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英文原文The cuttability of rock using a high pressure water jetPC HaganThe University of New South Wales (UNSW), SydneyMethods of improvement in the performance of mechanical rock cutting systems are continually being sought. One area being investigated is combining mechanical rock cutting tools with water jets. In this hybrid arrangement, the mass breakage mechanism of a rolling disc cutter or drag pick is coupled with the concentrated energy medium of a high pressure water jet. Research has indicated that the resultant improvements in performance of mechanical cutting tools could be due to the cutting of the rock surface through fracture and erosion thereby ameliorating the rock breakage process of the mechanical cutting tool. Damage by a water jet has been observed even in rock of high strength. This paper outlines a study on the sole use of water jets in cutting rock and the effects of changes in the principal variables of a water jet. An understanding of the characteristics and the relative importance of any changes in these variables is necessary to optimise the cutting performance of a hybrid system in terms of advance rate and energy expenditure. The variables considered in the study included nozzle diameter, water pressure, traversing speed and multiple passes of a water jet. Within the range of values studied for each variable, a change in water pressure was found to have the greatest impact on the level of surface damage in rock. Traversing speed, and to a lesser extent nozzle diameter, were also found to alter the magnitude of surface damage in rock. 1. INTRODUCTIOOne of the fundamental processes in mining is the liberation of minerals from the in situ rock mass. This can be achieved by a number of methods including abrasion and fracture. The causation of fracture in rock has long been associated with some form of mechanical indentation or the cyclic application of large impact forces. This is exemplified by the old hammer and tap method of rock drilling but it also underlies the more modern techniques of mechanical rock breakage with cutting tools such as picks and rolling disc cutters. This principle has not precluded other sometimes more subtle techniques from being used in the past such as the gentle knock in the correct orientation by a stonemason. Apart from mechanical indentation, noncontact mechanisms to initiate fracture in rock have also been developed. Probably the most significant being the detonation of an explosive within a confined space. Before the use of blackpowder, however, the ancient Chinese observed and adapted to their advantage the natural weathering process of exfoliation by accelerating the rapid changes in rock temperature with fire and water. A modern variation of this technique is the thermal jet lance (Fleming and Calaman, 1951). More recently Sellar (1991) has reported radiating rock with a pulsing laser to induce internal stress variations causing fracture wherein it was found that the pulse frequency was critical and should equal the resonance frequency of the rock. Common to all these techniques is an alteration in the internal state of stress such that bonds are broken and free surfaces created. Figure 1. View of several slots, or kerfs, cut in rock by a high pressure water jet.Another potential method of rock breakage reported to have significant potential is the application of high pressure water jets. High pressure water jets in this sense normally refer to pressures between 10 and 400MPa with a nozzle aperture of less than 1 mm. Harris and Mellor (1974) have reported that a rock surface can be significantly damaged or cut by a water jet at high pressure. As shown in Figure 1, this damage is normally in the form of a narrow slot of varying depth. Various theories have been developed to account for this damage including a theory on cavitational drag (Crow, 1973), energy balance in brittle fracture (Mohaupt and Burns, 1974) and material erosion (Rehbinder, 1976).During the early development work, it became apparent that water jets could not compete with conventional forms of rock fragmentation. A relatively large quantity of specific energy, in the order of 1000 MJ/m3, is required for this form of rock breakage which is several orders of magnitude greater than that required in conventional mechanical rock cutting. But it was found that water jets were useful when combined with conventional mechanical systems especially when cutting hard rock where tool life can be short (Hood, 1975). The most significant benefits derived from a hybrid cutting system being: a reduction in cutting forces. Fairhurst and Deliac (1986) reported an average force reduction of 30% in the cutting direction and somewhat greater reduction in the normal, or thrust, direction. increased