ENHANCED MODELLING OF INDOOR AIR FLOWS, TEMPERATURES, POLLUTANT EMISSION AND DISPERSION BY NESTING SUB-ZONES WITHIN A MULTIZONE MODEL

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1 ENHANCED MODELLING OF INDOOR AIR FLOWS, TEMPERATURES, POLLUTANT EMISSION AND DISPERSION BY NESTING SUB-ZONES WITHIN A MULTIZONE MODEL Zhengen Ren, BSc, MEng A thesis submitted to The Queen s University of Belfast for The Degree of Doctor of Philosophy Faculty of Engineering School of Computer Science September 2002

2 Table of Contents Abstract. Acknowledgements.. List of Symbols.. v vii viii Chapter 1: Introduction Indoor air quality Sources of indoor air contaminants Importance of indoor climate Room air distribution affects indoor air quality Room air and air contaminant distribution is important for industry Tools to determine indoor air quality Measurements Simulations Research objectives Outline of the thesis 13 Chapter 2: Simulation of Indoor Air Quality Introduction CFD The conceptual basis of CFD Development General capabilities of current models Difficulties with CFD Future developments Multizone models The conceptual basis of multizone models Development of multizone models General capabilities of current models Recent development and future work Zonal models The conceptual basis of zonal models Development Limits of current models Coupling CFD with multizone models Coupling zonal models with multizone models Summary 62 Chapter 3: Development of Air Flow Modelling in COwZ Introduction Methodology for nesting sub-zones within COMIS The concept of sub-dividing a room Standard sub-zones Mixed sub-zones Jets Thermal Plumes Thermal boundary layers Implementing sub-zones in COwZ 90 ii

3 3.4 Solution of mass balance equations Summary 95 Chapter 4: Development of Thermal Energy Modelling in COwZ Introduction Thermal modelling Thermal balance equations Thermal boundary conditions at solid surfaces Calculating convection coefficients Thermal solution procedure Numerical convergence and performance Summary 118 Chapter 5 Development of Source Emission Models in COwZ Introduction Review of previous studies on indoor source emissions Source emission models Introduction Non-boiling evaporation from pools Single component liquid spills on hard flooring Petroleum-based solvent spills on hard flooring VOC emission from indoor coating materials Gas and liquid releases Sink terms Solution procedure for pollutant emission and transport Summary 155 Chapter 6 Implementing New Features into COwZ Introduction Structure of the COwZ model Modifications of input file Structure of input file Modifications of problem description New air flow links Modifications of zone input Description of room heat convection between internal surfaces and air Description for source emission modelling Output file modification Modular code and numerical convergence Modular code Numerical convergence and performance Summary 184 Chapter 7 Evaluation and Application of COwZ Introduction Summary of evaluation cases presented in this chapter Prediction of air flow and temperature distributions within buildings A two-dimensional problem where COwZ results are compared with experimental data and those from zonal model 190 iii

4 7.3.2 A three-dimensional case where COwZ results are compared with experimental data and those from CFD and zonal model Modelling the flows and temperatures in a partitioned room Temperature distribution in an experimental atrium Prediction of pollutant concentration distributions within buildings Prediction of ventilation performance in an experimental ventilated room Prediction for a typical residential building An illustration of dispersing contaminant in a closed room Prediction of pollutant emission and dispersion within buildings Prediction of emission and dispersion for single solvent in a ventilated room Prediction of emission and dispersion of solvent mixture spill in a ventilated room Prediction emission and transport for indoor painted surfaces Conclusions 232 Chapter 8 Conclusions and Recommendations for Future Work Summary of achievements Detailed review of novel features Recommendations for future work 239 References 242 iv

