Patent Description:
In many known aerosol-generating systems, a liquid aerosol-forming substrate is heated and vaporised to form a vapour. The vapour cools and condenses to form an aerosol. In some aerosol-generating systems, such as electrically heated smoking systems, this aerosol is then inhaled by a user.

Typically, the liquid aerosol-forming substrate comprises several compounds which are vaporised when heated. These compounds may have different boiling points. For example, a liquid aerosol-forming substrate may comprise nicotine (with a boiling point of around <NUM> degrees Celsius at atmospheric pressure) and glycerol (with a boiling point of around <NUM> degrees Celsius at atmospheric pressure).

When a liquid aerosol-forming substrate with compounds having different boiling points is heated, compounds with lower boiling points may be vaporised before compounds with higher boiling points. Alternatively, or in addition, compounds with lower boiling points may be vaporised at a higher rate than compounds with higher boiling points.

This may be undesirable because interactions and combinations between different compounds may be limited. For example, a liquid aerosol-forming substrate may comprise a nicotine compound and an organic acid compound, these compounds having different boiling points. Both of these compounds may be vaporised. The nicotine in the liquid aerosol-forming substrate may form free base nicotine when it is vaporised. However, it may be desirable to generate an aerosol with nicotine salt rather than free base nicotine. In order to form this nicotine salt, the free base nicotine may be protonated by the vaporised organic acid. However, this protonation may be limited if the organic acid has not yet been vaporised until after nicotine has been vaporised, or is vaporised more slowly than is required to protonate a suitable proportion of the free base nicotine.

Further, vaporising some compounds of an aerosol-forming substrate more quickly than others may undesirably cause the properties of the aerosol generated to change over time, for example over the course of a puff on an aerosol-generating system. This may be because, towards the beginning of a puff, when a heating element is activated and rises in temperature, liquid aerosol-forming substrate close to the heating element may reach a first temperature at which a first compound with a lower boiling point is vaporised but a second compound with a higher boiling point is not vaporised. Then, later in the puff, liquid aerosol-forming substrate close to the heating element may reach a second temperature at which the second compound with the higher boiling point is vaporised. However, by this time, much of the first compound in the liquid aerosol-forming substrate close to the heating element may have already been vaporised. Thus, towards the start of a puff, the aerosol generated may comprise a larger proportion of the first compound and, later in the puff, the aerosol generated may comprise a larger proportion of the second compound.

Alternatively, or in addition, the properties of the aerosol generated may change over the course of several puffs. This may occur where compounds of the liquid aerosol-forming substrate are not vaporised at an appropriate rate. For example, a liquid aerosol-forming substrate may comprise X percent by mass of a first compound and Y percent by mass of a second compound. If the liquid aerosol-forming substrate is not vaporised to produce a vapour comprising a mass ratio of the first compound to the second compound of X to Y, then the composition of the liquid aerosol-forming substrate may change as vapour is generated. This may, in turn, lead to a change in the properties of the aerosol generated by the liquid aerosol-forming substrate.

<CIT> describes an atomizer having a combined wick and heater in which the wick can have a tapered end that engages the interior of a substantially basketshaped wire heater coil.

It is an aim of the invention to control the vaporisation of various compounds of a liquid aerosol-forming substrate, where these compounds have different boiling points.

The invention is defined in the appended independent claims; preferred embodiments are defined in the dependent claims. Different aspects of the present disclosure are further discussed; these aspects are not necessarily covered by the claims.

According to the present disclosure there is provided, a heater assembly for use in an aerosol-generating system. The heater assembly may comprise a retention material. The retention material may contain an aerosol-forming substrate. The retention material may contain an aerosol-forming substrate in condensed form. The aerosol-forming substrate may comprise a first compound and a second compound. The second compound may have a higher boiling point than the first compound. The heater assembly may comprise at least one airflow path defined through the retention material. The heater assembly may comprise at least one heating element. The at least one heating element may be shaped to define an interior volume. The interior volume may be filled with the retention material. The interior volume may have a cross-sectional area that decreases along a longitudinal axis. The at least one airflow path may pass through a first central region of the interior volume. The at least one airflow path may pass through a second central region of the interior volume. The first and second central regions may be spaced-apart along the longitudinal axis.

When used in an aerosol-generating system, the heater assembly may be configured to generate a vapour or aerosol by heating the aerosol-forming substrate. In particular, the heater assembly may be connectable to a power supply of the aerosol-generating system such that power may be supplied to the heating element. In some embodiments, heat may be generated by the heating element restively or inductively. The heat may be transferred to the retention material filling the interior volume by conduction. Alternatively or additionally, the heating element may generate an alternating magnetic field and the retention material may comprise or consist of a susceptor element and the susceptor of the retention material may be inductively heated by the heating element. The heating element may then be a coil. The heating element may be an inductor coil. In any case, the retention material may be heated. Therefore, the aerosol-forming substrate contained by the retention material may also be heated. This may vaporise the aerosol-forming substrate contained by the retention material. Vapour generated at both the first and second central regions of the retention material may enter the at least one airflow path. This vapour may cool to form an aerosol which may be inhaled by a user through a mouthpiece of the aerosol-generating system.

The heater assembly, and particularly the geometry of the interior volume defined by the heating element of the heater assembly, may provide areas of higher temperature and areas of lower temperature in the retention material. The first central region of the interior volume of retention material may be heated to a different temperature than the second central region. For example, the first central region may be heated to a lower temperature than the second central region.

Alternatively, or in addition, the heater assembly may provide areas which increase in temperature at a greater rate, and areas which increase in temperature at a lesser rate, in the liquid aerosol-forming substrate storage component.

Advantageously, the heater assembly may improve control of the vaporisation of the different compounds of the liquid aerosol-forming substrate. The heater assembly may result in the first and second compound of the aerosol-forming substrate being vaporised simultaneously at desirable rates. The first compound may be predominantly vaporised at one of the first or second central regions. The second compound may be predominantly vaporised at the other of the first or second central regions. The heater assembly may result in the first and second compounds of the liquid aerosol-forming substrate being vaporised in more preferable proportions. The heater assembly may provide generation of an aerosol with a more desirable composition. The heater assembly may provide more consistent generation of an aerosol with desirable properties.

The difference in temperature of the first and second central regions of the interior volume may arise because of the of the decreasing cross-sectional area of the interior volume defined by the heating element. As the first and second central regions are spaced-apart along the longitudinal axis, the cross-sectional area of the volume of retention material at the first central region may be different to the cross-sectional area of the volume of retention material at the second central region. Therefore, where the cross-sectional area of the interior volume of retention material is smaller, the heating element may be closer to a central region of the retention material. For example, if the cross-sectional area of the volume of the retention material at the first central region is smaller than the cross-section area of the volume at the second central region, the heating element may be closer to the first central region than the second central region.

