Post on 28-Mar-2015
transcript
Technical Investigation Department
METHOD FOR 3-D MODELLINGOF A MIXED FLOW PUMP
USING PHOENICS
D Radosavljevic
Introduction
Background information on the investigation
CFD and PHOENICS role
Modelling with PHOENICS
Results and analysis
Conclusions (simulation, project)
The Situation A major water supply project located in North
Africa (484 pumps).
Pumps reported to have been unused when first
put into service on this project.
The type of pump is defined as a wellpump of the
vertical submersible turbine type (7 stages).
Pumps were specified to meet a range of duties
for 25 years in relation to the envisaged drawdown
schedule.
The Situation
The Problem
During the course of approximately 3 years of
operation, pump performance problems were
encountered in a number of wells.
Upon withdrawal from the well, a pump was
observed to exhibit severe cracking and
corrosion, in particular in the region of the
upper pump bowl.
Cracking was also observed in the
corresponding corroded impeller.
The Problem
The Problem
Approach
Identifying the nature of the processes involved. The primary ones may be categorised as:
• - physical (clogging and abrasion);
• - chemical (clogging and electro-chemical corrosion);
• - microbial (clogging and microbially-induced corrosion);
Approach
Identifying the nature of the processes involved. Important subsidiary factors:
• - operational (steady loads (static water head), unsteady loads (water hammer), intermittent pumping and over-abstraction ;
• - structural and mechanical (design/construction and materials).
Approach
A number of separate studies defined including objectives to:
determine quasi-steady hydrodynamically-induced
loadings, using CFD analysis (PHOENICS);
determine other loadings from specification, such as
self-weight, torque and centrifugal;
apply all loadings to finite element analysis model
and determine individual and combined stresses;
GeometryNot supplied (proprietary vane design)
Perform sectioning of impeller and the bowl in order to take measurements.
Modelling in PHOENICS
Model one full stage of the pump as a single device;
Apply sliding grid with Multiblock. Rotating block - impeller and stationary block - bowl;
Advantage
Capture of full transient effects and (true) dynamic loading;
Modelling in PHOENICS
Problems (constraints of sliding MB)
no surface porosities allowed (vanes?);
only uniform grid in circumferential direction allowed;
only clock-wise rotation is allowed (pump rotates anti-clockwise).
Modelling in PHOENICS
Despite all the Problems !
Modelling in PHOENICS
Compromise approach
Treat impeller and the bowl as separate
components;
Steady simulation of the impeller;
Transient simulation of the diffuser with the
correct input flow field (impeller exit). (More
accurate rotor-stator interaction)
Modelling in PHOENICS
Impeller modelling
Steady;
BFC grid;
Single passage (1/6 of the flow volume);
Cyclic boundary at the exit (vaneless space);
2900 rpm (ROTA patch for rotational forces);
Wall friction, k- model;
Outlet flow field data saved in a file.
Modelling in PHOENICS Impeller modelling - velocity field
Modelling in PHOENICS
Stator modelling
Transient;
Inlet flow field cycles through impeller exit data;
BFC grid;
Single passage (1/7 of the flow volume);
Cyclic boundary at the exit and inlet (vaneless space);
Wall friction, k- model.
Modelling in PHOENICS Stator modelling - Ground
impellerdomain
statordomain
IX=NXst
IX=NXst-1
IX=3
IX=3IX=2
IX=1
IX=2IX=1
IX=NXim-1
IX=NXim
cyclic BC
cyclic BC
interface boundarystator inlet = impeller exit
IX=NXim-2flow
flow
IX=1
IX=NXim
absolute impellervane position t=0
relative impellervane position t=ti
= 0
shift
imp
new
t = 0
t = ti
Modelling in PHOENICS
Stator modelling - Numerics
Convergence generally within 500sw(/tstep);
Stator - start from steady solution in ‘aligned’ position;
10 hours CPU for the transient run;
Modelling in PHOENICS Stator modelling - velocity field
Modelling in PHOENICS Stator modelling - Assumptions
Impeller flow calculated in isolation - no interaction with the stator;
Cyclic condition ahead of stator inlet;
Assessment of accuracy
Pressure increase within impeller 3.6 bar;
Pressure increase within stator 2-3 bar;
5.71 bar per stage;
Torque 98.7 kW vs. 103kW (GXDRAG).
Modelling in PHOENICSTransient pressure field in vaneless space
Effect of the impeller
blade passing
NOTE:
Contour scaling at
plane values
CONCLUSIONS
PHOENICS
GROUND proved extremely valuable;
Allowed extensive modification to the calculation procedure;
Pump
Obtained estimates of the hydrodynamic loading within the pump;
Results do not identify any pronounced local peaks in pressure;
CONCLUSIONS