Field measurements on energy balance of an urban canyon in the summer season

Energy and Buildings, 15 - 16 (1990/91) 417 - 423

417

Field M e a s u r e m e n t s on E n e r g y B a l a n c e of an U r b a n C a n y o n in t h e Summer Season ATSUMASA YOSHIDA, KAZUHIDE TOMINAGA and SHIGERU WATATANI

Department of Mechanical Engineering, Okayama University, Okayama 700 (Japan)

ABSTRACT

Field measurements on the energy balance of an east-west oriented urban canyon were performed continuously for several days in the summer season, as preliminary research of the heat transfer in the urban atmospheric boundary layer. The urban canyon consisted of concrete buildings, 16 m high and partially covered with windows, standing face to face across an asphalt road 17m wide. The energy balance of the urban canyon is represented by the balance at the imaginary surface named "the top surface". The top surface is a plane above the canyon at the same level as the roof surface of the buildings. In the daytime, the energy budget into the top surface is much larger than that into the roof surface. At nighttime, there is no significant difference in the energy balance between the roof surface and the top surface.

1. INTRODUCTION

The meteorological conditions in an urban area are different from those in a rural area. In an u r b an area, a high t em pe r at ur e region is frequently formed and is called the heat island. This phenomenon is particularly noticeable in a big city and is caused by the artificial thermal environment. In order to elucidate the mechanism, there have been numerous studies by numerical simulation on the thermal s t r uc t ur e of ur ba n atmospheric bo u n d ar y layers, e.g., ref. 1. However, the bo u n d ar y condition at the complex surface in an u r b an area has not been examined thoroughly. The heat transfer within the surface boundary layer containing multistory buildings shown in Fig. 1 is complicated in comparison with t h a t above an even surface [2]. The basic surface unit in an urban area is 0378-7788/91/$3.50

Free .

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Atmosphere .

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.

Boundary z'" Layer 9!? Zt . . . . ~,~"~"ff~'- ~

Roof Level - - r ~ --

x _R4_ S..._ U ~._ S ~L_ R Soil Layer

Fig. 1. Schematic representation of the u r b a n atmos p h e r e - s u r f a c e system. R, S and U are rural, s u b u r b a n and u r b a n areas, respectively.

considered to be an urban canyon (Fig. 2) surrounded by concrete buildings face to face across an asphalt road [3]. It is important to clarify the characteristics of heat transfer of an urban canyon system as the preliminary stage of the physical understanding of urban climates. In the present study, the energy balance of an e a s t - w e s t oriented urban canyon is evaluated following a field investigation for several days in the summer season. The surface temperature, air temperature, solar irradiation, infrared radiation, conductive heat flux, wind direction and speed were measured continuously on the walls and windows of buildings, at the road and at the roof. The energy balance of the urban canyon is represented by t hat at the imaginary surface named "t he top surface", which is a plane above the canyon at the same level as the roof surface of the buildings.

f

%-

¸¸,,¸5 /////

Urban Canyon

Fig. 2. Schematic depiction of u r b a n canyons. ~ Elsevier Sequoia/Printed in The Netherlands

418 (South)

'~

:[~

~,

To ~

--'

- ....

.c-G)

(Norlh) T---

J~

E

-'~

,

Road

17m

Fig. 3. Elevation view of the canyon cross-section includ. ing instrument locations during 1987.

2. M E A S U R E M E N T SITE AND PERIOD

The field measurements were carried out in the central cross-section of a canyon located on the main campus of Kyoto University in Kyoto, Japan. There are several buildings of almost the same height on the campus. The canyon has sufficient length with a long axis in an e a s t - w e s t direction. Figure 3 shows the elevation view of the canyon cross-section. The canyon is formed by 4-story buildings, 16 m high, and an asphalt road 17 m wide. The constituent faces of the canyon are named as shown in Fig. 3. The imaginary surface is also named the top surface. The measured roof surface is made of concrete covered with thin gray rubber. The north wall of the canyon is covered with brown tile except for 35% covered with windows. Half of the south wall is made of concrete and the rest is covered by windows. The center of the canyon is an asphalt road. The measurement periods were August 2226, 1986, and August 14- September 10, 1987, except for rainy days.

