Multizone Airflow Research Articles

Prompt tracking of indoor airborne contaminant source location with probability-based inverse multi-zone modeling

Indoor air quality (IAQ) has a significant influence on occupants' comfort, health, productivity, and safety. Existing studies show that the primary causes of many IAQ problems are various airborne contaminants that either are generated indoors or penetrate into indoor environments with passive or active airflows. Accurate and prompt identification of contaminant sources can help determinate appropriate IAQ control solutions, such as, eliminating contaminant sources, isolating and cleaning contaminated spaces. This study develops a fast and effective inverse modeling method for identifying indoor contaminant source characteristics. The paper describes the principles of the probability-based adjoint inverse modeling method and formulates a multi-zone model based inverse prediction algorithm that can rapidly track contaminant source location with known source release time in a building with many compartments. The paper details the inverse modeling procedure with modification of an existing multi-zone airflow and contaminant transport simulation program. The application of the method has been demonstrated with two case studies: contaminant releases in a multi-compartment residential house and in a complex institutional building. The numerical experiments tested the source identification capability of the program for various contaminant sensing scenarios. The investigation verifies the effectiveness and accuracy of the developed method for indoor contaminant source tracking, which will be further explored to identify more complicated indoor contamination episodes.

Principles and applications of probability-based inverse modeling method for finding indoor airborne contaminant sources

Building indoor air quality (IAQ) has received growing attentions lately because of the extended time people spend indoors and the increasing reports of health problems related to poor indoor environments. Recent alarms to potential terrorist attacks with airborne chemical and biological agents (CBA) have further highlighted the research needs on building vulnerability and protection. To maintain a healthful and safe indoor environment, it is crucial to identify contaminant source locations, strengths, and release histories. Accurate and prompt identification of contaminant sources can ensure that the contaminant sources can be quickly removed and contaminated spaces can be effectively isolated and cleaned. This paper introduces a probability concept based prediction method—the adjoint probability method-that can track potential indoor airborne contaminant sources with limited sensor outputs. The paper describes the principles of the method and presents the general modeling algorithm and procedure that can be implemented with current computational fluid dynamics (CFD) or multi-zone airflow models. The study demonstrates the application of the method for identifying airborne pollutant source locations in two realistic indoor environments with few sensor measurement outputs. The numerical simulations verify the feasibility and accuracy of the method for indoor pollutant tracking applications, which forms a good foundation for developing an intelligent and integrated indoor environment management system that can promptly respond to indoor pollution episodes with effective detection, analysis, and control.

Evaluation of some assumptions used in multizone airflow network models

Multizone airflow network models assume that in a zone air temperature and contaminant concentrations are uniform, and air momentum effects are neglected. These assumptions could cause errors for airflow with strong buoyancy, large contaminant concentration gradient, or strong momentum. This study has found the correlations of the errors and some dimensionless air parameters. The assumption of uniform air temperature is acceptable when the dimensionless temperature gradient is smaller than 0.03. The assumption of uniform contaminant concentration is valid if the corresponding Archimedes number for the source zone is greater than 400. The assumption of neglecting air momentum effect is reasonable when the jet momentum effect is dissipated before reaching an opening in downstream.

Modeling Particle Dispersion under Human Activity Disturbance in a Multizone Indoor Environment

Human activity is an important factor influencing particle resuspension in the indoor environment. This work studies the applicability of a multizone airflow and contaminant transport model (CONTAM 2.1) in the simulation of indoor dispersion of particles under human activity disturbance. An iterative method is suggested to complement CONTAM 2.1, by tracking the transient particle concentration on floor surface due to dynamic deposition and resuspension process. A three-zone building with a heating, ventilation, and air conditioning system is used as the simulation case to test the convergence and accuracy of this algorithm under different particle-release scenarios. The algorithm shows a very fast convergence speed in the simulation. Comparisons of calculation results between the multizone model and the analytical model show good agreement and verify the accuracy of the multizone model simulation. The airborne particle concentration profiles and human breathing dose are also analyzed for the three-zone building model.

Inverse modeling methods for indoor airborne pollutant tracking: literature review and fundamentals

Reduction in indoor environment quality calls for effective control and improvement measures. Accurate and prompt identification of contaminant sources ensures that they can be quickly removed and contaminated spaces isolated and cleaned. This paper discusses the use of inverse modeling to identify potential indoor pollutant sources with limited pollutant sensor data. The study reviews various inverse modeling methods for advection-dispersion problems and summarizes the methods into three major categories: forward, backward, and probability inverse modeling methods. The adjoint probability inverse modeling method is indicated as an appropriate model for indoor air pollutant tracking because it can quickly find source location, strength and release time without prior information. The paper introduces the principles of the adjoint probability method and establishes the corresponding adjoint equations for both multi-zone airflow models and computational fluid dynamics (CFD) models. The study proposes a two-stage inverse modeling approach integrating both multi-zone and CFD models, which can provide a rapid estimate of indoor pollution status and history for a whole building. Preliminary case study results indicate that the adjoint probability method is feasible for indoor pollutant inverse modeling. The proposed method can help identify contaminant source characteristics (location and release time) with limited sensor outputs. This will ensure an effective and prompt execution of building management strategies and thus achieve a healthy and safe indoor environment. The method can also help design optimal sensor networks.

