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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Effect of field-aligned potential drop in a global magnetosphere-ionosphere coupling model

Effects of field-aligned potential drops on the magnetosphere-ionosphere coupling in a steady state are studied on a global ionospheric scale. It is shown that a constant-current generator can support a larger field-aligned potential drop than a constant voltage generator under similar conditions. The magnetospheric convection pattern is distorted more in the constant current generator case than in the constant voltage generator case. The main difference between a constant current generator and a constant voltage generator is found to lie in their ability to adjust the vorticity of the magnetospheric convection. The results show that a constant current generator allows the vorticity of the magnetospheric convection to adjust so that the field-aligned current can be kept constant under the loading influence of the field-aligned potential. On the other hand, a constant voltage generator by definition cannot adjust the vorticity of the magnetospheric convection to maintain the field-aligned current under the loading influence of the field-aligned potential.

Kan, J. R.↗
NASA NTRS1989

Efficient loads analyses of Shuttle-payloads using dynamic models with linear or nonlinear interfaces

An efficient method for the load analysis of Shuttle-payload systems with linear or nonlinear attachment interfaces is presented which allows the kinematics of the interface degrees of freedom at a given time to be evaluated without calculating the combined system modal representation of the Space Shuttle and its payload. For the case of a nonlinear dynamic model, an iterative procedure is employed to converge the nonlinear terms of the equations of motion to reliable values. Results are presented for a Shuttle abort landing event.

Spanos, P. D.↗
NASA NTRS1989

The thermal performance of heat pipes with localized heat input

The performance of heat pipes with localized heat input including the effects of axial and circumferential heat conduction under high and low working temperatures is investigated. The numerical results show that when heat pipes are spot heated, the peak temperature of the wall is greatly reduced and the surface can be protected from being burned out by the high heat flux. The boiling limitation becomes the most important limitation for this type of heat pipe. Numerical results for block heating a heat pipe with low working temperatures indicate a good agreement with existing experimental data. It is also shown that most of the input heat passes through the wall beneath the heated block.

Cao, Yiding↗
NASA NTRS1989

A numerical analysis of Stefan problems for generalized multi-dimensional phase-change structures using the enthalpy transforming model

An enthalpy transforming scheme is proposed to convert the energy equation into a nonlinear equation with the enthalpy, E, being the single dependent variable. The existing control-volume finite-difference approach is modified so it can be applied to the numerical performance of Stefan problems. The model is tested by applying it to a three-dimensional freezing problem. The numerical results are in agreement with those existing in the literature. The model and its algorithm are further applied to a three-dimensional moving heat source problem showing that the methodology is capable of handling complicated phase-change problems with fixed grids.

Cao, Yiding↗
NASA NTRS1989

The next 40 years in space standards

Methods used in producing, introducing, and using standards for civilian space research are examined, with special consideration given to the recommendations of the Consultative Committee for Space Data Systems (CCSDS) as an example of a set of standards best coordinated through the International Standard Organization (ISO). The paper uses an example of earth-observation data as a CCSDS application for an international coordination effort within the ISO and describes progress in achieving this objective. Lists of CCDSD member agencies and CCSDS observer agencies are included.

Sheetz, H. J.↗
NASA NTRS1990

Atmospheric pressure organometallic vapor-phase epitaxial growth of (Al/x/Ga/1-x/)0.51In0.49P (x from 0 to 1) using trimethylalkyls

This paper describes growth of (Al/x/Ga/1-x)0.51In0.49P layers (with x from 0 to 1) lattice-matched to (001)-oriented GaAs substrates by atmospheric-pressure OMVPE, using trimethylindium, trimethylaluminum, and trimethylgallium and PH3 as source materials in a horizontal reactor. Excellent surface morphologies were obtained over the entire range of Al compositions at a growth temperature of 680 C. Photoluminescence (PL) was observed for all samples with x values not below 0.52, with PL peak energies as high as 2.212 eV. The PL FWHM for Ga(0.51)In(0.49)P was 7.2 meV at 10 K and 35 meV at 300 K. At 10 K, the PL intensity was nearly a constant over the composition range from x = 0 to 0.52.