3. M E A S U R E M E N T METHOD FOR ENERGY BALANCE OF SURFACES

3.1. Roof, wall and road surfaces The parameters and the measuring instruments used during 1987 are listed in Table 1. They were set up on the walls of both buildings, on the northern and southern parts of the road 5 - 6 m away from each building, and on the roof of the south side building 2-3 m away from the edge, as shown in Fig. 3. In 1986, measurement points were added to the 1st and 3rd floors of both buildings. The twodimensional supersonic anemometers were placed 3.6 m above the roof and one meter

above the road. The air temperatures were measured 1.5 m above the surfaces, by using the double-shielded and aspirated thermometers with the sensor of T-type (copper-constantan) thermocouples (¢ 0.3 mm). In order to evaluate the heat transfer along the canyon, two thermometers were also placed 4 m above the road at a distance of 40 m apart with the measured cross-section between. The T-type thermocouples (¢ 0.1 mm) for the measfirement of the surface temperatures were mounted on the surfaces by transparent cellophane tape. All signals were recorded on a personal computer through a data logger at 10 s intervals and averaged over 10 min. Figure 4 shows the heat flux components at each surface, where K and L are the mean solar (shortwave) radiation flux and infrared (longwave) radiation flux from the atmosphere and the environmental surfaces, respectively. The boundary wavelength is about 3 #m. Incoming global solar radiation flux KS is composed of the direct component Kd~ and the diffuse component Kdi~. Latent heat flux can be negligible in this canyon system. In the case of the directions of heat transfer as shown in Fig. 4, the heat fluxes indicate positive values. As to the net radiation flux Q*, the direction of the positive value is the same as that in the case of K~. The energy balance and the radiation flux at each surface are expressed by the following equations: Q* + Qg + Qh = 0

(1)

Q*= K* + L * = ( K ~ - K$) + ( L $ - L T )

(2)

KT=rK$,

(3)

LT=e~T4+(1-e)L~

The values of K~, L~ and the conductive heat flux Qg in eqns. (1) and (2) were directly measured on the roof. The values of KT and L~" are obtained from eqn. (3). The total hemispherical reflectance for solar irradiation, r, had been measured previously. The total hemi-

~dir

~Kdi~

Q~ i

rlf,¢ /

LI, K¢

I

' Qh

% Fig. 4. Energy balance of the surface.

419 TABLE 1 Measured items and instruments Item

Instrument

Symbol

Number

Surface temperature

T[CC] thermocouple

~

11

Air temperature and humidity

Modified wet- and dry.bulb thermometer

,c -~

4

Room temperature

T[CC] thermocouple

~

4

Solar radiation (global component)

Pyranometer

(~

3

(diffuse component)

Pyranometer with a shade ring

~

2

Pyradiometer Heat flux plate

~ .m

2 11

O ~4 ~

4

Total radiation Conductive heat flux Conductive heat flux

Differential thermocouple

Wind direction and speed

Supersonic anemometer

spherical emissivity, 5, is assumed to be 0.95. Finally, the value of the convective heat flux Qh is derived from eqn. (1). The value of KS was measured by the pyranometer. The thermopiles used as the sensor were covered by a glass dome, which acted as a spectral filter to distinguish solar radiation from infrared (longwave) radiation. The value of Kdif was measured by the same type of pyranometer with a shade ring. The incoming total radiation flux was measured by the pyradiometer with a polyethylene dome. From this value and K I , the value of L$ was obtained. The value of Q~ was measured by the heat flux plate (75 × 75 × 0.7 mm) mounted on the gray surface by gray vinyl tape. The back was covered with a thin coat of silicone grease in order to decrease the gap between the plate and the surface. On the walls and the road, the value of KS was estimated from the measured values of K$ and Kdif on the roof considering the multiple reflection in the canyon. The value of L $ was estimated from the measured value of L~ on the roof and from the distribution of the surface temperatures T~ on the canyon faces. At the windows, solar radiation was considered not to be absorbed in the glass layer, and the values of Qg were measured by the differential thermocouples (~b 0.1 mm) because of the small temperature difference between the outside and inside surfaces. The other values were obtained in the same manner as in the case of the roof. The pyranometers and the pyra-

2

diometer on the road were used to monitor the estimated values of K$ and L$.

3.2. Top surface Figure 5 shows the schematic diagram of energy balance of the canyon system. The following equation is derived from the energy balance at the constituent surfaces of the canyon: fw all, Road (Q* + Qg + Qh)

dA

=

0

(4)

where, IWall, Road m e a n s the integration over the wall and road surfaces. As the absorption and the scattering of radiation can be neglected within the canyon space, the net radiation flux Q* is expressed by the following equation:

all, Road

op

= Q*(Top) • A(Top) QA'(IoP) Qh(lOp ) I[

Qg('l'o p/_.

Fig. 5. Energy balance of the urban canyon system.

