RU2191749C2 - Method of forming heterogeneities of concentration of particles suspended in liquid - Google Patents

Abstract

FIELD: technique of forming heterogeneities of concentration of particles suspended in liquid (biological particles inclusive) from 10^-1 to 10² mcm; cleaning liquid (molten metal inclusive) or concentration before subsequent use; ecology, sanitation, chemical industry, etc. SUBSTANCE: liquid is poured into reservoir where waves are excited, thus entrapping the particles; free surface of this liquid shall be ensured in reservoir and capillary-gravitational waves are excited on this surface. Travelling gravitational waves whose length exceeds depth of liquid in reservoir are excited at this. EFFECT: extended range of sizes of particles; reduced time required for procedure; reduced power requirements. 1 dwg

Description

The invention relates to techniques for creating inhomogeneities in the concentration of particles suspended in a liquid (including biological particles) in a size range from 10 ^-1 to 10 ² μm. It can be used for purification of liquids (including liquid metals) or for their enrichment before subsequent use, for waste disposal in various fields of science and technology: ecology, hygiene, chemical industry, nuclear energy, etc.

 There is a method of creating inhomogeneities in the concentration of particles suspended in a liquid, consisting in the fact that the liquid is placed in a vessel and excite waves entraining the particles in it [1]. In this method, standing ultrasonic waves are excited in the liquid, at the antinodes of which the concentration of particles is lowered, and in the nodes it is increased (compared to the average initial concentration).

In this method, the characteristic time of the drift (directional movement) of the particles to the nearest node of the ultrasonic wave, in itself quite large, depends on the size of the particles. At a given wave frequency, there is a certain particle size for which this time is minimal. With increasing or decreasing particle size, this time increases. That is, from a practical point of view, inhomogeneities of particle concentrations are formed only for a certain range of sizes of these particles (depending on the wave frequency), and this range does not exceed about one decade (the ratio of the maximum size to the minimum is not more than 10). In addition, in this method, areas of increased and reduced concentration smoothly transition into each other. Accordingly, the separation of the source liquid into "purified" and "enriched" is a rather difficult task. That is, for a "deep" separation, a multiple repetition of the process is necessary. In addition, in the method [1], the specific energy consumption is quite large (10 ² kWh / m ³ for a single separation) —the energy required to create concentration inhomogeneities per unit volume of the liquid.

 The closest technical solution is a method for creating inhomogeneities in the concentration of particles suspended in a liquid, which consists in placing the liquid in a vessel and exciting waves in it that entrain the particles [2]. In this method, either standing ultrasonic waves or traveling waves are excited in a liquid.

 If standing waves are excited, then in the antinodes of these waves, the concentration of particles is reduced, and in the nodes it is increased (compared to the average initial concentration). This case is similar to the method [1] with all the above disadvantages.

If waves (longitudinal) are excited, traveling in the 0X direction, then the vibrational velocity V _x of the fluid motion is described by the relation:
V _x = V _x0 sin (ωt-kx) (1),
where V _x0 is the amplitude, ω is the frequency, k is the wave number, k = 2π / Λ, Λ is the ultrasound wavelength.

Suspended particles, carried away by these waves, in addition to oscillatory movements, also make directional motion (drift) in the direction of wave propagation. The drift velocity V _d is determined by the relation: V _d = V $\genfrac{}{}{0ex}{}{\text{2}}{\text{x0}}$ / [2C (1 + ω ² τ ² )], where C is the wave propagation velocity (sound velocity in the medium), τ is the relaxation time of the particle velocity, τ = ρ _h D ² / 18η; ρ _h is the particle material density, D is the particle diameter (for nonspherical particles, the so-called hydrodynamic diameter), η is the viscosity of the liquid. That is, V _d depends on τ and, therefore, on D. If for particles of maximum size ω ² τ ² << 1, then all particles will have almost the same drift velocity V _d = V _x0 ² / 2C. For water, with ρ _h = 1 g / cm ³ and actually used frequencies of ultrasonic waves of 150-200 kHz (ω ≈ 10 ⁶ s ^-1 ), the condition ω ² τ ² ≤0.1 is fulfilled for particles with D≤2.4 μm. With increasing particle size, the drift velocity decreases rapidly. In particular, it will be half as much at D = 4.3 μm.

