Cooling device utilizing internal synthetic jets

Abstract

A synthetic jet cooling device (1) for cooling an object (5), comprising: a transducer (10) adapted to generate velocity waves, and an enclosure (4) arranged to receive the velocity waves via an actuated aperture (8). The enclosure (4) is sufficiently large to generate, at the actuated aperture (8), an internal synthetic jet inside the enclosure (4). Furthermore, the enclosure (4) is arranged to contain the object (5), thereby enabling cooling of the object (5) by the internal synthetic jet. The arrangement typically permits multifunctional use of an existing enclosure, containing the object to be cooled, both for its original purpose (e.g. a reflector in a lamp, or a LED backlight module) and as an enclosure generating internal synthetic jets, why the cooling device typically requires virtually no extra space and weight, and can be provided at a low cost.

Claims

1. A synthetic jet cooling device for cooling an object, the device comprising: a transducer for generating velocity waves, an enclosure for containing said object therein and arranged to receive said velocity waves via an actuated aperture, said enclosure being dimensioned to generate, at said actuated aperture, an internal synthetic jet inside the enclosure, thereby enabling cooling of said object by said internal synthetic jet, a housing enclosing a front of said transducer, whereby a second cavity is formed, wherein said enclosure and said actuated aperture are dimensioned to act as a resonating mass-spring system actuated by said transducer, such that fluid in said enclosure acts as a spring, and fluid in said actuated aperture acts as a mass and wherein a diameter of said actuated aperture is between 1/10 and ½ of the distance between an end of the actuated aperture and a first hot spot on the object, and wherein said actuated aperture is a bore of a tube attached to a loudspeaker coil of a loudspeaker comprising said transducer, and wherein said second cavity is connected to said second actuated aperture via a pipe having a length of λ/2, where λ is the wave length of the velocity waves generated by the loudspeaker.

2. A synthetic jet cooling device according to claim 1, wherein the stroke of the transducer is larger than the radius of the actuated aperture.

3. A synthetic jet cooling device according to claim 1, wherein the spatial volume change introduced by the transducer is ≧1% of the enclosure volume.

4. A synthetic jet cooling device according to claim 1, further comprising at least one cavity in communication with said enclosure via at least one actuated aperture, fluid in said at least one cavity being actuated by said transducer, wherein said at least one cavity is dimensioned to prevent the fluid therein from acting as a spring in a mass-spring system, and wherein a surface actuated by said transducer is larger than a surface of said at least one aperture.

5. A synthetic jet cooling device according to claim 1, wherein said actuated aperture is arranged in a member actuated by said transducer, said member being a wall, a membrane or a tube.

6. A synthetic jet cooling device according to claim 1, wherein said loudspeaker has a closed back, whereby a first cavity is formed by interior surfaces of said loudspeaker, a flange of said tube, and/or a loudspeaker membrane.

7. A synthetic jet cooling device according to claim 1, wherein said enclosure comprises at least one non-actuated aperture adapted to generate an additional internal synthetic jet inside said enclosure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

(2) FIG. 1 illustrates an embodiment of a cooling device for an UHP lamp where a loudspeaker, having an open back, actuates a tube whereby both an internal and an external jet is formed at the actuated aperture.

(3) FIG. 2 illustrates an embodiment of a cooling device for an UHP lamp where a loudspeaker actuates a tube. The loudspeaker has a closed back whereby a pumping cavity is formed boosting the internal jet.

(4) FIG. 3 illustrates an embodiment of a cooling device for an UHP lamp where a housing encloses the front of the loudspeaker, whereby a pumping cavity is formed boosting the internal jet.

(5) FIG. 4 illustrates an embodiment of a cooling device for an UHP lamp with pumping cavities on either side of the loudspeaker.

(6) FIG. 5 illustrates an embodiment of a cooling device for a LED backlight.

(7) FIG. 6 illustrates an embodiment of a cooling device for a LED spotlight.

