Simhydraulics (
in [Matlab]
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From: Arnaud Miege on 25 Jun 2010 12:34 "Lallaey khan" <sidh.sinha(a)yahoo.com> wrote in message <hp0iu8$kut$1(a)fred.mathworks.com>... > I have modelled a simple circuit with a fixed displacement pump filling a gas-charged accumulator. I have set the mechanical and volumetric efficiencies of the pump at 1 to make it ideal. The pump displacement and accumuator capacity is set at its default value but the specific heat ratio is set to 1 for isothermal process. I want to compare the results (accumulator pressure value at t=0) obtained from the simulation with my manual calculation. But the problem is that the results do not match with each other. In my view, the initial value of the pressure should confirm with the manual calculation and the simulation results, but they dont. Following is the description of the problem and my methodology: > > The equation for the accumualtor block is > Vf=Va[1-(Ppr/P)^1/k],----------------------(1) > > where Vf=fluid volume, Va=accumulator capacity, Ppr=Precharge pressure, P=accumulator inlet pressure, K-specific heat ratio > > for k=1 (isothermal process) and using the common engineering practice of setting the pre-charge pressure to 90% of minimum system pressure, i.e. Ppr=0.9P, the equation becomes; > Vf=0.1 Va--------------------------------------(2) > *This means that accumulator capacity parameter should not affect accumulator pressures.* > > Now, substituting (2) and a precharge pressure value of 90 bar in (1), yields the Accumulator inlet pressure value (P) to be 100 bar (for K=1). > Whereas, running the simulation yields this initial value of P to be more than 100 bar, Why is this happening? > > *If the simulation is run by making the accumulator capacity smaller, the simulation results in smaller pressure values for the same precharge value and constant pump displacement. How is this possible? > > > > Please please can anyone help me on this matter? I can provide the model file too.. just need to know how to upload it. > > Thanks in advance, P is the actual inlet pressure. As suggested in the other thread, use an ideal pressure source to control what the inlet pressure is. Also if you can share your model or at least a screenshot of it, that would help. There are various filesharing sites out there where you could upload it. HTH, Arnaud
From: Arnaud Miege on 26 Jun 2010 10:20 "Lallaey khan" <sidh.sinha(a)yahoo.com> wrote in message <hp0iu8$kut$1(a)fred.mathworks.com>... > I have modelled a simple circuit with a fixed displacement pump filling a gas-charged accumulator. I have set the mechanical and volumetric efficiencies of the pump at 1 to make it ideal. The pump displacement and accumuator capacity is set at its default value but the specific heat ratio is set to 1 for isothermal process. I want to compare the results (accumulator pressure value at t=0) obtained from the simulation with my manual calculation. But the problem is that the results do not match with each other. In my view, the initial value of the pressure should confirm with the manual calculation and the simulation results, but they dont. Following is the description of the problem and my methodology: > > The equation for the accumualtor block is > Vf=Va[1-(Ppr/P)^1/k],----------------------(1) > > where Vf=fluid volume, Va=accumulator capacity, Ppr=Precharge pressure, P=accumulator inlet pressure, K-specific heat ratio > > for k=1 (isothermal process) and using the common engineering practice of setting the pre-charge pressure to 90% of minimum system pressure, i.e. Ppr=0.9P, the equation becomes; > Vf=0.1 Va--------------------------------------(2) > *This means that accumulator capacity parameter should not affect accumulator pressures.* > > Now, substituting (2) and a precharge pressure value of 90 bar in (1), yields the Accumulator inlet pressure value (P) to be 100 bar (for K=1). > Whereas, running the simulation yields this initial value of P to be more than 100 bar, Why is this happening? > > *If the simulation is run by making the accumulator capacity smaller, the simulation results in smaller pressure values for the same precharge value and constant pump displacement. How is this possible? > > > > Please please can anyone help me on this matter? I can provide the model file too.. just need to know how to upload it. > > Thanks in advance, PS: don't forget that as stated in the documentation, we are dealing with gauge pressures here because of the presence of gas, so equation (1) becomes: Vf = Va*[1-((Ppr+Patm)/(P+Patm))^(1/k)] where Patm is the atmospheric pressure. HTH, Arnaud
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