GB2474352A

GB2474352A - Hygienic rapid heat transfer device

Info

Publication number: GB2474352A
Application number: GB1016870A
Authority: GB
Inventors: Simon Jelley; Andrew Chapman; Jon Spratley
Original assignee: 42 Technology Ltd; Heineken Supply Chain BV
Current assignee: 42 Technology Ltd; Heineken Supply Chain BV
Priority date: 2009-10-08
Filing date: 2010-10-07
Publication date: 2011-04-13
Anticipated expiration: 2030-10-07
Also published as: GB201016870D0; GB0917583D0; GB2474352B; WO2011042698A3; WO2011042698A2

Abstract

A heat transfer device used to cool or heat an object 4 comprises a thermal sink 5 used to cool or heat a flow of gas generated by a gas flow generator (fan) 2. Fan 2 communicates the heated or cooled gas through a cavity in the gas flow path, with the cavity receiving the object which in association with a wall of the gas flow path defines a restricted region having a gap distance 3 to promote turbulence in the gas flow and increase heat transfer. The object may be a drinking vessel having at least a portion of rapidly cooled to generate a frosted effect thereon. The cavity may be formed of a separate ducting 1 removable from the heat transfer device so that it may be cleaned or replaced with a different duct having a shape for a different item to be cooled or heated. The thermal sink may be copper coils communicating refrigerant or glycol based coolant. The device may be adapted to cool vials.

Description

Hygienic non-contact rapid heat transfer device This invention relates to the design of a heat transfer device that can rapidly and efficiently cool or heat items using only a gaseous medium. It relates to the cooling or heating of items where it is advantageous to avoid contact with liquids or solids; for example for sterility or hygiene.

Examples of such items include crockery and glassware for food and beverages, lab or medical equipment, pipetting tips, glassware or samples, soluble materials, porous materials, materials that are physically damaged by liquids or where wetting is to be avoided, electronics, materials with high coefficients of thermal expansion or items that need to be cooled to near or below the freezing point of water.

Rapid heat transfer is frequently achieved by using a liquid interface to create intimate contact with the surface of the item. However, in applications such as the frosting of drinking vessels prior to serving, such methods are undesirable. In particular, any chemicals required to suppress the freezing point of the cooling liquid would automatically prevent the creation of frost. Even wetting with pure water will affect the creation and appearance of the frosted layer.

Known alternative methods of achieving rapid heat transfer use a thermally conductive metal negative of the shape of the item to facilitate heat transfer or a flexible membrane to separate a liquid coolant from the item to be cooled. These approaches avoid wetting or contamination by glycol or similar freezing suppressants. A metal negative has a drawback in that the thermal interface at the boundary between the item and the cooling surface will resist heat transfer due to a trapped air double glazing' effect. In addition, if too much thermal expansion or contraction occurs, the item may be difficult to remove from a rigid negative. The cold surfaces may also be difficult to clean, allowing the negative or flexible membrane to pass contaminants, dirt or bacteria to the item being cooled.

Further problems exist in that before the item is present, condensation or ice may form on the neqative or flexible membrane; affecting fit, causing wetting and potentially ice adhesion to the item and providing an unhygienic environment. Due to problems with speed of frosting, frost appearance and hygiene implications, no such solutions are in widespread use today.

To avoid problems associated with solid or liquid contact, it is often advantageous to use a gaseous coolant. The drawback of using a gas as a heat transfer fluid is that gasses are generally far more thermally insulating than liquids and solids. Known arrangements involve forced convection using a fan or compressor to generate a gas flow.

Known methods of counteracting the low heat transfer rate in air or gas cooled devices to accelerate heat exchange include increasing the surface area of heat transfer and increasing the temperature difference. For heating applications, the temperature difference is commonly increased as achieving high temperatures is easy and avoids expensive and bulky fins. For cooling applications, where it is more difficult to achieve large temperature differences, increasing the surface area of the item being cooled is the more common approach, as is evident in computer processor heat sinks and car radiator design and the heat fins on air cooled engines and motors. Using expanded liquid CO2 to frost glasses (e.g. US 2,587,075) is one of the few examples of using a high temperature difference to achieve rapid cooling.

