Enthalpy effects on hypervelocity boundary layers

Abstract

More than 50 shots with air and 35 shots with carbon dioxide were carried out in the T5 shock tunnel at GALCIT to study enthalpy effects on hypervelocity boundary layers. The model tested was a 5° half-angle cone measuring approximately 1 meter in length. It was instrumented with 51 chromel-constantan coaxial thermocouples and the surface heat transfer rate was computed to deduce the state of the boundary layer and, when applicable, the transition location. Transitional boundary layers obtained confirm the stabilizing effect of enthalpy. As the reservoir enthalpy is increased, the transition Reynolds number evaluated at the reference conditions increases as well. The stabilizing effect is more rapid in gases with lower dissociation energy and it seems to level off when no further dissociation can be achieved. These effects do not appear when the transition location is normalized with the edge conditions. Further normalizing the reservoir enthalpy with the edge enthalpy appears to collapse the data for all gases onto a single curve. A similar collapse is obtained when normalizing both the transition location and the reservoir enthalpy with maximum temperature conditions obtained with BLIMPK, a nonequilibrium boundary layer code. The observation that the reference conditions seem more appropriate to normalize high enthalpy transition data was taken a step further by comparing the tunnel data with results from a reentry experiment. When the edge conditions are used, the tunnel data are around an order of magnitude below the flight data. This is commonly attributed to the fact that disturbance levels in tunnels are high, causing the boundary layer to transition prematurely. However, when the conditions at the reference temperature are used instead, the data come within striking distance of one another although the trend with enthalpy seems to be a destabilizing one for the flight data. This difference could be due to the cone bending and blunting observed during the reentry. Experimental laminar heat transfer levels were compared to numerical results obtained with BLIMPK. Results for air indicate that the reactions are probably in nonequilibrium and that the wall is catalytic. The catalycity is seen to yield higher surface heat transfer rates than the noncatalytic and frozen chemistry models. The results for carbon dioxide, however, are inconclusive. This is, perhaps, because of inadequate modeling of the actual reactions. Experimentally, an anomalous yet repeatable, rise in the laminar heat transfer level can be seen at medium enthalpies in carbon dioxide boundary layers.