James W. Axley

Multizone Airflow Modeling in Buildings: History and Theory

Multizone airflow modeling has provided the means to predict building air-change rates and airflow patterns in buildings and their HVAC systems. Given reliable input data and application to circumstances consistent with the assumptions behind the models, these modeling tools are capable of providing valuable insights into a number of important building performance characteristics, including ventilation efficiency, energy consumption, indoor-air pollutant transport, and smoke control. Computer hardware and software developments over the past two decades have led to simulation techniques that may be applied to integrated buildings and HVAC systems of arbitrary scale and complexity to investigate both steady and dynamic transport phenomena in a fast and efficient manner. However, multizone modeling is only appropriate for investigating spatially averaged airflow rates; when transport details within rooms are needed, other simulation tools, such as computational fluid dynamics, are required. This paper reviews the historical development of the multizone airflow-modeling theory and outlines the current state of the theory, considering both the nodal and port-plane approaches to the formulation of the theory, and several analytic methods based on these approaches.

Surface-drag flow relations for zonal modeling

Abstract Zonal models, combining simple cell-to-cell viscous loss, jet momentum, and convective energy transport relations appear to model the larger features of room airflow and thermal distribution fairly well. The power-law viscous loss relations commonly used in these models, however, have little physical justification and fail, by orders of magnitude, to model the resistance offered to airflow in simple flow regimes. An alternative approach based on a surface-drag viscous loss mechanism is presented that mitigates the deficiencies of the power-law approach, provides a family of cell-to-cell flow relations that includes linear members, and appears to capture room airflow structure more faithfully near-surfaces.

Achieving Natural and Hybrid Ventilation in Practice

Abstract Case studies provide essential evidence about the performance of buildings. They also illustrate the methods by which a technology can be implemented as well as highlighting problems. Various case study buildings (both new and retrofit) that incorporate mixed mode, natural ventilation and low energy cooling are reviewed in this paper. An outline of the tasks that ventilation is required to perform is also presented. The results show that many buildings perform well and can provide good thermal comfort and air quality for much of the occupied period. Various solutions have been introduced to extend the range and climate in which such buildings can operate. These include pre conditioning the air using underground labyrinths or buried pipes, the inclusion of pre-heating and cooling coils, and the use of thermal mass combined with night cooling. Design and operational faults include incorrect assumptions about heat gains, the failure of components, inaccessible components, structural failures and problems with outdoor air quality. These aspects are described in more detail in the paper.

Well-Posed Models of Porous Buildings for Macroscopic Ventilation Analysis

Abstract Macroscopic methods of building ventilation analysis developed in the past fifty years have proven to be accurate and thus useful for purposes of single-and multi-zone building infiltration, air quality, smoke spread, thermal comfort, and integrated HVAC/building ventilation system analysis. These methods fail, however, to provide the same level of accuracy when applied to the analysis of wind-driven airflow through porous buildings. While it may be unreasonable to rely solely on macroscopic methods to model the complex three-dimensional phenomena associated with airflow through porous buildings, the accuracy of these methods may be improved if the analyst poses the airflow problem in a physically consistent and complete manner with the numerical consequences of the formulation in mind. This paper considers a number of strategies to achieve these objectives including: a) the coupled use of mass and mechanical energy conservation principles, b) the application of total pressure boundary conditions, c) the selection of appropriate loss coefficients for flow-limiting openings, d) corrections for non-normal wind directions, and e) the impact of system model topology, nonlinearity, and initial solution estimate on solution numerics.

Embedded Detail Microscopic Models of Rooms within Macroscopic Models of Whole Building Systems

Abstract Established methods of computational fluid dynamics (CFD) have been applied to predict the details of airflow, contaminant dispersal and thermal transport within isolated zones, yet zone transport processes do not occur in isolation. They result from and interact with the bulk airflows from the larger whole-building systems in which they are embedded. As multi-zone models can reliably predict these bulk airflows, there is a growing interest in embedding detailed CFD models of specific zones within multi-zone models of the enclosing whole-building system to more faithfully account for these interactions and thus the details within the zone(s) of interest. This paper presents an analysis of an embedded CFD model and outlines some of the associated problems. A formulation is presented using unambiguous mathematical definitions of the coupling relations between the governing CFD and multi-zone equations made possible by using a port-plane approach to the multi-zone model. It is thereby shown that while the micro-to-macro coupling is straightforward, macro-to-micro coupling must remain indeterminate due to the fact that multi-zone models do not account for non-normal airflows to zone port-planes or for turbulent characteristics of the airflow that may be used by the embedded CFD model. The formulation of the embedded detail problem considered herein leads to the direct mathematical coupling of the semi-discrete Finite Element form of the CFD models used in the nonlinear algebraic enclosing multi-zone models. To investigate the limitations of embedded detail analysis the approach taken was applied to a hypothetical test problem configured to be sensitive to one obvious shortcoming of multi-zone models - their inability to account for non-normal inflow velocity components - and one less obvious shortcoming of CFD models - their tendency to model flow resistance differently and thus incompatibly with multi-zone models. The results indicated that these two shortcomings alone may critically limit the value of embedded detail analysis. Specifically, it appears that embedded detail analysis can not, in general, be expected to faithfully model flow detail in the CFD embedded zone nor model the larger macroscopic bulk flow structure correctly - even though these objectives may well be realized in special cases. Additional research is clearly needed to identify the special cases when embedded detailed analysis may be expected to be reliable. As such this paper should be of interest to not only the specialist in this emerging field, but to those seeking to employ or to fund research using these methods.