Drawbacks exist with these methods; heat fins are often expensive to manufacture and driving heat transfer with high temperature differences is thermodynamically inefficient.

Also, with items where altering the form to increase heat transfer is not favourable, such as glassware or other items where the shape is specified for other aesthetic or practical reasons, increasing the surface area with heat fins is not possible and the heat transfer needs to be high over the limited area available. Achieving controlled and consistent heat transfer with high temperature differences is difficult, leading to hot/cold spots on the item that could cause damage, and potentially inconsistent final temperatures. Other technologies for heating include microwaves or ultrasound but are also prone to over/inconsistent heating and hot spots.

To achieve the required heat flux for rapid cooling by increasing the temperature difference, it is possible to exploit the large temperature reduction which occurs during the expansion of liquefied compressed CO2. The temperature of the solid/gas mix of CO2 (-78.5°C) blown on to the glass makes the driving temperature difference high even as the glass cools below zero degrees Celsius. This allows very rapid cooling. However, drawbacks of this method include the safety of releasing significant quantities of CO2 into rooms, safety concerns bouL Lhe poterit or cod burns for the operator, and the cost of the liquefied CO2. The volume of CO2 required to give sufficient cooling capacity is very high due to the proportion of CO2 that escapes without cooling the glass. This poor use of potential cooling capacity, mixed with the thermodynamic inefficiency of such a high temperature difference make the system require a lot of C02 to achieve the desired cooling.

The present inventors have established that it is possible to accelerate heat exchange between a heat transfer gas and an object to be cooled or heated by creating a turbulent flow in the heat transfer gas in the region adjacent the object. Known rapid cooling applications which employ forced convection do not create a turbulent flow; instead, boundary layers of lamina flow remain which result in flow gradients and heat transfer that is uneven and relatively poor.

Thus, at its most general, the present invention provides methods and apparatus for rapidly cooling or heating an object using a gaseous heat transfer medium, by promoting turbulence within the heat transfer medium.

In a first aspect, the present invention provides a heat transfer device arranged to cool or heat a given object, the device having: a gas flow path arranged to circulate a gas flow; a gas flow generator arranged to generate a flow of a heat transfer gas within the gas flow path; a thermal sink or thermal source arranged to cool or heat a generated flow of heat transfer gas circulating within the gas flow path; a cavity in the gas flow path, the cavity being arranged to receive the given object to be cooled or heated to thereby create a restricted region in the gas flow path adjacent the object, the cavity being sized such that in the restricted region there is a gap distance between the object and a boundary wall of the gas flow path, the gap distance being selected to promote turbulence in the gas flow and thereby achieve a desired heat transfer rate.

Thus, it is possible to rapidly heat or cool an object, or a part of an object, using a gas heat transfer medium, without having to either increase the surface area of the object (e.g by applying fins) or create a high temperature difference between the object and the gas. This effect is achieved by promoting turbulence in a generated gas flow, as identified by the present inventors. The promoted turbulence induces chaotic mixing in the gas flow, with large numbers of rapidly moving eddy regions where the flow is perpendicular to the bulk flow and the cooled/heated surface. This mixing causes much improved heat transfer to or from the object to be cooled/heated, as can be predicted with the Dittus-Boelter heat transfer correlation. It is not necessary that the flow is entirely turbulent, and non-turbulent flow regions can be accepted.

Moreover, the turbulent flow is generated very simply and with a minimum number of device components. Simply by inserting an object to be cooled/heated into the cavity, the circulating gas flow becomes turbulent in the restricted region (i.e. region of reduced cross-sectional area) adjacent the object.

A number of optional features of the present invention will now be described. These features can be applied, either alone or in combination, to the first aspect of the invention, or to any of the other aspects of the invention described below.

The shape of the cavity preferably approximates the shape of the surface/surfaces of the object which is/are to be cooled or heated. The cavity may be a well, or recess, in the heat transfer device which is open at one end to the environment to allow the object to be inserted into the cavity and removed from it. Alternatively, the cavity may be a closed void within the device which is only open to the gas flow path.

