Experimental Investigation on the Effects of a Spacer Grid on Single- and Two-Phase Flows

www.ijnese.org International Journal of Nuclear Energy Science and Engineering Volume 4 Issue 2, June 2014

doi: 10.14355/ijnese.2014.0402.02

50

Experimental Investigation on the Effects of a

Spacer Grid on Single- and Two-Phase Flows Joshua Wheeler1, Adam Rau2, Ted Worosz3, Seungjin Kim4

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University

230 Reber Building, University Park, Pennsylvania 16802, USA

[email protected]; [email protected]; [email protected]; [email protected]

Received 15 April, 2014; Revised 20 May, 2014; Accepted 29 May, 2014; Published 23 June, 2014

© 2014 Science and Engineering Publishing Company

Abstract

Experiments are performed in a scaled 1x3 rod bundle

adiabatic air-water test facility to investigate the effects of a

spacer grid on single- and two-phase flows. A four-sensor

conductivity probe is used to obtain detailed measurements

of local time-averaged two-phase flow parameters, including

the void fraction and interfacial area concentration,

throughout the flow cross-section at four axial locations. The

local two-phase flow data indicates that the spacer grid

causes redistribution of the bubbles. In the transition from

bubbly to slug flows, the spacer grid is found to promote

bubble coalescence. In the slug to churn-turbulent transition,

however, the grid breaks up the larger bubbles, causing an

increase in the interfacial area concentration across the

spacer grid. Additionally, local velocity measurements in

single-phase liquid flow are performed with a laser Doppler

anemometer to investigate the change in the turbulence

structure across the grid. The axial turbulence intensity

measurements show the influence of the spacer grid

dimples, indicating an increased mixing effect from these

structures.

Keywords

Spacer Grid; Two-Phase Flow; Air-Water; Conductivity Probe;

Interfacial Area Concentration; Turbulence Intensity

Introduction

In a light water nuclear reactor core, the nuclear fuel

rods are supported by spacer grids. As such, spacer

grids are one of the major flow restrictions in the core

that affect the hydrodynamic and heat transfer

characteristics of the coolant flow. Understanding

these effects is required for best-estimate design and

analysis of the plant. In view of this, experimental

studies on the effects of spacer grids are indispensable

for improving the capability to predict the coolant

flow through the reactor.

Single-phase flows through rod bundles and spacer

grids have been investigated by many past researchers

(Rehme and Trippe, 1980; Neti et al., 1983; Yang and

Chung, 1998; Holloway et al., 2008; Caraghiaur et al.,

2009, Chang et al., 2008; Dominguez-Ontiveros et al.,

2012). Yang and Chung (1998) used a laser Doppler

anemometer (LDA) to investigate the effects of spacer

grids and mixing vanes on the turbulence structure in

a 5-by-5 spacer grid. Measurements were taken in the

center of the rod gap and in the center of the

subchannel. They concluded that the decay of the

turbulence intensity after the spacer grid is similar to

that downstream of mesh grids or screens. Similarly,

Caraghiaur et al. (2009) performed a study of multiple

flow parameters inside of a 24-rod fuel bundle with

spacer grids. Among the parameters studied was the

decay of axial turbulence intensity after the spacer

grid. The data showed that the turbulence intensity in

the side-wall subchannel increased sharply after the

spacer grid and then decayed monotonically, in

accordance with the correlations used by Yang and

Chung (1998). However, different behavior was

observed for the interior subchannel, implying that

turbulence intensity distribution is not exclusively

dependent on local channel geometry features. While

this study compared turbulence decay in the interior

and side-wall subchannels, the axial turbulence

intensity throughout the flow cross-section was not

collected in detail.

Comparatively few studies of two-phase flows

through rod bundles and spacer grids are available

that provide measurements of local two-phase flow

parameters, such as the void fraction, α, and interfacial

area concentration, ai. These parameters are of

particular importance since they are fundamental

International Journal of Nuclear Energy Science and Engineering Volume 4 Issue 2, June 2014 www.ijnese.org

51

geometric two-phase flow parameters that describe the

interfacial structure and, consequently, the interfacial

transfers of mass, momentum, and energy between the

phases. Of the available studies, Yun et al. (2008)

performed experiments in a large range of subcooled

boiling flow conditions. Measurements of local two-

phase flow parameters were acquired using a double-

sensor conductivity probe at multiple points within

the cross-section of an eighth-section of the central

subchannel of a 3x3 rod bundle. While this database

allows for study of the two-phase flow structure

around the rod, measurements were only performed

at one axial elevation, precluding study of the flow

development. Paranjape (2009) measured local two-

phase flow parameters using a four-sensor

conductivity probe at various elevations throughout

an eighth section of a 8x8 rod bundle with spacer

grids. However, owing to the near prototypic bundle

geometry, measurement locations in the subchannels

were limited to points along the rod-gap centerlines.

