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Formation mechanism of the pressure zone at the tip of the melt delivery tube during the spray forming process
Journal of Materials Processing Technology 137 (2003) 5–9
Formation mechanism of the pressure zone at the tip of the melt delivery tube during the spray forming process Chengsong Cui*, Fuyang Cao, Qingchun Li Harbin Institute of Technology, Heilongjiang Province, Harbin 150001, PR China
Abstract The gas pressures at the tip of the melt delivery tube in two typical atomizers are measured in this work. Gas dynamics is used to study the gas flow and pressure zone at the tip of the delivery tube, and a mechanism for the formation of pressurization and aspiration is proposed. The results show that the atomization efficiency of an atomizer with a Laval nozzle is superior to that of an atomizer with a converging nozzle. The gas jet expands strongly at the nozzle exit of a converging atomizer, while the Prandtl–Meyer angle of the jet is very small for a Laval nozzle. When the jet expands strongly, pressurization is likely to form at the tip of delivery tube. Aspiration is formed when the jet pressure at the exit of nozzle is equal to or smaller than the back pressure. # 2002 Published by Elsevier Science B.V. Keywords: Atomization; Melt delivery tube; Pressurization
1. Introduction Spray forming is an advanced processing technique of materials, which is capable of manufacturing near-net-shape products in the form of billets, tubes and sheets in a single, integrated operation [1]. Gas atomization of alloy melt is one of the crucial stages of this processing. For example, the gas pressure at the tip of the melt delivery tube may be markedly different under various processing conditions, i.e., pressurization or aspiration are both likely to be present in spray forming. Aspiration is favorable for drawing out the metal melt through the delivery tube, while pressurization blocks the flowing down of the melt. Therefore, investigation on the formation mechanism of gas pressure at the tip of the delivery tube is very important for controlling the spray forming process. In this paper, the gas pressure at the tip of delivery tube of two typical atomizers are measured, and the formation mechanism of pressurization and aspiration is proposed.
2. Experimental The pressure at the tip of the delivery tube during the practical atomization process is difficult to measure. Two types of atomizer with typical nozzle configuration (denoted *
Corresponding author.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 1 - 8
as Atomizer A and Atomizer B) were used for the simulation experiment in an open system without metal melt. The gas nozzle in Atomizer A is of converging structure whilst the nozzle in Atomizer B is of Laval structure. The configuration illustration of these atomizers is shown in Fig. 1, in which y is the jet impinging angle and d is the protrusion length of the delivery tube. Nitrogen was used as the atomization gas. The inner diameter of the delivery tube was 3.8 mm. The gas pressure at the tip of delivery tube was measured by means of a mercury manometer as shown in Fig. 2.
3. Experimental results Fig. 3(a) shows the pressure difference DP ðDP ¼ PE Pb , where PE is the gas jet pressure at the nozzle exit of the atomizer, and Pb is back pressure) at the tip of the delivery tube for Atomizer A as a function of atomization pressure Pa. It can be seen that the pressure at the tip of the delivery tube changes with the atomization pressure in different ways for various protrusion lengths d of the delivery tube. When d is shorter, the negative pressure at tip is smaller, while the delivery tube does not protrude at all ðd ¼ 0Þ, positive pressure is prone to be present. At the same time, the higher is the atomization gas pressure, the higher is the gas pressure at the tip. The effects of the configuration of the delivery tube on the tip pressure are shown in Fig. 3(b) and (c). The larger is the jet impinging angle, the greater is
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Fig. 1. Configuration of Atomizer A and Atomizer B.
