A special case of heat flow arises when the temperatures through the thickness of a solid body are changing as heat is added or removed. This non-steady or transient heat flow will occur, for example, when a thick slab of meat is to be cooled, or when sunlight strikes on a roof and heats the surface. When this hap­pens, some of the heat changes the temperature of the first layer of the solid, and the remaining heat passes on to the next layer, and so on. Calculations for heating or cooling times of thick solids consider the slab as a number of finite layers, each of which is both conducting and absorbing heat over successive periods of time. Original methods of solving transient heat flow were graphical, but could not easily take into account any change in the conductivity or specific heat capacity or any latent heat of the solid as the temperature changed.

Complicated problems of transient heat flow can be resolved by computer. Typical time-temperature curves for non-steady cooling are shown in Figures 16.1 and 16.3, and the subject is met again in Section 23.2.

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