Heat may be transferred from the heating element to the retention element by conduction. The temperature of the first and second central regions may depend on the shortest distance between the heating element and the respective central region of the retention material. The temperature of the respective central region of the retention material may increase as the shortest distance to the heating element decreases along the longitudinal axis. The temperature difference may be particularly pronounced when the heater assembly is not in thermal equilibrium, for example, following initial activation of the heating element at the beginning of a puff. This is because there may be a lag or delay between initial activation of the heating element to heat the retention material and the central regions of the retention element reaching maximum temperature. The extent of the lag or delay may depend on the thermal conductivity of the retention material. The greater the distance between the heating element and the respective central region, the longer the lag or delay to reach a maximum temperature.

Alternatively, the heating element may be configured to generate an alternating magnetic field and the retention material may comprise or consist of a susceptor. The temperature of the first and second central region may depend on the density or strength of the alternating magnetic field. The magnetic flux density or the magnetic field strength of the alternating magnetic field may increase as the cross-sectional area of the interior volume decreases. The magnetic flux density or the magnetic field strength of the alternating magnetic field may increase the closer the heating element is to the respective central region of the retention material.

The vaporised compounds of the aerosol-forming substrate may enter the at least one airflow path passing the first and second central region. This may advantageously ensure that the vaporised compounds pass directly into the airflow path rather than passing through other regions of the retention element that may be at different temperatures. The vaporised compounds in the at least one airflow path may mix or otherwise combine and form an aerosol.

The heating element may be in contact with retention material. The retention material may have a fibrous or spongy structure. The retention material may comprise a capillary material. The retention material may comprise a bundle of capillaries. For example, the retention material may comprise one or more of fibres, threads, and fine bore tubes.

The retention material may comprise sponge-like or foam-like material. The structure of the retention material may form a plurality of small bores or tubes, through which the liquid can be transported by capillary action.

The retention material may comprise any suitable material or combination of materials. Suitable materials include but are not limited to: a sponge or foam material, ceramic- or graphite-based materials in the form of fibres or sintered powders, foamed metal or plastics material, a fibrous material, for example made of spun or extruded fibres, such as cellulose acetate, polyester, or bonded polyolefin, polyethylene, terylene or polypropylene fibres, nylon fibres or ceramic. Preferably, the retention material may comprise a ceramic. The retention material may have any suitable capillarity and porosity so as to be used with different liquid aerosol-forming substrates having different physical properties.

The heating element may have a spiral shape. The longitudinal axis may be defined along a central axis of the spiral shape. The cross-sectional area of the interior volume defined by the spiral shaped heating element may decrease from a first end of the interior volume to a second end of the interior volume. The first central region of the retention material may be located towards the first end of the interior volume. The second central region of the retention material may be located towards the second end of the interior volume. Therefore, in operation, the temperature of the first central region may be lower than the temperature of the second central region. This may have the advantages described above.

In a preferred embodiment, the spiral shape of the heating element is a truncated spiral. A radius of curvature of the heating element may decrease along the longitudinal axis in the same direction that the cross-sectional area of the interior volume decreases.

The heater assembly may comprise a first heating element and a second heating element. The interior volume of the retention material may be defined between the first heating element and the second heating element.

A separation between the first heating element and the second heating element may decrease along the longitudinal axis in the same direction that the cross-sectional area of the interior volume decreases. Each of the first and second heating elements may comprise a series of connected surfaces. The surfaces may in turn be substantially parallel to the longitudinal axis and substantially perpendicular to the longitudinal axis. The first and second heating elements may be arranged such that the separation between the substantially parallel surfaces of the first and second heating elements decreases along the longitudinal axis. Adjacent surfaces may form the steps. In other words, a step may be formed by a surface that is substantially parallel to the longitudinal axis and a surface that is substantially perpendicular to the longitudinal axis. The first and second heating elements may each comprise a first step and a second step. The separation between the heating elements at the first step may be greater than at the second step. The first central portion of the retention material may be located between the first step of the first and second heating elements. The second central portion of the retention material may be located between the second step of the first and second heating elements. The first and second heating elements may comprise further steps, for example, the first and second heating elements may comprise a third step.

A minimum distance from the first central region to the heating element may be greater than a minimum distance from the second central region to the heating element.

The boiling point of the first compound may be between <NUM> and <NUM> degrees. The boiling point of the first compound may be about <NUM> degrees Celsius. The first compound may be nicotine.

The boiling point of the second compound may be between <NUM> and <NUM> degrees. The boiling point of the second component may be about <NUM> degrees Celsius. The second compound may be glycerol.

The interior volume may comprise a third central region spaced apart from the first and second central regions along the longitudinal axis. The third central portion may be located on an opposite side of the first central region compared to the second central region. In operation, the third central region may reach a lower temperature than the temperature of the first central portion and second central portion. The minimum distance from the third central region to the heating element may be greater than a minimum distance from the first central region to the heating element and greater than a minimum distance from the second central region to the heating element. It has already been described how providing regions of the interior volume having different temperature may improve the control of the vaporisation of various compounds of a liquid aerosol-forming substrate, where these compounds have different boiling points. By providing a third central region having an even lower temperature that the first region, the control of the vaporisation may advantageously be further improved.

Furthermore, the aerosol-forming substrate may comprise a third compound. The boiling point of the third component may be lower than the boiling point of the first compound. The boiling point of the third compound may be lower than the boiling point of second component. The boiling point of the third compound may be between <NUM> and <NUM> degrees Celsius, preferably between <NUM> and <NUM> degrees Celsius. Preferably, the boiling point of the third component may be <NUM> degrees Celsius. The third compound may be propylene glycol. At the low temperature of the third central region, the third component may advantageously be vaporised predominantly. By providing such a third central region, simultaneous vaporisation of each of the first, second and third compounds may be ensured.

The at least one airflow path may pass through a third central region of the interior volume, spaced-apart from the first central region and the second central region along the longitudinal axis. When the aerosol-forming substrate comprises a third compound, this may ensure that the vapour of all three compounds enters the at least one airflow path to be inhaled by a user.

