PULSATING COOLING SYSTEM
A cooling device comprising at least one transducer (1) having a membrane adapted to generate pressure waves at a working frequency, characterized by a first and a second cavity (3, 4), said transducer being arranged between said first and second cavities, such that said membrane forms an fluid tight seal between said cavities, each cavity having at least one opening (7, 8) adapted to emit a pulsating net output fluid flow, wherein said cavities and openings are formed such that, at said working frequency, a first harmonic fluid flow emitted by said opening(s) (7) of a first one of said cavities is in anti-phase with a second harmonic fluid flow emitted by said opening(s) (8) of a second one of said cavities, so that a sum of harmonic fluid flow from said openings is essentially zero. With this design, sound reproduction at the working frequency is largely cancelled due to the counter phase of the outlets resulting in a close to zero far-field volume velocity.
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The present invention relates to a pulsating cooling system, i.e. a cooling system where a transducer induces an oscillation creating a pulsating fluid stream that can be directed towards an object that is to be cooled. It may be advantageous to drive the system at, or at least close to, its resonance frequency, in order to obtain a high fluid velocity.
BACKGROUND OF THE INVENTIONThe need for cooling has increased in various applications due to higher heat flux densities resulting from newly developed electronic devices, being, for example, more compact and/or higher power than traditional devices. Examples of such improved devices include, for example, higher power semiconductor light-sources, such as lasers or light-emitting diodes, RF power devices and higher performance micro-processors, hard disk drives, optical drives like CDR, DVD and Blue ray drives, and large-area devices such as flat TVs and luminaries.
As an alternative to cooling by fans, document WO 2005/008348 discloses a synthetic jet actuator and a tube for cooling purposes. The tube is connected to a resonating cavity, and a pulsating jet stream is created at the distal end of the tube, and can be used to cool an object. The cavity and the tube form a Helmholtz resonator, i.e. a second order system where the air in the cavity acts as a spring, while the air in the tube acts as the mass.
Another example is given by N. Beratlis et al, Optimization of synthetic jet cooling for microelectronics applications, 19th SEMITHERM San Jose, 2003. Here a synthetic jet is disclosed having two diaphragms each communicating with the same orifice.
A pulsating fluid stream (typically air stream) of this kind has been found to be more efficient for cooling than laminar flow, as typically used in conventional cooling systems (e.g. cooling fans). The resonance cooling systems further require less space, and generates less noise.
However, in previously proposed systems, e.g. as disclosed in WO 2005/008348, a certain level of sound reproduction, related to the frequency of the oscillating air flow, remains.
SUMMARY OF THE INVENTIONIt is an object of the present invention to reduce the noise level in a pulsating cooling system even further.
According to the present invention, this and other objects are achieved by a cooling device comprising two cavities, the transducer being arranged between the two cavities, such that the membrane forms a fluid tight seal between the cavities, each cavity having at least one opening adapted to emit a pulsating net output fluid flow, wherein the cavities and openings are formed such that, at the working frequency, a first harmonic fluid flow emitted by the opening(s) of a first one of the cavities is in anti-phase with a second harmonic fluid flow emitted by the opening(s) of a second one of the cavities, so that a sum of harmonic fluid flow from the openings is essentially zero.
The transducer arranged between two cavities will act as a dipole, i.e. two acoustical sources in anti-phase. The invention is based on the idea that the harmonic parts of the sound from these two sources will cancel out. The non-harmonic parts, which represent the dominating part of the cooling effect, will not add coherently, and will thus not cancel out.
With this design, an improved cooling effect is achieved by means of an oscillating air stream, while at the same time sound reproduction at the working frequency is largely cancelled due to the counter phase of the outlets resulting in a close to zero far-field volume velocity. Consequently, the cooling system according to the present invention has significantly lower sound reproduction than prior art “synthetic jet” cooling devices.
