US20020079089A1

US20020079089A1 - Apparatus for enhancing condensation and boiling of a fluid

Info

Publication number: US20020079089A1
Application number: US09/808,999
Authority: US
Inventors: Byung-Ha Kang; Seo-Young Kim; Ho-Young Kim
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 2000-11-04
Filing date: 2001-03-16
Publication date: 2002-06-27
Also published as: KR100350363B1; KR20020035183A; JP2002156197A

Abstract

An apparatus enhances the condensation and boiling of a fluid in heat exchange machines, by directly applying time-periodic acoustic waves with a resonance oscillation frequency to liquid drops and/or bubbles formed on a solid surface, when the fluid is in the process of condensation or boiling, thereby effectively removing them therefrom. The apparatus comprises a signal generator for generating a driving signal based on a resonant oscillation frequency of at least one of the liquid drops and the bubbles; and a vibrator, in response to the driving signal, for providing an acoustic pressure wave to said at least one of the liquid drops and the bubbles, to thereby detach them from the solid surface.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a condensation and boiling system; and, more particularly, to an apparatus for enhancing condensation and boiling of a fluid in heat exchange machines, by employing the frequency characteristics of liquid drops and/or bubbles formed on a solid surface.

2. Description of the Related Art

There are various techniques for promoting condensation and boiling of a fluid, performed in heat exchange machines such as refrigerators, air-conditioners, and heaters. The techniques include: mechanically modifying a solid surface of, e.g., a tube or wall of the heat exchange machines, which is in contact with the flowing fluid in the process of condensation or boiling; directly vibrating a solid surface, such as a metal or mirror, with a resonant oscillation frequency of the solid part not of the fluid, as disclosed in U.S. Pat. No. 5,025,187; applying an electric field to a fluid; and coating the solid surface with a surfactant.

The surface modification technique mechanically creates a plurality of grooves on the solid surface to increase the total surface area thereof with which the fluid contacts. This technique, however, is not gaining popularity since its processing cost is considerably high. In addition, the pressure drop of the flow is increased because the grooves provided on the solid surface promotes the fouling and scaling in the condensation and boiling system during operation. The electric-field-applying technique has a problem of its own that a significantly high voltage of several tens of kilovolts (kV) is required for the generation of the electric field, thereby deteriorating the safety thereof.

In the surfactant-coating technique, the performance of condensation and boiling of the fluid becomes lower as the coated surfactant is gradually dissolved, after a certain time, causing serious environmental pollution due to the dissolved surfactant. The solid-vibration technique is difficult to apply for a shell-and-tube type heat exchange machine and also requires vibration of several tens of kilohertz to be imposed on the system, which consumes a significant amount of energy.

SUMMARY OF THE INVENTION

Therefore, the objective of the present invention is to provide an apparatus, which can be applicable to any condensation and boiling system employing various-shape heat exchange machines, for enhancing condensation and boiling of a fluid in a simple and a very cost-effective manner. In the apparatus of the present invention, time-periodic acoustic pressure waves are directly applied to liquid drops and/or bubbles formed on the solid surface, thereby effectively detaching the liquid drops and/or bubbles from the solid surface. Consequently, the condensation and boiling of the fluid, performed in the system, is enhanced.

In accordance with the present invention, there is provided an apparatus for enhancing condensation and boiling of a fluid in heat exchange machines by separating at least one of liquid drops and bubbles from a solid surface with which the fluid contacts, the apparatus comprising: a signal generator for generating a driving signal based on a resonant oscillation frequency of said at least one of the liquid drops and the bubbles; and a vibrator, in response to the driving signal, for providing an acoustic pressure wave to said at least one of the liquid drops and the bubbles, to thereby detach them from said solid surface.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The above and other objectives and features of the present invention will become apparent from the following description of a preferred embodiment given in conjunction with the accompanying drawings, in which: [0009]
FIG. 1 shows a block diagram of an apparatus for enhancing condensation and boiling of a fluid in accordance with the present invention; [0010]
FIG. 2 is a graphical representation illustrating the relationship between a minimum vibration velocity that causes liquid drop detachment and a vibration frequency imposed on the liquid drop; and [0011]
FIG. 3 presents photographs showing a series of removal process for the liquid drop in accordance with the present invention.[0012]

