alinik
۱۴ مهر ۱۳۸۸, ۰۰:۱۸
Two-phase cross-flow exists in many shell-and-tube heat exchangers. A detailed knowledge of the characteristics of
two-phase cross-flow in tube bundles is required to understand and formulate flow-induced vibration parameters such
as damping, fluidelastic instability, and random excitation due to turbulence. An experimental program was undertaken
with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures. The array is
made of relatively large diameter cylinders (38 mm) to allow for detailed two-phase flow measurements between
cylinders. Fiber-optic probes were developed to measure local void fraction. Local flow velocities and bubble diameters
or characteristic lengths of the two-phase mixture are obtained by using double probes. Both the dynamic lift and drag
forces were measured with a strain gauge instrumented cylinder.
r 2005 Elsevier Ltd. All rights reserved.Two-phase cross-flow exists in many shell-and-tube heat exchangers, for instance, in the U-tube region of nuclear
steam generators. A detailed knowledge of the characteristics of two-phase cross-flow in tube bundles is required to
understand and formulate flow-induced vibration parameters such as damping, hydrodynamic mass, fluidelastic
instability, and random excitation due to turbulence. The information is also required to validate tube-scale
thermal–hydraulic analyses and to understand local crud deposition mechanisms.
Prior to 1980, very little work had been done to study flow-induced vibration of tube bundles subjected to two-phase
cross-flow. Since then a few studies were conducted in this area. This work was reviewed by Pettigrew and Taylor
(1994). Since 1994, several researchers have contributed relevant results, in particular, Feenstra et al. (1995, 2002) in
Freon 11 two-phase flow, Mann and Mayinger (1995) in Freon 12 two-phase flow, and Nakamura et al. (2002),
Mureithi et al. (2002) and Hirota et al. (2002) in steam–water cross-flow. Also, comprehensive studies on vibration of
tube bundles subjected to both air–water and Freon 22 two-phase cross-flow were conducted at the Chalk River
Laboratories (Pettigrew et al., 2001, 2002). To our knowledge, no detailed measurements of two-phase flow in tube
arrays have ever been done.
An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to
simulate two-phase mixtures. The array, which has a pitch-to-diameter ratio of 1.5, is made of relatively large diametercylinders (38 mm). This results in larger gaps (19 mm) between cylinders to allow for detailed two-phase flow
measurements.
Fiber-optic probes were developed to measure local void fraction. Local flow velocities and bubble diameters or
characteristic lengths of the two-phase mixture are also obtained by using double probes. Both the dynamic lift and
drag forces were measured with a strain gauge instrumented cylinder. The results of these detailed two-phase flow and
force measurements are presented in this paper. An attempt is made to use this information to understand vibration
excitation mechanisms in two-phase cross-flowThe experiments were done in an air–water loop to simulate two-phase flows. The loop comprised a 25 l/s variable
speed pump, a magnetic flow meter, a 2500 l tank, a 250 l/s compressed air supply system and connecting piping as
shown in Fig. 1.
The compressed air was injected below a suitably designed mixer to homogenize and distribute the two-phase mixture
uniformly below the test-section. The air flow was measured with orifice plates connected to a differential pressure
transducer and electronic readout system. The loop was operated at room temperature and the pressure in the testsection
was slightly above atmosphericThe test-section, which has an essentially rectangular cross-section (99191mm2), is shown in Fig. 2. It consists of a
column of six 38mm diameter cylinders flanked on either side by half cylinders to simulate essentially the flow path in a
large array of cylinders in a rotated triangular configurationThe pitch-to-diameter ratio, P/D, was 1.5 resulting in an inter-cylinder gap of 19mm which allowed sufficient space
for detailed flow measurements. The test-section length-to-gap width ratio is 10, thus, adequate to maintain essentially
two-dimensional flow. The measurements were taken every millimeter with fiber-optic probes assembled within a
traversing mechanism. The tip of the probes could be positioned accurately with a micrometer head.
