Controlled Bubble Nucleation

Abstract

A heater with intentional surface aberrations spaced apart a predetermined distance allows for an efficient heating process by encouraging bubble nucleation at the sites of the aberrations. The spacing between aberrations creates a soft restriction to the size of the bubbles, which encourages a maximum bubble size. This reduces the likelihood of the creation of large bubbles that may inhibit heat transfer to the liquid. In embodiments where the liquid is a thin film over the heater, such as in ENDS applications, this structure can be used to skew droplet production to certain ranges of droplet sizes based on controlling the size of the bubbles.

Claims

1. A heater for heating a liquid having a set of physical characteristics including a viscosity and a boiling point, the heater comprising: a heating surface for transferring heat into the liquid; and first and second aberrations on the heating surface spaced apart by a distance determined in accordance with at least one of the set of physical characteristics and a heating objective.

2. The heater of claim 1 wherein the heating surface is planar.

3. The heater of claim 2 wherein the first and second aberrations are contained within a set of aberrations.

4. The heater of claim 3 wherein aberrations within the set of aberrations are spaced apart from each other by at least the determined distance.

5. The heater of claim 3 wherein the location of each aberration within the set of aberrations corresponds to the center of a circle having a radius of half the determined distance, and having a distance to the nearest next aberration of the determined distance.

6. The heater of claim 3 wherein the location of each aberration within the set of aberrations corresponds to the centre of a hexagon, and each hexagon corresponding to an aberration within the set of aberrations is tiled across the heating surface.

7. The heater of claim 1 wherein the heater is an element of a boiler.

8. The heater of claim 1 wherein at least one of the first and second aberrations is machined into the heating surface.

9. The heater of claim 1 wherein at least one of the first and second aberrations is created on the heating surface through at least one of: perforating the heating surface; stamping the heating surface; etching the heating surface; forging the heating surface with at least one aberration; and additively manufacturing the at least one aberration.

10. The heater of claim 1 wherein the heater comprises at least one of the following materials: aluminium; copper; steel; titanium; and nickel.

11. The heater of claim 1 wherein the heater is a linear heating element.

12. The heater of claim 11 wherein the heater is wound around a wick.

13. The heater of claim 12 wherein the wick comprises at least one of cotton, linen, nylon, other polymer based materials, hemp, wool, cellulose, glass fibers and carbon fibers.

14. The heater of claim 11 further comprising a third aberration, wherein the first and second aberrations are spaced apart by the determined distance, and the second and third aberrations are spaced apart by the determined distance.

15. The heater of claim 14 wherein the first, second and third aberrations are spaced apart to form a line.

16. The heater of claim 11 wherein the liquid is an e-liquid comprising at least one of vegetable glycerine, propylene glycol, nicotine and a flavoring.

17. The heater of claim 11 wherein the liquid is an e-liquid comprising a cannabinoid.

18. The heater of claim 11 wherein the heating objective is to constrain production of droplets over a threshold diameter to below a defined quantity.

19. The heater of claim 18 wherein the threshold diameter is 10 microns.

20. A pod for storing an atomizable e-liquid having a set of physical characteristics, the pod for use in a vaping device, having an airflow passage therethrough, the pod comprising: a reservoir for storing the atomizable e-liquid; a wick, in fluid communication with the reservoir for drawing the atomizable e-liquid across the length of the wick through capillary action; and a heater in contact with the wick, and aligned with the airflow passage, the heater further comprising: a heating surface for transferring heat into the atomizable e-liquid to atomize the e-liquid drawn across the wick; and first and second aberrations on the heating surface spaced apart by a distance determined in accordance with at least one of the set of physical characteristics and a heating objective.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the present invention will now be described in further detail by way of example only with reference to the accompanying figure in which:

[0032] FIG. 1A is a front view of a prior art pod for use in an electronic nicotine delivery system;

[0033] FIG. 1B is a side view of the pod of FIG. 1A;

[0034] FIG. 1C is a bottom view of the pod of FIG. 1A;

[0035] FIG. 2 is a cross section of the pod of FIGS. 1A and 1B along cut line A in FIG. 1B;

[0036] FIG. 3 is a cross section of an alternate pod design;

[0037] FIG. 4 is a side view of a conventional horizontal heater and wick assembly for use in an ENDS;

[0038] FIG. 5A is a side view of a linear heater according to an embodiment of the present invention;

[0039] FIG. 5B is a side view of the linear heater of FIG. 5A illustrating bubble creation;

[0040] FIG. 6 is a perspective view of a planar heater according to an embodiment of the present invention for use in a boiler-style heater;

[0041] FIG. 7 illustrates an example of the configuration of a planar heater such as the one illustrated in FIG. 6; and

[0042] FIG. 8 is a side view of a linear heater according to embodiments of the present invention as applied to a linear heater within a heater and wick assembly.

