Convection in gases

Properties are evaluated at the film temperature, and it is expected that this relation would be primarily applicable to calculations for free convection in gases. However, in the absence of more specific information it may also be used for liquids. We may note that for very low values of the Grashof-Prandtl number product the Nusselt number approaches a value of 2.0. This is the value which would be obtained for pure conduction through an infinite stagnant fluid surrounding the sphere. 

The purpose of insulation is to reduce total heat transfer from a. surface, not just conduction. A vacuum totally eliminates conduction but offers zero resistance to radiation, whose magnitude can be comparable to conduction or natural convection in gases (Fig. 7—35). Thus, a vacuum is no more effective in reducing heat transfer than sealing off one of the lanes of a two-lane road is in reducing the flow of traffic on a one-way road. 

Many familiar heal transfer application. involve natural convection as the primary mechanism of heat transfer. Some examples are cooling of electronic equipment such as power transistors, TVs, and DVDs heat transfer from electric baseboard heaters or steam radiators heat transfer from the refrigeration coils and power transmission lines and heat transfer from the bodies of animals and human beings. Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces. 

For free convection in gases, h is very low, 2-20 W/m K. For forced convection, h can be many orders of magnitude higher, controlled by the flow rate and fluid being convected. The Nusselt number Nu is an important dimensionless parameter related to the thermal conductivity and length scale of heat transfer to the convection coefficient ... 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Example 7 Radiation in Gases Flue gas containing 6 percent carbon dioxide and 11 percent water vapor by volume (wet basis) flows through the convection bank of an oil tube stiU consisting of rows of 0.102-m (4-in) tubes on 0.203-m (8-in) centers, nine 7.62-m (25-ft) tubes in a row, the rows staggered to put the tubes on equilateral triangular centers. The flue gas enters at 871°C (1144 K, 1600°F) and leaves at 538°C (811 K, 1000°F). The oil flows in a countercurrent direction to the gas and rises from 316 to 427°C (600 to 800°F). Tube surface emissivity is 0.8. What is the average heat-input rate, due to gas radiation alone, per square meter of external tube area ... 

Langmuir, I., The velocity of reactions in gases moving through heated vessels and the effect of convection and diffusion, J. Am. Chem. Soc., 30, 1742-1754 (1908). 

A small amount of heat will be transferred to the tubes by convection in the radiant section, but as the superficial velocity of the gases will be low, the heat transfer coefficient will be low, around 10 Wm-2oC-1. 

Conduction is the principal mechanism of heat transfer in solids. In liquids, conduction is frequently augmented by convection as circulating currents are set up due to density differences, while in gases, because of... 

Convection involves the transfer of heat by means of a fluid, including gases and liquids. Typically, convection describes heat transfer from a solid surface to an adjacent fluid, but it can also describe the bulk movement of fluid and the associate transport of heat energy, as in the case of a hot, rising gas. Recall that there are two general types of convection forced convection and natural (free) convection. In the former, fluid is forced past an object by mechanical means, such as a pump or a fan, whereas the latter describes the free motion of fluid elements due primarily to density differences. It is common for both types of convection to occur simultaneously in what is termed mixed convection. In such instance, a modified form of Fourier s Law is applied, called Newton s Law of Cooling, where the thermal conductivity is replaced with what is called the heat transfer coefficient, h ... 

In view of the above, it is clear that if the combustion of airflow is being adjusted on the basis of the oxygen content of the stack or convective section gases (i.e., from readings taken on an oxygen analyzer with probe located in the stack or convective section), it is very likely that there will be afterburn or secondary ignition in the stack or convective section. This scenario was discussed earlier in this chapter in the section on secondary combustion and afterburn. After all, the fire is supposed to be contained within the firebox, and not in the convec-... 

In gases and liquids, the rates of these diffusion processes can often be accelerated by convective flow. For example, the copper sulfate in the tall bottle can be completely mixed in a few minutes if the solution is stirred. This accelerated mixing is not due to diffusion alone, but to a combination of diffusion and convection. Diffusion still depends on the random molecular motions that take place over small molecular distances. The convective stirring is not a molecular process, but a macroscopic process which moves portions of the fluid over longer distances. After this macroscopic motion, diffusion mixes the newly adjacent portions of the fluids. 

The heat transfer to the immersed tubes is by a combination of radiation, convection from gases flowing past the tube, and transfer from particles in the vicinity of the tube surface. 

Convection. Heat is transferred by moving fluids and therefore only occurs in gases and liquids. A moving fluid takes thermal energy with it, and when it contacts another fluid or a surface at a different temperature, heat is transferred. There are two types of convection ... 

When, however, the diameter is increased above 10 cm., the speed of the flames is affected by the coming into play of another factor, namely, convection. This is noticeable with the fastest moving flames in tubes 10 cm. m diameter, the visible effect being a turbulence of the flame front. This is essentially a swirling motion in a direction nearly normal to the direction of translation of the flame front, which, as in tubes of smaller diameter, progresses at a uniform speed for about 150 cm. before backward and forward vibrations are set up. This swirling motion appears ab initio, and is due to rapid movement of the hot gases from below upwards by convection. In tubes of comparatively small diameter (5 to 9 cm.) this rapid movement is suppressed. 

Churchill, S. W., and M. Bernstein A Correlating Equation for Forced Convection from Gases and Liquids to a Circular Cylinder in Crossflow, J. Heat Transfer, vol. 99, pp. 300-306, 1977. 

Mass transfer can result from several different phenomena. There is a mass transfer associated with convection in that mass is transported from one place to another in the flow system. This type of mass transfer occurs on a macroscopic level and is usually treated in the subject of fluid mechanics. When a mixture of gases or liquids is contained such that there exists a concentration gradient of one or more of the constituents across the system, there will be a mass transfer on a microscopic level as the result of diffusion from regions of high concentration to regions of low concentration. In this chapter we are primarily concerned with some of the simple relations which may be used to calculate mass diffusion and their relation to heat transfer. Nevertheless, one must remember that the general subject of mass transfer encompasses both mass diffusion on a molecular scale and the bulk mass transport, which may result from a convection process. 

The value of the critical radius r, is the largest when k is large and h is smalF. Noting that the lowe.st value of h encountered in practice is about 5 W/m C for the case of natural convection of gases, and that the thermal conductivity of common insulating materials is about 0.05 W/m °C, the largest value of the critical radius we are likely to encounter is... 

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