Direct measurements of skin friction in high-enthalpy high-speed flows

Abstract

Nomenclature An experimental investigation was conducted to measure skin friction along the chamber walls of a SCRAMjet combustor model. Measurements were made in the Direct Connect Arcjet Facility (DCAF) SCRAMjet combustor at the NASA AMES Research Center in two separate test series. Flow conditions in the Applied Physics Laboratory built model ranged from total pressures of 275-400 psia (1900-2750 kPa) and total temperatures from 5800-8400 R (3222-4667 K). This gives enthalpies in the range of 1700-3300 BTU/lb, (3950-7660 KJ/kg) and simulated free stream Mach number from 9 to 13. A direct force measurement device was used to measure the small tangential shear force passing over a non-intrusive floating element. The floating head is mounted to a stiff cantilever beam arrangement with deflection due to the shear force on the order of O.ooO5 in (0.0125 mm). This small deflection allows the balance to be a non-nulling type. The design includes active cooling of the floating sensor head to eliminate non-uniform temperaturc effects between the head and the surrounding chamber wall. The key to this device is the use of a quartz tube cantilever with strain gages bonded at orthogonal positions directly on the surface at the base. A symmetric fluid flow was developed inside the quartz tube to provide cooling to the backside of the floating head. Bench tests showed that this did not influence the force measurement. Numerical heat transfer calculations were conducted for design feasibility and analysis, and to determine the effectiveness of the active cooling of the floating head. Skin friction coefficients between 0.0005-0.0053 were measured. Analysis of the measurement uncertainty in cold supersonic flow tests show that accuracy under 8% is achievable, but variations in the balance cooling during a particular test raiscd uncertainty up to 20% in these very hot flow tests during the first test series. Improvements to the strain gages and balance cooling reduced uncertainty for the second test series to under 15%. t/ Graduate Research Assistant, Member AIAA *Department Chairman and W. Martin Johnson Prolessor, '-i Fellow AIAA 0 1 9 9 a l y s l m ~ l n n i m ~ . s d A r r o a m i . . P As~-%JrcAUF&bWaamd Cf = skin friction coefficient D = diameter of the floating head G = gap around the floating head