CN109570517B - Design method of supersonic laval nozzle structure alloy melt atomizer - Google Patents

Abstract

A design method of an alloy melt atomizer with a supersonic Laval nozzle structure belongs to the field of alloy powder atomization preparation. The invention designs an ultrasonic laval nozzle structure atomizer by researching an ultrasonic laval wall surface curve, combining the characteristics of tight coupling atomization and according to a suction pressure criterion, a tight coupling criterion and a flow field speed criterion, and utilizes computational fluid dynamics software to simulate and research the influence of five main factors such as atomization air pressure, atomization gas temperature, gas nozzle hole center distance, gas injection angle, guide pipe extension length and the like on a flow field structure. And further, the structural parameters of the nozzle can be optimized, the phenomena of gas backflow and back spray are avoided, and the atomization efficiency is improved while the high-quality alloy powder production is guaranteed. The atomizer can prevent the occurrence of phenomena of blockage of a flow guide pipe, gas back-spraying and the like in the atomizer, reduce the generation of shock waves and turbulent flow, reduce the flying distance of gas, improve the speed of a flow field and reduce the loss of energy.

Description

Design method of supersonic laval nozzle structure alloy melt atomizer

Technical Field

The invention belongs to the technical field of alloy powder atomization preparation, and particularly relates to a design method of an alloy melt atomizer with a supersonic Laval nozzle structure.

Background

The gas atomization method is an important method for preparing high-performance metal and alloy powder, and compared with the traditional grinding and crushing method and the electrochemical method, the gas atomization method mainly has the advantages of narrow particle size distribution, high cooling rate, high powder sphericity, low impurity content and the like; the gas atomization method has wide applicability, and is suitable for preparing most metals and alloy powder except refractory metals such as tungsten, molybdenum and the like and very active metals. In recent years, more and more researches on gas atomization are carried out, and mainly the researches on an atomization flow field structure, powder granularity influence factors and melt crushing behaviors are focused. For example, after a fixed-width annular slit nozzle is researched, two typical flow field structures, namely "open vortex" and "closed vortex" exist in a flow field, and the two flow field structures are converted according to the structural parameters of the nozzle and the pressure of atomizing gas. As another example of simulating the interaction between the annular slot atomizing gas and the melt, it was found that the columnar melt moved along the nozzle centerline, the annular high velocity gas stream created a disturbance at the melt surface and pushed it downstream to form an unstable liquid layer, and the edges of the unstable melt stream broke up into small liquid bands, creating secondary fractures. At present, the flow field of the slit nozzle is researched more at home and abroad, while the flow field of the supersonic nozzle atomizing nozzle is researched less, and the system depth is not enough. The relevant patents are based primarily on qualitative improvements in nozzle design to address specific problems, and do not address quantitative design of critical structural dimensions of atomizer nozzles based on fundamental principles and problem oriented design criteria and rational simulation techniques. The supersonic nozzle structure atomizer is designed according to a suction pressure criterion, a tight coupling criterion and a flow field velocity criterion by combining the characteristics of a supersonic Laval wall surface curve and tight coupling atomization. And simulating and researching the flow field structures under different influence factors by using computational fluid mechanics software, and optimizing and designing the structural parameters of the nozzle.

Disclosure of Invention

The invention aims to provide a new method for designing an alloy atomizer, which is used for designing an alloy powder atomizer, can prevent the phenomena of blockage of a flow guide pipe, gas back-spraying and the like in the atomizer, and combines a Laval contraction-expansion type gas injection pipeline, wherein the pipe walls are connected by using a smooth curve, and after gas flow is sprayed out from a nozzle, the Prandtl-Meyer wave is not generated, so that the generation of shock waves and turbulent flow is reduced, the flying distance of the gas can be reduced to the maximum extent, the flow field speed is increased, and the energy loss is reduced.

