- Email: [email protected]
Effects of entrained air manner on cavitation damage
333
2011,23(3):333-338 DOI: 10.1016/S1001-6058(10)60120-5
EFFECTS OF ENTRAINED AIR MANNER ON CAVITATION DAMAGE* WU Jian-hua, LUO Chao College of Water Conservancy and Hydropwer Engineering, Hohai University, Nanjing 210098, China, E-mail: [email protected] (Received March 18, 2011, Revised April 16, 2011) Abstract: Early in 1953 the experiments by Peterka proved that air entrainment has effects on decreasing cavitation damage. This technology has been widely used in the release works of high dams since the inception of air entrainment in the Grand Goulee Dam in 1960. Behavior, mechanism and application of air entrainment for cavitation damage control have been investigated for over half century. However, severe cavitation damage happened due to complex mechanism of air entrainment. The effects of air entrainment are related to many factors, including geometric parameters, hydraulic parameters and entrained air manners. In the present work an experimental set-up for air entrainment was specially designed, the behavior of reducing cavitation damage was experimentally investigated in the three aspects of entrained air pressure, air tube aera and air tube number. The results show that magnitude of reduction of cavitation damage is closely related to the entrained air tube number as well as entrained air pressure, air tube aera, and that the effect through three air tubes is larger than that through single air tube although the entrained air tubes have the same sum of tube aera, that is, 1+1+1 > 3 . Therefore, it is important to design an effective manner of air entrainment. Key words: air entrainment, air pressure, air tube area, cavitation damage, entrained air manner
Introduction Air entrainment is a kind of effective and inexpensive measure to reduce cavitation damage. Early in 1953, Peterka conducted experiments to investigate the effects of air entrainment on the modification of cavitation damage. The finding revealed the cavitation damage of the material is greatly decreased when the air is entrained into the water on the surface of materials[1]. This is an important discovery and very useful for the development of the technology and application of cavitation damage control. In 1960 air entrainment was successfully used in the Grand Goulee Dam in America and at present this technology is applied in almost all the release works of high dam for the decrease of cavitation damage[2]. The effects of air entrainment are related to many factors, including geometric and hydraulic parameters, * Project supported by the National Natural Science Foundation of China (Grant No. 50879021), the National Science Fund for Distinguished Young Scholars (Grant No. 50925932), and the Ministry of Science and Technology of China (Grant No. 2008BAB19B04). Biography: WU Jian-hua (1958-), Male, Ph. D., Professor
and entrained air manners. Most of researches in the past focused on the effects of air concentration and, the form of aerators on the prevention from cavitation damage. The cavitation damage could be effectively reduced when the air concentration of 3-5% is provided in the flow on the surface of the materials. In the applications of entrained air technology, many kinds of aerators have been developed, such as U-type, V-type[3], dentform, and A-type downstream behind ramps[4] as well as the traditional forms with combinations of ramp, step and grove. All the purposes of the development of the different forms of aerators are to keep some air concentration on the interface between flow and structural surface to meet the working conditions of different release works, especially for the flow with either low flow Froude number[5] or small bottom slope of a spillway[6]. The air concentration has been paid attention to, and the estimation of cavity length ( L ) for the design of an aerator is an important issue since air flow (q) is directly related to flow velocity ( Vo ) and L , i.e., q = kVo L [7]. The control of the filling water in a cavity is another item due to affecting the effective cavity length[8,9]. Chen et al.[10] presented an idea that small air bubbles have better effects on the reduction of cavitation damage through
334
their experiments and that the functions of the air entrainment are much more closely correlated to the magnitude of the air bubbles entrained into the flow than the air concentration. They deemed that the less the air bubbles, the better are the effects of air entrainment under the conditions of the same air concentration. Behavior, mechanism and application of air entrainment have been investigated for over half a century. The recent investigations deal with the models of cavitation bubbles[11], the pit structure about cavitation erosion[12], and the interaction of a cavitation bubble and an air bubble[13]. However, severe cavitation damage happened, such as in the flood discharge tunnel in the Ertan Hydropower Project in China, due to complex mechanism of air entrainment[14]. This work aims at determining the effects of entrained air manners on decreasing cavitation damage as well as of entrained air pressure and air tube area. A test device was designed for different air entrainment conditions. The experiments were conducted through entrained air manners of single air tube and three air tubes to find the differences of various manners. 1. Experimental set-up and methodology The experiments were conducted in the HighSpeed Flow Laboratory at Hohai University (Nanjing, China). A special device was designed to reduce cavitation damage by means of entraining air into the surface of a sample for different manners.