Fig. 2. Set-up for gas dynamic characteristics.
the possibility of the presence of positive pressure. For a delivery tube with an impinging angle of 908 and 1208, it is almost impossible to form negative pressure when the protrusion length of the delivery tube is less than 4 mm and the atomization pressure is higher than 1.5 MPa. Fig. 4 shows the pressure at the tip of the delivery tube in Atomizer A as a function of protrusion length. It is obvious that positive pressure appears at the tip of the delivery tube when the atomization pressure is higher than 1.0 MPa and the protrusion length is less than 4.0 mm. In the gas atomization process, the pressure at the tip of the delivery tube has a great influence on the atomization of the metal melt. A reasonably designed atomizer should ensure that negative pressure is always present at the tip of the delivery tube for an atomization pressure within a wide range (0–3.0 MPa). However, the value of negative pressure should not be too large, since it may make the breaking effect of metal melt worse with too high a metal flow rate. It can be seen from Fig. 4 that the delivery tube in Atomizer A has to protrude to some extent in order to meet the above requirements: otherwise, if positive pressure is formed, the atomization process is difficult to perform. When the tube protrudes, the atomization gas jets will firstly impact on the outer surface of the tube and deflect, which will inevitably cause the loss of a portion of the gas kinetic energy and reduce the atomization efficiency. Especially for an atomizer with a larger jet impinging angle, the longer is the protrusion length, the greater is the loss of kinetic energy. The pressure change with atomization pressure at the tip of the delivery tube in Atomizer B is shown in Fig. 5. It is
Fig. 3. Pressure at the tip of the delivery tube in Atomizer A as a function of atomization pressure: (a) y ¼ 30 , (b) y ¼ 90 and (c) y ¼ 1200 .
seen that the pressure at the tip of the delivery tube is always negative over a wide range of atomization pressure and that the tube need not protrude. Only when the atomization pressure is greater than 3.7 MPa does a weak positive pressure occur. Compared with Atomizer A, Atomizer B has a wider negative pressure range and a smaller negative pressure value. Further, the tendency towards negative pressure at the tip of the delivery tube in Atomizer B becomes strong with the reduction of the gas jet impinging angle. The negative pressure characteristics of Atomizer B are similar to those of atomizer A with a protrusion length of 6 mm. The above experimental results indicate that Atomizer B with the configuration of a Laval nozzle can work properly when the protrusion length of the tube is 0 and can thus avoid the
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Fig. 4. Pressure at the tip of the delivery tube in Atomizer A as a function of tube protrusion length.
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Fig. 6. Deflection angle of the gas jet at the nozzle exit as a function of atomization pressure.
At the same time, the Prandlt–Meyer angle P(M) for the gas jet expansion can be formulated as [3]: sffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gþ1 g1 2 arctg ðM 1Þ arctg M 2 1 PðMÞ ¼ g1 gþ1 (2) where g is the specific heat ratio of gas and M the gas Mach number. From the above formulations, the gas expansion extent can be calculated. For Atomizer A, the gas jet at the nozzle exit expands with a deflection angle y and increases in speed from ME ¼ 1:0 to M2 y ¼ PðM2 Þ PðME Þ Fig. 5. Pressure at the tip of the delivery tube in Atomizer B as a function of atomization pressure.
dynamic energy loss from the impacting and deflecting of the gas jet. From this point of view, the configuration of Atomizer B is superior to that of Atomizer A.
