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Abstract

During the last few years, different research groups have been developing systems for the transition of abrasive water jet into ice abrasive water jet. The aim of this new technology is to make the technology cleaner from both practical and ecological points of view. Mineral abrasive is replaced with ice grains that melt away after the machining process, leaving the workpiece uncontaminated. Several different approaches to this technology were studied. Thermal aspects of integrating the ice abrasive water jet technology into commercially available machines were considered. The results and analyses of water temperature measurements on the ice abrasive water jet machine are presented in this article.

Keywords

Abrasive water jet ice grains ice jet machining subcooled water temperature measurements

Introduction

The use of high-speed water for material removal is not a recent idea and has been used for centuries in uses such as mining and soil removal. Because of their versatile ability to cut virtually all types of materials, today both water jet (WJ) and abrasive water jet (AWJ) technologies are seeing increased industrial implementation.

Both technologies have comparative advantages such as flexibility and low thermal influence on the workpiece, and also some inherent drawbacks, such as kerf taper, non-uniform roughness, and in the case of AWJ also contamination of the workpiece with embedded abrasive grains. In most applications, this is not a problem, since the workpiece is either machined afterwards or this issue is not important. A drawback in the case of pure WJ is that its material removal rate is significantly lower compared to AWJ and some of the thermal and electro-thermal nonconventional machining processes.

This article reports on a novel AWJ technology, more specifically on its thermal aspects. The technology is called ice abrasive water jet (IAWJ) or ice jet (IJ), and compared to conventional AWJ, it uses ice grains instead of mineral abrasive. It is expected that the material removal capability of this technology would be between pure WJ and AWJ. The motivation for its development is an approach towards more ecologically acceptable machining technologies as well as to offer new possibilities, especially in applications that demand minimal workpiece contamination with abrasive or heat, such as food processing and medical applications. A further application is machining workpieces with low melting points, such as polymers, where the low temperatures of IAWJ could offer better quality of the cut with reduced burr.

Several authors^1–8 have considered the possibility of substituting the mineral abrasives with ice grains. Generally, there are two principles of ice entrainment which are commonly accepted in the literature: (1) in situ creation of ice grains during the process, either from adiabatic drop of pressure after the water nozzle or from injection of cooling media,^5,6,8,9 and (2) creating ice grains a priori.^1,3,5,10 However, both principles are not equally feasible as will be shown later in the article. Figure 1 classifies these methods according to the production method.

Figure 1.

Taxonomy of the IAWJ and ice grain generation methods.

The article presents the results obtained from water temperature measurements taken at different parts of the custom-built WJ machine, specially adapted to be later used for IAWJ machining. The IAWJ prototype is a combination of a priori prepared ice particles injected into the cutting head together with gaseous cooling media and a subcooled WJ. In order to measure the thermal conditions that the ice grain abrasive will be exposed to, the temperatures of water were taken at normal operating conditions as well as when water under high pressure is cooled. Several research works are reported^4,8,11–13 which deal with water temperature measurement in order to analyse the AWJ process, but none deal specifically with temperature characterization of the AWJ machine at both normal and low temperatures.

Our research groups have performed preliminary work in this area⁴ by measuring temperatures on the commercial AWJ machine, which gave initial insight into thermal conditions on such machines. Kovacevic et al.¹¹ and Mohan et al.¹² studied the temperature distribution in the AWJ focusing nozzle in order to monitor the wear of the nozzle using an infrared thermography method. Kovacevic et al.¹¹ measured the temperature distribution for three differently worn focusing nozzles. Two relations were established: as the nozzle diameter increases due to wear, first the peak temperature decreases and second the position of the temperature peak moves towards the nozzle exit. Liu and Schubert¹³ measured the temperature on the focusing tube, on the flash AWJ system in order to control the temperature of the water for piercing different materials. Bach et al.⁸ monitored the temperature at the inlet and outlet of the pre-cooling system for their in-process ice particle generation system, where they cooled the water from 25 °C to −20 °C.

Based on the presented background and preliminary experiments,⁴ an experimental set-up was built in order to test the influence of pressure, high-pressure water temperature and orifice size on the water temperature variation throughout the IAWJ system.

Theoretical background

This section describes the physical properties of water and ice that are important for the IAWJ machining process. Depending on the principle of IAWJ generation being used, the water is either cooled down under high pressure and is later partially transformed into ice due to adiabatic pressure drop or it is frozen at atmospheric pressure to produce ice grain abrasive.

The water can be cooled below 0 °C at high pressures and still remain liquid, with its lowest temperature of −22.1 °C at pressure of 215.7 MPa,¹⁴ as shown in the phase diagram of water in Figure 2.

