Journal of Heat Island Institute International Vol. 12-2 (2017)

Academic Article

Development of a measurment instrument and its features
for estimating the degree of wetness caused by outdoor misting
Yusuke Uchida

Minako Nabeshima

Masatoshi Nishioka

Graduate School of Engineering, Osaka City University
Corresponding author: Minako NABESHIMA, nabeshima@eng.osaka-cu.ac.jp
ABSTRACT
Cooling by spraying mist has received widespread attention in Japan as a countermeasure against thedetrimental health effects of urban heat islands. Sprayed mist is expected to not only decrease the air temperatureby evaporation in the air but also promote evaporative heat dissipation from the skin by wetting. In this study, threecylindrical heated sol-air temperature (SAT) meters of the same configuration were prepared to measure the degreeof wetness of their surfaces inside the cloud produced by mist sprayer nozzles. After determining the differencesamong the three SAT meters, their appropriateness for wetness measurements was evaluated. A piece of wet paper
was pasted such that it covered 20% of the surface area of the SAT meter, corresponding to a wet area fraction of0.20. The wet area fraction was then calculated from the heat balance equation using the results obtained by thethree SAT meters and was estimated as 0.21, corresponding to an error of 5% relative to the actual value.
Key words : Urban heat island, Adaptation, Outdoor mist sprayers, Wet area fraction, SAT

Farnham, Emura, & Mizuno(4) have evaluated the effect of mist

1. Introduction

on reducing the surface temperature of the skin using a dry
Recently, urban heat islands have caused serious healthsilicone rubber skin analog that they developed to characterize a

problems in urban residents and visitors. Mist sprayers are a

misting fan system. Because their skin analog contains

common countermeasure used against heat stroke. Spraying mist

embedded heat flux and temperature sensors, they were able toin an outdoor space can change sensible heat to latent heat in the

directly and approximately measure the heat flux from the skin

air by the evaporation of the mist and cool the surrounding air.

surface. However, it was difficult to analyze the dependence of

Outdoor mist sprayers have been reported to reduce outdoor

heat flux on solar radiation and wind conditions.

(1)temperatures by approximately 1K . In the intervening space

The aims of the present study were to develop an instrument for

between indoor and outdoor environments, they have also been

determining the degree of wetness produced by outdoor misting(2)

reported to reduce the temperature by 2 - 3K .

and clarify the characteristics of this instrument. To this end, aOutdoor mist sprayers can also be expected to cool human

cylindrical heated sol-air temperature (SAT) meter that canbodies by the latent heat of vaporization. It has been shown thatmeasure the fraction of the surface area of the cylindrical meter

there is a correlation between the skin wetness and average skinthat has been wet by outdoor mist sprayers, hereafter referred to(3)temperature at the time of isothermal sensation . Thus, theas the wet area fraction, was developed in this study. It isaverage skin temperature, thermal sensation, or comfort can bepossible to estimate the degree of wetness of the SAT meter

predicted from the measured skin wetness. However, it isbased

difficult to separate the effect of mist from that of sweat, even if

environmental parameters. First, the individual differences inthe skin temperature of a person standing within the mist cloud

surface temperature among the measurements of three heatedis experimentally measured. Developing an evaluation method

SAT meters with the same configuration were determined.

based on physiological response of test subjects on an actualSecond, the measurement accuracy of the wet area fraction andmisting field is a complex task, and a simple method ofheat transfer characteristics was examined. However, in thisevaluating the cooling effects under misting conditions isstudy, the relationship between the wet area fraction of theneeded.

instrument and the wetness of human skin was not explored.

- 84 -

onlyonmeasurements

of

its

temperatureanddefined by Equation (6) (ISO 7726, 1998). Equations (4)–(6)were frequently used in this study.

2. Cylindrical heated SAT meter

After hcn and S were obtained, the latent heat flux Eg in Equation
A copper tube (H: 400 mm, D: 75 mm) was spray-painted black.

(1) was calculated. Finally, the wet area fraction of SAT meter I

Figure 1 shows the cylindrical heated SAT meter developed for

was evaluated using Equation (7).

use in this study. The shape of the SAT meter was designed tosimulate part of the human body, for example, the arm, the neck

3.2 Calculation of convective heat transfer coefficientor the shoulder. Because exposed parts of the upper body are

The heat balance equations for SAT meters I, II, and III are

possibly wet by misting, which causes an increase of a general

shown in Equations (8) - (10). SAT meter I was assumed to beskin wetness. The SAT meter has the same volume as a globe

under forced convection without misting. SAT meters II and III

probe with a diameter of 15 cm, enabling its possible futurewere assumed to be under natural convection without misting.

application as a globe probe substitute.