4. Discussion 4.1. Gas expansion at the nozzle exit It can be seen from previous research that the gas jet pressure PE at the nozzle exit in Atomizer A is usually greater than the back pressure Pb when the atomization gas pressure is in the range 0.1–4 MPa [2]. The gas jet always expands at the nozzle exit. The gas jet will be accelerated by expansion to a supersonic speed M2. In this case, the gas jet pressure will decrease to the value of the back pressure (about 0.1 MPa). For this expansion of gas jet, an equation, describing the relationship between the expanded gas pressure P2, the atomization pressure Pa and the expanded gas speed M2 can be written as [3]: P2 g 1 2 ðg=ðg1ÞÞ M2 ¼ 1þ (1) 2 Pa
(3)
By calculating the above three equations, the relationship between the deflection angle of gas jets and the atomization gas pressure for Atomizer A is as shown in Fig. 6. It can be seen that a strong deflection/expansion occurs at the nozzle exit even if the atomization pressure is small (1 MPa). In this way, the gas jet has a strong influence on the pressurization at the tip of delivery tube. In contrast, the gas jet expansion for Atomizer B with the Laval nozzle configuration is not so strong as that for Atomizer A with a converging nozzle. When the atomization pressure Pa is as high as 5.0 MPa, the gas expansion angle y is only about 3.58. This shows that the gas jets in atomizers with a Laval nozzle almost expand fully before discharging from the nozzle exit ðPE Pb Þ. The deflection angle of the gas jet is very small and has little pressurization effect on the pressure zone at the tip of the delivery tube. 4.2. Formation mechanism of the pressure zone 4.2.1. Atomizer A According to gas dynamics [3], the gas jet discharging from a converging nozzle under various experimental conditions has various flow patterns, depending on the difference between the jet pressure PE at the nozzle exit and the back pressure Pb. If PE is less than or equal to Pb, the gas jet issues as a cylindrical parallel stream, whilst it PE is larger
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Fig. 7. Pressurization at the tip of the delivery tube as the gas jet expands strongly at the nozzle exit.
than Pb, the gas jet expands as it discharges from the nozzle and the gas particles are accelerated radially. For Atomizer A, the gas jet pressure at the nozzle exit exceeds the back pressure (1 atm) when the atomization gas pressure Pa is higher than 0.19 MPa (with N2 as the atomization gas). This shows that the gas jet might expand after the nozzle exit under any practical spray forming conditions when the inner tube protrusion length is 0. Furthermore, the higher is the atomization gas pressure, the more intensive is the jet expansion, even in an explosion fashion. This will result in the accumulation of gas particles in the zone near to the tip of the delivery tube and make the gas pressure exceed 1 atm. Accordingly, pressurization will be present at the tip of the delivery tube as shown in Fig. 7. If the tube protrudes to a certain length, the gas jet will deflect after impacting on to the tube and the influence of jet expansion on the tip zone of the delivery tube will be reduced. The pressurization at the tip of the delivery tube will be restrained to some extent. 4.2.2. Atomizer B The flow pattern of gas jet from Laval nozzles is somewhat different from that of converging nozzles [3,4]. If PE is higher than Pb, the gas jet will expand through expansion waves at the nozzle exit and the jet pressure will decrease from PE to Pb. Similar to Atomizer A, the gas expansion will cause pressurization at the tip of the delivery tube. However, it is found that the diverging section in the Laval nozzle can make the jet expand more completely. Therefore, the gas jet pressure at the nozzle exit can be kept low. Even when a very high atomization pressure is used, the gas jet expansion after the nozzle exit is much weaker than that for Atomizer A. Severe pressurization is not likely to form at the tip of the delivery tube. If PE equals Pb, the gas jet will flow like a cylindrical parallel stream and neither expansion nor compression will occur, which is called the fully expanded state. In this case, the gas in the taper zone near to the tip of the delivery tube will be drawn by the high speed gas jet so that the air at the tip of delivery tube will become thin and the pressure will be
Fig. 8. Aspiration at the tip of the delivery tube due to the gas jet not expanding at the nozzle exit.
lower than the back pressure (the atmosphere). Therefore, the aspiration effect is prone to happen as shown in Fig. 8. For Atomizer B, although the gas jet has already been in a fully expanded state at the nozzle exit, the jet will continue to expand since the Laval nozzle exit is of a divergent shape and the flow direction is varied along the radius. However, the degree of jet expansion is very low and the gas boundary pressure is equal to the back pressure [2]. Consequently, it will not produce a pressurization effect on the tip of the delivery tube either. On the other hand, aspiration is likely to be present owing to the drawing effect of the gas jet. If PE is lower than Pb, two shock waves will be emitted at the nozzle exit and the jet will be compressed. Then the shock waves will be reflected from the free surface of the jet into expansion waves so that the jet will begin to expand again. This phenomena of expansion and compression is periodical [3,4]. During this periodical process, the gas jet pressure is lower than or equal to the back pressure, so it will still play the role of aspiration to the pressure zone at the end of the delivery tubeðDP < 0Þ. Obviously, the above analysis is consistent with the experimental results obtained in this investigation.