Fig.1 Sketch of experimental set-up for cavitation damage with entrained air function
1.1 Experimental set-up The experimental set-up consists of three parts, including a magneto-constriction oscillator, cooling water system and entrained air system (Fig.1). The magneto-constriction oscillator, with the power of 1 kW, presents oscillation with the frequency of 20 ± 10% kHz and amplitude of 50 ± 10% μm in order to make the testing samples damaged by cavitation. The
cooling water system keeps the testing water temperature in a certain range, at which the sample is located in order to reduce the effects of the temperature on the experimental results. The entrained air system is specially designed for this work, and includes an air pump to provide the air needed in air entrainment, an air flow control valve to adjust the flow and pressure of entrained air, an air pressure tank to make air pressure be stabilized, and an entrained air chamber and several tubes to send the air to the sample. All the samples are made from steel material of Q235 on the basis of the national standard of vibration cavitation erosion test[15]. 1.2 Experimental methodology and procedure The working conditions in the experiments are as follows: (1) The magneto-constriction oscillator: the frequency: 20 ± 10% kHz, and amplitude: 50 ± 10% μm. (2) The testing water temperature: 23 ± 0.5oC. (3) The weight loss and cleaning of a sample: the sample is cleaned before and after each test by means of acetone and is dried, then weighted by a high precision balance with the error of 0.01 mg to determine the magnitude of the cavitation damage of the sample. (4) The air pressure tank: the pressure of 0.00 MPa-0.10 MPa could be adjusted by the air pump and the air flow valve, and the air pressure is stabilized by this tank. (5) The sample is made as a kind of standard one, and its diameter is 16 mm. In this work the samples are all made from steel material of Q235. (6) The air is entrained by the air tubes on the entrained air chamber, and two kinds of entrained air manners are designed. One is the single air tube located along the centre of the sample, and the other is three air tubes arranged with triangle form. (7) The experiments have the eight cases as listed in Table 1. In Case 1 there is no air entrained to compare with the results of the other cases with air entrainment, that is, to check and examine the effects of the different entrained air pressures and air tube areas on reducing cavitation damage (see Cases 2-5 in Table 1). Each set of Cases 3 and 6, Cases 5 and 7, and Cases 4 and 8 in this table are designed to be of same entrained air tube areas, respectively, and those sets are to investigate the effects of different entrained air manners on cavitation damage control, i.e., the differences of entrained air by the single air tube and three air tubes with same total air tube area. 2. Results and discussions In this work three aspects of the effects were investigated by experiments on cavitation damage, which are entrained air pressure, air tube area and air tube number, respectively.
335
Table 1 Experimental cases Cases
Air tube aeras (mm2)
1
0.0
0.0
0.0
0.0
0.0
0.0
2
0.028×1
0.0
0.02
0.04
0.06
0.08
3
0.085×1
0.0
0.02
0.04
0.06
0.08
4
0.283×1
0.0
0.02
0.04
0.06
0.08
5
0.849×1
0.0
0.02
0.04
0.06
0.08
6
0.028×3
/
/
/
/
0.08
7
0.283×3
/
/
/
/
0.08
8
0.053+0.085+0.145
/
/
/
/
0.08
Pressures (MPa)
2.1 Effects of entrained air pressure Figure 2 is the variation of cavitation damage with time under different entrained air pressures, in which the entrained air tube area ( A ) is 0.085 mm2. It could be seen from this figure that the weight losses ( WL ) brought about by cavitation damage are approximately proportional to the exposed time ( t ) if t is larger than 1 h. Compared with the case of no air entrainment ( p = 0 MPa ), the cavitation damage is greatly decreased by air entrainment under the conditions of various pressures.