A design method of an alloy melt atomizer with a supersonic laval nozzle structure is characterized in that effects of atomization air pressure, atomization air temperature, air nozzle hole center distance, air injection angle and guide pipe extension length on a flow field structure are simulated and researched by utilizing computational fluid dynamics software through researching a supersonic laval wall surface curve and combining with the characteristics of tight coupling atomization according to a suction pressure criterion, a tight coupling criterion and a flow field speed criterion, nozzle structure parameters are optimally designed, air backflow and back injection are avoided, atomization efficiency and powder making quality are improved, in a cross section of the gas nozzle through the axis of a nozzle center hole, the extension line of the center line of a laval curve circumferential weld channel is intersected with the axis of the guide pipe center hole to form an included angle α which is the air injection angle, and α is 0-60 degrees.

Further, in a cross-sectional view of the gas nozzle through the central hole axis of the nozzle, the ratio of the width of the curve of the convergent section AB to the axial length is 1:1 to 1: 4.

Further, in the section of the gas nozzle passing through the central hole axis of the nozzle, the minimum axial width part of the expansion part where the expansion curves BC and CD are located is the throat diameter A0, the outlet is the outlet diameter A2, wherein A2/A0 is determined by the Mach number required by design, and A2/A0 value is 4:1-16: 1.

Furthermore, the extending length of the draft tube is the extending length of the outlet of the draft tube relative to the gas outlet platform, and the length value is 0-20 mm. The inner diameter of the draft tube is 4-12mm, and the tail end of the draft tube is round or square.

Further, the center-to-center distance of the gas nozzles is the center-to-center distance formed by the centers of the gas outlet circular seams, and the value of the center-to-center distance is 20-80 mm.

Furthermore, the supersonic annular seam nozzle atomized powder production designed by the invention has the main process parameters and ranges as follows: the pressure of the atomizing gas is 1-10MPa, the temperature of the atomizing gas is 27-500 ℃, the gas is nitrogen, argon, air and the like, the superheat degree of the melt is 0-400 ℃, and the melt flow is 0.05-1.00 Kg/min.

In order to achieve the above objects, the design criteria and technical solutions proposed by the present invention are as follows.

1. Design criteria for atomizers

(1) Suction pressure criteria

When the molten metal flows out of the draft tube, the molten metal is acted by gravity and pressure generated by pressure difference between the outlet of the draft tube and the metal liquid level. This pressure difference is defined as the suction pressure, i.e. the difference between the static pressure of the gas at the outlet of the draft tube and the ambient pressure of the crucible level. When the suction pressure is less than zero, the molten metal is sucked into the atomizer in an accelerated manner, which is favorable for the smooth proceeding of the atomization process. When the suction pressure is greater than zero, the flow rate of the molten metal flowing out of the flow guide pipe is slowed down. When the suction pressure is equal to the melt, and the suction pressure is equal to the melt gravity, the melt is stressed in balance, the flow rate is zero, the melt cannot flow out, and the atomization process is interrupted; when the static pressure of the gas at the outlet of the draft tube is further increased, the atomized gas may even flow back into the crucible through the draft tube, resulting in bubbling or back-spraying. The suction pressure depends on the static pressure at the nozzle of the flow guide pipe, so that the atomization process is carried out smoothly, the pressure at the nozzle of the flow guide pipe is controlled to be less than the standard atmospheric pressure, and the melt flow cannot be hindered. The critical atomization pressure is the atomization pressure corresponding to when the flow rate of the molten metal is zero, and is the maximum atomization pressure at which the atomization process is kept smooth. The suction pressure criterion is to ensure the atomization process to be carried out smoothly, and the suction pressure criterion should be satisfied when selecting atomization parameters such as gas atomization pressure, atomization gas temperature, nozzle hole center distance, spray angle, guide pipe extension length and the like, so that the atomization process can be carried out smoothly.

