___________________________________________________________________________
Numerical simulations of the liquid exchange process
in a chamber of a new hydraulic cylinder solution in the
aspect of the angular nozzle arrangement
Tomasz SIWULSKI
1), Urszula WARZYŃSKA
1)1)
Wrocław University of Science and Technology, Faculty of Mechanical Engineering, Wrocław
Abstract
The analysis of problems occurring in work machines operated in particularly difficult conditions is an irreplaceable source of knowledge about the degradation processes of their individual components or entire systems. The article presents a part of the results of research and development works carried out at the Faculty of Mechanical Engineering of Wrocław University of Science and Technology in the area of identification, analysis and development of new solutions for hydraulic cylinders. The basis for undertaking the work was the analysis of operational problems occurring in the machines operated in Polish underground mines of KGHM Polska Miedź S.A. The environment of machines operation in underground mines is usually characterized by a high ambient temperature, which is the result of the impact of a rock mass temperature and a difficult process of air exchange. At high ambient temperatures in machines equipped with hydrostatic systems, a significant phenomenon of accelerated degradation of hydraulic cylinders may be observed. The results of the analysis of this issue made it possible to develop a new technical solution for hydraulic cylinders. The research and development work of a new solution is being carried out in a very wide range of problems, and one of them is the analysis of the influence of the angular position of the nozzles on the obtained degree of liquid exchange in the cylinder chambers. The results of the conducted research in this area together with the discussion are contained in this article and are in the opinion of the authors an important stage in the process of designing and implementing the new cylinder solution.
Key words: hydraulic cylinder, liquid exchange in cylinder chambers, thermal analysis
Symulacje numeryczne procesu wymiany cieczy w komorze
nowego rozwiązania siłownika hydraulicznego w aspekcie kąta
ułożenia króćców
Streszczenie
Analiza problemów występujących w maszynach roboczych eksploatowanych w szczególnie trudnych warunkach jest niezastąpionym źródłem wiedzy o procesach degradacji poszczególnych ich elementów lub całych układów. W artykule przedstawiono część wyników prac badawczo-rozwojowych prowadzonych na Wydziale Mechanicznym Politechniki Wrocławskiej w obszarze identyfikacji, analizy oraz opracowywania nowych rozwiązań siłowników hydraulicznych. Podstawą podjęcia prac była analiza problemów eksploatacyjnych występujących w maszynach eksploatowanych w polskich kopalniach podziemnych KGHM Polska Miedź S.A. Środowisko pracy maszyn w kopalniach
podziemnych zazwyczaj charakteryzuje wysoka temperatura otoczenia, co jest wynikiem oddziaływania temperatury górotworu oraz utrudnionym procesem wymiany powietrza. W wysokiej temperaturze otoczenia w maszynach wyposażonych w układy hydrostatyczne zaobserwować można istotne zjawisko przyspieszonej degradacji siłowników hydraulicznych. Wyniki przeprowadzonej analizy tego zagadnienia umożliwiły opracowanie nowego rozwiązania technicznego siłowników hydraulicznych. Podjęte prace badawczo-rozwojowe nowego rozwiązania prowadzone są w bardzo szerokim spektrum zagadnień, a jednym z nich jest analiza wpływu ułożenia kątowego króćców na uzyskiwany stopień wymiany cieczy w komorach siłownika. Wyniki prowadzonych badań w tym zakresie wraz z omówieniem zawarte zostały w niniejszym artykule i są w opinii autorów istotnym etapem w procesie projektowania i wdrażania nowego rozwiązania siłowników.
Słowa kluczowe: siłownik hydrauliczny, wymiana cieczy w komorach siłownika, analiza
termiczna
Introduction
The dynamic development of hydrostatic drives of work machines observed recently has been proceeding in two main directions. The first one is based on increasing the power transmitted by the drive system by obtaining ever higher values of working pressures. The second clearly visible direction of the development is reduction of dimensions and weight of the components and entire systems. However, the development paths indicated above also bring negative effects in the form of increasing the amount of thermal energy generated by hydrostatic systems during operation. This is particularly unfavorable in the case of drives operated in conditions limiting the exchange of thermal energy with the environment, which occurs i.e. in deep mines. One of the elements particularly vulnerable to the influence of high temperatures of hydraulic oil is a hydraulic cylinder, because its construction prevents full liquid exchange, and thus drainage of high temperature liquid which may contain contaminants to a reservoir. Identification of the occurrence of this adverse phenomenon in machines operated in copper ore mines of KGHM Polska Miedź S.A. was the basis for undertaking research and development works carried out at the Faculty of Mechanical Engineering of the Wrocław University of Science and Technology (WrUST).
