WUFI Workshop at NTNU /SINTEF Fundamentals

Transcription

1 WUFI Workshop at NTNU /SINTEF 2008 Fundamentals

2 Contents: From steady-state to transient Heat storage and -transport Moisture storage and -transport Calculation of coupled transport Model limitations 2

3 From steady-state to transient complex (transient) scientific model restrictions and simplification userfriendly interface and pre-selection of input data steady-state model programming simple engineering software engineering software for skilled practitioners 3

4 From steady-state to transient Glaser / Dew Point Method Plot vapor pressure gradient for steadystate conditions Determine temperature gradient and plot saturation pressure gradient Adjust vapor pressure gradient so it does not cross saturation pressure and calculate from that influx and efflux of moisture Problems no heat and moisture storage no liquid flow no coupling of heat and moisture transfer 4

5 From steady-state to transient 5

6 From steady-state to transient 6

7 From steady-state to transient Surface temperature of west facing wall 7

8 By definition: an approximation of reality Safe Storage Capacity Wetting Wetting Drying John Straube

9 By definition: an approximation of reality Safe Storage Capacity Wetting Wetting No Drying John Straube

10 By definition: an approximation of reality Safe Storage Capacity Wetting Wetting Rain absorption penetration Built-in Condensation air convection vapor diffusion Drying Drainage Air convection Evaporation-Diffusion John Straube

11 From steady-state to transient construction details orientation inclination material properties ρ, c, λ, µ w = f(ϕ) D w = f(w) initial conditions e.g. construction moisture w φ = φ t H T = T t ( φ ) ( D φ + δ ) φ p p sat ( λ T) + h v δ ( φ p ) ( ) p sat climate conditions temperature, RH, radiation, precipitation, wind speed & direction dynamic temperature and moisture profiles heat and moisture fluxes 11

12 Contents: From steady-state to transient Heat storage and -transport Moisture storage and -transport Calculation of coupled transport Model limitations 12

13 Heat storage Heat storage of dry material h d = ρs cs ϑ Heat storage moist material h = h + m d h water Heat of fusion water (ice water) 13

14 Heat storage Due to correlation of RH and capillary pressure there is no ice formation below freezing limit φ e Ice formation depends on capillary pressure i.e. pore-size: in small pores the freezing temperature may lie well below 0 C (32 F) 14

15 Heat transport Transport mechanism Driving potential Heat conduction Latent heat flow Temperature Vapor diffusion with phase change Heat radiation Temperature 4 Convection Pressure and density differences 15

16 Energy balance h m = q + t p Heat conduction: q = λ T Latent heat source: p = h v g vapour 16

17 Contents: From steady-state to transient Heat storage and -transport Moisture storage and -transport Calculation of coupled transport Model limitations 17

18 Moisture storage No state of equilibrium above w f 18

19 Moisture storage Pressure plate 19

20 Vapour transport Transport mechanism Gas diffusion (Fick) Effusion (Knudsen) Convection Driving potential Partial vapour pressure Partial vapour pressure Pressure and density differences 20

21 Liquid transport Transport mechanism Capillary conduction Surface diffusion Seepage flow Driving potential Capillary suction (RH) Sorption layer thickness (RH) Gravitation 21

22 Moisture transport (vapour and liquid) Outdoor Indoor g δ = µ v p v g l = DO ϕ = D D (p w ) p g l pc c c (w) w Dϕ (w) ϕ 22

23 Contents: From steady-state to transient Heat storage and -transport Moisture storage and -transport Calculation of coupled transport Model limitations 23

24 Calculation of coupled transport Coupled transport equations Exponential increase of saturation pressure with temperature Heat transfer H T T t = ( ) ( λ T ) + h v δ ( φ p ) p sat Moisture depending thermal conductivity Enthalpy flow by vapour diffusion with phase change Coupled differential equations have to be solved numerically. Moisture transfer w φ φ = t ( φ ) ( D φ + δ ) φ p p sat 24

