Measurements of enthalpy in low-density arc- heated flows.

Abstract

The thermochemical state and the total stream enthalpy of are-heated gases expanding from reservoir pressures less than 0.5 atm were investigated. The flow was found to be in equilibrium at the arc-heater exit, and the chemical composition was frozen in the reservoir and nozzle. The total enthalpy of the inviscid core of the freejet was determined by three independent methods, and the values obtained by each method are compared. It is shown that if the chemical state of the flow is known, the enthalpy can be determined quite easily by the frozen sonic flow method. The chemical composition of the frozen flow was determined with an energy balance on the arc heater. The total enthalpy was also determined with stream enthalpy profiles and convective heating-rate measurements. When the calorimeters used to measure the heating rate were assumed to be fully catalytic, the enthalpy determined from these measurements and a heat-transfer theory were low by about 40%. This difference can be accounted for with the nonequilibrium theory. However, because of the complex relationship between heating rate and enthalpy for the nonequilibrium flows, the uncertainty of the surface reactivity, and the unknown effects of vorticity interaction on heat transfer, the heating-rate method is not recommended for determining the stream enthalpy in low-density flow. A = area of test model surface on which heating rates were measured; also, cross-sectional area in nozzle, m2 A* = cross-sectional area of nozzle throat, m2 C = defined by Eq. (8) c = mass fraction of dissociated species Cp = total specific heat, Mjoule/kg °K E = voltage, V F(T) = defined by Eq. (7) H = total enthalpy, (u^/2) + h, Mjoule/kg H$_ = total enthalpy at stream centerline, Mjoule/kg #T° = heat content at temperature T, Mjoule/kg ~Hhr = defined by Eq. (2), Mjoule/kg Hne = defined by Eq. (4), Mjoule/kg Hrn = defined by Eq. (3), Mjoule/kg AHf,T° = standard heat of formation at temperature T, Mjoule/kg h = static enthalpy, Mjoule/kg h° = heat of recombination, Mjoule/kg I = current, amp K = constant in Eq. (14), kwkg/Mjoule-m3/2 atm 1/2