Evaporation Performance Analysis forWater Retentive Material Based on
Outdoor Heat Budget and Transport Properties
S. Kinoshita, A. Yoshida and N. OkunoDept. Mechanical EngineeringOsaka Prefecture University
The 2nd International Conference onCountermeasures to Urban Heat Islands
Berkeley, California
September 21, 2009
Background
Recently, urban heat island is remarkable at almost metropolisesin Japan, and countermeasures for the phenomenon are urgentlydemanded.
Paving with water retentive material on streets or roadsTo diminish sensible heat flux and reduce air temperature byabsorbing latent heat of water retained in the material.
It is necessary for the promotion of the water retentive materialthat the method of performance evaluation of the material isestablished.
Rn lE
G
HRn lE H
G
Objectives
To evaluate the evaporation performance of waterretentive material outdoors in the condition simulatedpavement with the material. To evaluate the evaporation efficiency by measuring theheat budget on the surface of material. To measure heat and moisture transport properties andapply them to the numerical analysis in order to investigatethe evaporation behavior in detail.
Field MeasurementTemperature and humidity
sensor (height = 1m)Ultrasonic anemometer
(height = 1m)
50cm30cm
1.5
m
Water retentive material
Radiometer
Heat insulator
1.5m
9cm
Heat flow plate4
0cm
Plastic wrap
Wood frameSand
Thermocouple
Measuring object 50 water retentive material blocks in awood frame Size: 1.5m x 1.5m Sand layer is laid by assuming pavement
Measuring items Surface and internal temperatures ofwater retentive material with thermocouple Conductive heat flows near surface andat the bottom of material with heat flowplates Net radiation Air temperature and relative humidity Wind velocity
Data sampling rate: 1/20 sec-1
Averaging time: 10 minutes
Field Measurement
Water retentive materialPorous media
Raw materialsRecycled material: 80%
Stone powder (Ooyaishi), wasted molding sand, and etc.Clay: 20%
Porosity: 24.1%Water retention: 12.1 l/m3
Size: W300mm H150mm D50mmW150mm H150mm D50mm
Measurement site:Open terrace in Osaka Prefecture University,Sakai, Osaka
Date and measuring condition:Case 1:
Sep. 11, 2007Full wet condition
Case 2:Jul. 31, 2008After drying for one day
Field Measurement
Calculation of Evaporation Efficiency
Evaporation efficiency: β The relation between wind speed and convectiveheat transfer coefficient for measuring sensibleheat flux is pre-estimated in the dry condition.
α : Convective heat transfer coefficientTsur:Surface temperature, Tair: Air temperature
Rn : Net radiationlE : Latent heat fluxH : Sensible heat fluxG : Conductive heat flux into materialE : Evaporation ratel : Latent heat of waterqsat(Tsur) : Saturated specific humidity of watersurface based on surface temperatureqair : Specific humidity of atmospherek : mass transfer coefficient
Convective heat transfer coefficient
[W/m
2K]
Wind speed U [m/s]
= 4.86U + 6.02
(cor = 0.808)
!
!
May. 21st. 2008 Aug. 27th. 2007
May. 27th. 2008 Sep. 5th. 2007
0 1 2 3 4 50
10
20
30
40
50
GHRnlE !!=
llEE /=
))(( airsursat qTqk
E
!="
GRnH !=airsurTT
H
!="
02.686.4 += U!
JST
Tsur Tair Rh Relative humidity [%]
9 10 11 12 13 14 15 16 1725
30
35
40
45
50
0
20
40
60
80
100
JST
Temperature [
oC]
10 11 12 13 14 15 16 1725
30
35
40
45
50
0
20
40
60
80
100
Measured Results
JST
Heat flux [W/m
2]
10 11 12 13 14 15 16 17
0
200
400
600
800
JST
Rn
G
H
lE
9 10 11 12 13 14 15 16 17
0
200
400
600
800
Case 1: Full wet(Sep. 11th, 2007)
Case 2: After drying for one day(Jul. 31st, 2008)
Air and surface temperature
Heat budget
Although Rn in the morning ofSep. 11th is higher than that of Jul.31st, the surface temperature in fullwet condition is lower than thatafter drying condition.
Latent heat flux lE occupies mostof heat budget in both cases.
The ratio of latent heat flux to netradiation in case 2 is smaller thanthat in case 1. On the other hand,the ratio of sensible heat flux incase 2 is twice larger.