DETAILED DESCRIPTION

(8) FIG. 1 illustrates an embodiment of the present invention for cooling of an Ultra High Performance (UHP) lamp 1. The UHP lamp 1 comprises a paraboloid reflector 2, and a front glass 3 airtight attached to the reflector 2 thereby forming an enclosure 4. The reflector 2 is here made of glass and has a reflective interior surface. A quartz burner 5 can be arranged inside the UHP lamp 1. For a typical UHP lamp 1, there is a first hot spot 6 located at the front pinch of the quartz burner (i.e. the end of the quartz burner closest to the front glass), and a second hot spot 7 located in the middle of the quartz burner (i.e. halfway between the ends of the quartz burner). Note that, provided that the quartz burner is horizontally arranged, the heat distribution at the middle of the quartz burner is such that the second hot spot 7 is located in the upper part thereof, whereas a cold spot 20 will form in the lower part thereof.

(9) To prevent immanent crystallization these hot spots 6,7 require cooling, without cooling the cold spot 20. To enable cooling of the hotspots 6,7 an actuated aperture 8 and a non-actuated aperture 9 are arranged in the reflector wall.

(10) The actuated aperture 8 is arranged in the reflector wall in a region near the front glass and points towards the hot spot 6 at the front pinch of the quartz burner. Arranging the actuated aperture 8 near the front glass is advantageous as the temperature is lower here (there is a temperature gradient from approximately 200° C. at the actuated aperture 8 up to approximately 400° C. at the non-actuated aperture 9), and moreover space and flatness of the reflector wall simplifies manufacturing. By arranging the actuated aperture 8 at the enclosure upper part, it assists the natural convection loop. If instead placed at the lower part of the reflector it would counteract the natural convection loop. The distance of impingement is preferably such that sufficient vortex shedding of the synthetic jet is allowed.

(11) A transducer 10, here being a loudspeaker, is arranged at the actuated aperture 8. A tube 11 is attached to the loudspeaker coil 12, whereby the bore of the tube 11 forms the actuated aperture. Centering and aligning of the ceramic tube and the loudspeaker is facilitated by the protrusion 26 on the flange 24 that fits in the recess in the cone foil of the loudspeaker at the coil radius. The tube 11 is here a ceramic tube made of e.g. Alsint ceramics with coefficient of thermal expansion (CTE) less than or equal to the glass of the reflector, and can be fixed using suitable adhesive such as glue. The tube 11 fits, with clearance fit, in a hole in the reflector wall. The moment of inertia of the tube with flange is preferably minimized in order to prevent tilting resonance modes, that may induce contact between the tube and the hole in the reflector.

(12) In operation the stroke of the loudspeaker 10 results in a translational motion of the tube 11, introducing a volume change for the enclosure 4. The volume change is preferably ≧1% of the enclosure volume. If the loudspeaker stroke is larger than the radius of the actuated aperture, a jet flow may form at the actuated aperture 8. This results in an internal synthetic jet 12, which impinges on the hot spot 6 at the front pinch of the quartz burner. As the loudspeaker 10 has an opening 21 in the back of the loudspeaker there is also an external jet.

(13) As illustrated in FIG. 1, the non-actuated aperture 9 is arranged in the reflector wall and can, for example, be formed by simply drilling a hole in the reflector wall. The non-actuated aperture is not required, but may be advantageous as it allows cooling of multiple hot spots utilizing a single transducer. The non-actuated aperture 9 here points to the hotspot 7 in the middle of the quartz burner. The diameter of the non-actuated aperture is preferably between 1/10 and ½ of the distance between the non-actuated aperture 9 and the hot spot 7 in the middle of the quartz burner.

(14) The non-actuated aperture 9 differs from the actuated aperture 8 in that there is no transducer arranged to actuate the air. Instead the air in the non-actuated aperture 9 acts as mass driven by the air in the enclosure 4 which acts as a spring. As a result an internal synthetic jet will form at the non-actuated aperture 9, and impinge on the hot spot 7 in the middle of the quartz burner. Furthermore, an external jet forms at the non-actuated aperture.

(15) Each aperture 8,9 can be tapered towards the interior of the enclosure in order to boost the internal jet. Further, the edges of each aperture are preferably sharp to promote vortex shedding. By providing the surface of each aperture 8,9 with grooves shaped as a helix or by having an aperture in the form of an orifice protruding into the enclosure the turbulence of the jet may be further increased or the shedding of vortices promoted. Each aperture 8,9 may communicate with the ambient environment, but often the aperture 8,9 is in communication with an encapsulated volume outside the enclosure. This may be advantageous for example to prevent fouling or to confine mercury upon burner explosion of the UHP lamp. Alternatively, each aperture 8,9 can be equipped with a filter against dust and fouling. The filter may be remote.

