Conduction Through Solids in Series

3 CONDUCTION THROUGH SOLIDS IN SERIES 4.3A Plane Walls in Series 

In the case where there is a multilayer wall of more than one material present as shown in Fig. 4.3-1, we proceed as follows. The temperature profiles in the three materials A, B, and C are shown. Since the heat flow g must be the same in each layer, we can write Fourier s equation for each layer as 

the final equation is in terms of the overall temperature drop T, — 7 and the total resistance, -I- Rg -b.  

EXAMPLE 43-1. Heat Flow Through an Insulated fVall of a Cold Room A cold-storage room is constructed of an inner layer of 12.7 mm of pine, a middle layer of 101.6 mm of cork board, and an outer layer of 76.2 mm of concrete. The wall surface temperature is 255.4 K inside the cold room and 297.1 K at the outside surface of the concrete. Use conductivities from Appendix A.3 for pine, 0.151 for cork board, 0.0433 and for concrete, 0.762 W/m K. Calculate the heat loss in W for 1 and the temperature at the interface between the wood and cork board. 

Since the answer is negative, heat flows in from the outside. 

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Conduction through Several Bodies in Series Figrrre 5-1 ilhrstrates diagrammatically the temperatrrre gradients accompanying the steady condrrc4ion of heat in series throrr three solids. 

FIG. 5-1 Temperature gradients for steady heat conduction in series through three solids. 

In many applications of heat transfer two fluids at different temperatures are separated by a solid wall. Heat is transferred from the fluid at the higher temperature to the wall, conducted through the wall, and then finally transferred from the cold side of the wall into the fluid at the lower temperature. This series of convective and conductive heat transfer processes is known as overall heat transfer. 

Vibrations in solid or semi-solid material, or movement in liquid, can produce a series of minute changes in the pressure of the air which is in contact with the material. When this series of small air pressure variations reaches the ear and triggers a message in the auditory nerve, the brain registers this as sound. Sound waves can also travel as vibrations through solid materials such as steel, and Uquid materials such as water. We can hear the sound in steel if we put part of our skull in contact with the steel. This is known as bone conduction of the sound. Some of the pressure waves reaching us through the air can also travel to the inner ear by bone conduction rather than up the ear canal. 

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