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Applied Thermal Engineering 51 (2013) 144e154

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Numerical simulation for analyzing the thermal improving effect ofevaporative cooling urban surfaces on the urban built environment

Jin Wei a, Jiang He a,b,*

aCollege of Civil Engineering and Architecture, Guangxi University, 100 Daxue Road, Nanning, Guangxi 530004, PR ChinabKey Laboratory of Disaster Prevention and Structural Safety, Ministry of Education, 100 Daxue Road, Nanning, Guangxi 530004, PR China

h i g h l i g h t s

< We model two evaporative cooling strategies with ability to cool urban surfaces.< The proposed pavement model is validated using the experimental data.< The proposed models are integrated into a 3D-CAD-based thermal simulation tool.< A case study is carried out to demonstrate the applicability of the developed method.< The thermal improving effects can be quantified in terms of surface temperatures.

a r t i c l e i n f o

Article history:Received 28 June 2012Accepted 29 August 2012Available online 12 September 2012

Keywords:Evaporative coolingThermal modelingCAD modelSurface temperatureThermal environmentSimulation

* Corresponding author. College of Civil EngineerinUniversity, 100 Daxue Road, Nanning, Guangxi 537713232057; fax: þ86 7713236273.

E-mail address: kkj2010@qq.com (J. He).

1359-4311/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2012.08.06

a b s t r a c t

This paper presents a numerical simulation method for analyzing the thermal improving effect of urbansurfaces (pavement and building surfaces) with evaporative cooling effect on the urban built environ-ment. The numerical models for simulating the cooling effects of these cooling surfaces were proposedafter analyzing the experimental data. The surface wet condition of the moist pavement can be modeledas a function of the pavement water content. The cool building surface is coated with photocatalyst (TiO2)and sprinkled with water in order to cool external surfaces of the building. During sprinkling, the wettedbuilding surface can be modeled to be covered with a free water surface because the TiO2-coated surfacecan be completely covered with a thin film of water. The simulation algorithms for the two coolingstrategies were integrated into a 3D-CAD-based thermal simulation tool. The simulated surfacetemperature agrees well with the measured result for the cool pavement. As a case study, the coolingeffects of these cooling strategies applied to an urban area were simulated and quantified in terms ofsurface temperature reduction, mean radiant temperature (MRT) and heat island potential (HIP). Asa result, the thermal improving effect can be visualized on 3D-CAD models for the analyzed urban block.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Heating and heat storage of pavements and building envelopsby sunlight have been suggested to be one cause of urban heatisland formation. The reason for this is that the pavements andbuilding envelops in most cities are made of materials with lowalbedo and high heat capacity. This means that they absorb a greatdeal of solar heat during the day and that their surfaces remainwarmer than the surrounding air at night due to the absorbed solar

g and Architecture, Guangxi0004, PR China. Tel.: þ86

All rights reserved.4

energy. Consequently, reducing the heat absorbed by these urbansurfaces or lowering the surface temperature would be effective forurban heat island (UHI) mitigation. UHI mitigation strategies suchas cool roofs and cool pavements have been proposed in many paststudies [1e4]. The environmental improving and energy-savingeffects of these cooling strategies have also been studied throughfield investigations or using analytical methods. The present studyfocused on two passive cooling strategies with the ability to coolpavement and building surfaces by evaporation of water. The coolpavement is made of porous materials and has a capability ofwater-holding [5e7]. The cool building surface is a photocatalyst(TiO2)-coated surface which is sprinkled with water. This isa newly-developed cooling technology in Japan [8]. As reported inprevious studies [9,10], experiment results have shown that thethermal environment in the outdoor living spaces can be improved

Fig. 2. Photo of the experimental mock-up and experiment site.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154 145

from the application of the cooling system. In order to makea better use of the two cooling systems, it requires a quantitativeanalysis of the thermal improving effect at the design stage. Asa computer-aided simulation tool for supporting the thermalenvironment design, we attempt to use a 3D-CAD-based thermalsimulation tool [11]. This study was aimed to develop simulationmodels for the two cooling strategies and integrate the calculationalgorithms into the simulation tool. The methodology of thethermal modeling is described below and simulation results ofa case study for the cooling effects from the application of thesecooling strategies are also shown in the paper.

