Document v686pVKMZBaN4x83qY2MmnQMm

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 Contrasted to the thermal conduction mechanism, thermal convection involves, energy transfer by eddy mixing and diffusion2 in addition to conduction. This condition is pictured schematically in Fig. 1 which exhibits transfer from a pipe wall at surface temperature , to a colder fluid at a bulk temperature h. In the laminar sublayer, immediately adjacent to the wall, the heat transfer occurs by thermal conduction; in the transition region, which is called the buffer layer, eddy mixing as well as conduction effects are significant; in the eddy or turbulent region the major fraction of the transfer occurs by eddy mixing. In most commercial equipment the main body of the fluid is in turbu lent flow, and the laminar film exists at the solid walls only, as shown in Fig. 1. But in cases of low-velocity flow in small tubes, or witta viscous liquids such as heavy oil (low Reynolds' numbers),- the entire flow may be laminar. In these latter cases there.is no transition or eddy region. When the fluid currents are induced by sources external to the heat transfer region, as for example, a pump, the described solid to fluid heat CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER square foot, is proportional to the temperature gradient (dt)/(dx), degree Fahrenheit per foot. The proportionality factor is termed the thermal conductivity, k, Btu per hour per square foot per degree Fahrenheit per foot of thickness. The minus sign on the right side of the equation is introduced to indicate positive current flow in the direction of decreasing temperature. The physical significance of indicated quantities is illustrated further by the schematic diagram Fig. 2. ..................... It should be emphasized that the thermal conductivity used should be expressed in consistent units; either using the inch or foot throughout. Expressions of conductivity used in the heating field are usually, inconsistent in this sense, in that it is customary to refer to the con ductivity per square foot but for one inch of thickness. This custom has Fig. 1. Thermal Convection Conditions transfer is termed forced convection. In contrast, if the fluid currents are internally generated, as a result of non-homogeneous densities arising from the temperature variations, the heat transfer is termedfree convection. In the conduction and convection mechanisms heat is transferred as internal energy, i.e., the random molecular kinetic energy associated with the material temperature. For radiant heat transfer, however, a change in energy form takes place from internal energy.at the source to electro magnetic energy for transmission, then back to internal energy at the receiver. Since visible radiant energy exhibits characteristic wave lengths, the solution of thermal radiation problems is in many respects similar to the solution of problems in the field of illumination. The rate of thermal current flow (i.e., rate of heat transfer) corre sponding to the transfer mechanisms previously described, may be expressed by three rate equations. These are similar to Ohm's Law for electrical flow, the current flow through a resistance being proportional to the potential difference. Thermal Conduction Equation Equation 1 states symbolically that the thermal conduction current per unit transfer area normal to the flow, (dq)/(dA), Btu per hour per 'Absorption and Extraction, by T. K: Sherwood (McGraw-Hill Co.. 1937). 72 Fig. 2. Thermal Conduction in a Flat Slab been adopted for the reason that wall thicknesses are usually expressed in inches, whereas if expressed in feet, decimal or fractional thicknesses would result. When dealing with flat walls no complication is involved in using the inconsistent expression of conductivity. However, when curved or spherical walls are considered, considerable complication is involved. Therefore, in this discussion the consistent units of con ductivity expressed in Btu per hour per square foot per .degree Fahrenheit for onefoot of thickness are used throughout. Conductivity values obtained from Chapter 4 or Table 1 in this chapter, which are expressed in incon sistent units, must therefore be converted for use in the calculations of this chapter by dividing by 12. As an example, the conductivity of brick, expressed in inconsistent units as 5.0 in' Table 2 of Chapter 4, becomes 0.42 when used in the calculations of this chapter. Also, it should be. emphasized that in order to make the calculations and applications con sistent in this chapter, all dimensions of thickness must be expressed in feet. Thermal Convection Equation ^ Os -if) (2) This rate equation states that the thermal convection current per unit transfer area (dq)/(dA), Btu per hour per square foot, is proportional to 73