Computational Study of Hydraulic Accumulators

Abstract

The scope of this project is to make a CFD model to be used for getting a better understanding of accumulator discharge. To be able to establish the models credibility is there a need for validation of the models developed. First a model of one accumulator is to be validated by use of an experimental project as well as an analytical approach. This model is then to be adapted to a system of two accumulators in series to be studied and validated by a theoretical equation developed. The literature review reveal that the study of accumulator discharge for applications for the oil industry is not a area were many articles is presented, which makes the scope of this thesis more interesting. The first case studied is a single bladder-type accumulator, which is discharged through three different outlet areas and has an internal volume of 4 l, with a pre-charge pressure of 2 bar. The physical model developed is validated by experimental data, and a theoretical approach to establish the models credibility. The average discrepancies obtained from the results was below 12.56% for all cases modeled. These results shows a high level of agreement to the various validation points, which leads to significant credibility of the physical model developed. This model is then to be adapted to the following cases in this study. The second case studies discharge of a set of two 40 l accumulators placed in series with different pre-charge pressures, but equal charge pressure. The case studies the effect of changing the ambient temperature, outlet area of the discharge nozzle, and different outlet pressures (backpressure). The obtained results shows that the effect of various ambient temperature will not have a significant impact on the discharge pattern, but by changing the restrictions in the outlet nozzle the profiles obtained showed great deviation from the base case computation. Where the variation of outlet area of the nozzle had a bigger impact on the discharge than by changing the backpressure in the system. An observation from this case shows that the pressure drop in the accumulator with the lowest pre-charge had a rapid pressure drop in the first part of the discharge cycle. This pressure drop is caused by the flow pattern out of the given accumulator. The last case studied is a similar case as the one above, but with accumulator volumes of 10 l each. In this case the effect of changing the pre-charge order, as well as the backpressure was studied. The results obtained from this study shows that changing the pre-charge order will provide a more constant pressure delivery without a rapid pressure drop in the system. This will result in a more stable and predictable flow out of the hydraulic system studied. Even when changing the restrictions in the outlet nozzle, the results shows a more stable flow out of the system, with similar pressure profiles.