Fig.2 Variation of cavitation damage with time under different entrained air pressures ( A = 0.085 mm 2 )
between WL and entrained air pressure ( p ). For the profile of cavitation damage at the time of 1 h, WLs at the pressures of 0.04 MPa and 0.08 MPa, compared with the case of no air entrainment (i.e., p = 0 MPa ), are decreased by 10.0 mg and 13.4 mg, respectively. While the entrained air at 4 h gets the reduction of cavitation damage of 40.9 mg and 60.0 mg at the above-mentioned pressures. So it could be concluded that the entrained air pressure is an important factor to reduce the cavitation damage, and the effect of reducing cavitation damage is enhanced with higher air pressure. As a matter of fact, higher entrained air pressure produces higher air flow speed, provides much more air, and increases the air concentration of the water on the material surface so that the cavitation damage could be reduced. Figure 4 is the photos of the cavitation damage for samples of 4 h test at different pressures of entrained air. The effects of the air entrainment increase on the whole surface of the sample as the entrained air pressure increases. 2.2 Effects of entrained air tube area Figure 5 shows the variation of cavitation damage with air tube area ( A ) under different exposed times, in which the entrained air pressure is kept at 0.08 MPa. WLs drop rapidly with the increase of A when it is smaller than 0.03 mm2. While WLs decrease almost linearly when A increases if A > 0.03 mm 2 . Meanwhile, it could be found that their slopes become large as the time ( t ) increases. For t = 4 h , WLs are 25.4 mg and 8.8 mg at the tube areas A = 0.40 mm 2 and 0.85 mm 2 , respectively, that is to say, the magnitude of cavitation damage at A = 0.85 mm 2 is only about 1/3 of that at A =
Fig.3 Variation of cavitation damage with entrained air pressures
Figure 3 demonstrates clearly the relationship
0.40 mm 2 . Similar to the reasons with the changes of the entrained air pressure, the large air tube area gives much more air under same air pressure and then high air concentration for cavitation damage control.
336
Figure 4 Cavitation damage at various pressure of entrained air (4 h) Table 2 Results of cavitation damage for set one (mg)
t (h)
Cases
Entrained air mannar (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3
0.085×1
0.0
0.7
1.8
4.1
6.6
11.3
16.6
22.0
27.5
34.2
40.9
6
0.028×3
0.0
0.6
1.6
3.6
5.6
9.4
13.4
18.3
23.0
28.8
34.2
0
14
11
12
15
17
19
17
16
16
16
(WL1 − WL3 ) / WL1 × 100%
Table 3 Results of cavitation damage for set two (mg)
t (h)
Cases
Entrained air manner (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4
0.283×1
0.0
0.6
1.4
3.4
5.2
8.8
12.3
15.8
19.4
23.6
28.1
8
0.053+0.085+ 0.145
0.0
0.5
1.2
2.5
4.1
7.6
10.8
14.1
17.1
20.3
24.0
0
17
14
26
21
14
12
11
12
14
15
(WL1 − WL3 ) / WL1 × 100%
Table 4 Results of cavitation damage for set three (mg)
t (h)
Cases
Entrained air mannar (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
5
0.849×1
0.0
0.3
0.8
1.8
2.4
3.6
4.5
5.6
6.6
7.7
8.8
7
0.283×3
0.0
0.2
0.7
1.3
1.9
2.8
3.7
4.8
5.9
6.9
7.9
0
33
13
28
21
22
18
14
11
10
10
(WL1-WL3)/WL1×100%
Figure 6 is the photos of cavitation damage of the testing samples for different entrained air tube areas. The results of prevention from cavitation damage are obviously related to the entrained air tube areas.