Cao, D. S.↗
NASA NTRS1990

Analytical aerodynamic model of a high alpha research vehicle wind-tunnel model

A 6 DOF analytical aerodynamic model of a high alpha research vehicle is derived. The derivation is based on wind-tunnel model data valid in the altitude-Mach flight envelope centered at 15,000 ft altitude and 0.6 Mach number with Mach range between 0.3 and 0.9. The analytical models of the aerodynamics coefficients are nonlinear functions of alpha with all control variable and other states fixed. Interpolation is required between the parameterized nonlinear functions. The lift and pitching moment coefficients have unsteady flow parts due to the time range of change of angle-of-attack (alpha dot). The analytical models are plotted and compared with their corresponding wind-tunnel data. Piloted simulated maneuvers of the wind-tunnel model are used to evaluate the analytical model. The maneuvers considered are pitch-ups, 360 degree loaded and unloaded rolls, turn reversals, split S's, and level turns. The evaluation finds that (1) the analytical model is a good representation at Mach 0.6, (2) the longitudinal part is good for the Mach range 0.3 to 0.9, and (3) the lateral part is good for Mach numbers between 0.6 and 0.9. The computer simulations show that the storage requirement of the analytical model is about one tenth that of the wind-tunnel model and it runs twice as fast.

Cao, Jichang↗
NASA NTRS1991

Performance characteristics of a thermal energy storage module - A transient PCM/forced convection conjugate analysis

The performance of a thermal energy storage module is simulated numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid with low Prandtl numbers are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. The numerical results show that module geometry is crucial to the design of a space-based thermal energy storage system.

Cao, Y.↗
NASA NTRS1991

A PCM/forced convection conjugate transient analysis of energy storage systems with annular and countercurrent flows

Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.

Cao, Y.↗
NASA NTRS1991

Recombination of N4(+) ions with electrons

Using a modified high-pressure-afterglow/mass spectrometer apparatus similar to that described by Lee and Johnsen (1989), spectroscopic observations of afterglow helium plasmas, with N2 as a minor additive, were carried out in order to verify the mechanism suggested by Bates (1991) for dissociative recombination of electrons with N4(+) ions. It was found that dissociative recombination of electrons with N4(+) ions results in the formation of N2 molecules in the C 3Pi(u) (v = 0,1) state, with the recombination rate coefficient of (2.6 +/- 0.3) x 10 exp -6 cu cm/sec at 300 K.

Cao, Y. S.↗
NASA NTRS1991

Transient multidimensional analysis of nonconventional heat pipes with uniform and nonuniform heat distributions

A numerical analysis of transient heat pipe performance including nonconventional heat pipes with nonuniform heat distributions is presented. A body-fitted grid system was applied to a three-dimensional wall and wick model, which was coupled with a transient compressible quasi-one-dimensional vapor flow model. The numerical results were first compared with experimental data from cylindrical heat pipes with good agreement. Numerical calculations were then made for a leading edge heat pipe with localized high heat fluxes. Performance characteristics different from conventional heat pipes are illustrated and some operating limits concerning heat pipe design are discussed.

Cao, Y.↗
NASA NTRS1991

A study of high-temperature heat pipes with multiple heat sources and sinks. I - Experimental methodology and frozen startup profiles. II - Analysis of continuum transient and steady-state experimental data with numerical predictions

Experimental profiles for heat pipe startup from the frozen state were obtained, using a high-temperature sodium/stainless steel pipe with multiple heat sources and sinks to investigate the startup behavior of the heat pipe for various heat loads and input locations, with both low and high heat rejection rates at the condensor. The experimental results of the performance characteristics for the continuum transient and steady-state operation of the heat pipe were analyzed, and the performance limits for operation with varying heat fluxes and location are determined.

