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

!!"

#$$%

&'

(

()=)

2TR

PT

Pe

v

vSvSTR

Tgv

µ**

µ

gK wllg /, !"""" µµµµ =##+#=# !!

!!!!

"

#

$$$$

%

&

''''

(

)

****

+

,

-.

.

/=/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

!"

#$%

&

'

'(+

'

'+)*

+,-

.!"

#$%

&/

'

'(

'

'=

'

'

x

Tl

xg

xl

xt

Tc Tgg )( 00

µ01 µ

!"

#$%

&

'

'(

'

'+)*

+,-

.!"

#$%

&/

'

'(

'

'=

'

'!!"

#$$%

&

'

'

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