tool life. Taylor and Thimmons (1989) reported a doubling in tool life. Hood et al (1991) reported an appreciable tool life was obtained when cutting a hard rock with a UCS of more than 200MPa, where tool life was otherwise nonexistent. Morris and MacAndrew (1986) suggested that water jets reduced the rate of tool wear by cooling the highly stressed rockcarbide greater machine advance rates. Because of the reduction in thrust force,lager penetration per revolution can be attained for a machine with a given thrust capacity. Also the reduction in thrust requirements with water jets can compensate for the loss in cutting efficiency of worn cutter tools .lower machine vibration. Fowell et al (1988) reported significantly reduced vibration levels on a roadheader cutter boom.increased product size. It has been reported that the proportion of fines can be reduced and coarser rock debris produced (Wang and Wolgamont, 1978) .lower levels of reparable dust. Taylor et al (1989) found that dust make was 80% less during cutting with jet assistance then in cutting with conventional water spray systems. reduced occurrence of incendiary ignitions. Water in the cutting groove can dissipate frictional heat, hence lowering the possibility of gaseous ignition More recent work by Lin, Hagan and Roxborough (1990) has shown that greater efficiencies can be attained in water jet cutting of rock by focusing two or more jets below a rock surface. They reported largeragments were produced and that specific energy was reduced by nearly an order of magnitude. This work confirms the behavior predicted by Mazurkiewicz et al (1978) which they termed the jet accumulation phenomena and is an adaptation of the earlier work on shaped charges by Walsh et al (1953). To understand the mechanisms of water jet assistance in a hybrid cutting system, it is necessary to study the behavior of a water jet acting in isolation to break rock. Experiments were undertaken to assess the effects of a high pressure water jet in rock cutting. Such knowledge can be used to optimise the total extraction system so that the failure mechanism of the mechanical tool is most effectively complimented by a water jet .2. LABORATORY APPARATUSThe test program made use of a commercially available high pressure, low volume pump shown in Figure 2. Filtered town water was feed through one of two hydraulically actuated doubleended, reciprocating cylinders. Each cylinder had a 20:1 pressure intensification factor. Changes in water pressure were made by adjusting the outlet pressure of a variable displacement, pressurecompensated axial piston hydraulic pump. The unit was capable of delivering 4.7 L/min at pressures of up to 380 MPa.Standard industrial sapphire nozzles with a conical outlet were used to form the water jet. The discharge coefficient of the nozzles was 0.65. A range of nozzle aperture diameters was used in the experiments varying from 0.15 to 0.36 mm.Figure 2. View of the water intensifier unit and linear cuttingtable used in the test work.A linear cutting machine was used to move the rock samples with respect to a stationary water jet. This modified planer could accommodate rock samples with plan dimensions of 450 x 450 mm on a horizontal bed at velocities between 50 and 300 mm/s. A variable frequency controller was used to regulate the speed of the electric drive motor.TABLE 1 Material properties of test rockspropertyWoodlawn ShaleGosford SandstoneUCS (MPa)145 2741.8 4.4UTS (Brazilian) (MPa)11.7 3.72.95 0.53ES (GPa)36 36.3 2.7ED (GPa)26 69.2 0.6GD (GPa)10.0 2.6-Poissons ratio0.32 0.060.13 0.05Density - bulk (t/m3)2.73 0.012.21 0.05- grain (t/m3)2.77 0.01-Shore Hardness62 3-Schmidt Rebound No69 147 2Hacksaw Abrasiveness3.11 0.56-Porosity - apparent (%)0.5 0.19.4 1- true (%)1.8 0.3-3. MATERIAL PROPERTIES OF ROCKTwo rock types were used in the study; these were Woodlawn Shale and Gosford Sandstone. A summary of the material properties of these rocks is given in Table 1. Where applicable, the material properties were evaluated according to the suggested methods prescribed by the International Society for Rock Mechanics.4. TEST PROCEDURECutting of a rock mass by a water jet is termed water jet slotting or kerfs formation. A typical configuration involves a water jet traversing across the surface of a rock as is shown in Figure 3. The principal variables in cutting with a water jet include: jet variables: nozzle diameter, water pressure, nozzle discharge coefficient and water density all of which effect the water flow rate and jet velocityoperational variables: standoff distance, nozzle traverse speed, jet attack angle and number of multiple passes.Figure 3. Main variables in water jet cutting.Other variables include those of the rock (for example compressive strength, fracture toughness, porosity, grain size and