2. Methodology

2.1. Modelling of the evaporative cooling pavement

For predicting the surface temperature of the ground withevaporation, several precise models have been developed byconsidering simultaneously heat and moisture transfer [12,13]. It isreally hard to apply these models to a meso-scale simulation due tolarge computational load. On the other hand, some practical andsimplified models for the urban/built thermal environment simu-lation can be also found in the literature [14e17]. However, in theseworks, solar absorbance, evaporation efficiency, conductivity andheat capacity were considered to be constant. To reduce discrep-ancies in the prediction of the surface temperature, it is necessary toconsider the surfacewet conditionand influence fromwater contentof the cool pavement. From this view of point, the energy and massflow at the pavement surface can be modeled as shown in Fig. 1.

The energy balance equation at the pavement surface andconduction equation inside the pavement can bewritten by Eqs. (1)and (2) respectively. Evaporation (E) from the pavement surface iscalculated by Eq. (3). The water balance can be expressed by Eq. (4).The conductivity and volumetric specific heat of the pavement aregiven from theoretical equations for a medium composed of air andliquid and solid, respectively expressed by Eqs. (5) and (6).

aðuÞRs þ acðTa � TsÞ þ 3

�RL � sT4s

�� i$E ¼ �lð4Þ vT

vz(1)

vTvt

¼ lð4Þcr

v2Tvz2

(2)

E ¼ bðuÞ$accm

$ðXs � XaÞ (3)

Fig. 1. Heat and water balance at the pavement surface.

4ðtÞ ¼ 4ð0Þ �

Z

t

Edt

Vp(4)

lð4Þ ¼ l4ss $l4w

w l4aa (5)

czð4Þ ¼ cw$rw$4w þ cs$rs$ð1� hÞ (6)

To make clear the relationship between the surface wet condi-tion and water content, experiments were conducted using pave-ment mock-ups [3]. The pavementmock-up was made of pavementblocks with a size of L200 � W100 � H60 mm. Fig. 2 shows a viewof the experiment site. Table 1 lists main instruments and specifi-cations of sensor used in the experiment. Surface wet conditions ofthe pavement blocks were measured using a near-infrared aqua-meter. Surface wet conditions for more than 5 points on thepavement surface were observed, as illustrated in Fig. 3. The aver-aged value of themeasured data for test points was used in analysis.Fig. 4 gives photos of three surface wet conditions: (a) completelywet, (b) partly dry and (c) completely dry. The wet surface ratio isdetermined by Eq. (7):

u ¼ ðSx � SdÞ=ðSw � SdÞ (7)

Table 1List of main measurement sensors used in the experiment.

Measurement point Sensor type (sensitive range) Resolution/Accuracy

Dry-bulb and wet-bulbtemperature ofambient air

4: 0.3 mm T-type thermocoupleinside a mechanically-aspiratedcylinder

�0.1 �C

Solar radiation in theoutdoor location

Pyranometer(sensitive waveband: 0.3e2.8 mm)

�5%

Reflected solar radiation Spectroradiometer(sensitive waveband: 190e250 nm)

�2%

Wind speed anddirection

Wind vane anemometer(0.4e70 m/s, 0e540�)

�0.3 m/s, �5�

Precipitation Rain gauge (4: 200 mm) �0.5 mmSurface temperature Spot infrared thermometer

(sensitive wave: 8e14 mm)�0.5 �C

Surface wet ratio Near-infrared aquameter(sensitive wave: 1.2 mm,1.45 mm, 1.94 mm)

4: 25 mm

Weight Electronic balance(0e6.1 kg)

�0.1 g

Fig. 3. Schematic diagram for observing the surface wet condition.

