HVAC

As part of our continuing series on topics in the FDS 6 release notes, this blog post will focus on HVAC features in FDS 6. 

Through version 5.5.2, you could only specify pre-defined boundary conditions for inlet and outlet flows (i.e. temperature, species, and velocity/mass flow were explicitly defined in the input file).  There are a number of situations where these standard FDS inputs were not sufficient to model the behavior of a building: smoke can move through ducts to remote compartments and reduce their visibility, heating and cooling systems will turn on and off as temperatures change which will impact the movement and temperature of smoke, facilities that must maintain negative pressures can lose that ability as filters clog with soot, and in a multicompartment ventilation system the pressurization of a compartment due to a fire will change the flow rates in the system.  In addition to limitations in the current inputs, the FDS leakage model had stability issues with large leakage areas or complicated arrangements of leakage paths.

To address these limitations and to generally improve the ability of FDS to model buildings, an HVAC submodel was developed.  The first version of the model was released in FDS 5.5.3.  This model was limited in its functionality and did not work with MPI versions of FDS.  Development has since continued, and FDS 6 contains an improved HVAC model with additional functionality including being compatible with the MPI versions of FDS.

With a CFD code there are a couple of approaches one could take to model HVAC systems.  One could model a system by modeling each duct using the CFD solver.  This approach would be very costly given the number of grid cells that would be required to correctly resolve the flows and pressure drops in an HVAC system. Additionally, one would have to undergo the time consuming process of verifying that one was obtaining the correct pressure drops throughout the system.  A second approach is to use a specialized HVAC solver that treats the HVAC system in a simplified manner and couples it to the CFD solver.  This second approach was used.

The HVAC model in FDS 6 is a network HVAC model based on the solver found in MELCOR (a United States Nuclear Regulatory Commission code for analyzing containment buildings).  In brief, this model treats an HVAC system as a collection of nodes and junctions.  A junction would represent a duct, and a node would represent where two or more ducts connect or where a duct is connected to the remainder of  the FDS domain.  The model solves equations for the conservation of mass, energy, and momentum of the HVAC system.  For each junction, a velocity is predicted and for each node a pressure, temperature, and set of species mass fractions is predicted.  The particular solution method currently used does not account for transport delays in a duct.  That is, whatever mass and energy enters the system during a time step, also leaves the system in that same time step.  Multiple HVAC systems can be defined in one input file and the solver will attempt to identify independent systems (e.g. systems that do not have inlets or outlets in the same pressure zone) in order to reduce the computational cost.

The HVAC solver is not directly coupled to the FDS solvers for pressure and species transport.  Rather the HVAC solver uses the prior time step conditions to determine the boundary conditions at each inlet and outlet to the HVAC system and the solver returns a new set of boundary conditions that FDS then uses when computing the next time step.  For simulations with pressure zones, the HVAC solver uses the prior time step rate of zone pressure change to estimate the new end of time step pressure. Provided that the FDS solution does not vary greatly between time steps (so the HVAC solver estimated pressure is close to the actual pressure determined by FDS), this coupling approach is stable.  In general this is the case, FDS time steps are typically small enough that the pressure rise between time steps is small.

With the HVAC model one can define the follow components:

• Ducts with forward and reverse flow losses (ASHRAE and other handbooks contain tables of flow loss data for various types of ducts)
• Nodes  (e.g. tees, inlet and outlet vents, plenums, etc.) with flow direction dependent losses (as with ducts, values can be found in various handbooks).
• Fans with three fan models: constant flow, quadratic, and user defined.  The quadratic and user defined models will change the fan flow rate based on the inlet and outlet pressure of the fan.  This would allow, for example, FDS to reduce the flow into a compartment where a growing fire causes a pressure rise that a fan would have to work harder at to overcome.
• Dampers (currently only fully open or fully closed)
• Filters with the ability to define different removal efficiencies for different species as well as the impact of filter loading on the pressure drop across the filter
• Heating / cooling coils with either a fixed amount of heat exchange or an amount computed with a simple heat exchanger efficiency model

The HVAC model has resulted in changes to two other features of FDS:

1. The POROUS surface input has been removed.  This input was used to allow one to specify a fan in the computational domain that transferred species across it.  This is now handled by the HVAC model.  See the jet_fan.fds example case in the Flowfields subfolder in the FDS 6 Examples folder.
2. Leakage is now handled by the HVAC model.  A leak is merely some small gap, hole, etc. that allows flow from one room to another.  From a model point of view, this can be considered as a tiny duct that connects two compartments.  By using the HVAC model rather than the prior leakage model, instabilities due to large leakage areas or complex leakage paths are avoided.  Additionally, the prior leakage model could only transfer species if the compartments were separated by a POROUS surface otherwise ambient air was moved by the leakage.  By using the HVAC model, this limitation no longer exists as the HVAC model will compute the species mass transfer.  See the leak_test.fds example case in the HVAC subfolder in the FDS 6 Examples folder.

It is recommended that if you are using the HVAC model that you review the various examples in the FDS User's and Verification Guide; the inputs for which can be found in the HVAC subfolder of the Examples folder where FDS 6 is installed.