The heater assembly may use a resistive heating arrangement. The at least one heating element may be a resistive heating element. The heater assembly, and particularly the heating element, may be configured to be electrically connected or connectable to a supply of electrical current. The heating element may comprise or be formed from any material with suitable electrical and mechanical properties. Suitable materials include but are not limited to: semiconductors such as doped ceramics, electrically "conductive" ceramics (such as, for example, molybdenum disilicide), carbon, graphite, metals, metal alloys and composite materials made of a ceramic material and a metallic material. Such composite materials may comprise doped or undoped ceramics. Examples of suitable doped ceramics include doped silicon carbides. Examples of suitable metals include titanium, zirconium, tantalum and metals from the platinum group. Examples of suitable metal alloys include stainless steel, constantan, nickel-, cobalt-, chromium-, aluminium- titanium- zirconium-, hafnium-, niobium-, molybdenum-, tantalum-, tungsten-, tin-, gallium-, manganese- and ironcontaining alloys, and super-alloys based on nickel, iron, cobalt, stainless steel, Timetal®, iron-aluminium based alloys and iron-manganese-aluminium based alloys. Timetal® is a registered trade mark of Titanium Metals Corporation. The wires may be coated with one or more electrical insulators. Preferred materials may be <NUM>, <NUM>, <NUM>, <NUM> stainless steel, and graphite.

Additionally, combinations of the above materials may be used. For example, materials with a high resistivity may be combined with materials with a low resistivity. This may be advantageous if one of the materials is more beneficial from other perspectives, for example price, machinability or other physical and chemical parameters.

The resistance of the at least one heating element increases along the longitudinal axis in the same direction that the cross-sectional area of the interior volume decreases. In operation, this may advantageously provide a temperature gradient along the length of the heating element. The temperature of the heating element may increase along the longitudinal axis in the same direction that the cross-sectional area of the interior volume decreases. As explained above, the first and second central regions of the retention material are generally heated to different temperatures as a result of the geometry of the interior volume defined by the heating element. The temperature gradient along the heating element may increase the temperature difference between the first and second central regions of the retention material. Therefore, providing a heating element having such a temperature gradient may allow for increased control of the vaporisation of the first and second compounds.

The resistance of the heating element may increase as a result of the cross-sectional area of the heating element decreasing. Varying the cross-section, or cross-sectional area, of the heating element may result in different sections of the heating element reaching different temperatures. For example, in a resistive heating element, a section of the heating element having a smaller cross-sectional area may have a larger resistance, and may therefore be resistively heated to a higher temperature.

Alternatively, or in addition, this may provide areas which increase in temperature at a greater rate, and areas which increase in temperature at a lesser rate, in the liquid aerosol-forming substrate storage component. As explained above, this may lead to liquid aerosol-forming substrate compounds with higher boiling points and lower boiling points being vaporised simultaneously at desirable rates.

The heating element may extend between a first end and a second end. For example, the length of the heating element may extend between a first end and a second end. The heating element may have a first cross-sectional area at a first point between the first end and the second end. The heating element may have a second cross-sectional area at a second point between the first point and the second end. For example, the first cross-sectional area may be at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> percent less than, the second cross-sectional area.

A minimum cross-sectional area along a length of the heating element may be at least <NUM> percent less than a maximum cross-sectional area along the length of the heating element. A minimum cross-sectional area along a length of the heating element may be at least <NUM>, <NUM>, <NUM>, or <NUM> percent less than a maximum cross-sectional area along a length of the heating element.

A width or a thickness or both the width and the thickness of the heating element may vary along a length of the heating element.

The heater assembly may use an inductive heating arrangement. The use of inductive heating rather than a resistive arrangement provides improved energy conversion because of power losses associated with a resistive heater, in particular losses due to contact resistance at connections between the resistive heater and a power delivery system, are not present in inductive heating systems. In order to function, a restive heater is either permanently or replaceablely connected to a power source through leads provided within a device or heater assembly. Even with improved automated manufacturing techniques, resistive heater systems typically have a contact resistance at the leads which creates parasitic losses. Replaceable resistive heater devices may also suffer from a build-up of films or other materials that increase the contact resistance between contacts of a replaceable cartridge and the leads. In contrast, inductive heating systems do not require contact between the heating elements and the leads and therefore do not suffer from the contact resistance issue present in resistive heater devices.

The at least one heating element may be a susceptor. The heating element as a susceptor may be configured to be heatable by an alternating magnetic field. The alternating magnetic field may be generated by an aerosol-generating system. The alternating magnetic field may be generated by passing an alternating current through an inductor coil of the aerosol-generating system, the inductor coil surrounding the heater assembly in use. The alternating current may have any suitable frequency. The alternating current may be a high frequency alternating current. The alternating current may have a frequency between <NUM> kilohertz (kHz) and <NUM> megahertz (MHz).

Heat generated by the heating element may be transferred to the retention material. In such cases, the inductive heating arrangement may have the advantage that no electrical contacts need to be formed between the heating element of the heater assembly and another component, for example, to a power supply of the aerosol-generating system. Furthermore, the heater assembly may be manufactured more cheaply. For example, the heater assembly may be manufactured as part of a cartridge. Cartridges are typically disposable articles produced in much larger numbers than the devices with which they operate. Accordingly, reducing the cost of cartridges, even if it requires a more expensive device, can lead to significant cost savings for both manufacturers and consumers.

Alternatively, the retention material may comprise a susceptor element. The retention material may consist of a susceptor element. The at least one heating element may form or comprise an inductor coil configured to heat the susceptor element of the retention material. The at least one heating element may be electrically connected or connectable to a power supply. The power supply may be configured to generate an alternating current. The heating element may be configured to generate an alternating magnetic field. The temperature of the first and second central region may depend on the magnetic flux density or the magnetic field strength of the alternating magnetic field. The density or strength of the alternating magnetic field may increase as the cross-sectional area of the interior volume defined by the heating element comprising or forming an inductor coil decreases. The magnetic flux density or the magnetic field strength of the alternating magnetic field may increase the closer the heating element is to the respective central region of the retention material. Such an arrangement may have the advantage that a large temperature gradient is maintained in the interior volume even when the heater assembly has reached thermal equilibrium or maximum temperature.

The heater assembly may comprise a first heating element and a second heating element, wherein each of the first heating element and second heating elements comprise or form an inductor coil. For example, the first heating element and second heating element may be stepped, as described above. The interior volume of retention material may be defined between the first heating element and second heating element. The first heating element may be positioned opposite to the second heating element. Each of the first and second heating elements may comprise flat inductor coils. Providing two heating elements comprising inductor coils may advantageously increase the magnetic field strength or magnetic flux density. The inductor coils may be arranged to provide magnetic fields that add to one another within the interior volume. Similarly, the alternating current passed through the first and second heating elements may be configured to provide magnetic fields that add to one another within the interior volume. An alternating current of the same frequency may be passed through each of the first and second heating elements. The alternating current may be supplied by the same power supply.

