CHANSON, H. (2002). "Air-Water Flow Measurements with Intrusive Phase-Detection Probes. Can
we Improve their Interpretation ?." Jl of Hyd. Engrg., ASCE, Vol. 128, No. 3, pp. 252-255
(ISSN 0733-
9429)
.
AIR-WATER FLOW MEASUREMENTS WITH INTRUSIVE, PHASE-DETECTION
PROBES. CAN WE IMPROVE THEIR INTERPRETATION?
by H. CHANSON
1
INTRODUCTION
Interest in air-water flows have not diminished in recent years, as evident by the number of associated
papers published in the Journal of Hydraulic Engineering (ASCE) and other journals like the Journal
of Hydraulic Research (IAHR), the International Journal of Multiphase Flow and the Journal of Fluids
Engineering (ASME). For example, during the period January 1998 to July 2001, the Journal of
Hydraulic Engineering published nine papers on air-water flow measurements. Approximately twice
as many papers appeared in the International Journal of Multiphase Flow. The interest is accompanied
by frequent citations of very early, sometimes outdated articles. Such citations suggest that little
progress has been achieved in the last decades. To be sure, some articles are classics : for example
STRAUB and ANDERSON (1958). Their work was cited 24 times between 1985 and June 2001.
Another classic is WOOD (1983). It was cited 10 times between 1985 and June 2001 (source: Science
Citation Index Expanded™).
The writer believes that a particularly important issue is the often inadequate or incomplete
interpretation of air-water flow instrumentation by hydraulic engineers and researchers. The present
Forum Article briefly comments of the several common techniques for measuring air-water flows by
means of intrusive phase detection probes, and it describes a basic data processing method that readily
yields expanded information on air-water flow properties.
1
Reader, Fluid Mechanics, Hydraulics and Environmental Engineering, Department of Civil
Engineering, The University of Queensland, Brisbane QLD 4072, Australia.
CHANSON, H. (2002). "Air-Water Flow Measurements with Intrusive Phase-Detection Probes. Can
we Improve their Interpretation ?." Jl of Hyd. Engrg., ASCE, Vol. 128, No. 3, pp. 252-255
(ISSN 0733-
9429)
.
INTRUSIVE PHASE-DETECTION MEASUREMENT DEVICES
In hydraulic engineering, most air-water flows are characterised by large amounts of entrained air.
Void fractions are commonly larger than 5 to 10%, and flows are of high-velocity with ratios of flow
velocity to bubble rise velocity greater than 10 or even 20. Classical measurement probes (e.g. pointer
gauge, Pitot tube, LDA velocimeter) are affected by air bubbles and can produce inaccurate readings.
When void fraction C exceeds about 10 to 15%, and when the liquid fraction (1-C) is larger than about
5 to 10%, the most reliable probe is the intrusive phase detection probes, notably the optical fibre
probe and conductivity/resistivity probe (JONES and DELHAYE 1973, BACHALO 1994,
CHANSON 1997a). Intrusive probes are designed to pierce bubbles and droplets. The principle behind
the optical probe is the change in optical index between the two phases (CARTELLIER 1992,
CARTELLIER and BARRAU 1998). The principle behind the conductivity, or electrical probe, is the
difference in electrical resistivity between air and water. The resistance of air is one thousand times
larger than that of water and a needle resistivity probe gives accurate information on the local void
fluctuations (HERRINGE 1973, SERIZAWA et al. 1975, CHANSON 1997a).
Figure 1 illustrates examples of intrusive probe designs. There are two types : the single tip and dual-
tip probes. Their respective purposes are described below. Typical probe signals are shown in Figure
2. Although probe signal should be theoretically rectangular, probe response is not exactly square
because of the finite size of the tip, the wetting/drying time of the interface covering the tip and the
response time of the probe and electronics.
BASIC MEASUREMENTS AND DATA ANALYSIS
For both probe types, the probe outputs basically are the void fraction and bubble count rate. Dual-tip
probes also provide air-water velocity, specific interface area and distributions of chord-length. Figure
4 presents an example of void fraction, bubble count rate, velocity and specific interface area
CHANSON, H. (2002). "Air-Water Flow Measurements with Intrusive Phase-Detection Probes. Can
we Improve their Interpretation ?." Jl of Hyd. Engrg., ASCE, Vol. 128, No. 3, pp. 252-255
(ISSN 0733-
9429)
.
distributions measured in a skimming flow down a 16º stepped cascade (step height: 0.1 m, flow rate:
0.188 m
2
/s). The data presented in Figure 2 and subsequently in Figures 3 and 5 were recorded in the
same cross-section at y = 53 mm. Details are given in CHANSON and TOOMBES (2001a).
The air concentration or void fraction C is the proportion of time that the probe tip is in the air. Past
experience shows that the probe orientation with the flow direction has little effect on the void fraction
accuracy provided that the probe support does not affect the flow past the tip (e.g. SENE 1984). The
bubble count rate F is the number of bubbles impacting the probe tip. This measurement is sensitive to
probe tip size, bubble sizes, velocity and scanning rate, particularly when sensor size is larger than the
smallest bubble sizes. There is an unique relationship between bubble count rate and void fraction as
demonstrated by TOOMBES (2002).