The cooling device according to the present invention may be used for cooling a large variety of objects through directed outflow of various liquid or gaseous fluids, not only air. It is, however, particularly useful for air-cooling of such objects as electronic circuitry.
Each cavity may have only one opening, or have more than one opening. It is important however that the sum of harmonic contributions from all openings is essentially zero.
More than one transducer may be arranged between the cavities. For example, two, oppositely positioned transducers operating in counter phase will result in a larger air flow. By “oppositely positioned” is intended a situation where pressure waves from one transducer are directed into one cavity, while pressure waves of the other transducer are directed into the other cavity.
A “transducer” is here a device capable of converting an input signal to a corresponding pressure wave output. The input signal may be electric, magnetic or mechanical. Examples of suitable transducers include various types of membranes, pistons, piezoelectric structures and so on. In particular, a suitably dimensioned electrodynamic loudspeaker may be used as a transducer.
The distance between the openings should be short compared to the wavelength at the working frequency. For two sources (e.g. two openings) of strength A at a distance d from each other, the pressure p at distance r from these sources will be
where k is the wave number (ω/c) and θ is the angle of observation. In order to keep this pressure small, according to a preferred embodiment, the distance d is less than 0.2λ, and even more preferably less than 0.1λ.
There are no absolute requirements on the working frequency. However, the working frequency is preferably chosen such that the air velocities and air displacement through the openings have a local maximum, and typically this occurs in a neighborhood of a resonance frequency of the device, i.e. a frequency corresponding to a local maximum of the electric input impedance of the device (the transducer in combination with the cavities and openings). Typically the lowest such frequency is chosen.
Alternatively, the working frequency can be chosen such that the cone excursion of said transducer has a local minimum at this working frequency. Typically, this occurs at an anti-resonance frequency of the device, i.e. a frequency corresponding to a local minimum of the electric input impedance of the device.
One way of ensuring that the air velocities are of essentially equal size and in counter-phase is to provide equal circumstances for all air streams. For example, the cavities can be formed to have equal volume, and the openings can be formed to have equal cross section area. However, this is not a requirement, and canceling air streams may be achieved also with different sized cavities and/or openings.
According to one embodiment, the openings are connected to respective cavity via a channel (or pipe). This allows for more design freedom, as the channels can be formed to direct several air streams towards the same location, and with desired direction. For the same reason as above, the channels can be formed to have equal length and cross section area.
According to one embodiment, such channels are sufficiently long to act more as tube resonators. According to an alternative embodiment, the length of the channels is instead sufficiently short to allow the cavities to act as conventional Helmholtz resonators.
A channel connecting at least one opening of the first cavity can extend through the second cavity, so that this opening is located on the same side of said device as the openings of the second cavity. In a case where the cavities have essentially planar extension and are arranged on top of each other (i.e. like two discs on top of each other), such a design will enable locating all the openings on the top or bottom side of the device.
Two or more devices according to the present invention may be combined, to form a cooling arrangement with a multiple of two openings. The average distance between the openings of a first device and the openings of a second device is then subject to the same requirements as the distance between the two openings of each device, and should thus preferably be less than 0.2λ, and even more preferably less than 0.1λ.
According to this design, four (or more) outlets are arranged close to each other, in relation to the wavelength at the working frequency. This results in a further reduction of noise during operation of the cooling arrangement. This is partly due to a more perfect symmetry of geometry, because of the presence of two (mirrored but identical) transducers, and partly due to a better compensation of nonlinear distortion generated by the two identical loudspeakers.
This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.
The cooling system in
The volumes V1 and V2 and the form of the pipes 5, 6 are chosen such that in use, the transducer will act as a pressure wave dipole, cause a pulsating flows of fluid present in the cavities through the outlets which are essential equal and in counter phase. When driving the transducer at a working frequency, the two fluid flows will thus counteract each other, thereby suppressing any pressure waves escaping from the dipole (i.e. disturbing sound).