The at least one airflow path may comprise an airflow path defined through the retention material and passing through the first central region and second central region in a direction parallel to the longitudinal axis. The airflow path may be arranged such that, in use, air enters the retention material from the end having the largest cross-sectional area. In other words, the air may pass the cooler of the first or second central regions first. Incoming air may be considerably cooler than the retention material when the heater assembly is in operation. Therefore, air entering the retention material may advantageously cool the respective central region. When the air reaches the other of the first or second central regions, it may have been heated by the heater assembly and so the cooling effect of the air may have lessened. Therefore, such an arrangement of the airflow path may advantageously increase the difference in temperature between the first central region and the second central region.

Alternatively, the at least one airflow path may comprise a first airflow path defined through the retention material that passes through the first central region in a direction perpendicular to the longitudinal axis. The at least one airflow path may comprise a second airflow path defined through the retention material that passes through the second central region in a direction perpendicular to the longitudinal axis.

The heater assembly may comprise a reservoir for storing aerosol-forming substrate. The heater assembly may comprise a reservoir of aerosol-forming substrate. The reservoir may be configured to supply the aerosol-forming substrate to the retention material. The reservoir may be configured to supply the aerosol-forming substrate to the retention material where the interior volume filled by the retention material has the largest cross-sectional area. In use, this may advantageously increase the temperature difference between the first and second central region of the interior volume because the aerosol-forming substrate in the reservoir may be cooler than the aerosol-forming substrate in the retention material.

The retention material may store, or be configured to store, aerosol-forming substrate. The retention material may be in fluid communication with the reservoir.

The reservoir may comprise a reservoir housing containing the aerosol-forming substrate. A channel may be defined through the housing. The at least one airflow channel may pass through this channel.

According to the present disclosure there is also provided an aerosol-generating system comprising the heater assembly as described above. The aerosol-generating system may further comprise a power supply connectable to the heater assembly. The aerosol-generating system may further comprise a controller to control the power supplied to the heater assembly from the power supply. Thus, the controller may control heating of the heating element.

The power supply may be a battery. The power supply may be configured to supply power to the heating element. This may be to heat the heating element.

The power supply may be configured to supply power to the heating element to resistively heat the heating element. The aerosol-generating system may comprise an inductor coil.

The power supply of the aerosol-generating system may be configured to provide an alternating current to the inductor coil when the at least one heating element of the heater assembly forms or comprises an inductor coil. The power supply of the aerosol-generating system may be configured to provide an oscillating current to the inductor coil when the at least one heating element of the heater assembly forms or comprises an inductor coil. The oscillating current may be a high frequency oscillating current. As used herein, a high frequency oscillating current means an oscillating current having a frequency of between <NUM> and <NUM>.

The aerosol-generating system may comprise a cartridge, the cartridge comprising a cartridge housing containing the heater assembly. The cartridge may be configured to engage with the aerosol-generating device. The power supply may be configured to supply power to the heating element only when the cartridge is engaged with the aerosol-generating device.

The cartridge may comprise an air inlet. The cartridge may comprise an air outlet. The air inlet may be in fluid communication with the air outlet. The heating element may be disposed downstream of the air inlet. The heating element may be disposed upstream of the air outlet.

The cartridge may comprise first and second electrical contacts electrically connected to the heating element. The electrical contacts may comprise one or more of tin, silver, gold, copper, aluminium, steel such as stainless steel, phosphor bronze, tin alloyed with antimony, tin alloyed with zirconium, tin alloyed with bismuth, or tin alloyed with other components improving resistance to organic acids.

The electrical contacts may be configured to form an electrical connection with corresponding electrical contacts on an aerosol-generating device when the cartridge is engaged with the aerosol-generating device.

The reservoir optionally described in relation to the heater assembly may instead form part of the cartridge. The heater assembly and, particularly, the retention material of the heater assembly, may be in fluid communication with the reservoir.

The aerosol-generating system may further comprise an aerosol-generating device. The cartridge may be removably receivable in the aerosol-generating device.

The aerosol-generating device may comprise a device housing containing the power supply and the controller.

The aerosol-generating system may further comprise a mouthpiece in fluid communication with at least one airflow path, allowing a user to draw air through the at least one airflow path. The cartridge may comprise the mouthpiece. In use, when the cartridge is engaged with an aerosol-generating device, a user may puff on the mouthpiece of the cartridge. This may cause air to flow in through the air inlet, then across, over, past, or through the heater assembly or heating element, then through the air outlet.

The aerosol-generating system may be a handheld aerosol-generating system. The aerosol-generating system may be an electrically heated smoking system.

According to the present disclosure there is also provided a cartridge for use in the aerosol-generating system described above, the cartridge comprising a heater assembly as described above.

According to the disclosure there is also provided a method of using an aerosol-generating system, the aerosol-generating system comprising a retention material containing an aerosol-forming substrate in condensed form, the aerosol-forming substrate comprising a first compound and a second compound, the second compound having a higher boiling point than the first compound; at least one airflow path defined through the retention material; a heater assembly comprising at least one heating element shaped to define an interior volume, the interior volume filled with the retention material; a power supply; and a controller to control the power supplied to the heater assembly from the power supply; wherein the interior volume has a cross-sectional area that decreases along a longitudinal axis; and wherein the at least one airflow path passes through a first central region of the interior volume and a second central region of the interior volume, the first and second central regions being spaced-apart along the longitudinal axis; the method comprising activating the at least one heating element to heat the retention material such that the first central region is heated to a different temperature to the second central region.

The step of activating the at least one heating element may comprise heating the first central region to a temperature that is lower than the temperature of the second central region.

The step of activating the at least one heating element may comprise heating the first central region to a temperature that is at least <NUM> degrees Celsius lower than the temperature of the second central region.

The step of activating the at least one heating element may comprise heating the first central region of the of the interior volume to a temperature of between <NUM> and <NUM> degrees Celsius.

The step of activating the at least one heating element may comprise heating the second central region of the of the interior volume to a temperature of between <NUM> and <NUM> degrees Celsius.

The step of activating the at least one heating element may comprise heating aerosol-forming substrate such that a vapour is generated which enters into the at least one airflow path.

The vapour entering into the portion of the at least one airflow path passing through the first central region of the interior volume comprises the first compound in a higher amount by weight than the second compound.

The vapour entering a portion of the at least one airflow path passing through the second central region of the interior volume comprises the second compound in a higher amount by weight than the first compound.