surface roughness) and of the rock mass (for example structure). These variables are, however, site dependent and tend to be overridden by the jet and operational variables.The study was undertaken with a continuous water jet on the linear cutting machine described in 2. The principal goal was to study the basic aspects of a water jet in cutting rock. Only those variables that effect the hydraulic energy of a water jet and hence the energy available to initiate fracture or erode the rock were considered. It can be shown that the energy of a water jet acting per unit distance along a rock surface, or the specific hydraulic energy, can be calculated from Equation 1. where: 1） W = specific hydraulic energy, MJ/m = discharge coefficient = nozzle pressure, MPa = nozzle diameter, mm = nozzle traverse speed, mm/s = fluid density, kg/m3As indicated by Equation 1, water pressure and nozzle diameter are the two main jet variables. Other variables such as discharge coefficient, water temperature, addition of polymers or abrasive substances and pulsing of a jet were not studied. These alter the structure of a water jet and would only tend to further enhance performance. The range of water pressure values in the study was selected on the basis of the mechanical compressive strength of the rock. It has been observed that the minimum pressure required to initiate fracture, commonly referred to as the threshold pressure, is typically of the same order as the rock compressive strength. A spread of water pressure values about the equivalent compressive strength was selected to test this observation as well as to evaluate the effect of changes in energy of a water jet on slot depth. Of the operational variables, nozzle traverse speed and the number of multiple passes were examined. In each experiment the water jet standoff distance(shown in Figure 3 as the distance between the nozzle was equivalent to 70, 90 and 140 nozzle diameters for the three nozzles sizes used. Even though this was within the range of maximum effective jetting distance, the constant absolute standoff distance tended to give some advantage (in terms of additional effective jet energy) to the largest nozzle diameter. The parameters used to assess the effectiveness of changes in variables during the slotting of rock are slot depth and to a lesser degree, specific energy. Slot depth is the mean measured depth cut below the rock surface. The study was conducted in a mode which simulated a water jet preconditioning the rock surface where the actions of a water jet and tool are combined. The study involved three series of experiments. These are outlined in the following subsections. The test program was arranged to cover as wide a range of values as possible. Unless otherwise stated, five levels were selected for each variable which increased approximately in arithmetical progression. The test schedule was randomized to minimize any errors that may have arisen due to the heterogeneity of the rock. In each case, the rock was tested after being air dried for at least 48 hours.4.1Slot depth variationThe purpose of this test series was to determine the variability in depth along the length of a slot. The series involved measurement of slot depth in Gosford Sandstone. Three slots were formed in the sandstone at pressures of 100 MPa, 140 MPa and 210 MPa, at a fixed traverse speed of 50 mm/s and a nozzle diameter of 0.23 mm.4.2Principal variables in jet slottingThis series concerned three variables of a water jet in the slotting of Woodlawn Shale. Although slot depth was the main parameter, measurements were initially made also of slot width. Details of the experimental program are contained in Table 2.TABLE 2Variables in jet slotting test program rock type: Woodlawn Shalevariableunitleveltraverse speedmm/s50 100 150 200 250Water pressureMpa100 140 175 250Nozzle diametermm0.15 0.23 0.30The series was at first organised as a partial factorial program in which one variable was held constant while the other two were varied. Nozzle diameter was fixed at 0.23 mm while water pressure was varied at each level of traverse speed. Each combination of variables was replicated at least four times. The test series was later extended to a full factorial program where nozzle diameter was also varied for each combination of traverse speed and water pressure. This was done to verify the trends established at the original fixed nozzle diameter of 0.23 mm. The number of replications at the other two nozzle diameters of 0.15 mm and 0.30 mm was reduced from four to two.4.3Multiple passes of a jetThis series involved an examination of the effects of successive slot deepening in rock by multiple passes of a water jet. The tests were conducted in Gosford Sandstone. The series involved two parts. The first part involved the progressive deepening