5 Abstract This thesis is concerned with developing improved and more practical methods for modelling indoor air quality, which includes emission, transport and dispersion of indoor pollutants. The approach taken was to nest sub-zones within a multizone model and to add the necessary functionality to the combined program. Three significant innovations were devised and implemented within the multizone model COMIS to achieve the research objectives. The new program is called COMIS with sub-zones, abbreviated to COwZ. Firstly, sub-zones were nested within a multizone model and may be used for those rooms where extra detail is needed, while other well-mixed rooms are treated as single zones. This considerably enhances the accuracy from that of a standard multizone model as well as providing significant support for thermal simulation and zonal source emission modelling. For the prediction of air flows the key task is to calculate the airflow rates to and from adjacent sub-zones (or the outside). Fourteen new link methods were added to the 13 already available in COMIS. Nine of these are new for multizone models while five (associated with various jet inlets) have not previously been implemented in either zonal or multizone models. Collectively these methods can calculate air flows for a wide range of cases of practical interest. Secondly, a suitable thermal model has been developed and implemented in COwZ to account for the effects of temperatures on air flows and contaminant emission and dispersion. The key steps for thermal simulation are prediction of internal surface convection heat transfer coefficients and the solution of thermal energy equations. A pragmatic approach was established for calculation of convection coefficients according to the type and cause of the v

6 driving forces. After an extensive survey, 19 convection coefficient correlations have been incorporated in COwZ, which includes the latest modelling for mixed convection heat transfer. These methods can calculate convection heat transfer coefficients for most indoor air flows of practical interest. For the solution of thermal energy equations, two methods (Gaussian elimination with back substitution and TDMA Triangular Diagonal Matrix Algorithm) were selected for a whole building (room numbers are not limited) and for a single sub-divided room. These two methods were effective and did not suffer from convergence problems. Their execution times depend on the problem size and complexity. Thirdly, three types of zonal source emission modelling have been devised and implemented in COwZ. This means that the new program not only simulates pollutant transport under prescribed emission rates but also models pollutant emission and dispersion within buildings. These models include single- and multi-component liquid pools, solvent emissions from paints, and gas and liquid release jets. The models use local-scale data and avoid the assumption of current single-zone source emission modelling that each room is well-mixed and that the roomaverage concentration is a representative value for each room. This is a significant improvement for predicting pollutant emission, transport and dispersion from room-average values to local values. These adaptive modelling techniques advance the modelling of indoor air flows, temperatures, and contaminant emission, transport and dispersion between and within rooms of a whole building. A comprehensive and systematic evaluation of the new program has been completed which demonstrates that COwZ can be used to provide a level of detail and accuracy well beyond that of standard multizone models. vi

7 Acknowledgements I would first like to express the sincerest gratitude to my thesis supervisors, Mr. John Stewart and Professor Danny Crookes. This thesis would never have been written had they not believed in me and supported my studies. They inspired, motivated, and challenged me throughout the course of my studies. They have been mentors to me in the true sense of the word. I would like to thank the School of Computer Science for awarding me a research studentship and the Queen s University Environmental Science and Technology Research (QUESTOR) Centre for funding this project. Many thanks to the staff and students at these two departments for creating a stimulating, welcoming, and positive atmosphere. My colleagues in the Environmental Modelling Group at the QUESTOR Centre have been a support and source of encouragement. Special thanks to Claire Furlong, Ron Reimer, Micheal Collins and Tony Osborne for many interesting conversations. I would never have thought to write this thesis had my parents not taught me the value of education and hard work, and raised me to believe I could achieve anything I set my mind to. A very special thanks to my family and friends who have understood and accepted my distracted state and infrequent communications over the past three years. Finally, and most importantly, I wish to express my warmest appreciation and love for my wife, Ling Tu. I could not have accomplished my goal without her unfaltering support, encouragement, and inspiration. During the past three years, she stubbornly refused to allow my mind s stressed and exhausted state to distort reality. She also selflessly accepted my lack of attention and time. vii