The present work experimentally investigates the

effects of a spacer grid on single- and two-phase flows

in a scaled adiabatic air-water test facility. A flow

visualization study is performed over a wide range of

two-phase flow conditions to study the effects of the

grid on flow regime transition and to identify flow

conditions for further investigation. Measurements of

local two-phase flow parameters are performed with a

four-sensor conductivity probe (Kim et al., 2000) to

study the effect of the grid on their development.

Additionally, single-phase liquid velocity

measurements are performed with a laser Doppler

anemometer (LDA) to investigate the change in

turbulence structure across the grid. Before discussing

the experimental results, an overview of the

experimental facility is provided.

Experimental Facility

Scaling Considerations

The present experimental facility was designed to

study the effects of a spacer grid on adiabatic air-water

flows through a rod bundle at near ambient

conditions. Owing to the differences between the

experimental conditions and prototypic reactor

conditions, scaling considerations were made in the

designing the facility (Nedwidek, 2011; Green, 2012).

A conventional pressurized water reactor (PWR)

(Westinghouse, 2006) was chosen as the reference

system for the scaling. Although the reactor coolant

primarily remains single-phase liquid during normal

PWR operation, two-phase flows are possible during

accident scenarios. To compare the reference and

experimental systems, scaling ratios were calculated

as:

exp

]ref

R

(1)

Where ψ represents a generic parameter of interest.

The subscripts ref and exp indicate that the parameters

are evaluated based on the reference and experimental

systems, respectively.

The scaling methodology was to maintain the scaling

ratios of key hydrodynamic and geometric parameters

near unity to preserve similarity to single- and two-

phase flows in the reference system. The

hydrodynamic parameters that were considered

included the channel liquid Reynolds number (Ref),

the bubble Reynolds number (Reb), and the bubble

Weber number (Web). Since the bubble size under

prototypic reactor conditions is smaller than that in

air-water flows at near ambient conditions, it was

important to maintain the relative size of the channel

structures with respect to the bubble size in the facility

design. Therefore, nineteen geometric scaling ratios

were considered that compared various bubble and

system length scales for the reference and

experimental systems. The geometric scaling ratios

were optimized to be near unity through design

iterations of the experimental spacer grid and rod

bundle dimensions. This scaling approach preserved

the scale of the channel structures with respect to each

other and to the bubble size. Table 1 shows several of

the important geometric parameters and the

associated scaling ratios that were used in the facility

design. Details of the scaling study are given by

Nedwidek (2011) and Green (2012).

Facility Description

Fig. 1 shows images of the Spacer Grid Separate-

Effects Test Facility. The test section is constructed

from clear cast acrylic to form a rectangular channel

(13.65cm x 4.45cm). Within the channel, there is a 1x3

array of 3.18cm diameter clear acrylic rods separated

by a pitch of 4.45cm. The resulting overall channel

hydraulic diameter (Dh) is 2.23cm. The entrance of the

spacer grid section, which is 10Dh long, is located 27Dh

downstream of the air-water injector. The total

development length after the spacer grid is 109Dh.

Seven axial measurement ports are available along the

length of the test section to provide access to the flow

for instrumentation. One port is located 7Dh upstream

http://www.ijnese.org/


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of the entrance of the spacer grid, and the remaining

six ports are located 7, 16, 30, 45, 69, and 98Dh

downstream of the exit of the spacer grid.

FIG. 1 SCHEMATIC AND PHOTOGRAPH OF THE SPACER GRID

SEPARATE EFFECTS TEST FACILITY

The spacer grid section is designed to be

interchangeable with removable grid inserts so that

different grid designs can be investigated. In view of

the complicated geometry and proprietary designs of

prototypic spacer grids, a simplified spacer grid

design based on a reference PWR spacer grid is

implemented in the present study. The simplified grid

does not include swirl vanes, and the dimples and

springs that holds the rods in place are simulated as

circular arcs. Fig. 2 shows images of the experimental

spacer grid section and a diagram of a representative

subchannel showing the arrangement of the dimples

and springs. Note that the springs are located at the

mid-height of the grid, and the dimples are located

both at the bottom and top of the grid.