As used herein, the term "aerosol" refers to a dispersion of solid particles, or liquid droplets, or a combination of solid particles and liquid droplets, in a gas. The aerosol may be visible or invisible. The aerosol may include vapours of substances that are ordinarily liquid or solid at room temperature as well as solid particles, or liquid droplets, or a combination of solid particles and liquid droplets.

As used herein, the term "aerosol-forming substrate" refers to a substrate capable of releasing volatile compounds that can form an aerosol. The volatile compounds may be released by heating or combusting the aerosol-forming substrate.

The aerosol-forming substrate may comprise a plurality of compounds. The plurality of compounds may have different boiling points. For example, the aerosol-forming substrate may comprise a first compound with a first boiling point at atmospheric pressure and a second compound with a second boiling point at atmospheric pressure, the first boiling point being greater than the second boiling point. The aerosol-forming substrate may comprise a third compound with a third boiling point at atmospheric pressure.

The aerosol-forming substrate may comprise an aerosol former. As used herein, the term "aerosol-former" refers to any suitable compound or mixture of compounds that, in use, facilitates formation of an aerosol, for example a stable aerosol that is substantially resistant to thermal degradation at the temperature of operation of the system. Suitable aerosolformers are well known in the art and include, but are not limited to: polyhydric alcohols, such as triethylene glycol, <NUM>,<NUM>-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate.

The aerosol-forming substrate may comprise nicotine. The aerosol-forming substrate may comprise water. The aerosol-forming substrate may comprise glycerol, also referred to as glycerine, which has a higher boiling point than nicotine. The aerosol-forming substrate may comprise propylene glycol. The aerosol-forming substrate may comprise plant-based material. The aerosol-forming substrate may comprise homogenised plant-based material. The aerosol-forming substrate may comprise tobacco. The aerosol-forming substrate may comprise a tobacco-containing material. The tobacco-containing material may contain volatile tobacco flavour compounds. These compounds may be released from the aerosol-forming substrate upon heating. The aerosol-forming substrate may comprise homogenised tobacco material. The aerosol-forming substrate may comprise other additives and ingredients, such as flavourants.

As used herein, the term "liquid aerosol-forming substrate" is used to refer to an aerosol-forming substrate in condensed form. Thus, the "liquid aerosol-forming substrate" may be, or may comprise, one or more of a liquid, gel, or paste. If the liquid aerosol-forming substrate is, or comprises, a gel or paste, the gel or paste may liquidise upon heating. For example, the gel or paste may liquidise upon heating to a temperature of less than <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> degrees Celsius.

As used herein, the term "heating element" encompasses both an element that is configured to itself raise in temperature when supplied with power and an element that is configured to give rise to an increase in temperature of a coupled component when supplied with power, such as an inductor coil coupled to a susceptor element.

As used herein, a "susceptor element" means a conductive element that heats up when subjected to a changing magnetic field. This may be the result of eddy currents induced in the susceptor element and/or hysteresis losses. Possible materials for the susceptor elements include graphite, molybdenum, silicon carbide, stainless steels, niobium, aluminium and virtually any other conductive elements. Advantageously the susceptor element is a ferrite element. The material and the geometry for the susceptor element can be chosen to provide a desired electrical resistance and heat generation.

Examples will now be further described with reference to the figures in which the example of <FIG> is according to the present invention, whereas the examples of <FIG> and <NUM>-<NUM> are not encompassed by the claims:.

<FIG> shows a cross-sectional view of a first aerosol-generating system <NUM>. The aerosol-generating system <NUM> comprises an aerosol-generating device <NUM> and a cartridge <NUM>. In this example, the aerosol-generating system <NUM> is an electrically operated smoking system.

The aerosol-generating device <NUM> is portable and has a size comparable to a conventional cigar or cigarette. The device <NUM> comprises a battery <NUM>, such as a lithium iron phosphate battery, and a controller <NUM> electrically connected to the battery <NUM>. The device <NUM> also comprises two electrical contacts <NUM>, <NUM> which are electrically connected to the battery <NUM>. This electrical connection is a wired connection and is not shown in <FIG>. In <FIG>, the aerosol-generating device <NUM> is engaged with the cartridge <NUM>. In this example, the cartridge <NUM> is engaged with the aerosol-generating device <NUM> via a screw thread <NUM> of the cartridge <NUM> mated with a corresponding screw thread <NUM> of the aerosol-generating device <NUM>.

The cartridge <NUM> comprises first and second electrical contacts <NUM>, <NUM>, an air inlet <NUM>, an air outlet <NUM>, and a first heater assembly <NUM>. The air inlet <NUM> is in fluid communication with the air outlet <NUM>. The heater assembly <NUM> is positioned downstream of the air inlet <NUM> and upstream of the air outlet <NUM>. The heater assembly <NUM> comprises a retention material <NUM> in fluid communication with a reservoir <NUM> of liquid aerosol-forming substrate. The reservoir <NUM> is described herein as a separate component to the heater assembly <NUM>. However, in some embodiments, the reservoir <NUM> forms part of the heater assembly. The heater assembly <NUM> also comprises a spiral shaped heating element <NUM>. The first and second electrical contacts <NUM>, <NUM> are electrically connected to the heating element <NUM>. An airflow path <NUM> is defined through the centre of the retention material <NUM>.

In this system <NUM>, the liquid aerosol-forming substrate comprises around <NUM>% by weight glycerine, <NUM>% by weight propylene glycol, and <NUM>% by weight nicotine, though any suitable substrate could be used. At atmospheric pressure, nicotine has a boiling point of around <NUM> degrees centigrade, glycerine has a boiling point of around <NUM> degrees centigrade and propylene glycol has a boing point of around <NUM> degrees centigrade. Thus, when initially heating this liquid aerosol-forming substrate to form an aerosol, some systems may undesirably vaporise a disproportionately large amount of propylene glycol (which has the lowest boiling point of the compounds forming the substrate). This may lead to a less desirable aerosol being delivered to the user, such as an aerosol comprising a smaller proportion of nicotine than desired. This may also undesirably change the relative proportions of the compounds in the substrate over a longer time period. The present invention may eliminate or at least reduce these undesirable effects.