1. Background
The problem of high failure rate of hydraulic cylinders used in machines operating in underground mines of KGHM Polska Miedź S.A. in Poland was noticed and pre-described in publications a decade ago [2]. However, in the first period of development work, aimed at increasing the reliability of hydraulic cylinders, the activities focused on the replacement of individual cylinder elements, such as seals and piston rods, for elements made of materials much more resistant to the environment. The scope of changes was therefore limited to the material area, not taking into account the possibility of introducing fundamental changes in the design of a cylinder. However, with the passage of time, it turned out that the chosen development path made it possible to increase the reliability of hydraulic cylinders insufficiently and does not lead to the problem solution on a larger scale.
The first results of analyzes of damages and phenomena occurring in hydraulic cylinders during operation, which were carried out at the Faculty of Mechanical Engineering of WrUST, indicated that the degradation of the cylinders is mainly due to the effect of excessive temperature. Therefore, the analysis of the phenomenon of heat energy discharging from the cylinder during the operation was initiated. The research focused on the identification, mathematical description and simulations of the liquid exchange process in the cylinder chambers. As the analysis object, a classical cylinder supplied by hydraulic lines connecting it with a flow control element (e.g. a directional valve) was adopted. Therefore, the object of analysis was not the cylinder itself, but a system consisting of a cylinder and supply lines. The relationship between liquid flow divided into two domains with different temperatures that are subject to mixing during the operation of the cylinder was described. The analysis showed that the classical cylinder together with the supply lines, as a non-flow element to a certain extent, prevents the discharge of a certain volume of liquid outside the cylinder-supply lines system. In other words, during operation, a certain volume of liquid can always remain in the cylinder chamber system and supply lines. This phenomenon results in the lack of proper drainage of thermal energy contained in the liquid and the limitation of the possibility of removing contaminants from the cylinder chambers. This is an unfavorable effect, as a result of which the temperature of the cylinder and the liquid inside of it rises and is not registered by temperature sensors most often mounted in a reservoir. As a result, it is possible to exceed the boundary temperatures of the seals and their thermal degradation while not exceeding the operating temperature of the liquid measured in the tank. The described above negative effects in combination with the difficult process of removing contaminants from the cylinder chambers may result in faster degradation of the cylinders (Fig. 1). Particularly exposed to the accelerated degradation process are cylinders operated in high ambient temperatures, and such are the ones found in deep mines.
Fig. 1. Example of thermal degradation of the static seal of the hydraulic cylinder packing, visible splits of the sealing material [1]
The results of development works in the field of hydraulic cylinders, conducted by the authors at the Wrocław University of Science and Technology, enabled the development of new solutions for hydraulic cylinders [3]. The novel solutions allow to increase the level of reliability of the cylinders, and thus the efficiency of operation, the level of safety and reduce the complianceof the cylinders. Particular aspects of new solutions are described in more detail in previous publications [3, 5]. The basis of the novel idea is the connection of each cylinder chamber with two lines – supply and discharge, as opposed to the case of a classic solution with one nozzle at each chamber (Fig. 2). In addition, each hydraulic line is equipped with a control valve mounted in a block located on the cylinder. The valves are controlled by a signal from the central control system and are switched in pairs during normal operation of the cylinder. As a result, the cylinder is always supplied with liquid from the reservoir during operation and the liquid from the chambers is fully discharged. The research work was carried out for some time with the participation of an industrial partner, as a result of which the concept of the research stand developed at the Faculty of Mechanical Engineering of WrUST was implemented on the industrial side in the form of a specialized test rig for cylinders. The experimental tests allowed to positively verify the numerical model of fluid mixing in the piston-chamber of hydraulic cylinder [4]. Positive verification of the model made it possible to conduct further simulations, the results of which broadened knowledge in the area of hydrodynamic phenomena occurring in the cylinder chamber during operation. This article will present some of the results obtained by the simulations carried out on the verified numerical model.