5. Conclusions 1. The pressure at the tip of the delivery tube in an atomizer with a converging nozzle is influenced by atomization gas pressure, the protrusion length of the delivery tube and the atomization gas impinging angle. The tendency to pressurization for this type of atomizer is obvious. In contrast, aspiration always occurs at the tip of the delivery tube in an atomizer with a Laval nozzle. At the same time, the delivery tube does not need to protrude so that the dynamic energy loss from the impacting and reflecting of the gas jet on the tube can be avoided. 2. With the atomization gas pressure being used over a wide range in the practical spray forming process, the gas jet expands strongly at the nozzle exit of the converging
C. Cui et al. / Journal of Materials Processing Technology 137 (2003) 5–9
atomizer, while the Prandtl–Meyer angle of the jet is very small for a Laval nozzle. 3. The pressure characteristics at the tip of the delivery tube are closely related to the flow pattern of the gas jet. When the jet expands strongly at the exit of the nozzle, pressurization occurs at the tip of the delivery tube. Aspiration occurs when the jet pressure at the exit of the nozzle is equal to or smaller than the back pressure.
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References [1] A.G. Leatham, A. Lawley, Inter. J. Powder Metall. 29 (4) (1993) 323. [2] C.S. Cui, Spray deposition and its effect on the microstructure and tensile properties of rapidly solidified Al–Li alloy by spray deposition process, Ph.D. Thesis, Harbin Institute of Technology, Harbin, 1995. [3] M.J. Zucrow, J.D. Hoffman, Gas Dynamics, vol. 1, Wiley, New York, 1976. [4] Z. Ling, Gas Kinetics, Beijing Aeronautic University Press, Beijing, 1990.
Formation mechanism of the pressure zone at the tip of the melt delivery tube during the spray forming process Chengsong Cui*, Fuyang Cao, Qingchun Li Harbin Institute of Technology, Heilongjiang Province, Harbin 150001, PR China
Abstract The gas pressures at the tip of the melt delivery tube in two typical atomizers are measured in this work. Gas dynamics is used to study the gas flow and pressure zone at the tip of the delivery tube, and a mechanism for the formation of pressurization and aspiration is proposed. The results show that the atomization efficiency of an atomizer with a Laval nozzle is superior to that of an atomizer with a converging nozzle. The gas jet expands strongly at the nozzle exit of a converging atomizer, while the Prandtl–Meyer angle of the jet is very small for a Laval nozzle. When the jet expands strongly, pressurization is likely to form at the tip of delivery tube. Aspiration is formed when the jet pressure at the exit of nozzle is equal to or smaller than the back pressure. # 2002 Published by Elsevier Science B.V. Keywords: Atomization; Melt delivery tube; Pressurization
1. Introduction Spray forming is an advanced processing technique of materials, which is capable of manufacturing near-net-shape products in the form of billets, tubes and sheets in a single, integrated operation [1]. Gas atomization of alloy melt is one of the crucial stages of this processing. For example, the gas pressure at the tip of the melt delivery tube may be markedly different under various processing conditions, i.e., pressurization or aspiration are both likely to be present in spray forming. Aspiration is favorable for drawing out the metal melt through the delivery tube, while pressurization blocks the flowing down of the melt. Therefore, investigation on the formation mechanism of gas pressure at the tip of the delivery tube is very important for controlling the spray forming process. In this paper, the gas pressure at the tip of delivery tube of two typical atomizers are measured, and the formation mechanism of pressurization and aspiration is proposed.