The gas flow path has a varying cross-sectional area. Its cross-sectional area is typically smallest in the restricted region, in order to increase the flow velocity in this region and thereby induce turbulent effects. In the restricted region the gas flow path generally follows the contours of the surface/surfaces of the object to be cooled/heated, and so its route may be convoluted.

In the restricted region the gas flow path may be considered to be a stream, or channel, which travels adjacent the object to be heated/cooled. The gap distance may represent the depth of the stream or channel, i.e. the shortest distance from the surface of the object to the boundary wall of the gas flow path at any point along the restricted region.

The gap distance may be constant, i.e. uniform, along the length of the gas flow path in the restricted region (i.e. in the direction of travel of the gas flow). In such an arrangement the amount of cooling or heating achieved will be the same along the length of the gas flow path. Alternatively, and/or in addition, the gap distance may be constant in a direction at an angle, e.g. perpendicular, to the direction of travel of the gas flow path, so that the amount of cooling or heating is uniform in that direction.

To vary the arnouiit of cuoing or heatrig along the threcton of the ges flow poth, e.g. to achieve a graduated frosted effect along a drinking glass, the gap distance may vary with distance along the restricted region of the gas flow path. Thus, non-uniform heating or cooling patterns may be achieved in the direction of gas flow.

To achieve non-uniform heating or cooling patterns in a direction at an angle, e.g. perpendicular, to the direction of gas flow the restricted region may include a plurality of parallel channels adjacent the object to be cooled or heated, each channel being separated from its neighbouring channel by a dividing wall and each channel being arranged such that its gap distance is different to the gap distance of its neighbouring channel. Thus, the amount of cooling or heating achieved in regions of the object adjacent neighbouring channels is different.

Using different gap distances to effect different rates of cooling in neighbouring gas flows to achieve non-uniform heating or cooling patterns can have the effect of restricting flow via a smaller gap distance, with the gas instead following the path of least resistance via a larger gap distance. To minimise this restrictive effect the restricted region may include a plurality of parallel channels adjacent the object to be cooled or heated, each channel being arranged such that its gap distance is different to the gap distance of its neighbouring channel, and at least one channel including one or more fins projecting into the channel to achieve a localised reduced gap distance.

The device may include one or more locating members projecting into the cavity to locate the object within the cavity to thereby achieve the selected gap distance. The locating members are preferably very thin plate-like members, i.e. planar members with a very small thickness, aligned with the gas flow so that they have minimal aerodynamic effects.

The cavity may be formed in a ducting member, or ducting insert, which is separable from the heat transfer device. In this way, it is possible to remove the insert -which comes into contact with the cooled/heated object via the cavity -for cleaning. Further, the insert may be replaced with an alternative insert with a differently shaped cavity, arranged to receive a differently shaped object. In this way, a user may use a single device comprising expensive hardware such as the gas flow generator and thermal sink/source to heatlcool many differently shaped and sized objects.

The cavity may be arranged to receive only a portion of the object, whereby said portion of the object is heated or cooled. Such an arrangement is particularly suitable for applications where it is desirable to retain one or more portions of an object at room temperature, e.g. to permit easy handling by a user.

The heat transfer device may include a housing having an opening for allowing passage of the object into or out of the cavity, the opening being bounded by a gas deflector arranged to deflect a gas flow within the gas flow path away from the opening. Thus, objects can be inserted into and removed from the device without the necessity of closing the opening with a lid but while still minimising loss of cooled or heated gas from the device. Alternatively, the opening may be bounded by a seal, such as a rubber seal, which contacts the object when it is inserted in the device to thereby prevent loss of cooled or heated gas from the device. The gas deflector is considered to be particularly desirable means of preventing gas loss because it does not suffer from the known disadvantages of seals, which can be unhygienic and can deteriorate over time.

In preferred embodiments the gap distance is 5mm or less, and preferably 2mm or more, or 3mm or more.