25 Calculation of coupled transport dx Conservative discretisation: implicit finite volumes Influx Efflux Iterative coupling by subsequent solution of transfer equations with under-relaxation t A ρ ρ = a + Sources / Sinks Matrix-solver: Thomas- Algorithm (1D) or ADI (2D) 25

26 Calculation of coupled transport Material properties Initial conditions Climatic data Time steps Construction Numerical grid Input Surface transfer Control parameters New time step Update thermal coefficients Calculate temperature field Update hygric coefficients Calculate moisture field No convergence Yes Temperature fields Heat fluxes Output Moisture fields Moisture fluxes 26

27 Calculation of coupled transport Total Water Content Simulation results may be presented as temporal fluctuation (courses) of water content (total or individual layers) or temperature and RH at monitor positions 27

28 Calculation of coupled transport Film viewer Simulation results may also be presented as local distributions at certain time intervals of water content, temperature and RH or as successive profiles (film) 28

29 Calculation of coupled transport Experimental validation Prerequisites: well defined material parameters (measured for the same material) hourly recording of boundary conditions (natural climate) periodic or continuous readings of total water content (weighing) and moisture profiles (e.g. NMR- scanning) IBP weather station 29

30 Calculation of coupled transport Seal Exposed side Sandstone Insulation Comparison of total water content 30

31 Calculation of coupled transport NMR-Scanner Comparison of moisture distributions during a test period of 80 days 31

32 Calculation of coupled transport Experiment Calculation Comparison of long-term moisture profiles 32

33 Walls: brick masonry wall elements Exposed masonry elements with construction moisture: good agreement with periodic weight recordings before driving rain gauge was replaced Good agreement during the first 6 months afterwards odd behavior of concrete samples 33

34 Laboratory tests (1D) Different types of natural sandstone were brought in close capillary contact by kaolin and sealed for water absorption tests Water uptake was recorded by weighing and NMR scanning 34

35 Laboratory tests (1D) Sqrt of time Sqrt of time 35 Water absorption [kg/m²] Water absorption [kg/m²]

36 Laboratory tests (1D) simulation results smoothed to account for NMR resolution Water content [kg/m³] Water content [kg/m³] Position [mm] Position [mm] 36

37 Calculation of coupled transport CSB wall with exterior insulation Practice case: drying of construction moisture 37

38 Calculation of coupled transport Experimental validation of temperature and RH fluctuations CSB wall with exterior MW insulation (ETICS) RH (capacitive sensor) & temperature (PT100) 38

39 Calculation of coupled transport Temperatur [ C] Temperature [ C] Messung Measurement Rechnung Calculation Measured and simulated temperature fluctuation at the exterior surface of the insulation layer of west facing wall

40 Calculation of coupled transport Relative Feuchte RH [%] [%] Relative Feuchte Relative Humidity - Westseite Measurement Messung Calculation Rechnung Measured and simulated RH fluctuation at exterior surface of the insulation layer of west facing wall 40

41 Contents: From steady-state to transient Heat storage and -transport Moisture storage and -transport Calculation of coupled transport Model limitations 41

42 Limitations of the Model Every model has its shares of limitations. The user must be aware of what the model can do and cannot do. 42

43 Model limitations Disregarded phenomena significant e.g. air convection 43

44 Model limitations Boundary conditions far away from normal situation Temperatures >> 50 C (e.g. in case of fire) 44

45 Model limitations Boundary conditions change very rapidly Local sorption equilibrium is not achieved due to heavy fluctuations 45

46 Model limitations Property changes due to contamination e.g. salt Example: clay brick with NaCl Dry-Cup Wet-Cup Vapour diffusion affected by salt Water retention increased by salt Ref.: Poul Klenz Larsen 46

47 WUFI Workshop at NTNU /SINTEF 2008 Fundamentals