Fig. 5. Correlation between pavement’s water content and surface wet ratio from themeasured data for the days without rain between Apr. 2005 and Aug. 2006.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154146

here u is the wet surface ratio, Sx is a value read from the near-infrared aquameter under various wet conditions, Sd is the readvalue for a completely dry surface and Sw is the read value fora completely wet surface. Wet surface ratios u for a completely dryand a wet surface are normalized to 0 and 1.0, respectively. Formeasuring the water content of a pavement block, three pavementblocks were selected as measurement targets. Each test block wascut to three segments at two elevations of 8 mm and 28 mm fromthe top surface. The water content was calculated from themeasured values of weight for these segments, and density in thecompletely dry condition. The solar reflectance of the pavementblock was measured in the laboratory using a spectroradiometerwith a waveband of 190e2500 nm.

Figs. 5e7 were prepared from the experimental data. As shownin Fig. 5, the surface wet ratio can be expressed as a function ofwater content and an empirical relation between them wasformulated by regression analysis. This figure represents 165measured values under different surface wet conditions over oneyear from Apr. 2005 to Aug. 2006. The correlation exhibited in thisfigure is 0.95 and the root mean square error is 2%.

The correlation between the solar absorptance and surface wetratio is illustrated in Fig. 6. From a regression analysis, the relationbetween the solar absorptance and surface wet ratio can be ob-tained, as shown in Fig. 6. A characteristic can be found that thesolar absorptance is in direct proportion to the surface wet ratio,i.e., the surface gets wetter and its solar absorptance goes higher.This figure reveals a good agreement between measured data andpredicted values. The correlation coefficient reached 0.99 and theroot mean square error is 0.01 for the solar absorptance.

The correlation between the evaporation efficiency and surfacewet ratio was investigated using the following method. The surfacetemperature, surface wet ratio, rate of evaporation for the testpavement block and the environmental parameters were measuredunder different wet conditions. Applying these measured data to

Fig. 4. Photos of the pavement surface for three wet conditions

Eq. (3), a scatter plot was obtained as shown in Fig. 7. The regressionanalysis gives the relation between the evaporation efficiency andsurface wet ratio in the form of a logarithmic function. Some ofdeviations between modeled and measured values may be due tothe reason that the measurements were conducted in the outdoorlocation and outdoor conditions (especially wind condition) wereunsteady in most cases. The correlation coefficient is 0.94 and theroot mean square error is 0.13 for the evaporation efficiency.

2.2. Validation of the proposed pavement model using theexperimental data

The modeling of the pavement described above was integratedinto the simulation tool. Weather data measured during threeexperimental days from Aug. 11 to 13 of 2005 was used as inputdata for the simulation. Instant values from all sensors wererecorded automatically every 10 min. Hourly values were preparedfrom the measured data and used as the simulation input data andanalysis data. The pavement mock-up was saturated by sprinklingat the night of Aug. 10. The saturated water content was known tobe 23% by measurement. The experiment was carried out in an un-shaded outdoor location. As a calculation condition, the depth ofthe pavement was set to be 6 cm, assuming a layer of soil witha depth of 100 cm beneath.

Fig. 8 is a comparison between simulated and measured resultsof the surface temperature and water content. This figure showsa good agreement between simulation and measurement. Themaximum difference between the simulated and measured surfacetemperature was lower than 1 �C and 1.5 �C in the daytime andnighttime, respectively. A slightly greater difference was found in

: (a) completely wet, (b) partly dry and (c) completely dry.

Fig. 6. Correlation between surface wet ratio and solar absorptance.

Fig. 8. Comparison between simulated and measured results for three experimentaldays without rain.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154 147

the period from the nighttime of the second day to the earlymorning of the third day. These deviations may be due toa phenomenon of increase in the surface wet ratio during thenighttime without surface condensation. This case was not consid-ered in the present simulation model. The analysis gives that thestandard deviation for the simulated surface temperatures is 0.4 �C.