It is noted that the principle is not limited any particular fluid, but the present description will be based on a device operated in air, i.e. a device that generates oscillating air streams.
According to the illustrated example, this is ensured by letting respectively V1 and V2, Lp1 and Lp2, and Sp1 and Sp2 have the same value.
By keeping the distance d short compared to the wavelength, e.g. less than 0.1λ, where λ is the wavelength in air corresponding to the working frequency, the air pressure radiating from the dipole is kept very small.
The volumes V1 and V2 and the form of the pipes 5, 6 (from
According to an exemplifying embodiment, a device may have the following properties:
For this device,
Another embodiment is illustrated in
Yet another embodiment is depicted in
The holes 24, 25 need not be arranged on opposite sides of the cavities. As shown in
In another variant of the device in
As a general comment, it is noted that the number of channels from each cavity must not be equal. For example, in the embodiments in
According to yet another embodiment, illustrated in
The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the number of transducers may be increased further, and the placement and form of openings and channels may be varied depending on the application.
Further, the transducer may be implemented in micro electromechanical system (MEMS) technology, i.e. realized on a very small scale. More specifically, on such a small scale, an entire cooling device, including transducer, cavities, openings and any channels, can be completely embodied in silicon using e.g. etching technology. Such a device can advantageously be integrated with an IC to be cooled, e.g. a micro processor. By providing cooling by means of a cooling device on the same scale as the object to be cooled, the cooling may be made more efficient. Of course, a silicon device can be combined with additional channels connected to the silicon substrate.
Claims
1. A cooling device comprising at least one transducer (1) having a membrane adapted to generate pressure waves at a working frequency, characterized by
- a first and a second cavity (3, 4), said transducer being arranged between said first and second cavities, such that said membrane forms a fluid tight seal between said cavities,
- each cavity having at least one opening (7, 8) adapted to emit a pulsating net output fluid flow,
- wherein said cavities and openings are formed such that, at said working frequency, a first harmonic fluid flow emitted by said opening(s) (7) of a first one of said cavities is in anti-phase with a second harmonic fluid flow emitted by said opening(s) (8) of a second one of said cavities, so that a sum of harmonic fluid flow from said openings is essentially zero.
2. The device according to claim 1, wherein each cavity has more than one opening.
3. The device according to claim 1, wherein two transducers (34, 35) are arranged in opposite positions between said cavities (31, 32).
4. The device according to claim 1 wherein a distance d between any two openings is less than 0.2λ, and preferably less than 0.1λ, where λ is the wave length in said fluid corresponding to the working frequency.
5. The device according to claim 1 wherein said working frequency is chosen such that velocities of said first and second harmonic flows have a local maximum at this working frequency.
6. The device according to claim 1 wherein said cavities (3, 4) have essentially equal volume.
7. The device according to claim 1 wherein said openings (7, 8) have essentially equal cross section area.
8. The device according to claim 1 wherein said openings are connected to respective cavity via a channel (5, 6).
9. The device according to claim 8, wherein said channels (5, 6) have essentially equal length.
10. The device according to claim 8, wherein said channels (5, 6) have essentially equal cross section.
11. The device according to claim 8, wherein a channel connecting at least one opening of said first cavity extends through said second cavity, so that said at least one opening is located on the same side of said device as the openings of said second cavity.
12. The device according to claim 1, realized using micro electromechanical system (MEMS) technology.
13. The device according to claim 12, wherein the transducer is formed by etching a silicon substrate.
14-15. (canceled)
Type: Application
Filed: Nov 27, 2007
Publication Date: Jan 28, 2010
Applicant: KONINKLIJKE PHILIPS ELECTRONICS N.V. (EINDHOVEN)
Inventors: Ronaldus Maria Aarts (Eindhoven), Joris Adelbert Maria NIEUWENDIJK (Eindhoven), Antonius Johannes Jo Wismans (Eindhoven)
Application Number: 12/515,999
International Classification: F28D 15/00 (20060101);