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring now to FIG. 1, there is shown an [0013] apparatus 100 for enhancing condensation and boiling of a fluid, performed in heat exchange machines, in accordance with the present invention. As shown in FIG. 1, the apparatus 100 comprises a signal generator 102, an amplifier 104, and a vibrator 106. Preferably, they may be designed as one integrated unit. The signal generator 102 produces an electrical signal with a resonant oscillation frequency corresponding to the natural oscillation frequency of liquid drops and/or bubbles 120. The electrical signal may be a sinusoidal, saw-tooth, or rectangular wave. The liquid drops and/or bubbles 120 are formed on a solid surface 110 when a fluid is in the process of condensation or boiling. The solid surface 110 is contacting with the fluid during the condensation or boiling process thereof.
Thereafter, the electrical signal is provided to the [0014] amplifier 104, which amplifies it to a predetermined signal level. The amplified signal is provided to the vibrator 106 as a driving signal. In response to the driving signal, the vibrator 106 generates time-periodic acoustic pressure waves with the same frequency as that of the electrical signal. The time-periodic acoustic pressure waves are then applied to the liquid drops and/or bubbles 120 formed on the solid surface 110. When the frequency of the imposed vibration coincides with the resonant oscillation frequency of the liquid drops or bubbles 120, the liquid drops or bubbles 120 oscillate very violently to eventually disengage from the solid surface 110 (hereinafter, the liquid drops and bubbles 120 are referred to as liquid drops for the purpose of simplicity). Consequently, the condensation or boiling process of the fluid can be enhanced as a result of an increasing area where the surrounding fluid could contact with the solid surface 110.
The [0015] vibrator 106 may be implemented by using an acoustic speaker, which is capable of easily generating the time-periodic acoustic pressure waves. Alternatively, there may be employed a piston, cam, membrane, or flap associated with a motor, or a piezoelectric device, for the same purpose.
Referring to FIG. 2, there is a graphical representation illustrating the relationship between a minimum vibration velocity that causes drop detachment and the vibration frequency imposed on the [0016] liquid drops 120 formed on the solid surface 110.
In FIG. 2, each point depicted as a circle represents a mean value of minimum vibration velocities of the [0017] vibrator 106, causing drop detachment at each vibration frequency applied to the liquid drops 120 as mentioned above. Each point is measured by increasing the vibration amplitude from zero at a fixed vibration frequency. An error bar marked on each point represents the standard deviation of positive and negative to the vibration velocity. The liquid drops 120 are composed of water and each volume of those equals that of a sphere with the diameter of, approximately, 1.3 millimeter (mm).
As well known in the art, when the liquid drops [0018] 120 float in the air, the natural oscillation frequency ƒ thereof may be written as follows: $\begin{matrix} f = {\frac{1}{2 π} [n (n - 1) (n + 2) \frac{σ}{ρ a^{3}}]}^{\frac{1}{2}} & Eq . (1) \end{matrix}$
wherein n is a vibration mode number for determining the shape of a vibration and σ, ρ, and a represent the surface tension, density, and diameter of each of the [0019] liquid drops 120, respectively.
Similarly, when bubbles float in the air, the natural oscillation frequency ƒ thereof may be written as follows: [0020] $\begin{matrix} f = {\frac{1}{2 π} [(n + 1) (n - 1) (n + 2) \frac{σ}{ρ a^{3}}]}^{\frac{1}{2}} & Eq . (2) \end{matrix}$
wherein n is a vibration mode number for determining the shape of a vibration and σ, ρ denote the surface tension and density of a surrounding liquid, respectively, and a is the diameter of each of the bubbles. (See, H. Lamb, Hydrodynamics, 6th Ed. Cambridge University Press, Cambridge, England(1932), p.475.) [0021]
In case that the liquid drops [0022] 120 are waters, σ is to 0.0717 N/m, and ρ is to 1000 kg/m³. Calculating the natural oscillation frequency ƒ of the liquid drops 120 by substituting the values of σ, ρ, and a into Eq. (1), the natural oscillation frequency ƒ becomes 80 Hz when the vibration mode number of the liquid drops 120 is two.
It is assumed that the resonant oscillation frequency of the liquid drops [0023] 120 contacting with the solid surface 110 is similar to that of the liquid drops 120 floating in the air. Then, the resonant oscillation frequency of the liquid drops 120 formed on the solid surface 110 will approximate to 80 Hz. This is consistent with a measured point B in the proximity of 80 Hz, as shown in FIG. 2. Consequently, the measured point B proves to be the resonant oscillation frequency of the liquid drops 120 when the vibration mode number thereof is two. And, a measured point A with a lower frequency than that of the measured point B is supposed to be a resonant oscillation frequency when the vibration mode number thereof is one.
In addition, the fact that the minimum vibration frequency for drop detachment is locally minimum at points A and B indicates that the vibrations whose frequencies correspond to the points A and B effectively oscillate the [0024] liquid drops 120 to cause them to disengage from the solid surface 110. On the contrary, in the frequency ranges other than those near the points A and B, higher vibration velocity is required to induce drop detachment. Therefore, it is clear that imposing vibrations at resonant oscillation frequencies of the liquid drops 120 are crucial in effectively promoting the drop disengagement process, thereby enhancing the condensation and boiling of the fluid.
Referring to FIG. 3, there is presented photographs showing a series of removal processes at the point A for the [0025] liquid drops 120 formed on the solid surface 110, in accordance with the present invention. The photographs are taken by using a commercially available high-speed camera. The resonant frequency applied to the liquid drops 120 is 25 Hz, approximately, wherein the vibration is induced by the time-periodic acoustic pressure waves in accordance with the present invention. As shown in FIG. 3, a surface area where the liquid drops 120 contact with the solid surface 110 is repeatedly increased and decreased depending on the vibration of the liquid drops 120 between time intervals of −205 to −5 millisecond (ms). At time intervals of −3 to 0 ms, the surface area is abruptly decreased, thereby removing the liquid drops 120 from the solid surface 110. The time is represented as a relative time by setting a time, at which the liquid drops 120 disengage from the solid surface, to zero.
As described above, the apparatus in accordance with the present invention dramatically decreases the energy consumption of a system employing a condensation and/or boiling process of a fluid, by enhancing the condensation and/or boiling of the fluid in the aforementioned manner. Also, the apparatus may be used in a radiator or cooling tower to remove the liquid drops and/or bubbles that cause erosion and scaling in the heat exchange machines. Further, the apparatus in accordance with the present invention could be integrated as one unit so that it may be applicable for a small-scale system employing the condensation and/or boiling process of the fluid. [0026]
While the present invention has been described and illustrated with respect to a preferred embodiment of the invention, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad principles and teachings of the present invention which should be limited solely by the scope of the claims appended hereto. [0027]

