Ultrasonic fluid pressure generator

An ultrasonic fluid pressure generator for generating high pressure head in a fluid. The ultrasonic fluid pressure generator comprises a transducer comprising a piezoelectric actuator and a displacement amplifier, the displacement amplifier having a fluid channel therethrough, the displacement amplifier being connected to the piezoelectric actuator at one end and having a free vibrating tip at another end; a reflecting condenser disposed at the vibrating tip of the displacement amplifier to form a gap between the vibrating tip and a reflecting surface of the reflecting condenser; and a casing configured for establishing a standing wave in the fluid contained within the casing, the transducer and the reflecting condenser being at least in part within the casing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/SG2009/000488 filed Dec. 22, 2009, the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an ultrasonic fluid pressure generator. It particularly relates to an ultrasonic fluid pressure generator for generating high fluid pressure head for use as a pump, a pressure regulator, a hydraulic actuator or a microfluidic device.

BACKGROUND OF THE INVENTION

Rotary centrifugal pumps are conventionally used in industrial applications to induce flow of fluids via a pressure difference. The maximum pressure head that can be obtained depends on the external diameter of the impeller and the speed of the rotating shaft. Consequently, for high pressure head applications, a large rotary centrifugal pump is required, leading also to high power consumption.

However, it is often not feasible to use a large-sized pump especially where space is a constraint. Furthermore, it is desirable to have as low a power consumption as possible to improve efficiency and save energy.

Due to its valveless nature, ultrasonic pumps have been proposed. As shown inFIG. 1(a)(prior art), an ultrasonic pump1comprises chiefly a tube2with a plate3positioned at a gap G from the tip4of the tube2. Either the tube2or the plate3is ultrasonically vibrated so as to create a displacement D in the gap G. This generates a pressure P in a region of the fluid5immediately between the tip4and the plate3, thereby pushing water into the tube2as shown by the block arrow. The pressure P generated is a function of several parameters such as the gap G, internal diameter ID of the tube2, vibration amplitude D and vibration frequency ƒ used. In an alternative embodiment, the ultrasonic pump comprises the tube2with an insertion rod6as shown inFIG. 1(b)(prior art).

As an example, an ultrasonic pump from Precision and Intelligence Laboratory of the Tokyo Institute of Technology uses a bending disk transducer to vibrate the plate3. This achieved a maximum pump pressure of about 2 mH2O (or 20 kPa) with a vibration velocity of 1.0 m/s and a gap size of 10 μm, obtaining a maximum flow rate of 22.5 mL/min with input power of 3.8 W. Another ultrasonic pump from the same source uses a vibrating tube2(with or without the insertion rod6) to achieve a similar maximum pump pressure. Although prototypes have been developed, the maximum pump pressure is still low for many practical applications, such as micro channel cooling.

SUMMARY OF THE INVENTION

According to a first aspect, there is provided an ultrasonic fluid pressure generator for generating high pressure head in a fluid. The ultrasonic fluid pressure generator comprises a transducer comprising a piezoelectric actuator and a displacement amplifier, the displacement amplifier having a fluid channel therethrough, the displacement amplifier being connected to the piezoelectric actuator at one end and having a free vibrating tip at another end; a reflecting condenser disposed at the vibrating tip of the displacement amplifier to form a gap between the vibrating tip and a reflecting surface of the reflecting condenser; and a casing configured for establishing a standing wave in the fluid contained within the casing, the transducer and the reflecting condenser being at least in part within the casing.

The reflecting condenser is preferably configured for focusing sound waves and improving sound pressure magnitude between the vibrating tip and the reflecting condenser, and may include a rod projecting from the reflecting surface into the fluid channel of the displacement amplifier without contacting the displacement amplifier. The reflecting condenser may further be configured to moveably engage the casing for adjusting pressure magnitude in the fluid.

The displacement amplifier preferably has a decreasing external dimension from the end connected to the piezoelectric actuator to the end having the free vibrating tip.