The surface temperature and physical environment factors wereA 2-m linear heater was attached to the inside of the copper tube

substituted into the heat balance equations for SAT meters II andusing aluminum tape, as shown in Figure 2. Figure 2 shows anIII, which are respectively given by Equations (9) and (10),exploded view of the inside of the copper tube. T typeallowing these equations to be solved simultaneously for thethermocouples that can measure the inside surface temperature

unknown quantities hcn and S. These calculated values were thenat six points were attached between the linear heaters at 25 mm

input into Equation (8), and the forced convective heat transferintervals. Three of the measurement points are arranged incoefficient hcf was evaluated. This experimental value of hcf wastandem at heights of 100, 200, and 300 mm from the bottom ofcompared with the theoretical convective heat transfer

the copper tube on both the front and back sides. Eight pieces ofcoefficient of the cylinder calculated using Equations (11) - (13).

insulation (H: 50 mm, D: 75 mm; Figure 3) were inserted into

Equations (12) is the average Nusselt number for a cylinder in athe copper tube such that no empty space remained.

cross flow recommended by Zukauskas(5) .

3. Theory

4. Methods

3.1

Calculation of the wet area fraction in a

4.1 Surface temperature measurement

verification experiment

In this study, three cylindrical heated SAT meters were created.

Equations (1)–(3) are the surface heat balance equations for SATTo evaluate the latent heat flux and wet area fraction from themeters I, II, and III. SAT meter I was assumed to be set under aheat balance equations for each meter, the accuracy of themist sprayer nozzle inside the mist cloud. SAT meters II and III

surface temperature measurement was confirmed for the SAT

were assumed to be outside of the mist cloud. Simultaneously

meters. The three SAT meters were examined to determine ifsolving Equations (2) and (3) using substitution yielded thethere were any individual differences in surface temperature

natural convective heat transfer coefficient hcn and solar

among them. The experiment required 30 min for the conditions

radiation S; the other parameters in these equations are

to reach a steady state, and the output power of the heater wasmeasureable. The radiant heat transfer coefficient is given by

531.0 W/m2.

Equations (4) and (5), and the average radiation temperature is- 85 -75

5075

400400heat insulatingmaterial

Fig. 1 Cylindrical heated SAT meterℎ𝑐𝑐 (𝑇𝑠1 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠1 − 𝑇𝑟 ) + 𝐸𝑔 = 𝐻𝑖𝑖𝑖𝑖𝑖1 + 𝑆(2)ℎ𝑐𝑐 (𝑇𝑠3 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠3 − 𝑇𝑟 ) = 𝐻𝑖𝑖𝑖𝑖𝑖3 + 𝑆
𝑇𝑚 =𝑇𝑟 = �(3)+ 273.15)3𝑇𝑔 + 𝑇𝑟
24

�𝑇𝑔 + 273.15� + 2.50 × 108
× 𝑢0.6 �𝑇𝑔 − 𝑇𝑎 �𝐸𝑔 = 𝐿𝐿ℎ𝑐 �𝑃𝑔∗ − 𝑃𝑎 �𝑤

SuRe
D(1)

ℎ𝑐𝑐 (𝑇𝑠2 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠2 − 𝑇𝑟 ) = 𝐻𝑖𝑖𝑖𝑖𝑖2 + 𝑆ℎ𝑟 = 4𝜎(𝑇𝑚Fig.3 Insulation piece

Fig.2 Exploded view of insideλv
Pr, PrsNuLRPg*Pa(4)(5)

14� − 273.15ℎ𝑐𝑐 (𝑇𝑠1 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠1 − 𝑇𝑟 ) = 𝐻𝑖𝑖𝑖𝑖𝑖1 + 𝑆

Received solar radiation
Wind speed
Reynolds number
Representative length
Thermal conductivity of air
Kinematic viscosity coefficient
Prandtl number, Prs is evaluated at Ts
Average Nusselt number
Lewis coefficient
Saturated water vapor pressure
Water vapor pressure of air
Stefan-Boltzmann constant

(6)

σ(7)

4.2 Wet area fraction measurement

W/m2
m/s
mW/(m･K)
m²/s
K/kPakPakPa
W/(m2･K4)

The estimation accuracy of the wet area fraction of the SAT(8)meter was verified using a piece of wet Japanese paper (L: 300(9)mm, W: 63 mm) pasted on the surface of the device so that itℎ𝑐𝑐 (𝑇𝑠3 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠3 − 𝑇𝑟 ) = 𝐻𝑖𝑖𝑖𝑖𝑖3 + 𝑆(10)covered 20% of the surface area, corresponding to a wet area

