JPH01215354A - Crushing and coating device - Google Patents

Abstract

PURPOSE:To efficiently carry out crushing and coating by specifying the ratio of the length to diameter of a throat at the throat part of an injector and the expanding angle of the diffuser on the high-pressure gas injection side of a diffuser part. CONSTITUTION:The crushing zone A for crushing and coating particulate material is formed between the injectors 4 and 5, and a classifying zone B is connected to the crushing zone through a crushed material passage 6 and a coarse- grain return passage 7. High-pressure gas is supplied to jet nozzles 2 and 3 from a high-pressure gas source 10. The particulate raw material from the particulate material supply source 1 is untrained by a jet from at least one between the jet nozzle 2 and 3, and supplied. The ratio LT/Dr of the length LT to diameter Dr of the throat at the throat parts 4b and 5b of the injectors 4 and 5 is controlled to 0-5, and the diffuser expanding angle Q2 on the high- pressure gas injection side of the diffuser parts 4c and 5c is controlled to 5-11 deg.. As a result, crushing and coating can be efficiently carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a grinding and coating device using an opposed jet mill, and particularly relates to a grinding and coating device for grinding and coating products such as pharmaceuticals, foods, dyes, waxes, +4J finger pigments, synthetic river fat, Toner, magnetic material. Organic/inorganic chemicals, mineral materials, ceramics, biochemical materials, and/or powder particles and/or coating materials are gasified at high pressure or at normal pressure, and the liquefied gas is liquefied at high pressure. speed of sound, speed of sound,
Alternatively, it relates to a technique that is effective for producing fine particles or ultrafine particles by supplying the particles into a supersonic air stream and pulverizing the particles by colliding the particles with each other due to the shearing action of the air stream and acceleration of the particles.

Further, the present invention uses an opposed jet nozzle to jet a solution of a pigment 1, a surfactant, a wax, a film agent, etc. from a two-fluid nozzle having an atomization function. The present invention relates to a technique that is effective when used to coat a pulverized product, or to perform coating simultaneously with crushing and dispersing fine particle agglomerated powder.

[Conventional technology]

Regarding grinding and coating using an opposed jet mill, for example, Japanese Patent Publication No. 48-42905, Japanese Patent Application Laid-open No. 5
It is disclosed in Japanese Patent Publication No. 8-137449, Japanese Patent Application Laid-Open No. 1988-102455, Japanese Patent Publication No. 61-27104, etc.

By the way, the principle of the opposed jet mill is as follows. In other words, an opposed jet mill has a jet nozzle that suctions and crushes particles supplied from a feeder, a supply injector that suctions and accelerates the pulverized material, and an opposed jet nozzle that returns oppositely to each other on their center line and suctions and crushes coarse particles. and an opposing injector that performs suction and acceleration, pulverization is performed by the suction collision action of the potential core of each jet nozzle and the head-on collision action of particles accelerated by the opposing injectors in the pulverization zone, and the crushed material is pulverized. The material is guided from the material passage to the classification zone, and the swirling flow created in this zone classifies fine powder and coarse particles, and the fine powder is taken out of the thread as a pulverized product from the discharge port and collected by a dust collector. on the other hand,
The coarse particles pass through the return path and are crushed by the opposing jet nozzle and the opposing injector, and are further crushed in the crushing zone by colliding with the particle group accelerated to crush by the supply jet nozzle and the supply injector. This recycling pulverization is continued until the pulverized material is classified in the classification zone, turned into fine powder with a size smaller than the classification diameter, and taken out of the system.

This grinding principle and mechanism is similar to other jet mills such as U.S. Pat.
Unlike the jet mill disclosed in No. 6, which is a type in which accelerated particles are collided with each other, a head-on collision type is used, and therefore the probability of two collisions is extremely high. Furthermore, since each of the opposing jets is accelerated by an injector, the particle velocity is higher than that of other jet mills, and the force is greater than the particle crushing stress, so the stress probability is also increased.

Since pulverization efficiency is proportional to the probability of collision and the probability of stress, opposed jet mills have higher pulverization efficiency than ordinary jet mills, breaking through the 3μm wall of the pulverization limit diameter of ordinary jet mills, and achieving 0.5~ It is possible to produce pulverized products with a diameter of 1 μm.

