A steady state approach to calculation of valve pressure rise rate ...

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TechSoft Engineering & SVS FEM If we take into account some simplifications: dynamic effects are avoided, valve at fixed poppet stroke behaves like an orifice which has quadratic function behavior for wide range of flow rates, we can design the valve characteristic from several CFD steady state calculations. This approach is shown for charge pressure relief valve. spring pretension mechanism nut with guide R guide pump end cap outlet channel connected to pump case case pressure marked by blue (p case ) R seat springs balance chamber poppet gallery (channel) charge pressure marked by red (p charge ) inlet branch of the gallery for measurement as well as CFD purposes these branches are closed for measurement as well as CFD purposes Fig. 2. Design of charge pressure relief valve
Konference ANSYS 2011 2. Charge pressure relief valve (CPRV) The function of CPRV is to maintain charge pressure of the closed hydrostatic system at a designated level above case pressure (Sauer-Danfoss, 2011). The CPRV is a direct acting poppet valve (Fig. 2). It opens and discharges fluid to the pump case when pressure exceeds the designated level, p charge , at a channel called gallery. The valve poppet has a cone which acts against a seat created in the pump end cap (Fig. 2). The poppet guiding is provided by a guide designed in the nut. The 2 springs provide force which acts in direction to close the valve. Pretension of one spring can be adjusted by a bolt to reach desired level of the cracking pressure. The pretension force can be calculated from equation: ( ) , (1) where and radii are evident from Fig. 2. As the poppet moves, the spring force corresponds to equation: where is spring rate and is poppet stroke. , (2) 3. CFD model As mentioned above, the steady state approach is used here. The CFD models (Fig. 3) are prepared for 3 poppet strokes (0.5 mm, 1.5 mm and 3 mm). There is used combination of hexahedral, tetrahedral and wedge mesh types created using ANSYS ICEM CFD. The CFD analysis is performed in ANSYS Fluent. Material properties of hydraulic oil Shell Tellus 46 are used. The fluid is assumed as incompressible. Based on previous investigation, experience and comparison with testing, laminar model is chosen in this specific case. Pressure outlet boundary condition is applied to the pump housing passage. The pressure constant for all the calculated cases. Velocity inlet boundary condition is experienced as suitable and stable for this task. Inlet velocity and flow rate are related to each other by simple relation: , (3) where is inlet flow rate, is inlet velocity magnitude and is inlet area. The velocity magnitude (flow rate respectively) is ramped to several values for each the stroke model. At least, 3 discrete velocity magnitudes are needed for each the stroke to be able approximate results by quadratic function. As a result from each the CFD calculation is: pressure at inlet ( ), flow force induced by oil to the poppet ( ). is For reference, pressure contours and streamlines in region of control passage are shown in Fig. 4 for 1 mm poppet stroke and 70 lmin -1 flow rate case.
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A steady state approach to calculation of valve pressure rise rate ...

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