𝑃𝑃 0.25
𝑁𝑢 = 0.26𝑅𝑅 0.6 𝑃𝑃 0.37 � �,𝑃𝑃𝑠(12)ℎ𝑐𝑐 (𝑇𝑠2 − 𝑇𝑎 ) + ℎ𝑟 (𝑇𝑠2 − 𝑇𝑟 ) = 𝐻𝑖𝑖𝑖𝑖𝑖2 + 𝑆
𝜆𝑁𝑢ℎ𝑐𝑐 =𝐷𝑢𝑢
𝑅𝑅 =𝜈𝑃𝑃
≈1𝑃𝑃𝑠indoor environment under natural convection. A piece of wet
Japanese paper was pasted in the position shown in Figure 4 on
the front side of SAT meter I, which was presumed to have been
inhomogeneously wetted by misting. Its surface temperature was(13)then measured, and the latent heat flux and wet area fraction
were estimated from the heat balance equations (Equations (1)–hc = 0.865u2+12.3u+6.48
Table of symbols
Ta
Air temperature
Tg
Globe temperature
TsSurface temperature
Tr
Average radiation temperature
Estimated convection heat transfer
hccoefficient approx.
Theoretical convection heat transfer
hctcoefficient
Forced convection heat transfer
hcf
coefficient
Natural convection heat transfer
hcncoefficienthrRadiant heat transfer coefficient
EgLatent heat flux
WWet area fraction
HinputAdditional sensible heat flux

fraction of 0.20. The three SAT meters were installed in an(11)(3)). The environmental parameters are given in Table 1, and the(14)experimental conditions and unknowns are given in Table 2. The
experimental layout under natural convection conditions is°C°C°C°C

shown in Figure 5. During the experiment, the Japanese paper
was kept wet by using a dropper.

W/(m2･K)

4.3

Convectiveheattransfercoefficientmeasurement

2W/(m ･K)

To estimate the convective heat transfer coefficient, the three

W/(m2･K)

SAT meters were installed in an indoor environment. TheW/(m2･K)

experimental layout is shown in Figure 6, and the unknowns areW/(m2･K)
W/m2
W/m2

listed in Table 3. SAT meter I was set in front of a misting fan,and its front side was facing the wind. The distance between thefan and SAT meter I was increased from 2 to 5 m in increments- 86 -of 1 m.

Fig.6 Experimental layout for the measurement of theconvective heat transfer coefficientFig. 4 Position in which the wet Japanese paper was pasted. F

Table 3: Experimental conditions and unknowns

and B indicate the front and back sides, respectively.

No.
Convection

Table 1: Physical environment parameters

Physicalenvironment
factorMeasurement

instruments

temperaturehumidity

Temperature /humidity sensor
3D ultrasonicanemometerwind speed
Globe
temperature

Recordinginterval

Additional
heat flux
Unknown
valueⅠwithH input12531.0W/m
h cnSw531.0W/m2

531.0W/m2

265.5W/m2

Unknownnumberh cf
Sh cn
Sh cn
S

5.1 Surface temperature measurement
Figures 7 and 8 show the distribution of the surface temperature

Ⅲwithout
H input3

531.0W/m

265.5W/m2

h cn
Sh cn
S2

Ⅲ
NaturalH input3

5．Result

Globe sphereⅡ
without
H input2

Ⅱ
NaturalH input2

1sec

Table 2: Experimental conditions and unknowns
No.
Wet paper

Additional
heat flow

ⅠForcing
H input1of the three SAT meters. The surface temperature of the lower
part of each of the meters was lower than that of the middle andupper parts because there was some heat loss from the lower
edge of the SAT meter. There was little difference between thesurface temperatures of the middle and upper parts. The
maximum temperature difference among the SAT meters wasless than 1.0 K for both the middle and upper parts.
In this study, the surface temperature data of the lower and upperparts were not considered in the calculations; only the data of themiddle part were analyzed to solve the heat balance equation on
the surface.
5.2 Estimation of wet area fraction using Japanesepaper
Figure 9 shows the average surface temperatures of SAT meters
I, II, and III over a period of 30 s under steady state conditions.
Figure 10 shows the estimated wet area fraction and latent heatFig.5 Experimental layout for the estimation of the wet area

flux of SAT meter I. The true value of the wet area fraction wasfraction under natural convection conditions

0.20.
The wet area fraction was estimated to be 0.25 and 0.17 on thefront and back sides, respectively, yielding an average of 0.21
for both sides. The difference between the estimated and truevalues was 0.01, corresponding to an error of 5%. This
demonstrates that the wet area fraction can be evaluated using
the heat balance equations and the average temperature of the- 87 -Ⅰtemperature[℃]

