Proceedings of IMECE 2003 2003 ASME International Mechanical Engineering Congress and RD&D Expo November 15-21, 2003, Washington D.C. USA IMECE2003-42924 MODELING AND EXPERIMENTAL STUDY OF FLOW FORCES FOR UNSTABLE VALVE DESIGN Qinghui Yuan Dept. of Mechanical Engineering University of Minnesota Minneapolis, Minnesota 55455 Email: qhyuan@me.umn.edu Perry Y. Li Dept. of Mechanical Engineering University of Minnesota Minneapolis, Minnesota 55455 Email: pli@me.umn.edu ABSTRACT Single stage electrohydraulic flow control valves are cur- rently not suitable in high flow rate and high frequency appli- cations. This is due to the very significant flow induced forces and the power/force limitation of electromagnetic actuators that directly stokes the spool. An unstable valve has been proposed that can utilize the flow forces to achieve fast responses at high flow rate. In this paper, we model the flow forces, including both steady and transient, of a directional flow control valve for in- compressible and viscous fluid. In particular, the viscosity effect and non-orifice flux are investigated. The new models have been verified by CFD analysis to be more accurate than the old mod- els. The paper also presents a systematic experimental study on the flow forces, in particular on the steady flow forces. The es- timates according to our new models, revised slightly due to the limitation of the experiment, are consistent with the experimen- tal results. Both the experimental results and the modeling esti- mation show that, for an unstable valve with negative damping length, both transient and steady flow forces can help to achieve the higher spool agility. The satisfactory modeling and experi- mental study on the flow forces give us a grounding for the future research of unstable valve design. Keywords: Unstable valve, damping length, transient flow force, steady flow force, viscosity, computational fluid dynamics (CFD) THIS RESEARCH IS SUPPORTED BY THE NATIONAL SCIENCE FOUNDATION ENG/CMS-0088964. 1 Introduction In a single stage electrohydraulic flow control valve, the main spool is stoked directly by solenoid actuators. In high flow rate applications, there is a stringent power/force require- ment for solenoids because flow forces produced at high flow rate are very significant. The dimensional limitation of valve re- sults in the power/force restriction of the solenoids. Therefore, single stage valves are not suitable for high performance applica- tions. In these situations, multi stage valves are generally used in which the spools are driven by one or more pilot stage hydraulic valves. However, multi stage valves tend to be more susceptible to contamination, are more expensive in terms of manufacturing and maintenance, and are therefore less reliable than single stage valves. Our research aims at improving the flow and bandwidth ca- pabilities of single stage valves by alleviating the high demands for solenoid actuators. The approaches we adopt is to utilize fluid flow forces to increase the agility of the spool, so that less power/force is needed from the solenoids to achieve fast spool response. Generally speaking, flow forces can be classified as ei- ther steady or transient. Steady flow forces are those experienced by the spool when it is at rest, while transient forces are the extra ones exerted by fluid when the spool is in motion. In our previous study, we investigated the transient flow force both theoretically and experimentally [6]. In [2], we show using computer simu- lation that the valves configured to be unstable can have faster step responses under solenoid saturation than their stable coun- terparts. Also less positive power but more negative power (brak- ing) power is required to track a sinusoidal flow rate. A basic 1 Copyright c 2003 by ASME