(5)

420

where Q*(Top) means the net radiation flux at the top surface, and A(Top) is the area of the top surface with a unit width in a long-axis direction. The conductive heat flux at the top surface, Qg(Top), is defined by the following equation: fw all, R o a d

VgdA

Q~(Wop) • n(Top)

(6)

The solar radiation toward the room through the window is considered to be included in the integration of Q~. As to the sensible heat flux Qh, the following relation is derived, because the heat storage in the canyon space and the heat transfer along the canyon QA (Fig. 5) are neglected, as a result of the measured data of the air temperature difference shown in Fig. 6 as described later: Qh dA

fw

Qh(Top) • A(Top)

(7)

all, R o a d

The equation of the energy balance at the top surface is derived from eqns. (4) -(7), and is expressed by the following: Q*(Top) + Q~(Top) + Qh(Top) = 0

(8)

The energy balance of the canyon is represented by that at the top surface.

4. RESULTS AND DISCUSSION

4.1. Air temperature and surface temperature Figure 6(a) shows the air temperature difference in the canyon system. In this Figure, T(Road) and T(Roof) are the air temperatures at 1.5m above the road and the roof, respectively. T(East) and T(West) are the air

04-- (a)

-0~

0

T(WesO-T(East)

/T(_Roof)-l(Road) 4 i I

~

I

4

8

12 hours

Time

~,l(West)I !

August 27

r

l(Road) I

16

J i

20

_~

24

Fig. 6. (a) Air temperature difference between locations above the roof and within the canyon. (b) Wind speed and direction above the roof.

temperatures at 4m above the road in the canyon at a distance of 40 m apart with the measurement cross-section between. The air temperature differences in the canyon (T(West) - T(East) and T(West) - T(Road)) were less than 0.3 K in the horizontal and the vertical direction regardless of the wind condition shown in Fig. 6(b). In addition to the results, it is known from the measured data of (T(Roof) - T(Road)) that the air temperature inside the canyon is nearly equal to that outside the canyon. Figure 7(a) and (b) show the outside surface temperatures Ts at several points in the canyon system on a fine day except around noon and a cloudy day, respectively. The shade on the south side building remained almost in the same position during the middle of the day. The values of T~ at the sunlit positions except the windows, especially at the roof and the north side of the road due to the small solar zenith angle, are remarkably higher than the air temperature T ~ in the daytime on a fine day. On a cloudy day, the surface temperature difference between the roof and the road is noticeable and the value of T~ on the north wall of the 4th floor is slightly higher than that on the wall of the 1st floor because of the difference in the diffuse solar radiation. As to T s on the north wall of the 1st and 4th floors, the reverse tendency is shown on a fine day in comparison with a cloudy day, due to the reflected solar radiation and the infrared radiation from the sunlit road. On a fine day, the sign of temperature difference between Tair and Ts on the southern part of the road or on the south wall (the 1st floor) is reversed in comparison with the other position. This result (Tair> Ts) is mainly caused by the warming of the whole air volume in the canyon by the sensible heat transfer from the sunlit road and the north wall. While operating the air conditioner on the 4th floor of the north-side building, the value Ts of the window is affected by the room temperature. In the daytime the distribution of Ts on the walls is considered to be uniform from a viewpoint of heat transfer in the canyon system. The values of T~ on the shaded parts and the windows are nearly equal to Tair. At nighttime, the values of Ts are always slightly higher than Tair and the difference in

421

4.2. Daily variation of energy balance at the roof surface and the top surface

50

i,/

,.o0

RReO~d(N)

~oL . . . . . ~°"

. . . . .~l ...

/

-.-"";~"~-.

~

20 i I

TS

~°, - - -

North

,~ . . . . . . ~ 4 ~

Wail

~

F ~

i .... To,,

2ci

/

20L

I

1

I

0

% 8

4

I

[ 12

Time

!

I 16

1

I 20

24

hours

(a)

50F

s

N

Roof ~, Road

AO"~ .... Rood(N)