JST
Evaporation rate E [g/m
2s]
10 11 12 13 14 15 16 170
0.05
0.1
0.15
0.2
0
0.2
0.4
0.6
0.8
1
JST
E
Evaporation efficiency!
!
9 10 11 12 13 14 15 16 170
0.05
0.1
0.15
0.2
0
0.2
0.4
0.6
0.8
1
Case 1: Full wet(Sep. 11th, 2007)
Case 2: After drying for one day(Jul. 31st, 2008)
Measured Results
Evaporation rate Evaporation efficiency
E and β after drying is lowerthan those in wet condition.
It is revealed that the effect of surface temperature decreases by water retention andthe evaporation efficiency deteriorates as water retentive material becomes drier.
It is necessary to sustain the evaporation by water supply and to construct simultaneously watersupply system in utilization of water retentive material.For this purpose, numerical analysis of heat and moisture transport inside of material is needed.
Fundamental Equations
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(
()=)
2TR
PT
Pe
v
vSvSTR
Tgv
µ**
µ
gK wllg /, !"""" µµµµ =##+#=# !!
!!!!
"
#
$$$$
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&
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(
)
****
+
,
-.
.
/=/TP
T
PTR
vS
vSv
Tgg
µ00µ !1
0, =!!+!=! TlTlTgT """" !!
Estimate relation
ψ : volume moisture contentµ : moisture chemical potentialρ : densityρw : density of waterλ : thermal conductivityPvS : saturated vapor pressureK : hydraulic conductivityλ': moisture conductivity
λ'µ : moisture conductivity related to µ gradientλ'T: moisture conductivity related to temperature gradient
!"
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'
'(+
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'+)*
+,-
.!"
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'
'(
'
'=
'
'
x
Tl
xg
xl
xt
Tc Tgg )( 00
µ01 µ
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'+)*
+,-
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'=
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'
'
x
T
xg
xxtTw 0
µ0
µ
µ
12 µ
Moisture conservation
Energy conservation
Schematic of measurement instrument
Thermal Conductivity λUnsteady hot-wire method based on JIS R2616
Relation between thermal conductivityand moisture content of material
Moisture content [kg/kg %]Thermal conductivity [W/mK]
= 0.085 + 0.911
(cor = 0.991)
!"
"
!
measured value
averaged value
0 2 4 6 8 10 120
1
2
3
Evaluation of Transport Properties
Data logger
Electrically conductive
lead (enamel)
Thermocouple
Specimen
Hot wire
(nichrome)
Constant
temperature/humidity bath
DC power source
DC power source
for current control
0 20 40 60 80 1000
1
2
3[!10-11
]
Mois
ture
conducti
vit
y [k
g/m
sPa]
Averaged relative humidity![%]
0 96 192 288 384 480-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
Lapsed time [hour]
Vari
ati
on o
f w
eig
ht
[g/h
our]
0% 32.8% (No.1) 32.8% (No.2) 75.3% 100%
Measured result of moisture conductivity
Average1.96 x 10-11 kg/msPa
! 78
90
d=
13
.3
! 86
Saturated solution of
salt
pv2
pv1 (T = 25deg.C, Rh = 55%) Thermohygrostat
Specimen
G
! 78! 78
90
90
d=
13
.3d
= 1
3.3
! 86
Saturated solution of
salt
pv2
pv1 (T = 25deg.C, Rh = 55%) ThermohygrostatThermohygrostat
SpecimenSpecimen
G
Evaluation of Transport PropertiesMoisture Conductivity
Using saturated salt solution based on JIS A1324ë!
Schematic of measurement instrument
Weight change per unit time
Permeability [10
-4 mm/s ]
Water level difference [mm]
No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
No.9
0 200 400 600 800 1000
5
10
15
20
K = 6.12×10-5 mm/s
Water
Specimen
!67mm
!40.8mm
11
30
mm
Wat
erle
vel
dif
fere
nce
"h
220mm
Vinyl chloride pipe
Rubber gasket packing
(thickness = 3mm)
Flange
Acrylic tank
(thickness = 10mm)
Standpipe
Evaluation of Transport PropertiesHydraulic Conductivity K
Falling head permeability test based on JIS A1218
Schematic of measurement instrument
Measured result of hydraulic conductivity
TN-2 !N-2
TN-1 !N-1
TN !N
T1 !1
T2 !2
T3 !3
Atmosphere
Water retentive
material
Sand
!x
!x
2
Tair, !air
U
x
Rn
Discretization of fundamental equations using 2nd order centraldifference for diffusive terms.Euler explicit method for time marching
Net radiation: Rn , Wind speed: U ,Air temperature: Tair , Relative humidity: RH (Chemical potential of moisture in air: µair )
Numerical Conditions
Δx = 1mmΔt = 10ms
Initial and Boundary Conditions
Initial profiles of Temperature: linear interpolation of measured values Moisture chemical potential: uniform profile based on bulkwater content at the beginning of measurement and waterretention curve modeled by van Genuchten
Boundary condition of top surface
Boundary condition of bottom surface Measured temperature Moisture transport: impermeable
1=! TRC
vm"#
#µ
!!µ"
"#=# v
m
P
T
Pv
mT
!