of a slot and measurement after each successive pass of a water jet. A total of twelve consecutive passes were made over the same slot. Each test was performed at a fixed nozzle diameter of 0.23 mm, traverse speed of 150 mm/s and water pressure of 210 MPa. The relatively high level of water pressure was chosen to ensure that even the strongest regions within the rock would be cut by a water jet. In the second part, water pressure was varied with nozzle diameter and traverse speed fixed at 0.23 mm and 150 mm/s respectively. Measurements of slot depth were made after one and then five consecutive passes of a water jet for each level of pressure. Each test was replicated at least three times. Details of the experimental program are contained in Table 3.TABLE 3 Level of variables in multiple pass testsnozzle diameter: 0.23 mmtraverse speed: 150 mm/srock type: Gosford Sandstonevariableunitlevelwater pressureMpa70 140 210 275 345no. of passes1 55. RESULTS5.1Slot widthThere were little discernible changes in slot width with either water pressure or traverse speed. This is in agreement with previous research. A casual observation made during the tests indicated that the incidence of surface spalling tended to decrease with increasing water pressure.5.2Slot depthFigure 4 illustrates the extent of the variation in slot depth. The irregularity along the slot base when reported in absolute terms of standard deviation was found to increase with pressure from 0.66 through to 0.83 and finally 1.13 mm. The coefficient of variation in the mean slot depth decreased with water pressure from 50% to 33% and finally 24%; that is the variability in slot depth tended to decrease with water pressure and consequentially with slot depth.Figure 4. Superimposition of the depth profiles for three slots formed by a water jet at three different water pressures in Gosford Sandstone.This indicates that within the rock matrix, there exist regions of high strength material that are highly resistant to fracture and erosion by a water jet. Although slot depth increased with water pressure, it competes against the variation in toughness within the rock where the latter can dominant the forces of a water jet. The effectiveness of the high water pressures diminishes with depth causing greater absolute variations in depth.5.3Effect of nozzle diameterSlot depth was found to increase with nozzle diameter as shown in Figure 5. Evidently as water pressure increases, the effect of nozzle diameter on slot depth becomes more significant. Hence, nozzle diameter is not the insignificant variable that has sometimes been supposed.Figure 5. Effect of nozzle diameter on slot depth at different pressures in Woodlawn Shale.Traverse speed was fixed at 150 mm/s.Nikonov (1971) has reported that slot depth increases linearly with nozzle diameter. Based on these results, such a relation would imply a positive slot depth at zero nozzle diameter. It would appear more likely, however, that slot depth would vary as some power function of nozzle diameter such that: 2）where: h = slot depth, mm k = constantand m is some value less than 1.In this rock it appears there is a pressure, somewhat less than 100 MPa, at which there is little variation in slot depth with nozzle diameter and, where the water jet is ineffective in cutting rock. This may equate with the threshold pressure concept referred to earlier in 4.As was shown in Equation 1 both nozzle diameter and water pressure influence the specific hydraulic jet energy and hence the level of energy available to cut rock. Figure 5 confirms also, as might be expected, that for a given amount of energy more benefit can be got from increasing pressure rather than by increasing nozzle diameter. For example, the two points A and B shown in Figure 5 are points of equal hydraulic energy but B is five times deeper than A. Point B has an effective nozzle diameter equal to only twothirds of A and is about double the water pressure. Therefore, in terms of maximizing slot depth for a given level of energy, greater benefits are gained from higher pressures than larger nozzle diameters (and hence flow rates).It is worth noting that each curve for the different pressures appears to approach a limiting slot depth, indicating that there is an optimum nozzle diameter above which no useful increase in slot depth can be gained and that this diameter increases with water pressure. The four curves also indicate a common intercept which can be termed the critical diameter such that:where: = critical nozzle diameter, mmat h = 0This critical diameter is the minimum diameter necessary to cause any significant failure in rock. It can be expected that the value of critical diameter will vary between rock types and be dependent on This critical diameter is the minimum diameter necessary to cause