2.3. Description of the building surface with a water film

The building surface with a water film is an application of thesuper-hydrophilicity of the TiO2 coating. The contact angle of waterwith the TiO2-coated surface is several dozen degrees. The irradi-ation of UV light from sunlight then causes the contact angle todecrease gradually, until ultimately the contact angle reaches 0�.Therefore the entire TiO2-coated surface is easily covered withwater. As a result, a thin water film can be formed on the surfaceand evaporation of water throughout the surface can thus beachieved.

In the present study, a TiO2 coating was applied to the walls thatreceive high amounts of solar radiation (e.g., east-, south-, or west-facing walls, including windows) and sloped roofs, because lesssunlight is irradiated on north-facing walls and water cannot easilyflow off of a flat roof. TiO2-coated shading screens were used infront of walls with pent roofs or verandas.

Fig. 7. Correlation between surface wet ratio and evaporation efficiency.

The water system is briefly described as follows. As shown inFig. 9, water is sprinkled from outlets at the top of a wall or roofallowing it to flow down the wall or roof surfaces. All of watercollected at the bottom of the wall drains into awater tank which isconnected to the rainwater pipe. Water for sprinkling is pumped upfrom thewater tank. Sprinkled water collected inwater collectors isreused. Collected rainwater is also reused for sprinkling, and wateris supplied automatically from the water service pipe if collectedrainwater is unavailable.

Fig. 9. Schematic description of the building surface with water film.

Fig. 10. Energy flow at the surface with water film.

Fig. 11. Structure diagram f

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154148

2.4. Modeling of the surface with moving water film

Experimental studies have been carried out in the buildings towhich the cooling system was applied. For example, Naganumaet al. [9] have conducted experiments and quantified the thermalparameters for predicting the surface temperature and evaporationof an alumni-finished wall with a TiO2 coating. Takeda et al. [10]have conducted an experimental study and performed a numer-ical simulation to analyze the reduction effects of indoor temper-ature and cooling load for an experimental building with TiO2-coated surfaces. However, these previous studies have not quanti-tatively evaluated the thermal improvement effect of the coolingsystem on the surrounding environment. Based on the analysis ofthe measured results from the previous studies, a thermal modelfor the surface with water film was proposed and described below.

Fig. 10 illustrates the energy flow paths at the surface coveredwith a water film. The energy exchange equations at node i for thewaterfilmand surface canbewrittenbyEqs. (8) and (9) respectively.

or the simulation tool.

Fig. 12. (a) Albedo of the external surfaces, (b) heat transmissions and (c) capacity for the walls and roofs are visualized on the 3D-CAD models of the analyzed area.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154 149

Table 2Calculation conditions used in the simulation.

Commercial/Office building Residential house

Indoorair-conditioning

Cooling setpoint: 26 �CPeriod: 7:00e24:00

Cooling setpoint: 26 �CPeriod: 19:00e24:00

Sprinkling buildingsurface

Watering period: 9:00e18:00Water supply ¼ 12 kg/h

Weather data Hourly weather data for Tokyo

Fig. 13. Simulation results of surface temperatures for three points AeC on thebuilding surfaces during a summer period of August 5e8.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154150

Heat transfer caused by water flow is considered and expressed atthe right of Eq. (8), where compared with the water flow rate (Wq),evaporation is insufficient, so that the decrease in water during theflow fromwater outlets to water collectors can be neglected.

acðTa � TwðiÞÞ þ 3

�RL � sT4wðiÞ

�� awðTwðiÞ � TsðiÞÞ

� i$acCm

ðXsðiÞ � XaÞ ¼ CwWqvTwvz

(8)

aRs þ awðTwðiÞ � TsðiÞÞ ¼ �lvTvx

(9)

Convection coefficient aw is determined from Eq. (10):

aw ¼ Nulwz

(10)

Nu is computed from an empirical equation, i.e. Eq. (11)(JohnsoneRubesin equation).

Nu ¼ 0:0296Re0:8Pr0:6 (11)

Re is calculated by Eq. (12).

