Measurements of surface exhalation characteristics of porous fibre reinforced composites.

Rocket combustion chambers are subject to extremely high thermal loads. To protect the chamber walls from these loads, an efficient cooling is necessary. As a highly effective cooling mechanism, transpiration cooling promises advantages over conventional regenerative and active cooling techniques like film cooling. In combination with porous ceramic matrix composites (CMC), longer durability due to low coefficient of thermal expansion, lower manufacturing costs and possible weight reductions are possible, all while retaining the performance of regenerative cooled combustion chambers. One parameter determining the effectiveness of this cooling mechanism, is the exhalation characteristic of the coolant from the porous surface. Measurements of this characteristic are shown for a carbon fibre reinforced carbon (C/C) and single free jets. For C/C it is shown, that possible defects like blocked pores or delaminations can be detected by measuring the dynamic pressure of the outflow. Furthermore, it is shown, that the exhalation characteristic for C/C is dependent on the through flow direction. The derivation of mass flow through the sample from these measurements reveals an quantitative overestimation of the velocity distribution. By simulating the measurement, it is shown, that one important aspect is the spatial integration of the measured values over the sensor area. But it is also shown, that this effect is not enough to explain the overestimation and additional effects need to be considered