Aerosol-generating system using the venturi effect to deliver substrate to a heating element

Abstract

An aerosol-generating system is provided, including an air inlet and an air outlet; a liquid storage portion having a liquid outlet and being configured to hold a liquid aerosol-forming substrate; an air flow passage from the air inlet to the air outlet past the liquid outlet, wherein the air flow passage is shaped such that there is a pressure drop within the air flow passage at the liquid outlet when air flows from the air inlet to the air outlet through the air flow passage; and a heating element disposed within the air flow passage between the liquid outlet and the air outlet. There is also provide a cartridge for an aerosol-generating system and a method of generating aerosol from a liquid aerosol-forming substrate.

Claims

1. An aerosol-generating system, comprising: an air inlet and an air outlet; a liquid storage portion having a liquid outlet and being configured to hold a liquid aerosol-forming substrate; an air flow passage from the air inlet to the air outlet past the liquid outlet, wherein the air flow passage is shaped such that there is a pressure drop within the air flow passage at the liquid outlet when air flows from the air inlet to the air outlet through the air flow passage, the pressure drop creating suction at the liquid outlet that draws liquid out of the liquid outlet and into the air flow passage; and a heating element disposed within the air flow passage between the liquid outlet and the air outlet.

2. The aerosol-generating system according to claim 1, wherein the airflow passage has a restricted cross-section at the liquid outlet relative to the air inlet.

3. The aerosol-generating system according to claim 1, wherein the liquid storage portion provides a sealed enclosure for the aerosol-forming substrate such that a fluid cannot enter or exit the sealed enclosure except through the liquid outlet.

4. The aerosol-generating system according to claim 1, wherein the liquid storage portion comprises an air inlet valve configured to allow air to enter the liquid storage portion when in an open position but not when in a closed position.

5. The aerosol-generating system according to claim 4, wherein the system is configured such that the valve is controlled to be in a closed position when a predetermined flow rate of air flows through the air flow passage.

6. The aerosol-generating system according to claim 1, wherein the liquid storage portion comprises an annular housing, and wherein the air flow passage extends through the annular housing.

7. The aerosol-generating system according to claim 1, wherein the liquid outlet is annular.

8. The aerosol-generating system according to claim 1, wherein the heating element spans the air flow passage and is fluid permeable.

9. The aerosol generating system according to claim 8, wherein the heating element comprises a mesh of heater filaments, or an array of heater filaments, or a fabric of heater filaments, or a combination thereof.

10. The aerosol-generating system according to claim 1, further comprising an electrical power supply connected to the heating element, wherein the heating element is configured to be resistively heated.

11. The aerosol-generating system according to claim 1, further comprising a disposable cartridge and a device, wherein the disposable cartridge comprises the liquid storage portion.

12. The aerosol-generating system according to claim 11, wherein the disposable cartridge further comprises the heating element.

13. An aerosol-generating system, comprising: an air inlet and an air outlet; a liquid storage portion having a liquid outlet and being configured to hold a liquid aerosol-forming substrate; an air flow passage from the air inlet to the air outlet past the liquid outlet, wherein the air flow passage is shaped such that there is a pressure drop within the air flow passage at the liquid outlet when air flows from the air inlet to the air outlet through the air flow passage, the pressure drop creating suction at the liquid outlet that draws liquid out of the liquid outlet and into the air flow passage without aid of a liquid-retaining material extending beyond the liquid outlet; and a heating element disposed within the air flow passage between the liquid outlet and the air outlet.

14. The aerosol-generating system according to claim 13, wherein the airflow passage has a restricted cross-section at the liquid outlet relative to the air inlet.

15. The aerosol-generating system according to claim 13, wherein the liquid storage portion provides a sealed enclosure for the aerosol-forming substrate such that a fluid cannot enter or exit the sealed enclosure except through the liquid outlet.

16. The aerosol-generating system according to claim 13, wherein the liquid storage portion comprises an air inlet valve configured to allow air to enter the liquid storage portion when in an open position but not when in a closed position, and wherein the system is configured such that the valve is controlled to be in a closed position when a predetermined flow rate of air flows through the air flow passage.

17. The aerosol-generating system according to claim 13, wherein the liquid storage portion comprises an annular housing, wherein the air flow passage extends through the annular housing, and wherein the liquid outlet is annular.

18. The aerosol-generating system according to claim 13, wherein the heating element spans the air flow passage and is fluid permeable, and wherein the heating element comprises a mesh of heater filaments, or an array of heater filaments, or a fabric of heater filaments, or a combination thereof.

19. The aerosol-generating system according to claim 13, further comprising an electrical power supply connected to the heating element, wherein the heating element is configured to be resistively heated.

20. The aerosol-generating system according to claim 13, further comprising a disposable cartridge and a device, wherein the disposable cartridge comprises the liquid storage portion and the heating element.

Description

(1) Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic cross-section of a system in accordance with a first embodiment;

(3) FIG. 2 is a cross-section of the cartridge of FIG. 1;

(4) FIG. 3 is an exploded view of the cartridge of FIG. 2; and

(5) FIG. 4 is a lateral cross-section of the cartridge of FIG. 2.