Figure 2.

Phase diagram of ice for the range of water pressures and temperatures of interest.^14,15

As the ice grains are going to be used as an abrasive, temperature dependence of properties such as hardness, flowability and elastic modulus is important.

To produce the ice grains, water is either atomized into water droplets and frozen producing spherically shaped grains or frozen into block and crushed down to the desired size to produce sharp edged grains. Research shows that it is feasible to use ice as abrasive in processes such as dry ice blasting,^16–18 air ice blasting^{3,10,17,19,20} or ice water slurry jetting.^1,17,21,22 However, the cutting efficiency of such abrasive processes has not yet been determined.

The hardness of ice at low temperatures is difficult to measure and the available data are scarce. Several studies^20,23,24 report that the hardness of ice grains increases with decreasing temperatures. The lowest temperature at which the hardness of ice was measured and reported is −78.5 °C, with the value of 6 Mohs.²⁴ For comparison, softer mineral abrasives such as olivine have hardness of 6.5 Mohs while harder abrasives such as aluminium oxide have hardness of 9 Mohs.²⁵ This suggests that, from the point of hardness, ice at low temperatures is a suitable replacement for abrasive material, provided that it could be kept at low enough temperatures until it reaches the workpiece. Ice grain abrasive to be used in our IAWJ prototype will be cooled with liquid nitrogen to −195.8 °C.¹⁴ The hardness of ice grains at this temperature could not be found in the literature, but it is expected to be even higher.

However, as the generated ice grains are small in diameter, they tend to warm up and could potentially melt when injected into the high-speed WJ, thus reducing their hardness and other mechanical properties. The rate of thermal exchange for ice grain of 0.1 mm in diameter, injected into the 200-MPa WJ, was calculated by Truchot et al.⁵ They estimated that it would take 0.2 ms for the grain to change its temperature from −194 °C to 0 °C in the jet of water with 40 °C. This time is in the same order of magnitude as the time it takes for the jet created at that pressure to travel through the cutting head.

It can be concluded that for the process such as IAWJ, it would be beneficial for the water to be cooled to as low temperatures as possible before injecting the ice grains.

Deformation constants such as compression and toughness strength, Poisson’s ratio and Young’s modulus are not determined very often even for mineral abrasives.²⁶ The same is true for ice abrasive, where these kinds of data at low temperature are even scarcer. In all cases, Young’s modulus is given most often and is therefore easiest to compare. In the temperature range from −40 °C to −3 °C, ice behaves as an almost perfectly elastic body.²⁷ Young’s modulus of ice is 9.36 GPa at −40 °C²⁸ which is also characterized by moderate anisotropy.²⁹ The modulus increases with decreasing temperatures. In comparison, Young’s modulus for softer mineral abrasives such as olivine is around 125 GPa.³⁰

The thermal conductivity coefficient is also temperature dependent and increases with lower temperatures.¹⁴ This is important during the injection phase of abrasive into the cutting head where ice grains get accelerated by the high-temperature WJ.

It can be concluded that in order to achieve maximum efficiency of the cutting process, the temperature of ice grains hitting the workpiece should be as low as possible. Furthermore, the data in this section suggest that the principle of IAWJ generation with partial phase transformation of the WJ due to the adiabatic pressure behind the water nozzle is possible. However, the temperature of ice grains created in such a way would be relatively high, and thus lacking the proper mechanical properties to be effectively used as an abrasive. This principle should be used in combination with the injection of pre-generated ice grain abrasive in order to provide the best temperature conditions until such abrasive reaches the workpiece. To summarize, controlling the temperatures of the WJ is a critical step in controlling the IAWJ machining process.

Experimental set-up

In order to perform the water temperature measurements on our IAWJ prototype, a custom-made two-axis AWJ cutting system was designed and assembled in the laboratory of the authors and is shown in Figure 3. It is equipped with a high-pressure direct drive pump (P-2040; Omax, Kent, WA, USA), with maximal pressure up to 280 MPa and flow rate of 3.2 L/min.

Figure 3.

The experimental set-up, showing the major components of the IAWJ prototype.

The cutting head (ALLFI Wasserstrahltechnik GmbH, Schlüsslberg, AT) was connected to the high-pressure pump using two high-pressure pipe lines controlled by two three-way valves V2 and V3 and a check valve V4, so that the water could be directed either directly to the cutting head or through the heat exchanger unit, positioned in between, as shown in Figure 4. A cooling compressor with 7.3 kW (Danfoss Trata d.o.o, Ljubljana, SI) of cooling power was used to cool the cooling medium, a glycol–water solution, down to −25 °C. Submerged in this solution were high-pressure pipes that were bent into a coil. The temperature of the cooling medium was regulated in order to achieve the desired temperature before the cutting head. Pipes leading from the heat exchanger to the cutting head were thermally insulated using 20-mm-thick insulating foam (Armaflex; Armacell Engineered Systems, Münster, DE).

