Phase change heat storage composite superhard abrasive grinding wheel

Abstract

The application provides a phase change heat storage composite superhard abrasive grinding wheel, which comprises a grinding wheel base and abrasive materials arranged on the outer periphery of the grinding wheel base. An annular cavity is arranged on the near-outer end region of the grinding wheel base. A heat dissipation fin is arranged in the annular cavity. The inner end and both side ends of the heat dissipation fin are in contact with the grinding wheel base. The outer end of the heat dissipation fin extends to the outer wall surface of the annular cavity but does not contact the outer wall surface. The outer wall surface of the annular cavity is provided with a layer of foamed metal. A working medium is injected into the annular cavity. The phase change heat storage composite superhard abrasive grinding wheel has the advantages of high heat transfer coefficient, large heat capacity, good structural strength and effective regulation and control of the temperature of the grinding arc region.

Description

Technical Field

[0001] This invention relates to the field of grinding wheel technology, and more specifically, to a phase change thermal storage composite superhard abrasive grinding wheel. Background Technology

[0002] High-efficiency grinding processes are prone to generating high temperatures, which can cause thermal damage to the workpiece surface, such as residual stress, altered layers, and microcracks. Increasing the amount of grinding fluid and the jet velocity are currently the main methods to suppress grinding burns and improve grinding efficiency in order to cool the workpiece promptly. However, the negative impacts of excessive grinding fluid use, such as increased production costs, environmental pollution, and personnel health, cannot be ignored.

[0003] In recent years, people have begun to improve the thermal properties of grinding wheels themselves, trying to enhance the heat transfer effect of grinding wheels on the arc zone during grinding by improving the thermal conductivity and heat capacity of the grinding wheels themselves. This allows more grinding heat to enter the grinding wheel and be discharged through the grinding wheel, ultimately reducing the temperature of the grinding arc zone and reducing the use of grinding fluid.

[0004] One current research direction is to add heat pipes to the base of the grinding wheel, thereby improving the thermal conductivity of the grinding wheel through the phase change heat transfer effect of the heat pipes, and at the same time, adding heat storage materials inside the heat pipes to increase the internal heat capacity of the heat pipes.

[0005] For example, in patent application CN201610865211.0, entitled "Green Dry Grinding Phase Change Heat Storage Grinding Wheel and its Manufacturing Method Thereof," a heat pipe is added to the grinding wheel, and a working fluid is filled inside the heat pipe. A ring-shaped heat storage element is also added inside the heat pipe. However, its limitations are: the heat transfer distance inside the heat pipe is relatively long, and the contact area when the vaporized fluid condenses on the surface of the heat storage element is small. When the gas-liquid phase change heat transfer process is not smooth, the condensation heat exchange between the heat storage element and the vaporized fluid is insufficient, thus affecting the heat transfer rate and the heat storage capacity of the heat storage element. Furthermore, the large space of the heat pipe also affects the structural strength of the grinding wheel. In simpler terms, the practical problem encountered in application is that when the grinding wheel is grinding, the working fluid vaporizes. However, due to the long movement path of the vaporized fluid and the small condensation area provided by the heat storage element, the condensation heat dissipation is insufficient. This increases the thermal resistance, thus affecting the grinding wheel's own heat transfer capacity and the conduction of heat in the grinding arc area.

[0006] For example, in the invention patent with patent application number CN201811635039.5, entitled "A Novel High Thermal Conductivity Phase Change Heat Storage Superhard Abrasive Grinding Wheel and Its Preparation Method," the structure inside the heat pipe was improved by filling the heat pipe with a large amount of foamed metal and injecting working fluid into the annular cavity. The aim was to utilize the large specific surface area and good permeability of the foamed metal to accelerate heat transfer and gradually transfer it to the inner end of the grinding wheel. However, it still has limitations. During use, it was found that the heat capacity of the foamed metal was insufficient, and after the working fluid vaporized, the foamed metal would cause difficulties in the return flow of the condensed working fluid, which would lead to the drying of the hot end of the grinding wheel. In the long term of use, the heat at the hot end could not be efficiently transferred in the form of phase change convection heat transfer.