In addition, as described in Japanese Patent Publication No. 61-27104,
By replacing the opposed jet nozzle with a suitable coating material supply means, pulverization or coating of dispersed particles can be easily carried out. The coating material supply means can be composed of a specially constructed two-fluid nozzle that entrains and accelerates a solution of Pigment 9 surfactant, wax, etc. as a coating material and atomizes it with high-pressure air, a coating material tank, and a pump. .

[The problem that the invention aims to solve]

However, the opposed jet mill has the following problems.

(1) When pulverizing and coating adhesive particles, such as wax, low-melting substances such as magnesium stearate, and charged raw powder such as fine particles of high-molecular organic substances, etc., with an opposed jet mill, the injector section attached to,
Sticking and fusion phenomena occur, resulting in a decrease in crushing and dispersion ability, and eventually a phenomenon in which the system becomes clogged and becomes inoperable.

(2) Abrasive powders, such as alumina and titanium oxide.

When pulverizing powder of entertainment oxide such as zirconia, violent collisions at the injector section inevitably cause abrasion particles of the mill material to be mixed into the powder, causing contamination. This phenomenon becomes a particularly serious problem in pharmaceuticals, foods, and biotechnology, and the jet mill-based grinding and coating equipment cannot be used as a product even if the desired grinding and coating are obtained.

(3) When attempting to efficiently obtain sub-micron particles using a opposed jet mill, there are structural constraints that prevent the classification diameter of the classification zone from becoming small, and the return to the pulverization coating chamber. Since the amount of powder cannot be adjusted, grinding efficiency and coating efficiency cannot be improved.

The inventors of the present invention conducted intensive research to fundamentally solve these problems of opposed jet mills, and discovered the following facts.

In other words, the two opposing injectors of a conventional opposed-type jet mill have a cylindrical shape with a constant diameter at the inlet and outlet, and the ratio of the injector length to the injector diameter is several times more than ten times. Collision of particles occurs, but since the particle velocity is small, the pulverization efficiency does not improve beyond a certain value. Furthermore, the present inventors have discovered that the flow pattern of the fluid in this area is extremely poor, and the generation of vortices results in a significant loss of jet energy.

An object of the present invention is to provide a technique that can efficiently perform pulverization and/or coating.

Other objects of the invention are to reduce contamination;
The objective is to provide a technology that allows obtaining high quality milled and/or coated products.

Still another object of the present invention is to provide a technique capable of pulverizing and/or coating ultrafine particles.

[Means to solve the problem]

The pulverization and coating device of the present invention has a ratio of the throat length Lt of the throat section that is used for the injector for accelerating the high-pressure gas and the powder and granular material supplied together with the high-pressure gas and the throat diameter /Dr from 0 to 5. Also, set the diffuser expansion angle θ2 on the high pressure gas ejection side of the diffuser part to 5~
It was formed at 11 degrees.

Further, the ratio d/D of the diffuser length Ld of the diffuser portion to the throat diameter is preferably 1OOsO4.

[Effect]

According to the above-described means, the ratio Lr/Dr of the throat length to the throat diameter of the throat portion of the injector is 0 to
5. By setting the defenizer expansion angle θ2 to 5 to 11 degrees, the structure of the injector part is extremely ideal from the viewpoint of fluid dynamics and high-speed aerodynamics, and the throat part that is unnecessary for particle acceleration is eliminated. As the length becomes shorter, the energy of the jet jet ejected from the defnizer section is improved, so the particle velocity is accelerated and the collision speed of the powder increases, and the jet is sufficiently expanded. The pulverization and coating efficiency of powder and granules can be significantly improved, and adhesion and contamination of powder and granules during pulverization can also be prevented.

Ratio between defenizer length d and throat diameter Ld/Dr
By forming the particle size between 10 and 30, the particle velocity becomes even higher, allowing more efficient grinding and coating.

These and other objects and advantages of the present invention will become more apparent from the following detailed description of the embodiments illustrated in the accompanying drawings.

〔Example〕

FIG. 1 is an enlarged partial cross-sectional view of the main parts of an opposed jet mill type crushing and coating device that is an embodiment of the present invention, FIG. 2 is a schematic vertical cross-sectional view of the overall structure, and FIGS.
Fig. 6 is a partial explanatory diagram of various embodiments of the coarse particle return passage, Fig. 7 is a schematic cross-sectional view taken along the line ■-■ in Fig. 2-1 Fig. 8 is an enlarged partial cross-section of an embodiment of the product discharge port. 9 are schematic illustrations of the injector section of the apparatus of the present invention, and FIGS. 10 to 12 are graphs showing the results of experiments conducted using the injector structure of FIG. 9.