Figure 4.

Schematic representation of the experimental set-up.

In order to measure and regulate the temperatures of different parts of the high-pressure water system, six 1.5-mm-thick K-type and one 0.5-mm T-type class 1 thermocouples (TCs) were used. TCs were connected to a 24-bit USB-2416-4AO data acquisition card (DAQ) (Measurement Computing, Norton, MA, USA), with 16 differential channels for thermocouple readings, all with cold junction compensation. The data from the DAQ were acquired and stored using a program created in LabVIEW program package (National Instruments, Austin, TX, USA). Different positions of the TCs are shown in Figure 4.

The temperatures from the high-pressure water were measured indirectly from the surface of the high-pressure stainless steel pipes, due to the extreme pressures inside. These measuring points were thermally insulated using a 10-mm-thick and 150-mm-long layer of Armaflex foam. For better contact, heat conductive thermal paste was used. To measure the low temperatures of the cooling medium, a T-type TC was used, and all the other points were measured using K-type TCs.

TC1 was used to constantly measure the ambient temperature while TC2 measured the temperature of tap water entering the high-pressure pump. The water temperature upon exiting the high-pressure pump was measured by TC3. TC4 was positioned downstream the pressure regulating valve V1. The temperature of the cooling fluid was measured by TC7 and was used to regulate the temperature measured by TC5. The latter was positioned on the pipe upstream of the collimation tube before the cutting head. No temperatures were measured between TC4 and TC5. The heat exchange in this section was reduced by using the insulation foam and compensated by changing the temperature inside the heat exchanger unit in such a way that it produced the desired temperature at TC5. For the measurements where no cooling was used, the heat transfer with the ambient air can be neglected due to negligible temperature differences and the use of insulation. Direct measurement of the WJ temperature was not carried out due to extreme conditions; instead, the WJ was captured in the catching tank filled with water that dissipated its energy. The water inside the catcher was slowly exchanged with the water from the WJ, and its temperature was measured by TC6.

The parameters of water pressure, water nozzle diameter size and cooling temperature were varied in order to determine the effect on the temperature of the water at different parts of the machine and the final jet temperature. Three different water pressures of 200, 250 and 280 were used in order to determine how different pressure loads affect the temperature of water exiting the high-pressure pump and the WJ temperature. According to the phase diagram of water in Figure 2, the pressure of 200 MPa is the pressure where the water can be cooled down the most. However, higher pressure is desirable in order to increase the productivity of the IAWJ, and for that reason, two higher pressures were also chosen, 280 MPa being the limit for the pump used.

Two different water nozzles with diameters 0.15 and 0.20 were used in order to determine the effect of different nozzle size and water volume flows on (1) the pump work load and (2) the friction in the water nozzle which influences the temperature of the high-pressure water and the WJ. Different nozzle sizes also have different nozzle velocity coefficients³¹ which also change with different pressures and can affect the jet temperature. The calculated water volume flows at different conditions are shown in Table 1.

Table 1.

Volume flow of water at different pressures and water nozzles.

No.	Pressure (MPa)	Water nozzle diameter (mm)	Volume flow of water (L/min)
1	200	0.20	1.15
2		0.15	0.65
3	250	0.20	1.27
4		0.15	0.71
5	280	0.20	1.34
6		0.15	0.75

Three different temperatures were set on the collimation tube in order to observe how the cooling of the water influences jet formation. For this reason, the temperature of the jet was measured with and without cooling, where the water temperature before the collimation tube was set to 0 °C and −10 °C. These values were selected in order to have a stable experiment for the selected pressures and nozzle diameters; otherwise, the water can freeze at lower temperatures using the highest pressure parameter.

The water temperature readings were acquired by running the IAWJ machine at each given parameter set. When the temperatures had stabilized, a minimum of 100 data samples were taken at the rate of 1 Hz, which were then averaged. Each set of experiments was repeated three times.

In order to evaluate the measurement results in the catching vessel, the temperatures at this point were also calculated analytically with respect to the temperature measured before the water nozzle. The temperatures in this area are of the most interest from both AWJ and IAWJ points of view and deserve a more detailed analysis.

Theoretical velocity v_th of the WJ is derived from the Bernoulli equation

$v_{th} = \sqrt{\frac{2 \cdot p_{0}}{ρ_{1}}}$ (1)

where p₀ is the water pressure before the water nozzle and ρ₁ is the density of the water after the nozzle for which 1000 kg/m³ was used.

