Device for generating a CO.SUB.2 .snow jet

Abstract

The invention relates to a device for generating a CO.sub.2 snow jet, comprising an expansion channel (6) which extends in a flow direction (14) for generating a CO.sub.2 gas/CO.sub.2 snow mixture based on liquid CO.sub.2, said expansion channel having an inlet opening (18) for supplying liquid CO.sub.2 and an outlet opening (22) for discharging the CO.sub.2 gas/CO.sub.2 snow mixture. The device also comprises a nozzle for generating an outer jet which surrounds and accelerates the CO.sub.2 gas/CO.sub.2 snow mixture discharged from the outlet opening of the expansion channel. The expansion channel has multiple channel sections (36a, 36b, 36c, 36d, 36e) arranged one behind the other in the flow direction, wherein the expansion channel cross section (40) that lies on a plane orthogonal to the flow direction changes locally in a particular transition or transition region (38a, 38b, 38c, 38d, 38e, 38f) between the channel sections, and the expansion channel (6) cross section (46d) at the upstream end (48d) of a particular channel section (36d) is larger than the expansion channel (6) cross section (46c) at the upstream end (48c) of the channel section (36c) arranged upstream of said channel section (36d) in the flow direction (14).

Claims

1. A device (2) for generating a CO2 snow jet (4), comprising an expansion channel (6) which extends in a flow direction (14) for generating a CO2 gas/CO2 snow mixture (8) based on liquid CO2, said expansion channel (6) having an inlet opening (18) for supplying liquid CO2 and an outlet opening (22) for discharging the CO2 gas/CO2 snow mixture (8), the device (2) comprising a nozzle (26) for generating an outer jet (28) which surrounds and accelerates the CO2 gas/CO2 snow mixture (8) discharged from the outlet opening (22) of the expansion channel (6), wherein the expansion channel (6) has multiple channel sections (36a, 36b, 36c, 36d, 36e, 36f, 36g) arranged one in front of the other in the flow direction (14), which widen continuously over an extension of each said channel section (36a, 36b, 36c, 36d, 36e, 36f, 36g) in the flow direction (14), an expansion channel (6) cross section (40) that lies on a plane orthogonal to the flow direction (14) tapers suddenly radially inwardly in transition between each said adjacent channel sections (36a, 36b, 36c, 36d, 36e, 36f, 36g) in the flow direction (14), to form an annular screen surface (50) to serve as a collision surface for snow particles that have already been formed, such that the snow particles, when they hit the screen surface are compacted and grow together to form larger snow agglomerates, the expansion channel cross section at an upstream end of each said channel section is smaller than the expansion channel cross section at a downstream end of a channel section arranged immediately upstream of each said channel section in the flow direction (14), and the expansion channel (6) cross section (46d) at the upstream end (48d) of each said channel section (36d) is larger than the expansion channel (6) cross section (46c) at the upstream end (48c) of the channel section (36c) arranged immediately upstream of each said channel section (36d) in the flow direction (14), the expansion channel (6) is formed by at least one cavity of a workpiece, said workpiece having a wall delimiting the expansion channel (6), the transition tapers suddenly radially inwardly with respect to that wall, the expansion channel is configured such that a two-phase flow arises in the course of the phase conversion of liquid CO2 into gaseous CO2 that takes place in the expansion channel, said CO2 liquid phase flowing annularly at the edge of the expansion channel and the CO2 gas phase flowing in a central region of the expansion channel, the change in cross-section in the transition (38a, 38b, 38c, 38d, 38e, 38f) brings about a pressure jump, as a result of which turbulences are generated in the flow within the expansion channel, which mixes up the CO2 liquid phase and the CO2 gas phase such that in the course of the intermixing of the gas and liquid phases, the liquid phase is accelerated by the faster flowing gas phase such that snow particles resulting from the accelerated liquid phase emerge from the outlet opening of the expansion channel at increased velocity.

2. The device (2) of claim 1, wherein the particular transition (38a, 38b, 38c, 38d, 38e, 38f) extends in the flow direction (14) over a length of from 0 mm to 5 mm.

