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論文集 ISME TOKYO 2000 Volume?

 事業名 第6回舶用機関国際シンポジウムの開催
 団体名 日本マリンエンジニアリング学会  


Such being the case, various water droplet separating device are employed such as cyclones, bent plate separators, wire-mesh demisters, washers, electro filters and so on.

At first, plate type separator called also eliminator or inertial force type separator is widely used in the industrial field because of their simple construction and of their high efficiency, In this study the equations of motion of droplets in the first part from the separator inlet are solved analytically(1). The exit conditions of the stream are supposed to be the inlet conditions for the second bent part of the separator. In this way the efficiency of the whole machinery is calculated for any number of bendings. And the experimental verification of these theoretical results is also carried out (2).

Next the separation characteristics of the separator by Coulomb force are studied. The Coulomb force, which is one of the basic physical forces, is of acting between two charged particles. Since the magnitude of the force is in proportion to the product of the amounts of electric charge on the particles, the important factor for moving the particles can be the amount of electric charge. The mechanism of separation by the Coulomb force is that droplets charged with negative ions flowing in between a couple of electric poles are collected on the positive pole. Consequently, it is expected that even the very small droplets can be separated by controlling the voltage between the poles of the test section. It has been confirmed from several experiments using both plate type poles and cylindrical poles that the separation efficiency of more than 80% is achieved for the range of diameter of 5μ m to 15μm and the separation performance of the cylindrical pole is better than that of the plate type poles(3). The purpose of the study is to propose the appropriate and the most effective length and diameter of the test section by theoretically analyzing the behavior of a charged droplet flowing through the test section. And the experiments are also carried out(4).

Finally, the separator is the wire-mesh demister, which is also an inertial force type separator as the eliminator, simple in construction, readily mountable to equipment, and has a considerably small pressure loss. It is generally said to be more advantageous than the eliminator in separating the droplets within the range of sizes 1μm to 10μm. A demister of this type in actual service consists of 20 to 100 layers of wire meshes spaced at intervals of several millimeters and is usually installed in a horizontal position in the midst of a vertically rising air-flow. Considerably extensive experimental studies have so far been made for the multi-layer type demisters. However rather few studies have ever been made on the simplest type of demister with a few layers including one. In this study the experiments are carried out with a few layers type demister, and the results obtained are compared with the previously published theoretical studies(5).

 

2. ELIMINATOR

 

2.1 Assumption

The following assumptions are made:

(1) Gravity effects are negligible.

(2) Droplets experience a resistance to their motion in the air according to Stokes law.

(3) Droplets are spherical, and there is no interaction among them.

(4) The direction of air-flow is perpendicular to the inlet surface of the separator, and the air-flow between the bent plates and for each bent part is uniform.

(5) Droplets flow into the separator has the same velocity as air.

(6) The effect of diffusion caused by the difference in droplets density is negligible.

(7) Conglomeration of droplets is due to their impingement on the plates. Their reentrainiment is negligible.

(8) Air viscosity is negligible.

 

2.2 Droplet Trajectory

The drag force FD that is exerted on a droplet suspended in the air-flow is assumed to have the form:

298-1.gif

Most plates can be represented by the zigzag from shown in Fig. 1.

From assumptin (2), CD=24/Rep, Rep=(ua-up)Dpa. Therefore the equation of motion of droplet is as follows:

298-2.gif

whereK=18νaρap

These equations can be integrated with initial conditions

x=0, y=yi, upx=upx,i and upy=upy,i at t=0.

The droplet velocity upx,upy and the droplet position x,y are then obtained, and each variable is made dimensionless and the parameters α and β are introduced as follows:

 

u*p=up/U, u*a=ua/U, t*= U・t/{1・cos ( θ/2)}

x*=x/{1・cos (θ/2) }, y*=y/ {1・sin (θ/2) },    (4)

p*=p/{1・sin (θ/2) }, α=UDp2/{K・1・cos (θ/2) }

β=exp (-1/α)

 

Therefore, upx, upy and x, y can be expressed by dimensionless equations as:

u*px=u*ax-(u*ax-u*px,i) βt* (5)

u*pyx=u*ay-(u*ay-u*pyi) βt* (6)

x*=u*axt*-(u*ax-u*px,i) (1-βt*) /1n (1/β) (7)

y*=u*ayt*cot (θ/2)-(u*ay-u*py,i) (1-βt*) ×cot (θ/2)+y*i (8)

where the subscript i denotes the inlet condition at each bent part.

Since air is incompressible, the following equations are obtained for all parts in the bent plates:

 

298-3.gif

Frg 1 x, y-distance of bent plates

 

 

 

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更新日: 2020年6月27日

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