In a second aspect the present invention provides a heat transfer device arranged to cool at least a portion of a drinking vessel to generate a frosted effect thereon, the device having: a gas flow path arranged to circulate an air flow; a gas flow generator arranged to generate an air flow within the gas flow path; a thermal sink arranged to cool a generated air flow circulating within the gas flow path; a cavity in the gas flow path, the cavity being arranged to receive at least a portion of the drinking vessel to thereby create a restricted region in the gas flow path adjacent the drinking vessel, the cavity being sized such that in the restricted region there is a gap distance between the drinking vessel and a boundary wall of the gas flow path, the gap distance being selected to promote turbulence in the gas flow and thereby achieve a desired heat transfer rate.

Such an arrangement has many advantages over known glass frosting devices. In particular, there is no requirement for unhygienic and otherwise unsatisfactory contact between the glass and a liquid or solid heat transfer medium, it is not necessary to generate a high temperature difference, and the drawbacks of using liquefied compressed CO2 as a medium are avoided.

In a third aspect the present invention provides a method of cooling or heating an object using a heat transfer gas, the method including the steps of: generating a circulating flow of the heat transfer gas around a gas flow path; cooling or heating the gas flow; achieving a desired heat transfer coefficient by locating the object in the cooled or heated gas flow to thereby create a restricted region of the gas flow path adjacent the object and to thereby achieve a gap distance between the object and a boundary wall of the gas flow path in the restricted region which promotes turbulence in the restricted region, whereby the turbulent gas flow cools or heats the object.

In a fourth aspect the present invention provides a method of determining the gap distance of the heat transfer device according to the first or second aspects, the method including the steps of: for each a plurality of initial gap distances: estimating the volumetric flow rate of a gas flow within the gas flow path; estimating the heat transfer coefficient; and predicting the heat transfer rate using the estimated volumetric flow rate and heat transfer coefficient, correlating the predicted heat transfer rates with the initial gap distances to determine an optimal heat transfer rate and a corresponding gap distance; and providing the corresponding gap distance as the predetermined gap distance.

In this way, the optimal, or near-optimal, gap distance can be selected to achieve a desired heat transfer rate, based on a desired amount of cooling or heating.

The method may include, for each of the initial gap distances, calculating the pressure loss caused by the restricted region and estimating the volumetric flow rate based on the calculated pressure loss and the pressure-flow operating parameters of the gas flow generator. The method may also include, for each of the initial gap distances, calculating the Reynolds number and the Nusselt number for the gas flow and estimating the heat transfer coefficient using the Dittus-Boelter heat transfer correlation.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. I shows s schematic vertica! cross-secJona! view of a glass frosting pprtus according to a first embodiment of the present invention; Fig. 2 shows a partial vertical cross-sectional view of a glass frosting apparatus according to a second embodiment of the present invention; Fig. 3 shows an isometric view of the apparatus of Fig. 2; Fig. 4 shows a schematic vertical cross-sectional view of a bottle cooling apparatus according to a third embodiment of the present invention; Fig. 5 shows schematic cross-sectional views of a clamshell-type cooling apparatus according to a fourth embodiment of the present invention; and Fig. 6 shows a schematic cross-sectional view of a vial thermal cycling apparatus according to a fifth embodiment of the present invention.

In each of the illustrated embodiments of the present invention heat is rapidly transferred to or from an object 4 using a highly turbulent, gaseous heat transfer fluid such as air.

The highly turbulent gas flow generates a high heat transfer coefficient without the need to increase the surface area of the object or have a very high temperature differential. In addition, the surface of the item can be cooled uniformly or unevenly, as required, to create the desired cooling pattern. Accordingly this invention allows designed and repeatable cooling across the surface. This is particularly useful when applied to frosting glasses in that a visible frosted coating can be achieved without cooling the entire glass, so that less heat needs to be removed for a similar visual effect.

The embodiments of the invention include the following three main components: A. Driving Fan (2): The driving fan 2 recirculates the heat transfer gas at high speed, enabling the ducting 3 (discussed below) to generate the turbulence that drives the heat transfer. It needs to generate a high flow at a sufficient pressure to accelerate the heat transfer gas to high speed and overcome the restriction in the ducting 3.

