Prateen V. Desai

A time‐dependent model for railgun plasma armatures

In this paper a one‐dimensional, time‐dependent model is described for the analysis of railgun plasma armatures. The model assumes that the armature is isothermal, and that the electrical conductivity and degree of ionization in the plasma are uniform and constant in time. The model is applied to the analysis of three problems—armature initiation, armature response to a change in current, and armature response to a change in mass. In each case, the perturbation induces damped oscillations in the armature length and projectile acceleration about the corresponding steady‐state values. The period of the oscillations is, for early times, approximately equal to 3leq/a0, where leq is the equilibrium armature length corresponding to the steady‐state solution and a0 is the acoustic propagation velocity in the plasma. The exponential decay time for the cases studied ranges from 140 to 200 μsec. Although most calculations are completely numerical, a partly analytic, limiting‐case perturbation solution of the govern...

CFD Simulation of Multi-Dimensional Effects in an Inertance Tube Pulse Tube Refrigerator

Two entire Inertance Tube Pulse Tube Refrigerator (ITPTR) systems operating under a variety of thermal boundary conditions were modeled using a Computational Fluid Dynamics (CFD) code. Each simulated ITPTRs included a compressor, an after cooler, a regenerator, a pulse tube, cold and hot heat exchangers, an inertance tube, and a reservoir. The simulations represented fully coupled systems operating in steady-periodic mode. The objectives were to ascertain the suitability of CFD methods for ITPTRs and to examine the extent of multidimensional flow effects in various ITPTR components. The results confirmed that CFD simulations are capable of elucidating complex periodic processes in ITPTRs. The results also showed that a one-dimensional modeling is appropriate only when all the components in the system have large length-to-diameter (L/D) ratios. Significant multi-dimensional flow effects occur at the vicinity of component-to-component junctions, and secondary-flow recirculation patterns develop when one or more components have small L/D ratios.

Corona Discharge Anemometer and Its Operational Hypothesis

The paper presents an outline of the physical mechanism of corona discharge between a highly stressed anode and a large plate cathode as applied to the operation of a new type of anemometer. An operational hypothesis is proposed for the anemometer with reference to its kinematic and electrostatic principles of performance. Experimental results are presented to justify the hypothesis and the calibration of the probe. A discussion of the probe sensitivity to yaw of gas stream is included.

Working Fluid State Properties Measurements in Medium and High Frequency Cryocoolers

The measurement of the internal thermodynamic states in operational cryocoolers has been an experimental challenge of long standing. An experimental data acquisition system is described which uses high frequency pressure and hot wire anemometry transducers to measure the pressure, temperature, and mass flow within a working Stirling cycle refrigerator operating in the 20 to 60 hertz frequency range. Problems with establishing refrigerator hermeticity and instrumentation calibrations are discussed and the solutions which were used are detailed. A subset of the empirical data obtained by the experimental effort is shown in order to demonstrate how working fluid equations of state can be used to describe refrigeration component performance and estimate how components contribute to overall system characteristics.

Asymptotically matched sheath and presheath in a dense plasma

Asymptotically matched solutions for electoun and ion density, electron and ion velocity, and electric potential are obtained in the boundary region of a dense low-temperature plasma adjacent to perfectly absorbing walls-walls that absorb, without reflection, incident electrons and ions. Ledling-order composite solutions, valid throughout the boundary, region, are constructed from solutions in three subdomains distinguished by different physical length scales: the geometrie length. the ion mean free path and the Debye length. The composite solutions are used to assess the impact of electron-ion recombination in the ionization nonequilibrium region on sheath and presheath profiles, and on quantities evaluated at the wall. While, at leading order, the velocity profiles throughout, the boundary region are not influenced by recombination, the density and potential profiles are significantly altered when recombination is included. These results show that the region of rapid change in these profiles lies closer to the wall when recombination is explicitly included in the model. The influence of recombination on the presheath potential, and consequently the wall potential, is found to scale as the natural logarithm of the recombination length. The broadening of the density profile results in a larger flux of ions accelerating through the sheaths and impacting on the wall. The influence of recombination on the ion power flux to the wall is found to state with the inverse recombination length. This scaling influences the prediction of surface erosion rates in technological applications that utilize these plasmas.

Compensation for refractive-index variations in laser Doppler anemometry.

Techniques to compensate for index of refraction variations in the application of a laser Doppler anemometer are examined. For discontinuous plane-layered media a method of discrete elements is employed. An alternative set of equations is derived for continuous cylindrically layered media and simplified to make comparisons with available results for a single cylinder. Results of velocity measurements in a Plexiglas model of a cylindrically layered nuclear fuel assembly are presented to establish a positioning accuracy of the method to within 0.025 cm.

A Comparative Evaluation of Numerical Models for Cryocooler Regenerators

Various models for predicting the flow and heat transfer in a porous cryocooler regenerator have been proposed in the literature. One such model utilizes a semi-implicit set of equations after making some simplifying assumptions, resulting in a momentum equation that is decoupled from the energy and continuity equations. This work addresses concerns with a semi-implicit model based on scale analysis. An important result is that the pressure gradient term in the energy equation, which has been neglected in the semi-implicit model, is leading order.

A two-fluid analysis of filmwise condensation in tubes

Abstract Thermal and hydrodynamic behavior of steady, annular two-phase flows, typified by filmwise condensation in circular tubes, are examined. Conservation equations of mass, axial momentum, and energy are derived for the liquid and vapor phases, as well as mass and energy balances for the interface between the phases. The model remains valid for either a laminar or a turbulent vapor core, the liquid film being laminar, but contains more unknowns than the available equations. Closure is obtained by first selecting dependent variables for which boundary conditions are known at the tube inlet or exit, assuming radial profiles of velocity and temperature in each phase, requiring the assumed profiles to satisfy the definitions of the dependent variables and appropriate radial boundary conditions, and finally evaluating the remaining unknowns by using the velocity and the temperature profiles. Computed values of the heat transfer coefficient obtained for the turbulent vapor core/laminar liquid film case are shown to provide good agreement with experimental results.

Comparison of partition function calculations for metal plasmas

Three methods are commonly used to handle the unobserved energy levels in calculating the properties of plasmas. The contribution of the levels may be simply ignored, or the energies of the levels may be estimated by either the hydrogenic or the Rydberg–Ritz approximation. In this paper we compare the predictions of plasma properties obtained with each of these methods for Cu and Al plasmas for temperatures of 10 000–50 000 K and pressures from 0.1 to 600 atm. The validity of simplifying assumptions used to handle the unobserved levels has been documented, and regimes where each method is most appropriate have been identified.

Measurement of Anisotropic Hydrodynamic Parameters of Pulse Tube or Stirling Cryocooler Regenerators

Pulse tube refrigeration (PTR) systems are often modeled as one‐dimensional flow fields. However, recent computational fluid dynamics (CFD) — based investigations have shown that multi‐dimensional flow effects can be significant in the regenerator of a PTR, especially when the aspect ratio of the regenerator is small. Anisotropic hydrodynamic parameters of regenerators are therefore needed for the realistic simulation of their multi‐dimensional flow phenomena. In this paper we report on measurements of the lateral or radial permeability and Forchheimer’s inertial coefficient of a widely used PTR regenerator filler. Using helium as the working fluid, steady‐state pressure drops were measured over a wide range of flow rates in annular test sections that contained regenerator fillers. The aforementioned hydrodynamic parameters were then obtained by comparing the data with the results of CFD calculations that simulated the test sections and their vicinity. CFD simulations of the experiments were performed ite...