Arnold J. Kelly

Neutron radiographic study of limiting planar heat pipe performance

Abstract A condition limiting the maximum heat transfer rate attainable by heat pipes has been investigated experimentally. Using neutron radiographic techniques, a detailed investigation of the vaporization processes which occur interior to the wick structure of a planar heat pipe employing water as a working fluid was conducted for a number of wicks having different mean pore sizes and for several angles of inclination of the wick. The neutron radiographic system employed permitted measurements of the liquid layer thickness, in the wick, to be made with an accuracy of 0·006 in. for thicknesses up to 0·125 in. of water, and with a lateral resolution of 0·0135 in. Two models of the heat transfer process occurring in planar heat pipes were postulated and analytically formulated. The first model assumed that evaporation occurred only from the upper surface of the wick whereas the second assumed that vapor was generated at the wicks base and released solely from the sides of the wick. With a secondary assumption of variable pore size, the second model proved to be more realistic for correlating the test date than the first. The apparent variation of pore size, indicated by the data to be a function of heat transfer rate, has been interpreted as a manifestation of the “Leverett effect”.

The application of the triple probe method to MPD thruster plumes

The triple probe method is an attractive technique for measuring axial and radial profiles of electron temperature T(e) and electron number density n(e) in magnetoplasmadynamic (MPD) thruster plumes. In this paper the performance and applicability of the triple probe to MPD thruster plumes is evaluated by identifying sources of error, correction methods, and error estimation techniques. Included in the analysis is an adaptation of Laframboises exact calculations to the triple probe, aligned with the flow vector, and a preliminary investigation of the effect of flow transverse to the probe axis. In MPD thrusters, the triple probe method can provide measurement accuracy of T(e) wintin about 10 percent and n(e) within about 60 percent. 52 refs.

Electrostatic atomization—Experiment, theory and industrial applications

Experimental and theoretical research has been initiated at the Princeton Plasma Physics Laboratory on the electrostatic atomization process in collaboration with Charged Injection Corporation. The goal of this collaboration is to set up a comprehensive research and development program on the electrostatic atomization at the Princeton Plasma Physics Laboratory so that both institutions can benefit from the collaboration. Experimental, theoretical and numerical simulation approaches are used for this purpose. An experiment consisting of a capillary sprayer combined with a quadrupole mass filter and a charge detector was installed at the Electrostatic Atomization Laboratory to study fundamental properties of the charged droplets such as the distribution of charges with respect to the droplet radius. In addition, a numerical simulation model is used to study interaction of beam electrons with atmospheric pressure water vapor, supporting an effort to develop an electrostatic water mist fire‐fighting nozzle.

The adverse effect of perpendicular ion drift flow on cylindrical triple probe electron temperature measurements

The cylindrical triple probe method is an attractive technique for measuring electron temperatures (Te) and electron number densities (ne) in a variety of plasmas sources. In practice, however, the cylindrical triple probe can be sensitive to sources of error that affect all Langmuir probe techniques. In particular, the presence of an ion drift velocity component that is perpendicular to the probe axis has been known to result in erroneous measurements of ne. Less obvious, however, is that ion flow perpendicular to the probe has a significant effect on the indicated Te. The purpose of this note is to make researchers aware of such an effect and to demonstrate a technique which can mitigate it. The approach taken to investigate this phenomenon was to make Te measurements in the plume of a 20 kW magnetoplasmadynamic thruster with the probe oriented at several angles with respect to the local ion flow.

MICROINSTABILITIES IN A 10-KILOWATT SELF-FIELD MAGNETOPLASMADYNAMIC THRUSTER

Abstract : Theoretical studies indicate that a variety of microscopic and macroscopic plasma instabilities can significantly affect the performance of the magnetoplasmadynamic (MPD) thruster, yet experimental evidence for their existence is extremely limited. The objective of this research was to provide experimental evidence for the existence of microinstabilities in a 10-kW-class, self-field MPD thruster. The lower- bound of the range of thruster operating currents investigated in this study was set by the requirement that the electromagnetic thrust component be much greater than the electrothermal thrust component. The upper-bound of the operating current was set by the initial observation of high-frequency thruster voltage oscillations, which is usually associated with the onset of excessive thruster erosion. Experimental evidence for two microinstabilities was obtained in the near-field plume of the thruster: 1) the generalized lower hybrid drift instability and 2) the electron cyclotron drift instability.