In the glass frosting device of Figs. 1 to 3, the driving fan 2 is a 24W backward curved radial blower, chosen to match the space requirements of system and the flow-pressure requirements of the ducting to achieve the required cooling performance. In different implementations other fan types and powers may be preferred.

B. Thermal sink/source (5): According to the direction of heat transfer required, the thermal sink/source 5 cools or warms the heat transfer gas as it is recirculated.

In the glass frosting embodiments shown in Figs. 1 to 3, multiple copper coils provides a heat sink 5, the copper coils being either directly cooled by refrigerant or cooled by glycol based coot ant.

It is preferred to have a relatively large surface area, low restriction thermally conductive heat exchanger to exchange heat with the gas. For cooling devices this could be driven by peltier, refrigerant, secondary coolant or a large thermal store. Heating could be performed by any heater technology or a large thermal store.

C. Ducting (1): The ducting I generates turbulence in the heat transfer gas across the entire surface of the item to create rapid heat exchange. The ducting 1 may be arranged to provide a uniform gas flow over the whole surface, to thereby achieve uniform cooling or heating, or to provide localised flow and localised cooling or heating.

The method of achieving the rapid and repeatable heat exchange is by controlling the flow velocity and turbulence of the heat transfer gas as it travels over the item surface to fulfil the requirements of heat transfer. To implement these flow characteristics, the ducting 1 is shaped as a pseudo-negative of the item 4 to be cooled/heated such that when placed around the item 4, there is a specified gap 3 between the ducting I and the item 4.

The item 4 is held in place, and the gap 3 maintained, by thin aerodynamic supports (7) to ensure correct location within the ducting 1. This allows brand protection as the ducting 1 will only work effectively for one shape of item (e.g. a glass froster will only frost one brand's glass shape, as other glass shapes will either not cool quickly because the gap is incorrect or will not physically fit in the device). Such physical brand protection that prevents competitor products being used in the device is advantageous in industries where the equipment owner cannot control the usage of their equipment by third parties.

The chosen gap width depends on the size and shape of the item to be cooled or warmed and the flow-pressure curve for the driving fan. In the embodiments of Figs. I to 3, for cooling and frosting the top 60mm of a beverage pint glass 4, a gap of between 3mm and 5mm across the cooled area was found to achieve the required flow and heat transfer. Testing has achieved frosting times of 15 seconds when coc!ng from 20°C and estimations predict this can be further reduced to under 10 seconds.

Another advantageous feature of the ducting 1 is that it can be easily removable, allowing it to be cleaned easily. For example, ducting 1 of the glass frosting embodiment of Figs. 1 to 3 may be removable and washable in a glass washer, making it easier for the bar staff to keep the unit hygienic. Fig. 3 shows a view of removed ducting 1, with the drinking glass 4 located therein. Another advantage of having the ducting removable is that it allows for the system to be changed to work with a different item shape. With the glass froster example (Figs. 1 to 3), the brewery can change the shape of their glassware by swapping the interchangeable ducting blocks. The expensive part of the system with the fridge circuit and fan does not therefore need to be changed. This would also allow mass production of one system to work with many different items (or glass types), with only the ducting being produced in smaller runs.

When designing ducting for an item, the gap size is specified to create the required level of cooling at each point (constant for cooling/heating evenly; varied for uneven cooling/heating). Decreasing the gap reduces the cross-sectional area of the flow, causing it to increase velocity by the Bernoulli effect. This increases the turbulence and so increases the heat transfer. However, reducing the gap too far causes too much restriction in the flow, and a large back pressure to the fan, slowing the whole flow down.

The optimum gap size in the glass cooling system of Figs. 1 to 3 was calculated to be 3.5mm; this must be determined for each vessel type and is strongly dependant on item shape and cooled area, as well as the fan type and power.