The crushing and coating device of this example has a structure of an opposed jet mill type, and the powder raw material supplied from a hopper l (powder supply source) connected to a metered feeder (not shown) is fed by an opposed jet mill. It is configured to crush and/or coat by the impinging action of the air flow.

A supply jet nozzle 2 for sucking and supplying the powder raw material and ejecting a jet stream; The opposing jet nozzles 3 for suctioning and entraining are disposed facing each other on the same axis, that is, the same center line.

A supply injector 4 and a counter injector 5 for accelerating the jet stream from each jet nozzle 2.3 are interposed on the same axis between the pair of jet nozzles 2.3.

Moreover, between both injectors 4 and 5, there is a crushing zone A.
is formed. In this crushing zone A, the jet airflow from the jet nozzle 2.3 is transmitted to the injector 4.
．． The hopper 1 is accelerated and collides head-on through the hopper 1, and the powder raw material from the hopper 1 is pulverized and/or coated.

The upper side of the crushing zone A is connected to the classification zone via a crushed material passage 6 for raising the crushed material, and this classification zone B is connected to the opposed jet nozzle 3 via a coarse particle return passage 7.
It is connected to the jet stream passage in front of the.

The classification zone B separates fine powder and coarse powder by the swirling flow formed inside the classification zone B.The fine powder, which has been pulverized to a predetermined classification diameter, is produced as a pulverized product in the depths of the approximate center of the classification zone B. It is discharged out of the system from a side discharge port 8 through a discharge pipe 9, and is collected by a product recovery device or a dust collector (not shown). On the other hand, coarse particles that have not reached the desired classification diameter are sucked by the opposed jet nozzle 3 from the classification zone B through the coarse particle return passage 7, accelerated by the opposed injector 5, and crushed again in the crushing zone A. , sent to the classification zone. Such recycling of coarse particles is repeated until the coarse particles become fine powder with a classification diameter or less.

In this embodiment, both the injectors 4 and 5 have a unique structure. These injectors 4.5 have their inlet portions 4a, 5a, i.e. jet/) nozzles 2.
The side where the jet stream from 3 enters is tapered at an angle of 01, and the throat portions 4b and 5 continue to the inlet portion.
b has a throat diameter of the same diameter, and the diffuser portions 4c and 5c on the outlet side following the throat portion are expanded in cross-sectional shape into a curved line or a straight line at an expansion angle θ2.

The present inventors have focused on the fact that the throat length Ly, which is the axial length of the throws 14b and 5b, and the throat diameter D7, which is the diameter (inner diameter) thereof, play an extremely important role for grinding efficiency, etc. After various experiments,
Ratio between Lt and DT Lv/D? It has been found that a range of =0 to 5 gives very good results for grinding and coating efficiency. This L? For the ratio of /Dr, a smaller value within the range θ to 5 is better for particles with strong adhesion, and any large value can be used for powder and granules with weak adhesion, but for filtration exceeding 5, the jet stream The throat portions 4b and 5b act as resistance against this, making it impossible to obtain a sufficient acceleration effect.

Furthermore, it has been confirmed through experiments by the inventor that the expansion angle θ on the exit side of the definizer sections 4c and 5c also has a very large effect on the particle velocity. That is,
According to experiments conducted by the inventor, this expansion angle θ2 is 5
It is preferable that the expansion angle θ2 be in the range of ~11 degrees, and the volume to the crushing zone becomes large at the expansion angle θ2 within this range.

The jet stream ejected from 5b is sufficiently tensioned,
Further, the amount of residence in the crushing zone A increases and the residence time becomes sufficiently long, so that crushing and coating can be performed efficiently. On the other hand, if the expansion angle θ2 is smaller than 5, there will not be enough volume for the crushing zone and sufficient expansion of the jet will not be achieved, so it is difficult to achieve good crushing and coating as described above. I can't.

