Concept of Water Mist System and Control
This development is based on the idea that if it is possible to supply evapotranspiration from plants into the air in the form of fine particle size water mist that easily evaporates, it may help to mitigate the heat island effects by utilizing the cooling effects it produces, without depending on green.
The installation examples of the Water mist system increase, and a performance evaluation result is provided by actual field experiment. According to these results, air temperature fall down of 3-4 degree C could be obtained by using Water mist system. In addition, it is proved that water mist system have influenced to thermal comfortable.
As for the control parameter of the Water mist system, air temperature, humidity, the wind velocity, the rain fall are considered. Especially, air temperature and humidity must be discussed because other parameter will not be determined by cooling effect of water mist.
Table 1. Empirical Control Strategy for Spray
To be start
(If all conditions shown below are
satisfied)
To be stopped
(If any conditions shown below are
satisfied) Outdoor
air temp.
31 Degree C. above 30 Degree C. under Outdoor
air
Humidity
60% Rh under 70% Rh above
Air velocity
When mean velocity during 10 minutes is
under 3.0 m/s
3.0 m/s above
Rain fall No rain Rain
Vapor air 33℃
31℃ City Water
Water Pump Valve Control Equipment
Optional
Sensor (Optional)
Dry Bulb
Air Humidity Air Velocity
Solar radiation Water Mist
Obtain
Evaporative cooling
Outline of System
5 10 15 20 25 30
15 20 25 30 35
Dry Bulb ℃
Abs. Humidityg/kg'
100% 90% 80%
70% 60%
Mist Spray Zone
(Empirical)
Water Mist Spray Condition
CFD Analysis
50m
4m 15m 15m 3.5m
7.5m
horizontal solar
radiation: 363W/㎡ inflow velocity: 0.1m/s
spray position
X Z Y
atmospheric pressure
Mass flow rate 0.83 g/s Water temperature 28.0 °C
Spray cone angle 50 ° Injection pressure 6 MPa
Boundary Conditions
Thermal conductivity : 0.11 W/m·K Solar absorptance :10.8 %
Solar transmittance : 13.7% Roof top
surface (z=4m) PVC-coated
glass-fiber
plain-weave Heat transfer coefficient : 23 W/m2·K Thermal conductivity : 1.4W/m·K
Solar absorptance : 60 % Ground
surface (z=0m) Concrete
paved Heat transfer coefficient : 23 W/m2·K Upwind : air velocity, 0.1 m/s
Upwind / Downwind flow
boundary Downwind : atmospheric pressure Free-slip
Side surface
(y=0,
y=15) Adiabatic boundary
Boundary conditions
Spraying conditions
Diagram of calculation domain
Fluent 6.3 was used for this numerical analysis. The Discrete Phase Model was used and we considered the interaction between the mist particles and air, including heat transfer, phase changes, and the momentum conservation law. Also, we adopted the pressure-swirl atomizer model to analyze the nozzle spray conditions shown below.
Analysis Results
Temperature difference contour diagram at outdoor temperature
and humidity conditions of 34°C and 60% RH, respectively
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0 0.2 0.4 0.6 0.8 40%RH 60%RH 70%RH 75%RH
残存粒子量 [g]
高さ [m]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0 0.2 0.4 0.6 0.8 60%RH 70%RH 75%RH 80%RH
残存粒子量 [g]
高さ [m]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
残存粒子量 [g]
高さ [m]
GL 0.25～0.00 GL 0.75～0.50 GL 1.25～1.00 GL 1.75～1.50 GL 2.25～2.00 GL 2.75～2.50 GL 3.25～3.00 GL 3.75～3.50
高さ [m]
Mass of Remaining Particles [g]
(a) D.B. 30℃ (b) D.B. 34℃ Height [m]
Histograms of mass of remaining particles under different humidity at specific outdoor
temperature
X=13.0m X=26.0m
Z=1.5m Z=0.0m Z=4.0m [ Y=7.5m ]
X=13.0m X=26.0m Y=4.0m
+1.0 0.0 -1.0 -2.0
Diff. of Temp. [K] Ambient Temp.
X=13.0m X=26.0m
Z=1.5m Z=0.0m Z=4.0m [ Y=7.5m ]
X=13.0m X=26.0m Y=4.0m
X=13.0m X=26.0m
Z=1.5m Z=0.0m Z=4.0m [ Y=7.5m ]
X=13.0m X=26.0m Y=4.0m
a. 30℃/80%RH
b. 30℃/60%RH
c. 34℃/60%RH
Discussion and Conclusion
5 10 15 20 25 30
15 20 25 30 35
Dry Bulb ℃
Abs. Humidityg/kg'
100% 90% 80%
70% 60%
DI 85 DI 80
DI 75 Mist Spray Zone (Empirical)
No Spray,
But uncomfort
5 10 15 20 25 30
15 20 25 30 35
Dry Bulb ℃
Abs. Humidityg/kg'
100% 90% 80%
70% 60%
SET* 35 SET* 30
SET* 25 Mist Spray Zone (Empirical)
No Spray,
But uncomfort
5 10 15 20 25 30
15 20 25 30 35
Dry Bulb ℃
Abs. Humidityg/kg'
100% 90% 80%
70% 60%
Mist Spray Zone (Proposal)
In the case of 80% humidity, the particles that descended near the ground evaporated, creating a large temperature decrease in the space. However, in the case of 70% humidity, the particles vaporized rapidly, and the particles evaporate at higher level than the case of 80% humidity.
This phenomenon also affected the remaining height of particles. It was found out that the air humidity is a important element when the mist system is controlled.
There is no spray zone outside of empirical spray zone with considering SET* and discomfort index(DI).
The proper control strategy for humidity is 70% rh, and air temperature is 28.5 or 29 degree Celsius.