The approximate required gap can be estimated by performing the following calculations for a range of gap sizes, Calculate the head loss caused by the restriction and iterate to solve with the pressure-flow curve of the fan. This can be done using the annular equivalence to flow between two plates for axisymmetric systems where the gap is far smaller than the radius. This will give the volumetric flow rate during operation. Using double the gap width as the characteristic length, calculate the Reynolds number and Nusselt number, and use the Dittus-Boelter heat transfer correlation to estimate the heat transfer coefficient. By calculating the thermal mass flow of air and the estimated heat-flow, it is possible to correct for the air warming up more with lower flow rates, and the resulting heat-flow predictions should give a local peak indicating the estimated optimal gap width. More complicated systems may require the application of computational fluid dynamics to predict flow velocities and heat transfer coefficients.

Using different gap widths to effect different rdtes of cuung wi ao affect the restriction of the air flow, i.e. the resistance met by the air flow. This restrictive effect makes it difficult to achieve parallel flow paths with different cooling/heating requirements; i.e. neighbouring parallel flow paths having different gap sizes or different lengths/restrictions. Such non-axisymmetric systems are more difficult to design because the gas naturally tends to take the path of least resistance so that a flow route with a higher resistance, e.g. one with a smaller gap size, which should generate a faster flow may in fact experience a lower flow rate than a neighbouring flow route with a lower resistance. The gas inlet and outlet port locations must be specifically chosen to minimise such flow path differences. To further reduce the effects of different parallel path restrictions, narrow restrictive bands with very small gap widths can be used to separate neighbouring flow routes, or channels, so as to increase restriction where needed. Moreover, such narrow restrictive bands can be included within individual flow channels to control the flow resistance/restriction.

For axisymmetric cooling/heating of axisymmetric shapes, the parallel path restriction problem is avoided altogether, allowing easy 1-dimensional design of the gap.

Being able to selectively cool only the top part of the glass, a similar visual effect can be reached without the same requirement for energy use and refrigeration capacity, and without making the bottom of the glass uncomfortably cold for the bar person to hold.

There are many possible embodiments for the device to enable parallel usage, different cooling/heating methods and performance, and different ducting types for different shapes of item or usage requirements (such as one handed operation).

Examples of ducting type options include: open top (Fig. 1, 4), where the item can be placed in or removed easily; clamshell (Fig. 5), where the ducting is closed on the item to entirely surround it; or a plastic holder which doubles as the ducting when connected with an air supply 8 and return 9 to a fan and heat sink/source, for example a holder (Fig. 6) for a batch of vials 13 that can be connected to a unit 12 that rapidly freezes, incubates or thermally cycles them. This could incorporate a simple seal system 10 that seals empty spaces 11.

The unit can have one or many cooling stations which may be run together in parallel or independently switchable depending on usage requirements.

Where the requirements for uniformity or control of cooling/heating power are lower, less complicated ducting designs can meet the cooling/heating requirements allowing simpler and cheaper manufacturing, easier item insertion, versatile ducting (eg. also used as a r L.I4......' I IUIUI) al IU I'JVVI t%Jl'I cli I'.,'.'J.

Recirculating air avoids wasting cooling/heating that is not transferred immediately to the object.

For open top embodiments, either a conformable seal or an aeroscreen 6, i.e. air flow deflector, can be used to direct the air to recirculate. As the system is a closed loop, the fan should not create a significant pressure difference at the top, so preventing mixing of recirculating and fresh air is the main aim of this screen or seal.

If maintaining a pure atmosphere is important, a pure gas supply could be used to supply the heat transfer gas. This could be recirculated for efficiency or, if needed for purity, could be used once with a heat exchanger between the waste and inlet gas streams.

All of the above are examples and a wide range of specific embodiments and applications can be envisaged, within the scope of the appended claims, as interpreted

by the description and drawings.