The piezoelectric actuator may have a tubular configuration, and preferably comprises a fluid channel therethrough, the fluid channel of the piezoelectric transducer being in fluid connection with the fluid channel of the displacement amplifier.

The transducer is preferably affixed to the casing at its nodal position.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An ultrasonic fluid pressure generator10capable of generating high pressure head as shown inFIG. 2, which is an exemplary embodiment of the invention, will now be described. As a result of the high pressure head that can be produced, the ultrasonic fluid pressure generator10may serve not only as a fluid pump, but may also be used as a pressure regulator, a hydraulic actuator or a microfluidic device.

As shown inFIG. 2, the exemplary embodiment of the ultrasonic fluid pressure generator10comprises a transducer15, a reflecting condenser40and a casing50enveloping the transducer15and the reflecting condenser40. The transducer15further comprises a piezoelectric actuator20and a displacement amplifier30.

The transducer15is configured for effecting one-dimensional longitudinal vibration in a fluid12contained within the casing50so that as sound waves propagate in the fluid12, pressure patterns are generated in the fluid12. Preferably, the transducer15has a power consumption as low as 1 Watt, a frequency range of 10 to 100 kHz and a vibration amplitude with an operational vibration velocity range of 0 to 5 m/s. The piezoelectric actuator20which serves as a driving component of the transducer15may be of a multilayer piezoelectric stack20as shown, or have a tubular configuration. Total length of the transducer15may be a multiple of a half a wavelength, while length of the piezoelectric actuator20is preferably a multiple of a quarter or half of a wavelength. The piezoelectric actuator20is preferably clamped between the displacement amplifier30and an end-cap60as shown.

The displacement amplifier30of the transducer15is connected to the piezoelectric actuator20at one end32while having a free vibrating tip34at another end. The displacement amplifier30has a fluid channel36therethrough, and is preferably made of a metal such as titanium or an equivalent for generating high vibration velocity while being corrosion resistant. The displacement amplifier30is configured to have a decreasing external dimension38from the end32connected to the piezoelectric actuator20to the end having the free vibrating tip34. In this way, high vibration amplitude is achieved at the vibrating tip34while requiring lower vibration velocity of the piezoelectric actuator20. Consequently, less heat is generated by the piezoelectric actuator20, thereby improving reliability of the transducer15. In the preferred embodiment, the piezoelectric actuator20and the end-cap60also comprise fluid channels26and66respectively, wherein all the fluid channels36,26,66are in fluid connection with one another, thereby forming a continuous through-hole in the transducer15as shown inFIG. 2.

By providing a displacement amplifier30with a vibrating tip34of a reduced cross-sectional area compared to the piezoelectric actuator20, an overall vibration amplification ratio of about 15 to 20 is obtained. This results in high pressure generation in the fluid12as pressure becomes focused at a region13of the fluid12around a rim39of the tip34as shown inFIG. 3(a), where arrows indicate direction of fluid flow and dashed lines indicate a maximum pressure region13.

Impedance of the fluid pressure generator10is therefore adjusted by providing the displacement amplifier30so as to lower power required of the piezoelectric actuator20. Ensuring a smooth decrease in external dimension38of the displacement amplifier30results in lower overall system energy loss and also reduces bending vibration of the displacement amplifier30.

The reflecting condenser40engages the casing50to form a seal41between the reflecting condenser40and the casing50. The reflecting condenser40comprises a reflecting surface42that is preferably circular in shape and large enough to cover the cross-sectional area of the amplifier tip34. The reflecting surface42may be flat as shown, or also curved. The reflecting condenser40is disposed at the vibrating tip34of the displacement amplifier30so as to form a gap46between the vibrating tip34and the reflecting surface42, as shown inFIG. 3(b). Downward vertical flow as shown inFIG. 3(a)is thus reduced or eliminated by the reflecting surface42as can be seen in the absence of downwardly directed arrows inFIG. 3(b). The size of the gap46may be adjusted by configuring the reflecting condenser40to moveably engage the casing50for adjusting pressure magnitude in the fluid region13, wherein movement of the reflecting condenser40may be actuated by appropriate means such as adjustment screws.