3. The device (2) of claim 2, wherein the particular transition (38a, 38b, 38c, 38d, 38e, 38f) extends in the flow direction (14) over a length of from 0 mm to 4 mm.

4. The device (2) of claim 1, wherein the expansion channel (6) has a maximum cross-sectional area surface of less than 5.0 mm.sup.2 on a plane orthogonal to the flow direction (14).

5. The device (2) of claim 4, wherein the expansion channel (6) has a maximum cross-sectional area surface of less than 3.0 mm.sup.2 on a plane orthogonal to the flow direction (14).

6. The device (2) of claim 1, wherein the expansion channel (6) is formed by at least one cavity of a workpiece.

7. The device (2) of claim 6, wherein the expansion channel (6) is formed by a cylindrical tubular body (10).

8. The device (2) of claim 1, wherein the transition or transition region (38a, 38b, 38c, 38d, 38e, 38f) form a screen surface (52) oriented orthogonal to the flow direction (14) or inclined to the flow direction (14).

9. The device (2) of claim 1, wherein each said channel section (36a, 36b, 36c, 36d, 36e, 36f, 36g) widens conically in the flow direction (14).

Description

(1) In the drawing:

(2) FIG. 1 is a schematic view of an embodiment of a device for generating a CO.sub.2 snow jet in a partially sectioned view;

(3) FIG. 2a is a side view of a tubular body half of a tubular body delimiting an expansion channel of the device according to FIG. 1 in a first configuration;

(4) FIG. 2b is a section designated IIb in FIG. 2a in an enlarged view;

(5) FIG. 2c is a sectional view of the section according to

(6) FIG. 2b along the section line IIc-IIc shown in FIG. 2b;

(7) FIG. 2d is a perspective view of the tubular body half of the tubular body according to FIG. 2a;

(8) FIG. 3a is a side view of a tubular body half of a tubular body delimiting an expansion channel of the device according to FIG. 1 in a further configuration;

(9) FIG. 3b is a section designated IIIb in FIG. 3a in an enlarged view;

(10) FIG. 3c is a perspective view of the tubular body half of the tubular body according to FIG. 3a;

(11) FIG. 4a is a side view of a tubular body half of a tubular body delimiting an expansion channel of the device according to FIG. 1 in a further configuration;

(12) FIG. 4b is a section designated IVb in FIG. 4a in an enlarged view;

(13) FIG. 4c is a perspective view of the tubular body half of the tubular body according to FIG. 4a.

(14) FIG. 1 is a schematic view of an embodiment of a device, which is denoted as a whole by the reference sign 2 and is partially shown for generating a CO.sub.2 snow jet 4.

(15) The device 2 has an expansion channel 6 which is used to generate a CO.sub.2 gas/CO.sub.2 snow mixture 8 based on liquid CO.sub.2. In the case shown by way of example, the expansion channel 6 is formed by a cavity in a cylindrical tubular body 10. The tubular body 10 and the expansion channel 6 extend along a central axis 12 in a flow direction 14.

(16) At the upstream end 16 thereof, i.e. at the end thereof which is upstream in the flow direction 14, the expansion channel 6 has an inlet opening 18 for supplying liquid CO.sub.2.

(17) The expansion channel 6 also has an outlet opening 22 at the downstream end 20 thereof, i.e. at the upstream end thereof in the flow direction 14, for discharging the CO.sub.2 gas/CO.sub.2 snow mixture 8 formed in the expansion channel 6.