!"=" ##
( ) ( ) RnTTllx
Tg
xl
x
TsurairTsurairTgg +!"++!"=#
$
%&'
(
)
)"+*
+
,-.
/!
)
)"!
)
)! )( 00µµ01
µ11 µµ
( ) ( )surairTsurairT TTx
Tg
x!"+!"=
#
#"!$
%
&'(
)!
#
#"! *µµ*+
µ+ µµ
JST
!
Evaporation efficiency
(measured)
(calculated)!
!
9 10 11 12 13 14 15 16 170
0.1
0.2
0.3
0.4
0.5
0.6
Distance from surface [m]
Moisture content [wt %]
10:00 11:00 12:00 13:00 14:00 15:00 16:00
Initial
0 0.01 0.02 0.03 0.04 0.0512.5
13
13.5
14
14.5
15
Evaporation efficiency is the largest at thebeginning, and decreases with the lapse oftime, which agrees with the result of fieldmeasurement.
Evaporation efficiencyMoisture content profile in materialInput data: Measured value in Sep. 11th, 2007
Numerical Results
Evaporation rate at the material surface is faster.Moisture content gradually decreases with theprogress of surface drying. Evaporation rate isthe largest at the beginning of drying anddecreases with the lapse of time.
Bulk moisture content at the end of measurement: Calculation: 12.9wt% Measurement: 11.6wt%
JST
Temperature [
oC]
Tsur (measured)
Tsur (calculated)
9 10 11 12 13 14 15 16 1720
25
30
35
40
45
Numerical Results
The numerical model using in this study can express internal moisture transfer ofthe water retentive material, but remains scope for improvement in heat transfer.
Calculated Heat budgetSurface temperature
Calculated temporal change of temperaturequantitatively agrees with measured one, butthe value of calculated temperature is about5deg.C lower than measured temperature.
Heat flux [W/m
2]
JST [hour]
Rn (mea) G (cal)
H (cal) lE (cal)
9 10 11 12 13 14 15 16 17-200
0
200
400
600
800
G (calculation)H (calculation)
lE (calculation)
Rn (measure)
Latent heat flux lE at the top surface isoverestimated, and sensible heat flux andconductive heat flux are underestimated.
Summary
1. The effect of surface temperature of water retentive materialdecreases by water retention, and the evaporation efficiencydeteriorates as water retentive material becomes drier.Evaporation efficiency can be applied for the performanceevaluation of water retentive material.
2. Numerical analysis using simultaneous heat and moisturetransfer equations can express internal moisture transfer of thewater retentive material, but remains scope for improvementin heat transfer because of overestimation of latent heat flux byevaporation on the material surface.
Water Retentive Material
Properties in catalog (300×150×50mm3)
Calculation12 l / m3Waterretention
JISA520915%Waterabsorption
MethodMeasuredvalue
Raw materials Recycled material: 80%
Stone powder (Ooyaishi), wasted molding sand, and etc.Clay: 20%
Pore diameter [ m]µ
Incremental intrusion [mL/g]
Cumulative intrusion [mL/g]
Incremental intrusion
Cumulative intrusion
10-3
10-2
10-1
100
101
102
103
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0
0.05
0.1
0.15
0.2
Pore Diameter Size Profile
Mercury porosimeter (Porosity: 30.02%)
X-ray CT image (Porosity: 27.54%)
Measurement of Pore diameter size profile of water retentive materialwity Mercury porosimeter and X-ray CT ( Measured in JFCC)
Peak of Pore diametersize profile
10~100µm size pore isdominant
X-ray CT image
(Binarization processingimage)
Pore diameter [ m]µ
Pore volume [ m
3 ]
µ
Pore number
Pore volume
Pore number
0 50 100 150 200 250 300
0.5
1
1.5(!108)
0
500
1000
1500
2000
2500
3000
Pore