20~ i e~)40b~" - -

Ts

North

4F I . . . . 4F{WindOw) F --~F • ~

~

IF

40~-

,

50u,hc:~Wo,,

---- 4F(WlndOw) -

II~

C~O N

Figure 8(a) and (b) show the daily variation of energy balance components of the roof and the top surfaces averaged over one hour on a fine day. The net solar radiation K* of the top surface is larger t han t hat of the roof surface due to the multiple reflection in the canyon. The net longwave (infrared) radiation L* of both surfaces is always negative. The amount of L* of the top surface is smaller t han t hat of the roof surface in the daytime, because the surface temperature of the shaded parts in the canyon is much lower t han t hat of the roof surface as shown in Fig. 7. Therefore, the net radiation flux Q* of the top surface is much larger t han t hat of the roof surface. The conductive heat transfer towards the inside is active in the canyon compared with t hat at the roof. In addition to the effect of the canyon structure, the tendency is emphasized by the fact t hat the solar radiation t hrough the windows is included in the heat flux Q~ of the top surface. The position flux represents the heat storage component. The ratio of the convective heat flux Qh of the top surface to t hat of the roof surface is smaller t han the ratio of increase of the surface areas, because the heat transfer of the shaded parts is quite inactive compared with the roof surface as mentioned above, and the wind speed in the canyon is generally weaker

N

1ooo

%

5

i ~ I [Q ) Roof 4-

~

I

I

I August

i27 i

Q*

--~Qh

500

I

I

1F

~.:__~ . . . . . . . . . . . . 201 0

I

E

4

I

i

8 Time

12

16

: t 20

\{7",

0:

24

hours

"1-

-50C

(b)

,

1000

Fig. 7. Daily variation of surface temperatures on the canyon walls, road and roof, together with the air temperature within the canyon in (a) fine weather, and (b) cloudy weather.

~

I

I

Q*

i

t

i

I

I

I

1

1


500 o

o,

u-

the position is relatively small. On a fine night, it is recognized t h a t the slope of T 8 on the r o o f vs. time is large in comparison with the slopes of the positions in the canyon. This result indicates t h a t the canyon s t r u c t u r e prevents the heat release.

I

(b) Top

0

....

~~

~

~

3=

-500 i

r

4

1

1

8 Time

~ 112 r I'6 1

20

24

hours

Fig. 8. Daily variation of energy balance components averaged over one hour for (a) the roof surface, and (b) the top surface.

422

than that outside the canyon. It is imagined that the value of Qh of the top surface is affected by the wind direction. This problem will be investigated in the future. At nighttime, there is no significant difference in the energy balance components between the roof and the top surfaces in spite of the large difference in the daytime. The convective heat transfer is not active on both surfaces, because the temperature difference between the surface and the air is smaller than that in the daytime and the wind speed is generally much weaker. The stored heat during the daytime is mainly released from the surface by infrared radiation. Figure 9 shows the temporal variation of the ratios Q*(Top)/Q*(Roof) and Qg/Q* of both surfaces except for the several hours with the small amount of Q*. Q*(Top) and Q*(Roof) are the radiation fluxes at the top and the roof surfaces, respectively. The ratio Q*(Top)/Q*(Roof) increases with time in the afternoon due to the large difference of the surface temperature. At nighttime, the ratio approaches unity. The ratio Qg/Q* decreases with time in the afternoon. The conductive heat transfer becomes inactive, because the inside temperature of the walls and the road rises continuously late in the afternoon. There is no significant difference of the ratio Qg/Q* between the top and the roof surfaces. Figure 10 shows the changes of the surface temperature ATe~At and of the conductive heat flux AQg/At. The surface temperature at the top surface is defined by L T(Top)

=

aT 4

(9)

There is a difference in the peak time and the 2.0

i

TOP

August 27

= = = ROOf

~z z,

200

/~

~oo~

~AE/At , \, ' _ ~

-

,

0

o

<:-£

-100

-8 20 Time

-200 24

hours

Fig. 10. Daily variation of the changes of the surface temperature and the conductive heat transfer at the roof surface and the top surface averaged over one hour.

change of the sign between the roof and the top surfaces. One of the causes is that the sunlit position in the canyon changes with time in the morning and in the evening.

4.3. Energy balance at the roof surface and the top surface in the daytime and at nighttime Figure 11 shows the diurnal total of convective heat flux ZQh averaged over the whole sunlit wall (north wall) except the windows, the shaded wall (south wall) and the windows. The value of EQh at the north wall increases linearly with the increase of the diurnal total of the incoming solar irradiation on the roof EKe, while the values at the windows and south wall are independent of EK$. The heat transfer at the window and the shaded position is obviously inactive in the daytime. It is expected from the results of the surface temperature. Figure 12 shows the mean energy balance components at the roof and the top surfaces in the daytime (10:00-16:00) and the nighttime (00:00-05:00) during the measurement peri-

i

August 27 -

g

-

Q*(Top)/Q*(Roof)

rr

~E30

Qg/Q~ ~ " - ~ - . O

2C

-Top .... Roof

O

o

~

; Time

o North Wail x North Windows • South Wall

Daytime

6 P4

,'2 ,; 2'0 2~

10

hours

Fig. 9. Temporal variation of the ratio of radiative h e a t transfer between the top surface and the roof surface, and the comparison of the temporal variation of the conductive h e a t transfer to the radiative h e a t t r a n s f e r at both surfaces averaged over one hour.