The probe assemblies were installed at four principal positions in the array as shown in Fig. 2. These positions are
henceforth called lower and upper 601 for the narrow gaps between cylinders and lower and upper 901 for the larger
flow areas between upstream and downstream cylinders. One cylinder was instrumented with strain gauges to measure
the dynamic drag and lift forces due to the two-phase flowFig. 3 shows a double fiber-optic probe, which comprises two fiber-optic probes inserted in one stainless tube. Each
probe has a conical tip and is made of an optical fiber of 170 mm diameter. It acts as a phase sensor based on the
different level of light reflection between air and water. Two flow conditions were investigated in detail, i.e., 50 and 80%
volumetric void fraction at a nominal pitch flow velocity, Up, of 5 m/s.
For each measurement, the probe data were recorded for a period of 20 s at a 2106 Hz sampling rate. A data
analysis software was developed to obtain the time, Ti, at which the ith gaseous particle touches the probe, and the
duration of this contact, ti, as schematically illustrated in Fig. 4. The void fraction can be obtained from either probeBoth the dynamic lift and drag forces were measured with a strain gauge instrumented cylinder in the fourth position
from the upstream end of the test-section (Fig. 2). The instrumented cylinder was cantilevered and surrounded by rigid
tubes. Two pairs of diametrically opposite strain gauges were installed in the cylinder at 901 from each other to measure
the forces in the flow direction (drag) and in the direction normal to the flow (lift). The strain gauges were connected to
strain indicators. Before the instrumented cylinder was inserted into the test-section, the static strain–force relation was
determined via a careful calibration. The signals were routinely analyzed on an OR38 8-32 channel real-time multianalyzer/
recorder coupled to a lap-top computerTypical detailed measurements along the lower 601 line across the narrowest gap between cylinders are shown in Fig.
6. Due to assembly problems, the probe tips were not exactly on the 601 and 901 lines. In fact the tips were roughly 2mm
downstream of the lines. Although this will be corrected in future tests, we do not believe this slight misalignment
affected much the results. The measurements were taken every millimeter across the 19mm gap. The measurements
were remarkably stable as may be seen by the lack of scatter in the data. The measurements of void fraction, bubble
velocity (gas-phase velocity) and bubble size (characteristic lengths of the two-phase mixture) are shown in Figs.
6(a)–(c), respectively. These measurements correspond to homogeneous flow conditions of 80% void fraction and gap
flow velocity of 4.33 m/s (pitch velocity of 5 m/s). The average measured gap bubble velocity and void fraction are
respectively, 4.55 m/s and 73%. This indicates nearly homogeneous two-phase flow conditions. The slip between the gas
and liquid phase is small. The bubble sizes range from 0.5 to 5 mm.
The results for all the flow measurements are summarized in Fig. 7. It shows that the flow velocity is relatively
uniform across the lower and upper 601 gaps between cylinders. The flow velocity distribution in the lower and upper
901 space between cylinders is much less uniform. There is also a region of low flow velocity immediately upstream and
downstream of the cylinders. It also shows an abrupt increase in velocity at a position corresponding to a transition
between a more stagnant region between tubes and the main stream of the flow. It appears from both these
measurements and visual observation that the flow streams through the available flow path between cylinders. This flow
path is outlined with dash lines in Fig. 7. The region of low flow and the transition are more pronounced in between the
half-tubes near the wall of the test-section than in the centre of the test-section. This is not surprising, since the wake
between cylinders in the centre of the test-section is quite unsteady due to the absence of a solid boundary.
The void fraction distribution in the narrow 601 gaps is nearly uniform for the 80% void fraction tests whereas it is
much less uniform for the 50% tests (Fig. 7(b) versus (a)). Interestingly, the void fraction distribution for the lower 601
gap is the mirror image of that for the upper 601 gap. This is expected because of symmetry. The void fraction is
generally lower in the more stagnant regions between cylinders.
The bubble size distribution in the narrow 601 gaps is very similar to the void fraction distributions in the same
locations (Fig. 7(e) versus (a) and Fig. 7(f) versus (b)). This is reasonable because the bubble size should be consistent
with the void fraction. The bubble size distribution in the lower and upper 901 space is essentially similar to the void
fraction distributions in the same location except in the centre of the test-section for the upper 901 measurement (Fig.