(16) The enclosure may also have one or more air exhausts (not illustrated) equipped with check valves to improve gas exchange, flow pattern and vortex shedding.

(17) FIG. 2 illustrates another embodiment of the invention. Here, a ring 22 is arranged at the back of the loudspeaker 10. The ring seals the back of the loudspeaker so that a cavity 16 is formed by the interior surfaces of the loudspeaker, the flange 24 of the tube and the loudspeaker membrane 25. The cavity 16 is in communication with the enclosure 4 via the actuated aperture 8. The cavity 16, which is sufficiently small to prevent the air in the cavity from acting as a spring in a mass-spring system, modifies the jet formation criterion into
s>r.sub.aperture.Math.A.sub.aperture/A.sub.pump
where s is the stroke of the transducer (referring to the air rather than the tube) r.sub.aperture is the radius of the aperture, A.sub.aperture is the area of the aperture. A.sub.pump is the area of the actuated surface.

(18) The actuated surface, A.sub.pump, is here formed by the area of the flange 24 and membrane 25 facing the cavity, and is typically about 50 times the area of the actuated aperture, A.sub.aperture. This pumps the air and boosts jet formation even with modest loudspeaker stroke. Indirectly this also affects the non-actuated aperture 9 as it increases the volume change in the enclosure 4.

(19) In operation the flange 24 of the tube and the membrane 25 together pump the air in the cavity. Air flows from the part of the cavity near the membrane 25 around the coil 12 to the part of the cavity near the flange 24 to the tube. This flow cools the loudspeaker coil.

(20) FIG. 3 illustrates yet another embodiment of the invention. Here a housing 27, enclosing the front of the loudspeaker 10, has been airtight attached to the loudspeaker 10 to form cavity 28. The housing has an opening forming the actuated aperture 33. The housing 27 preferably has a CTE that matches the UHP lamp, and can be made of for example ceramics. As illustrated in FIG. 3, the housing can be in the form of a tube with a flange, wherein a protrusion 19 on the flange of the tube is attached to the outer edge of the loudspeaker 10. Note that the housing 27 does not move with the stroke of the loudspeaker coil. Furthermore, the connection of the housing 27 to the reflector 2 preferably is airtight.

(21) The cavity 28, which is sufficiently small to prevent the air in the cavity from acting as a spring in a mass-spring system, modifies the jet formation criterion into
s>r.sub.aperture.Math.A.sub.aperture/A.sub.pump
where s is the stroke of the transducer, r.sub.aperture is the radius of the aperture, A.sub.aperture is the area of the aperture. A.sub.pump is the area of the loudspeaker membrane.

(22) The area of the loudspeaker membrane 25, A.sub.pump, is typically about 50 times the area of the aperture, A.sub.aperture, and thus boosts the vortex shedding and cooling considerably. Indirectly this also affects the non-actuated aperture 9 as it increases the volume change in the enclosure 4.

(23) Parameters for two exemplifying embodiments are specified in table 1 below. The first exemplifying embodiment refers to an embodiment having a vibrating tube with a pumping cavity as described above with reference to FIG. 2. The second exemplifying embodiment refers to an embodiment having a loudspeaker arranged in front of a pumping cavity as described above with reference to FIG. 3.

(24) TABLE-US-00001 TABLE 1 Vibrating tube with pumping Loudspeaker in front of cavity (FIG. 2) pumping cavity (FIG. 3) Focal length [m] 0.007 0.007 Reflector radius [m] 0.0325 0.0325 Reflector half length [m] 0.0305 0.0305 Reflector half width [m] 0.027 0.027 Burner radius [m] 0.0045 0.0045 Burner length [m] 0.04 0.04 Tube length [m] 0.006 0.006 Aperture radius [m] 0.0015 0.0015 Loudspeaker stroke [m] 0.0012 0.0012 Loudspeaker radius [m] 0.0107 0.0107 Temperature [K] 300 300 Cavity volume [cm.sup.3] 1 1

(25) Note that in the embodiment having a vibrating tube (depicted in FIG. 2), the resonance frequency of the loudspeaker is preferably tuned to coincide with the resonance frequency of the Helmholtz resonator by adjusting the mass of the tube 11.

