U factors
U factors are an import tool for evaluating the thermal performance of a particular surface. Suppose we want to evaluate the amount of heat passing through the area highlighted in blue in the figure below:
We define the U-factor of this surface as:
U-f = F / ΔT / L [W/m²K]
Where:
F: heat flux passing through the surface;
ΔT: temperature difference of the project; there must be only two temperatures among the various boundary conditions used in the project, otherwise the software will not be able to identify this value;
L: reference length. Particular attention must be paid to that parameter, which can be determined in different ways:
automatic: L will be the maximum between the projection of the surface along the two coordinate axes (in this case A);
projected X: projection of the surface along the X axis (A);
projected Y: projection of the surface along the Y axis (B);
custom rotation: projection of the surface along an axis rotated counterclockwise from the X axis;
The surfaces along which calculate the factors U must be identified in two ways:
In both cases (material or boundary condition), you can choose a "U-factor surface"; alongside this choice there is a button ">>" that will give you access to the U factors management window. In this window you can set be the various parameters of the U factors:
name;
factors group (not available in Frame Simulator): all the U factors with the same "factors group" will be grouped and used to determine a separate ψ (see below).
U-factors and linear thermal transmittance computation (not available in Frame Simulator)
For each group of U factors will be calculated a different linear thermal transmittance ψ. With reference to the following figure:
There
are four internal boundary conditions (A, B, C, D) and an external
one (E). A and B belong to two different U factor surfaces but they
are part of the same group of U factors. The same applies to C and D
boundary conditions. In this way, the software will calculate two
different ψ for the two groups of U factors; please note that the
sum of U factors' ψ is very similar to the global ψ value
(the small difference is due to the fact that in the calculation of
the global ψ approximations are reduced analyzing the outer
boundary).
As can be seen in the figure, two different section
elements have been created: one for the upper part and one for the
lower one. It is very important that the horizontal line of the two
section elements intersects a boundary of the respective group of U
factors. The project shown in the figure can be loaded from the
examples folder (file
example11.mos).