So, in the method under consideration, the range of particle sizes for which concentration inhomogeneity is formed is small. In addition, drift velocities are small in it. In particular, for water V _d = 0.01 mm / s at an acoustic wave intensity of 1 W / cm ² [2]. It should be noted that for the case of standing waves, the average particle drift velocity (to the nearest wave node) is an order of magnitude lower than the drift velocity in a traveling wave.

The energy W necessary for the "cleaning" of small particles per unit volume of the liquid is, in order of magnitude, determined by the relation: W = ρC ² , where ρ is the density of the liquid. For water, W≈2.10 ⁹ J / m ³ = 600 kWh / m ³ . A somewhat smaller, but of the same order of magnitude value of W is also characteristic of [1]. Note that for air W≈10 ⁵ J / m ³ = 0.03 kWh / m ³ .

 The technical result that can be obtained by carrying out the invention (the purpose of the invention) is to expand the range of particle sizes for which concentration inhomogeneities are formed, to reduce the formation time of these inhomogeneities, and to reduce the specific energy consumption for creating these inhomogeneities.

 This result is achieved by the fact that the liquid is placed in a vessel and waves that entrain particles are excited in it, the liquid is placed in a vessel so that a free surface of this liquid exists, and capillary-gravitational waves are excited on this surface.

 In particular, traveling gravitational waves are excited on this surface, the length of which obviously exceeds the depth of the liquid in the vessel.

 The drawing shows a diagram of a device for implementing the method.

 The device comprises a vessel 1 equipped with a source of mechanical vibrations 2 and an absorber (damper) of vibrations 3. A liquid 4 with suspended particles having a free surface 5 is placed in this vessel.

 The device operates by the proposed method as follows.

 The liquid 4 is placed (filled) in the vessel 1 so that there is a free surface 5 - the interface between the liquid - gas (air). Generator 2, operating at a certain frequency f (f = ω / 2π, ω is the cyclic frequency), drives the fluid adjacent to it (in the X0Z plane), and as a result, a capillary-gravitational wave of a certain length Λ is excited on surface 5 (shown dashed line). The absorber 3 ensures the absence of reflection of the wave from the corresponding end of the vessel 1, as a result of which the traveling wave mode is realized (in the direction indicated by the arrow).

If the wavelength Λ satisfies the relation k ² = (2π / Λ) ² << ρg / σ (k is the wave number; ρ, σ is the density and surface tension coefficient of the liquid; g is the acceleration of gravity), then the waves are called gravitational; if k ² >> ρg / σ, the waves are called capillary. In the intermediate case, they speak of capillary-gravitational waves (Landau L. D., Lifshits E. M. Hydrodynamics. - M.: Nauka, 1986. - P. 342). Accordingly, the frequency ω satisfies the condition ω ⁴ << ρg ³ / σ for gravitational waves and ω ⁴ >> ρg ³ / σ for capillary waves. In particular, for water in the first case ω≤30 s ^-1 (f≤5 Hz) and in the second ω≥108 s ^-1 (f≥17 Hz).

So, the wavelength is chosen to satisfy the relation (2π / Λ) ² << ρg / σ. In addition to the indicated condition, the wavelength Λ is chosen to obviously exceed the depth h of the liquid in the vessel, Λ >> h (kh << 1). In this case, the so-called long waves are realized with the propagation velocity U: U = (gh) ^1/2 (Landau L.D., Lifshits E.M. Hydrodynamics. - M .: Nauka, 1986. - P.60). Accordingly, the horizontal V _x and vertical V _z components of the fluid velocity are:
V _x ≅V _x0 [1-k ² z (2h-z) / 2] sin (ωt-kx);
V _z ≅V _x0 k (hz) cos (ωt-kx).
Here V _x0 is the amplitude of the velocity on the surface of the liquid (z = 0). In particular, if k ² h ² / 2≤0.1 (Λ / h≥14), then the values of V _x over the entire depth of the liquid (for different z) will differ from each other by no more than 10%. In this case, the amplitude of the vertical component on the surface of the liquid is half that of V _x0 . However, this component is not of interest for the problem under consideration.

So, the horizontal component of the velocity can be approximately represented in a form similar to (1): V _x = V _x0 sin (ωt-kx).
However, the propagation velocity of this wave is the value U = (gh) ^1/2 defined above, and not the speed of sound C. Accordingly, "small" suspended particles will drift at a speed of V _d = V _x0 ² / 2U = (X ₀ k) ² U / 2, where X ₀ is the amplitude of the displacements (determined by the generator 2), V _x0 = X ₀ ω. It is significant that U is several orders of magnitude lower than the speed of sound C.