Fig.5 Variation of cavitation damage with entrained air tube area
2.3 Effects of entrained air number Tables 2-4 give three sets of the experimental results of cavitation damage at different numbers of entrained air tubes, and their entrained air pressures are all at 0.08 MPa. Each table includes two kinds of the entrained air manners, i.e., the air entrained by the single and three tubes, but the sum of the entrained air tube areas are same. In those tables, (WL1 − WL3 ) /
WL1 × 100% expresses the difference of cavitation damage for two kinds of the entrained air manners, while the subscripts 1 and 3 in (WL1 − WL3 ) / WL1 × 100% are the number of entrained air tubes for different entrained air manners. It could be seen from Tables 2-4 that the effects of decreasing cavitation damage by three entrained air tubes are better than that by single air tube, and in
337
Fig.6 Cavitation damage at various tube area of entrained air (4 h)
Fig.7 Cavitation damage at different entrained air manners (4 h)
those tables, their sums of the tubes are 0.085 mm2, 0.283 mm2 and 0.849 mm2, respectively. It could be concluded that under the conditions of same entrained air tube area for those three sets of the results, there exists the fact that the effect of the entrained air manner of 1 + 1 + 1 is larger than that of 3. Meanwhile, it should be noted that small air tube area has better effect on reductions of cavitation damage, such as 16% from Table 2 ( A = 0.085 mm 2 ) at time of 4 hrs, 10% from Table 4 ( A = 0.849 mm 2 ). This phenomenon may confirm the results of Chen et al.’s, that is, the effects of reducing cavitation damage are related to the air bubble diameters entrained[10]. Figure 7 is the photos of the above-mentioned experiments. It could be seen that there are much more than 3 regions of prevention from cavitation damage by three air tubes, and larger extension of cavitation damage control than that by single air tube although they have the same sums of the air tube aeras.
References [1]
[2]
[3]
[4]
[5]
[6]
3. Conclusion The experimental investigations of cavitation damages have been conducted in the experimental set-up with special designs, under different experimental conditions of different entrained air manners, including entrained air pressure, air tube area and air tube number. Higher air pressure and larger air tube area have better effects of cavitation damage control due to providing much air and higher air concentration of the flow on the surface of materials. The prevention from cavitation damage by three air tubes is much more effective than that by single air tube, and the fact could be clearly expressed that 1 + 1 + 1 > 3 for the same entrained air pressure and same sum of air tube areas.
[7] [8]
[9]
[10]
PETERKA J. A. The effect of entrained air on cavitation pitting[C]. Proceedings Minnesota International Hydraulics Convention. Minnesota, Minneapolis, USA, 1953, 507-518. WU Jian-hua, WU Wei-wei and RUAN Shi-ping. On necessity of placing an aerator in the bottom discharge tunnel at the Longtan Hydropower Station[J]. Journal of Hydrodynamics, Ser. B, 2006, 18(6): 698-701. WANG Hai-yun, DAI Guang-qing and YANG Qing et al. Experimental study on V-type aerator for spillway tunnel with inlet raised[J]. Journal of Hydraulic Engineering, 2005, 36(11): 1371-1374(in Chinese). WU Wei-wei, WU Jian-hua and RUAN Shi-ping. Study of the geometries of aerators for flat-bottom discharge tunnels[J]. Journal of Hydrodynamics, Ser. A, 2007, 22(4): 397-402(in Chinese). YANG Yong-sheng, YANG Yong-quan and SHUAI Qing-hong. Hydraulic and aeration characteristics of the low Froude number flow a step aerator[J]. Journal of Hydraulic Engineering, 2002, 33(2): 27-31(in Chinese). SU Pei-lan, LIAO Hua-sheng and QIU Yue et al. Experimental study on a new type of aerator in spillway with low Froude number and mild slop flow[J]. Journal of Hydrodynamics, 2009, 21(3): 415-422. WU Jian-hua, RUAN Shi-ping. Cavity length below chute aerators[J]. Science in China, Series E: Technological Sciences, 2008, 51(2): 170-178. MOHAGHEGH A., WU Jian-hua. Hydraulics of discharge tunnel service gate: The flow regimes[C]. Proceedings of the 9th International Conference of Hydrodynamics, Shanghai, 2010, 726-732. ZHANG Li-heng. Experimental investigation on backwater in cavity pocket of flow aeration types to prevent cavitation erosion[D]. Master Thesis, Chengdu: Sichuan University, 2006(in Chinese). CHEN Xian-pu, XI Ru-ze and SHAO Dong-chao et al. New concept of air entrainment for cavitation damage control[J]. Journal of Hydraulic Engineering, 2003, 34(8): 70-74(in Chinese).