In use, a user puffs on the air outlet <NUM> of the cartridge <NUM>. At the same time, the user presses a button (not shown) on the aerosol-generating device <NUM>. Pressing this button sends a signal to the controller <NUM>, which results in power being supplied from the battery <NUM> to the heating element <NUM> via the electrical contacts <NUM>, <NUM> of the device and the electrical contacts <NUM>, <NUM> of the cartridge. This causes a current to flow through the heating element <NUM>, thereby resistively heating the heating element <NUM>. In other examples, an air flow sensor, or pressure sensor, is located in the cartridge <NUM> and electrically connected to the controller <NUM>. The air flow sensor, or pressure sensor, detects that a user is puffing on the air outlet <NUM> of the cartridge <NUM> and sends a signal to the controller <NUM> to provide power to the heating element <NUM>. In these examples, there is therefore no need for the user to press a button to heat the heating element <NUM>.

As the user puffs on the air outlet <NUM> of the cartridge <NUM>, air is drawn into the air inlet <NUM>. This air then travels through a channel <NUM> defined through the reservoir <NUM> and then through airflow path <NUM> defined through the retention material <NUM> and towards the air outlet <NUM>. This flow of air entrains the vapour formed by the heating element <NUM> heating liquid aerosol-forming substrate in the liquid aerosol-forming substrate storage component <NUM>. This entrained vapour then cools and condenses to form an aerosol. This aerosol is then delivered to the user via the air outlet <NUM>. As liquid aerosol-forming substrate in the retention material <NUM> is heated, vaporised, and entrained in the air flow, liquid aerosol-forming substrate from the reservoir <NUM> travels into the retention material <NUM>. This liquid aerosol-forming substrate from the reservoir <NUM> effectively replaces the vaporised liquid aerosol-forming substrate. The liquid aerosol-forming substrate from the reservoir <NUM> may be drawn into the retention material <NUM>, at least partly, by capillary action. This is because the liquid aerosol-forming substrate storage component <NUM> is a capillary material having a fibrous structure. The retention material <NUM> in this example is a capillary material having a fibrous structure. In the example shown in <FIG>, the capillary material is formed form polyester, though any suitable material could be used.

The heater assembly is shown more clearly in <FIG> which illustrate the heater assembly <NUM> separately from the rest of the aerosol-generating system.

<FIG> shows a perspective view of the heater assembly <NUM> which shows how the heating element <NUM> is spiral shaped, having the form of a truncated spiral. The spiral shape of the heating element <NUM> defines an interior volume which is filled with retention material <NUM>. The cross-sectional area of the interior volume decreases along a longitudinal axis defined through the centre of the spiral shape. The retention material <NUM> filling the interior volume has a truncated cone shape. The heating element <NUM> is a strip of material. In this example, the material is stainless steel, though any suitable material could be used.

The heating element <NUM> has a uniform cross-sectional area along its length and a uniform thickness and width along its length. The heating element <NUM> has a uniform resistance along its length. Therefore, in use and when power is supplied to the heating element <NUM>, the heating power of the heating element <NUM> is substantially constant along the length of the heating element <NUM>. However, because of the geometry of the interior volume defined by the heating element <NUM>, the centre of the retention material comprises areas of higher temperature and areas of lower temperature. This is shown more clearly in <FIG>.

<FIG> shows a cross-sectional view of the heater assembly <NUM>. Three central regions of the interior volume filled with retention material <NUM> are indicated by dotted lines. The three central regions are spaced apart along a longitudinal axis. A first central region <NUM> is located between a second central region <NUM> and a third central region <NUM>. The second central region <NUM> is located towards a first end of longitudinal axis, where the cross-sectional area of the interior volume is smallest. The third central region <NUM> is located towards a second end of the longitudinal axis where the cross-sectional area of the interior volume is largest. In use, air passing through the airflow path <NUM> passes sequentially through the third central region <NUM>, the first central region <NUM> and the second central region <NUM>.

As described above, the cross-sectional area of the interior volume of retention material <NUM> decreases along the longitudinal axis. Heat is transferred from the heating element to the retention element by conduction and so the temperature of the first and second central region depends on the shortest distance between the heating element and the respective central region of the retention material. Where the cross-sectional area of the volume of retention material is smaller, the heating element <NUM> is closer to the centre the retention material. As shown in <FIG>, the second central region <NUM> closer to the heating element <NUM> than the first central region <NUM> which, in turn, is closer to the heating element <NUM> than the third central region <NUM>. This results in non-uniform heating of the retention material filling the interior volume with the third central region <NUM> being heated to a lower temperature than the first central region <NUM> which, in turn, is heated to a lower temperature than the second central region <NUM>.

The difference in temperature between the first, second and third central regions <NUM>, <NUM> and <NUM> results in the vapour generated at first central region <NUM> comprising a different proportion of nicotine by weight compared to the vapour generated at the second and third central regions <NUM>,<NUM>. Similarly, the vapour generated at the first central region comprises glycerol and propylene glycol in different proportion by weight compared to the second central region and third central region. In particular, propylene glycol, which has the lowest boiling point, is vaporised at the coolest third central region, nicotine which has a higher boiling point than propylene glycol is predominantly vaporised at the intermediate first central region <NUM> and glycerol, which has the highest boiling point, is predominantly vaporised at the hottest second central region <NUM>.

The temperature difference between the central regions of the retention material <NUM> are particularly pronounced when the heater assembly is not in thermal equilibrium, for example, following initial activation of the heating element. This is because there is a lag or delay between the initial activation of the heating element to heat the retention material and the central regions of the retention element reaching a maximum temperature. The extent of the lag or delay depends on the thermal conductivity of the retention material. The greater the distance between the heating element and the respective central region, the longer the lag or delay to reach a maximum temperature.

The airflow path <NUM> passes through the first, second and third central regions <NUM>,<NUM>,<NUM> in a direction parallel to the longitudinal axis. The airflow path <NUM> is arranged such that, in use, air enters the airflow path from the end of the retention material <NUM> having the largest cross-sectional area. In other words, passing through third central region <NUM> first. In operation, the third central region <NUM> is the coolest. Incoming air is considerably cooler than the retention material <NUM> when the heater assembly is in operation. Therefore, air entering the airflow path may advantageously act to cool the third central region. When the air reaches the first and then second central regions <NUM>,<NUM>, it will have been heated by the heater assembly and so the cooling effect of the air progressively lessons. Therefore, such an arrangement of the airflow path <NUM> increases the difference in temperature between the third central region, first central region and second central region.

<FIG> show a second heater assembly <NUM> comprising a first heating element <NUM> and a second heating element <NUM>. <FIG> shows a perspective view and <FIG> shows a cross-sectional view of the second heater assembly <NUM>. Both the first heating element <NUM> and the second heating element <NUM> are stepped and comprise three steps configured such that the heating elements get progressively closer together. This is shown most clearly in <FIG>. Between the first and second heating elements <NUM>, <NUM> is defined an interior volume which is filled with retention material <NUM>. The cross-sectional area of the interior volume decreases along a longitudinal axis as a result of the stepped heating elements. The longitudinal axis is defined through the centre of the interior volume. The first and second heating elements <NUM>,<NUM> are both shaped strips of material. In this example, the material is stainless steel, though any suitable material could be used.

