Characterization of Supersonic and Subsonic Plasma Flows

Abstract

Tests of thermal protection materials in different plasma facilities often show markedly different results, owing, in part, to poor definition of test conditions and to misunderstanding of heat transfer scaling in different facility types. Recent work has been performed to compare stagnation point heat transfer testing results in arcjet and inductively coupled plasma facilities. Given that the free stream flow in conventional constricted-arc heated facilities is typically supersonic and the flow in inductively coupled plasma facilities is typically subsonic, care must be taken in determining equivalent test conditions that will enable valid comparisons. Thus, the different stagnation point heat transfer scaling for these facilities are examined. Laser spectroscopic techniques have been used to investigate and characterize plasma streams in both inductively-coupled and constricted- arc plasma facilities; however, the procedure for determining the relevant flow property values is different for the two types of facilities, owing to differences in the nature of the applicable governing equations. These differences are also examined in a first qualitative comparison of two- photon laser-induced fluorescence results obtained in a constricted-arc facility with those from on an inductively-coupled plasma facility. Introduction Plasma facilities play a key role in the development of planetary entry and re-entry vehicles because they can duplicate the heat load of even the most demanding trajectories. These facilities also offer the possibility of investigating non-equilibrium thermo-chemical processes since they can provide steady flow conditions for long test times. Steady flow conditions mean that the energy deposition rate on a test article can be maintained at a steady level for test times on the order of minutes. Two different types of plasma facilities, constricted arc heated and inductively-coupled, have been developed for moderate to large scale testing and development of thermal protection materials. Although the technologies of the two types of facilities are quite different, both use a common approach to duplicating aerothermal conditions for stagnation point heating, and both types of facilities are moving toward the use of spectroscopic measurement techniques to characterize the plasma stream. In the case of Inductively-Coupled Plasma (ICP) facilities, this suggests the possibility of measuring species fluxes in the reacting zone at the surface of the test article, which are needed to better understand gas/surface interactions. Irrespective of their construction, all plasma test facilities must answer the same fundamental questions about the state of the test gas, and about how test conditions are related to the expected flight environment. 1 The objective of the present paper is to compare constricted-arc and ICP facilities from the perspectives of thermal environment simulation for stagnation point heat transfer and for plasma stream characterization. Test results from three facilities are used to support the discussion with examples. The three facilities are an ICP facility: the VKI Plasmatron; and two constricted arc plasma facilities: the L3K Plasma Tunnel of DLR, and the 20 MW Aerodynamic Heating Facility (AHF) of NASA Ames Research Center. In the following section, the basic features of constricted arc and ICP plasma test facilities are discussed, and salient features of the three facilities mentioned in the present paper are given. A subsequent section is used to describe stagnation point heat transfer scaling in subsonic and supersonic flows, and this is followed by a discussion of comparative heat transfer measurements made at the same stream enthalpy and pressure conditions using standard calorimetry in the two types of facility. In the third section the use of spectroscopic techniques to measure important flow properties is addressed, and strategies to combine such measurements with conventional probe based measurements to characterize the thermo-chemical state of the plasma stream are compared for the two facility types, with measurement examples.