[0007] In summary, the common problem they face is that the thermal conductivity and heat capacity properties of the grinding wheel itself are greatly affected by factors such as grinding conditions, grinding wheel structure, and heat transfer state of working fluid phase change. The grinding wheel cannot simultaneously improve the two important thermal properties of thermal conductivity and heat capacity, as well as ensure stability during the grinding process.

[0008] Based on the above issues, it can be found that improving the thermal conductivity of the grinding wheel itself while taking into account its heat capacity is the core and basic requirement for improving the thermal properties of the grinding wheel. It is also the key to realizing the use of the grinding wheel to enhance heat transfer in the grinding arc zone, reduce grinding temperature, and effectively control it.

[0009] In order to solve the above problems, people have been seeking an ideal technological solution. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of existing technologies by providing a phase change thermal storage composite superhard abrasive grinding wheel with strong heat transfer capacity, large heat storage capacity, good structural strength, and the ability to effectively control the temperature of the grinding arc zone.

[0011] The basic design concept of this invention is as follows: First, this invention is based on the boiling / condensation heat transfer principle, which has the characteristics of small heat exchange area, high heat transfer coefficient, and small required heat transfer temperature difference. Currently, boiling heat transfer is widely used and occupies an important position in the field of enhanced heat transfer. Second, for convective heat transfer using gas / liquid two-phase change in a closed container, since the heat capacity of the phase change heat transfer part is very small, once the container accumulates heat due to insufficient heat dissipation, it is easy to cause a rapid increase in the internal gas pressure and temperature of the container, further affecting its heat transfer performance. At the same time, it is also difficult to effectively control the temperature at the heat input end.

[0012] Based on the above principles, this application sets a sealed annular cavity around the outer periphery of the grinding wheel matrix, lays a thin layer of foamed metal on the outer wall of the annular cavity, and injects an appropriate amount of low-boiling-point working fluid into the annular cavity, thereby forming the basic structure of boiling heat transfer. During the grinding process, when grinding heat is transferred to the grinding wheel, the microporous structure of the foamed metal effectively promotes the nucleation process of the liquid working fluid, allowing it to quickly enter the nucleation boiling state while maintaining a small temperature difference between the working fluid and the outer wall of the annular cavity. Simultaneously, heat dissipation fins are filled inside the annular cavity. The tail and side faces of the fins contact the grinding wheel substrate, while the outer ends of the fins are close to the foamed metal. When the vaporized fluid moves towards the inner hole of the grinding wheel under the driving force of the pressure difference, it quickly contacts the foamed metal due to the proximity of the fins. The large surface area of ​​the fins ensures rapid and sufficient condensation and heat dissipation of the vaporized fluid. The fins then further transfer the heat to the grinding wheel substrate and achieve final heat dissipation through convective heat transfer between the grinding wheel end face and the surrounding environment. Furthermore, when the heat conduction from the grinding wheel is insufficient, the fins can temporarily store the heat through self-heating, thus preventing rapid temperature changes at various points during the heat transfer process. In addition, since the grinding wheel with an annular cavity has many thin-walled structures, the heat dissipation fins also play a supporting and reinforcing role, thereby improving and ensuring the strength of the grinding wheel.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a phase change thermal storage composite superhard abrasive grinding wheel, comprising a grinding wheel substrate and abrasive disposed on the outer periphery of the grinding wheel substrate, wherein an annular cavity is provided in the near-outer region of the grinding wheel substrate, and heat dissipation fins are installed in the annular cavity, wherein the inner end and the two side ends of the heat dissipation fins contact the grinding wheel substrate, and the outer end of the heat dissipation fins extends toward the outer wall of the annular cavity but does not contact it, wherein a layer of foamed metal is disposed on the outer wall of the annular cavity, and a working fluid is injected into the annular cavity.

[0014] Based on the above, the distance between the outer end of the heat dissipation fin and the outer wall of the annular cavity is 10mm-30mm, and the thickness of the foam metal is 0.2mm-1.5mm.

[0015] Based on the above, the pore size of the foamed metal is 20 PPi-80 PPi, and the porosity of the foamed metal is higher than 95%.

[0016] Based on the above, the boiling point of the phase change working fluid is between 10℃ and 50℃.

