WU Jian-hua, GOU Wen-juan

College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China, E-mail: jhwu@hhu.edu.cn

Critical size effect of sand particles on cavitation damage*

WU Jian-hua, GOU Wen-juan

College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China, E-mail: jhwu@hhu.edu.cn

(Received February 3, 2013, Revised February 6, 2013)

The critical size of the sand particles in liquid is determined by means of the special vibratory apparatus, and it is related to various effects on the cavitation damage. The increase of the sand size or concentration would aggravate the cavitation damage if their sizes are larger than this critical size, conversely, this damage would be relieved.

cavitation damage, sand particle, sand size, abrasion

The cavitation and its damage have been investigated for about 130 years since the term “cavitation”was first used by Froude, Barnaby and Parsons in connection with the propeller performance breakdown of early steamships[1]. The normal operations would be seriously affected due to the cavitation damages in the hydro-machinery[2], the hydraulic structures[3], the navigation structures[4], the ship propellers[5], and other related structures. The mechanism of the cavitation damage is much more complicated if there are sands in the flow. A large number of investigations were conducted, focusing on the effects of the hardness, the concentration and sizes of the sand particles, and the temperature and the viscosity of the liquid, on the cavitation damage, however, the actual role of the sand size and concentration remains to be illustrated in aggravating or relieving the cavitation damage.

The investigations on the effect of the sand particle concentration show that the damage would be increased with the increase of the sand concentration in the liquid due to the abrasion wear of the sands to the structural surfaces[6,7], but it is also shown that for the solid particles, such as those of aluminum, molybdenum, and nickel, the damage may be relieved with the increase of the concentration when their concentration is above some level[8]. The effects of the particle size were experimentally investigated[8-10], and Chen et al.[10]show that the most severe damage happens in the solution containing 500 nm particles in the range of 100 nm-1.2 μm. Moreover, the damage is affected by the liquid properties, like the temperature and the viscosity. The crest value of the temperature is taken when the most damage occurs, such as 50oC, as seen from the results by Kwok et al.[11], and 65oC from the results of Auret et al.[12]. The liquid viscosity changes when some additives are added to the liquid, like polymer, which would change the cavitation damage[13,14]. The effects of the sands, however, remain to be explored.

Fig.1 Schematic diagram of the special vibratory apparatus

In the present work, experiments were carried out to study the effects of the sand particles on the cavitation damage, mainly focusing on the particle size, and the concentration, in the high-speed flow laboratory in Hohai University, Nanjing, China. Figure 1 is the schematic diagram of a special vibratory apparatus with the power of 1 100 W, the vibration frequency of19.6±0.5 kHz, and the amplitude of 50 μm, in which a sand particle moving device is used to ensure a uniform distribution of the particles in the liquid. The samples for the cavitation damage were made of AMST 1045 carbon steel. The experimental liquid, the mixture of distilled water and sand, was controlled at the temperature of 25±1.0oC by a cooling water system, and the sands came from the Nanjing reach of the Yangtze River. Three concentrations of sand were tested: 25 kg/m3, 50 kg/m3, and 85 kg/m3for five sizes of sand with d50=0.531 mm, 0.253 mm, 0.063 mm, 0.042 mm and 0.026 mm. The duration of the test is 4 h.

Fig.2 Variations of WL with C and d50

Figure 2 is the results of the effects of the sand particle size and concentration on the cavitation damage, in whichWL is the weight losses of the samples, andC is the particle concentration in the liquid, where d50=0 mm, means that the results are for the distilled water without sand particles.

Comparing with the result for the distilled water without the sand particles, the critical size of the sand particles was determined:DC=0.048 mm, and the curves could be divided into two groups, the curves where the particle size is larger than the critical size (d50=0.063 mm, 0.253 mm and 0.531 mm) and the curves where the particle size is less than the size (d50=0.026 mm and 0.043 mm).WL( C)has a positive slope for the group d50＞DC, and the larger the sand particle size and concentration, the larger is the slope. While for the group d50＜DC,WL( C)has a negative slope, which means that the cavitation damage decreases with the decrease of the sand size or the increase of concentration. The opposite effects of the particle sizes may be related to the changes of the liquid viscosity, and in its turn to the action of the microjet on the structural surfaces when the cavitation occurs[8,13]. The sand particle effects in liquids are important for the prediction of the cavitation damage.

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10.1016/S1001-6058(13)60350-9