Firing Durations Research Articles

POC net, a subclass of Petri nets, and its application to timed Petri nets

Abstract Liveness is one of the most important properties of the Petri net analysis. This property is concerned with a capability for firing of transitions. On the other hand, place-liveness is another notion related to liveness, which is concerned with a capability for having tokens in places. Concerning these liveness and place-liveness problems, this paper suggests a new subclass of Petri net, ‘POC nets’, as a superclass of AC nets and DC nets. For this subclass, the equivalence between liveness and place-liveness is shown and a sufficient condition for liveness for this POC net is derived. Then the results are extended to liveness problem of timed Petri nets which have transitions with finite firing durations and the earliest firing rule. Although liveness of a (non-timed) Petri net is neither necessary nor sufficient condition for liveness of a timed Petri net, it is shown that liveness is preserved if the net has POC structure. Furthermore, it is pointed out that if a POC net satisfies some additional condition, Petri net liveness is equivalent to timed Petri net liveness. Finally, it is shown that liveness of timed POC nets with TC structure and the earliest firing rule is decidable with deterministic polynomial time complexity.

Motor unit regulation of mammalian pharyngeal dilator muscle activity.

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Properties of clay-sand-rice husk ash mixed bricks

Bricks were made from clay-sand mixes with different percentages of rice husk ash and burnt in a furnace for different firing times. The firing durations at 1000°C were 2 hours, 4 hours and 6 hours. The effects of rice husk ash contents on workable mixing water content, Atterberg limits, linear shrinkage, density, compressive strength and water absorption of the bricks were investigated. Test results indicated that lightweight bricks could be manuctured with rice husk ash without any deterioration in the quality of bricks. Further, the compressive strength of the bricks was higher with rice husk ash contents. The optimum firing duration was found to be 4 hours at 1000°C. It is shown that bricks made of clay-sand-rice husk ash mixes can be used in load bearing walls.

Experimental investigations of the 10 N catalytic hydrazine thruster

Experimental investigations of the 10 N catalytic hydrazine thruster are reported. These thrusters find applications in orbit raising functions of a spacecraft. The hardware was realized and tested in a vacuum chamber (10 −3 mbar vacuum) for its performance. When tested at the design propellant supply pressure of 21.5 bar the thruster developed 10.25 N thrust at an operating chamber pressure of 16.4 bar. The thruster was also tested for off-design conditions (24, 18 and 14.5 bar propellant supply pressures) of operation to determine the steady-state performance. The chamber pressure and vacuum thrust follow more or less a linear law with the propellant-supply pressure. The thruster was also tested for its response characteristics for short (100 ms) firing durations at various propellant-supply pressures (15.5, 18.8, 22.5, 25.6 and 29.5 bar) and the experimental results are reported and discussed. The hydrazine was injected at the room temperature (300 ±5 K).

Single motor unit activity in the diaphragm of cat during pressure breathing.

In this study we analyzed the breath-by-breath activity of single motor units in the diaphragm slip of allobarbital-anesthetized cats during quiet breathing and during continuous positive- and negative-pressure breathing. Our objective was to determine whether single motor units, on the basis of their activities, can be separated into discrete subpopulations or whether they fall on a continuum analogous to that of motor units of hindlimb muscles. The firing profiles of each unit were characterized for each pressure level by the onset and peak firing frequencies, onset latency, duration of firing, number of impulses per breath, and minimal frequency, when appropriate. Units with shorter onset latencies had higher peak frequencies, longer firing durations, and increased firing frequencies than did units with longer onset latencies. These comparative relationships persisted even though the activity of every motor unit was altered during pressure breathing. During positive-pressure breathing onset latencies were lengthened, and durations of firing were shortened with little change in onset or peak frequencies. Late units might be silenced. During negative-pressure breathing onset latencies were shortened, and durations of firing were lengthened, sufficiently in some cases to fill the expiratory pause. In addition, previously inactive units were recruited late in inspiration for short, relatively high frequency bursts during inspiration. The results support the concept that the phrenic motoneuron pool is comprised of three discrete subpopulations.

A radiation-cooled nuclear rocket nozzle for long firing durations

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TRANSPIRATION COOLING WITH LIQUID METALS

The development of hybrid propulsion concepts at the Naval Ordnance Test Station has created the requirement for a lightweight nozzle system capable of withstanding gas temperatures in excess of 6200°F for burn times longer than 30 sec. A series of 40 nozzle-material test firings illustrated that tungsten nozzle insert systems provided the best operation. The inserts tested included tungsten metal-oxide compacts and tungsten infiltrated with metals selected on the basis of their latent heats of vaporization and boiling points. Temperaturetime profiles taken during firings indicated that heat transfer rates were lower with the tungsten-metal systems than for unmodified tungsten, which indicated the possibility that transpiration or mass transfer cooling was being realized. A laboratory program was initiated to subject infiltrated tungsten samples to a plasma-arc heat source. It was found that, during exposure to high heat fluxes, temperatures were reduced by inducing flow of the molten infiltrant by pressurizing the back side of the specimen. This technique, in combination with ablative sacrificial materials, offers an excellent nozzle system for long-time firing durations in hybrid and solid propellant rockets.

New Materials

An ablative‐cooled thrust chamber assembly for liquid‐fuelled rocket engines capable of withstanding extended firing durations has been developed by United Technology Corporation, and a 2,200 lb. thrust engine using this chamber has been continuously fired for 4·5 mins. The engine achieved an accumulated duration of 7·2 mins. with a refiring.