2. Experimental The pressure at the tip of the delivery tube during the practical atomization process is difficult to measure. Two types of atomizer with typical nozzle configuration (denoted *
Corresponding author.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 1 - 8
as Atomizer A and Atomizer B) were used for the simulation experiment in an open system without metal melt. The gas nozzle in Atomizer A is of converging structure whilst the nozzle in Atomizer B is of Laval structure. The configuration illustration of these atomizers is shown in Fig. 1, in which y is the jet impinging angle and d is the protrusion length of the delivery tube. Nitrogen was used as the atomization gas. The inner diameter of the delivery tube was 3.8 mm. The gas pressure at the tip of delivery tube was measured by means of a mercury manometer as shown in Fig. 2.
3. Experimental results Fig. 3(a) shows the pressure difference DP ðDP ¼ PE Pb , where PE is the gas jet pressure at the nozzle exit of the atomizer, and Pb is back pressure) at the tip of the delivery tube for Atomizer A as a function of atomization pressure Pa. It can be seen that the pressure at the tip of the delivery tube changes with the atomization pressure in different ways for various protrusion lengths d of the delivery tube. When d is shorter, the negative pressure at tip is smaller, while the delivery tube does not protrude at all ðd ¼ 0Þ, positive pressure is prone to be present. At the same time, the higher is the atomization gas pressure, the higher is the gas pressure at the tip. The effects of the configuration of the delivery tube on the tip pressure are shown in Fig. 3(b) and (c). The larger is the jet impinging angle, the greater is
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Fig. 1. Configuration of Atomizer A and Atomizer B.
Fig. 2. Set-up for gas dynamic characteristics.
the possibility of the presence of positive pressure. For a delivery tube with an impinging angle of 908 and 1208, it is almost impossible to form negative pressure when the protrusion length of the delivery tube is less than 4 mm and the atomization pressure is higher than 1.5 MPa. Fig. 4 shows the pressure at the tip of the delivery tube in Atomizer A as a function of protrusion length. It is obvious that positive pressure appears at the tip of the delivery tube when the atomization pressure is higher than 1.0 MPa and the protrusion length is less than 4.0 mm. In the gas atomization process, the pressure at the tip of the delivery tube has a great influence on the atomization of the metal melt. A reasonably designed atomizer should ensure that negative pressure is always present at the tip of the delivery tube for an atomization pressure within a wide range (0–3.0 MPa). However, the value of negative pressure should not be too large, since it may make the breaking effect of metal melt worse with too high a metal flow rate. It can be seen from Fig. 4 that the delivery tube in Atomizer A has to protrude to some extent in order to meet the above requirements: otherwise, if positive pressure is formed, the atomization process is difficult to perform. When the tube protrudes, the atomization gas jets will firstly impact on the outer surface of the tube and deflect, which will inevitably cause the loss of a portion of the gas kinetic energy and reduce the atomization efficiency. Especially for an atomizer with a larger jet impinging angle, the longer is the protrusion length, the greater is the loss of kinetic energy. The pressure change with atomization pressure at the tip of the delivery tube in Atomizer B is shown in Fig. 5. It is
Fig. 3. Pressure at the tip of the delivery tube in Atomizer A as a function of atomization pressure: (a) y ¼ 30 , (b) y ¼ 90 and (c) y ¼ 1200 .
seen that the pressure at the tip of the delivery tube is always negative over a wide range of atomization pressure and that the tube need not protrude. Only when the atomization pressure is greater than 3.7 MPa does a weak positive pressure occur. Compared with Atomizer A, Atomizer B has a wider negative pressure range and a smaller negative pressure value. Further, the tendency towards negative pressure at the tip of the delivery tube in Atomizer B becomes strong with the reduction of the gas jet impinging angle. The negative pressure characteristics of Atomizer B are similar to those of atomizer A with a protrusion length of 6 mm. The above experimental results indicate that Atomizer B with the configuration of a Laval nozzle can work properly when the protrusion length of the tube is 0 and can thus avoid the
C. Cui et al. / Journal of Materials Processing Technology 137 (2003) 5–9
Fig. 4. Pressure at the tip of the delivery tube in Atomizer A as a function of tube protrusion length.
