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

Academic Article

Performance Evaluation of Convective Heat Transfer on Leaf
Surface by Model Specimens
11Shinichi KINOSHITA*

Atsumasa YOSHIDA**1 Department of Mechanical Engineering, Osaka Prefecture University
Corresponding author: Shinichi KINOSHITA, kinosita@me.osakafu-u.ac.jp
ABSTRACT
Urban vegetation is considered to be one of the countermeasures to urban heat island. A recent study
highlighted that approximately 50% of the net radiation absorbed by an isolated plant is transferred to theatmosphere as sensible heat flux. This suggests that the surfaces of leaves have a high sensible heat transferperformance. Since the sensible heat transfer in forced convection depends upon the size of the heating surface andthe direction of wind, the heat transfer coefficient of a leaf’s surface is greater than that of a flat plate with the same
area as total area including in a tree crown. The angle of attack of the leaf surface and the oscillatory motion in
response to wind speed fluctuations are also potential factors affecting the heat transfer coefficient. In this study, theinfluences of these factors on the heat transfer coefficient of leaf surfaces are evaluated using the heating flat platemodel specimens. The evaluation of heat transfer coefficients for flat plates is performed in a wind tunnel, which isconnected to an induced draft fan.
Key Words : Urban heat island, Urban vegetation, Sensible heat transfer on leaf surface, Heat transfer coefficient,
Model specimen, Experiment, Wind tunnel

atmosphere(12). Asawa et al. evaluated the convective heat

1. Introduction

transfer coefficient of the entite crown of an isolated tree underUrban vegetation, such as trees planted along streets, building

outdoor conditions, by measuring the whole tree heat balances ofrooftops, and walls, is considered to be a countermeasure to thetwo individual trees that differed in their traspiration rate(13). In

effects of urban heat island. Urban vegetation has many benefits,

both of these studies, the resulting heat transfer coefficients ofnotably, it causes atmospheric cooling by absorbing net radiation

isolated plants are larger than those found using existing

through transpiration, due to the latent heat of moisture. As theempirical formula such as Jürges’s formula. This suggests thatmitigative effect of urban vegetation on the thermal environment

the leaf surface of a plant has a high performance of sensible(1, 2, 3)

in urban areas has been identified, several studies on theheat transfer. The tree crown is an aggregate of small leaf

heat exchange performance between tree crowns or green belt

surfaces contained within it, and the heat transfer performance ofareas and the surrounding air have been conducted(4, 5). It can be

the entire crown is considered to be due to the heat transfer

asssumed that a leaf surface within a tree crown is much smaller

performance of individual leaves and the interactions between

than the scale of an entire tree, and has superior characteristics

them. Since the sensible heat transfer in forced convectionof heat and mass transfer to the atmosphere on its own. In thedepends on the size of heating surface and the direction of wind ,field of mechanical engineering, empirical and theoreticalthe heat transfer coefficient of the leaf surface is larger than that

relationships for turbulent heat transfer coefficients on a flatof a plate with the same area as total area including in a treeplate have been established previously(6, 7). In the agricultural orcrown. The angle of attack of the leaf surface and the oscillatoryarchitectural fields, the heat and mass transfer characteristics ofmotion in response to wind speed fluctuations are also potential

a leaf’s surface have also been experimentally and numericallyfactors that affect the sensible heat transfer.

(8, 9, 10, 11). In a recent study, it was shown thatIn this study, the influences of these factors on the heat transfer

approximately 50% of the net radiation absorbed by an isolated

coefficient of leaf surfaces are evaluated by heating flat plate

plant in an urban area is transferred as sensible heat flux into themodel specimens. The effect of the specimen size, angle relativeevaluated- 68 -

Butterfly valve

Temperature measuring point

Hot-wire anemometer

100Winddirection

To blower
Supporting section

x200

800mm

Fig.2 Flat plate model specimen and temperature measuring
Fig. 1 Technical sketch of the experimental apparatus

positions.

to wind direction, and wind speed fluctuation on the sensible

Thermocouple

heat transfer coefficient was parametrically evaluated using aWind direction

Hot wire anemometer60

wind tunnel.

