Rich mixtures

In a general manner, diesel engines, jet engines, and domestic or industrial burners operate with lean mixtures and their performance is relatively insensitive to the equivalence ratio. On the other hand, gasoline engines require a fuel-air ratio close to the stoichiometric. Indeed, a too-rich mixture leads to an excessive exhaust pollution from CO emissions and unburned hydrocarbons whereas a too-lean mixture produces unstable combustion (reduced driveability and misfiring). 

Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

You may have noticed that most soft drinks contain high fructose corn syrup Corn starch is hy drolyzed to glucose which is then treated with glu cose isomeraseto produce a fructose rich mixture The... 

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

The behavior of rich mixtures is compHcated by the entrainment of air at the burner port that sustains combustion of hot combustion products of the primary flame near the port. The blowoff velocity is found to increase continuously with ( ), or richer mixtures are more stable with respect to blowoff. They also have a lesser tendency toward flashback. Hence, a Bunsen flame has more latitude for stable operation if the primary mixture is rich. For this... 

Hydrocarbons and carbon monoxide emissions can be minimised by lean air/fuel mixtures (Fig. 2), but lean air/fuel mixtures maximize NO emissions. Very lean mixtures (>20 air/fuel) result in reduced CO and NO, but in increased HC emissions owing to unstable combustion. The turning point is known as the lean limit. Improvements in lean-bum engines extend the lean limit. Rich mixtures, which contain excess fuel and insufficient air, produce high HC and CO concentrations in the exhaust. Very rich mixtures are typically used for small air-cooled engines, needed because of the cooling effect of the gasoline as it vaporizes in the cylinder, where CO exhaust concentrations are 4 to 5% or more. 

The firing controls that best ensure an air-rich mixture are often referred to as metering type controls, because gas flow and air flow are metered, thus the fuel-air ratio is controlled. The fuel-air ratio is the most important factor for safe, economical firing, so it is better to control it directly. Do not settle for low budget controllers that... 

Flare A device used to burn rich mixtures of combustible waste gases containing pollutants. 

Tests are needed to determine the effects of multicomponent lean and rich mixtures on the performance of deflagration and detonation flame arresters. Combustion of lean mixtures can result in spin and galloping detonations which have more focused and higher pressures, and thus are of greater concern with respect to the structural integrity of flame arresters and other pipeline devices (e.g., fast-closing valves). Lean mixtures are more prevalent than stoichiometric mixtures in most manifolded vent systems. 

Rich mixture A mixture of flammable gas or vapor and air in which the fuel concentration is above the fuel s upper limit of flammability (UFL). 

This will generally be tr-ue as we proceed to look at other alkanes as the number of carbon atoms increases, so does the boiling point. All the alkanes with four car bons or less are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. We are all faniliar- with propane tanks. These are steel containers in which a propane-rich mixture of hydrocar bons called liquefied petroleum gas (LEG) is maintained in a liquid state under high pressure as a convenient clean-burning fuel. 

The efficiency of the three-way catalytic converter is also a function of air/fuel ratio. At the stoichiometric air/fiiel ratio of 14.7 kilograms of air per kilogram of fuel, the relative air/fuel ratio known as X equals 1.0. Figure 1 illustrates catalytic converter efficiency for each pollutant as a function of relative air/fuel ratio X (where a positive X indicates a lean mixture and a negative X indicates a rich mixture). The closer the mixture stays to stoichiometric, the more efficient the catalyst at reducing the combined emissions of the three pollutants. 

Such tight mixture control is beyond the capability of the traditional carburetor. Consequently, after sorting through a number of alternatives, industry has settled on closed-loop-controlled port-fuel injection. Typically, an electronically controlled fuel injector is mounted in the intake port to each cylinder. A sensor in the air intake system tells an onboard computer what the airdow rate is, and the computer tells the fuel injectors how much fuel to inject for a stoichiometric ratio. An oxygen sensor checks the oxygen content in the exliaust stream and tells the computer to make a correction if the air/fuel ratio has drifted outside the desired range. This closed-loop control avoids unnecessary use ot an inefficient rich mixture during vehicle cruise. 

