Temperature in diffusion

In lean hydrogen mixtures the flame temperature in diffusive combustion is higher than in normal propagation and so, in a wide region of concentrations (from 4 to 9% H2 in a mixture of hydrogen with air), normal propagation is impossible, only diffusive combustion is possible. Our point of view is in accord with the observed properties of flame propagation in this concentration interval. 

Fuiita H, "Diffusion of Organic Vapours in Polymers above the Glass Temperature", in "Diffusion in Polymers" (see Gen Ref) (1968), Chap. 3, pp 75-106. 

FTIR analyses were done on Nicolet Nexus FTIR Raman spectrometer at room temperature in diffusion-reflectance mode (2 wt% HPC in KBr). Solid-state NMR analyses were carried out on a Bruker 400 MHz spectrometer. TGA-DSC analyses were performed on Setaram TGA-DSC 111 apparatus by heating the samples up to 600°C in N2 at heating rate of 5°C/min. The mass changes and heat flow were recorded against the temperature. HR-TEM and EDX analyses were performed using a JEOL 2010 microscope, operating at 200 kV. 

Fujita, H., Organic Vapors Above the Glass Transition Temperature, in Diffusion in Polymers, J. Crank and G. S. Park (eds.), Academic Press, London, 1968. 

Note that this method of standardizing D values makes no allowance for the possibility that a molecule may change size, shape, or solvation with changes in temperature. In the next section we shall survey the behavior of polymeric materials in an ultracentrifuge. We shall see that diffusion coefficients can be... 

The driving force in diffusion involves differences in the concentration of the diffusing substance. The molecular diffusion of a gas into a hquid is dependent on the characteristics of the gas and the hquid, the temperature of the hquid, the concentration deficit, the gas to hquid contact area, and the period of contact. Diffusion may be expressed by Pick s law (13,14) ... 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

Actual temperatures in practical flames are lower than calculated values as a result of the heat losses by radiation, thermal conduction, and diffusion. At high temperatures, dissociation of products of combustion into species such as OH, O, and H reduces the theoretical flame temperature (7). Increasing the pressure tends to suppress dissociation of the products and thus generally raises the adiabatic flame temperature (4). 

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. 

The effect of temperature on diffusivities in zeolite ciystals can be expressed in terms of the Eyring equation (see Ruthven, gen. refs.). 

The specific conductivities of molten salts are frequently represented, as a function of temperature by an AtTlrenius equation, but it is unlikely that the unit step in diffusion has a constant magnitude, as in the coiTesponding solids and the results for NaCl may be expressed, within experimental eiTor, by the alternative equations... 

Ammonium reineckate (Reineckate salt) [13573-16-5] M 345.5, m 270-273 (dec). Crystd from water, between 30° and 0°, working by artificial light. Solns of reineckate decompose slowly at room temperature in the dark and more rapidly at higher temperatures or in diffuse sunlight. 

Many stainless steels, however, are austenitic (f.c.c.) at room temperature. The most common austenitic stainless, "18/8", has a composition Fe-0.1% C, 1% Mn, 18% Cr, 8% Ni. The chromium is added, as before, to give corrosion resistance. But nickel is added as well because it stabilises austenite. The Fe-Ni phase diagram (Fig. 12.8) shows why. Adding nickel lowers the temperature of the f.c.c.-b.c.c. transformation from 914°C for pure iron to 720°C for Fe-8% Ni. In addition, the Mn, Cr and Ni slow the diffusive f.c.c.-b.c.c. transformation down by orders of magnitude. 18/8 stainless steel can therefore be cooled in air from 800°C to room temperature without transforming to b.c.c. The austenite is, of course, unstable at room temperature. Flowever, diffusion is far too slow for the metastable austenite to transform to ferrite by a diffusive mechanism. It is, of course, possible for the austenite to transform displacively to give... 

NOx emission requirements. This injeetion of steam reduees the temperature in the hot seetion, thus redueing the amount of NO produeed. When sprayed through the fuel nozzle, this steam ean impinge on the liner, thus ereating a temperature gradient, whieh ean lead to eraeks. Steam injeetion— whether it is required for NO eontrol or for extra power (5% steam by weight will produee 12% more work and inerease effieieney a few pereent)— must injeet steam into the eompressor diffuser to be safe and effeetive. This proeess will allow the steam to be fully mixed with the air before it enters the eombustor, redueing the ineidenee of liner failures due to steam injeetion. 

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