8 List of Symbols a Crack flow coefficient {m 2 /s Pa n } ach Room air change per hour {h -1 } A Area {m 2 } Ar b Archimedes number The jet thickness where velocity u is equal to half the maximum velocity {m} B Slot length {m} c Speed of sound in the gas {m/s} c d Discharge coefficient or flow coefficient c p Air specific heat at constant pressure {J/kg K) c v Air specific heat at constant volume {J/kg K) c µ Empirical constant in k - ε model C Concentration of contaminant or flow coefficients {kg/m 3 }or {m 3 /s Pa n } C 1 C 2 Empirical constant in k - ε model or jet model Empirical constant in k - ε model or jet model C d Empirical permeability constant {m/s Pa n } C p Pressure coefficient D Molecular diffusion coefficient for the contaminant {m 2 /s} or {m 2 /h} D h Hydraulic diameter {m} D P Pipe diameter {m} f Friction factor viii

9 g Gravitational acceleration {m/s 2 } Gr Grashof number H Specific enthalpy or enthalpy {J/kg }or{j} H l Height of liquid above puncture {m} h Height of zone or jet {m} h 0 Slot width {m} i, j Grid indices in x and y directions k Thermal conductivity or turbulence kinetic energy {W/m K}or {m 2 /s 2 } k a The sink rate constant {m/s} k d Desorption rate constant {1/s} K Mass transfer coefficient {m/s} l A length scale {m} m mass flow rate {kg/s} MW Molecular weight {g/mol} n flow exponent N Fan rotating speed or number of VOCs in the product {1/s}or {-} N f N l N r N z The total number of solid surfaces within a zone The total number of flow paths between two adjacent zones The sequence number of the key word in the string KEYS The total of the zones within the solution domain p Pressure {Pa} P Perimeter {m} Pr Prandtl number = c p µ/k q Heat source {W}or {W/m 3 } q(x) Air flow rate {m 3 /s} ix

10 q conv Convective heat transfer between wall surface and air {W} Q Infiltration rate {m 3 /s} r Radius or gas specific heat ratio = c p /c v {m} or {-} Rex Relaxation factor for thermal simulation R Gas constant {J/g K} Re R p Reynolds number Reactivity (in pollutant transport equation) R u Universal gas constant {J/mol K} S Source emission rate or volumetric contaminant generation rate {kg/s}or {kg/m 3 s} Sc Schmidt number = µ /(ρ D) Sink Sink absorption rate {kg/s} t Time {s} T Temperature {K} T Reference temperature {K} u Velocity component in x-direction {m/s} u j Velocity components expressed in tensor notation {m/s} U Velocity at nozzle opening {m/s} v Velocity component in y-direction {m/s} w Velocity component in z-direction {m/s} W Width or amount of solvent {m} or {mol} x, y, z Cartesian coordinates {m} x j Cartesian coordinates expressed in tensor notation {m} Xh y i Relative humidity Mole fraction of component in the product x

11 Greek symbols α Convection heat transfer coefficient {W/m 2 K} λ Wall thermal conductivity or dimensionless friction factor {W/m K}or { } η The filter effect of a link Γ Turbulent heat diffusion {Pa.s} ρ Fluid density {kg/m 3 } µ Air viscosity {Pa.s} µ t Eddy viscosity {Pa.s} ν Kinematic viscosity {m 2 /s} β The thermal expansion coefficients of air {K -1 } p Pressure difference {Pa} t Time interval {s} T Temperature difference { C} φ An independent variable (temperature, pressure, turbulent energy, etc.) φ k Convective heat emission {W} Φ Heat flux {W} τ Shear stress {N/m 2 } ε Dissipation rate of turbulence energy or wall surface emittance {m 2 /s 2 }or {-} ζ Dimensionless coefficient of duct fitting δ Boundary layer thickness {m} δ ij Kronecker delta; 0 when i j, 1 when i = j σ Stefan-Boltzman constant {W/m 2 K 4 } σ t σ c σ k Turbulent Prandtl number Empirical constant in k - ε model Empirical constant in k - ε model xi

12 σ ε Empirical constant in k - ε model Subscripts 0 Initial state a A cr Atmospheric state Component A Critical state i, j Zone number l m o p s w Flow link or liquid Maximum Outside of a building Pollutant p Source or saturation condition Wall surface Superscripts ' The fluctuation of a solution variable about its time-mean value Time-mean quantity of a solution variable xii