The Ce-Cg aromatic hydrocarbons—though present in crude oil—are generally so low in concentration that it is not technically or economically feasible to separate them. However, an aromatic-rich mixture can be obtained from catalytic reforming and cracking processes, which can be further extracted to obtain the required aromatics for petrochemical use. Liquefied petroleum gases (C3-C4) from natural gas and refinery gas streams can also be catalytically converted into a liquid hydrocarbon mixture rich in C6-C8 aromatics. 

When mixed with air, LPG can form a flammable mixture. The flammable range at ambient temperature and pressure extends between approximately 2 per cent of the vapor in air at its lower limit and approximately 10 per cent of the vapor in air at its upper limit. Outside this range, any mixture is either too weak or too rich to propagate flame. However, over-rich mixtures resulting from accidental releases can become hazardous when diluted with air. At pressures greater than atmospheric, the upper limit of flammability is increased but the increase with pressure is not linear. 

When an engine idles or mns with an over-rich mixture the combustion process is imperfect and soot will... 

Another liquid contaminant is unburned fuel. A poor-quality fuel, for example, may contain high boiling point constituents that will not all burn off in the combustion process and will drain into the sump. The practice of adding kerosene to fuel to facilitate easy starting in very cold weather will eventually cause severe dilution of the lubricating oil. Excessive use of over-rich mixture in cold weather will mean that all the fuel is not burnt because of the lack of oxygen and again, some remains to drain into the sump. 

R.22 Separates into oil-rich mixture at top and refrigerant-rich mixture at bottom Fully miscible 1177... 

The strict control of oxides of nitrogen required for 1976 cars can be partially met by operating the engine with rich mixtures, and by using spark retardation and exhaust gas recirculation (EGR) to reduce the peak... 

It can also be noted that the slope of the S , .,f-Ka plot reflects the combined effect of stretch rate and non-equidiffusion on the flame speed. Figure 4.1.6 clearly shows that the flame response with stretch rate variation differs for lean and rich mixtures. In particular, as Ka increases, the S for stoichiometric and rich mixtures increases, but decreases for the mixture of equivalence ratio = 0.7. This is because the effective Lewis numbers of lean w-heptane/ air and lean /so-octane/air flames are... 

In all the mixtures, the flame speed increased almost linearly with an increase in the maximum tangential velocity. The value of the slope in the Vf-y(,max plane was almost unity for the stoichiometric mixtures, however, the slope became smaller for the lean and rich mixtures. The flame to the core diameter ratio decreased with the increasing Vg The ratio was around unity in the stoichiometric mixtures, while it was smaller than unity in the lean and rich mixtures. 

The effect of natural gravity on flammability limits has been known for a long time. The difference between flammability limits for downward and upward flame propagation was first observed by White [26], for hydrogen/air mixtures. Subsequently, similar effects were also found for other mixtures. For propane flames, the lean flammability limit for both downward and upward propagation was observed to be = 0.53. The rich limits were = 1.64 for downward and = 2.62 for upward propagation. Such wide gap between the flammability limits for rich mixtures is explained in... 

The radius of curvature of flame is shown in Figure 6.1.7 as a function of the quenching distance (Figure 6.1.7a) and of the equivalence ratio (Figure 6.1.7b). The radius was determined from the flame pictures. For lean mixtures, the radius increases linearly with the channel width, both for the downward and upward propagating flames. For rich mixtures and downward propagation, the increase is linear for quenching distances up to Dq = 7 mm, but the increase is not as steep as that of lean mixtures. However, the increase accelerates. For rich... 

Figure 2.5 Dynamics of oxygen chemisorption at Cu(110) and Mg(0001) surfaces based on XPS and HREELS leading to oxidation of ammonia and imide chemisorbed species. The reactive oxygen is a transient state Cr(s). With ammonia-rich mixture Pathway 2 is dominant whereas for oxygen-rich mixtures Pathway 1 dominates at 295 K.

If ignition of fuel-rich mixture occurs, the release will bum as a fireball. Burning will occur primarily in the outer layer of the fuel-rich cloud. As the buoyancy of the hot gases increases, the burning cloud rises, expands, and assumes a spherical shape. Damage is again caused by direct flame contact and radiant heat. 

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