Re ¼ Uwzn

(12)

ac is considered to be a function of wind speed and is given by theJurges equation [18]. A detailed description of the modelingmethodology can be found in the previous paper [19].

2.5. Description of the simulation tool

The simulation process of the simulation tool used in the anal-ysis is illustrated in Fig. 11. The simulation is performed using 3D-CAD models for buildings, trees and other structures in the areabeing analyzed. The 3D-CAD models are generated usinga commercial 3D-CAD system called VectorWorks. Applicationprograms for mesh generation and data assignment were inte-grated in the 3D-CAD system. Three-dimensional spatial forms ofbuildings and two dimensional ground surfaces are divided intomesh grids. Thermophysical data of construction materials such asalbedo, conductivity and solar transmittance are assigned to thegrids. An automatic mesh-dividing process with a spatial resolutionof 0.05e5 m (a practical size is 0.1e0.4 m) has been designed andonly uniform mesh can be used in the present version of the tool.Three-dimensional radiation (solar radiation and longwave radia-tion) from surroundings (the sky, ground and surroundings) wasconsidered in the heat balance calculation for each mesh.Conduction heat was assumed to be transferred in one directionwhich is normal to the mesh surface. The external surfacetemperature for each grid can be determined by solving anunsteady-state one-dimensional heat balance equation in thenormal direction of the surface. A backward-differencemethodwasused and the calculation time-step used in this study is 5-min. Onerunning simulation runs for five days at a 5-min time-step in orderto obtain a periodic steady-state solution with initial conditions ofperiodic weather data. The calculation results of the surfacetemperature for the 5th day are output for analysis.

In the heat balance equation, direct solar radiation, sky solarradiation and reflected solar radiation are considered to be short-wavelength radiation irradiated on the surface. The reflected solarradiation includes both specular reflection and diffuse reflection.Atmospheric radiation and long-wavelength radiation from thesurroundings are considered as long-wavelength radiation. Skysolar radiation and atmospheric radiation are calculated from the

sky factor for each mesh. Convective heat transfer is calculated onthe assumption that ambient air temperature and wind velocity areuniformly distributed in the outdoor spaces of the analyzed area. Amore detailed description of the calculation methodology can befound in Reference [11].

As outputs of the simulation, the temperatures of all externalsurfaces can be predicted and visualized on a color 3D display. Fromthe calculated results of the surface temperatures, themean radianttemperature (MRT) and the heat island potential (HIP) can be alsoestimated. The HIP is defined as an index by which to express theamount of sensible heat flux emitted from an urban area into thesurrounding atmosphere and is calculated by Eq. (13) [20].

HIP ¼

Z

all surfaces

ðTs � TaÞds

A(13)

The MRT at a height of 1.5 m above the ground was used toevaluate the thermal radiation effect on a person in outdoor humanactivity spaces. It is defined as the uniform temperature of animaginary enclosure inwhich radiant heat transfer from the humanbody or object equals to the radiant heat transfer in the actual non-uniform enclosure. The MRT is calculated by Eq. (14).

MRT ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXNs

i¼1

FiT4si4

vuut (14)

Fig. 14. Distributions of surface temperature for Case 1 and Case 2 at 12:00 and 15:00 on August 7.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154 151

3. Case study

3.1. Study area and simulation conditions

An urban area in downtown Tokyowas selected for analysis. It isan area consisting of commercial/office and residential buildings.Simulations for the following two cases were performed based onthe 3D-CAD models for the area.

Case 1: without the application of the two cooling strategies.Case 2: with the application of the two cooling strategies.

In Case 2, the evaporative cooling pavement was applied to thesidewalks and groundwithin the area. The evaporative cooling wallwas applied to thewalls that do not face north andhavenot veranda.The evaporative cooling roof was applied to the sloped roofs. Fig. 12shows the 3D models on which thermophysical properties (albedo,

Fig. 16. Profile of MRT at 1.5 m height above the ground for a passage (aej) shown inFig. 12. The graph was prepared from the simulated results for 12:00.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154152

heat transmission, heat capacity) for the walls and roofs and otherstructures are visualized. Input conditions for the simulation aregiven inTable 2. Auniformmesh size of 0.2m for this case studywasused in the simulation. The computing time for a single simulationwas approximately 2 h with a desktop PC (its CPU specifications:Intel Xeon, 3.06 GHz, 3 GB RAM). For simulations, a PC with thefollowing CPU specifications is required at least: 2.0 GHz, 2 GB RAM,single processor.