(6) FIG. 1 is a schematic cross-section of an aerosol-generating system in accordance with an embodiment of the invention. The system shown in FIG. 1 is an electrically operated, handheld smoking system, often referred to as an e-cigarette. The system comprises a device 10 and a cartridge 20 which, together with a disposable mouthpiece 50, form the smoking system.

(7) The device comprises a housing 12 containing a battery 14, such as a lithium iron phosphate battery, control electronics 16, a cavity 15 for receiving a portion of the cartridge 20 and air inlets 18. The device has a circular cross-section and comprises a plurality of air inlets 18 disposed around the circumference of the device housing 12. The cavity 15 has a screw thread (not shown) for engaging a corresponding screw thread on the cartridge 20. However it should be clear that many other types of connection between the cartridge and the device could be used. The battery 14 and control electronics 16 provide electrical power to the cartridge through electrical connections (not shown), as will be described. Again, any type of connection can be used to provide the electrical contact between the cartridge and device, such as a snap-fit, inference fit, or bayonet type connection.

(8) The cartridge 20 is shown engaged with the device 10 in FIG. 1 but is shown separately and in more detail in FIGS. 2, 3 and 4.

(9) The cartridge 20 has an outer cartridge housing 21. A liquid storage portion 30 having a liquid storage housing 34 is provided inside the outer cartridge housing 21. The liquid storage portion housing is annular and an airflow passage 22 is formed through its centre. The airflow passage has an inlet end which has a narrowing portion 40 so that the airflow passage within the liquid storage housing is constricted relative to the airflow through the inlet end. The orifice in the inlet plate 41 has a radius of 1 mm whereas the airflow channel within the liquid storage housing has a radius of 0.375 mm.

(10) A reservoir of liquid aerosol-forming substrate is held between inner and outer walls of the liquid storage housing 34. A spigot 32 extends into the reservoir to define a constricted liquid flow path 36 for liquid from the reservoir to a liquid outlet 38 in the air flow passage. The liquid outlet 38 is substantially annular, as best seen in FIG. 4. The inner diameter of the liquid outlet at the base of the liquid storage portion is around 1.5 mm, the outer diameter of the liquid outlet is around 1.75 mm. Small slots or openings (not shown) are provided in the base of the reservoir to properly locate the spigot and ensure that the spigot is centred relative to the reservoir.

(11) The air flow passage immediately downstream of the liquid outlet is defined by the spigot, and widens in a divergent portion 42.

(12) A heating element 26 is supported on the spigot 32 downstream of the liquid outlet. The heating element is a mesh formed from 304L stainless steel, with a mesh size of about 400 Mesh US (about 400 filaments per inch). The filaments of the mesh have a diameter of around 16 μm. The mesh is connected to electrical contacts 46 that are formed from copper. The electrical contacts 32 are provided on a polyimide substrate 44. The filaments forming the mesh define interstices between the filaments. The interstices in this example have a width of around 37 μm, although larger or smaller interstices may be used. The open area of the mesh, i.e. the ratio of the area of interstices to the total area of the mesh is advantageously between 25 and 56%. The total resistance of the heater assembly is around 1 Ohm. The mesh provides the vast majority of this resistance so that the majority of the heat is produced by the mesh. In this example the mesh has an electrical resistance more than 100 times higher than the electrical contacts 46.

(13) An aerosol-forming chamber 28 is provided downstream of the heater. The aerosol-forming chamber 28 is a region in which vapour from the heater can cool and condense to form aerosol before exiting the air outlet 24 into a user's mouth.

(14) As most clearly seen in FIG. 3, the outer cartridge housing is formed in two parts to permit assembly. Lower cartridge housing 21a supports the liquid storage portion, spigot and heater assembly. Upper cartridge housing 21b defines a mouthpiece portion of the cartridge and holds the aerosol-forming chamber and the air outlet 24. The disposable mouthpiece 50 is positioned around the upper cartridge housing, as shown in FIG. 1. The upper and lower cartridge housings are secured to each another by a pair of threaded bolts 23 and corresponding nuts (not shown).

(15) The cartridge housing and the device housing may comprise any suitable material or combination of materials. In this example, polypropylene, polyetheretherketone (PEEK) is used.

(16) The removable mouthpiece 50 may mimic the filter of a conventional cigarette in look and feel. For example, the removable mouthpiece 50 may be formed from cellulose acetate, rubber, or plastic, such as polyethylene or polypropylene or a mixture of both, and may be covered with a paper layer.

(17) In operation, when a user puffs on the mouthpiece portion, air is drawn through the airflow passage from the air inlets 18 to the air outlet 24. The air is drawn through the airflow passage past the liquid outlet, through the heater to the aerosol-forming chamber. The pressure of the airflow at the liquid outlet is lower than atmospheric pressure and crucially lower than the pressure of the air 35 within the liquid storage portion. This pressure difference causes liquid aerosol-forming substrate to be drawn out of the liquid outlet into the airflow passage.