any significant failure in rock. It can be expected that the value of critical diameter will vary between rock types and be dependent on size, porosity and permeability. If a linear relation were assumed between slot depth and nozzle diameter then the critical diameter would tend to zero. If on the other hand the trend is as indicated in Figure 5, which is similar for all four water pressures, then for Woodlawn Shale: dc 0.08 mm hence Equation 2 becomesFigure 6 shows a graph of normalised slot depth against nozzle diameter. Normalised slot depth is a measure of slot depth expressed in relative units of nozzle diameters and is a parameter used in some Equations to predict slot depth, as for example the General Cutting Equation. The graph shows that over the range of nozzle diameters studied, normalised slot depth remains constant for a given water pressure, that is normalised slot depth is independent of nozzle diameter. Similar relations were also found for the other two traverse speeds of 50 and 250 mm/s. Based on the model of Equation 2 this would be expected if m = 1.Figure 6. Effect of nozzle diameter on normalized slot depth at different pressures.Although a slight benefit in the form of an increase in slot depth is evident at a speed of 150 mm/s, it is not significant nor is it consistent with the other two traverse speeds. 5.4Effect of water pressureFigures 7a, 7b and 7c are graphs of water pressure against slot depth at different nozzle diameters for the three traverse speeds.Figures 7a, 7b and 7c. Effect of water pressure on slot depth at different nozzle diameters and traverse speeds.Over the range of water pressures studied, the results suggest a near linear relation between water pressure and slot depth. This linear relation has also been observed by Brook and Summers (1969) in sandstone at pressures up to 70 MPa and, by Harris and Mellor (1974) in granite at pressures up to 400 MPa. The graphs suggest also that slot depth is dependent on nozzle diameter and traverse speed, where the rate of increase in slot depth increases with nozzle diameter and inversely as the traverse speed. There is clear evidence from each of the graphs of a threshold water pressure and that it appears to be independent of nozzle diameter. For this rock type, the value of the threshold water pressure is about 60 80 MPa. Although this water pressure is equivalent to about half of the mean uniaxial strength of this rock, it is of a similar magnitude to the minimum measured value of compressive strength. It is also equivalent to about three times the mean uniaxial tensile strength or about twelve times the minimum measured value of tensile strength. The value of the threshold pressure appears to be only marginally effected by traverse speed. Crow (1973) noted that the threshold pressure should increase with traverse speed. Considering the fivefold increase in speed, the small increase in threshold pressure shown in each of the graphs does not appear very significant. The variation in normalised slot depth with water pressure appears to be linear as shown in Figure 8. This holds equally true for all nozzle diameters since it was earlier shown in Figure 6 that for a given pressure there is little variation in normalised slot depth with nozzle diameter.Figure 8. Effect of water pressure on normalised slot depth at different traverse speeds.Hence, normalised slot depth can be expressed in terms of water pressure such that: 3）Where:=water pressure, MPaassuming a linear relation this can be rewritten as: which applies for the case whenwhere :=standoff distance, mmAn intercept can be observed on the water pressure axis in Figure 8 where normalised slot depth equals zero. The value of this intercept is equal to the threshold pressure. This intercept is also shown to increase slightly with traverse speed. Hence, threshold pressure is equal to some function of nozzle traverse speed as was earlier predicted by Crow (1973) that is: 4）where: =threshold pressure, MPaFigure 9 shows a plot of normalised slot depth against water pressure where pressure is expressed in terms of the intercept on the pressure axis from in Figure 8. Figure 9. Effect of normalised water pressure on normalised slot depth.Hence the plot should show an intercept on the normalised pressure axis equal to unity.Figure 9. Effect of normalised water pressure on normalised slot depth.Rearranging Equation 3 in terms of normalised pressure: 5）substituting on the basis of the observed linear relation: 6）as the lines pass through the point (1,0) then a = bsubstituting this relation into Equation 6 and multiplying both sides by the constant : 7）withthat is equal to the inverse of the gradient of normalised slot depth against normalised water pressure. TABLE 4Values of threshold pressure and pressure constant at different