[0043] In the above described figures like elements have been described with like numbers where possible.

DETAILED DESCRIPTION

[0044] In the instant description, and in the accompanying figures, reference to dimensions may be made. These dimensions are provided for the enablement of a single embodiment and should not be considered to be limiting or essential. Disclosure of numerical range should be understood to not be a reference to an absolute value unless otherwise indicated. Use of the terms about or substantively with regard to a number should be understood to be indicative of an acceptable variation of up to ±10% unless otherwise noted.

[0045] In the above-described prior art, heating a liquid to a boiling point is done through the application of heat to the liquid, typically by passing an electric current through a heating element to generate heat. Liquids will convert to a gas phase when heated past their boiling point. Typically, a liquid will somewhat uniformly increase in temperature and driving the heater to a higher temperature will result in a greater overall temperature (a higher average temperature), but the temperature of the liquid will not exceed the boiling point until (almost) all the liquid has approached the boiling point. This can be observed in a kettle, where the application of more heat will result in the liquid approaching the boiling point faster, but will not cause the temperature of the liquid to exceed the boiling point. However, on a more local scale, boiling of the liquid nearest the heater may occur without bringing the temperature of all the liquid to the boiling point. Because the generation of vapor bubbles is part of the heat transfer process, the size of the generated bubbles can have an effect on the efficiency of the heat transfer process.

[0046] As a bubble forms and expands, the heater can increase the temperature of the vapor above the boiling point of the liquid, which causes the size of the bubble to increase. This may not have an effect on the temperature of the liquid outside the bubble. Accordingly, when trying to improve the efficiency of heat transfer into a liquid, there may be an optimal size of bubble. Bubbles smaller than this size may not transfer sufficient heat to the rest of the liquid as they travel through the liquid (possibly a function of a bubble with an insufficiently small surface area), and bubbles above this size may not have the efficiency of a set of smaller bubbles at absorbing heat and creating vapor from the liquid. Furthermore, in thin-film heating applications where the liquid being heated takes the form of a thin film across the heater, the size of the bubble generated has a relationship to the size of the droplets created during the rupturing of the bubbles. In some applications, this may result in a messy boiling process, and in others, such as those related to ENDS it may result in the generation of sub-optimal droplet sizes.

[0047] FIG. 5A illustrates a linear heating element 100 according to one embodiment of the present invention. Where in prior art implementations, surface defects within a heater may create irregularities, in the illustrated embodiments intentional aberrations 102 are formed within the surface of heater 100. In some embodiments the aberrations 102 are less than 1 mm in size. In the illustrated embodiment, the aberrations 102 form peaks, and create a crevice 106 with the rest of the surface of heater 100. In some embodiments, this can be formed by cutting or slicing into the heater 100. The height and the angle at which the aberration 102 leaves from the heater 100 can vary between embodiments. As shown on heater 100, there are a plurality of aberrations, each one spaced apart from a previous aberration 102 by a distance 104. In some embodiments the distance 104 between aberrations 102 may be between 60 microns and 2 mm. It should be understood that the distance 104 acts as a regulator on the maximum size of the bubble produced, and the size of the bubble (in conjunction with the properties of the liquid being heated) will define the expected distribution of droplet sizes created from the rupturing of the bubble. The distance 104, the height of the aberration 102 and its angle of departure from the heater 100 may all vary in accordance with properties of the liquid to be heated, including a viscosity, the specific heat capacity of the liquid, the desired maximum size of bubble and other such properties.

[0048] As shown in FIG. 5B, as heat is applied through heater 100, a bubble 108 will form around aberration 102. This may be related to the crevice 106 providing heating on multiple sides, or it may be related to other requirements for nucleation. As two adjacent bubbles 108 form, spaced apart by distance 104, they will come into contact with each other when the distance 110 between their centers is approximately the inter-aberration distance 104. It should be apparent from FIG. 5B, that the bubbles will each have a radius that is approximately half of the distance 104.