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[11] [12]
[13]
M∅RCH K. A. Cavitation nuclei: Experiments and theory[J]. Journal of Hydrodynamics, 2009, 21(2): 176-189. BAI Li-xin, XU Wei-lin and ZHANG Fa-xing et al. Cavitation characteristics of pit structure in ultrasonic field[J]. Science in China, Series E: Technological Sciences, 2009, 52(7): 1974- 1980. XU Wei-lin, BAI Li-xin and ZHANG Fa-xing. Interaction of a cavitation bubble and an air bubble with a rigid boundary[J]. Journal of Hydrodynamics, 2010, 22(4): 503-512.
[14] [15]
RUAN Shi-ping. Hydraulic characteristics of chute aerators for release works[D]. Ph. D. Thesis, Nanjing: Hohai University, 2008(in Chinese). WU Jian-hua, CHAI Gong-chun. Effects of N+ implantation on surface modification of cavitation damage for 0Gr13Ni9Ti SS of turbine materials[J]. Journal of Fluids Engineering, Trans. ASME, 1997, 119(3): 204-208.
2011,23(3):333-338 DOI: 10.1016/S1001-6058(10)60120-5
EFFECTS OF ENTRAINED AIR MANNER ON CAVITATION DAMAGE* WU Jian-hua, LUO Chao College of Water Conservancy and Hydropwer Engineering, Hohai University, Nanjing 210098, China, E-mail: [email protected] (Received March 18, 2011, Revised April 16, 2011) Abstract: Early in 1953 the experiments by Peterka proved that air entrainment has effects on decreasing cavitation damage. This technology has been widely used in the release works of high dams since the inception of air entrainment in the Grand Goulee Dam in 1960. Behavior, mechanism and application of air entrainment for cavitation damage control have been investigated for over half century. However, severe cavitation damage happened due to complex mechanism of air entrainment. The effects of air entrainment are related to many factors, including geometric parameters, hydraulic parameters and entrained air manners. In the present work an experimental set-up for air entrainment was specially designed, the behavior of reducing cavitation damage was experimentally investigated in the three aspects of entrained air pressure, air tube aera and air tube number. The results show that magnitude of reduction of cavitation damage is closely related to the entrained air tube number as well as entrained air pressure, air tube aera, and that the effect through three air tubes is larger than that through single air tube although the entrained air tubes have the same sum of tube aera, that is, 1+1+1 > 3 . Therefore, it is important to design an effective manner of air entrainment. Key words: air entrainment, air pressure, air tube area, cavitation damage, entrained air manner
Introduction Air entrainment is a kind of effective and inexpensive measure to reduce cavitation damage. Early in 1953, Peterka conducted experiments to investigate the effects of air entrainment on the modification of cavitation damage. The finding revealed the cavitation damage of the material is greatly decreased when the air is entrained into the water on the surface of materials[1]. This is an important discovery and very useful for the development of the technology and application of cavitation damage control. In 1960 air entrainment was successfully used in the Grand Goulee Dam in America and at present this technology is applied in almost all the release works of high dam for the decrease of cavitation damage[2]. The effects of air entrainment are related to many factors, including geometric and hydraulic parameters, * Project supported by the National Natural Science Foundation of China (Grant No. 50879021), the National Science Fund for Distinguished Young Scholars (Grant No. 50925932), and the Ministry of Science and Technology of China (Grant No. 2008BAB19B04). Biography: WU Jian-hua (1958-), Male, Ph. D., Professor
and entrained air manners. Most of researches in the past focused on the effects of air concentration and, the form of aerators on the prevention from cavitation damage. The cavitation damage could be effectively reduced when the air concentration of 3-5% is provided in the flow on the surface of the materials. In the applications of entrained air technology, many kinds of aerators have been developed, such as U-type, V-type[3], dentform, and A-type downstream behind ramps[4] as well as the traditional forms with combinations of ramp, step and grove. All the purposes of the development of the different forms of aerators are to keep some air concentration on the interface between flow and structural surface to meet the working conditions of different release works, especially for the flow with either low flow Froude number[5] or small bottom slope of a spillway[6]. The air concentration has been paid attention to, and the estimation of cavity length ( L ) for the design of an aerator is an important issue since air flow (q) is directly related to flow velocity ( Vo ) and L , i.e., q = kVo L [7]. The control of the filling water in a cavity is another item due to affecting the effective cavity length[8,9]. Chen et al.[10] presented an idea that small air bubbles have better effects on the reduction of cavitation damage through