[0017] Based on the above, the heat dissipation fins include an annular body and a ring of fins arranged in a circular array around the annular body, and the heat dissipation fins are an integral structure.

[0018] Based on the above, the connection area between the heat dissipation fins and the annular cavity is coated with a high thermal conductivity structural adhesive and then cured by heating and insulation to achieve the connection.

[0019] Based on the above, one sidewall of the annular cavity is an annular end cap mounted on the grinding wheel base. The grinding wheel base has a liquid injection hole communicating with the annular cavity. The grinding wheel base is provided with a sealing plug for sealing the liquid injection hole. The heat dissipation fins are provided with through holes corresponding to the liquid injection hole. The diameter of the through holes is 2mm-3mm.

[0020] Based on the above, the foamed metal is bonded to the outer wall surface of the annular cavity by structural adhesive.

[0021] Based on the above, the outer wall surface of the annular cavity is a smooth wall surface or an irregular wall surface. When calculating the distance between the outer end of the heat dissipation fin and the outer wall surface of the annular cavity, the outer wall surface at the corresponding position of the heat dissipation fin is used as the reference. In particular, in irregular wall surfaces, this distance is variable.

[0022] This invention has outstanding substantive features and significant progress compared to the prior art. Specifically, this invention has the following advantages:

[0023] 1. Heat dissipation fins are designed in the annular cavity of the grinding wheel. The heat dissipation fins occupy most of the space in the annular cavity and have a large surface area. Based on the unique fin structure, it can be close to the outer wall of the annular cavity while meeting the space requirements of the annular cavity. After the working fluid enters the nucleus boiling heat transfer state, the vaporized fluid can quickly and fully contact the heat dissipation fins, thereby ensuring condensation and heat dissipation. Since the heat dissipation fins themselves have a large heat capacity and high thermal conductivity, they can also quickly absorb heat and transfer it to the grinding wheel substrate, so that the heat can be dissipated to the external environment through the grinding wheel substrate. Therefore, the heat dissipation fins can act as an effective cold end for the boiling heat transfer of the working fluid, causing the vaporized fluid to condense quickly and flow back to the inner wall of the outer circle of the annular cavity. This structure will not increase the thermal resistance due to insufficient / inadequate condensation of the vaporized fluid, thus affecting the overall heat transfer capacity of the grinding wheel.

[0024] 2. A layer of foamed metal is placed on the outer wall of the annular cavity. The foamed metal is not used for heat transfer, but rather its microporous structure is used to facilitate bubble nucleation, allowing the working fluid to enter the nucleation boiling state more quickly. Combined with the low boiling point of the working fluid and the fact that sufficient condensation and heat dissipation are ensured, the liquid working fluid and the outer wall of the annular cavity only need to maintain a small temperature difference to enter the heat transfer cycle of vaporization nucleation, boiling, heat transfer, condensation, heat dissipation, and condensation working fluid recirculation. This improves and ensures the heat transfer rate, thereby enhancing the overall heat transfer capacity of the grinding wheel. Attached Figure Description

[0025] Figure 1This is a schematic diagram of the overall structure of the phase change thermal storage composite superhard abrasive grinding wheel in this invention.

[0026] Figure 2 This is a partial structural cross-sectional view of the phase change thermal storage composite superhard abrasive wheel in this invention.

[0027] Figure 3 This is a partial cross-sectional view of the phase change thermal storage composite superhard abrasive wheel in this invention.

[0028] Figure 4 Schematic diagram of the working principle of phase change thermal storage composite superhard abrasive grinding wheel for heat transfer / storage.

[0029] Figure 5 This is a schematic diagram of the convective heat transfer principle of a phase change thermal storage composite superhard abrasive grinding wheel.

[0030] In the diagram: 1. Grinding wheel base; 2. Abrasive; 3. Annular cavity; 4. Heat dissipation fins; 5. Foam metal; 6. Annular end cap; 7. Injection hole; 8. Sealing plug; 9. Through hole; 10. Outer wall surface; 11. Working fluid; 12. Vortex cooling gun. Detailed Implementation

[0031] The technical solution of the present invention will be further described in detail below through specific embodiments.

