The thermal response of an enclosure to periodic excitation: The CIBSE approach: Fid-Bau Portal

The thermal response of an enclosure to periodic excitation: The CIBSE approach

Davies, M.G.

AbstractIn order to relate the temperatures and heat flows at the surface of a slab when it undergoes sinusoidal excitation, five quantities (conductivity, specific heat, density, thickness and driving period) have to be specified. They can however be reduced to just two groups, (i) the characteristic admittance, a, which has the usual units of thermal transmittance (W/m²K) but also takes account of the fact that heat flows and temperatures are not in phase, and (ii) the cyclic thickness, τ, a non-dimensional form of the thickness of the slab. The slab transmission matrix can be expressed in terms of a and τ, and the overall transmission matrix for a wall, including films, is formed by multiplying the slab matrices. From this, the wall admittance and cyclic transmittance can be found. An example is given. Equations are then developed for the response of an enclosure when subjected to periodic excitation. It is shown that the response of a wall can be expressed exactly in thermal circuit terms, either as a pair of lumped elements (units W/K) (to express admittance), or as three pairs of lumped elements to express two admittances and the cyclic transmittance. The relations between various temperature and heat flows in an enclosure can be shown graphically in the form of vector diagrams. The special status of ventilation in estimating room response is mentioned. These considerations serve to support methods currently advanced in the CIBSE Guide to estimate periodic changes in room temperature.