334
their experiments and that the functions of the air entrainment are much more closely correlated to the magnitude of the air bubbles entrained into the flow than the air concentration. They deemed that the less the air bubbles, the better are the effects of air entrainment under the conditions of the same air concentration. Behavior, mechanism and application of air entrainment have been investigated for over half a century. The recent investigations deal with the models of cavitation bubbles[11], the pit structure about cavitation erosion[12], and the interaction of a cavitation bubble and an air bubble[13]. However, severe cavitation damage happened, such as in the flood discharge tunnel in the Ertan Hydropower Project in China, due to complex mechanism of air entrainment[14]. This work aims at determining the effects of entrained air manners on decreasing cavitation damage as well as of entrained air pressure and air tube area. A test device was designed for different air entrainment conditions. The experiments were conducted through entrained air manners of single air tube and three air tubes to find the differences of various manners. 1. Experimental set-up and methodology The experiments were conducted in the HighSpeed Flow Laboratory at Hohai University (Nanjing, China). A special device was designed to reduce cavitation damage by means of entraining air into the surface of a sample for different manners.
Fig.1 Sketch of experimental set-up for cavitation damage with entrained air function
1.1 Experimental set-up The experimental set-up consists of three parts, including a magneto-constriction oscillator, cooling water system and entrained air system (Fig.1). The magneto-constriction oscillator, with the power of 1 kW, presents oscillation with the frequency of 20 ± 10% kHz and amplitude of 50 ± 10% μm in order to make the testing samples damaged by cavitation. The
cooling water system keeps the testing water temperature in a certain range, at which the sample is located in order to reduce the effects of the temperature on the experimental results. The entrained air system is specially designed for this work, and includes an air pump to provide the air needed in air entrainment, an air flow control valve to adjust the flow and pressure of entrained air, an air pressure tank to make air pressure be stabilized, and an entrained air chamber and several tubes to send the air to the sample. All the samples are made from steel material of Q235 on the basis of the national standard of vibration cavitation erosion test[15]. 1.2 Experimental methodology and procedure The working conditions in the experiments are as follows: (1) The magneto-constriction oscillator: the frequency: 20 ± 10% kHz, and amplitude: 50 ± 10% μm. (2) The testing water temperature: 23 ± 0.5oC. (3) The weight loss and cleaning of a sample: the sample is cleaned before and after each test by means of acetone and is dried, then weighted by a high precision balance with the error of 0.01 mg to determine the magnitude of the cavitation damage of the sample. (4) The air pressure tank: the pressure of 0.00 MPa-0.10 MPa could be adjusted by the air pump and the air flow valve, and the air pressure is stabilized by this tank. (5) The sample is made as a kind of standard one, and its diameter is 16 mm. In this work the samples are all made from steel material of Q235. (6) The air is entrained by the air tubes on the entrained air chamber, and two kinds of entrained air manners are designed. One is the single air tube located along the centre of the sample, and the other is three air tubes arranged with triangle form. (7) The experiments have the eight cases as listed in Table 1. In Case 1 there is no air entrained to compare with the results of the other cases with air entrainment, that is, to check and examine the effects of the different entrained air pressures and air tube areas on reducing cavitation damage (see Cases 2-5 in Table 1). Each set of Cases 3 and 6, Cases 5 and 7, and Cases 4 and 8 in this table are designed to be of same entrained air tube areas, respectively, and those sets are to investigate the effects of different entrained air manners on cavitation damage control, i.e., the differences of entrained air by the single air tube and three air tubes with same total air tube area. 2. Results and discussions In this work three aspects of the effects were investigated by experiments on cavitation damage, which are entrained air pressure, air tube area and air tube number, respectively.