(2) Tight coupling design rule

On the premise of complying with the suction pressure criterion, a tight coupling design criterion is provided for optimizing the structural design of the nozzle, and the criterion is as follows: under the condition of ensuring that the pumping pressure is less than zero, the atomizing gas is acted into the metal melt in the shortest distance as far as possible. Compared with a common nozzle, the principle greatly shortens the flying distance of the atomizing gas, reduces the loss of gas kinetic energy, ensures that a melt is crushed more sufficiently, and reduces the median particle size and the standard deviation of particle size distribution of the atomized powder.

(3) Flow field velocity criterion

On the premise of following a suction pressure criterion and a tight coupling design criterion, a nozzle structure capable of obtaining a larger flow field speed is selected to obtain the larger flow field speed, so that the melt is favorably and fully crushed to obtain finer powder.

2. Atomizer design technical scheme

(1) Laval nozzle design

The gas nozzle of the atomizer of the invention adopts a Laval nozzle with a contraction-expansion structure. The nozzle structure can accelerate gas to supersonic velocity to obtain high-speed airflow parallel to the outlet, expansion compression shock waves are not generated in the airflow, energy loss and turbulent flow are reduced, and a uniform and concentrated gas flow field is obtained. The inner wall of the spray pipe is formed by two Laval curves which are symmetrical about the central axis of the spray pipe, and the central axis of the gas nozzle and the central axis of the guide pipe form a certain angle inclination, namely a gas injection angle. The laval curve consists of a contraction part AB, an expansion part BC and a CD section, wherein the point B is a throat part, and the inclined plane connecting the flow guide pipe and the gas nozzle is a side surface formed by extending the tangent line of the outlet of the gas nozzle. The annular seam-shaped gas spray pipe is formed by an inclined curved surface formed by a Laval curve rotating 360 degrees around the central axis of the draft tube. In the invention, a contraction section curve AB of the Laval curve is determined by adopting a quintic curve, and expansion section curves BC and CD are determined by adopting an analytical method based on characteristic line design. The expansion segment BCD utilizes an analytical method to determine a curve so as to enable the curve to be smoothly connected, and the analytical method can divide the expansion curve into a BC section and a CD section. According to the invention, inert gases such as nitrogen and the like are used as atomizing gases, and the corresponding ratio of the outlet area to the throat area A2/A0 is designed according to the Mach number of an outlet and the type of the gases and by combining an isentropic flow formula. The specific structure of the laval nozzle is shown in fig. 1 and 3.

(2) Software simulation setup

The simulation of the atomizing flow field structure is completed by commercial computational fluid dynamics software ANSYS FLUENT, a k-epsilon model is selected as a turbulence model for flow field simulation, argon, nitrogen and air can be used as atomizing gas, the density of the gas is set to be in an ideal gas state, the viscosity is calculated by a Sutherland formula, and a Coupled solver is selected because the flow of the atomizing gas is a high-speed compressible problem. The distribution information of the atomizing gas flow field can be obtained according to the simulation setting, the flow field distribution rule and the characteristics of the reflux area are analyzed, and the optimized atomizer structure and the optimized technological parameter range are obtained.

3. Effect of Primary parameters and Primary Structure sizing

Atomization pressure is a major factor affecting the gas atomization process, which not only affects the gas kinetic energy, but also indirectly affects the mass flow ratio and powder surface finish. The kinetic energy of the atomized gas is in direct proportion to the temperature, the gas temperature is increased, the flow field speed of the gas can be obviously increased, and the yield of fine powder is increased. The nozzle hole center distance is the diameter of the outlet of the annular Laval nozzle. The spraying angle is the included angle between the axis of the outlet of the Laval nozzle and the axis of the guide pipe. The length of the draft tube extension is the distance that the draft tube outlet protrudes relative to the laval nozzle outlet horizontal plane. The center distance of the nozzle hole and the spraying angle. The spraying angle and the extension length of the flow guide pipe are closely related to the static pressure and the suction pressure of the backflow area, and the structure of the backflow area is directly influenced.