[0049] As bubbles 108 come into contact with each other, they provide an impediment to continued growth. This will encourage either the departure of the bubbles 108 from the surface of the heater, or it will encourage the rupturing of the bubbles 108. The difference between these two situations is largely associated with whether the liquid is a thin film or a boiler-like setup. Those skilled in the art will appreciate that the distance 104 between aberrations 102 may be selected in accordance with a desired bubble size. This bubble size may be associated with physical characteristics of a liquid being heated, such as the viscosity of the liquid, the boiling point of the liquid, the specific heat capacity of the liquid, and other such physical characteristics. The heat to which the heater 100 is heated may also be taken into account in determining the bubble size, as may be the rate at which the heater 100 itself is heated. A heating objective may also be considered. In some embodiments, the heating objective may be to minimize the time or power required to heat the liquid to a specific temperature or state. In other embodiments, the manner in which a bubble ruptures may be taken into account when setting a heating objective. If different sizes of bubbles create different distributions of droplet sizes when they rupture, a desired droplet size, or a desired profile of droplet sizes may define the heating objective. In one example, in an ENDS system, droplets of a diameter in excess of 10 microns may be associated with spitback, so a heating objective may be to create a bubble that ruptures to minimize production of droplets in excess of 10 microns. It should be understood that different liquids may require different bubble sizes to satisfy the same final objective, so the heating objective may be associated with physical properties of the liquid. It should also be understood that the above description of a 10 micron spitback threshold may be based on properties of a specific e-liquid, and for other e-liquids a different spitback threshold may be required.

[0050] FIG. 6 illustrates a boiler 120 making use of a heater 122 to heat a liquid within walls 126. To allow for the improved heat transfer offered by a heater having controlled aberrations, the planar heater 122 has a series of aberrations 124, similar to aberrations 102 on linear heater 100. Aberrations 124 are spaced apart from each other so that, similarly to the spacings 104 on linear heater 100, they are approximately equidistant to each neighbor. Those skilled in the art will appreciate that any one of a number of different arrangements of aberrations 124 on the heater surface 122 can be used, and that maintaining equidistance between closest neighbors is neither necessary nor essential in each embodiment.

[0051] In operation, liquid is contained within walls 126 of the boiler 120, and heat is applied through the heater 122. As the liquid heats, aberrations 124 serve as nucleation points for the bubble to form. The bubbles, once formed, will grow outwards from the aberrations. As the bubbles encounter each other, each bubble will restrain the growth of the adjacent bubbles. If there is sufficient liquid within boiler 120 to ensure that the bubbles are fully submerged when they reach this size, they will separate from the heater and rise through the liquid. As noted earlier, heat transfer can occur through having these bubbles break from the heater 122, and rise through the liquid. By placing the aberrations in specific locations, the size of these bubbles can be largely controlled. It should be understood that due to a number of different factors, the nucleation process may not be completely regular, and as such, not every aberration will serve as a nucleation point starting at the same time. As such, some bubbles may form with a larger or a smaller radius, but the differences in these bubble sizes will be bounded by the location of other aberration locations.

[0052] FIG. 7 will be used in an explanation of how aberrations can be located on the surface of a heater to control bubble size. As will be apparent, it may not be feasible to arrange each aberration to be the nucleation point of a spherical bubble equidistant to the nucleation point of all neighboring bubbles. This would require the effective tiling of circles on a flat plane. Although it is possible to lay the circles beside each other with no overlap, there will be regions outside these circles. To maximize the placement of the aberrations, it may be preferred to replace the circle associated with the radius of a bubble, with a corresponding hexagon as shown in FIG. 7. In one embodiment, each of the hexagons 122, 124 could have at its center an aberration to allow for the creation of a simple tiling that allows for bubbles of a defined size. In another embodiment, the heater 122 has aberrations only within hexagons labelled 124. Each of these hexagons 124 is surrounded by a heater hexagon 122 that has no aberration. This grouping of 7 hexagons is then tiled. This is a geometric arrangement used in a number of other fields, including in the determination of the placement of base stations in mobile networks, to create roughly circular cells. Those skilled in the art will appreciate that the particular tiling geometry used can be varied, with the understanding that the size of the bubbles created will largely depend on the distance between the heater cells 124 that have an aberration.

[0053] By heating heater 122 with a given power, and thus given temperature, it is possible to encourage nucleation to occur at defined points in the surface of heater 122 corresponding to the location of a surface aberration 124. This allows for a heating surface with regularly spaced aberrations designed to promote nucleation. The aberrations can take a number of different forms, just as they can be tiled in a number of different ways. In the illustrated embodiments, the aberrations can be formed by cutting or slicing into the heater creating a crevice that allows for heating of the liquid from more than one surface at a given time. In other embodiments, the aberrations can be perforations in the surface of the heater, while in other various machining techniques can be used to create the aberrations. In some embodiments, chemical or laser etching can be applied to the surface of the heater to create aberrations that can be used to encourage nucleation. In some embodiments, a roughened patterning can be applied to a smooth heating surface to create regions that act as surface aberrations that act as nucleation sites. In such an embodiment, it may be desirable to restrict the size of the roughened region so that there is greater consistency in the sizing of the bubbles that are created. In other embodiments, a heater can be formed with the aberrations present, such as through a forging or stamping process. In further embodiments the aberrations can be formed through techniques such as additive manufacturing where the aberrations are built in layers upon the heating surface.

