| OCR Text |
Show 5.9.9 Therefore the rate of heat transfer to the 125p droplets will be at least 2.4 times as great as the 200u droplets. Due to the smaller diameter and volume, the droplets will be vaporized at an even higher rate, and the fact that there are almost 14 times the number, more turbulence will be generated with flames initiated at their surfaces. Note on Fig. 3, 150 M sec. vs. 30 M sec. which is 5 times more. With high viscosity materials, the nozzle orifices have to be made larger to nunimize pressure drop, erosion and blockage. Therefore it is important that nozzle designs allow a shearing action of the liquid to break it into many smaller diameter particles. Most nozzles designed to atomize viscous liquids use a pneumatic fluid (steam or compressed air) to properly break up the stream into droplets which can be carried into the combustion zone. (Fig. 4,5) TURBULENCE Proper mixing of the ccmbustion air (oxygen) with the liquid droplets is necessary. As the liquid is vaporized and superheated to ignition temperature, the oxygen reacts with the hydrocarbon vapor to allow release of the energy available. As this occurs, there is a sudden rise in temperature caused by the oxidation reaction and the release of energy. An increase in the gas velocity occurs in the zones surrounding the droplet, permitting increased mixing and finalizes the oxidation reaction (see Fig. 6.) With low boiling hydrocarbons, this reaction occurs rapidly at the initial boiling point temperature and the rapid rise to reaction temperatures (1500°F to 3000°F) will provide the necessary heat sink to the incoming colder liquid droplets. As the viscosity of the liquid increases, droplet sizes tend to get larger, and the 90% boiling point of the hydrocarbon also is at a much higher level. In order to completely vaporize and superheat the droplet, more time is needed to reach this stage. Increased turbulence created by high intensity burners permits this reaction to be achieved rapidly. |
