indoor air quality – combining air humidity with construction moisture

205 Pages·2005·3.42 MB·English

INDOOR AIR QUALITY – COMBINING AIR HUMIDITY WITH

CONSTRUCTION MOISTURE

PATRICK KEN AMISSAH
INDOOR AIR QUALITY – COMBINING AIR HUMIDITY WITH
CONSTRUCTION MOISTURE

Patrick Ken Amissah B.Sc.(Eng)Hons. M.Sc.(Eng)

A thesis submitted for the
Degree of Doctor of Philosophy

Department of Mechanical Engineering
Energy Systems Research Unit
University of Strathclyde
Glasgow, U.K.

April 2005
Indoor Air Quality: Combining Air Humidity with Construction Moisture
Acknowledgements
List of Symbols
Chapter 1 Indoor air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Introduction ………………………………………………… 1
1.2 Factors affecting indoor air quality . . .. . . .. . . . .. . . . .. . . . . … … 2
1.3 Impacts on building performance and health . . . . . . . . . . . . . . . . . 3
1.4 Health aspects of indoor air quality ………………………………..4
1.5 Research objectives and thesis outline . . . . . . . . . . . . . . . . . . . . . . 8
Chapter 2: Review of heat, air and moisture transfer modelling ……. . . . . …12
2.1 Porous medium heat and mass transfer . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Moisture flow model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 13
2.1.2 Heat, air and moisture transport through porous medium . . . . . . 20
2.1.3 The IEA Annex XIV enquiry on HAM codes . . . . . . . . . . . . . . . . 21
2.2 Zone moisture transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
2.2.1 Air humidity and energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Chapter 3: Review of indoor air flow modelling with CFD . . . . . . . . . . .. . . 27
3.1 Introduction …………………………………………………….. 27
3.2. Co-located grid system ……………………………………………47
3.2.1 Numerical solution and algorithm ………………………… ……47
3.2.2 SIMPLE algorithm for a co-located grid arrangement ………….. 53
3.3 Turbulence modelling …………………………………………… 60
3.3.1 Near-wall treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ….. 66
3.4 Application of CFD to indoor air quality ………………………… 79
3.5 Commonalities between HAM and CFD models . . . . . . . . . . . . . . 82

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Chapter 4: Review of boundary layer theory . . . . . . . . . . . . . . . . . . . . . . . ….86
4.1 Convective surface transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
4.2. Boundary layer classification . . . . . . . . . . . . . . . . . .. . . .. . . . . .. . 87
4.2.1 Velocity boundary layer . . . .. . . . .. .. . . . . . . . . . . . . . . . . . . . . . . 88
4.2.2 Thermal boundary layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.2.3 Concentration boundary layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90
4.2.4 Transfer equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.3 Heat and Mass Transfer Analogy . . . . . . . . . . . . . . . . . . . . . . . . 94
4.3.1 Correlations for convective mass transfer coefficient . . . . . .. . . 96
4.4 Boundary conditions as they relate to buildings . . . . . . . . . . . 101
Chapter 5: Conflation mechanism ………………………………………… 104
5.1 Coupling approaches….………………………………………… 104
5.1.1 Staged coupling . ……………………………………………….. 104
5.2 The conflation problem. ……………………………………… 108
5.2.1 The conflation technique ……………………………………… 110.
5.2.2 Two-time step dynamic coupling ………………………………..116
5.2.3 Coupling of 1D HAM model to 3D CFD model ……………….. 122
5.3 Configuration mechanism …………………………………… 126
5.3.1 Factors that impact on the convection heat and mass transfer … 127
coefficient
5.4 Moisture control algorithm………………….…………………….129
5.4.1 Free convection flow …………….……………..………………. 133
5.5 Moisture desorption/absorption at the interface ……………… 138
5.5.1 CFD model adjustments………………………………………… 138
5.5.2 HAM model adjustments…………………………………………147
Chapter 6: Conclusion and Future work . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .150
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6.1 Conclusion ………………………………………………………. 150
6.2 Future work ……………………………………………………… 153
6.2.1 Coding ………………………………………………………. 153
6.2.2 Material properties ……………………………………………… 154
6.2.3 Moisture transport models …………………………………. ……154
6.2.4 Experiments ………………………………………………………154
APPENDICES
A: Mathematical model for moisture transport ………………………………… 155
B: Overview - I.E.A. Annex XIV classification of HAM codes ………………….165
C: The sequential solution procedure …………………………………………… 167
D: Alternative near-wall treatments ……………………………………………….170
E: Boundary layer theory - significant dimensionless parameters ………………. 173
F. Characterisation of the energy transfer mechanism at internal surfaces ………. 177
by ESP-r
REFERENCES …………………………………………………………………….184

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Acknowledgements
In appreciation of the endless support and various forms of assistances given to me during
the period of my studies within the Energy Systems Research Unit of the University of
Strathclyde, Glasgow, acknowledgement is hereby given. I wish to express my sincere
gratitude to Professor Joe Clarke for his expert guidance and encouragement throughout the
study. My thanks go also to the government of Ghana for their support with the Scholarship
grant for the programme. To my ESRU colleagues, Lori, Nick, John, Paul, Andy, Cameron,
Ian and Kathleen, I am indebted for all forms of assistances. Finally, I am grateful to my
wife and family for their support and help during the study.