[0054] Although the above discussion has largely centered on a heater having a substantially planar heating surface, these teachings can also be applied to linear heaters, and to heaters designed to heat thin films of liquids, not just pools of liquid.

[0055] FIG. 8 illustrates the application of the above described inventive concepts to a heater for use in conjunction with a wicking substrate in an ENDS or other vaporizer system. A wick 150 is in contact with a heater 152 and draws e-liquid 154 from a reservoir across the wick 150 so that it is in contact with heater 152. Heater 152 may be referred to as a linear heater as its width is relatively minor in comparison to its length. In some embodiments, a linear heater may be a wire, whose length dominates its width or diameter. The heater 152 may be a metal wire, such as a copper wire, an aluminium wire, a steel wire, a titanium wire, a nickel wire or wire of other such materials. As noted in earlier discussions, this creates a surface film of e-liquid 154 that covers heater 152. When power is applied across heater 152, it increases in temperature to cause the volatilization of the thin film of e-liquid 154 atop the heater 152. The e-liquid lost to the volatilization process is replaced by wick 150 typically through capillary action.

[0056] As shown in the magnification within FIG. 8, heater 152 has regularly spaced aberrations 156 that create a crevice 158 between the aberration 156 and the heater 152. As heat is applied, the e-liquid within crevice 158 will be heated by both heater 152 and the aberration 156. This may increase the likelihood of bubble nucleation at this location. Those skilled in the art will appreciate that this will increase the likelihood of bubbles forming at the interface between each of the aberrations 156 with the heater 152. The regular spacing of the aberrations will result in bubbles forming much as shown in FIG. 5B. The spacing of the aberrations 156 result in bubbles that will rupture after contacting each other as the formed bubbles will generally be larger in diameter than the depth of the e-liquid film. However, one skilled in the art will note this depends on capillary action replenishing the surface, viscosity, and surface tension of the liquid to be heated.

[0057] Those skilled in the art will appreciate that by controlling the size of the bubbles within a vaporizer system, the size of droplets created by the rupturing of the bubbles can be constrained. These droplets are entrained within the airflow for delivery to the user. By controlling the size of droplets formed, effects such as spitback can be reduced, if not avoided, not by filtering out droplets of a given size, but instead by preventing the creation of many of these droplets. As noted earlier, there are many techniques that can be used for spitback mitigation and often they are used in conjunction with each other. By applying a mechanism to reduce the creation of spitback sized droplets, other known spitback mitigation techniques can be used if there is concern that in given environments spitback sized droplets may still be generated.

[0058] Those skilled in the art will appreciate that the above teaching can be applied in a variety of different styles of vaporizer devices including those with either horizontal or vertical wicks, and those that use conventional reservoirs or those that make use of reservoirs that are filled with a so-called cartomizer matrix used to hold the e-liquid in a sponge like element. Although discussed in the context of a nicotine based e-liquid, it should be understood that e-liquids used to deliver cannabinoids like one or both of Cannabidiol (CBD) and tetrahydrocannabinol (THC) also exhibit this tendency. It should be understood that there is often a wide variation in the thickness or viscosity of e-liquids held within a cartomizer based ENDS pod and the e-liquids used to deliver cannabinoids. Different e-liquids with different compositions may have different spitback thresholds. Different e-liquid compositions thus may require different spacings between the aberrations in the surface of the heater. Similarly, a change in the design of a vaping pod may also necessitate a change in the inter-aberration spacing. It should also be understood that in addition to the location of the aberrations on the heater, the size and shape of the aberrations may also require modification with changes in the design of the pod or the properties of the liquids in question. It should also be understood that the temperature to which the heater is heated may also be an input factor in selecting the size and shape of the aberrations.

[0059] In the ENDS embodiments discussed above, the e-liquid is stored in a replaceable pod that stores the e-liquid and draws the e-liquid from a reservoir into a wick, and then through the wick to a heater. It should be understood that in some embodiments, the wick may be vertically oriented, that is parallel (and possibly co-axial) with the direction of airflow through the pod. In other embodiments the reservoir may contain a cartomizer matrix that holds the e-liquid within the pod for delivery to the wick. In further embodiments, the reservoir storing the e-liquid, the heater and the wick may be integrally formed with the device. In such embodiments, the reservoir may be sealed so that the vaping device is a one-time use device. In other embodiments the reservoir may provide a port through which a user can refill the reservoir, and where at least one of the wick and heater may be replaceable by a user.

[0060] In the instant description, and in the accompanying figures, reference to dimensions may be made. These dimensions are provided for the enablement of a single embodiment and should not be considered to be limiting or essential. The sizes and dimensions provided in the drawings are provided for exemplary purposes and should not be considered limiting of the scope of the invention, which is defined solely in the claims.