Anode power deposition in an applied-field segmented anode MPD thruster

Anode heat flux measurements of a water cooled segmented anode applied-field MPD thruster were made to investigate anode heat transfer phenomena. Pure argon and argon-hydrogen mixtures were used as propellants for a variety of thruster currents, propellant mass flow rates, and axial applied magnetic field strengths. The thruster was operated in two modes; with all four segments active, and with two of the segments floating. In addition, thrust and specific impulse were determined for each operating condition. The results show that the heat flux to the anode increases monotonically with axial magnetic field strength and thruster current. Between 50 and 75 percent of the anode heat flux is transported by the current carrying electrons. Convective and radiative heat transfer account for the remaining portion of the power deposited in the anode. The addition of hydrogen to the argon propellant results in the reduction of the fraction of anode power deposited by the anode fall to a level equivalent to that deposited by convection and radiation.

Energy deposition in low-power coaxial plasma thrusters

An experimental examination of energy deposition in self-field, coaxial plasma thrusters revealed that the thrust efficiency ranged from 2-9 percent and that the dominant losses resulted from electrode heating and propellant ionization. Sensible enthalpy and radiative losses were negligible. Thruster specific impulse increased with current, ranging from 550-1750 seconds. Spectroscopic studies of the plume plasma showed that the electron temperature ranged from 0.5-2.5 eV and that the dominant species were singly and doubly ionized argon. Attempts to raise thruster efficiency by increasing the chamber pressure resulted in reduced electrode losses and lowered I(sp), but the thrust efficiency decreased because of a current redistribution that lowered the thrust beyond expectations. 25 refs.

Low Charge Density Electrostatic Atomization

A detailed analysis of the conditions required for low charge density spray development is presented. It is shown that in addition to the requirement that spray droplet size exceed 10 ¿m, it is also necessary that a criterion which relates fluid viscosity to density and surface tension be satisfied. For typical hydrocarbon liquids, this criterion is met when the viscosity is less than about 250 mPa·s. The basic descriptive equations for low charge density spraying are derived and their implications discussed. A characteristic voltage for spray development is defined. This permits the establishment of general limits for all electrostatic spray device terminal behavior. The analysis reveals that when an electrostatic spray system enters the low charge density regime, it exhibits a behavioral pattern that is similar to a first-order phase transition. The nature and basis of this behavior are not understood and should be the subject of a future theoretical study.

Numerical simulation of MPD thruster flows with anomalous transport

Anomalous transport effects in an Ar self-field coaxial MPD thruster are presently studied by means of a fully 2D two-fluid numerical code; its calculations are extended to a range of typical operating conditions. An effort is made to compare the spatial distribution of the steady state flow and field properties and thruster power-dissipation values for simulation runs with and without anomalous transport. A conductivity law based on the nonlinear saturation of lower hybrid current-driven instability is used for the calculations. Anomalous-transport simulation runs have indicated that the resistivity in specific areas of the discharge is significantly higher than that calculated in classical runs. 21 refs.

Anode Power Deposition in Quasisteady Magnetoplasmadynamic Thrusters

Spatially resolved anode heat flux measurements of a pulsed quasisteady magnetoplasmadynamic (MPD) thruster have been made by embedding thermocouples to the inner surface of a hollowed anode. Results obtained using argon propellant at mass flow rates of 4 and 16 g/s with the thruster operating at currents between 824 kA, are presented. These operating conditions correspond to thruster power levels of between 340 kW and 6 MW. In addition, floating probe measurements serve as a second means of estimating the magnitude of anode power deposition. Electron temperature and anode current densities were measured which, with heat flux measurements, permit an estimate of the anode fall to be made. The results of this work show that for moderate values of J2/m (<100 kA2-s/g), the anode fall voltage increases linearly with thruster current and its independent of propellant mass flow rate. The fraction of thruster power deposited into the anode is 42% at 1 MW and less than 20% at 6 MW. At any given operating condition, an inverse relationship is observed between the local anode fall and the local anode current density.