technical investigation department. method for 3-d modelling of a mixed flow pump using phoenics d...

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