335
Table 1 Experimental cases Cases
Air tube aeras (mm2)
1
0.0
0.0
0.0
0.0
0.0
0.0
2
0.028×1
0.0
0.02
0.04
0.06
0.08
3
0.085×1
0.0
0.02
0.04
0.06
0.08
4
0.283×1
0.0
0.02
0.04
0.06
0.08
5
0.849×1
0.0
0.02
0.04
0.06
0.08
6
0.028×3
/
/
/
/
0.08
7
0.283×3
/
/
/
/
0.08
8
0.053+0.085+0.145
/
/
/
/
0.08
Pressures (MPa)
2.1 Effects of entrained air pressure Figure 2 is the variation of cavitation damage with time under different entrained air pressures, in which the entrained air tube area ( A ) is 0.085 mm2. It could be seen from this figure that the weight losses ( WL ) brought about by cavitation damage are approximately proportional to the exposed time ( t ) if t is larger than 1 h. Compared with the case of no air entrainment ( p = 0 MPa ), the cavitation damage is greatly decreased by air entrainment under the conditions of various pressures.
Fig.2 Variation of cavitation damage with time under different entrained air pressures ( A = 0.085 mm 2 )
between WL and entrained air pressure ( p ). For the profile of cavitation damage at the time of 1 h, WLs at the pressures of 0.04 MPa and 0.08 MPa, compared with the case of no air entrainment (i.e., p = 0 MPa ), are decreased by 10.0 mg and 13.4 mg, respectively. While the entrained air at 4 h gets the reduction of cavitation damage of 40.9 mg and 60.0 mg at the above-mentioned pressures. So it could be concluded that the entrained air pressure is an important factor to reduce the cavitation damage, and the effect of reducing cavitation damage is enhanced with higher air pressure. As a matter of fact, higher entrained air pressure produces higher air flow speed, provides much more air, and increases the air concentration of the water on the material surface so that the cavitation damage could be reduced. Figure 4 is the photos of the cavitation damage for samples of 4 h test at different pressures of entrained air. The effects of the air entrainment increase on the whole surface of the sample as the entrained air pressure increases. 2.2 Effects of entrained air tube area Figure 5 shows the variation of cavitation damage with air tube area ( A ) under different exposed times, in which the entrained air pressure is kept at 0.08 MPa. WLs drop rapidly with the increase of A when it is smaller than 0.03 mm2. While WLs decrease almost linearly when A increases if A > 0.03 mm 2 . Meanwhile, it could be found that their slopes become large as the time ( t ) increases. For t = 4 h , WLs are 25.4 mg and 8.8 mg at the tube areas A = 0.40 mm 2 and 0.85 mm 2 , respectively, that is to say, the magnitude of cavitation damage at A = 0.85 mm 2 is only about 1/3 of that at A =
Fig.3 Variation of cavitation damage with entrained air pressures
Figure 3 demonstrates clearly the relationship
0.40 mm 2 . Similar to the reasons with the changes of the entrained air pressure, the large air tube area gives much more air under same air pressure and then high air concentration for cavitation damage control.