y150

40502. Experimental wind tunnel apparatus

OxA suction type wind tunnel, with a blower installed on its

Model specimen

200

downstream side, is used in this study. The cross section of theduct is rectangular, with dimensions of 200 × 150 mm. The
mean wind speed can be varied between 0 and 7 m/s byoperating two butterfly valves. The model specimen is supportedFig.3 Location of the model specimen and measuring devices.

near the center of the duct, and the angle of attack to themainstream flow direction can be adjusted by rotating it in themeasuring devices located within the wind tunnel. The wind

pitch direction. Figure 1 shows a technical sketch of the general

speed is set at various speeds, approximately 1, 2, 3, 4, and 5 m/s.

arrangement of the wind tunnel apparatus. The wind velocity

A thermocouple and a hot wire anemometer are located 50 mm

near the model specimen is measured using a hot wire

downstream from the leading edge of the model specimen andanemometer.

60 mm and 40 mm above the specimen surface, respectively, inorder to measure the wind temperature and speed as reference

3. Effect of the flat plate size on the heat transfer

values for the evaluation of the heat transfer coefficient.

coefficient

Figure 4 shows the effect of the wind speed on local and averageheat transfer coefficients. The local heat transfer coefficient

The size of plant leaves differs between plant species. It can beincreases as the distance from the leading edge of the specimen

considered that plant crowns consist of small sized flat plates ofdecreases, and as wind speed increases. The average heatleaves, and that higher heat transfer coefficients of foliage aretransfer coefficient is calculated by integrating the temperature

due to the size of the leaf.

profile from the leading edge to position x, which corresponds to

The model specimen is a flat plate of ABS plastic, on which

the effect of the model specimen size on the heat transfer

aluminum foil is adhered. It is then heated using electricalcoefficient. As shown in Fig. 4, it is found that the heat transfer

resistance heating. The flat plate specimen is installed in thecoefficient increases as the size of the model specimen, whichwind tunnel, and the heat transfer coefficient is calculated bycorresponds to leaf size, decreases.

recording data for measured wind speed, surface temperature
and air temperature. The size of the model specimen is 100 ×4. Effect of the angle of attack on the heat transfer

200 mm. In order to measure the surface temperature profile,coefficient

copper constantan thermocouples are located at nine points witha separation of 20 mm, starting from the leading edge. The

The effect of variations of the angle of attack on the heat transfer

thermocouples are also located along the center of the specimen,coefficient of the leaf surface is evaluated by heating the flatas shown in Fig. 2. It is assumed that heating on the upper andplate model specimen in the wind tunnel. Measurements arelower surfaces is uniform and of the same amount.

taken at a range of different angles for the heating flat plate

Figure 3 shows the location of the model specimen and themodel specimen. Figure 5 shows the location of the measuring- 69 -Thermocouple

6.1m/s4.7
3.2
1.9
0.930Wind100mm

θModel specimen

7520
1001540150

Heat transfer coefficient [W/m2/K]

500246810 12 14 16 18Fig. 5 Location of model specimen for evaluation of the effectMeasuring position [cm]of angle of attack.

(a) Local heat transfer coefficient

5040

Heat transfer coefficient[W/m2/K]

Heat transfer coefficient [W/m2/K]

50

6.1m/s
4.7
3.2
1.9
0.930

20100051015204030
201000250-degree
102030

Average position [cm]246Measuring position[cm]8(a) Positive angle(b) Average heat transfer coefficient

50Heat transfer coefficient[W/m2/K]

Fig. 4 Effect of wind speed on the heat transfer coefficient.
specimen and measurement devices, which is similar to thatfound in the previous section. The size of the model specimen is
100 × 100 mm. The horizontal plane is set as the base, and theangle between the specimen surface and the plane is θ.
Measurements are taken in seven conditions from θ = +30° to-30°, at 10° intervals. The wind speed for the condition of θ = 0degrees is set as the base, and this value is used for calculating
the heat transfer coefficient. The thermocouple is located on thesurface of the lower side, and θ is controlled such that the wind
is directly impinging against the surface and is positive. The

