Heat transfer mechanisms radiation

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

Also, the pyrolyser construction plays an important role in the rate of attaining the Teq temperature. This construction may determine the way in which heat is transferred to the sample. This transfer process can be understood by evaluating the heat transfer mechanisms, which are conduction, convection, and radiation. The heat transfer rate q in J/s = W (watts) by unidirectional conduction for a small element of a material having the area A and thickness dx is given by Fourier s law ... 

Faeth, G. M., Gore, J. R, Chuech, S. G., and Jeng, S.-M. "Radiation from Turbulent Diffusion Flames." Annual Review of Fluid Mechanics Heat Transfer 2 (1989) 1-38. 

Heat and mass are transferred between a solid surface and a fluid by similar mechanisms heat transfer by radiation is not considered unless reactions occur well above 400 °C in industrial reactors rim with pellet sizes larger than 6 mm [7, 9]. For present purposes, the addition of radiation effects is not necessary. 

The basic heat transfer mechanisms are conduction, convection, and radiation. The heat transfer can be achieved through direct or indirect contact of streams. It can happen between two streams or multiple streams, (n this chapter, we will concentrate on heat transfer through conduction and convection, indirect conlBct, and between two flow streams. Direct contact heat transfer equipment such as cooling tower and heat transfer by radiation... 

The heat transfer in fluidized beds of monodisperse particle has been extensively investigated in the past. Heat transfer in a packed/fluidized bed with an interstitial fluid may involve many mechanisms as shown in Fig. 1 (Yagi and Kunii, 1957). These mechanisms can be classified into three heat transfer modes in fluidized beds fluid—particle or fluid—wall convection particle-particle or particle—wall conduction and radiation. Different heat transfer models are developed for these mechanisms, as described in the following. 

Drying temperatures may range up to 1000 K, the limiting temperature for most common structure metals. At the higher temperatures, radiation becomes an important heat-transfer mechanism. 

There will also be heat loss from tire substrate due to convection cuiTents caused by the teirrperamre differential in the suiTounding gas phase, but this will usually be less than the radiation loss, because of the low value of the heat transfer coefficient, / , of gases. The heat loss by this mechanism, Qc, can be calculated, approximately, by using tire Richardson-Coulson equation... 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

Many everyday heat flows, such as those through windows and walls, involve all three heat transfer mechanisms—conduction, convection, and radiation. In these situations, engineers often approximate the calculation of these heat flows using the concept of R values, or resistance to heat flow. The R value combines the effects of all three mechanisms into a single coefficient. 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

In many of the applications of heat transfer in process plants, one or more of the mechanisms of heat transfer may be involved. In the majority of heat exchangers heat passes through a series of different intervening layers before reaching the second fluid (Figure 9.1). These layers may be of different thicknesses and of different thermal conductivities. The problem of transferring heat to crude oil in the primary furnace before it enters the first distillation column may be considered as an example. The heat from the flames passes by radiation and convection to the pipes in the furnace, by conduction through the... 

The transfer of heat by radiation in general can be said to occur simultaneously with heat transfer by convection and conduction. Transfer by radiation tends to become more important than that by the other two mechanisms as the temperature increases. It is useful to gain an appreciation of the basic definitions of the energy flux terms, the surface property terms and their relationships while discussing radiative heat transfer. With this objective, reference may be made to Table 3.4 in which these are presented. 

The vial heat transfer coefficient is the sum of heat transfer coefficients for three parallel heat transfer mechanisms (1) direct conduction between glass and shelf surface at the few points of actual physical contact, Kc (2) radiation heat exchange, Kr, which has contributions from the shelf above the vial array to the top of the vials, Krt, and from the shelf upon which the vial is resting, Krb and (3) conduction via gas-surface collisions between the gas and the two surfaces, shelf and vial bottom, Kg ... 

In normal atmospheric conditions, fire usually is initialed by a combustible material coming in contact with a heat source. The spread of fire occurs due to direct flame impingement or the transfer of heat to the surrounding combustible materials. Heat transfer occurs by three principal mechanisms - conduction, convection, and radiation. Conduction is the movement of heat through a stationary medium, such as solids, liquids or gases. Steel is a good conductor of heat as is aluminum, therefore they can pass the heat of a fire if left unprotected. 

Heat transfer is always from hot to cold. The main mechanisms of heat transfer in a hydrocarbon are thermal radiation and direct flame contact (convection). Heat transfer to personnel can cause burns. Heat transfer to equipment and structures can lead to failure of hydrocarbon-containing equipment that can further feed the fire. 

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