Claims

Claims: 1. A heat transfer device arranged to cool or heat a given object, the device having: a gas flow path arranged to circulate a gas flow; a gas flow generator arranged to generate a flow of a heat transfer gas within the gas flow path; a thermal sink or thermal source arranged to cool or heat a generated flow of heat transfer gas circulating within the gas flow path; a cavity in the gas flow path, the cavity being arranged to receive the given object to be cooled or heated to thereby create a restricted region in the gas flow path adjacent the object, the cavity being sized such that in the restricted region there is a gap distance between the object and a boundary wall of the gas flow path, the gap distance being selected to promote turbulence in the gas flow and thereby achieve a desired heat transfer rate.
2. A heat transfer device according to claim 1, wherein the gap distance is constant in a direction at an angle to the direction of gas flow.
3. A heat transfer device according to claim 1 or claim 2, wherein the gap distance is constant with distance along the restricted region of the gas flow path.
4. A heat transfer device according to claim I or claim 2, wherein the gap distance varies with distance along the restricted region of the gas flow path.
5. A heat transfer device according to any of the preceding claims, including one or more locating members projecting into the cavity to locate the object within the cavity to thereby achieve the selected gap distance.
6. A heat transfer device according to any of the preceding claims, wherein the cavity is formed in a ducting member which is separable from the heat transfer device.
7. A heat transfer device according to any of the preceding claims, wherein the cavity is arranged to receive only a portion of the object, whereby said portion of the object is heated or cooled.
8. A heat transfer device according to any of the preceding claims, wherein the restricted region includes a plurality of channels adjacent the object to be cooled or heated, each channel being separated from its neighbouring channel by a dividing wall and each channel being arranged such that its gap distance is different to the gap distance of its neighbouring channel.
9. A heat transfer device according to any of the preceding claims, wherein the restricted region includes a plurality of channels adjacent the object to be cooled or heated, each channel being arranged such that its gap distance is different to the gap distance of its neighbouring channel, and at least one channel including one or more fins projecting into the channel to achieve a Jocalised reduced gap distance.
10. A heat transfer device according to any of the preceding claims, wherein the gas flow path is convoluted in the restricted region.
11. A heat transfer device according to any of the preceding claims, including a housing having an opening for allowing passage of the object into or out of the cavity, the opening being bounded by a gas deflector arranged to deflect a gas flow within the gas flow path away from the opening.
12. A heat transfer device according to any of the preceding claims, wherein the gap distance is 5mm or less.
13. A heat transfer device according to claim 12, wherein the gap distance is 2mm or more.
14. A heat transfer device arranged to cool at least a portion of a drinking vessel to generate a frosted effect thereon, the device having: a gas flow path arranged to circulate an air flow; a gas flow generator arranged to generate an air flow within the gas flow path; a thermal sink arranged to cool a generated air flow circulating within the gas flow path; a cavity in the gas flow path, the cavity being arranged to receive at least a portion of the drinking vessel to thereby create a restricted region in the gas flow path adjacent the drinking vessel, the cavity being sized such that in the restricted region there is a gap distance between the drinking vessel and a boundary wall of the gas flow path, the gap distance being selected to promote turbulence in the gas flow and thereby achieve a desired heat transfer rate.
15. A method of cooling or heating an object using a heat transfer gas, the method including the steps of: generating a circulating flow of the heat transfer gas around a gas flow path; cooling or heating the gas flow; achieving a desired heat transfer rate by locating the object in the cooled or heated gas flow to thereby create a restricted region of the gas flow path adjacent the object and to thereby achieve a gap distance between the object and a boundary wall of the gas flow path in the restricted region which promotes turbulence in the restricted region, whereby the turbulent gas flow cools or heats the object.
16. A method of determining the gap distance of the heat transfer device according to any of claims I to 14, the method including the steps of: for each a plurality of initial gap distances: estimating the volumetric flow rate of a gas flow within the gas flow path; estimating the heat transfer coefficient; and predicting the heat transfer rate using the estimated volumetric flow rate and heat transfer coefficient, correlating the predicted heat transfer rates with the initial gap distances to determine an optimal heat transfer rate and a corresponding gap distance; and providing the corresponding gap distance as the predetermined gap distance.
17. A method according to claim 16, including, for each of the initial gap distances, calculating the pressure loss caused by the restricted region and estimating the volumetric flow rate based on the calculated pressure loss and the pressure-flow operating parameters of the gas flow generator.
18. A method according to claim 16 or claim 17, including, for each of the initial gap distances, calculating the Reynolds number and the Nusselt number for the gas flow and estimating the heat transfer coefficient using the Dittus-Boelter heat transfer correlation.