Fig. 2. A schematic diagram of a new design of hydraulic cylinder with a control system and a model of prototype hydraulic cylinder [3]
2. Numerical models
Numerical simulations were performed with the use of Computational Fluid Dynamics (CFD) based on the immersed solid algorithm, which is commonly applied in the construction of numerical models in transient analyses, in the case when fluid domain has variable geometry. The analysis of the state of knowledge has shown that in the international scientific papers there are no studies dealing directly with this subject, however, the phenomena occurring in the cylinder chamber seem to be
analogous to the widely described processes of fuel injection into the combustion chamber of piston engines [6, 7]. The properties of the working liquid have been assumed in accordance with the specifications of hydraulic oil HLP46, which was also used during the experimental tests. The parameters of the oil have been assumed to be constant and independent on temperature and pressure (Table 1).
Table 1. Parameters of the hydraulic oil used in the simulations
Kinematic viscosity Density Specific heat Heat conductivity
[mm2/s] [kg/m3] [J/kgK] [W/mK] 44.2 (for T=40°C) 900 1880 0.134
Geometric and numerical models of the hydraulic cylinder were prepared accordingly to the previously confirmed models by real tests [4]. During the tests, the piston rod in its initial position was extended by l0=330mm, the work cycle began
with the retraction of the piston, and the stroke length was s=145mm. As the boundary conditions in the present simulations, the temperature of the liquid in the cylinder and in the power supply lines was equal to T2=80°C. The cylinders were
powered with oil with the temperature T1=40°C. Oil flow velocity set as a boundary
condition at the inlet resulted from the actual flow rate during the tests: at the rod-end inlet it was v1=1.2m/s, while at the cap-end inlet it was v2=1.24m/s. Piston
velocity during retraction was v1t=0.014m/s, and during extension was v2t= 0.011m/s.
The boundary condition at the outlet was defined as free outflow with pressure p=1atm. On the outer walls of the model, an adiabatic wall was applied that prevented heat transfer from the fluid domain to the environment.
This article describes the results of simulations performed for a piston chamber for three different calculation examples of the mutual position of the supply and discharge nozzles of the new solution of a cylinder – angles of 180°, 90° and 20° between nozzles were analyzed (Fig. 3).
Fig. 3. The geometrical models for flow simulations differing in angular position of inlet and discharge nozzles
The analyzes were carried out in order to determine the influence of angular positioning of the nozzles in the piston chamber, which is a particularly important factor in the design of the valve block mounted on the hydraulic cylinder. In order to simplify the construction of the system, the developers tend to the solution with one valve block, however, this solution requires to locate the supply and discharge nozzle at a relatively small distance from each other and with a small angle spacing. This construction at first seems to be less advantageous from the point of view of the liquid exchange process in the cylinder chamber in comparison with a location of nozzles on opposite sides of a cylinder.
3. Simulation results
Each simulation consisted of three work cycles starting from the retraction movement of the piston and the scope of the analysis was an observation of fluid mixing and evaluation of thermal energy discharge rate from the piston chamber of a cylinder. Therefore, the important information was the average temperature of liquid flowing out of the system from the outlet of a piston chamber during the second and the third retraction movement. In Fig. 4 the plot of average temperature calculated at the piston-chamber outlet versus time is depicted for the whole three-cycle analyses of different angular spacing of nozzles.
Fig. 4. Plot of average temperature of oil calculated at the outlet of piston chamber during three cycles of piston movement for 180°, 90° and 20° angular spacing of inlet and outlet
Based on the results depicted in Fig. 4, the small differences may be seen in the average temperature calculated at the outlet of a piston chamber during the retraction stroke. The results clearly indicate that the influence of the angular spacing of the nozzles in the plane perpendicular to the axis of a hydraulic cylinder is not an important parameter and their location should be dependent on the cylinder design. However, the simulations showed a significant effect of a shift of the nozzles from the main axis of a cylinder. Research in this area is still carried out, and exemplary results are presented for the nozzles located parallel to the axis of a cylinder (Fig. 8).
The results of the analyses are shown in the form of temperature contour plots obtained after each extension stroke (Figs. 5-8). The process of fluid mixing with different initial temperatures may be seen according to different nozzles arrangement.