(26) The undamped Helmholtz frequency, f.sub.H, of the cooling device can be described as:

(27) $f_H\approx \frac{c}{2\pi }\sqrt{\frac{1}{V}\underset{i}{.Math.}\frac{A_i}{L_i+1.5.Math.r_i}},\mathrm{where}$ V is the volume of the enclosure less the volume of the quartz burner, A.sub.i is the cross-sectional area of the aperture L.sub.i is the length of the aperture r.sub.i is the aperture radius i is the number of the aperture and, c is the speed of sound in the gas (here 20√T where T is the temperature in K)

(28) The parameters in table 1 results in the following calculated values.

(29) TABLE-US-00002 TABLE 2 Vibrating tube with pumping Loudspeaker in front of cavity (FIG. 2) pumping cavity (FIG. 3) Helmholtz frequency [Hz] 259 259 Air velocity [m/s] 31.07 31.69 Volume displacement [m.sup.3] 1.69 .Math. 10.sup.−6 1.73 .Math. 10.sup.−6 Relative volume displacement 2.2% 2.2% Quality factor 7.8 7.8 Damped Helmholtz frequency 251 251 [Hz] Sound pressure [dB] 64 64

(30) The calculated sound intensity is 64 dB. However, in practical experiments the perceived noise turns out to be less.

(31) FIG. 4 illustrates a combination of the two preceding embodiments (which were describe with reference to FIGS. 2 and 3). Here both sides of the loudspeaker 10 has been closed to create a double action pump. A ring 22 seals the back of the loudspeaker 10 so that a first cavity 16 is formed by the interior surfaces of the loudspeaker, the flange 24 of the tube 11 and the loudspeaker membrane 25. The first cavity 16 is in communication with the enclosure 4 via a first actuated aperture 8 where a synthetic jet is formed. Furthermore, a housing 27, encloses the front of the loudspeaker 10 forming a second cavity 28. The housing here has an opening connected to a second actuated aperture 33 by pipe 29. The additional synthetic jet formed at the second actuated aperture 33 can be utilized to cool an additional hot spot, such as the hot spot 7 at the middle of the quartz burner. By having the length of the pipe 29 to be λ/2, where λ is the wave length of the velocity waves generated by the loudspeaker 10, both apertures 8,33 will breathe simultaneously to enable Helmholtz resonance. Note that the housing 27 does not move with the loudspeaker coil.

(32) According to another embodiment of the invention, there are two adjacent Helmholtz resonators with an aperture in an actuated common wall. This allows cooling of at least one hot spot with clean recirculating air, preventing fouling and dust.

(33) According to yet another embodiment, there are two or more actuated apertures arranged in an enclosure to reduce the audible noise (e.g. by acting as a dipole or quadrapole) and/or be utilized to impinge on a multitude of hot spots. It is recognized that a transducer is already a dipole by itself as long as both sides of the transducer are in communication. Thus, two transducers can make up a quadrupole.

(34) FIG. 5 illustrates an embodiment of the invention for a LED backlight. Here the LED backlight module 32, may act as a Helmholtz resonator. The LED backlight module 32 may be split into a multitude of compartments (each compartment comprising a subset of LEDs) to improve resonance and/or to operate the compartments pair wise in counter phase to create dipole for noise reduction. Internal synthetic jets here impinge on the heat sinks of the LEDs to force convection.

(35) FIG. 6 illustrates a LED spotlight, where internal synthetic jets are utilized to cool the dielectric in the capacitor of the power converter which is temperature critical (typically 70-80° C. max).

(36) The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended claims.

(37) For example, instead of using a tube attached to loudspeaker coil, a perforated membrane could be actuated by the transducer. The membrane may be specularly reflective to fit into, for example, an existing UHP-reflector. Yet another alternative would be an actuated wall having an aperture. It would also be possible to omit or utilize more than one non-actuated aperture. Furthermore, the cooling device may be used for cooling a large variety of objects through outflow of various liquid or gaseous fluids, not only air.

(38) Although the resonance frequency of the device has been in order of magnitude 100 Hz for the described embodiments, the resonance frequency can also be designed to be below the audible range (subsonic) or above the audible range (supersonic) to achieve little audible noise during operation. Furthermore, the cooling device may comprise automatic resonance frequency tuning, as disclosed in WO 2005/027569.