While a short rod R alone inserted into the fluid channel36of the transducer15reduces horizontal flow as shown inFIG. 3(c), too long a rod R by itself will halt fluid flow up the fluid channel36as a result of downward flow being greater than upward flow around the rod R. In the preferred embodiment of the fluid pressure generator10of the present invention, therefore, the reflecting condenser40has a ⊥-shape, comprising a rod44together with the reflecting surface42as shown inFIG. 3(d). The rod44projects from the reflecting surface42into the fluid channel36of the displacement amplifier30without contacting the displacement amplifier30. By providing the ⊥-shaped reflecting condenser40, useless flow in both the downward and horizontal directions is reduced or eliminated. A well defined flow path is thus created with the use of the ⊥-shaped reflecting condenser40together with the displacement amplifier30, thereby increasing efficiency.

By providing the reflecting surface42together with the rod44, the rod44may be of unlimited length within the fluid channel36of the displacement amplifier30as downward flow is prevented by the reflecting surface42. However, when the length of the rod44is a multiple of a quarter of the wavelength, the pressure wave is more focused at the vibrating tip34.

The ⊥-shaped reflecting condenser40also reduces the area of pressure distribution when compared to using only the reflecting surface42alone (FIG. 3(b)) or the short rod R alone (FIG. 3(c)). This is due to the ⊥-shaped reflecting condenser40providing a corner ring47that focuses energy generated by the transducer15. In the preferred embodiment as shown inFIG. 3(d), the corner ring47has a sharp right angle which focuses pressure between itself47and the amplifier tip34. This produces a new area of focusing below the vertical flow path that more effectively directs fluid12into the fluid channel36. Other embodiments of the corner ring47such as a concave design may be provided to focus the pressure wave more effectively.

As shown inFIG. 2, the transducer15and the reflecting condenser40are enveloped by the casing50. The casing50is configured for establishing a standing wave in the fluid12contained in a liquid cavity56within the casing50. The liquid cavity56is defined or bound by the casing50, the displacement amplifier30, and the reflecting condenser40. The transducer15and the reflecting condenser40should therefore be at least in part within the casing50. For example, in an alternative embodiment, the piezoelectric actuator20may be external to the casing50. Wavelength of the standing wave established in the liquid cavity56may range from zero to infinity in any direction.

The casing50is provided with at least an inlet52for in-flow of the fluid12. In the embodiment shown inFIG. 2, the casing50is also provided with an outlet54for liquid out-flow, the outlet54being connected to the end-cap60of transducer15via an out-flow connecting tube58. The casing50is preferably cylindrical in shape and may have an inner diameter less than a quarter wavelength and a liquid cavity length being multiples of half a wavelength so as to create resonance of the fluid12in the cavity56. The casing50should be made of an acoustically hard material such as aluminium in order to reflect the sound wave generated in the fluid12, so as to reduce energy loss induced in the fluid12. In the preferred embodiment, the transducer15is affixed to the casing50to form a seal at a nodal position of the transducer15itself. The inlet52should be positioned on the casing so as not to affect the standing wave condition created in the fluid12. Alternative embodiments of the casing50are shown inFIG. 4, wherein the casing50may be spherical, semi-spherical, stepped, conical, and so forth.

By establishing a standing wave condition in the fluid12, the casing reduces power consumption required by the transducer15. This in turn increases sound pressure at the amplifier tip34. In an ideal case, the standing wave condition would not affect power consumption and vibration displacement of the transducer15as all the power will be reflected from the boundary. By forming a seal between the casing50and the transducer15, as well as a seal between the casing50and the reflecting condenser40, the generated sound wave is confined within the liquid cavity56. The displacement amplifier30thus forms a first order focusing, the reflecting condenser40a second order focusing and the casing50a third order focusing.