The influence of five main factors, namely atomizing air pressure, atomizing air temperature, gas nozzle hole center distance, gas spraying angle and honeycomb duct extension length, on the flow field structure is simulated and researched by utilizing computational fluid dynamics software. The following are found: (1) the typical flow field structure of the supersonic nozzle is different from that of the traditional nozzle, expansion waves are not formed after the air flow is sprayed out of an air flow outlet, and a closed vortex flow field structure is not formed; a typical conical gas recirculation zone occurs below the draft tube, and the temperature of the flow field will drop as the gas expands and accelerates. The flow field structure under different pressures is very similar, but the flow field velocity increases with the increase of the air pressure, the area increases, and the static pressure of the flow guide pipe keeps a lower value firstly with the increase of the pressure and then gradually rises. (2) The kinetic energy of the atomizing gas is related to the temperature, the velocity of the flow field can be linearly increased by increasing the gas temperature, and meanwhile, the flow field temperature is increased, so that the method is an important method for refining the powder granularity. (3) Along with the increase of the hole center distance of the gas nozzles, the area of the backflow area is gradually increased, the flow field speed is reduced, and the static pressure at the outlet of the flow guide pipe is also gradually reduced. (4) The gas jet angle can obviously influence the flow field structure, the velocity of the flow field is increased along with the increase of the angle, and the reflection action is enhanced after the gas meets, so that strong reflected waves and a Mach disk are formed. However, when the angle is increased, the static pressure at the outlet of the draft tube is increased, the suction pressure is reduced, and the critical atomization pressure is reduced. Therefore, the gas injection angle should not be too large, and is preferably controlled to be about 30 degrees. (5) The extension length of the flow guide pipe is also an important factor influencing the flow field structure, the flow field speed is slightly increased along with the increase of the extension length, the static pressure in a backflow area has a change trend of first decreasing and then increasing, the minimum value is reached when the static pressure is 6-8mm, and the extension is controlled in the range, so that the larger suction pressure is favorably obtained.

Through simulation calculation and optimization, the finally given main atomization parameter range is as follows: the pressure of atomizing gas is 1-10 MPa; the temperature of atomizing gas is 27-500 ℃; the gas is nitrogen, argon, air, etc.; the superheat degree of the melt is 0-400 ℃; the melt flow is 0.05-1.00 Kg/min. The final major structural size ranges given are: the gas injection angle is 0-60 degrees; the distance between the centers of the holes of the atomizer is 10-80 mm; the extending length of the draft tube is 0-20mm, the inner diameter of the draft tube is 4-12mm, and the tail end of the draft tube can be round, square and the like; the ratio of the width of the contraction section of the Laval nozzle to the axial length is 1:1-1: 4; the ratio of the outlet diameter of the Laval nozzle to the diameter of the throat is determined by the Mach number required by the design and is 4:1-12: 1.

The invention designs an ultrasonic laval nozzle structure atomizer by researching an ultrasonic laval wall surface curve, combining the characteristics of tight coupling atomization and according to a suction pressure criterion, a tight coupling criterion and a flow field speed criterion, and utilizes computational fluid dynamics software to simulate and research the influence of five main factors such as atomization air pressure, atomization gas temperature, gas nozzle hole center distance, gas injection angle, guide pipe extension length and the like on a flow field structure. And further, the structural parameters of the nozzle can be optimized, the phenomena of gas backflow and back spray are avoided, and the atomization efficiency is improved while the high-quality alloy powder production is guaranteed. The alloy powder atomizer designed by the method can prevent the phenomena of blockage of a flow guide pipe, back spraying of gas and the like in the atomizer, and the method combines a Laval contraction-expansion type gas spraying pipeline, the pipe wall is connected by using a smooth curve, the gas flow can be accelerated to a supersonic speed state to obtain high-speed gas flow with a parallel outlet, and the gas flow does not generate Prandtl-Meyer waves after being sprayed out from a nozzle, so that the generation of shock waves and turbulent flow is reduced, the flying distance of the gas can be reduced to the maximum extent, the flow field speed is increased, and the energy loss is reduced.