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.

Feasibility of wireless measurements for semi-empirical multizone airflow model tuning

The feasibility of wireless measurements for rapid tuning of a multizone airflow model was extensively investigated using a small-scale residential building as a test site. The main objective of the research was to provide fundamental information, such as measurement time and cost for various measurement techniques, needed for selecting an appropriate field measurement strategy. Potential advantages of the wireless measurement approach were evaluated. These include: portability and flexibility, fast equipment setup, time synchronized data collection, negligible interruption to the occupants’ activity during the measurement, and installation time and cost savings. Currently, the high equipment cost of the wireless approach, relative to more conventional techniques, may offset the noted advantages and be the highest barrier to the wide use of wireless sensors in field measurements. Consequently, conventional hand-held measurements can be more economical, but the stability of the building HVAC system operating status should be considered before selecting the hand-held measurement approach. System stability and the ambient and internal building load status to which it is responding have significant impact on the model tuning usefulness of this approach

Effects of HVAC System and Building Characteristics on Exposure of Occupants to Short-Duration Point Source Aerosol Releases

This paper presents results from the simulation of localized, short-duration bioaerosol releases in a hypothetical building similar to a dormitory or barracks using public domain multizone air flow and contaminant dispersion modeling software. The primary purpose of the modeling was to generate example exposure data to be used in the development of a comprehensive microbial risk assessment methodology. However, these results are also of intrinsic interest for what they reveal about the contribution of various building characteristics to risk from airborne contaminants. A variety of parameters were varied, including building construction, heating, ventilating, and air-conditioning (HVAC) system design, and release characteristics, among others. Results of these simulations demonstrate the variability of exposure possible under different scenarios and, more particularly, the impact that HVAC design decisions can have on risk. Although a single building and restricted set of scenarios was investigated, several general conclusions could be drawn regarding factors, such as HVAC zoning and filter maintenance, that intrinsically contribute to vulnerability reduction.

Validation of a Coupled Multizone-CFD Program for Building Airflow and Contaminant Transport Simulations

Current multizone airflow network models assume air momentum effects, contaminant concentrations, and air temperatures are uniformly and homogeneously distributed in a zone of a building. These assumptions can cause errors for zones where air and/or contaminant are not well mixed. A coupled multizone-CFD program has been developed to improve the multizone model by applying a CFD model to those poorly mixed zones and the multizone model to the remaining zones. This paper validates the coupled multizone-CFD program by using experimental data obtained in a four-zone facility with nonuniform distributions of air momentum effects, contaminant concentrations, and air temperatures. The calculated results by the coupled program generally agreed with the experimental data although discrepancies exist in some cases. The coupled multizone-CFD simulations used less computing time than the CFD simulations for the whole flow domain.

Contrasting the capabilities of building energy performance simulation programs

For the past 50 years, a wide variety of building energy simulation programs have been developed, enhanced and are in use throughout the building energy community. This paper is an overview of a report, which provides up-to-date comparison of the features and capabilities of twenty major building energy simulation programs. The comparison is based on information provided by the program developers in the following categories: general modeling features; zone loads; building envelope and daylighting and solar; infiltration, ventilation and multizone airflow; renewable energy systems; electrical systems and equipment; HVAC systems; HVAC equipment; environmental emissions; economic evaluation; climate data availability, results reporting; validation; and user interface, links to other programs, and availability.

A model-based approach to natural ventilation

This paper presents an in situ calibrated model-based approach to the control of natural ventilation in buildings. Specifically, the potential of empirically-based and numeric air flow models is explored as part of a hybrid building controls scheme. Toward this end, two case studies are described. In one case, the underlying predictive model is based on the statistical treatment of data obtained from a set of in situ measurements in a typical office space. In the second case, a multi-zone air flow model is used to predict the fresh air volume flows into the building interior. The resulting air flow models can be used as an integrated part of a control method for natural ventilation devices to predict the implications of alternative control options.