(18) The approximate volume of liquid drawn into the airflow from the liquid outlet can be calculated using Poiseuille's equation. For the sake of calculation, consider a 1 second puff of 40 ml.

(19) The velocity of the air travelling through the unconstructed section and the constricted section of the airflow channel is calculated as follows:
Velocity.sub.(2):(40 mm.sup.3*10.sup.3)/[3.14*(1 mm).sup.2]=12.7*10.sup.3 mm/s
Velocity.sub.(1):(40 mm.sup.3*10.sup.3)/[3.14*(0.375 mm).sup.2]=90*10.sup.3 mm/s

(20) From these velocities a pressure differential can be calculated:
P.sub.2−P.sub.1=ρ/2(v.sub.2.sup.2−v.sub.1.sup.2)=½(90.sup.2−12.7.sup.2)=approx. 4000 kg mm.sup.−1 s.sup.−2

(21) In this example, the liquid droplets originate from an annular tube with width 0.25 mm, whose “area equivalence” for the cartridge we can be estimated by multiplying the ring's circumference (2*π*r=2*π*1 mm) by its width of 0.25 mm:
Approximate Area(of liquid delivery)=2πr*width=2*3.14*1*0.25 mm=1.57 mm.sup.2

(22) Thus the approximate “radial equivalence” for the sake of estimating liquid delivery is:
(1.57/π).sup.0.5=0.75 mm

(23) The liquid viscosity for the liquid aerosol-forming substrate can be estimated from the liquid's composition, in this example: PG (52%), Glycerin (20%), Water (15%) and nicotine (5%)
Approximately:0.6*52+0.2*1.4+0.15*1.0022+0.05*1.004=32 Pa s

(24) Finally, the liquid delivery can be calculated using the Pressure Differential (ΔP), the radial equivalence (r), the liquid viscosity (μ) and the length of the liquid flow path (L):
Q=(ΔP*πr.sup.4)/(8 μL)=[4000*π*(0.75*10.sup.−3).sup.4]/(8*32*14)=between 0.8 mm.sup.3 s.sup.−1 and 1 mm.sup.3 s.sup.−1.

(25) It can be seen that key parameters for determining the volume of droplet delivery are the surface area of the liquid outlet in the cartridge the pressure drop at the liquid outlet and the length of the liquid flow path. The liquid flow path is to some extent limited by the overall length of the cartridge and for a handheld system would desirably be between 10 mm and 30 mm. It can also be seen from this equation that the viscosity of the liquid aerosol-forming substrate is also a significant factor, such that if the liquid viscosity increases then to achieve the same liquid delivery the dimensions of the liquid outlet and/or liquid path would need to be altered.

(26) The liquid aerosol-forming substrate in the airflow is conveyed to the mesh heating element. The mesh heating element, which may be activated in response to a sensed user puff, vapourises the liquid aerosol-forming substrate as it contacts or passes through the heating element. The vapourised substrate and heated air then passes into the aerosol-forming chamber where it cools to form an aerosol. The aerosol is then drawn out of the air outlet 24 and into the user's mouth.

(27) As a user puffs on the system and liquid is drawn out of the liquid storage portion, the pressure inside the liquid storage portion drops. In order to provide consistent liquid delivery from puff to puff, the pressure inside the liquid storage portion is advantageously allowed to return to its initial pressure, typically atmospheric pressure, between puffs. The liquid outlet may be large enough to allow air bubbles to enter the liquid storage portion through the liquid outlet between puffs. Alternatively, a pressure relief valve 48 may be included in the liquid storage portion that opens when the pressure difference between the inside of the liquid storage portion and outside the liquid storage portion exceeds a threshold pressure difference. This is illustrated in FIGS. 2 and 3. The pressure relief valve 48 may be controlled to remain closed during each user puff.

(28) A system as described has several advantages over prior systems. It is a mechanically robust system that does not require winding a heater around a flexible wicking material. It eliminates the potential for burning or charring of a capillary material in contact with a heating element. It eliminates the need for a liquid retaining material and so reduces manufacturing costs and a manufacturing step. It also eliminates a potential aerosol fading issue found with capillary based systems, whereas the liquid is depleted the amount of liquid delivered to the heating element is reduced, similar to the fading of a felt-tipped pen.

(29) When compared to piezoelectric based delivery systems, it is energy efficient, as it uses the depressurization occurring during puffing to deliver the liquid droplets to the heater. It also allows the user's puffing behaviour to control the amount of liquid delivered rather than the amount of liquid being metered by a piezoelectric valve.

(30) The exemplary embodiments described above illustrate but are not limiting. In view of the above discussed exemplary embodiments, other embodiments consistent with the above exemplary embodiments will now be apparent to one of ordinary skill in the art. For example, the embodiment described is an electrically operated smoking system, but the invention may be applied to any type of aerosol-generating system, and different liquid and airflow geometries may be used.