Principal Research ResultsWind Tunnel Experiment for Predicting the Visible Plume Region from aWet Cooling Tower−Toward the Application of an Environmental Impact Assessment−
Background
Recently, in Japan, an independent power producer (IPP) has built thermal power plants with wet mechanical cooling towers
instead of the conventional sea water-cooled system. Visible plume from the wet cooling tower produces some significant atmosphericeffects, such as the reduction of visibility to air and ground, ice formation on surfaces, cloud initiation, and augmentation of precipitation. It is therefore of great importance to predict the visible plume region from the cooling tower in an environmental impact assessment. A wind tunnel experiment is considered to be one of the predominant methods, but no one has estimated the visible plume region generated from the vapor of the cooling tower.

Objectives
The objectives of this study are to develop detailed and simple methods for the wind tunnel experiments to predict the visible plume region from a wet cooling tower.

Principal Results
1. Development of a detailed method to predict the visible plume region
The diffusions of water vapor and heat emitted from the cooling tower in wind tunnel experiment are tracked using a tracer
gas. The water vapor mixing ratio is estimated using the instantaneous concentration of the tracer gas, which is measured using highresponse flame ionization detectors. A visible plume region is defined as all area where the instantaneous water vapor mixing ratio islarger than the instantaneous saturation water vapor mixing ratio (Figure 1). The results obtained by the present wind tunnel experiments are in good agreement with the some observations as shown in Figure 2.
2. Development of simple method to predict the visible plume region
In above method, since the instantaneous concentration must be measured, the specific measuring instrument, which is not
usually applied in the present environmental impact assessment, is needed. Therefore, we experimentally introduced the approximate
equation of the instantaneous high concentration, which is a function of time-averaged concentration and the spreading width, and proposed a simple method to predict the visible plume region using only time-averaged concentration. The visible plume region obtainedby simple method are in good agreement with those obtained by detailed method (Figure3), and it indicates that it is possible to evaluate the visible plume region by measuring only time-averaged concentration using the normal measuring instrument.

Future Developments
We will investigate the validity of the present method for a real cooling tower that has many stacks, and aim to apply the present method to an environmental impact assessment.
Main Researcher: Takenobu Michioka, Ph. D.,
Research Scientist, Atmospheric Environment Sector, Environmental Science Research Laboratory
Reference
Michioka et al. Wind tunnel experiments of visible plume from a cooling tower. -Development of visible plume prediction methodfrom a single stack- Komae Research Laboratory Rep. No. T030011 (2004)60

2. Environment - Measures to regional environmental problems

The diffusions of water vapor emitted
from a cooling tower in wind tunnel
experiment are tracked using tracer gas. A
visible plume region is justified as the area
where the water vapor mixing ratio islarger than the saturation water vapor
mixing ratio.

Fig.1 Schematic of the present method400observation (Policastro et al. 1981)
experiment

Visible plume height Hv [m]Visible plume length L v [m]600

40020004050607080

observation (Policastro et al. 1981)
experiment2000409050607080

Relative humidity at ground level, RH [%]Relative humidity at ground level, RH [%](a) Visible plume length(b) Visible plume height90

Fig.2 Comparison of observations and wind tunnel experiments
The visible plume length and height obtained by the present wind tunnel experiments are compared with someobservations. Here, in the present method, since the cloud is generated at a high altitude of a few meters under a highrelative humidity at the ground level, we cannot justify whether it is a visible plume from the cooling tower or acloud. However, the present method can predict the visible plume region at low altitude, it is hence possible to estimate
the reduction in visibility on elevated roads and buildings, which is very important in environmental impact assessment.

Vertical distance [m]400wind velocity
8 m/s
ground temperature
15℃
300 ground relative humidity 80%2001000simple method
detailed method
0200

400

It is possible to evaluate the visible plumeregion by measuring only time-averaged
concentration using the normal measuring
instrument. Therefore, it is expected toapply the present simple method toenvironmental impact assessment.

600

downwind distance [m]Fig.3 Visible plume region (Comparison of simple and detailed methods)

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