Spark initiation

Reduction of pressure reduces the intensity of explosion,1 and for each gaseous mixture there appears to be a critical pressure, below which explosions will not take place. This pressure is a function of the chemical composition and proportions of the gases, the moisture content and the initial spark impulse. The completeness of combustion likewise falls with the pressure. Thus, for example, in one series of experiments with mixtures of methane and air, it was observed 2 that under a pressure of 40 mm. of mercury, 6 per cent, of the gas combined, whereas under 61 mm. 30 per cent, combined. 

Gas Grills Using outdoor grills is a popular way to cook. Gas grills burn either natural gas or propane that is mixed with air. The initial spark is provided by a grill starter. Propane is more commonly used for fuel because it can be supplied in liquid form in a portable tank. Combustion of liquid propane also releases more energy than natural gas. 

The initial spark for proline catalysis was provided independently and simultaneously by two groups in 1971. Hajos and Parrish on the one hand (Scheme 5.1), and Eder, Sauer and Wiechert (Scheme 5.2) on the other developed an asymmetric aldol cyclisation of triketones such as 1 to bicyclic allq l ketones 2. In the former report, (S)-proline was applied at 3 mol%, a low organocatalyst loading, even to date. The quantitative cyclisation reaction was completed in the reasonable time of 20 h, and provided the product in 93.4% ee. Dehydration to enone 3 completed the synthesis of a valuable building block in steroid chemistry. 

In spite of the above logical rationalization, the solution to the problem of purine synthesis came in a very roundabout way. You may say by serendipity. The initial spark emerged from a discussion between G. E. Hutchinson, from Yale, P. Abelson and myself (during the 1st International Oceanographic Congress held in New York in 1959) on the nature of the polymer formed by electrical discharges in Miller s experiment. Because of... 

Another condition to keep gasoline engines running smoothly is that the fuel-air mixture start burning at a precise time in the combustion cycle. An electrical spark starts the ignition. The remainder of the fuel-air mix should be consumed by a flame front moving out from the initial spark. 

It is not yet clear whether biotin is a cofactor no biotin was detected in highly purified preparations of the enzyme. Malate easily converts to oxaloacetate. The formation of oxaloacetate by one of these two pathways is significant, because it provides the initial spark for the citrate cycle, since oxaloacetate is needed as partner for the condensation reaction with acetyl-CoA. Furthermore, oxaloacetate mediates the resynthesis of glucose (see below). It should be pointed out, finally, that pyruvate can be transformed to alanine (Chapt. VIII-10) by transamination so that pyruvate also represents a link to the metabolism of protein. 

The rupture process of a soap film is of some interest. In the case of a film spanning a frame, as in Fig. XIV-15, it is known that rupture tends to originate at the margin, as shown in the classic studies of Mysels [207, 211]. Rupture away from a border may occur spontaneously but is usually studied by using a spark [212] as a trigger (a-radia-tion will also initiate rupture [213]). An aureole or ridge of accumulated material may be seen on the rim of the growing hole [212, 214] (see also Refs. 215, 216). Theoretical analysis has been in the form of nucleation [217, 218] or thin-film instability [219]. 

By use of a piezoelectric device, as in a gas lighter, a small spark containing electrons and ions can be produced. If the spark is introduced into the gas in a discharge tube, it will provide the extra initial electrons and ions needed to start a continuous discharge. A plasma torch is frequently lit (started) in this fashion. 

This region is often referred to as the Townsend breakdown region, in which — with little or no further change in voltage — the current can rise by several orders of magnitude, e.g., from Kh to 10" A. There is usually a spark produced during the initiation of this process. The current flow is controlled by the size of the resistance in the external voltage circuit. 

All methods of plasma production require some electrons to be present as electric-discharge initiators. For a plasma torch, the initiating electrons are introduced from a piezoelectric spark directed into argon gas flowing in the interval between two concentric quartz tubes. 

Flammability and Explosivity. Ozone is endothermic, thus it can bum or detonate by itself and represents the simplest combustible and explosive system. The concentration threshold for spark-initiated explosion ofHquid ozone in oxygen at —183° is 18.6 mol % O the concentration limit... 

The ignition temperature or autoignition temperature is the minimum temperature of a flammable mixture that is required to initiate or cause self-sustained combustion without ignition from an external source of energy such as a spark or flame (ASTM D2155). 

A minimum volatihty is frequently specified to assure adequate vaporization under low temperature conditions. It can be defined either by a vapor pressure measurement or by initial distillation temperature limits. Vaporization promotes engine start-up. Fuel vapor pressure assumes an important role particularly at low temperature. For example, if fuel has cooled to —40°C, as at arctic bases, the amount of vapor produced is well below the lean flammabihty limit. In this case a spark igniter must vaporize enough fuel droplets to initiate combustion. Start-up under the extreme temperature conditions of the arctic is a major constraint in converting the Air Force from volatile JP-4 to kerosene-type JP-8, the military counterpart of commercial Jet Al. 

For a point spark source, the flame volume is initially spherical and the critical ignition volume is determined by calculating the rate of change of flame volume with respect to radius compared to the rate of change of volume of the combustion products (eq. 

For a line spark source, the flame volume is initially cylindrical with the cylinder length equal to the separation distance between the electrodes. Thus, for a cylindrical flame, = e, and the critical ignition volumes are equation 7 for a spherical flame and equation 8 for a cylindrical flame where = critical ignition volume, m /kg e = thickness of flame front, m and d = flame height, m. 

The autoignition temperature is the minimum temperature for a substance to initiate self-combustion in air in the absence of a spark or flame. The temperature is no lower than and is generahy considerably higher than the temperature corresponding to the upper flammabihty limit. Large differences can occur in reported values determined by different procedures. The lowest reasonable value should be accepted in order to assure safety. Values are also sometimes given in oxygen rather than in air. 

Rapid reinsertion of the capacitors as soon as the fault coiiditions are removed is an important requirement to retain the stability of the system. This can be achieved with the use of an additional ZnO. non-linear resistance (ZnO being the latest in this field compared to SiC. which was used earlier), across the capacitor banks (Figure 26.12). Generally, the ZnO resistor will be adequate to dampen the fault current without initiating the spark gap. and w ill limit the overvoltage across the capacitors. It will also permit automatic reinsertion of the capacitors as soon as the fault conditions are removed without causing a delay. The spark gap will serve as a backup to the ZnO resistor in the event of very severe faults. 

---