4030201000base wind speed is set to approximately 5 m/s.
Figure 6 presents the experimental results. For the condition of apositive angle, the local heat transfer coefficient shows a similar

0-degree-10-20-30

462Measuring position[cm]8(b) Negative angletendency to that of the horizontal location case , and decreases asFig. 6 Local heat transfer coefficients in the condition changing

it approaches the trailing edge of the specimen. However, for thethe angle of the specimen.

condition of a negative angle, the opposite outcome appears in- 70 -Hot wire anemometer

Heat transfer coefficient[W/m /K]

Rotating vane

50

2

Heating plate

To blower

Acceleration sensor
Holding partWind

Fig.7 Location of model specimen for the evaluation ofoscillatory motion effects.

403020100certain cases, and the overall tendency is complicated.

4139Frequency[Hz]18(a) Support with rigid arm

5. Effect of oscillatory motion on the heat transfer

Heat transfer coefficient[W/m /K]

coefficient
2Plant leaves oscillate as the wind blows. It has been reported that50

the thickness of the boundary layer on a leaf’s surface becomesthinner owing to this oscillation than it would otherwise be in astationary condition. It is also understood that the efficiency ofphotosynthesis improves as the wind blows, due to the reductionof resistance in gaseous species exchanges. The effects of leaf
oscillation are evaluated within this study by subjecting the flat
plate to an oscillatory air current(14). In this section, the effect ofoscillatory motion on sensible heat transfer performance isevaluated.
Figure 7 shows the experimental apparatus. A heating flat platemodel specimen is installed at the end of a flexible beam of ABS

403020100plastic, which extends from the downstream side to the upstream

413
9Frequency[Hz]18side of the wind tunnel, and wind speed is varied using a rotating(b) Support with flexible beamvane located on the upstream side of the blower. The frequency

Fig. 8 Experimental results for the average heat transfer

and acceleration of the flat plate oscillation are measured using

coefficient in different conditions of air current oscillation.

an acceleration sensor. The fluctuation frequency can becontrolled within the range of 4 to 60 Hz, by altering the speed

oscillation frequency experiment, relatively large oscillations ofof the rotating vane. The size of the specimen is 100 × 100 mm.

the flexible beam were observed, and a relatively higher heatExperiments are performed for two cases relating to the support

transfer coefficient of the plate specimen supported with theapparatus of the model specimen; rigid arms connected to thebeam were measured, than that supported with the rigid arm.

wind tunnel wall as shown in Figs.1 and 3, and the flexible
plastic beam described above. Four cases of air current

6. Summary

oscillation frequency are applied; 4, 9, 13, and 18 Hz. 4 Hz
corresponds to the characteristic frequency of the flexible beam.

In this study, the influences of leaf size, angle of attack, andFigure 8 shows the experimental results for the average heat

oscillatory motion on the heat transfer coefficient of a leaf’stransfer coefficients. In the case of the 18 Hz air current

surface are evaluated using heated flat plate model specimens.

oscillation frequency experiment, the oscillatory motion of theThe local heat transfer coefficient increases with an increase in

specimen is found to be negligible, and the difference of heatwind speed, and decreases as the measuring position moves

transfer coefficients between the fixed plate specimen andalong the wind direction from the leading edge.

movable one is minor. In the case of the 4 Hz air current

The influence of the angle of attack of a leaf’s surface on the- 71 -(1998), pp.255-267.

heat transfer coefficient has been evaluated. It was found that thelocal heat transfer coefficient profile of the specimen becomes(11) T. Defraeye, P. Verboven, Q.T. Ho and B. Nicolai,

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Convective heat and mass exchange predictions at leafof the oscillatory motion in response to wind speed fluctuation

surfaces:

was evaluated using the same measurements, by fixing theComputers and Electronics in Agriculture, 96 (2013),

Applications,methodsandperspectives,

pp.180-201.

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Measurement of the Energy Budget for an Isolated Plant

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