•

•

~•°~

• x

o ~

,b

:'~ Z K ~,

•

2b

2s

MJ I m 2

Fig. 11. Diurnal total of convective h e a t flux at the wall and window surfaces.

423

Daytime

Nighttime

Fig, 12. Mean energy balance components at the roof

surface and in the urban canyon system in the daytime (00:00-05:00) and the nighttime (10:00-16:00) during the measurement period. ods. T h e a m o u n t s of the c o m p o n e n t s are normalized by the n e t r a d i a t i v e h e a t t r a n s f e r Q* at the r o o f surface in the daytime. A b o u t 40% of Q* at e a c h surface is d i r e c t l y r e l e a s e d by the c o n v e c t i o n in the daytime. T h e a m o u n t of the s t o r e d h e a t in the c a n y o n is a b o u t 1.5 times as m u c h as t h a t at the roof. The s o u t h e r l y wind blows most f r e q u e n t l y above the r o o f d u r i n g the m e a s u r e m e n t period. T h e r e f o r e , the c o n v e c t i v e h e a t transfer in the c a n y o n may be a c t i v a t e d b e c a u s e the wind blows on the sunlit wall. H o w e v e r , the a m o u n t of Qh at the top surface is n o t so large in c o m p a r i s o n w i t h t h a t at the r o o f surface. At nighttime, the a m o u n t of r a d i a t i v e h e a t t r a n s f e r is n e a r l y equal to t h a t of c o n d u c t i v e h e a t transfer. T h e r e is no significant difference in the e n e r g y b a l a n c e c o m p o n e n t s b e t w e e n the r o o f surface and the top surface. As the effect of the sky r a d i a t i o n cooling on the r o o f surface is small in the s u m m e r season, the effect of the c a n y o n s t r u c t u r e u p o n the h e a t t r a n s f e r is not clearly recognized.

5. CONCLUDING REMARKS Field m e a s u r e m e n t s on the h e a t t r a n s f e r in the s u m m e r season were c a r r i e d o u t in a typical two-dimensional u r b a n c a n y o n w h i c h was the basic surface u n i t in an u r b a n area. T h e c a n y o n was o r i e n t e d east a n d west w i t h a long axis. T h e r a t i o of the building h e i g h t to the r o a d w i d t h (the aspect r a t i o ) is a l m o s t equal to unity. T h e e n e r g y b a l a n c e of the r o o f and t h a t of the c a n y o n system are c o m p a r e d and

discussed in detail. T h e results o b t a i n e d are s u m m a r i z e d as follows: (1) In the daytime, the e n e r g y b u d g e t into the c a n y o n is a b o u t 1.5 times as m u c h as t h a t into the r o o f surface. T h e h e a t t r a n s f e r of the s h a d e d walls is negligible in c o m p a r i s o n with the sunlit walls. (2) At nighttime, t h e r e is no significant difference in the e n e r g y b a l a n c e c o m p o n e n t s b e t w e e n the r o o f and the c a n y o n system.

ACKNOWLEDGEMENT T h e a u t h o r s wish to t h a n k Associate Professor Y a s u t o N a k a m u r a of K y o t o U n i v e r s i t y for his v a l u a b l e advice and provision of the measurement instruments.

LIST OF SYMBOLS A

area

K, K* solar r a d i a t i o n flux, net solar radiaL,L*

Q* Qg Qh r Tair

T, (T

t i o n flux i n f r a r e d r a d i a t i o n flux, net i n f r a r e d r a d i a t i o n flux net r a d i a t i o n flux c o n d u c t i v e h e a t flux c o n v e c t i v e h e a t flux total h e m i s p h e r i c a l r e f l e c t a n c e for solar r a d i a t i o n air t e m p e r a t u r e surface t e m p e r a t u r e t o t a l h e m i s p h e r i c a l emissivity for infrared radiation Stefan-Boltzmann constant

REFERENCES 1 K. E. Torrance and J. S. W. Shum, Time-varying energy consumption as a factor in urban climate, Atmos, Environ., 10 (1976) 329 - 337. 2 I. Uno et al., An observational study of the structure of the nocturnal urban boundary layer, Boundary-Layer Meteorol., 45 (1988) 59- 82. 3 M. Nunez and T. R. Oke, The energy balance of an urban canyon, J. Appl. MeteoroL, 16 (1977) 11- 19.