5.3 Relationship between wind speed and convection
heat transfer coefficient
Figure 11 shows the relationship between the wind speed and theconvective heat transfer coefficient, where the the solid line is anapproximation obtained by fitting the experimental data. Figure
11 also includes theoretical data for reference. At wind speeds ofless than 4.3 m/s, the approximation given by Equation (14)resulting from the experiment which is mentioned in 4.3 isapplicable. The experimental values were approximately 1.5

6867
66
65
64
6362

Ⅱ

Maximum
temperature
difference

Ⅲ

Front

Ⅰ

lower
62.2

middle
66.4

upper
66.7

Ⅱ

64.2

67.1

66.1

Ⅲ
Maximum
temperature
difference

62.8

66.5

66.4

2.0

0.7

0.6

times larger than the theoretical values. Because the theoreticalFig.7 Observed surface temperatures of the front sides of thevalues are evaluated for a cylinder in a uniform flow of thethree SAT meters

3.0
2.5
2.0
1.5
1.0
0.5
0.0

Maximum temperature difference[℃]

SAT meter.

calculated from the SAT meter which was put in a non-uniformⅠ

flow by the fan.
temperature[℃]

The approximate formula for the convection heat transfer
coefficient is a function of wind speed, which is useful to reduce
measurement devices. For example, there is a method of using apair of SAT meters, one placed in a mist cloud and the otherplaced outside. The received solar radiation S and latent heatflux Eg are the only unknowns in simultaneous equations (1) and(2). In this way, the wet area fraction of the SAT meter can be
estimated using a fewer measurement devices.
6．Conclusions

6867
66
65
64
63
62

Ⅱ

Maximum
temperature
difference

Ⅲ

BackⅠ

lower
63.3

middle
66.6

upper
66.8

Ⅱ

64.2

66.9

66.1

Ⅲ
Maximum
temperature
difference

63.5

66.5

66.2

0.9

0.4

0.7

3
2.5
2
1.5
1
0.5
0Fig. 8 Observed surface temperatures of the back sides of thethree SAT meters

A measuring instrument for the characterization of the cooling
effect of outdoor misting was developed in this study. The
following conclusions about the essential features of theinstrument were obtained from the experimental results obtainedin this study.
(1)

It was confirmed that the wet area fraction could be
accurately estimated using the cylindrical heated SAT
meter by covering a known fraction of the surface with
wet Japanese paper. There was a difference of 0.01
between the estimated and true values, corresponding to anerror of 5%.

(2)

An approximate formula for the convective heat transfer

Fig.9 Average observed surface temperatures of the three SAT

coefficient of the SAT meter was proposed for use at windmeters used to estimate the wet area fraction

speeds ranging from 0 to 4.3 m/s.
In future work, a measurement system and method of using theinstrument developed in this study will be developed. Because
simple examination is best, fewer measurement devices aredesired in an actual misting field.

- 88 -

Maximum temperature difference[℃]infinite width. On the other hand, the experimental values are(Received Apr. 30, 2017, Accepted Jul. 7, 2017)

Fig.10 Latent heat flux and wet area fraction of SAT meter I
Experimental value
Convection heat transfer
coefficient[W[W/(m2K)]

50

Theoretical valuey = -0.865 x2 + 12.3 x + 6.48
R² = 0.994

4030
20
1000

123
wind speed[m/s]45

Fig.11 Relationship between wind speed and convective heattransfer coefficient from experimental values and theoreticalvalues

References(1)

Hayashi, A., et al., Study on heatislandcontrol with water
mistsprayer(Part2)

ImprovementofOutdoor

Environment with Water Mist Sprayer, Proceedings of the2004 meeting on the Society of Heating, Air-ConditioningandSanitaryEngineersofJapan,

No.2(2004),

pp.1039-1042(2)

Yoon, G., et al., Validation of cooling effectiveness andCFD simulation: Study on cooling system by using dry
mist, Journal of Environmental Engineering (Transactionof Architectural Institute of Japan), Vol.73, No.633 (2008),
pp.1313-1320(3)

Mochida, T., et al., Fundamental Study concerning Equal
Thermal Sensation, Proceedings of the 20th Symposium onJapanese Society of Human-Environment System (1996),
pp.31-34(4)

Farnham, C., Emura, K. and Mizuno, T., Evaluation ofcooling effects: outdoor water mist fan, BUILDINGRESEARCH & INFORMATION, Vol.43, No.3(2015),
pp.334-335(5) Zukauskas, A., Heat Transfer from Tubes inCrossflow, Advances in Heat Transfer, Vol.8 (1972),
pp.93-106- 89 -