7(e) versus (a) and Fig. 7(f) versus (b)). This requires further investigation......
two-phase cross-flow in tube bundles is required to understand and formulate flow-induced vibration parameters such
as damping, fluidelastic instability, and random excitation due to turbulence. An experimental program was undertaken
with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures. The array is
made of relatively large diameter cylinders (38 mm) to allow for detailed two-phase flow measurements between
cylinders. Fiber-optic probes were developed to measure local void fraction. Local flow velocities and bubble diameters
or characteristic lengths of the two-phase mixture are obtained by using double probes. Both the dynamic lift and drag
forces were measured with a strain gauge instrumented cylinder.
r 2005 Elsevier Ltd. All rights reserved.Two-phase cross-flow exists in many shell-and-tube heat exchangers, for instance, in the U-tube region of nuclear
steam generators. A detailed knowledge of the characteristics of two-phase cross-flow in tube bundles is required to
understand and formulate flow-induced vibration parameters such as damping, hydrodynamic mass, fluidelastic
instability, and random excitation due to turbulence. The information is also required to validate tube-scale
thermal–hydraulic analyses and to understand local crud deposition mechanisms.
Prior to 1980, very little work had been done to study flow-induced vibration of tube bundles subjected to two-phase
cross-flow. Since then a few studies were conducted in this area. This work was reviewed by Pettigrew and Taylor
(1994). Since 1994, several researchers have contributed relevant results, in particular, Feenstra et al. (1995, 2002) in
Freon 11 two-phase flow, Mann and Mayinger (1995) in Freon 12 two-phase flow, and Nakamura et al. (2002),
Mureithi et al. (2002) and Hirota et al. (2002) in steam–water cross-flow. Also, comprehensive studies on vibration of
tube bundles subjected to both air–water and Freon 22 two-phase cross-flow were conducted at the Chalk River
Laboratories (Pettigrew et al., 2001, 2002). To our knowledge, no detailed measurements of two-phase flow in tube
arrays have ever been done.
An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to
simulate two-phase mixtures. The array, which has a pitch-to-diameter ratio of 1.5, is made of relatively large diametercylinders (38 mm). This results in larger gaps (19 mm) between cylinders to allow for detailed two-phase flow
measurements.
Fiber-optic probes were developed to measure local void fraction. Local flow velocities and bubble diameters or
characteristic lengths of the two-phase mixture are also obtained by using double probes. Both the dynamic lift and
drag forces were measured with a strain gauge instrumented cylinder. The results of these detailed two-phase flow and
force measurements are presented in this paper. An attempt is made to use this information to understand vibration
excitation mechanisms in two-phase cross-flowThe experiments were done in an air–water loop to simulate two-phase flows. The loop comprised a 25 l/s variable
speed pump, a magnetic flow meter, a 2500 l tank, a 250 l/s compressed air supply system and connecting piping as
shown in Fig. 1.
The compressed air was injected below a suitably designed mixer to homogenize and distribute the two-phase mixture
uniformly below the test-section. The air flow was measured with orifice plates connected to a differential pressure
transducer and electronic readout system. The loop was operated at room temperature and the pressure in the testsection
was slightly above atmosphericThe test-section, which has an essentially rectangular cross-section (99191mm2), is shown in Fig. 2. It consists of a
column of six 38mm diameter cylinders flanked on either side by half cylinders to simulate essentially the flow path in a
large array of cylinders in a rotated triangular configurationThe pitch-to-diameter ratio, P/D, was 1.5 resulting in an inter-cylinder gap of 19mm which allowed sufficient space
for detailed flow measurements. The test-section length-to-gap width ratio is 10, thus, adequate to maintain essentially
two-dimensional flow. The measurements were taken every millimeter with fiber-optic probes assembled within a
traversing mechanism. The tip of the probes could be positioned accurately with a micrometer head.