Fig. 5. Visualization of the fluid mixing process in the piston chamber with 180° angular spacing of nozzles after each extension stoke
Fig. 6. Visualization of the fluid mixing process in the piston chamber with 90° angular spacing of nozzles after each extension stoke
Fig. 7. Visualization of the fluid mixing process in the piston chamber with 20° angular spacing of nozzles after each extension stoke
Fig. 8. Visualization of the fluid mixing process in the piston chamber with cap-side nozzles position after each extension stoke
The difference in the average temperature of oil discharged from the system in the second and third retraction stoke is shown in Figs. 9 and 10.
Fig. 9. Plot of average temperature of oil calculated at the outlet of piston chamber during the
second retraction stroke for 180°, 90°, 20° angular spacing and cap position of inlet and
Fig. 10. Plot of average temperature of oil calculated at the outlet of piston chamber during
the third retraction stroke for 180°, 90°, 20° angular spacing and cap position of inlet and
outlet nozzles of a hydraulic cylinder
The average temperature values in time of the second and the third retraction stoke are compiled in Table 2.
In order to quantitatively compare the analysis results, the SPOC parameter described in previous publications was adopted [3]. It has been defined as the ratio of the difference in temperature increase of the liquid discharged from the system of compared structures: the second compared solution (Ta2 – T1) and the first base
solution (Ta1-T1), to the temperature difference between the liquid in the supply line
and the liquid initially located in the cylinder (ΔTp = T2 -T1). This parameter makes it
possible to compare the amount of heat discharged from the system based on the average temperature of the liquid measured at the outlet of the system. The SPOC parameter was calculated only for the second cycle in relation to the initial values maintained during the first retraction stroke. A constant value of specific heat of the liquid was assumed and the assumption that during the full cycle of retraction and extension of the cylinder, the sum of mass and volume of liquid supplied and discharged from the chamber is equal to zero.
(1)
where:
T1 – the temperature of the fluid flowing into the cylinder system with the power
supply lines, T1=40°C,
T2 – the initial temperature of the cylinder system with the power supply lines,
T2=80°C,
Ta - the mean temperature of the fluid flowing out of the system, measured at
the outlet during each one retraction stroke [°C],
ΔTp - difference between the power supply line fluid and the fluid originally
In this case, as base solution the cylinder with 180° angular spacing between inlet and outlet nozzle was chosen. In Table 2. the SPOC parameters as well as mean temperatures of oil flowing out of piston chamber of hydraulic cylinder are compared.
Table 2. Comparison of the average temperatures of oil and SPOC parameter for different nozzle positions of a hydraulic cylinder
2nd retraction stroke 3rd retraction stroke
Nozzle position 180° 90° 20° Cap 180° 90° 20° Cap Ta [°C] 64.61 64.81 65.05 65.89 53.78 53.70 53.76 55.15
SPOC [%] - 0.49 1.11 3.20
The general interpretation of the results is based on the observation that the higher average temperature of oil calculated at the outlet, the better removal of heat from the cylinder chamber. The simulation results showed that the smaller angular spacing of nozzles is more favorable than the location of nozzles on the opposite side of a cylinder. The most advantageous is the solution with both inlet and outlet nozzles located at the cap side of piston chamber. The SPOC parameter shows the improvement of heat dissipation by fluid mixing of about 3% in the case of cap position of nozzles comparing to the opposite location after one piston stroke.
Summary
The results of the numerical simulations showed that the angular position of the supply and discharge nozzles in the plane perpendicular to the axis of the cylinder does not have a significant impact on the process of draining the liquid from the piston chamber during operation. Therefore, it has been proved that placing the nozzles and liquid flow lines in one valve block mounted on the cylinder is the most proper solution from the point of view of the liquid exchange process. In addition, the use of a single valve block significantly increases the applicability of the new cylinder solution, especially in mining machines in which the available space is usually significantly reduced. Additional numerical simulations indicated that if a liquid stream acts in a direction parallel to the cylinder axis, it increases the discharge of heated liquid contained in the piston chamber of the cylinder. These conclusions indicate the validity of continuing research in this direction, which will result in the development of a full mathematical model describing the process of fluid exchange in the cylinder chambers, proposing the best shape and position of the supply and discharge nozzles and conducting the analysis of solid impurities removal from the chamber volume. The results of in-depth research and development work in this area will be presented in subsequent publications.
The results presented in the article, which are only a part of the more extensive work, clearly indicate that the level of achieved knowledge is most likely to predispose the solution to the application process in mining machinery.
References
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