<FIG> shows three central regions of the interior volume defined by the heating elements <NUM>,<NUM>. The three central regions are indicated by dotted lines. The three central regions are spaced apart along the longitudinal axis. A first central region <NUM> is located between a second central region <NUM> and a third central region <NUM>. The second central region <NUM> is located towards a first end of longitudinal axis, where the cross-sectional area of the interior volume is smallest. The third central region <NUM> is located towards a second end of the longitudinal axis where the cross-sectional area of the interior volume is largest. In use, air passing through the airflow path <NUM> passes sequentially through the third central region <NUM>, the first central region <NUM> and the second central region <NUM>. In use, air passing through the airflow path <NUM> passes sequentially through the third central region <NUM>, the first central region <NUM> and the second central region <NUM>.

Therefore, like the first heater assembly, the second heater assembly provides an arrangement where the first, second and third central portions are different directions
<FIG> shows a third heater assembly <NUM>. The third heater assembly <NUM> comprises two airflow paths <NUM>, <NUM>, rather a single airflow path as shown in the first and second heater assemblies. The interior volume defined by the heating element <NUM> is filled with retention material <NUM>. A first airflow path <NUM> is defined through a first central region <NUM> of the interior volume. A second airflow path <NUM> is defined through a second central region <NUM> of the interior volume. Of course, the interior volume comprises other central regions between and either side of the first and central regions <NUM>,<NUM>. However, <FIG> does not show an airflow path defined through these central regions. In some embodiments, the heater assembly <NUM> can comprise additional airflow paths passing through different central regions of the retention material.

In all other respects, the third heater assembly <NUM> is the same as first heater assembly <NUM>.

<FIG> shows a fourth heater assembly. The fourth heater assembly <NUM> comprises a spiral shaped heating element <NUM>. The spiral shaped heating element differs from the heating element <NUM> of the first heater assembly in that the width of the heating element decreases along the longitudinal axis in the same direction that the cross-section area of the interior volume decreases. In all other respects, the fourth heater assembly <NUM> is the same as the first heater assembly <NUM>.

The resistance of the heating element <NUM> increases as the width decreases. Therefore, in use, the heating element gets hotter along its length. This increases the temperature difference of the first and second central regions. Therefore, providing a heating element having such a temperature gradient allows for increased control of the vaporisation of the first and second compounds.

<FIG> shows a schematic, cross-sectional view of a second aerosol-generating system <NUM>. The aerosol-generating system <NUM> comprises an aerosol-generating device <NUM> and a cartridge <NUM> incorporating a second heater assembly <NUM>. In this example, the aerosol-generating system <NUM> is an electrically operated smoking system.

The aerosol-generating device <NUM> is portable and has a size comparable to a conventional cigar or cigarette. The device <NUM> comprises a battery <NUM>, such as a lithium iron phosphate battery, and a controller <NUM> electrically connected to the battery <NUM>. The device <NUM> also comprises an induction coil <NUM> electrically connected to the battery <NUM>. The device <NUM> also comprises an air inlet <NUM> and an air outlet <NUM> in fluid communication with the air inlet <NUM>.

The cartridge <NUM> comprises an air inlet <NUM>, an air outlet <NUM>, and a second heater assembly <NUM>. The air inlet <NUM> is in fluid communication with the air outlet <NUM>. The heater assembly <NUM> is positioned downstream of the air inlet <NUM> and upstream of the air outlet <NUM>. When the cartridge <NUM> is engaged with the aerosol-generating device <NUM>, as shown in <FIG>, the air outlet <NUM> of the device <NUM> is adjacent to the air inlet <NUM> of the cartridge <NUM>. Thus, in use, when a user puffs on the air outlet <NUM> of the cartridge <NUM>, air flows through the air inlet <NUM> of the device <NUM>, then through the air outlet <NUM> of the device <NUM>, then through the air inlet <NUM> of the cartridge <NUM>, then past the heater assembly <NUM>, then through the air outlet <NUM> of the cartridge <NUM>.

In this system <NUM>, the liquid aerosol-forming substrate comprises around <NUM>% by weight glycerine and <NUM>% by weight nicotine, though any suitable substrate could be used. At atmospheric pressure, nicotine has a boiling point of around <NUM> degrees centigrade and glycerine has a boiling point of around <NUM> degrees centigrade. Thus, when initially heating this liquid aerosol-forming substrate to form an aerosol, some systems may undesirably vaporise a disproportionately large amount of nicotine (which has the lowest boiling point of the compounds forming the substrate). This may lead to a less desirable aerosol being delivered to the user. This may also undesirably change the relative proportions of the compounds in the substrate over a longer time period. The present invention may eliminate or at least reduce these undesirable effects.

In <FIG>, the cartridge <NUM> is engaged with the aerosol-generating device <NUM>. In this example, the cartridge <NUM> is engaged with the aerosol-generating device <NUM> via apertures <NUM>, <NUM> which form a snap-fit connection with corresponding protrusions <NUM>, <NUM> on the aerosol-generating device <NUM>.

The heater assembly <NUM> comprises a spiral shaped heating element <NUM> and a retention material <NUM> filling an interior volume defined by the spiral shaped element. This is a similar arrangement to the first heater assembly <NUM> described in relation to <FIG>. The heating element <NUM> comprises a strip of a susceptor material. In this example, the susceptor material is aluminium, though any suitable susceptor material could be used.

The retention material <NUM> in this example is a capillary material having a fibrous structure. The capillary material is formed form polyester, though any suitable material could be used.

In use, a user puffs on the air outlet <NUM> of the cartridge <NUM>. At the same time, the user presses a button (not shown) on the aerosol-generating device <NUM>. Pressing this button sends a signal to the controller <NUM>, which results in the battery <NUM> supplying a high frequency electrical current to the induction coil <NUM>. This causes the induction coil <NUM> to create a fluctuating electromagnetic field. The heating element <NUM> is positioned within this field. Thus, this fluctuating electromagnetic field generates eddy currents and hysteresis losses in the heating element <NUM>. The heating element <NUM> is therefore inductively heated. In other examples, an air flow sensor, or pressure sensor, is located in the device <NUM> and electrically connected to the controller <NUM>. The air flow sensor, or pressure sensor, detects that a user is puffing on the air outlet <NUM> of the cartridge <NUM> and sends a signal to the controller <NUM> to supply the high frequency electrical current to the induction coil <NUM>, thereby heating the heating element <NUM>. In these examples, there is therefore no need for the user to press a button to heat the heating element <NUM>.