336
Figure 4 Cavitation damage at various pressure of entrained air (4 h) Table 2 Results of cavitation damage for set one (mg)
t (h)
Cases
Entrained air mannar (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3
0.085×1
0.0
0.7
1.8
4.1
6.6
11.3
16.6
22.0
27.5
34.2
40.9
6
0.028×3
0.0
0.6
1.6
3.6
5.6
9.4
13.4
18.3
23.0
28.8
34.2
0
14
11
12
15
17
19
17
16
16
16
(WL1 − WL3 ) / WL1 × 100%
Table 3 Results of cavitation damage for set two (mg)
t (h)
Cases
Entrained air manner (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4
0.283×1
0.0
0.6
1.4
3.4
5.2
8.8
12.3
15.8
19.4
23.6
28.1
8
0.053+0.085+ 0.145
0.0
0.5
1.2
2.5
4.1
7.6
10.8
14.1
17.1
20.3
24.0
0
17
14
26
21
14
12
11
12
14
15
(WL1 − WL3 ) / WL1 × 100%
Table 4 Results of cavitation damage for set three (mg)
t (h)
Cases
Entrained air mannar (mm2)
0.0
0.25
0.5
0.75
1.0
1.5
2.0
2.5
3.0
3.5
4.0
5
0.849×1
0.0
0.3
0.8
1.8
2.4
3.6
4.5
5.6
6.6
7.7
8.8
7
0.283×3
0.0
0.2
0.7
1.3
1.9
2.8
3.7
4.8
5.9
6.9
7.9
0
33
13
28
21
22
18
14
11
10
10
(WL1-WL3)/WL1×100%
Figure 6 is the photos of cavitation damage of the testing samples for different entrained air tube areas. The results of prevention from cavitation damage are obviously related to the entrained air tube areas.
Fig.5 Variation of cavitation damage with entrained air tube area
2.3 Effects of entrained air number Tables 2-4 give three sets of the experimental results of cavitation damage at different numbers of entrained air tubes, and their entrained air pressures are all at 0.08 MPa. Each table includes two kinds of the entrained air manners, i.e., the air entrained by the single and three tubes, but the sum of the entrained air tube areas are same. In those tables, (WL1 − WL3 ) /
WL1 × 100% expresses the difference of cavitation damage for two kinds of the entrained air manners, while the subscripts 1 and 3 in (WL1 − WL3 ) / WL1 × 100% are the number of entrained air tubes for different entrained air manners. It could be seen from Tables 2-4 that the effects of decreasing cavitation damage by three entrained air tubes are better than that by single air tube, and in
337
Fig.6 Cavitation damage at various tube area of entrained air (4 h)
Fig.7 Cavitation damage at different entrained air manners (4 h)
those tables, their sums of the tubes are 0.085 mm2, 0.283 mm2 and 0.849 mm2, respectively. It could be concluded that under the conditions of same entrained air tube area for those three sets of the results, there exists the fact that the effect of the entrained air manner of 1 + 1 + 1 is larger than that of 3. Meanwhile, it should be noted that small air tube area has better effect on reductions of cavitation damage, such as 16% from Table 2 ( A = 0.085 mm 2 ) at time of 4 hrs, 10% from Table 4 ( A = 0.849 mm 2 ). This phenomenon may confirm the results of Chen et al.’s, that is, the effects of reducing cavitation damage are related to the air bubble diameters entrained[10]. Figure 7 is the photos of the above-mentioned experiments. It could be seen that there are much more than 3 regions of prevention from cavitation damage by three air tubes, and larger extension of cavitation damage control than that by single air tube although they have the same sums of the air tube aeras.
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3. Conclusion The experimental investigations of cavitation damages have been conducted in the experimental set-up with special designs, under different experimental conditions of different entrained air manners, including entrained air pressure, air tube area and air tube number. Higher air pressure and larger air tube area have better effects of cavitation damage control due to providing much air and higher air concentration of the flow on the surface of materials. The prevention from cavitation damage by three air tubes is much more effective than that by single air tube, and the fact could be clearly expressed that 1 + 1 + 1 > 3 for the same entrained air pressure and same sum of air tube areas.
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