3.2. Simulation results

3.2.1. Surface temperature distributionSimulations were performed using hourly weather data of

Tokyo for a summer period. Fig. 13 is the simulated surfacetemperatures for Point A (awest-facingwall surface), Point B (a roofsurface) and Point C (a pavement surface) during the period of Aug.5e8. Positions of Points AeC are indicated in Fig. 12. In Case 2, thepavement was saturated bywatering at 0:00 of Aug. 6. Thewall androof were sprinkled between 9:00 and 18:00 from Aug. 6 to 8 inwhich each day is sunny. From the top graph of Fig. 13, it can beseen that the roof temperature (Point B) for Case 1 exceeded 60 �Cat noon andwasmore than 20 �C higher than that (the cooling roof)for Case 2. The wall surface temperature (Point A) for Case 1reached 40 �C in the afternoon. On the other hand, the wall surfacetemperature for Case 2 was lowered below 30 �C. As seen in thebottom graph of Fig. 13, the surface temperature (Point C) of thepavement (Case 1) without the application of the evaporativecooling pavement was about 5 �C and 3 �C higher than those forCase 2 in the daytime and nighttime respectively.

Fig. 15. Distribution of MRT for Case 1 and Case 2 at 12:00.

Fig. 14 is surface temperature distributions for 12:00 and 15:00on Aug. 7 which is one day later after the pavement was watered.From this figure, it can be easy to find the reduction effect of thesurface temperature for Case 2 due to the application of the coolingstrategies. As shown in the bottom thermograph of Fig. 14, it can bealso noted that the surface temperatures of the shaded pavementfor Case 1 and Case 2 were slightly higher and lower than ambientair temperature, respectively.

3.2.2. Distribution of mean radiant temperature (MRT)Fig. 15 presents MRT distributions at a height of 1.5 m from the

ground for Case 1 and Case 2 at noon on Aug. 7. As seen in this figureit is obvious that the MRTs for Case 2 are 2e6 �C lower than that forCase 1 in outdoor locations. A greater MRT reduction effect can befound in the shaded outdoor locations near the walls. In order toquantify theMRT reduction effect, theMRT profile for a passage (aej) within the analyzed areawas output and presented in Fig.16. Thisfigure shows that the MRT at the passage for Case 1 is higher thanthat for Case 2. The differences in MRT between two cases were4.3 �C for a SeN-oriented shaded street canyon (Lines aeb), 6.5 �C atamaximumfor a SeN-orientedun-shaded street canyon (Lines eef),2.3�C for an EeW-oriented shaded street canyon (Lines hei), 2.7 �Cfor a SeN-oriented tree-shaded sidewalk (Lines iej), respectively.

3.2.3. Diurnal variation of heat island potential (HIP)Diurnal variations in the HIP for Aug. 7 are indicated in Fig. 17.

The HIP for Case 1 reachedmore than 30 �C in themid-daytime andwent down to 2e3 �C at night. The nighttime HIP is a positive value,whichmeans that there is sensible heat flux flowing from the urbanarea to the atmosphere, in other words, the urban air is warmedduring the nighttime. On the other hand, the HIP for Case 2 was lessthan 5 �C during the watering (sprinkling) period of 9:00e18:00 inthe daytime and went down below 0 �C in the late afternoon. Inother words, Case 2 could create a greater HIP reduction than thatfor a lawn-planted ground in the daytime. The negative values ofHIP at night suggest that the urban air is cooled in the nighttime forCase 2.

Fig. 17. Diurnal variation of HIP for Cases 1e2 on August 7.