As the heating element <NUM> is heated, areas of higher temperature and areas of lower temperature are created in the retention material <NUM>. The creation of areas of higher temperature and areas of lower temperature causes compounds of the liquid aerosol-forming substrate with higher boiling points and lower boiling points in the retention material <NUM> to be vaporised simultaneously. This effect was described above in relation to the aerosol-generating system shown in <FIG>.

As the user puffs on the air outlet <NUM> of the cartridge <NUM>, air is drawn into the air inlet <NUM>. This air then travels through a channel <NUM> defined through the reservoir <NUM> and then through airflow path <NUM> defined through the retention material <NUM> and towards the air outlet <NUM>. This flow of air entrains the vapour formed by the heating of liquid aerosol-forming substrate in the retention material <NUM>. This entrained vapour then cools and condenses to form an aerosol. This air then travels towards the air outlet <NUM>. This flow of air entrains the vapour formed by heating of the liquid aerosol-forming substrate by the heating element <NUM>. This entrained vapour then cools and condenses to form an aerosol. This aerosol is then delivered to the user via the air outlet <NUM>.

<FIG> shows a sixth heater assembly <NUM> that uses an alternative inductive heating mechanism. The sixth heater assembly <NUM> comprises a first heating element <NUM> and a second heating element <NUM>. Both the first heating element <NUM> and the second heating element <NUM> comprise an electrically insulating substrate made of polyamide. The electrically insulating substrate has a stepped shape comprising three steps. The steps are configured such that the heating elements get progressively closer together. Each heating element <NUM>,<NUM> comprises an induction coil <NUM> in the form of a flat spiral on each of the steps. The induction coil <NUM> is attached to, or embedded in, the electrically insulating substrate.

Between the first and second heating elements <NUM>, <NUM> is defined an interior volume which is filled with retention material <NUM>. The cross-sectional area of the interior volume decreases along a longitudinal axis as a result of the stepped heating elements. The longitudinal axis is defined through the centre of the interior volume. An airflow path <NUM> is defined through the centre of the retention material, parallel to the longitudinal axis. This is a similar arrangement to that shown in <FIG>. However, in this embodiment, inductor coils <NUM> in the shape of flat spiral coils are formed on each of the heating elements.

Further, the retention material <NUM> consists of a susceptor material. In this embodiment, the susceptor material consists of a foamed metal, but any suitable material can be used. The heating element comprising the induction coils are connected to a battery of an aerosol-generating system. This is not shown in <FIG>.

In use, a signal is sent to a controller of the aerosol-generating system, which results in the battery supplying a high frequency electrical current to the induction coils <NUM> of the heating elements <NUM>, <NUM>. This causes the induction coils <NUM> to create a fluctuating electromagnetic field. The retention material <NUM> consisting of susceptor material is positioned within this field. Thus, this fluctuating electromagnetic field generates eddy currents and hysteresis losses in retention material <NUM>. The retention material <NUM> is therefore inductively heated.

The temperature of retention material <NUM> depends on the magnetic flux density or the magnetic field strength of the alternating magnetic field. The magnetic flux density between the induction coils increases as the induction coils get closer together. Therefore, as described in relation to previous embodiments, the retention material is heated to different temperatures in different zones, corresponding to the different steps of the heating elements <NUM>,<NUM>. A first central region of the retention material <NUM> is heated to a different temperature than a second or third central region. This is not shown in <FIG>.

<FIG> shows a cross-sectional schematic view of a seventh heater assembly <NUM>. The seventh heater assembly <NUM> comprises a first heating element <NUM> and a second heating element <NUM>. As in the heater assembly <NUM> of <FIG>, both the first heating element <NUM> and the second heating element <NUM> comprise an electrically insulating substrate made of polyamide. The electrically insulating substrate of both the first heating element <NUM> and the second heating element <NUM> is stepped and comprises three steps configured such that the heating elements get progressively closer together. Each heating elements <NUM>,<NUM> comprises an induction coil (not shown in <FIG>) in the form of a flat spiral on each of the steps.

Between the first and second heating elements <NUM>, <NUM> is defined an interior volume which is filled with retention material <NUM>. The cross-sectional area of the interior volume decreases along a longitudinal axis as a result of the stepped heating elements. The longitudinal axis is defined through the centre of the interior volume. An airflow path <NUM> is defined through the centre of the retention material <NUM>, parallel to the longitudinal axis. This is a similar arrangement to that shown in <FIG>. However, in the embodiment shown in <FIG>, the retention material does not consist of a susceptor material. Instead a susceptor material is provided in the form of a cylindrical mesh <NUM> surrounding the airflow path <NUM>.

In use, a signal is sent to a controller of the aerosol-generating system (not shown in <FIG>), which results in a battery supplying a high frequency electrical current to the induction coil of the heating elements <NUM>,<NUM>. This causes the induction coil <NUM> to create a fluctuating electromagnetic field. The retention material <NUM> consisting of susceptor material is positioned within this field. Thus, this fluctuating electromagnetic field generates eddy currents and hysteresis losses in cylindrical mesh <NUM>. The cylindrical mesh <NUM> is therefore inductively heated and that heat is conducted to the retention material <NUM> to heat aerosol-forming substrate contained in that retention material <NUM>.

Claim 1:
A heater assembly (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) for use in an aerosol-generating system (<NUM>; <NUM>), the heater assembly comprising:
a retention material (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) containing an aerosol-forming substrate in condensed form, the aerosol-forming substrate comprising a first compound and a second compound, the second compound having a higher boiling point than the first compound;
at least one airflow path (<NUM>; <NUM>; <NUM>; <NUM>) defined through the retention material; and
at least one heating element (<NUM>; <NUM>, <NUM>; <NUM>; <NUM>; <NUM>, <NUM>; <NUM>, <NUM>) shaped to define an interior volume, the interior volume filled with the retention material;
wherein the interior volume has a cross-sectional area that decreases along a longitudinal axis; and
wherein the at least one airflow path passes through a first central region (<NUM>; <NUM>; <NUM>; <NUM>) of the interior volume and a second central region (<NUM>; <NUM>; <NUM>; <NUM>) of the interior volume, the first and second central regions being spaced-apart along the longitudinal axis,
wherein the resistance of the at least one heating element increases along the longitudinal axis in the same direction that the cross-sectional area of the interior volume decreases.