Velocity measurement is based upon the successive detection of air-water interfaces by two tips. This
technique assumes that the probe tips are aligned along a streamline and that bubbles and droplets are
little affected by the leading tip. In turbulent air-water flows, the successive detection of a bubble by
each tip is highly improbable, and therefore it is common to use a cross-correlation technique (e.g.
CROWE et al. 1998). The time-averaged air-water velocity equals :
V =
∆x
T
(1)
where ∆x is the distance between tips (Fig. 1) and T is the time for which the cross-correlation
function is maximum (Fig. 3). The shape of the cross-correlation function provides further information
on the velocity fluctuations. The turbulent intensity may be derived from the broadening of the cross-
correlation function compared to the auto-correlation function (KIPPHAN 1977, CHANSON and
TOOMBES 2001b). Figure 3 shows an example of auto- and cross-correlation functions for the flow
case mentioned above.
The measurement of air-water interface area is a function of void fraction, velocity, bubble size and
bubble count. Specific air-water interface area, a, is defined as the air-water interface area per unit
volume of air and water. It may be estimated from the air bubble size in monosize bubbly flows :
a = 6 *
C
∅
(2)
CHANSON, H. (2002). "Air-Water Flow Measurements with Intrusive Phase-Detection Probes. Can
we Improve their Interpretation ?." Jl of Hyd. Engrg., ASCE, Vol. 128, No. 3, pp. 252-255
(ISSN 0733-
9429)
.
where ∅ is the bubble diameter. Measurements with intrusive probes do not provide bubble diameters
but bubble chord lengths (Fig. 1). For any bubble shape, size distribution and chord length
distribution, the mean chord length size (i.e. number mean size) equals C*V/F by continuity. The
specific air-water interface area may then be estimated as:
a =
4 * F
V
(3)
Equation (3) is valid for bubbly flows. In regions of high air content (C > 0.3 to 0.4), the flow
structure is more complex and the result is not exactly the true specific interface area. Then, a becomes
simply proportional to the number of air-water interfaces per unit length of air-water mixture : that is,
a ∝ 2*F/V.
Remarks
Some studies suggested that interfacial velocities may be measured with a single-tip probe based upon
the voltage rise time associated with a water-air transition (e.g. CARTELLIER 1992). This technique
is, however, restricted to specific applications and probe designs. Several studies show that it does not
work with most probes, for large void fractions or high velocities (e.g. SENE 1984, CUMMINGS
1996).
The probe signals may be analysed to provide bubble and droplet chord size distributions (e.g.
CHANSON 1997b, 1999), though the amount of data processing is significant. Figure 5 presents the
probability distribution functions of bubble chord sizes and water chord sizes in 0.5 mm intervals. The
data were measured at the same location and for the same flow conditions as those in Figures 2 and 3.
Bubble chord sizes are indicated in white, and water chord sizes in black. The last column indicates
the probability of chord size exceeding 20 mm. Further signal processing may yield information on
air-water structures including bubble/droplet clusters (e.g. CHANSON and TOMBES 2001a).
CHANSON, H. (2002). "Air-Water Flow Measurements with Intrusive Phase-Detection Probes. Can
we Improve their Interpretation ?." Jl of Hyd. Engrg., ASCE, Vol. 128, No. 3, pp. 252-255
(ISSN 0733-
9429)
.
APPLICATION TO AIR-WATER MASS TRANSFER
Air-water flows in hydraulic structures have great potential for aeration enhancement of flow, because
of the large interfacial area generated by entrained bubbles as inferred by Figure 4. This consideration
is worthy commenting on in the context of using intrusive probes for measuring air-water flow.
The mass transfer rate of a chemical across an interface varies directly as the coefficient of molecular
diffusion, the negative gradient of gas concentration and the interface area. If the chemical of interest
is volatile (e.g. oxygen), the transfer is controlled by the liquid phase and the gas transfer of the
dissolved chemical may be expressed as :
∂
∂t
C
gas
= k
L
* a * (C
sat
- C
gas
) (4)
where k
L
is the liquid film coefficient, a is the specific surface area, C
gas
is the local dissolved gas
concentration and C
sat
is the concentration of dissolved gas in water at equilibrium (e.g. GULLIVER
1990). Equation (4) accounts for the effect of air bubble entrainment and the drastic increase in
interfacial area. Many studies have assumed implicitly that the term (k
L
*a) is a constant. For example,
this assumption is made by all but one relevant papers published in the Journals of Hydraulic
Engineering and Environmental Engineering (ASCE) during the period January 1998 and July 2001.
The assumption is incorrect. Detailed studies showed that the mass transfer coefficient, k
L
, in
turbulent gas-liquid flows is almost constant regardless of bubble sizes and flow situations.(e.g.
KAWASE and MOO-YOUNG 1992). But the interface area varies greatly along a hydraulic structure
as a function of the air-water flow properties, as explained above. If the air-water interface area is
measured, the integration of the mass transfer equation may provide a genuine, accurate estimate of
aeration performances (e.g. TOOMBES and CHANSON 2000).
CONCLUDING COMMENT
During the period January 1998 to July 2001, the Journal of Hydraulic Engineering published nine
papers on air-water flow measurements. All, but one paper, presented only void fraction data, though
