Advanced Building Simulation

92 Hensen
Table 4.1 Summary of prediction potential (�� � none, �� � very good) for airflow modeling
levels in the context of displacement ventilation system
Aspect A B C
Cooling electricity �� �� ��
Fan capacity �� �� ��
Whole body thermal comfort � �� �
Local discomfort, gradient �� � ��
Local discomfort, turbulence �� �� ��
intensity
Ventilation efficiency �� 0 ��
Contaminant distribution � � ��
Whole building integration �� �� ��
Integration over time �� �� ��
Note
A � fully mixed zones; B � zonal method; C � CFD (Hensen et al. 1996).
thermal counterpart. This means that the information demands of the energy conservation
formulations can be directly satisfied. Second, the technique can be readily
applied to combined multi-zone buildings and multi-component, multi-network plant
systems. Finally, the number of nodes involved will be considerably smaller than that
required in a CFD approach and so the additional CPU burden is minimized. The
remainder of this chapter will focus on the zonal method.
4.2 Zonal modeling of building airflow
This approach is known under different names such as zonal approach, mass balance
network, nodal network, etc., and has successfully been implemented in several software
packages such as CONTAMW, COMIS, and ESP-r. The method is not limited
to building airflow but can also be used for other building-related fluid flow
phenomena such as flow of water in the heating system, etc.
In this approach, during each simulation time step, the problem is constrained
to the steady flow (possibly bidirectional) of an incompressible fluid along the connections
which represent the building and plant mass flow paths network when
subjected to certain boundary conditions regarding pressure and/or flow. The problem
reduces therefore to the calculation of fluid flow through these connections with
the nodes of the network representing certain pressures. This is achieved by an iterative
mass balance approach in which the unknown nodal pressures are adjusted
until the mass residual of each internal node satisfies some user-specified criterion.
Information on potential mass flows is given by a user in terms of node descriptions,
fluid types, flow component types, interconnections, and boundary conditions.
In this way a nodal network of connecting resistances is constructed. This may then
be attached, at its boundaries, to known pressures or to pressure coefficient sets that
represent the relationship between free-stream wind vectors and the building external
surface pressures that result from them. The flow network may consist of several
decoupled subnetworks and is not restricted to one type of fluid. However, all nodes
and components within a subnetwork must relate to the same fluid type.