Using Viscosity to Improve Spool Agility

In single stage valves, the main spools are stroked directly by solenoid actuators. They are cheaper and more reliable than multistage valves. Their use, however, is restricted to low bandwidth and low flow rate applications due to the limitation of the solenoid actuators. Our research focuses on alleviating the need for large solenoids in single stage valves by advantageously using fluid flow forces. For example, in a previous paper, it was proposed to improve the spool agility by inducing unstable transient flow forces by the use of negative damping lengths. In the present paper, we consider the effect of fluid viscosity on steady flow forces for both positive and negative damping lengths. Using fundamental momentum and CFD analysis, it is shown that viscosity affects significantly the steady flow forces. Specifically, for positive damping lengths, viscosity increases the steady flow force, whereas for negative damping lengths, viscosity has the tendency of reducing steady flow forces. Thus, in a viscous environment, negative damping lengths have the positive influence of increasing spool agility through both transient and steady flow forces.