traverse speeds.Traverse speed(mm/s)Threshold pressure(MPa)pressure constant,，5068.80.11415082.30.14025086.70.171Equation 7 is similar to the dimensionless cutting Equation derived by Veenhuizen et al (1978). Values for threshold pressure and are given in Table 4. Figures 10a, 10b and 10c. Effect of traverse speed on slot depth at different nozzle diameters and water pressures.5.5Effect of traverse speedGraphs of the variations in slot depth with nozzle traverse speed are shown in Figs 10a, 10b and 10c. The graphs indicate an inverse relation between traverse speed and slot depth such that slot depth decreases with traverse speed. Harris and Mellor (1974) found similar trends whereby at traverse speeds greater than 300 mm/s, slot depth was insensitive to changes in traverse speed but as speed decreased below 200 mm/s, slot depth increased dramatically. On the basis of the apparent trends in Figure 10 it can be shown that:Or8） Where k is some function of water pressure and c tends to zero. If the relation were a simple hyperbolic expression as shown in Equation 8, there would be a tendency with increasing speed for the slot depth to approach zero. Intuitively this is to be expected since for any given pressure, a speed must eventually be reached at which the jet will be ineffective in damaging the rock surface. Each of the curves in Figs 10a, 10b and 10c is consistent with this hypothesis but there are strong indications that useful penetrations could still be achieved at quite high traverse speeds. Figure 11. A schematic diagram of a water jet moving normally across a rock surface.To define the relation between traverse speed and slot depth consider an idealized case of a water jet with a jet velocity, moving normally across a flat surface at a nozzle traverse speed, as is illustrated in Figure 11. Based on this arrangement it can be shown that 9）where:=water jet velocity/sand as jet velocity is a function of water pressure and nozzle diameter: 10）Equation9 is similar to the energy balance Equation developed by Mohaupt and Burns (1974) and to the General Cutting Equation where a linear relation was assumed between normalised slot depth and the inverse of traverse velocity. But the Equation does not take into account the material properties of rock. Figure 12 is a graph of these terms for the four pressure levels and three nozzle diameters. Figure12. Effect of inverse traverse speed on normalised slot depth.Figure 12. Effect of inverse traverse speed on normalised slot depth.Although the linear functions shown in Figure 12 appear to be lines of best fit, they do not correlate with the expectations at either extremity of the function. First, it could be expected that as traverse speed approaches zero, the slot depth should approach some limiting depth. With a simple linear relation no convergence to some limiting depth takes place as the inverse of traverse speed approaches infinity (that is as traverse speed approaches zero). Secondly, it could be expected that as traverse speed increases then depth would tend to zero. As the inverse of traverse speed approaches zero (that is a speed approaches infinity) then depth approaches some finite positive depth. Hence, a simple linear model of these terms is insufficient to describe the relation between traverse speed and depth. A better model to describe the variation in traverse speed with slot depth is that proposed by Veenhuizen et al (1978) where normalised slot depth varies as the inverse of the ratio of jet and traverse velocities. This model has been incorporated into the modified dimensionless cutting Equation of Enever and Tooley (1985). In their Equation, normalised slot depth is assumed to vary as the square root of the ratio of jet velocity to traverse speed that is: 11）5.6Effect of multiple jet passesThe final set of tests involved successive multiple passes of a water jet over the same groove. This has several advantages where with a particular jet configuration, a desired slot depth may not be achieved in one pass. Multiple passes of the jet allow time for the water and rock debris to drain leaving the surface exposed for reapplication and shock impact by the jet. This is beneficial since some of the energy would otherwise be absorbed by water in the slot. Figure 13 shows the effect of multiple passes by a water jet on the cumulative increase in slot depth. The graph shows that also depth tends to some upper limit with the number of passes. Though the initial increases in slot depth are large and of the order of 70%, this rate of increase quickly diminishes after five passes to an insignificant level. Figure 13. Effect of multiple pass slotting by a water jet on cumulative slot depth.Figure 14 shows how the coefficient of variation in slot depth decreases and tends to stabilise with the numbers of