(18) To generate the CO.sub.2 gas/CO.sub.2 snow mixture 8, liquid CO.sub.2 is fed, for example and preferably at an outlet pressure of approx. 60 bar or, when supplied from low-pressure tanks, of approx. 20 bar by device components (not shown) via the inlet opening 18 to the expansion channel 6 in the flow direction 14. Over the extension of the expansion channel 6 in the flow direction 14, the pressure drops from the outlet pressure at the inlet opening 18 to ambient pressure (approx. 1 bar) at the outlet opening 22. As the pressure falls, a step-by-step phase conversion from liquid CO.sub.2 to gaseous CO.sub.2 takes place with simultaneous cooling of the mixture. In the course of the phase conversion, a two-phase flow is formed, the liquid phase flowing annularly at the edge of the expansion channel 6 and the gas phase flowing in a central region of the expansion channel 6. If the pressure falls below the triple point of CO.sub.2 (5.185 bar), the remaining liquid phase is converted step-by-step into solid CO.sub.2 in the form of CO.sub.2 snow crystals 22, so-called CO.sub.2 snow. There are then three phases. The resulting CO.sub.2 snow crystals 24 are entrained by the gas flow, accelerated and, together with the CO.sub.2 gas, are discharged from the outlet opening 22 as a CO.sub.2 gas/CO.sub.2 snow mixture 8. As explained at the outset, larger snow crystals, in particular a larger proportion of snow, are formed by the changes in cross section and the pressure jumps caused thereby.

(19) The device 2 also has a nozzle 26 for generating an outer jet 28 from a carrier gas, for example and preferably from ultra-pure air or ultra-pure nitrogen. In the case shown by way of example, the nozzle 26 is designed as an annular nozzle which preferably concentrically surrounds the tubular body 10. By way of example and preferably, the nozzle 26 can be designed as a Laval nozzle in a manner known per se. The outer jet 28 surrounds the CO.sub.2 gas/CO.sub.2 snow mixture 8 discharged from the outlet opening 22 of the expansion channel 6 and accelerates it to a CO.sub.2 snow jet 4.

(20) As explained above and in a manner known per se, the CO.sub.2 snow jet 4 can be used to remove filmic and/or particulate impurities 30 from a workpiece surface 32.

(21) FIG. 2a to 4c show different configurations of the expansion channel 6 in detail. In each of the illustrated configurations, the tubular body 10 delimiting the expansion channel 6 is, for example and preferably, formed from, preferably two, preferably identical, tubular body halves 34. FIG. 2a to 5b each show such a tubular body half 34 in different views. The tubular body halves 34 are produced, for example and preferably, from plastics material, for example in a vacuum casting method. The tubular body halves 54 can be joined to one another, for example, by means of ultrasonic welding, laser welding, gluing, crimping or by pressing the tubular body halves 54 into an outer tube.

(22) In each of the configurations shown, the expansion channel 6 has a plurality of channel sections 36a, 36b, 36c, 36d, 36e, 36f, 36g arranged one behind the other in the flow direction 14. The expansion channel 6 also has transitions or transition regions 38a, 38b, 38c, 38d, 38e, 38f arranged between the particular channel sections 36a, 36b, 36c, 36d, 36e, 36f, 36g. In the transitions or transition regions 38a, 38b, 38c, 38d, 38e, 38f, the expansion channel 6 cross section 40 which lies on a plane orthogonal to the flow direction 14 changes locally. The change in cross section in the particular transition or transition region 38a, 38b, 38c, 38d, 38e, 38f each brings about a pressure jump, as a result of which turbulences are generated in the flow within the expansion channel 6, which mixes up the phases present in the expansion channel 6, in particular the CO.sub.2 liquid phase and the CO.sub.2 gas phase.

(23) First of all, a particular channel section is considered: In the configuration according to FIG. 2a to 2d, the channel sections 36a, 36b, 36c, 36d, 36e widen conically over their particular extension in the flow direction 14. In front of these channel sections 36a, 36b, 36c, 36d, 36e, a feed section with a constant cross section is provided by way of example. By way of example and preferably, the channel sections 36a, 36b, 36c, 36d, 36e are designed as frustoconical, i.e. conically widening, cavities within the tubular body 10. As shown by way of example in FIG. 2b for the channel section 36c, the expansion channel 6 cross section 42c at the downstream end 44c of a particular channel section 36c is larger than the cross section 46c at the upstream end 48c of this channel section 36c.

