Performance characteristics of a multiple- arcjet wind tunnel.

Abstract

This paper describes the operational characteristics of a low-density wind tunnel that uses six arc jets operating thermally in parallel but with separate electric supplies. Programable power controls permit operation at constant total equilibrium enthalpy for open-loop timevarying mass flow rate (mt) programs that simulate real-time altitude changes. Impact pressure fluctuations of 3% rms for low mass-flow rates rapidly decay to 1% as the programed m t is increased. Measured Pilot and static pressures for m t = 156 g/sec and a dimensionless enthalpy ho/RTs of approximately 150 are in good agreement with the machine computations. These computations also show that the flow freezes slightly beyond the throat. Microwave and Langmuir probe measurements of static electron density indicate that this quantity also freezes approximately at the throat. Boundary-layer impact-pressure profiles have been found to satisfy the Blasius velocity profile for more than an order-of-magnitude variation of m. Assuming the Blasius distribution for velocity to hold in the boundary layer, isentropic core densities and velocities were calculated. The velocities determined in this way agreed reasonably well with velocities determined by electron-beam-probe data and machine-computed results. NomenclatureA = area, m 2 a = speed of sound, m/sec C = coefficient of enthalpy scaling E = efficiency F = rh c /mt h = enthalpy, joules/kg; h 0 /RT s = normalized specific total enthalpy (dimensionless) / = current, amp K = constant of proportionality k = qi/p m , Eq. (8) M = Mach number m = mass-flow rate = pvA, kg/sec N = electron concentration, per m 3 n = number of arcs used P = electric power, w p = pressure, N/m 2 ; p 0 i = plenum pressure; p 0 2 = impact pressure q = dynamic pressure = pv*/2 = ypM z /2, N/m 2 q = heat-transfer rate, w/cm 2 R = gas constant, 288 m 2 /sec 2 -°K r = radius (from centerline), m; r' = radius of total flowfield S = entropy, joules/°K T = temperature, °K; T s = reference temperature, 273.1 °K for ho/RTs t = time, sec V = voltage, v v = velocity, m/sec x = normalized boundary-layer thickness, (r f -r)/(r f -r c ) Z* = compressibility factor for gas at the nozzle throat a = coefficient of compounding rate, sec -1 7 = ratio of specific heats 8, 6* = boundary-layer thickness and displacement thickness, respectively, m = P1/P2 € = Presented as Paper 68-229 at the