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Fig. 6. Deflection angle of the gas jet at the nozzle exit as a function of atomization pressure.
At the same time, the Prandlt–Meyer angle P(M) for the gas jet expansion can be formulated as [3]: sffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gþ1 g1 2 arctg ðM 1Þ arctg M 2 1 PðMÞ ¼ g1 gþ1 (2) where g is the specific heat ratio of gas and M the gas Mach number. From the above formulations, the gas expansion extent can be calculated. For Atomizer A, the gas jet at the nozzle exit expands with a deflection angle y and increases in speed from ME ¼ 1:0 to M2 y ¼ PðM2 Þ PðME Þ Fig. 5. Pressure at the tip of the delivery tube in Atomizer B as a function of atomization pressure.
dynamic energy loss from the impacting and deflecting of the gas jet. From this point of view, the configuration of Atomizer B is superior to that of Atomizer A.
4. Discussion 4.1. Gas expansion at the nozzle exit It can be seen from previous research that the gas jet pressure PE at the nozzle exit in Atomizer A is usually greater than the back pressure Pb when the atomization gas pressure is in the range 0.1–4 MPa [2]. The gas jet always expands at the nozzle exit. The gas jet will be accelerated by expansion to a supersonic speed M2. In this case, the gas jet pressure will decrease to the value of the back pressure (about 0.1 MPa). For this expansion of gas jet, an equation, describing the relationship between the expanded gas pressure P2, the atomization pressure Pa and the expanded gas speed M2 can be written as [3]: P2 g 1 2 ðg=ðg1ÞÞ M2 ¼ 1þ (1) 2 Pa
(3)
By calculating the above three equations, the relationship between the deflection angle of gas jets and the atomization gas pressure for Atomizer A is as shown in Fig. 6. It can be seen that a strong deflection/expansion occurs at the nozzle exit even if the atomization pressure is small (1 MPa). In this way, the gas jet has a strong influence on the pressurization at the tip of delivery tube. In contrast, the gas jet expansion for Atomizer B with the Laval nozzle configuration is not so strong as that for Atomizer A with a converging nozzle. When the atomization pressure Pa is as high as 5.0 MPa, the gas expansion angle y is only about 3.58. This shows that the gas jets in atomizers with a Laval nozzle almost expand fully before discharging from the nozzle exit ðPE Pb Þ. The deflection angle of the gas jet is very small and has little pressurization effect on the pressure zone at the tip of the delivery tube. 4.2. Formation mechanism of the pressure zone 4.2.1. Atomizer A According to gas dynamics [3], the gas jet discharging from a converging nozzle under various experimental conditions has various flow patterns, depending on the difference between the jet pressure PE at the nozzle exit and the back pressure Pb. If PE is less than or equal to Pb, the gas jet issues as a cylindrical parallel stream, whilst it PE is larger
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C. Cui et al. / Journal of Materials Processing Technology 137 (2003) 5–9
Fig. 7. Pressurization at the tip of the delivery tube as the gas jet expands strongly at the nozzle exit.