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154 153

4. Conclusions

Simulation models for a pavement and a building surface withevaporative cooling effect were developed. Validation of theproposed modeling methodology was discussed using the experi-mental data. The proposed simulation models were integrated toa 3D-CAD-based thermal simulation tool for predicting and eval-uating the thermal improving effect of these cooling strategies. Themain findings from this study can be summarized as follows:

The surface wet condition of the moist pavement can bemodeled as a function of the pavement water content, and itsempirical equation can be derived from the analysis of the exper-imental data. The correlation between the surface solar absorp-tance and surface wet ratio can be also formulated from theexperimental data. A logarithmic correlation was found betweenthe surface wet ratio and evaporation efficiency. The simulatedtemperature of the cooling pavement based on the proposedsimulation model agreed well with the measured result. Thewetted building surface is a TiO2-coated surface with a thin layer ofwater which can be modeled as a free water surface.

A case study for simulating the thermal improving effects of thetwo cooling strategies was carried out. An urban area in downtownTokyo was selected as a study area. Compared with the casewithout the application of the cooling strategies, in the case withthe application of the cooling strategies, the surface temperaturesof the sunlit pavement, building wall and roof can be lowered bymore than 5 �C, 7 �C, 20 �C during the daytime on a sunny summerday, respectively. Furthermore, aMRT reduction of 2e6 �C and a HIPreduction of 20e30 �C can be realized during the daytime.

Simulation results such as surface temperature and MRT can bevisualized in color images using this simulation tool. From theseimages, a designer can easily understand where is cooled, howmany degrees theMRT value could be reduced in the passage space.

In our future work, a study on energy performance of theproposed cooling systems and comparison with other cooling strat-egies is planned.Moreover, the influence of nocturnal increase in thesurfacewet ratio on the prediction precision of the pavement surfacetemperature will be considered for future model improvement.

Acknowledgements

The authors would like to acknowledge the financial supportfrom the Scientific Research Foundation of Guangxi University (No.XGZ110356) during this paper preparing.

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Nomenclature

as: solar absorptanceA: horizontal area for an analyzed area (m2)cs: specific heat (J/(kg K))cm: specific heat of humid air (J/(kg K))cw: water specific heat (J/(kg K))ds: surface area of a mesh (m2)E: evaporation (kg/(m2 s))Fi: view factor from a point to surface ih: wall height (m)i: number of meshNs: number of surrounding surfacesNu: Nusselt numberPr: Prandtl numberRe: Reynolds numberRL: long-wave radiation (W/m2)Rs: solar radiation (W/m2)s: small area (m2)Sd: a value read from the near-infrared aquameter for a completely dry surfaceSw: a value read from the near-infrared aquameter for a completely wet surfaceSx: a value read from the near-infrared aquameter for a wet surfacet: time (s)T: temperature (K)Ta: outdoor air temperature (K)Ts: surface temperature (K)Tw: temperature of water film (K)Uw: velocity of water flow (m/s)Wq: water supply (kg/s)Vp: volumetric rateXa: specific humidity mixing ratio at temperature Ta (kg/kg(DA))Xs: specific humidity mixing ratio at temperature Ts (kg/kg(DA))z: distance from water outlet (m)ac: surface convection coefficient (W/(m2 K))aw: surface convection coefficient (W/(m2 K))b: evaporation efficiencyε: emissivity4: volumetric water content4a: air volumetric rate4s: solid volumetric rate4w: water volumetric rateh: percentage void volume of the total volume

J. Wei, J. He / Applied Thermal Engineering 51 (2013) 144e154154

i: latent heat (J/kg)l: thermal conductivity (W/(m K))la: air thermal conductivity (W/(m K))ls: solid thermal conductivity (W/(m K))lw: water thermal conductivity (W/(m K))

rs: solid density (kg/m3)rw: water density (kg/m3)s: StefaneBoltzmann constant (W/(m2 h K4)n: kinematic viscosity (m2/s)u: surface wet ratio