Physical Sciences

Magnetic Field Would Reduce Electron Backstreaming in Ion Thrusters

Erosion of accelerator grid could also be reduced.

The imposition of a magnetic field has been proposed as a means of reducing the electron backstreaming problem in ion thrusters.

Electron backstreaming refers to the backflow of electrons into the ion thruster. Back- streaming electrons are accelerated by the large potential difference that exists between the ion- thruster acceleration electrodes, which otherwise accelerates positive ions out of the engine to develop thrust. The energetic beam formed by the backstreaming electrons can damage the discharge cathode, as well as other discharge surfaces upstream of the acceleration electrodes. The electron-backstreaming condition occurs when the center potential of the ion accelerator grid is no longer sufficiently negative to prevent electron diffusion back into the ion thruster. This typically occurs over extended periods of operation as accelerator-grid apertures enlarge due to erosion. As a result, ion thrusters are required to operate at increasingly negative accelerator-grid voltages in order to prevent electron backstreaming. These larger negative voltages give rise to higher accelerator-grid erosion rates, which in turn accelerates aperture enlargement. Electron backstreaming due to accelerator-grid-hole enlargement has been identified as a failure mechanism that will limit ion-thruster service lifetime.

Posted in: Briefs, TSP, Physical Sciences, Nuclear energy, Magnetic materials, Spacecraft

MEMS-Based Piezoelectric/Electrostatic Inchworm Actuator

Nanometer steps could be concatenated into overall travel of hundreds of microns.

A proposed inchworm actuator, to be designed and fabricated according to the principles of microelectromechanical systems (MEMS), would effect linear motion characterized by steps as small as nanometers and an overall range of travel of hundreds of microns. Potential applications for actuators like this one include precise positioning of optical components and active suppression of noise and vibration in scientific instruments, conveyance of wafers in the semiconductor industry, precise positioning for machine tools, and positioning and actuation of microsurgical instruments.

Posted in: Briefs, TSP, Physical Sciences, Calibration, Microelectricmechanical device, Microelectromechanical devices, Sensors and actuators, Microelectricmechanical device, Microelectromechanical devices, Sensors and actuators

Predicting and Preventing Incipient Flameout in Combustors

Increases in acoustic signals could trigger rapid adjustments to prevent flameouts.

A method of predicting and preventing incipient flameout in a combustor has been proposed. The method should be applicable to a variety of liquid- and gas-fueled combustors in furnaces and turbine engines. Until now, there have been methods of detecting flameouts after they have occurred, but there has been no way of predicting incipient flameouts and, hence, no way of acting in time to prevent them. Prevention of flameout could not only prevent damage to equipment but, in the case of aircraft turbine engines, could also save lives.

Posted in: Briefs, TSP, Physical Sciences, Heating, ventilation, and air conditioning systems (HVAC), Heating, ventilation, and air conditioning systems (HVAC), Prognostics, Combustion chambers, Gas turbines, Fire prevention

Detecting Conductive Liquid Leaking From Nonconductive Pipe

A capacitive detector is scanned over the ground above the pipe.

A method that can be implemented with relatively simple electronic circuitry provides a capability for detecting leakage of an electrically conductive liquid from an electrically nonconductive underground pipe. Alternatively or in addition, the method can be applied to locate the pipe, whether or not there is a leak. Although the method is subject to limitations (some of which are described below), it is still attractive as an additional option for detecting leaks and locating pipes without need for extensive digging.

Posted in: Briefs, TSP, Physical Sciences, Integrated circuits, Sensors and actuators, Integrated circuits, Sensors and actuators, Conductivity, Hoses

Hot Films on Ceramic Substrates for Measuring Skin Friction

Low-thermal-conductivity ceramic substrates, based on Space Shuttle tile technology, serve to increase sensitivity.

Hot-film sensors, consisting of a metallic film on an electrically nonconductive substrate, have been used to measure skin friction as far back as 1931. A hot film is maintained at an elevated temperature relative to the local flow by passing an electrical current through it. The power required to maintain the specified temperature depends on the rate at which heat is transferred to the flow. The heat-transfer rate correlates to the velocity gradient at the surface, and hence, with skin friction. The hot-film skin friction measurement method is most thoroughly developed for steady-state conditions, but additional issues arise under transient conditions.

Posted in: Briefs, Physical Sciences, Measurements, Sensors and actuators, Sensors and actuators, Heat transfer, Heat transfer, Thermal testing

Nulling Infrared Radiometer for Measuring Temperature

A microwave-radiometer self-calibration principle would be adapted to measurement of infrared.

