Ventilation 1 Building performance modelling


CIBSE publications KS6 – Comfort


KS16 – How to manage overheating in buildings


Both are introductions and are available from the CIBSE bookshop at www.cibse.org/bookshop, £21 for members, and £42 for non-members.


Temperature distributions around the IESD-Fiala model compared with simplifi ed shapes for occupants showing differences in surface temperature and buoyant plume structures


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CFX solver and the IESD-Fiala model is achieved using CFX Command Language (CCL), CFX Expression Language (CEL) and Junction Box routines. CCL is used to defi ne numerical parameters for controlling the coupled simulation, while CEL functions are used to dynamically assign boundary conditions at the body surface based on data from the IESD-Fiala model. Junction Box routines, linked to specifi c events in the CFD solution cycle, are used to extract information from the CFD solver and to exchange data with the thermal comfort model. The coupled system begins by setting an initial guess


for the fl ow fi eld in the CFD model. In response to this, surface temperatures and perspiration rates are calculated for each of the 59 body parts by the IESD- Fiala model and are passed to the CFD model to be used as boundary conditions. Once the CFD solution has achieved a suffi cient degree of stability, or convergence, information about the local environment close to the body surface is extracted by a Junction Box routine and passed to the IESD-Fiala model, which determines the body’s response to these new conditions. Updated surface temperatures and perspiration rates are then returned to the CFD model and the CFD solution process resumed. This data exchange is repeated until the coupled system achieves a sufficient degree of overall convergence; that is, when changes to the body surface temperatures between consecutive data exchanges and the CFD residual values (equation errors) are suffi ciently small. This two-way data transfer is thought to be particularly


important when modelling naturally ventilated spaces where air velocities are low, because the effect that a human body has on the local environment is potentially more signifi cant than in other environments where velocities are often higher. The two illustrations (see left and page 63) show the


coupled system in use to simulate natural ventilation in a school classroom. The model comprises open


64 CIBSE Journal September 2010


windows with cross ventilation to a stack driven purely by buoyancy forces generated by the room occupants. The detailed image is taken from a simulation that was used to investigate the differences exhibited by the new coupled system alongside a more traditional, simplifi ed shape model of a human with a constant heat fl ux. Although still in progress, the work demonstrates


how a coupled model approach can lead to a more accurate prediction of the convective-radiative split at the human body surface to generate a more buoyant plume and ultimately a greater ventilation rate in buoyancy-driven natural ventilation. And if this is not enough to whet the appetite of CFD modellers, the coupled system also predicts dynamic thermal sensation using one of the world’s most respected and widely validated cybernetic models of human heat transfer and thermoregulation. The work continues and is now being enhanced


to include the effects of breathing to provide a more accurate means of investigating indoor air quality and pollutant dispersion. ●


Dr Malcolm Cook and Dr Tong Yang are based at the Department of Civil and Building Engineering, Loughborough University, UK. Dr Paul Cropper is with the Institute of Energy and Sustainable Development at De Montfort University, UK. www.lboro.ac.uk/departments/cv


Reference: Fiala D. Dynamic Simulation of Human Heat Transfer and Thermal Comfort. PhD Thesis, De Montfort University, 1998.


Acknowledgements The authors wish to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for supporting this research through grant ref. EP/C517520/2. The authors also wish to acknowledge the technical support provided by Dr Yehuda Sinai (formerly of CFX Technical Services, ANSYS Europe Ltd), Mr Chris Staples (CFX Technical Services, ANSYS Europe Ltd) and Dr Dusan Fiala (ErgonSim – Comfort Energy Effi ciency, Stuttgart, Germany).


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