The velocity coefficient η of the nozzle gives the relation between the theoretical and the actual velocity of the jet v

$η = \frac{v}{v_{th}}$ (2)

Since the WJ is completely stopped in the catching vessel, a total transformation of the jet’s kinetic energy to heat is assumed

$\frac{1}{2} m \cdot v^{2} = m \cdot c_{p} \cdot Δ T$ (3)

$Δ T = \frac{v^{2}}{2 \cdot c_{p}} = \frac{{(η \cdot v_{th})}^{2}}{2 \cdot c_{p}} = \frac{η^{2} \cdot p_{0}}{ρ_{1} \cdot c_{p}}$ (4)

$T_{cv} = T_{n} + Δ T$ (5)

where m is the mass of the fluid, c_p is the specific heat of the water(4200 J/kg/K was used) and ΔT is its temperature change. This temperature difference is added to the temperature of the water before the water nozzle T_n in order to calculate its temperature inside the catching vessel T_cv. The analytical results were compared to measurements as can be observed in the graph in Figure 6.

Results and discussion

The temperature measurement results are presented in Figure 5; temperature profiles are presented at different TC positions and different operational pressures, while in Figure 6 relations between the input temperature and the one inside the catching vessel for the two nozzles at different pressures are shown. The latter are most relevant from the IAWJ point of view, as these are the conditions that ice particles will be exposed to.

Figure 5.

Temperatures of water measured at different positions and different operational pressures at (a) 200 MPa, (b) 250 MPa, and (c) 280 MPa.

Figure 6.

The relation between the input (before the water nozzle) and output (after the water nozzle) temperatures for different nozzle sizes and pressures used.

The results show that the ambient temperature at TC1 has almost no effect on the final water temperature at TC6 although it must be noted that when cooling the water, the higher ambient temperatures can again heat up the water inside the pipes. This can be solved by additional insulation of the high-pressure pipes.

The temperature of water entering the high-pressure pump at TC2 has a small effect on the output temperature. This effect is further reduced with the length of the high-pressure pipe. From the readings at TC3, which measures the exit temperature from the high-pressure pump, a rise of 8 °C to 10 °C was recorded and was dependent only on the set pressure, rising by 1.5 °C for every additional 50 MPa. If the water is not cooled afterwards, then this directly affects the temperature in the catching tank.

Water temperature measurements after the pressure regulating valve V1 at TC4 show a decrease of around 1 °C which is due to the heat exchange with surrounding air. Therefore, the heat generated at the valve itself does not affect the temperature of the high-pressure water.

The temperature of the cooling medium measured by TC7 was omitted from the charts for better clarity of the results. The temperature of the glycol solution was set to either −8 °C or −21 °C in order to achieve 0 °C and −10 °C, respectively, before the water nozzle.

Figure 5 shows that the temperature in the catching vessel at TC6 remains almost the same for a given pressure and is independent of the used water nozzle diameter. Also, the temperature difference in the catching vessel for different water nozzles was found to be small and falls within the measurement uncertainty.

Figure 6 shows the relation between the input and output temperatures of the water before and after the nozzle. It can be seen that this relation is linear. In the case where the water was not cooled, some temperature fluctuations can be seen at the input temperature. These are due to several factors already mentioned above, the main one being the change in temperature contribution from the pump at different pressures. It can be concluded that the temperature in the catching vessel is affected by the pressure and the temperature of the water flowing through the water nozzle and not by the diameter of the nozzle. This is additionally verified if equation (5) is used to calculate the temperature difference. The measurements closely agree with the analytical values when the velocity coefficient of 0.90 is applied, as the maximum difference was less than 4%.

The data analysis indicates that the main generation of heat occurs during water compression inside the high-pressure pipe and inside the catching vessel. Cooling the water before the water nozzle proved successful and the recorded data show a linear relation between the water temperature entering the water nozzle and the one recorded inside the catching vessel. Linear temperature change in the catching vessel was also recorded for different pressures regardless of the nozzle diameter.

The measurements after the three-way valve pressure regulating valve V1 at TC4 showed that the heat generated at the valve has no relevant effect on the water temperature in the pipes. The excess water flowing out of the valve at ambient pressure must therefore be the only reason for the heating of the valve.

The temperature inside the catching tank turned out to be almost independent of the water nozzle size and was affected mainly by the water pressure and the temperature before the water nozzle.