A nulling, self-calibrating infrared radiometer is being developed for use in noncontact measurement of temperature in any of a variety of industrial and scientific applications. This instrument is expected to be especially well-suited to measurement of ambient or near-ambient temperature and, even more specifically, for measuring the surface temperature of a natural body of water. Although this radiometer would utilize the long-wavelength infrared (LWIR) portion of the spectrum (wavelengths of 8 to 12 µm), its basic principle of operation could also be applied to other spectral bands (corresponding to other temperature ranges) in which the atmosphere is transparent and in which design requirements for sensitivity and temperature-measurement accuracy could be satisfied.

Posted in: Briefs, Physical Sciences, Measurements, Water, Thermal testing, Industrial vehicles and equipment, Marine vehicles and equipment

Assessment of Models of Chemically Reacting Granular Flows

A report presents an assessment of a general mathematical model of dense, chemically reacting granular flows like those in fluidized beds used to pyrolize biomass. The model incorporates submodels that have been described in several NASA Tech Briefs articles, including "Generalized Mathematical Model of Pyrolysis of Biomass" (NPO-20068) NASA Tech Briefs, Vol. 22, No. 2 (February 1998), page 60; "Model of Pyrolysis of Biomass in a Fluidized-Bed Reactor" (NPO-20708), NASA Tech Briefs, Vol. 25, No. 6 (June 2001), page 59; and "Model of Fluidized Bed Containing Reacting Solids and Gases" (NPO-30163), which appears elsewhere in this issue. The model was used to perform computational simulations in a test case of pyrolysis in a reactor containing sand and biomass (i.e., plant material) particles through which passes a flow of hot nitrogen. The boundary conditions and other parameters were selected for the test case to enable assessment of the validity of some assumptions incorporated into submodels of granular stresses, granular thermal conductivity, and heating of particles. The results of the simulation are interpreted as partly affirming the assumptions in some respects and indicating the need for refinements of the assumptions and the affected submodels in other respects.

Posted in: Briefs, TSP, Physical Sciences, Computer simulation, Mathematical models, Thermodynamics, Thermodynamics, Biomaterials

Numerical Study of Pyrolysis of Biomass in Fluidized Beds

A report presents a numerical-simulation study of pyrolysis of biomass in fluidized-bed reactors, performed by use of the mathematical model described in "Model of Fluidized Bed Containing Reacting Solids and Gases" (NPO-30163), which appears elsewhere in this issue of NASA Tech Briefs. The purpose of the study was to investigate the effect of various operating conditions on the efficiency of production of condensable tar from biomass. The numerical results indicate that for a fixed particle size, the fluidizing-gas temperature is the foremost parameter that affects the tar yield. For the range of fluidizing-gas temperatures investigated, and under the assumption that the pyrolysis rate exceeds the feed rate, the optimum steady-state tar collection was found to occur at 750 K. In cases in which the assumption was not valid, the optimum temperature for tar collection was found to be only slightly higher. Scaling up of the reactor was found to exert a small negative effect on tar collection at the optimal operating temperature. It is also found that slightly better scaling is obtained by use of shallower fluidized beds with greater fluidization velocities.

Posted in: Briefs, TSP, Physical Sciences, Design processes, Mathematical models, Production, Gases, Thermal testing

Integrated Arrays of Ion-Sensitive Electrodes

Electronic "tongues" would "taste" selected ions in water.

The figure depicts an example of proposed compact water-quality sensors that would contain integrated arrays of ion-sensitive electrodes (ISEs). These sensors would serve as electronic "tongues": they would be placed in contact with water and used to "taste" selected dissolved ions (that is, they would be used to measure the concentrations of the ions). The selected ions could be any or all of a variety of organic and inorganic cations and anions that could be regarded as contaminants or analytes, depending on the specific application. In addition, some of the ISEs could be made sensitive to some neutral analytes (see table).

Posted in: Briefs, TSP, Physical Sciences, Sensors and actuators, Sensors and actuators, Water quality, Test equipment and instrumentation

Model of Fluidized Bed Containing Reacting Solids and Gases

This model can be used to optimize designs and operating conditions.

A mathematical model has been developed for describing the thermofluid dynamics of a dense, chemically reacting mixture of solid particles and gases. As used here, "dense" signifies having a large volume fraction of particles, as for example in a bubbling fluidized bed. The model is intended especially for application to fluidized beds that contain mixtures of carrier gases, biomass undergoing pyrolysis, and sand. So far, the design of fluidized beds and other gas/solid industrial processing equipment has been based on empirical correlations derived from laboratory- and pilot-scale units. The present mathematical model is a product of continuing efforts to develop a computational capability for optimizing the designs of fluidized beds and related equipment on the basis of first principles. Such a capability could eliminate the need for expensive, time-consuming predesign testing.

Posted in: Briefs, TSP, Physical Sciences, Computational fluid dynamics, Mathematical models, Optimization, Thermodynamics, Thermodynamics, Gases

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