Let h = 10 cm, Λ = 140 cm, X ₀ = 1.4 cm. Then U = 100 cm / s, ω = 4.5 s ^-1 , V _x0 = 6.3 cm / s, V _d = 0 , 2 cm / s. That is, the drift velocity is two orders of magnitude higher than in an acoustic wave at an intensity of 1 W / cm ² . The energy consumption W = ρU ² in the proposed method is significantly (six orders of magnitude!) Lower than in the methods [1, 2]. If U≤1 m / s, then W≈10 ³ J / m ³ = 3.10 ^-4 kWh / m ³ .

Let us evaluate the possible particle size ranges, which can be considered “small”. The sizes D of the particles should be much larger than the mean free path of the molecules of the medium [2], so that for a liquid D≥0.1 μm. The upper limit is determined by several factors. In this case, the wave frequency is small, ω = 4.5 s ^-1 (f = ω / 2π = 0.7 Hz). Therefore, the most "stringent" condition is: Re≤1, where Re is the Reynolds number for the particle. For the above parameter values, D≤200 μm. That is, all particles in the indicated size range will have the same drift velocity (V _d = 0.2 cm / s).

The traveling wave mode is realized only when the wave attenuation is relatively small. That is, the kinematic viscosity ν of the liquid must satisfy the condition: ν≤2.5 • 10 ^-4 h ² ω (in this case, at a distance equal to the wavelength, the oscillation amplitude decreases by no more than 10%). For the above parameters ν≤0.11 cm ² / s, i.e. the drift under consideration is possible not only in liquids with low viscosity (in water, etc.), but also in liquids whose viscosity is an order of magnitude higher.

В этом случае дрейф частиц реально имеет место лишь в поверхностном слое жидкости глубиной порядка Λ. Это обстоятельство может использоваться для создания неоднородностей концентрации "легких" частиц (ρ_ч<ρ), сосредоточенных в поверхностном слое жидкости. А поскольку затухание таких волн существенно меньше, чем "длинных" волн, то предлагаемый способ может быть применим для жидкостей с весьма большой вязкостью ν≤4 см²/c (типа глицерина).It should be noted that for kh >> 1, the wave decays rapidly with increasing Z,

In this case, particle drift actually takes place only in the surface liquid layer with a depth of the order of Λ. This circumstance can be used to create inhomogeneities in the concentration of "light" particles (ρ _h <ρ) concentrated in the surface layer of the liquid. And since the attenuation of such waves is significantly less than the "long" waves, the proposed method can be applied to liquids with a very high viscosity ν≤4 cm ² / s (such as glycerol).

If a standing wave is excited in the vessel (for example, absorber 3 is absent, and the vessel is a multiple of an integer number of waves), then, similarly to [1], the concentration of particles for which ωτ≈1 will be increased in nodes and decreased in antinodes. This case can also be used for some practical problems, since the specific energy consumption (compared with the case of traveling waves) will decrease in proportion to the increase in the ratio of the vessel length to the wavelength. For relatively large particles, the frequency ω should be relatively large (ω≈10 ² s ^-1 ). As noted above, waves with such frequencies are called capillary-gravitational.

Thus, the proposed method allows to create inhomogeneities in the concentration of particles suspended in a liquid due to the drift of these particles in the field of a capillary-gravitational wave. The drift velocity can be 10 ⁰ mm / s - two orders of magnitude higher than in the known methods [1, 2]. The range of particle sizes (from 10 ^-1 μm to 10 ² μm) is significantly wider than in the known methods. The specific energy consumption for creating concentration inhomogeneities (10 ³ J / m ³ ) is six orders of magnitude lower than in the known methods. This method can be used for purification of liquids (including liquid metals) or for their enrichment before subsequent use, for waste disposal in various fields of science and technology: ecology, hygiene, chemical industry, nuclear energy, etc.

Sources of information
1. RF patent 2079345, class B 01 D 51/08, 1997.

 2. Yu.N. Rare beard. On the theory of drift of small particles in acoustic fields. - Kiev: National Academy of Sciences of Ukraine. The main astronomical observatory. Preprint GAO-95-2R - 1995. - 48 p. (prototype).

Claims (1)

 A method of creating inhomogeneities in the concentration of suspended particles, which consists in placing a liquid in a vessel and exciting traveling waves on the surface of the liquid, entraining the particles, characterized in that they excite waves whose length obviously exceeds the depth of the liquid in the vessel.