The temperatures measured in the catching vessel at 280 MPa in this article are higher than those measured by thermographic camera on the surface of the focusing tube in the authors’ previous work,³² which used a 0.25-mm water nozzle at 300 MPa. In our previous work,⁴ it was determined that the temperature on the focusing nozzle is indeed lower than that inside the catching vessel. This difference was attributed to the air entering the focusing nozzle through the mixing chamber, surrounding the jet inside. The bigger the difference between the water nozzle and focusing nozzle diameter, the bigger this temperature difference becomes as more cold air surrounds the jet.

The lowest temperatures recorded inside the catching vessel were 30 °C at 200 MPa and −10 °C before the water nozzle. The cutting head itself was covered with frozen condensed water and was therefore not heated significantly during the process due to friction in the nozzle. This leads to the conclusion that the jet itself also remains cold on the exit from the nozzle and that the friction in the nozzle has a small or negligible effect on the jet temperature. This result is expected, as the nozzle velocity coefficient is usually between 0.97 and 0.99.³¹ It is assumed that the outer layer of the WJ heats up as it passes through the air due to friction, while the rest heats up inside the catching vessel where the kinetic energy gets transformed into internal energy. It is assumed that this is the reason why calculated values agree best when a lower than normal velocity coefficient of 0.90 is used as presented in Figure 6. The temperature inside the catching vessel is assumed to be similar to the one occurring on the cutting front and is important when cutting food or when used in medical applications due to potential bacterial growth. In theory, the water temperature at 200 MPa could be lowered by additional 10 °C, as can be seen in Figure 2. This means that if the entire kinetic energy of the jet would be transformed into internal energy, the lowest temperature the IAWJ technology could achieve on the cutting front would be around 20 °C or colder if the energy transfer is not complete.

The survival time of ice grain with 0.1 mm diameter, reported by Truchot et al.⁵ which was determined for a jet temperature of 40 °C, should increase significantly at −10 °C, and even more if the ice grains are produced with diameters larger than 0.5 mm as proposed by Kovacevic et al.³³ for the injection method.

The temperatures in the jet core are low enough that ice crystals could be formed as suggested by some authors,⁵ but the mechanical properties of ice at these temperatures would not be very good in terms of efficient material removal, as was also noted by Bach et al.⁸ during their experiments.

In the end, it should be noted that the process exhibits some problems with starting and stopping the cold jet because the water freezes inside the pipes when the pump is turned off and the pressure drops. This would need to be solved in order to reach the industrial readiness of the process.

Conclusion

A holistic view of the water temperature–affecting zones on the IAWJ prototype machine was presented in the article. The water temperature measurements on different parts of the system have been used to identify the places of heat generation. While the heat generation on the cutting front cannot be avoided, the results show that cooling the water under high pressure lowers the jet temperature in a linear fashion. The water under high pressure is heated only during its compression in the high-pressure pump; afterwards, its temperature is changed (usually cooled) only by the heat exchange with the ambient air through the pipe walls. Flowing through the pressure regulating valve had no effect on the temperature. No heat generation in the water nozzle could be observed as it was covered with frozen condensation during the tests at −10 °C. The temperature can be lowered even further; however, stability of the process worsens when approaching −20 °C, especially when using water nozzles with smaller diameters as a result of lower flow rates.

It was found that water temperature is not significantly changed when passing through the water nozzle. If the water is cooled before the water nozzle, then the WJ temperature remains low as well, meaning that the ice particles could be formed inside. However, according to the revised literature, because temperatures were above −20 °C, the ice particles formed in such a way that they would not have adequate mechanical properties for machining purposes. This means that the direct phase transformation approach to generating the IAWJ would be inefficient. It is therefore advisable to cool the water only as a means to create better conditions for the injected ice abrasive. Among others, IAWJ technology has great potential in the food and medical industries for applications, where the temperatures during cutting are desired to be as low as possible to prevent bacteria growth. The results show that cooling the water could bring the temperatures on the cutting front using the cold WJ below 20 °C. It is expected that addition of the ice abrasive in the next stage will increase the cutting efficiency of the cold WJ, which still needs to be verified.

Footnotes

This prototype was developed as a part of an international European project titled ‘Ice jet environmental technology pilot plant for drastically reducing waste produced by abrasive water jet cutting techniques’.

Academic Editor: Mohammad Reza Salimpour

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research,authorship and/or publication of this article.

Funding

This work was partially supported by the Slovenian Ministry of Higher Education,Science and Technology with the program Innovative production systems (ARRS,Grant No. 1000-08-603-231 and Grant No. P2-0248) and partially by the European Commission in the frame of LIFE + financial instrument (LIFE08 ENV/ES/000167). The program was carried out in Spain and Slovenia from 2010 to 2013.

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