March 6 - 8, 2017 | Berlin
LEADING ENGINEERING SIMULATION CONFERENCE
Increasing demands for high-performance screw pumps in oil and gas as well as other applications require deep understanding of the fluid flow inside the machine. Important effects on the performance such as dynamic losses, influence of the leakage gaps, presence and extent of cavitation are difficult to observe by experiments. However, it is possible to study such effects using well validated computational fluid dynamics (CFD) models. The novel structured numerical mesh consisting of a single computational domain for moving screw pump rotors was recently developed and integrated in the grid generation tool SCORGTM to allow 3D CFD simulation of such machines The STAR-CCM+® solver was used to predict pressure, temperature and velocity fields within the machine. This allowed calculation of instantaneous mass flow rates, rotor torque, local pressure field, velocity field and other performance indicators including the indicated power. A calculation model for the bearing friction losses was introduced to account for mechanical losses. The geometry of the inlet and outlet passages and piping system are taken into consideration to evaluate their influences on the pressure distribution and shaft power. The paper also shows the influence of rotor clearances on the pump performance.
The CFD model was validated by comparing the numerical results with the measured performance obtained in the experimental test rig for a set of discharge pressures and rotational speeds. Validation includes comparison of mass flow rates, shaft power and efficiency under variety of speeds and discharge pressures. The predicted results match well with the measurements. The results also showed that that the radial clearances have larger influence on the mass flow rate than the interlobe clearance indicating that a correct design of the screw pump plays significant role in minimizing required power consumption.
Furthermore, to evaluate effects of cavitation, the VOF (Volume of Fluid) Method has been adopted for dealing with the liquid-gas two-phase flow, while the bubble dynamics was handled by a homogenous cavitation model. By changing the rotation speed and discharge pressure, the intensity, distribution area and variation of cavitation at different rotor angle were obtained. The influence of cavitation on the performance of a screw pump in terms of the mass flow rate, pressure distribution, rotor torque and the shaft power have been analysed and discussed.
The analysis presented in this paper contributes to better understanding of the working process inside the screw pump and offers design improvements and optimisation of positive displacement rotary screw machines.