The probe assemblies were installed at four principal positions in the array as shown in Fig. 2. These positions are
henceforth called lower and upper 601 for the narrow gaps between cylinders and lower and upper 901 for the larger
flow areas between upstream and downstream cylinders. One cylinder was instrumented with strain gauges to measure
the dynamic drag and lift forces due to the two-phase flowFig. 3 shows a double fiber-optic probe, which comprises two fiber-optic probes inserted in one stainless tube. Each
probe has a conical tip and is made of an optical fiber of 170 mm diameter. It acts as a phase sensor based on the
different level of light reflection between air and water. Two flow conditions were investigated in detail, i.e., 50 and 80%
volumetric void fraction at a nominal pitch flow velocity, Up, of 5 m/s.
For each measurement, the probe data were recorded for a period of 20 s at a 2106 Hz sampling rate. A data
analysis software was developed to obtain the time, Ti, at which the ith gaseous particle touches the probe, and the
duration of this contact, ti, as schematically illustrated in Fig. 4. The void fraction can be obtained from either probeBoth the dynamic lift and drag forces were measured with a strain gauge instrumented cylinder in the fourth position
from the upstream end of the test-section (Fig. 2). The instrumented cylinder was cantilevered and surrounded by rigid
tubes. Two pairs of diametrically opposite strain gauges were installed in the cylinder at 901 from each other to measure
the forces in the flow direction (drag) and in the direction normal to the flow (lift). The strain gauges were connected to
strain indicators. Before the instrumented cylinder was inserted into the test-section, the static strain–force relation was
determined via a careful calibration. The signals were routinely analyzed on an OR38 8-32 channel real-time multianalyzer/
recorder coupled to a lap-top computerTypical detailed measurements along the lower 601 line across the narrowest gap between cylinders are shown in Fig.
6. Due to assembly problems, the probe tips were not exactly on the 601 and 901 lines. In fact the tips were roughly 2mm
downstream of the lines. Although this will be corrected in future tests, we do not believe this slight misalignment
affected much the results. The measurements were taken every millimeter across the 19mm gap. The measurements
were remarkably stable as may be seen by the lack of scatter in the data. The measurements of void fraction, bubble
velocity (gas-phase velocity) and bubble size (characteristic lengths of the two-phase mixture) are shown in Figs.
6(a)–(c), respectively. These measurements correspond to homogeneous flow conditions of 80% void fraction and gap
flow velocity of 4.33 m/s (pitch velocity of 5 m/s). The average measured gap bubble velocity and void fraction are
respectively, 4.55 m/s and 73%. This indicates nearly homogeneous two-phase flow conditions. The slip between the gas
and liquid phase is small. The bubble sizes range from 0.5 to 5 mm.
The results for all the flow measurements are summarized in Fig. 7. It shows that the flow velocity is relatively
uniform across the lower and upper 601 gaps between cylinders. The flow velocity distribution in the lower and upper
901 space between cylinders is much less uniform. There is also a region of low flow velocity immediately upstream and
downstream of the cylinders. It also shows an abrupt increase in velocity at a position corresponding to a transition
between a more stagnant region between tubes and the main stream of the flow. It appears from both these
measurements and visual observation that the flow streams through the available flow path between cylinders. This flow
path is outlined with dash lines in Fig. 7. The region of low flow and the transition are more pronounced in between the
half-tubes near the wall of the test-section than in the centre of the test-section. This is not surprising, since the wake
between cylinders in the centre of the test-section is quite unsteady due to the absence of a solid boundary.
The void fraction distribution in the narrow 601 gaps is nearly uniform for the 80% void fraction tests whereas it is
much less uniform for the 50% tests (Fig. 7(b) versus (a)). Interestingly, the void fraction distribution for the lower 601
gap is the mirror image of that for the upper 601 gap. This is expected because of symmetry. The void fraction is
generally lower in the more stagnant regions between cylinders.
The bubble size distribution in the narrow 601 gaps is very similar to the void fraction distributions in the same
locations (Fig. 7(e) versus (a) and Fig. 7(f) versus (b)). This is reasonable because the bubble size should be consistent
with the void fraction. The bubble size distribution in the lower and upper 901 space is essentially similar to the void
fraction distributions in the same location except in the centre of the test-section for the upper 901 measurement (Fig.
7(e) versus (a) and Fig. 7(f) versus (b)). This requires further investigation......