passes. The variability in slot depth after eight passes is probably a reflection of the variation in strength of the test rock. Differences in strength dominate the slot profile at depth because of the energy losses within the slot through friction with the sidewalls, etc.Figure 14. Effect of multiple pass slotting on the variation in slot depth.A graph of the effect of water pressure on multiple pass cutting is shown in Figure 15. The results are consistent with the trend in Figure 13 where for a given water pressure, the cumulative slot depth increases with the number of passes. The graph indicates that the benefits of multiple pass slotting improve with water pressure. As the water pressure increases, so the rate of increase in cumulative slot depth with the number of passes also increases. In addition, the coefficient of variation in slot depth is found to decrease with an increase in water pressure. Figure15. Effect of water pressure on slot depth in multiple pass slotting.Figure 15. Effect of water pressure on slot depth in multiple pass slotting.6. CONCLUSIONBased on the results of the test program, the following of passes. The variability in slot depth after eight passes is probably a reflection of the variation in conclusions can be made. 1. There is little discernible change in slot width with any of the variables tested other than nozzle diameter. 2. Slot depth increases marginally with nozzle diameter at low water pressures but tends to become more significant as the pressure increases. There is evidence of an optimum nozzle diameter above which no useful increase in slot depth is found. This optimum diameter tends to increase with water pressure. 3. The results infer the existence of a critical nozzle diameter. This diameter is defined as the minimum nozzle diameter necessary to affect any significant damage to a rock surface. In Woodlawn Shale, the critical diameter is equal to 0.08 mm. 4. Slot depth increases linearly with water pressure. The slope of this curve increases directly with nozzle diameter and inversely as the traverse speed. 5. There appears to be a minimum water pressure necessary to cut rock. This pressure, termed the threshold water pressure, appears to be only marginally affected by nozzle traverse speed. The threshold pressure is of similar order as the minimum measured value of compressive strength or about twelve times the lowest measured value of tensile strength. Values for threshold pressure were found to vary from 68.8 to 86.7 MPa for traverse speeds ranging from 50 to 250 mm/s. 6. There appears to be a linear variation in normalised slot depth with water pressure which takes the form was found to equal approximately 0.14 for Woodlawn Shale though the value varied slightly with traverse speed. 7. Slot depth decreases with traverse speed in a hyperbolic fashion. Normalised slot depth varies as the inverse of traverse speed to some power. Even though a simple linear model may appear to correlate well with the data, it is limited in its prediction of slot depth. 8. Slot depth can be increased by multiple passes of a water jet. Despite this it can be an inefficient process especially after several passes. Total slot depth becomes asymptotic to some depth value with the number of passes. Although increases in slot depth with each pass can be large and of the order of 70%, the increase in depth quickly reduces after five passes. The efficiency of multiple passes in terms of slot depth tends to increase with water pressure. ACKNOWLEDGMENTThe assistance of Professor FF Roxborough, Ross Marbles and the technical staff of the School of Mining Engineering, The University of New South Wales, is acknowledged as well as the financial support of CRA. SYMBOLS: discharge coefficient : critical nozzle diameter, mm : nozzle diameter, mm : water pressure, MPa : threshold pressure, MPa : fluid density, kg/m3 : standoff distance, mm : water jet velocity, m/s : nozzle traverse speed, mm/s W : specific hydraulic energy, MJ/m k : constant : pressure constant h : slot depth, mm中文翻译岩石的高压水流切割性PC Hagan新南威尔士大学（UNSW）,悉尼岩石切割系统的力学表现形式中有许多改善的方法正在不断的被提出。其中一个正在研究的领域就是将岩石的力学切割工具与水射流结合起来。在这种“混合”装置安排中，具有滚动圆形刀片或拖齿结构的巨大破裂装置与高压水射流作为介质的集中能量结合起来。研究调查表明，岩石切割工具的表现性能得到提高得益于切割岩石表面时，利用岩石本身的破裂和侵蚀，而这些裂缝和侵蚀在切割过程中改善了岩石破坏进程。由水射流形成的破坏甚至在高强度岩石中已经得到研究。这篇论文概括介绍了一个单独利用水射流切割岩石的研究和水射流主要变量改变所导致的结果。就改善效率和能量耗费而言，对特点和这些变量改变的相关重要程度有一个好的理解对于完善一个混合系统的切割性能具有重要意义。研究中考虑到的变量有：喷嘴直径，水压，喷头横向移动速度以及水射流的重复冲击次数。在对各变量重要性研究中发现，水压的改变被认为对岩石表明破坏程度具有最重要的影响。喷头横向移动速度和缩小喷嘴直径同样被发现可以改变岩石表面破坏量的大小。1.简介采矿工程中的一个基本的步骤就是将处于原岩体中的矿物释放出来。这一过程可以通过包括摩擦和破坏在内的一系列方法达成。岩石中这种破坏的结果长期以来被认为是与一些形式的力学压缩或巨大破坏力的循环加载有关的。这些都被岩石钻孔中古老的“锤子和凿岩”方法得到例证，但是这同样是基于利用诸如凿和圆形刀片等更加现代的岩石破坏技术。这个准则并没有阻碍过去一些精细的技术的利用，例如石匠在正确的方向上进行温和的敲击。除了力的压缩以外，引起岩石的初始破坏的无接触机制也同样得到发展。或许，最重要的贡献就是有限空间内的进行的爆破。在黑火药使用前，古代中国人发现火和水加速能改变岩石温度，并利用这个特点加快岩石剥落物的自然风化。