Drawings

FIG. 1 is a drawing showing a wall curve of a Laval nozzle

Wherein: the curve AB is a gas contraction section curve; r1 is the constrictor inlet radius; l1 is the axial length of the constrictor; point B is the throat part of the Laval nozzle; r0 is the throat radius; curves BC and CD are expansion curves; the point C is an expansion curve connecting point and is a curve inflection point, and the two curves are smoothly connected; l2 is the axial length of the expanded section; r2 is the diverging section exit radius; r1 and r2 are radii of analytical curves; theta is an included angle corresponding to the arc length.

FIG. 2 is a schematic view of an atomizing nozzle

The device comprises an atomizer fixing hole 1, an annular gas storage cavity 2, a flow guide pipe 3, a tangential gas inlet 4, a Laval gas spray pipe 5, a gas nozzle hole center distance D, a gas injection angle α and a flow guide pipe extending length h.

FIG. 3 is an enlarged view of a Laval nozzle section of the atomizer

The alphabetical meaning is the same as in fig. 1.

Detailed Description

Simulation calculation shows that when the atomization pressure is 1.7MPa, the gas does not expand after being sprayed when the atomization pressure is low, and only a small amount of expansion occurs after the gas outlet when the atomization pressure is increased to 3.2 MPa. The increase in the pressure of the atomizing gas gradually increases the velocity of the flow field from 615m/s to 674m/s, but the structure of the gas flow field is basically similar. Unlike conventional gas lances, laval lance atomizers do not exhibit the so-called "closed vortex structure" when the pressure increases. This is because due to the special structure of the converging-diverging laval tube, the gas has been fully expanded and accelerated in the tube, and the high-speed gas is ejected tangentially along the outlet, and no longer forms a periodic expansion-compression wave. When the atomization pressure is lower, the static pressure change in the reflux area is smaller and is kept at a lower value of 70-80KPa, when the atomization pressure reaches 8MPa, the static pressure at the outlet of the draft tube reaches 110KPa, and the critical atomization pressure can be greatly improved due to the expansion characteristic of the Laval nozzle. Due to the Joule-Thomson effect, the temperature of the gas is reduced after expansion along with the gradual acceleration of the gas, the lowest temperature is reduced along with the increase of the atomization gas pressure, and when the atomization gas pressure is increased from 1.7MPa to 3.2MPa, the temperature of a flow field is reduced from 108.4K to 70.5K. The pressure and temperature in the flow field are opposite to the velocity distribution trend of the gas in the flow field, namely, the corresponding temperature and pressure are lower in the area with higher velocity. An inverted cone-shaped gas reflux area with negative speed is arranged below the flow guide pipe, the maximum speed in the reflux area can reach 250m/s, and the turbulent flow structure is formed by the reverse reflux of subsonic gas at the edge of supersonic gas flow under the action of pressure difference.

The temperature T is increased from 300K to 400K, the maximum speed of the flow field is increased by 100m/s, the amplification is 15.3%, the flow field speed and the gas temperature are almost linearly increased, and the temperature of the flow field is also increased. Thus, for the same mass flow rate of the melt, a greater gas kinetic energy can be obtained by increasing the atomizing gas temperature, reducing the average particle size of the atomized powder. The change of the gas temperature does not cause the change of the atomization flow field structure, and only influences the size of the flow field speed and the temperature.

When the hole center distances of the gas nozzles are respectively 10.4mm and 19.4mm, the maximum speed of the flow field is reduced along with the increase of the hole center distances of the nozzles, the maximum speed is reduced from 645m/s to 623m/s, the speed in the reflux area is increased, the range is obviously enlarged, and the area of the speed flow field is also enlarged. The hole center distance D of the gas nozzles can obviously influence the static pressure at the outlet of the flow guide pipe, the static pressure of the backflow area is gradually reduced along with the increase of the hole center distance of the gas nozzles, and when the value is increased to 19.4mm, the static pressure is not reduced continuously when the static pressure is reduced to about 69KPa, and the static pressure tends to be stable. In the aspect of promoting the smooth outflow of the melt, the increase of the hole center distance of the nozzle is beneficial to increasing the suction pressure and promoting the outflow of the melt; from the viewpoint of improving the crushing efficiency of atomized melts, the reduction of the flow field speed and the increase of the flow field area are not beneficial to the rapid and concentrated action of gas energy on the melts, so the distance between the centers of the gas nozzles should be controlled not to be too small or too large.