Further, when the expansion angle θ becomes a value larger than 11, the function of the diffuser sections 4c and 5c as accelerating sections becomes insufficient, and sufficient particle acceleration cannot be obtained. In addition, the value of the expansion angle θ2 is based on the adhesion of the powder, #5I
It is selected within the above range depending on factors such as crushability and classification diameter.

Furthermore, according to the inventor's experiments, the definizer section 4c,
It has been found that a good particle velocity can be obtained by setting the ratio La/Dr of the axial length of the tube 5c to the throat diameter D7 to a range of 10 to 30.

By using the injector 4.5 of this example,
The suction pulverization in the potential core of the jet nozzle 2.3 and the acceleration of the pulverized material in the injector 4.5 are performed smoothly, and the collision speed between the opposing jets that suspend the pulverized material is 2 to 4 times that of the conventional type. Therefore, the stress probability becomes high and the crushing efficiency improves.

In addition, in FIG. 2, the symbol lO indicates jet nozzle 2.
11 is a high pressure gas source for supplying high pressure gas to 11.
．． 12 is a high-pressure gas flow rate regulating valve, and 13.14 is a pressure gauge.

In the crushing and coating apparatus of this embodiment, a jet nozzle 15 as a return flow rate adjusting means is provided in the middle of the coarse particle return passage 7, and this jet nozzle 1
The high pressure gas to 5 is supplied from the high pressure gas source 10 to the flow rate regulating valve 1.
6.

By variably adjusting the flow rate of the high-pressure gas from this jet nozzle 15, the amount of coarse particles sucked into the coarse particle return passage 7 from the classification zone B can be adjusted, and good pulverization efficiency can be obtained. .

In other words, in order to increase the collision probability while keeping the stress probability of crushing efficiency constant, the ratio between the supply amount F and the return amount R is an extremely important factor. This ratio is constant depending on the amount of gas sucked in and the proportion of coarse particles larger than the classification diameter, so in conventional opposed jet mills, the grinding efficiency reaches a ceiling at a certain point. In this embodiment, in order to improve this phenomenon, the jet nozzle 15 is used as a return flow rate adjusting means, and the optimum R/F can be selected, so that the crushing efficiency is improved. According to experiments, results have been obtained in the case of the present invention in which the throughput is increased by 1.5 to 2 times under the same crushing conditions as in the conventional type.

As this return flow rate adjusting means, in addition to the jet nozzle 15 described above, various means as shown in FIGS. 3 to 6 can be used.

In the embodiment shown in FIG. 3, a ring nozzle 17 is provided in the coarse particle return passage 70 as a return flow rate adjusting means, and a high pressure gas source 18 is provided.
By controlling the flow rate of high-pressure gas supplied to the ring nozzle 17 via the flow rate adjustment valve 19, an optimum powder return flow rate is ensured.

The embodiment shown in FIG. 4 uses a rotatable damper member 20 as the return flow rate adjusting means. This damper member 2
0 can adjust the return flow rate of the granular material returned via the return path 7 by rotating it by a desired angle using an actuator such as a motor (not shown).

In the embodiment shown in FIG. 5, as the return flow rate adjusting means,
For example, an expansion member 21 made of rubber or resin is used to variably adjust the return flow rate through the coarse particle return passage 7 by expanding and contracting the expansion member 21.

The embodiment of FIG. 6 uses a Neufle plate 22 as a return flow rate adjustment means, and this buckle plate 22 is connected to the high pressure gas source 1.
By being oscillated by the air cylinder 24 from 8 through the electromagnetic valve 23, the flow rate of the powder returned to the coarse particle return passage 7 can be adjusted.

Furthermore, in this embodiment, the discharge port 8 of the classification zone B
The structure is also uniquely designed.

That is, during the pulverization coating, the airflow containing the pulverized material flowing from the pulverization zone 8 through the pulverized material passage 6 flows as a swirling flow within the classification zone B, and the particles dispersed in the airflow flow in this swirling flow. The state of movement is determined by the balance between the centrifugal force F caused by this and the force generated by the inward flow toward the discharge pipe 9.