这种技术的一个现代变化就是热的水射流切割。更近一些，Sellar报告称，利用激光脉冲来辐射岩石以便引诱内部应力变化导致破坏，同时发现脉冲频率处于临界状态并且应该等于岩石的共振频率。这些所有的技术共同的是内部应力结构的改变，导致粘结破坏，自由面生成。另外一种极具潜力的岩石破坏方法就是高压水射流的应用。高压水射流在这种意义下所指代的是压力在10400MPa，喷头缝隙小于1mm。Harris和Mellor称岩石表面能被具有高压力的水射流严重破坏或切割。如图1，这些破坏是正常的具有不同深度的细槽。为了解释这种破坏，许多种理论被提出：例如空化阻力理论，脆性破坏和材料侵蚀处的能量平衡理论。图1.被高压水射流切割岩石产生的切缝。在早期的研究工作中，水射流明显不能与传统的岩石碎片相对抗。在这种岩石破坏中需要一个相对大量在1000等级的特定能量，这个能量比传统机械岩石破坏所需能量大好几个等级。但是发现当和传统机械系统结合起来时，特别是当切割硬岩时切割工具寿命较短，水射流十分有用。从一个混合切割系统中获得的最重要的好处是：切割力的减小。Fairhurst 和Deliac声称在切割方向上平均减少30%的力，并且在正应力和切应力方向有更大的减少量。延长了切割工具的寿命。Taylor 和Thimmons称工具寿命会延长一倍。Hood和其他一些人发现当切割USC超过200MPa的硬岩时，在这种情况下工具仍然具有客观的寿命，否则工具寿命是不存在的。Morris和MacAndrew则认为水流冷却了，高度压缩的岩石碳化物界面从而减少了工具的磨损。大型机械提高效率。由于降低了切向应力，所以在给定切应力的条件下每次都会有更大的正向渗透力被装在机器上。同时，水射流在切割方向上需求的降低可以补偿坏掉的切割工具的切割效率。更轻微的机器震动。Fowell和一些人发现这个系统能够明显地降低掘进机轰鸣声的的震动等级。增加了产品的尺寸。据称碎石的体积减小，同时产生更加粗糙的岩石碎片。更低等级的可呼吸性粉尘量。Taylor等人发现相比传统的水流喷射系统，利用水流助力可以降低80%的粉尘量。降低了可燃物燃烧的发生。切槽中的水能够驱散摩擦产生的热量，因而能够降低气态燃烧的可能性。更近的一些由Lin,Hagan和Roxboough所做的工作表明，在水射流切割岩石时，将两股或者更多股的水流置于岩石表面下方能偶获得更高的效率。在这个过程中会产生大量的碎片，并且能够产生一系列等级的特定能量。这个结论证实了由Mazurkiewicz等人所预言的“射流积累效应”，同时也与早期由Walsh等人主导的形状实验相适应。要理解在混合切割系统中水射流辅助的机制，就必须研究水射流单独作用时切割岩石时的行为。许多实验就是用来评定高压水在切割岩石时的作用的。这些知识可以用来改善整个开采系统，以便机械工具的破坏机制能够最有效的被水射流完善。2.实验室机械该测试计划利用了一种如图2中所示的，用于商业的高压、低音泵。净化过的居民用水由液压动力、两端往复运动的两个滚筒中的一个提供。每个滚筒都有20:1的压力强化因素。水压力的改变可通过调解多重替换、压力补偿轴流式水泵的压力出水口来实现。控制单元可以以达到380MPa的压力输送4.7L/min的水流。用工业标准锥形蓝宝石喷头形成水流。水管出水效率是0.65。管孔直径从0.15到0.36mm的一系列水管运用于实验中。图2试验中所用到的水力增强单元和线性切割桌面。在一个稳定水流状态下，用直线型切割机切割岩石试样。这种修改过的刨床可以为标准规格450X450mm的岩石试样提供一个速率在50300mm/s的实验环境。为了控制电机的速率使用了不同频率的控制器。表1 待测岩石的材料性质性质木草坪页岩戈斯福德砂岩单轴抗压强度 (MPa)145 2741.8 4.4极限抗拉强度(巴西法) (MPa)11.7 3.72.95 0.53ES (GPa)36 36.3 2.7ED (GPa)26 69.2 0.6GD (GPa)10.0 2.6-泊松比0.32 0.060.13 0.05密度(t/m)2.73 0.012.21 0.05颗粒 (t/m)2.77 0.01-肖氏硬度62 3-施密特回弹系数69 147 2锯形磨损3.11 0.56-表面孔隙性 (%)0.5 0.19.4 1- true (%)1.8 0.3-3.岩石的材料性质研究中使用了两种类型的岩石：木草坪页岩和戈斯福德砂岩。两种岩石材料的性质总结在表1中给出。岩石的性质依据国际岩石力学协会有关规定的方法测得，可以使用。4.测试程序用水力切割岩体被称为水力开槽或者切割构成。如图3所示，典型的结构包括用水流穿过岩石表面。用水流进行切割时主要变量包括：水流变量：管口直径、水压、喷头喷射效率、水的密度这些所有影响水流等级和速率的因素实验变量：与表面间距离，水管喷头横向移动速度，水流冲击角度以及管道数量图3水力切割中的主要变量。其他一些变量包括岩石（例如抗压强度、裂隙强度、孔隙性、颗粒尺寸以及表面粗糙度）和岩体（例如结构）的变量。这些变量在位置上依赖并且有可能过度依赖于水流和实验变量。该研究利用了连续水流在第2部分讲到的线性切割机器上进行。主要目的是研究水流切割岩石是的基本方面。只考虑这些影响到水流的液压能量并且此种能量足以开始破坏或侵蚀岩石的变量。可以看到在岩石表面每移动一个单位距离所消耗的能量，或者特指的液压能量也可以利用方程1算得这里： 1)W=特定的液压能，MJ/m =喷射效率 =水管压力，MPa =喷嘴直径，mm =喷嘴横向移动速度，mm/s =液体密度，kg/m由方程1可以看出，水压和喷嘴直径是最主要的两个变量。其他变量诸如喷射效率，水温，聚合附加物或研磨料物质以及水流的脉冲并不研究。这些变量改变了水流的结构并且因而导致更进一步的表现。研究中，水压力值的范围选择依据的是岩石的力学抗压强度。观察到岩石开始破裂的最小压力，也就是通常被称为临界值，与岩石的抗压强度顺序具有典型的相似性。为了测试这个观察项目，同时估计在一个切缝深度上水流能量的改变引起的效果，选用了一系列与抗压强度相同的水压力值。在操作变量中，检查了水管喷头横向移动速度和重复冲击次数的数量。在每个试验中，水流的相隔距离（如图3中所示，喷射出口到岩石的距离）修正为21mm。这就意味着岩石和出水口之间的有效孔隙近于14mm。平衡距离分别为70,90和140的三种的管直径的水管在试验中得到应用。尽管这在最大影响喷射距离范围之内，持续的绝对平衡距离有可能给最大水流直径带来优势（就额外的有效水流能量而言）。用来评价在岩石裂缝过程中各变量变化有效性的参数有裂缝深度和更低级别的特别能量。切缝深度是在岩石表面下的主要可测量深度。该研究是以一种模仿水流事先调节岩石表面通过在切割工具前并且在一条直线上形成切缝的方式来操作的。这种模式与水流的从在哦和工具结合起来相互作用是不同的。该研究包括三个系列的试验。这些实验在以下的一些小部分中概述。测试计划尽可能广泛的覆盖一系列数值。如果不是另外声明，每个变量都选择了五个级别，是近似于数值方式的增长。为了尽可能减少由岩石杂质所引起的误差，测试计划是随机的。在每种情况下，岩石放置于空气中至少48小时干燥处理。4.1 切割深度的变化这个测试系列的目的是判断随着裂缝长度的变化裂缝深度的变化情况。这个系列包括了在戈斯福德砂岩中裂缝深度的测量。在控制的喷头横向移动速度为50mm/s，喷嘴直径0.23m，压力分别为100MPa，140MPa和210MPa下砂岩形成三条裂缝。4.2水流切割中的主要变量这个系列测试在用水流切割木草坪页岩时关注三个变量。尽管切割深度是主要变量，最初也测量了切割宽度。试验项目的详细情况参照表2.表2水流切割测试中的变量岩石类型：木草坪页岩变量单位等级喷头横向移动速度mm/s50 100 150 200 250水压Mpa100 140 175 250喷嘴直径mm0.15 0.23 0.30这个系列最初设计为因子程序，其中一个变量保持稳定，另外两个因子变化。喷嘴直径修正为0.23mm，水压以不同等级的喷头横向移动速度变化。每个变量组合至少重复四次。该系列测试最终延伸为全变量计划，即对于任何一个喷头横向移动速度和水压组合，喷嘴直径也发生变化。这样做的目的是为了证明建立最初喷嘴直径为0.23mm的基础上的发展趋势。另外两种喷嘴直径0.15mm和0.30mm的试验重复次数由4次减少为2次。4.3水流的多次冲击这个系列测试包括检测多次水流连续加深切缝的效果。这个试验是在gosford 砂岩上做。测试包括两部分。第一部分包括逐渐加深切缝和每股水流冲击后的测量。总共12股连续的水流在同一个切缝上通过。每次测试规定喷嘴直径为0.23mm，喷头横向移动速度为150mm/s，水压210MPa。采用相对较高水压的目的是为了保证岩石内部最坚硬的部分能够被水流切开。第二部分，将喷头横向移动速度和喷嘴直径分别设定为150mm/s和0.23mm并且改变水压。每次水流冲击后测量一下裂缝深度，然后再以不同的水压连续进行五次切割。每个测试进行至少三次。试验项目的具体内容见表3.表3在多次冲击测试中变量的等级喷嘴直径：0.23mm喷头横向移动速度：150mm/s岩石类型：戈斯福德砂岩变量单位等级水压Mpa70 140 210 275 345冲击次数1 55.结果5.1 切缝宽度对于水压或喷头横向移动速度而言，裂缝宽度出现了很小可辨别的变化。这与之前的研究一致。测试中一次偶然的观察预示着随着水压的增加，岩石表面的破碎的概率有可能减少。5.2 切缝深度图4表明了切缝变化的范围。就标准误差而言，随着水压的增加切缝底部不规则宽度从0.66增加到0.83直到1.13mm。变量的协同作用在这期间随着水压的减小，切缝深度从50%到33%最终到24%；这就是随着水压减少切缝深度的相对可变性并且导致影响切深。图4.三种不同水压力情况下gosford 砂岩切缝深度叠加的剖面这就意味着在岩石模型内部，存在着具有很高强度的材料介质，能很强烈的抵制由水流引起的破裂和侵蚀。尽管切割深度随着水压增加，水压还是要与岩石内部的强度进行比较，因为在岩石中这部分对于水流的力量具有支配作用。高水压在切割深度上的效果减半导致了绝对深度的变化。5.3喷嘴直径的影响效果由图5可以看出，切割深度随着喷嘴直径的增加而增加。很明显，随着水压增加，喷嘴直径在切割深度上的影响变得更加重要。因此，喷嘴直径不像之前某些时候所想象的那样不重要。图5.在木草坪页岩中不同水压力值下喷嘴直径对切缝深度的影响。喷头横向移动速度设定为150mm/s.Nikonov（1971）曾提到，切割深度随着喷嘴直径线性增长。依据这些结论，这样一个关系预示着在起点喷嘴直径时一个肯定的切割深度。然而，更有可能的是，切割深度会随着喷嘴直径的某些力的功能而不同，例如： 2）其中：h=切缝深度，mmk=常量m是一个小于1的值。在这块岩石中出现了一个略微小于100MPa的压力，在这个压力值下，水流对于切割岩石几乎不具有效果，并且在切缝深度上几乎没有改变。这可能就是与之前4中所提到的临界压力概念相同。如方程1中所示，喷嘴直径和水压都会影响特定的水射流液压能，并且因此该能量可以切开岩石。正如我们可能所期待的，图5也证明，给定确定的能量值，增加水压力值要比增加喷嘴直径更有好处。例如，图5中的A点和B点具有同样的液压能量值，但是B点要比A点深许多。B点的有效喷嘴直径大约是A点的2/3，但是水压却近于A点的两倍。因此，就给定一个能量值要达到最大切缝深度而言，更高的水压要比更大的喷嘴直径会更有利（因而还有流动指标）。必须注意到每次不同的压力值进行的切割可能会接近一个有限的切