Predicting size-resolved particle behavior in multizone buildings

We compare model predictions to measurements of SF 6 and environmental tobacco smoke particle concentrations in a three-room chamber experiment. To make predictions of multi-room aerosol transport and fate, we linked a multizone airflow model (COMIS) with an indoor aerosol dynamics model (MIAQ4). The linked models provide improved simulation capabilities for predicting aerosol concentrations and exposures in buildings. In this application, we found that the multizone air flow model was vital for predicting the inter-room airflows due to temperature differences between the rooms and when air-sampling pumps were operating during the experiment. Model predictions agree well with measurements, as shown by several comparison metrics. However, predictions of airborne ETS concentrations are slightly lower than measurements. This is mostly attributable to under-stating the source release amount, which we specified independently from literature estimates. Model predictions of ETS particle-size distributions agree with measurements; size bins with the peak concentrations are slightly over-predicted initially, but agree thereafter.

Simulated Performance of Natural and Hybrid Ventilation Systems in an Office Building

Past research on natural ventilation has revealed that the application of pure natural ventilation systems may be limited in the United States by issues such as climate suitability, humidity control, and reliability. However, hybrid (or mixed-mode) ventilation systems offer the possibility of attaining energy savings in a greater number of buildings and climates through the combination of natural ventilation systems with mechanical equipment. The objective of this study is to investigate the potential energy and indoor environmental performance of natural and hybrid ventilation alternatives in low- to mid-rise US commercial buildings in a variety of US climates. In this effort, the National Institute of Standards and Technology (NIST) reviewed hybrid ventilation approaches and conducted simulations to predict and compare the indoor environmental and energy performance of natural, hybrid, and mechanical systems in an otherwise similar building. Due to the strong interaction of airflow and heat transfer in naturally ventilated buildings, a coupled multi-zone airflow and thermal simulation tool was used to model the systems in a five-story office building in five US cities. Overall, the natural ventilation system performed adequately in San Francisco and Los Angeles, although some tolerance for imperfect thermal and indoor air quality (IAQ) control is required. Natural ventilation system performance was poor in the more challenging climates of Boston, Minneapolis, and Miami due to poor thermal control, unreliable ventilation, or high heating loads. The hybrid ventilation system improved on the performance of the natural ventilation system in all climates, with dramatic improvements in some. Compared to the mechanical system, the hybrid system saved significant amounts of fan energy, reduced cooling loads, or reduced both fan and cooling loads in all climates but often resulted in higher heating loads. Although the hybrid system provided acceptable thermal control, the mechanical system provided more consistent control, as expected. The hybrid ventilation system provided better IAQ control, as indicated by CO2 concentrations, in most but not all cases.

Nonlinear Dynamic Analysis of Natural Ventilation in a Two-Zone Building: Part A—Theoretical Analysis

The nonlinear behavior and solution multiplicity of airflow and natural ventilation in multi-zone buildings are significant for building ventilation and smoke control design. A two-zone building with four openings is considered in this study. Nonlinear dynamic behaviors are analyzed, and we report on the fixed points (steady-state solutions) and their stability and provide a description of the bifurcation of the fixed points and an estimation of the separating curve between two stable fixed points. Different flow modes were identified with the same physical and boundary conditions. A multi-zone airflow and thermal coupled program was also used to evaluate the results. There was good agreement between the numerical and theoretical solutions, except that the unsteady-state solutions could not be predicted by the multi-zone program. These results may have significant implications for multi-zone modeling of natural ventilation and smoke or contaminant spread in buildings, as well as allow us to gain a good understanding of physical flow behavior and solution multiplicity.

POWBAM0 Mechanical Power Balances for Multi-zone Building Airflow Analysis

Conventional methods of multi-zone airflow analysis ignore mechanical energy conservation in forming the system equations governing building airflows. As a result, airflows computed using these methods generally violate this fundamental conservation principal and thereby falsely create or destroy kinetic energy within building zones. While the impact of this fundamental oversight has yet to be fully evaluated, it need not be tacitly accepted. This paper will present a rigorously detailed theoretical approach that imposes both mass and mechanical energy conservation to the problem of multi-zone airflow analysis and, through comparisons with results computed using computational fluid dynamics (CFD) and the conventional approach to multi-zone airflow analysis, will demonstrate that a power balance approach can predict building airflows that satisfy both of these fundamental conservation principles while offering greater capabilities than that of the conventional approach. Furthermore, it will be shown that the conventional approach is a special limited case of the practical application of the power balance approach. A computational prototype, POWBAM0, based on the theory will be introduced.