In this case, if the particles are coarse, F>D, and the classification zone B
The supplied particles jump out further and move along the peripheral wall of the classification chamber, are suctioned and pulverized by the opposed jet nozzle 3 from the coarse particle return passage 7, are accelerated by the opposed injector 5, and are further accelerated by the supply jet nozzle 2 and the supply injector 4. The particles are collided with the particles in the grinding zone 8 to perform grinding. On the other hand, fine particles become FED and are taken out of the system as a pulverized product through a discharge pipe 9 connected to a discharge port 8 of the classification zone B. In this case, the particle size of F=D is called the cut size. In this way, the classification diameter is determined by the velocity of the swirling flow and the velocity ratio of the inward flow, and this classification diameter is proportional to the square root of the ratio of the diameter of the discharge pipe 9 and the diameter of the classification zone B. It is known that it becomes smaller.

As described above, the ratio D − /D + of the discharge pipe diameter and the classification zone diameter Dl is an important factor that defines the classification diameter, and the smaller D − /D I is, the smaller the classification diameter is.

In order to perform efficient pulverization when you want to obtain submicron-level pulverized material, D-/D+ should be set to 0 to improve classification performance.
．． I would like to reduce /D to about 2 to 0.15, but with a conventional opposed jet mill, /D is 0.2 to 0.1.
If it is set to 5, the air resistance in this part becomes large, air and powder are ejected from the hopper part 5 of the jet mill, and the operation cannot be continued.

Therefore, in this embodiment, as shown in FIG.
A discharge port member 25 is provided at the inlet of a discharge pipe 90 connected to the discharge pipe 90. The artificial part 25a of the discharge port member 25 is formed into a curved surface or a combination of straight lines to make the classification vortex close to the Archimedean vortex suitable for the purpose of classification, and a cylindrical part 25b is provided in the middle. In order to recover the energy of the jet stream at that part, the enlarged part has an enlarged angle θ3. By providing the outlet member 25 with this structure, it is possible to operate even when the D-/D+ ratio is reduced to about 0.15, and a clean classification vortex is created, making it possible to efficiently produce submicron particles. can.

As described above, according to the crushing and coating apparatus of this embodiment, the structure of the injector 4.5 is Lt/Dv
= 0 to 5, expansion angle θ of differential sizer parts 4c and 5c,
By setting Ld/Dr to 5 to 11 degrees and Ld/Dr to 10 to 30, the collision speed at the time of opposite collision increases, improving the crushing efficiency and eliminating the adhesion of adhesive powder during crushing, resulting in improved abrasiveness. Powder contamination is also reduced.

Further, since the powder concentration in the grinding zone A can be appropriately maintained by the return flow rate adjusting means of the coarse particle return passage 7, efficient grinding and coating can be performed.

Furthermore, by making the shape of the classification outlet 8, that is, the inlet shape of the discharge pipe 9 connected to the outlet 8, suitable for the flow in the classification zone B, the diameter of the classification can be made smaller, making it more efficient. It is possible to create and coat submicron particles.

Additionally, the synergistic effect of combining these improvements results in smoother grinding and coating of adherent, abrasive powders and significantly greater grinding capacity for the same energy compared to traditional opposed jet mills. We are seeing improved results. Furthermore, it has become possible to efficiently grind and coat ultrafine particles of 3 μm or less, which have conventionally been said to be difficult to grind.

However, the present invention is not limited to the embodiments described above, but can be applied to the inclination, shape, and position of the passages of the classification zone and the grinding zone when vertical opposed jet mills are arranged horizontally. It can also be applied when changing etc.

In addition to metals such as iron and stainless steel, the materials of the nozzle, injector, discharge port, and main body of the device include rubber, plastic, and their linings, alumina,
It may be made of a wear-resistant material such as corundum, silicon carbide, silicon nitride, or zirconia.

〔Example〕

The inventors have created various test injectors 4 as shown in FIG. The structural dimensions of each part are shown in Table 1. Using this test injector 4, the influence on the particle velocity at the outlet of the differentializer was confirmed through experiments.

Glass bulbs of 50 μm and 80 μm were used as samples, and compressed air with a nozzle pressure of 4 kg/c++f G and an air temperature of 1.8 to 21°C was ejected from the jet nozzle 2, and the transparent tube section at the outlet of the definizer section 4C was Photographs were taken using the instantaneous flash method, and particle velocities were determined from particle trajectories.

Figure 10 shows the investigation of the throat length/throat diameter (Lt/D7) and the particle velocity at the exit of the diffuser section 4c. It was found that these conditions maximize the speed.

Figure 11 shows the ratio of differential equalizer length/throat diameter.

