Incipient Cavitation in a Steering Rotary Valve
By Jesús Esarte, Centro Multidisciplinar de Innovación y Tecnología de Navarra CEMITEC, Noain-Navarra, SPAIN, and Miguel Marcotegui, TRW Automotive, Pamplona-Navarra, SPAIN
A schematic of the TRW rotary valve |
The standards for comfort in the automotive industry have reached such a high level that any noise detected by the components can cause them to be rejected. In particular, manufacturers of hydraulic steering systems, in their ongoing drive towards technological specialization and perfection,have faced noise problems that usually must be solved through experimental means. Among the different kinds of noises generated within a hydraulic steering system, that coming from the rotary valve has been of special interest. It is well known that this noise appears as a result of the cavitation that the oil suffers when flowing through the narrow, convergent and divergent sections of the valve. In a recent project, the Research and Technological Center of Navarre, Spain (CEMITEC),in collaboration with TRW Automotive, have developed a 2D steering valve model in order to better understand the cavitation phenomenon and be able to predict if, where, and when cavitation is going to take place.
A TRW rotary valve was used for the investigation.The valve has twelve chambers machined on its outer surface, six on the stator and six on the rotor,with a 60°radial disposition. Hydraulic steering oil enters the valve radially at high pressure through three pressure inlets in the stator, and flows through narrow passages (convergent and divergent) in the system either to the left or the right side of the hydraulic piston, depending on which direction the steering wheel is twisted.
It is experimentally known [1] that cavitation occurs in certain locations inside the rotary valve, where the oil flows through a passage and there is an accompanying drop in pressure. However, any ge o-metrical change or modification to the operating conditions makes it impossible to predict whether or not cavitation will take place unless new bench tests are carried out. Experimental procedures such as this are both time-consuming and expensive. For this reason, it is important to have a computational model that is capable of predicting the appearance and severity of cavitation.
Cavitation predictions by the model (left and center) and by experiments (right) in the divergent section |
Two-dimensional FLUENT models were created to study cavitation for convergent and divergent channels. The geometry of a single passage was meshed with 100,000 cells, and the boundary conditions were set so that the oil flowed in one direction or the other. The standard k-ε model was used for turbulence. In the first round of simulations, the cavitation phenomenon resulting from shear layer instability was predicted by FLUENT,and the results were in very good agreement with experimental data. The model was unable to predict any cavitation produced as a consequence of an extreme pressure reduction, as the experiments had shown. In order to improve the CFD model, the oil viscosity was modified according to the Ostwald law for non-Newtonian fluids. This law states that the fluid viscosity falls drastically once its shear rate exceeds a certain value. From this point the viscosity keeps a constant but lower value. When the viscosity drops, the pressure is reduced, giving rise to the onset of cavitation. By introducing this revised viscosity law through user-defined functions (UDF's), the model predict-ed the cavitation generation in the divergent section, exactly as the experiments had shown.
Cavitation predictions by experiments (left) and the model (center and right) in the convergent section |
Now that the model has been validated, the next step will be to determine the cavitation intensity asa function of the operating conditions (flow regime). To do this, the cavitation parameter, K, will be used. This parameter is defined as the ratio of the maximum pressure drop to the pressure recovery that the fluid experiences on its way through the convergent and divergent sections [2, 3].
Cavitation parameter, K, vs. fluid velocity for the case where the rotor is twisted by a torsion angle of 2° with respect to the stator; cavitation begins only after the point of incipient cavitation has been passed |
For a specific torsion angle of the rotary valve (2°),there is a particular operating condition or flow regime beyond which cavitation begins. Beyond this point of so-called incipient cavitation, or Ki, as the velocity is increased, the cavitation intensity rises while the cavitation parameter, K, decreases.
References
- Behrens, H.W.; Harpole, G.M.: Review of Hiss Noise Research Results. TRW Steering Division, March 1992.
- McCloy, D.: Cavitation and Aeration: The Effect on Valves and Systems. Fluid Power Institute, Milwaukee School of Engineering.
- Rau, J.; Miller, L.: Reduction of Flow-Induced Noise Dueto Cavitation in an Integral Power Steering Gear Rotary Valve”. TRW Cross Gear Division, 1989.
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