Along with the increase of the gas injection angle, the reflection effect is enhanced after the high-speed gas meets the lower part of the draft tube, and the backflow area is compressed to form a stronger reflected wave and a Mach disk. The velocity of the flow field also increases with increasing angle from 618m/s to 635 m/s. The static pressure in the recirculation zone increases significantly with increasing gas injection angle and as the injection angle increases to 35 and 40, the static pressure in the recirculation zone reaches 116.7KPa and 175.4KPa, respectively, above a standard atmospheric pressure at which melt flow is impeded. Therefore, in order to enable the fluid to be smoothly remained and enable the gas to better act on the melt in a centralized way, the gas injection angle is properly controlled to be about 30 degrees after comprehensive analysis.

Increasing the length of the draft tube extension compresses the size of the recirculation zone. When the extension length is increased from 0mm to 12mm, the speed of the flow field is slightly increased from 618m/s to 633m/s, and the influence on the whole flow field structure is small. The length of the flow guide pipe mainly influences the pressure distribution in the backflow area, and along with the continuous increase of the length of the flow guide pipe, the static pressure in the backflow area tends to decrease firstly and then increase under different atomization pressures. The greater the atomization pressure, the greater the magnitude of the drop. When the atomization air pressure is 3.6MPa, the pressure in the reflux area is reduced to 53.8KPa from 117.3KPa when the extension length of the guide pipe is 0mm, and the reduction amplitude reaches 54.1%. Therefore, the static pressure in the reflux area can be obviously reduced by increasing the extension of the guide pipe to a certain extent, and higher suction pressure is obtained.

Claims (2)

1. A design method of an alloy melt atomizer with a supersonic laval nozzle structure is characterized in that effects of atomization air pressure, atomization air temperature, air nozzle hole center distance, air injection angle and draft tube extension length on a flow field structure are simulated and researched by utilizing computational fluid dynamics software through researching a supersonic laval wall surface curve and combining with the characteristics of tight coupling atomization according to a suction pressure criterion, a tight coupling criterion and a flow field speed criterion, nozzle structure parameters are optimally designed, air backflow and back injection are avoided, and atomization efficiency and powder making quality are improved;

in the section view of the gas nozzle passing through the axis of the central hole of the nozzle, the ratio of the width of the curve AB of the contraction section to the axial length is 1:1-1: 4;

in the section of the gas nozzle passing through the axis of the central hole of the nozzle, the minimum part of the axial width of the expansion part where expansion curves BC and CD are located is the throat diameter A0, and the outlet is the outlet diameter A2, wherein A2/A0 is determined by the Mach number required by design, and A2/A0 value is 4:1-16: 1;

the extending length of the draft tube is the extending length of the outlet of the draft tube relative to the gas outlet platform, and the length value is 0-20 mm; the inner diameter of the draft tube is 4-12mm, and the tail end of the draft tube is round or square;

the gas nozzle hole center distance represents the hole center distance formed by the centers of the gas outlet circular seams, and the value of the hole center distance is 20-80 mm.

2. The design method of the supersonic laval nozzle structure alloy melt atomizer according to claim 1, wherein the designed supersonic annular seam nozzle atomization milling has the main process parameters and ranges as follows: the pressure of the atomizing gas is 1-10MPa, the temperature of the atomizing gas is 27-500 ℃, the gas types are nitrogen, argon and air, the superheat degree of the melt is 0-400 ℃, and the melt flow is 0.05-1.00 Kg/min.

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