Abstract:
The understanding of the flow behaviour such as the flow regimes is important
in multiphase flow metering for verification of the test meters especially during
the reproducibility tests, as the meter could be transferred among different test
flow loops or moved from one location to another within a flow loop. As the pipe
geometry and configurations may vary for different testing laboratories and on
the field, proper understanding of effect of geometrical variances on multiphase
flow behaviour is deemed important for proper assessment of multiphase flow
meter (MPFM) performance and as well developing testing protocols for
commercial flow meters. To improve the performance assessment of MPFM,
adequate understanding of the influence of pipe configurations on multiphase
flow behaviour in a typical multiphase flow loop is important in order to design a
flow loop for the purpose of calibration and validation of MPFM. To obtain this
knowledge, a systematic study of flow characteristics transitioning from the
horizontal to the vertical section in a typical MPFM testing installation with
varying upstream and downstream configuration is needed to provide guidance
on proper designing of MPFM calibration flow loop.
To this aim, an experimental study was carried out in a typical MPFM flow loop
which consists of 19.2 m long horizontal section followed by a 2.6 m long
vertical section. All the sections are at industrial scale, being made of inner
diameter (ID) of 0.077 m clear PVC pipe that allows for gas-liquid two-phase
flow behaviour to be observed and determined. The alteration of upstream and
downstream geometries of the flow loop are also carried out to investigate the
effect of geometrical variances on the flow. Air and water are the fluids used for
this study.
The result of the study showed that the pipe configuration has significant effect
on smooth stratified flow. The stratified flow regime observed in conventional
straight pipe in horizontal section for low superficial velocities was observed to
be absent in the present work. Instead, unstable wavy-slug (UWS) flow regime
was observed. None of typical horizontal flow regime maps considered in this
work were able to correctly predict UWS flow regime. The void fraction in the
horizontal section was observed to be influenced by the pipe configuration due
to liquid accumulation in the horizontal section. This could contribute to
measurement uncertainties of phase fractions in the horizontal section.
Analyses of the experimental results showed that no significant change in flow
regimes was observed in the horizontal section with different development
lengths of 100D and 200D (D is the pipe diameter) from the gas injection points.
This suggests that a length of 100D may be sufficient development length for
air-water two-phase flow in the horizontal section for such flow loop.
Furthermore, more liquid accumulation is observed in 200D as compared with
100D case, which leads to lower void fraction in 200D development length.
Downstream effect of the pipe configuration due to backward flow of the liquid
phase was noticed to have significant effect on the flow structure in the
horizontal section as observed in the probability density function (PDF)
signature of the flows. The experimental investigation of effect of blind tee
length on pressure fluctuation has shown that the 90-degree bend (equivalent to
a blind length of 0D) has the highest-pressure fluctuation while the blind tee with
0.154 m clearance (2D length) has the lowest pressure fluctuation. The
magnitude of pressure fluctuation is observed to be higher for intermittent flows
than that of separated flows. The influence of blind tee length on pressure
fluctuation tends to decrease with distance away from the blind tee in straight
pipes.
A set of guidelines for the MPFM test flow loop were proposed based on the
outcome of the current studies.