As shown in Table 1 below, with the casing alone, improvement in sound pressure can be up to two times the pressure obtained without the casing50, as a result of the casing50forming a reflective boundary condition in the fluid12. Using the casing50together with the reflecting condenser40, the sound pressure can be increased by 14 times as the casing50and reflecting condenser40together restrain and focus the sound wave in a limited space within the casing50, thereby producing high static pressure which induces fluid flow towards the outlet54.

To appropriately configure the fluid pressure generator10for optimizing performance, the piezoelectric transducer15is represented as an electric circuit model as shown inFIG. 5, where each section of the transducer15, i.e. the displacement amplifier30, the piezoelectric actuator20and the end-cap60are each represented by an appropriate electric circuit component accordingly.

In the circuit, Ztipis the radiation impedance at the amplifier tip34. Zendis the back load from the air. Cois clamped capacitance of the piezoelectric actuator20, Rois dielectric resistance, φ is electromechanical conversion coefficient (φ=S/L·d33/s33E), νtipand νendare the vibration velocities at the amplifier tip34and an end of the actuator20, respectively. The parallel and series impedances Z inFIG. 5are given by the following expressions:

In the above expressions, ρi, ci, Si, ki, li(i=1, 2, 3, 4) are density, sound speed, area of cross section, wave number and length for each section respectively, while n is the number of elements in the piezoelectric stack forming the piezoelectric actuator20. Before solving the circuit, the following parameters are defined:

The circuit is then solved to obtain important parameters as listed below, where:

impedance of vibration system is

Z=Zf⁢ZbZf+Zb+Z3⁢a(16)
velocity at the end is

vend=Z4⁢aZ4⁢a+Z11⁢ZfZf+Zb⁢φ⁢⁢VZ(17)
velocity at the tip34is

vtip=Z7Z7+Zg⁢ZbZf+Zb⁢φ⁢⁢VZ(18)
and power consumption of the transducer15is

Table 2 below shows experimental performance results of the fluid pressure generator10under different conditions.

It can be seen that where a flat reflecting condenser is used without a casing, the ultrasonic pressure generator10is effectively the same as the prior art ultrasonic fluid pump as shown inFIG. 1(a)(prior art) and achieves only a pressure head of 1.6 mH2O.

However, by providing the casing50together with the ⊥-shaped reflecting condenser40in the ultrasonic pressure generator10of the present invention, for the same flow rate of 9.2 mL/min, a pressure head of 24 mH2O is achieved while power consumption is reduced from 1.5 W to 0.6 W. This is an improvement of 15 times the pressure head that can be obtained by a known ultrasonic pump, while reducing power consumption by 2.5 times.

Furthermore, as shown in Table 3 below, in comparison with three different centrifugal pumps, it can be seen that for an equivalent power consumption of around 1 W, the RS M200-S-SUB having small external dimensions of 15.7×15.7×28.5 mm can only reach a pressure head of 1.9 mH2O, while the ultrasonic fluid pressure generator10of the present invention achieves a maximum pressure head of 30 mH2O, an improvement of nearly 16 times for the same power consumption.

Comparing the ultrasonic fluid pressure generator10of the present invention with a centrifugal pump of similar size such as the SWIFTECH MCP655, the centrifugal pump consumes some 24 times more power while achieving a pressure head of about 10 times less.

To achieve a similar pressure head as the ultrasonic fluid pressure generator10of the present invention, it can be seen that a much bigger centrifugal pump such as the ZHEJIANG LEO CO., LTD., micro centrifugal pump will be required, which consumes over 1000 times the power used by the ultrasonic fluid pressure generator10of the present invention.

The performance of the ultrasonic fluid pressure generator10of the present invention thus greatly exceeds that of all known embodiments of existing ultrasonic fluid pumps, as well as known embodiments of centrifugal pumps having an equivalent size, or power consumption, or pressure head output.

It should be appreciated that the invention has been described by way of example only and that various modifications in design and/or detail may be made without departing from the scope of this invention.