Faghri, A.↗
NASA NTRS1991

Identification of dislocation etch pits in n-type GaAs by NIR transmission microscopy

An optical method is described for identifying dislocation etch pits in n-type GaAS, using near-IR brightfield transmission microscopy. Dislocations are revealed in a nondestructive manner through contrasts that are likely due to impurity decoration of the dislocation lines. By subjecting the same wafers to a photoetching technique, it was established that each etch pit on the surface is associated with a dislocation and that the termination of each (decorated) dislocation is an etch pit.

Cao, X. Z.↗
NASA NTRS1991

Task sequence planning in a robot workcell using AND/OR nets

An approach to task sequence planning for a generalized robotic manufacturing or material handling workcell is described. Given the descriptions of the objects in this system and all feasible geometric relationships among these objects, an AND/OR net which describes the relationships of all feasible geometric states and associated feasibility criteria for net transitions is generated. This AND/OR net is mapped into a Petri net which incorporates all feasible sequences of operations. The resulting Petri net is shown to be bounded and have guaranteed properties of liveness, safeness, and reversibility. Sequences are found from the reachability tree of the Petri net. Feasibility criteria for net transitions may be used to generate an extended Petri net representation of lower level command sequences. The resulting Petri net representation may be used for on-line scheduling and control of the system of feasible sequences. A simulation example of the sequences is described.

Cao, Tiehua↗
NASA NTRS1992

Closed-form analytical solutions of high-temperature heat pipe startup and frozen startup limitation

Previous numerical and experimental studies indicate that the high-temperature heat pipe startup process is characterized by a moving hot zone with relatively sharp fronts. Based on the above observation, a flat-front model for an approximate analytical solution is proposed. A closed-form solution related to the temperature distribution in the hot zone and the hot zone length as a function of time are obtained. The analytical results agree well with the corresponding experimental data, and provide a quick prediction method for the heat pipe startup performance. Finally, a heat pipe limitation related to the frozen startup process is identified, and an explicit criterion for the high-temperature heat pipe startup is derived. The frozen startup limit identified in this paper provides a fundamental guidance for high-temperature heat pipe design.

Cao, Y.↗
NASA NTRS1993

Simulation of the early startup period of high-temperature heat pipes from the frozen state by a rarefied vapor self-diffusion model

The heat pipe startup process is described physically and is divided into five periods for convenience of analysis. The literature survey revealed that none of the previous attempts to simulate the heat pipe startup process numerically were successful, since the rarefied vapor flow in the heat pipe was not considered. Therefore, a rarefied vapor self-diffusion model is proposed, and the early startup periods, in which the rarefied vapor flow is dominant within the heat pipe, are first simulated numerically. The numerical results show that large vapor density gradients existed along the heat pipe length, and the vapor flow reaches supersonic velocities when the density is extremely low. The numerical results are compared with the experimental data of the early startup period with good agreement.

Cao, Y.↗
NASA NTRS1993

A numerical analysis of high-temperature heat pipe startup from the frozen state

Continuum and rarefied vapor flows co-exist along the heat pipe length for most of the startup period. A two-region model is proposed in which the vapor flow in the continuum region is modeled by the compressible Navier-Stokes equations, and the vapor flow in the rarefied region is simulated by a self-diffusion model. The two vapor regions are linked with appropriate boundary conditions, and heat pipe wail, wick, and vapor flow are solved as a conjugate problem. The numerical solutions for the entire heat pipe startup process from the frozen state are compared with the corresponding experimental data with good agreement.

Cao, Y.↗
NASA NTRS1991

New optical approaches to the quantitative characterization of crystal growth, segregation and defect formation

Elemental and compound semiconductors were characterized using new optical approach based on NIR microscopy in conjunction with computational image analysis and contrast enhancement. The approach made it possible to perform a quantitative microsegregation analysis of GaAs and InP. NIR dark file illumination in transmission mode makes it possible to detect submicron precipitates in semiinsulating GaAs.

Carlson, D. J.↗