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Abstract
The project aims to improve the modelling of moisture transfers at internal surfaces by
linking the finite volumes representing the Heat, Air and Moisture (HAM) and
Computational Fluid Dynamics (CFD) domains. Conflation of both models facilitates the
detailed study of moisture flow as it impacts on indoor air quality and occupant health. The
thesis lays down the conceptual framework for the subsequent development of an indoor air
quality analytical tool. The work thus improves the modelling of construction feature risk
assessment, for example, moisture absorption and desorption at the internal fabric surfaces
in as much as it relates to indoor air quality. Through such an improvement, an indoor air
quality analytical tool for the prediction of time-varying temperature/humidity conditions at
specific locations within the building is enabled and subsequently these conditions may be
related to the likely occurrence of mould. Humidity in indoor spaces is one of the most
important factors in the determination of indoor air quality. High indoor humidity is a major
contributor to the accumulation of moisture in the building envelope. This often results in
dampness within the building envelope and subsequent health-related problems for the
occcupants. Moderation of the indoor relative humidity, temperature and moisture content
of the indoor air amongst others is a pre-requisite for a healthy building because it affects
the perception of indoor air quality, thermal comfort, occupant health (asthma, respiratory
illness, etc), building durability, material emission and energy consumption. Excessively
high relative humidity promotes the growth of moulds and mildew on building surfaces.
The basis for the envisaged conflation evolves around the boundary layer theory as it
pertains to the velocity, thermal and concentration profiles associated with flow parallel to a
flat surface, a phenomenon which is recognised as being similar in nature to buoyancy-
driven convective heat transfer within building enclosures (White 1988). Within the
framework of modelling of indoor air flows, the conflated modelling approach is very much
dependent upon the treatment of the internal surface convection, for example, in the
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conflation of HAM and CFD models. This is referred to as the pivot point for the
handshaking between HAM and CFD modelling domains. Within the framework of this
project, the pivot point refers to the treatment of surface convection mass transfer at the
internal surface to facilitate the hand shaking between HAM and CFD modelling domains.
The two-time step coupling approach based on the loose coupling algorithm is adapted for
the conflation. The technique involves a process whereby the HAM and CFD models are
processed independently but exchange information at the interface at every time-step. The
numerical method for the solution of the Navier-Stokes equations is based on the co-located
grid arrangement, whereby all flow variables are defined in the centre of the grid cells. The
transport equations are integrated for each grid cell and the Gauss Theorem applied to yield
an integral over the cell face. These cell face integrals are then approximated using
interpolation of the cell centred data. For the resolution of flow in the near-wall regime, the
Low-Reynolds number k-ε turbulence model is used. A configuration mechanism with a
rules-based moisture control algorithm to facilitate the handshaking of the HAM and CFD
domains is presented. Methods for the solution of problems due to moisture migration
across the interface, which are effected through variation of the convective mass transfer
coefficient, h , through variation of the standard k-ε turbulence model, namely the low-
m
Reynolds number model with its embedded wall damping functions and through adjustment
of the source terms of governing transport equations of the CFD and HAM models are also
discussed.

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List of Symbols
A area
a dry air
C vapour concentration in the air
C species molar concentration
A,∞
C local friction factor
f
C specific heat at constant pressure
p
C vapour concentration at the surface
s
C concentration of water vapour
w
c specific heat capacity, specific mass concentration

D diffusion coefficent
D mass diffusivity of species, e.g. A and B
AB
D moisture diffusivity of water vapour with respect to moisture content, w
w
(moisture content mass by volume, kg/m3)
D moisture diffusivity of water vapour with respect to moisture content, u
u
(moisture content mass by mass, kg/kg)
E total energy
f friction factor
f , f ,wall damping functions
µ 1
g vapour flux
v
Hin shorthand notation for the sum of the advective and viscous terms
i
h average convective heat transfer coefficient
h local convective heat transfer coefficient
c
h enthalpy of liquid, fluid enthalpy
f
h enthalpy of evaporation
fg
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h convective mass transfer coefficient
m
h enthalpy of moisture flux source
s
h enthalpy of vapour
v
J vapour mass flux
J diffusion flux of species, j’
j’
J Colburn j-factor for heat transfer
H
J Colburn j-factor for mass transfer
M
J , vapour flux
v
K hydraulic conductivity
K thermal moisture permeability
θ
k thermal conductivity, kinetic energy of turbulence
k air permeance
a
k , effective thermal conductivity
eff
k fluid phase thermal conductivity
f
k moisture permeability
m
k solid medium thermal conductivity
s
Le Lewis number
l mixing length
m
M molecular weight
m mass fraction of species j’
j’
m mass flux, mass flow rate
N total molar transfer
Nu Nusselt number
n ” molar flux of species A
A
n thickness of porous medium
n air change rate
vent
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