Multi-zone modeling of probable SARS virus transmission by airflow between flats in Block E, Amoy Gardens

More than 300 residents of a private high-rise housing estate were infected with severe acute respiratory syndrome within a short period during the 2003 epidemic in Hong Kong. The outbreak occurred after the identified index patient visited a flat on a middle floor in Block E of the Amoy Gardens estate on two nights. Approximately 45% of the subsequently infected people resided in Block E, while the other 55% of infected cases mainly resided in six other blocks close to Block E. The distribution of the infected flats in Block E conformed to a non-uniform spatial pattern. Probable environmental causes for airborne transmission associated with the air movements between flats in Block E are identified. The well-established multi-zone airflow modeling method was used to analyze the virus-laden bio-aerosol dispersion between flats through door and window leakage areas in Block E under six different scenarios. The distribution of infection risk in Block E matched with the virus concentrations in flats predicted with the use of multi-zone modeling. Our study shows the importance of ventilation design in high-rise residential apartments. The present study on the Amoy Gardens outbreak presented a scenario in which crowded living spaces might lead to infection disasters. There is a need to improve the current sanitary drainage design and maintenance standards to avoid any leakage of foul gas into the indoor environments. Our study revealed the need for a review of indoor air quality and ventilation design in buildings including offices, homes and hotels. The study has implications to public health in, for example, the control of other airborne respiratory infectious diseases such as influenza, and in bio-terror safety in buildings.

Validation of multizone IAQ model predictions for tracer gas in a townhouse

To provide additional validation data for the multizone airflow and contaminant model, CONTAMW, experiments were performed in an occupied three-storey townhouse in Reston, VA. A tracer gas, sulfur hexafluoride (SF6), was manually injected within one room of the house and the concentration of SF6 was then measured in nine rooms. This same process was then recreated in CONTAMW and the resulting predictions were statistically compared to the measured values. A total of 10 experiments were conducted and simulated between May 2000 and June 2001. In four cases, the heating and air-conditioning system fan was operating. SF6 was injected in the recreation room (basement level), the kitchen= dining room (main level) and the master bedroom (upstairs level). A statistical comparison of measurements and predictions was performed per ASTM D5157 (ASTM 1997) for all cases. Comparisons were made for overall zone average concentrations and individual zone transient concentrations. The results for zone average concentrations were very good with many cases meeting most or all of the D5157 criteria. Several cases showed a poor to fair correlation between average measurements and predictions due to discrepancies with a single zonemdashthe main floor bathroommdashbut excluding that zone resulted in these cases meeting or nearly meeting the D5157 criteria. Comparisons of individual zone transient concentrations were mixed with many good to excellent cases but also numerous fair to poor. Zones other than the bathroom had occasional poor comparisons between predictions and measurements but no consistent discrepancies. The predicted SF6 concentration averaged over all zones and cases was within 10% of the average measured concentration.

Monitoring and Modelling Indoor Air Quality and Ventilation in Classrooms within a Purpose-Designed Naturally Ventilated School

This paper investigates the performance in use of thermal passive ventilation stacks in the classrooms of a school during the summer period. Measurements of air temperature, carbon dioxide (CO₂) and air velocity were carried out from which ventilation rates in the classrooms and stacks were calculated. It was found that the airflow through the passive stacks was a significant proportion of the classroom air exchange rate and metabolic CO₂ was lower in classrooms with passive stacks. The predictions of a multi-zone airflow computer model were compared with measured values. Multi-zone modelling indicated that passive stacks could significantly increase ventilation in classrooms, which would otherwise be ventilated via single side ventilation through openable windows on one external wall. It was found that if the zones are accurately represented, multi-zone modelling is able to provide a good indication of the general trends of airflow rates both for the whole classroom and the flow through passive stacks.

Monitoring and Modelling Indoor Air Quality and Ventilation in Classrooms within a Purpose-Designed Naturally Ventilated School

This paper investigates the performance in use of ther mal passive ventilation stacks in the classrooms of a school during the summer period. Measurements of air temperature, carbon dioxide (CO2) and air velocity were carried out from which ventilation rates in the class rooms and stacks were calculated. It was found that the airflow through the passive stacks was a significant pro portion of the classroom air exchange rate and metabolic CO2 was lower in classrooms with passive stacks. The predictions of a multi-zone airflow computer model were compared with measured values. Multi-zone modelling indicated that passive stacks could significantly increase ventilation in classrooms, which would otherwise be ventilated via single side ventilation through openable windows on one external wall. It was found that if the zones are accurately represented, multi-zone modelling is able to provide a good indication of the general trends of airflow rates both for the whole classroom and the flow through passive stacks.

Computational aspects of nodal multizone airflow systems

The multizone approach to steady-state airflow problems models a building as a network of discrete mass flow paths. A nodal formulation of the problem writes the governing equations in terms of the unknown pressures at the points where the flow paths connect. This paper proves conditions under which the nodal equations yield symmetric positive-definite matrices, guaranteeing a unique solution to the flow network. It also establishes relaxed conditions under which a nodal airflow system yields asymmetric matrices with positive eigenvalues, guaranteeing at least one solution. Properly exploiting the system properties greatly reduces the cost of numerical solution. Thus, multizone airflow programs such as C ONTAM and C OMIS depend on symmetric positive-definite systems. However, the background literature neglects or simplifies the underlying assumptions, does not assert existence and uniqueness, and even contains factual errors. This paper corrects those errors, states the implicit assumptions made in the programs, and discusses implications for modelers and programmers.