/D7 shows the relationship between particle velocity and L, /D =
It has been found that a range of 10 to 30 is the maximum particle velocity (collision velocity).

Figure 12 shows the relationship between the expansion angle θ2 of the diffuser section 4C and the particle velocity at the diffuser outlet, where θ2 = 5 to 1.
It was found that the range of 1 degree is the condition that maximizes the particle velocity (collision velocity).

〔Effect of the invention〕

According to the configuration of the present invention as described above, the following effects can be obtained.

(1) Even if the adhesive powder is crushed at room temperature, there is almost no adhesion to the injector part, so ultrafine particles can be efficiently crushed and/or coated.

[2) Since there is almost no contamination due to polishing, a finely pulverized product and/or a finely coated product of good quality can be obtained.

(3) Since the pulverization efficiency is high and the classification point can be shifted to small particle sizes, submicron-level pulverized products and/or coated products thereof can be easily obtained.

These effects have made it possible to grind ultrafine powders of pharmaceuticals, foods, waxes, etc., which have relatively low melting points and avoid contamination, and to coat them.

Also, synthetic resins, toners, and magnetic materials that are difficult to crush.

It has become possible to efficiently grind mineral materials, ceramics, etc. into ultrafine powder.

[Brief explanation of the drawing]

FIG. 1 is an enlarged partial cross-sectional view of the main parts of an opposed jet mill type crushing and coating device that is an embodiment of the present invention, FIG. 2 is a schematic vertical cross-sectional view of the overall structure, and FIGS.
Fig. 6 is a partial explanatory diagram of one embodiment of the coarse particle return passage, Fig. 7 is a schematic cross-sectional view taken along the line ■-■ in Fig. 2, and Fig. 8 is an enlarged partial cross-section of one embodiment of the product discharge port. 9 are schematic illustrations of the injector section of the device of the present invention, and FIGS. 10 to 12 are graphs showing experiments and results conducted using the injector structure of FIG. 9. 1. Pa... hopper (powder supply source), ゛ 2.
...Supply jet □ nozzle, 3... Comparative jet nozzle, 4... Supply injector, 4a, 5a... Population section, 4b, 5b... Throat section, 4c, 5c... Differential unit, 5... Opposing injector, 6... Crushed material passage, 7... Coarse particle return passage, 8... Discharge port, 9 ... Discharge pipe, 10 ... High pressure gas source, 11.12 ... Flow rate adjustment valve, 13.14 ... Pressure gauge, 15 ... Jet nozzle, 16 ...Flow rate adjustment valve 17 ... Ring nozzle, 18 ... High pressure gas source, 19 ... Flow 1 adjustment valve, 20 ... Damper Member, 21... Expansion member, 22... Baffle plate, 23... Solenoid valve, 24... Air cylinder, 25... Exhaust Outlet member, 25a...Population part, 25b...Cylindrical part, 25c...Outlet part, A...Crushing zone, B......Classification zone. Patent Applicant Freund Sangyo Co., Ltd. Agent Patent Attorney Dai Tsutsui Sodo Patent Attorney Hide Matsukura Sanema Patent Attorney Toshio Nakano Figure 2 Figure 3 Figure 4 Figure 5 Figure 9 ↓ Figure 10 LT/D Figure 7 Figure 8 Figure 11 Figure 12

Claims (1)

[Claims] 1. A pair of jet nozzles facing each other and arranged on a coaxial line; a throat portion located on the axis of the jet nozzles between the two jet nozzles and through which high-pressure gas passes and accelerates; A pair of injectors having a diffuser portion, a crushing zone formed between both injectors, in which powder and granules are crushed and coated, and connected to the crushing zone via a crushed material passage and a coarse particle return passage. a high-pressure gas source that supplies high-pressure gas to the jet nozzle, and a powder supply source that supplies powder and granule raw material along with a jet stream from at least one of the jet nozzles, The ratio L_r/D_r of the throat length L_r and the throat diameter D_r of the throat portion of the injector is 0 to 5, and the diffuser expansion angle θ_2 on the high pressure gas ejection side of the diffuser portion is 5 to 11 degrees. Features grinding and coating equipment. 2. The crushing and coating apparatus according to claim 1, wherein the ratio L_d/D_r of the diffuser length L_d of the diffuser section to the throat diameter D_r is set to 10 to 30.