[0032] like Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown, a phase change thermal storage composite superhard abrasive grinding wheel includes a grinding wheel base 1 and an abrasive 2 disposed on the outer periphery of the grinding wheel base. An annular cavity 3 is provided in the near-outer region of the grinding wheel base 1. Heat dissipation fins 4 are installed in the annular cavity 3. The inner end and two side ends of the heat dissipation fins 4 contact the grinding wheel base 1, and the outer end of the heat dissipation fins 4 extends toward the outer wall surface 10 of the annular cavity 3 but does not contact it, with a distance of 10mm-30mm.

[0033] The outer wall 10 of the annular cavity 3 is bonded with a layer of foam metal 5 by structural adhesive. The thickness of the foam metal 5 is 0.2mm-1.5mm, the pore size of the foam metal 5 is 20PPi-80PPi, and the porosity of the foam metal 5 is higher than 95%. The working fluid 11 is injected into the annular cavity 3. The boiling point of the working fluid 11 is between 10℃ and 50℃. In this embodiment, isopentane is used.

[0034] In this embodiment, the main function of the foam metal 5 is only to improve the boiling efficiency of the working fluid. By utilizing the microporous structure of the foam metal, which is conducive to bubble nucleation, the working fluid and the wall of the tube only need to maintain a small temperature difference to enter the nucleation boiling state, and it is not used for heat storage or heat conduction. In this embodiment, the working fluid can enter the boiling state when the temperature reaches 28°.

[0035] The heat dissipation fins 4 include an annular body 41 and a ring of fins 42 arranged in a circular array outside the annular body. The heat dissipation fins 4 are an integral structure and can be made of aluminum alloy, copper or steel. The connection area between the heat dissipation fins 4 and the annular cavity 3 is coated with a high thermal conductivity structural adhesive and then heated and cured to achieve the connection.

[0036] The main functions of the heat dissipation fins are fourfold: First, they serve as the cold end of the working fluid phase change convective heat transfer, used for the condensation of the vaporized working fluid; second, they act as heat storage and heat transfer elements, absorbing the heat transferred by the phase change working fluid and gradually transferring the heat outward through the grinding wheel substrate 1 via their connection with the grinding wheel substrate 1. When temperature transfer is not smooth, they can also temporarily store the heat using their high heat capacity; third, they provide space for phase change heat transfer by utilizing the gaps between the heat dissipation fins, and the large surface area allows the vaporized phase change working fluid to fully contact the heat dissipation fins for rapid condensation. At the same time, the heat dissipation fins are as close as possible to the outer wall surface 10 to shorten the boiling heat transfer path; fourth, the heat dissipation fins effectively support the thin-walled structure at the annular cavity of the grinding wheel, providing structural strength to the grinding wheel.

[0037] Specifically, one sidewall of the annular cavity 3 is an annular end cap 6 mounted on the grinding wheel base. The grinding wheel base 1 has a liquid injection hole 7 communicating with the annular cavity. The grinding wheel base 1 is provided with a sealing plug 8 for sealing the liquid injection hole 7. The heat dissipation fins 4 are provided with through holes 9 corresponding to the liquid injection hole 7. The diameter of the through holes 9 is 2mm-3mm.

[0038] Working principle:

[0039] After the processing begins, the grinding wheel rotates at high speed. At this time, a small amount of working fluid is injected and forms a thin liquid film on the outer wall of the annular cavity under the action of centrifugal force. The thickness of the liquid film is slightly higher than the thickness of the foam metal by about 2 mm. At the same time, the heat from the grinding arc zone enters the annular cavity 3 of the grinding wheel from the abrasive layer. The outer wall of the annular cavity 3 can be approximated as the boiling heat exchange surface in the pool.

[0040] Because the microporous structure of the foam metal is conducive to bubble nucleation, the working fluid and the outer wall of the annular cavity only need to be maintained at a small temperature difference to enter the nucleation boiling state. In this embodiment, the boiling temperature is about 28°C.

[0041] like Figure 4As shown, the vaporized working fluid moves rapidly toward the inner hole of the grinding wheel under the driving force of the pressure difference. Since the outer end of the heat dissipation fins is close to the foam metal, the vaporized working fluid can quickly come into contact with the heat dissipation fins. At the same time, the large surface area of ​​the heat dissipation fins is used to quickly and fully condense and dissipate heat from the vaporized working fluid. The condensed working fluid returns to the outer wall of the annular tube under the action of centrifugal force, ready for the next boiling heat transfer.