Claims

What is claimed is:

1. An apparatus for enhancing condensation and boiling of a fluid in heat exchange machines by separating at least one of liquid drops and bubbles from a solid surface with which the fluid contacts, the apparatus comprising:

means for generating a driving signal based on a resonant oscillation frequency of said at least one of the liquid drops and the bubbles; and

means, in response to the driving signal, for providing an acoustic pressure wave to said at least one of the liquid drops and the bubbles, to thereby detach them from said solid surface.

2. The apparatus according to claim 1, wherein the resonant oscillation frequency corresponds to a first natural oscillation frequency ƒ₁of each of the liquid drops, and

the first natural oscillation frequency ƒ₁is calculated as follow:

f_{1} = {\frac{1}{2 π} [n (n - 1) (n + 2) \frac{σ}{ρ a^{3}}]}^{\frac{1}{2}}

wherein n is a vibration mode number for determining the shape of a vibration; and σ, ρ, and a represent the surface tension, the density, and the diameter of said each of the liquid drops, respectively.

3. The apparatus according to claim 1, wherein the resonant oscillation frequency corresponds to a second natural oscillation frequency ƒ₂of each of the bubbles, and the second natural oscillation frequency ƒ₂is calculated as follow:

f_{2} = {\frac{1}{2 π} [(n + 1) (n - 1) (n + 2) \frac{σ}{ρ a^{3}}]}^{\frac{1}{2}}

wherein n is a vibration mode number for determining the shape of a vibration; and σ, ρ denote the surface tension and the density of a surrounding liquid, respectively, and a is the diameter of said each of the bubbles.

4. The apparatus according to claim 1, further comprising means for amplifying the driving signal to a predetermined signal level.

5. The apparatus according to claim 4, wherein the signal generation means, the providing means, and the amplification means are integrated as one unit.

6. The apparatus according to claim 1, wherein the driving signal includes one of a sinusoidal, triangular, saw-tooth, and rectangular wave signals.

7. The apparatus according to claim 1, wherein the providing means includes a membrane, piston, acoustic speaker, and piezoelectric device.

8. The apparatus according to claim 2, wherein the acoustic pressure wave has a resonant oscillation frequency that depends on the first natural oscillation frequency ƒ₁, wherein the liquid drops are vibrated with said resonant oscillation frequency.

9. The apparatus according to claim 3, wherein the acoustic pressure wave has a resonant oscillation frequency that depends on the second natural oscillation frequency ƒ₂, wherein the bubbles are vibrated with said resonant oscillation frequency.

10. The apparatus according to claim 1, wherein the resonant oscillation frequency is several tens of hertz (Hz).