(24) As shown by way of example in FIG. 2b for the channel sections 36c and 36d, in the present configuration the expansion channel 6 cross section 46d at the upstream end 48d of a particular channel section 36d is larger than the expansion channel 6 cross section 46c at the upstream end 48c of the channel section 36c arranged upstream of said channel section 36d in the flow direction 14. The expansion channel 6 cross section 46c,d at the upstream end 48c,d of a particular channel section 36c, 36d becomes therefore larger, viewed in the flow direction 14, from channel section 36c to channel section 36d.

(25) Now the transitions between the channel sections are considered:

(26) In a particular transition 38a, 38b, 38c, 38d between the channel sections 36a, 36b, 36c, 36d, 36e according to the configuration in FIG. 2a to 2d, the expansion channel 6 tapers suddenly radially inward. As shown by way of example in FIG. 2b for the channel sections 36c and 36d, the expansion channel 6 cross section 46d at the upstream end 48d of a particular channel section 36d is smaller than the expansion channel 6 cross section 42c at the downstream end 44c of the channel section 36c arranged upstream of this channel section 36d in the flow direction 14.

(27) By way of example and preferably, the particular transition 38a, 38b, 38c, 38d in the present case each forms an annular screen 50, a screen surface 52 being oriented orthogonally to the flow direction 14 (see FIGS. 2b and 2c).

(28) FIG. 3a to 3c show a further different configuration of the expansion channel 6, in which the channel sections 36a, 36b, 36c, 36d, 36e, 36f and a feed section provided by way of example have a constant cross section orthogonal to the flow direction 14 over their particular extension in the flow direction 14. As shown by way of example in FIG. 3b for the channel sections 36c and 36d, the cross section 54d of a channel section 36d arranged downstream in the flow direction 14 is larger than the cross section 54c of the channel section 36c arranged upstream of this channel section 36d in the flow direction 14. The particular channel sections 36a, 36b, 36c, 36d, 36e, 36f are designed, for example and preferably, as circular cylindrical cavities in the tubular body 10.

(29) In the configuration according to FIG. 3a to 3c, the particular transitions 38a, 38b, 38c, 38d, 38e between the channel sections 36a, 36b, 36c, 36d, 36e, 36f are designed as transition regions 38a, 38b, 38c, 38d, 38e; they thus have in each case an extension in the flow direction 14. By way of example and preferably, each transition region 38a, 38b, 38c, 38d, 38e has a first section 56 which tapers conically inwards radially and a second cylindrical section 58 adjoining the conical section 56 in the flow direction 14 (in FIG. 3b shown by way of example for the transition region 38c). The particular transition regions 38a, 38b, 38c, 38d, 38e interconnect the channel sections 36a, 36b, 36c, 36d, 36e, 36f in terms of flow.

(30) In the configuration according to FIG. 3a to 3c, the particular transition region 38a, 38b, 38c, 38d, 38e also forms an annular screen 50, a screen surface 52 being oriented inclined to the flow direction 14 (see FIG. 3b).

(31) FIG. 4a to 4c show a further different configuration of the expansion channel 6, in which the expansion channel cross section in the particular transition region 38a, 38b, 38c, 38d, 38e, 38f is initially increased in the flow direction 14. By way of example and preferably, the particular transition region 38a, 38b, 38c, 38d, 38e, 38f is designed as an oval-like bulge (see FIG. 4b). Viewed in the flow direction 14, in the case shown by way of example, a particular extension of the transition regions 38a, 38b, 38c, 38d, 38e, 38f in the flow direction is reduced from transition region to transition region (see FIG. 4a).

(32) The channel sections 36a, 36b, 36c, 36d, 36e, 36f, 36g each have a constant cross section over their particular extension in the flow direction 14 in the configuration shown in FIG. 4a to 4b, the cross section 54d of a channel section 36d arranged downstream in the flow direction 14 being larger, however, than the cross section 54c of the channel section 36c arranged upstream of this channel section 36d in the flow direction 14 (shown in FIG. 4b by way of example for the channel sections 36c and 36c).