Significance and Use
The purpose of this test method is to provide a standard calculation of the stagnation enthalpy of an aerodynamic simulation device using the heat transfer theory and measured values of stagnation point heat transfer and pressure. A stagnation enthalpy obtained by this test method gives a consistent set of data, along with heat transfer and stagnation pressure for ablation computations.
Scope
1.1 This test method covers the calculation from heat transfer theory of the stagnation enthalpy from experimental measurements of the stagnation-point heat transfer and stagnation pressure.
1.2 Advantages
1.2.1 A value of stagnation enthalpy can be obtained at the location in the stream where the model is tested. This value gives a consistent set of data, along with heat transfer and stagnation pressure, for ablation computations.
1.2.2 This computation of stagnation enthalpy does not require the measurement of any arc heater parameters.
1.3 Limitations and ConsiderationsThere are many factors that may contribute to an error using this type of approach to calculate stagnation enthalpy, including:
1.3.1 TurbulenceThe turbulence generated by adding energy to the stream may cause deviation from the laminar equilibrium heat transfer theory.
1.3.2 Equilibrium, Nonequilibrium, or Frozen State of GasThe reaction rates and expansions may be such that the gas is far from thermodynamic equilibrium.
1.3.3 Noncatalytic EffectsThe surface recombination rates and the characteristics of the metallic calorimeter may give a heat transfer deviation from the equilibrium theory.
1.3.4 Free Electric CurrentsThe arc-heated gas stream may have free electric currents that will contribute to measured experimental heat transfer rates.
1.3.5 Nonuniform Pressure ProfileA nonuniform pressure profile in the region of the stream at the point of the heat transfer measurement could distort the stagnation point velocity gradient.
1.3.6 Mach Number EffectsThe nondimensional stagnation-point velocity gradient is a function of the Mach number. In addition, the Mach number is a function of enthalpy and pressure such that an iterative process is necessary.
1.3.7 Model ShapeThe nondimensional stagnation-point velocity gradient is a function of model shape.
1.3.8 Radiation EffectsThe hot gas stream may contribute a radiative component to the heat transfer rate.
1.3.9 Heat Transfer Rate Measurement An error may be made in the heat transfer measurement (see Method E 469 and Test Methods E 422, E 457, E 459, and E 511).
1.3.10 ContaminationThe electrode material may be of a large enough percentage of the mass flow rate to contribute to the heat transfer rate measurement.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
