothermodynamic heat flux to an object. To date, no attempt has been made to optimise the shape of objects specifically to enhance the aerothermodynamic heat flux to spacecraft equipment.
A set of simple shape adaptations have been tested in the DLR H2K cold wind tunnel on a PEEK model in order to determine whether a noticeable increase in heating can be obtained using specific shapes. The shape concepts examined were holes, facets, steps and grooves, which were selected on the basis of likely effectiveness from sparse literature data, and their applicability to spacecraft components.
Three different sizes of hole were drilled in a sphere cap to investigate the holes concept. The use of holes did produce a significant increase in downstream heating, up to a factor of seven over the original level. Although this was a relatively local effect, a pattern of holes can be used to provide significant heating over a sizeable area. Where the holes are closed, the heat flux increase is smaller, but still approximately a factor of two.
Two cylinders with different sizes/frequencies of radius reducing steps were tested. The stepped surfaces show a clear increase in heating over a cylinder of the same radius. There is an increase in heating at both edges of the step, and the flux in the centre of the step is still above that which would be obtained on the equivalent cylinder. Along the centreline, heat flux increases of approximately a factor of two are observed. The augmentation is even higher at angle of attack where the steps are facing the flow.
Three different faceted objects were tested, from a hexagonal prism, to a 24-gonal prism. The overall heating for these shapes does not appear to be significantly higher than obtained for a cylinder, but there is a significant increase in the heat flux along the facet ridges. For the hexagon, a local increase of a factor of four is observed.
One grooved cylinder was tested, but this had a range of groove patterns, with different spacings, widths and depths. This approach was shown to be highly successful with approximately a factor of two heat augmentation. At angle of attack, this approach is also very promising as the boundary layer is not able to grow, and the grooves act as new leading edges. This results in high heat fluxes along the length of the grooved surface, where a smooth cylinder shows a dramatic reduction in heating as the streamlength increases.
These concepts will be applied to samples to be tested in the hot L2K wind tunnel to verify the demise performance enhancements in the next stage of the work.