[0042] Meanwhile, the heat dissipation fins themselves have a large heat capacity and high thermal conductivity. The heat dissipation fins further transfer the absorbed heat to the grinding wheel substrate through the sides and tail end, and finally the heat is discharged through convective heat transfer between the grinding wheel substrate and the external environment. In practical applications, such as... Figure 5 As shown, vortex cooling guns 12 are installed on both sides of the annular cavity outside the grinding wheel to enhance convective heat transfer; in addition, when the heat is not sufficiently conducted outward, the heat can be temporarily stored by utilizing the large heat capacity of the heat dissipation fins.

[0043] Through the above process, both the thermal conductivity and heat capacity of the grinding wheel are improved, ultimately enhancing the thermal properties of the grinding wheel.

[0044] The following mathematical proof will illustrate the importance of the improved thermal conductivity and heat capacity properties, as well as the theoretical basis for the aforementioned modifications.

[0045] During grinding, the abrasive layer on the outer cylindrical surface of the grinding wheel can be considered as the heat input end, and the grinding arc area as the heat source. Considering the structure of the phase change heat storage composite grinding wheel, and neglecting the contact thermal resistance between the grinding surface of the grinding wheel and the workpiece surface, the overall heat transfer coefficient of the grinding wheel can be approximated as a heat transfer process from the outside to the inside of a composite cylindrical wall.

[0046] Therefore, if the grinding wheel cavity is taken as the main part of heat transfer, the entire heat transfer process mainly includes two modes: heat conduction and phase change heat transfer. The overall heat transfer coefficient can be expressed as:

[0047]

[0048] In the formula: λs is the thermal conductivity of the grinding wheel matrix material, h1 is the boiling heat transfer coefficient on the outer wall of the annular tube, h0 is the condensation heat transfer coefficient of the vaporized substance on the surface of the heat dissipation fins and the tube wall, D is the diameter of the grinding wheel, D1 is the outer diameter of the grinding wheel tube, S is the surface area of ​​the heat dissipation fins, and R' is the thermal resistance of the phase change heat transfer space inside the tube.

[0049] Compared to traditional pure carbon steel-based grinding wheels, the heat transfer process involves only heat conduction. Comparing the heat transfer coefficient of a conventional grinding wheel with a cavity of the same size, it can be expressed as:

[0050]

[0051] In the formula: D2 is the inner diameter of the annular cavity of the grinding wheel.

[0052] Assuming the length (radial) of the annular cavity inside the composite grinding wheel and the length (radial) of the heat dissipation fins are both 35 mm, and the wall thickness of the outer surface of the grinding wheel is 1.0 mm, compared to a traditional single-layer superhard abrasive grinding wheel, the thermal resistance of the heat conduction part of the outer surface of the grinding wheel and the thermal resistance of the phase change heat transfer part inside the cavity in equation (1) are both very small values ​​and can be ignored. In addition, the surface area S of the heat dissipation fins per unit width is much larger than the surface area of ​​the working fluid boiling heat transfer, and the heat transfer coefficient h0 of the vaporized working fluid during condensation is also slightly higher than the boiling heat transfer coefficient h1 due to the surface roughness. Therefore, the thermal resistance of condensation heat dissipation inside the cavity is also small and can also be ignored. In summary, under a certain grinding wheel structure, the total heat transfer coefficient of the phase change heat storage composite grinding wheel mainly depends on the boiling heat transfer coefficient h1 on the outer surface of the cavity. In practice, the thermal conductivity of the composite grinding wheel can be increased by 2-3 times compared to the carbon steel grinding wheel of the same specification.

