Hot particles

Besides the nature of a primary photochemical step, of great kinetic importance is the excess energy released in this step. For a continuous absorption spectrum or for predissociation, the excess energy is defined as 

The excess energy of the primary photochemical decomposition products at the moment of their generation makes them more reactive than the same species in the state of thermal equilibrium with the ambient gas. Particles with an elevated energy store are referred to as hot particles. 

The photochemical generation of hot particles with their elevated reactivities is recognized in particular by the temperature independence of the reaction rate. 

Electronically excited molecules generated in the primary photochemical step can also be formally called hot. The resemblance between electronically excited and hot particles shows also in that both are capable of thermalization. 

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The output of a primer includes hot gases, hot particles, a pressure pulse (which, in some cases, may be a strong shock), and thermal radiation (Refs 6, 12 13). Some of the aspects of primer output which have been measured as a means of characterization include the following (a) vol of gas emitted (b) the impulse imparted to a column of Hg by the pressure pulse ... 

National Academy of Sciences-National Research Council, Health Effects of Alpha-Emitting Particles in the Respiratory Tract, Report of AD Hoc Committee on Hot Particles, U. S. Environmental Protection Agency, EPA 520/4-76-013, October 1976. 

Exposure of hot particle contaminated soil (sampled 15 km north of the Chernobyl Nuclear Power Plant) to film X, Y and Z of a Differential Autoradiographic Imaging Sandwich. The original film size is 85 x 10 x 10 mm, exposure time was 14 days. Area (a) shows irradiation from a 90Sr dominated hot particle whereas irradiation in area (b) is caused by a 137Cs dominated hot particle. 

The light emitted by a candle originates from hot particles of soot in the flame these soot particles strongly absorb and thence re-emit it as visible light. By contrast, the gas flame of a kitchen oven is paler, despite being hotter than a candle flame, and does not emit much light owing to an absence of soot. 

When a metallized energetic material is burned as a propellant igniter in a rocket chamber, a consequence of the aforementioned production of metal oxides as hot condensed particles is that there is very tittle associated pressure increase. However, the surface of the propellant grain in the chamber is ignited by the hot particles and a stable burning pressure is established. Typical metallized pyrolants used as igniters are shown in Table 11.1. 

Metal particles incorporated into a gas-generating pyrolant act as flame holders to keep the flame in the ramburner. Fach metal particle flows with the combustible gas and becomes a hot metal or metal oxide particle. Since the flow velocity of such a hot particle is lower than that of the combustible gas, the flow velocity of the combustible gas just downstream of the hot particle is decreased due to the aerody-... 

The 40 mm or so of delay fuse burns noisily for several seconds and when the end of the fuse is reached, smoke, flame and hot particles are showered onto the main gunpowder filling which promptly explodes. The explosion creates a pressure pulse which operates over a few thousandths of a second but which is enough to shatter the tube and clay plug and enter the air as a blast wave. 

On the other hand, for molecules, the electronic transitions result in bands lO SOnm in width due to the changes in vibrational energy levels which also occur. A third type of radiation emitted by stars in the near-UV visible near-IR region is a continuum emission originating from hot particles e.g. hot AI2O3 particles) but this is considered to be grey body radiation and does not contribute to the colour of the star. 

GAS COOL Cooling from 2500 C -ve MgO (solid) CO COjrN OjH O Na(g)- Na(s)-i Na(l) Na- Na20 Continuum radiation from hot particles vis. radiation ceases 900 C Some emission due to energy transfer from zone 4 Mg droplets bum out Main cooling rone... 

COMBUSTION About 2500 C to2700 C ++ve Mg(l,g) MgO(s,l) Na(g) 0,0,Hj0,0H CO,COyNj,NO Visible emission zone from Na, Mg, MgO plus continuum radiation from hot particles High temperatures e ite Na atoms to emit at 590nm. Mg droplets move across rone Mg bums rapidly in diffusion flame surrounding liquid droplets... 

CONTINUUM THERMAL RADIATION Radiation which involves the transfer of heat by electromagnetic waves, confined to the relatively narrow window of the electromagnetic spectrum (i.e. visible light around 400 nm to infrared light around 1000 nm). The hot particles in and above hrework flames contribute to this type of radiation. 

Thus our first conclusion if the harmful effects of thermal shock, or sintering of the catalyst surface, or drop in selectivity, do not occur with hot particles, then we would encourage nonisothermal behavior in exothermic reactions. On the other hand, we would like to depress such behavior for endothermic reactions. 

There is general agreement that both fronts accelerate, and that the process is cumulative by virtue of their mutual interaction. This is promoted by turbulence, diffusion of hot particles ahead of the flame front, and passage of reflected shocks and rarefactions thru the "processed medium (Ref 3, p 130). Advance of the flame front thru it differs more and more from the initial laminar burning... 

These tests are more varied in their nature and complexity than are the input tests. This difference may be attributed to the fact that the output of a firing train component may be req uired to produce a wide variety of effects to detonate a lead or booster by shock to do mechanical work in driving a firing pin to initiate a detonator by shock, flame, or hot particles to ignite a delay by flame hot particles or to lock a train in the armed position by moving detents or expanding confining walls... 

Combustible systems can be ignited by sparks, hot wires, flames, hot particles, heated surfaces and many other sources. Ignition by sparks hot wires are of particular interest in connection with explosives and we will therefore consider them in more detail below... 

The basic construction has been modified in addnl patents. One change added a refractory bead to the top of the wire (inside the lamp) to prevent burn thru and to disperse the hot particles of initiating material. Addnl patents covering the initiation of the Magi cube are 3535064, 3540819, 3540820, 3597603, 3597604, 3602618, 3625641, 3667992, 3699021, 3700377, 3730669, 3734679 Refs Public Affairs Dept, SyIvania Electric Products Inc, 730 Third Ave, New York NY 10017 2) G. Cohn, Expls Pyrots 6(12), 1973... 

SC 80-1 Hot Particles on the Skin Assessment of Exposures from Therapy Control of Indoor Radon... 

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