Mixing-zone combustion

If such a process continues to accelerate, the combustion mode may suddenly change drastically. The reactive mixture just in front of the turbulent combustion zone is preconditioned for reaction by a combination of compression and of heating by turbulent mixing with combustion products. If turbulent mixing becomes too intense, the combustion reaction may quench locally. A very local, nonreacting but highly reactive mixture of reactants and hot products is the result. 

The quality and yield of carbon black depends on the quality of the feedstock, reactor design, and input variables. The structure is controlled by the addition of alkali metals to the reaction or mixing zones. Usual practice is to use aqueous solutions of alkali metal salts such as potassium chloride or potassium hydroxide sprayed into the combustion chamber or added to the make oil in the oil injector. Alkaline-earth compounds such as calcium acetate that increase the specific surface area are introduced in a similar manner. 

Toshiba, in collaboration with Tokyo Electric Power Company, has developed a hybrid catalytic combustion. Here only a part of the fuel is converted heterogeneously on the catalyst. The system consists of a pre-combustion mixing zone, a low-temperature catalyst zone, and a gas-phase combustion zone. The fuel-air mixture is controlled to maintain the temperature of the catalyst below 800 C, because the catalyst is not stable above the temperature. More fuel is added downstream to attain the final combustion temperature. The function of the catalyst is to be a source of additional "pre-heat" to support the lean, homogeneous down-stream combustion. 

When the kinetics of the combustion reactions are taken into consideration, there are also other design shapes and operating ranges for the FLOX burners. Even undesired spontaneous reactions between air and fuel can be suppressed, inasmuch as temperature in the mixing zone is kept low or flow velocities are well above flame... 

For low values of R (lower than 1), the flame is usually attached to the burner tube and stabilizes in the wake of the burner. Increasing the fuel jet velocity, usually, does not cause flame lift-off. In this configuration, the mixing and combustion characteristics are dominated by two-zone flame structures on the lee side of the burner and coherent structures evolving from the upwind side of the jet [16]. Huang and coworkers [16,44,45] further... 

A specially designed combustion tube allows the sample to be injected at much higher carrier flow rates than can be achieved with the standard combustion tube design. The sample inlet and furnace mixing zone promote complete sample oxidation. A proper combustion tube is typically constructed of quartz. 

Because of the relatively low temperatures in the fuel-air mixing zone near the flame lift-off, the experimentally observed soot formation cannot be explained in terms of fuel pyrolysis. To account for the soot observed in this zone, Dec and Coy [13] hypothesize that there is a standing fuel-rich premixed flame just upstream from the liquid phase, where the fuel vapor and air mixture reaches an equivalence ratio between two and five. Such a fuel-rich combustion zone is ideal for soot production, because the combustion products contain an abundance of excess fuel... 

All of the dry low-NO, combustors currently available for combustion turbines utilize the lean pre-mix principle of operation, which creates a homogeneous, fuel-lean mixture of fuel and air prior to combustion. This mixture is then introduced to the combustion zone of the combustion chamber at a controlled velocity sufficiently higher than the local speed of flame propagation to prevent flashback into the pre-mix zone. However, the pre-mixture velocity must be low enough to avoid blowing the whole flame downstream (Schreiber, 1991). 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

Flame Types and Their Characteristics. There are two main types of flames diffusion and premixed. In diffusion flames, the fuel and oxidant are separately introduced and the rate of the overall process is determined by the mixing rate. Examples of diffusion flames include the flames associated with candles, matches, gaseous fuel jets, oil sprays, and large fires, whether accidental or otherwise. In premixed flames, fuel and oxidant are mixed thoroughly prior to combustion. A fundamental understanding of both flame types and their stmcture involves the determination of the dimensions of the various zones in the flame and the temperature, velocity, and species concentrations throughout the system. 

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