[0053] When a phase-change heat storage composite grinding wheel dissipates heat in the grinding arc region, the grinding arc region and the grinding wheel form a heat transfer system. According to the principles of heat transfer and the law of conservation of energy, when grinding heat rapidly enters the grinding wheel, the grinding wheel needs to dissipate heat to maintain the stability of its own system. If the incoming heat cannot be dissipated in time, the remaining heat will be stored in the grinding wheel, causing the heat transfer system to continuously heat up. At this point, the heat capacity of the grinding wheel itself plays a crucial role in regulating the temperature rise rate of the heat transfer system. For the internal cavity of the grinding wheel's heat transfer body, its heat capacity is composed of a combination of various substances, including liquid working fluid, gaseous working fluid, and fin material. Its equivalent heat capacity can be expressed as:

[0054]

[0055] m0=ρ c V c +ρ l V l +ρ v V v (3)

[0056] In the formula: Q is the heat accumulated inside the grinding wheel cavity, ΔT is the temperature rise caused by each substance inside the cavity, m0 is the total mass of each substance inside the cavity, ρc, ρl, and ρv correspond to the densities of the heat dissipation fins, liquid working fluid, and gaseous working fluid, respectively, and Vc, Vl, and Vv correspond to the volumes of the heat dissipation fins, liquid working fluid, and gaseous working fluid, respectively.

[0057] Because the gaseous working fluid has a low density and the injected amount of the liquid working fluid is also small, the heat capacity of these two parts can be ignored. Therefore, the heat capacity inside the grinding wheel cavity mainly depends on the properties and mass of the heat dissipation fins. Since the heat dissipation fins and the grinding wheel substrate are in contact, there is also a heat transfer effect between them. Therefore, the grinding wheel substrate also participates in heat storage. For this composite form of heat dissipation fins and steel substrate, the heat capacity of the composite grinding wheel is actually quite close to that of the traditional pure carbon steel substrate.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A phase change thermal storage composite superhard abrasive grinding wheel, characterized in that: The device includes a grinding wheel substrate and abrasive disposed on the outer periphery of the grinding wheel substrate. An annular cavity is provided near the outer end of the grinding wheel substrate, and heat dissipation fins are installed within the annular cavity. The inner and outer ends of the heat dissipation fins contact the grinding wheel substrate, while the outer ends of the heat dissipation fins extend towards but do not contact the outer wall of the annular cavity. A layer of foamed metal is disposed on the outer wall of the annular cavity, and a working fluid is injected into the annular cavity. The distance between the outer end of the heat dissipation fins and the outer wall of the annular cavity is 10mm-30mm, and the thickness of the foamed metal is 0.2mm-1.5mm. The heat dissipation fins act as heat transfer and heat storage elements, absorbing the heat transferred by the phase change working fluid and gradually transferring the heat outward through the grinding wheel substrate via their connection with the grinding wheel substrate. When temperature transfer is inefficient, the heat dissipation fins temporarily store heat. Simultaneously, the gaps between the heat dissipation fins provide space for phase change heat transfer.

2. The phase change thermal storage composite superhard abrasive wheel according to claim 1, characterized in that: The foam metal has a pore size of 20 PPi-80 PPi and a porosity of over 95%.

3. The phase change thermal storage composite superhard abrasive wheel according to claim 2, characterized in that: The boiling point of the phase change working fluid is between 10°C and 50°C.

4. The phase change thermal storage composite superhard abrasive grinding wheel according to claim 1, 2, or 3, characterized in that: The heat dissipation fins include an annular body and a ring of fins arranged in a circular array around the annular body, and the heat dissipation fins are an integral structure.

5. The phase change thermal storage composite superhard abrasive wheel according to claim 4, characterized in that: The connection areas between the heat dissipation fins and the two sides of the annular cavity are coated with high thermal conductivity structural adhesive and then cured by heating.

6. The phase change thermal storage composite superhard abrasive grinding wheel according to claim 1, 2, 3, or 5, characterized in that: One sidewall of the annular cavity is an annular end cap mounted on the grinding wheel base. The grinding wheel base has a liquid injection hole communicating with the annular cavity. The grinding wheel base is provided with a sealing plug for sealing the liquid injection hole. The heat dissipation fins are provided with through holes corresponding to the liquid injection hole. The diameter of the through holes is 2mm-3mm.

7. The phase change thermal storage composite superhard abrasive wheel according to claim 2, characterized in that: The foamed metal is bonded to the outer wall of the annular cavity using structural adhesive.

8. The phase change thermal storage composite superhard abrasive wheel according to claim 7, characterized in that: The outer wall of the annular cavity is either a smooth wall or an irregularly shaped wall.

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