than Pb, the gas jet expands as it discharges from the nozzle and the gas particles are accelerated radially. For Atomizer A, the gas jet pressure at the nozzle exit exceeds the back pressure (1 atm) when the atomization gas pressure Pa is higher than 0.19 MPa (with N2 as the atomization gas). This shows that the gas jet might expand after the nozzle exit under any practical spray forming conditions when the inner tube protrusion length is 0. Furthermore, the higher is the atomization gas pressure, the more intensive is the jet expansion, even in an explosion fashion. This will result in the accumulation of gas particles in the zone near to the tip of the delivery tube and make the gas pressure exceed 1 atm. Accordingly, pressurization will be present at the tip of the delivery tube as shown in Fig. 7. If the tube protrudes to a certain length, the gas jet will deflect after impacting on to the tube and the influence of jet expansion on the tip zone of the delivery tube will be reduced. The pressurization at the tip of the delivery tube will be restrained to some extent. 4.2.2. Atomizer B The flow pattern of gas jet from Laval nozzles is somewhat different from that of converging nozzles [3,4]. If PE is higher than Pb, the gas jet will expand through expansion waves at the nozzle exit and the jet pressure will decrease from PE to Pb. Similar to Atomizer A, the gas expansion will cause pressurization at the tip of the delivery tube. However, it is found that the diverging section in the Laval nozzle can make the jet expand more completely. Therefore, the gas jet pressure at the nozzle exit can be kept low. Even when a very high atomization pressure is used, the gas jet expansion after the nozzle exit is much weaker than that for Atomizer A. Severe pressurization is not likely to form at the tip of the delivery tube. If PE equals Pb, the gas jet will flow like a cylindrical parallel stream and neither expansion nor compression will occur, which is called the fully expanded state. In this case, the gas in the taper zone near to the tip of the delivery tube will be drawn by the high speed gas jet so that the air at the tip of delivery tube will become thin and the pressure will be
Fig. 8. Aspiration at the tip of the delivery tube due to the gas jet not expanding at the nozzle exit.
lower than the back pressure (the atmosphere). Therefore, the aspiration effect is prone to happen as shown in Fig. 8. For Atomizer B, although the gas jet has already been in a fully expanded state at the nozzle exit, the jet will continue to expand since the Laval nozzle exit is of a divergent shape and the flow direction is varied along the radius. However, the degree of jet expansion is very low and the gas boundary pressure is equal to the back pressure [2]. Consequently, it will not produce a pressurization effect on the tip of the delivery tube either. On the other hand, aspiration is likely to be present owing to the drawing effect of the gas jet. If PE is lower than Pb, two shock waves will be emitted at the nozzle exit and the jet will be compressed. Then the shock waves will be reflected from the free surface of the jet into expansion waves so that the jet will begin to expand again. This phenomena of expansion and compression is periodical [3,4]. During this periodical process, the gas jet pressure is lower than or equal to the back pressure, so it will still play the role of aspiration to the pressure zone at the end of the delivery tubeðDP < 0Þ. Obviously, the above analysis is consistent with the experimental results obtained in this investigation.
5. Conclusions 1. The pressure at the tip of the delivery tube in an atomizer with a converging nozzle is influenced by atomization gas pressure, the protrusion length of the delivery tube and the atomization gas impinging angle. The tendency to pressurization for this type of atomizer is obvious. In contrast, aspiration always occurs at the tip of the delivery tube in an atomizer with a Laval nozzle. At the same time, the delivery tube does not need to protrude so that the dynamic energy loss from the impacting and reflecting of the gas jet on the tube can be avoided. 2. With the atomization gas pressure being used over a wide range in the practical spray forming process, the gas jet expands strongly at the nozzle exit of the converging
C. Cui et al. / Journal of Materials Processing Technology 137 (2003) 5–9
atomizer, while the Prandtl–Meyer angle of the jet is very small for a Laval nozzle. 3. The pressure characteristics at the tip of the delivery tube are closely related to the flow pattern of the gas jet. When the jet expands strongly at the exit of the nozzle, pressurization occurs at the tip of the delivery tube. Aspiration occurs when the jet pressure at the exit of the nozzle is equal to or smaller than the back pressure.
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References [1] A.G. Leatham, A. Lawley, Inter. J. Powder Metall. 29 (4) (1993) 323. [2] C.S. Cui, Spray deposition and its effect on the microstructure and tensile properties of rapidly solidified Al–Li alloy by spray deposition process, Ph.D. Thesis, Harbin Institute of Technology, Harbin, 1995. [3] M.J. Zucrow, J.D. Hoffman, Gas Dynamics, vol. 1, Wiley, New York, 1976. [4] Z. Ling, Gas Kinetics, Beijing Aeronautic University Press, Beijing, 1990.