FIG. 2 IMAGES OF THE EXPERIMENTAL SPACER GRID

Although the channel walls influence the flow around

the 1x3 array of rods unlike in a prototypic system,

this initial configuration was selected to allow for

unobstructed access to the flow area around the

simulant fuel rods. As such, this configuration

provides the ideal condition for applying optical

instrumentation, including a high speed video camera

for flow visualization and a laser Doppler anemometer

for local liquid velocity measurements, around the

rods and within the spacer grid section. Furthermore,

a traversing unit, shown in Fig. 3, was designed to fit

in the measurement ports to precisely position a four-

sensor conductivity probe (Kim et al., 2000)

throughout the channel cross-section. This capability

enables detailed measurements of local time-averaged

two-phase parameters to be acquired around the

simulant fuel rods.

TABLE 1 EXAMPLE GEOMETRIC SCALING PARAMETERS

Parameter (ψ) Description Reference (ψref) Experimental (ψexp) Scaling Ratio (ψ]R)

P/Drod Fuel Rod Pitch to Diameter Ratio 1.378 1.4 0.98

LSG/Dh,ch Spacer Grid Length to

Subchannel Hydraulic Diameter Ratio 6.539 6.539 1.00

Dh,ch/Db,sph Subchannel Hydraulic Diameter to

Maximum Spherical Bubble Diameter Ratio 14.067 14.726 0.96

Dh,SG/Db,sph Spacer Grid Hydraulic Diameter to

Maximum Spherical Bubble Diameter Ratio 6.479 6.783 0.96

Spring

Grid Strap

Dimple

Representative Subchannel

Top View from Back Front View


53

FIG. 3 LOCAL CONDUCTIVITY PROBE TRAVERSING UNIT

The facility is an adiabatic air-water test facility

capable of generating single- and two-phase flows.

Experiments are performed at 20°C and near

atmospheric pressure conditions. Filtered water is

supplied to the test section from a 2270L accumulator

tank by a 10HP centrifugal pump. A variable

frequency controller is used to set the pump speed

and, consequently, adjust the liquid flow rate. An air

compressor supplies dry air to the facility that is

regulated to a gauge pressure of 414kPa. The air is fed

to three sintered stainless steel spargers in the dual-

stage air-water injector to generate bubbles for two-

phase flow conditions. The spargers have an average

pore size of 10μm, and they are the same diameter as

the acrylic rods. Each of the three rods sits flush on

top of a sparger such that the bubbles are injected

around the circumference of the rods without

obstruction. The water supply is split into main and

auxiliary liquid flows. The main liquid flow is

distributed amongst twelve lines to provide uniform

liquid injection and is varied to set the desired flow

condition. The auxiliary liquid flow is fed at a constant

rate into annular regions around the air spargers in the

injector to shear off bubbles. This provides a controlled

method of injecting a uniform bubble size at the inlet

of the test section independent of the main liquid flow

rate. After passing through the test section, the flow

enters a two-phase separator, where the air is allowed

to exhaust to the room and the water returns to the

accumulator tank.

Instrumentation

The air flow rate is controlled by set of rotameters with

accuracies of ±3% of their full scale readings. The

liquid flow rate is measured by an electromagnetic

flow meter with an accuracy of ±0.5% of the flow rate.

At each measurement port, pressure taps are available

to acquire pressure measurements with a differential

pressure transducer with an accuracy of ±0.04% of the

measurement. In two-phase flow experiments, a four-

sensor conductivity probe is used to measure local

time-averaged two-phase flow parameters including

the void fraction (α), interfacial area concentration (ai),

bubble velocity (vg), and bubble Sauter-mean diameter

(DSm) (Kim et al., 2000).

An integrated backscatter LDA system capable of one-

dimensional velocity measurements is used to

measure the local axial velocity in single-phase liquid

flow. The light beam is generated by a 600nm

wavelength 35mW laser; other characteristics of the

LDA are listed in Table 2. The LDA is mounted on a

two-dimensional traversing system. The traversing

system has a resolution of 1µm. In positioning the

LDA measurement volume within the channel, the

differences in the refractive indices of air, acrylic, and

water are accounted for by employing Snell’s law. To

perform the single-phase liquid velocity

measurements, the flow is seeded with polyamide

seeding particles that have a mean diameter of 20µm

and specific gravity of 1.04.

TABLE 2 LDA CHARACTERISTICS

Measurement Length 300 mm

Beam Width 2.0 mm

Measurement Volume 0.1mm x 0.1mm x1.0mm

Maximum Measureable Velocity 27 m/s

Measurable Velocity Fluctuation 0.7µm/s-4.6mm/s

Experimental Results and Discussion

Two-Phase Flow Visualization Study

Flow visualizations are performed with a high-speed

video camera to visually observe the effect of the

spacer grid on flow regime transition and to identify

conditions of interest for local conductivity probe

measurements. Videos are captured in flow conditions

in the superficial liquid velocity (jf) range of 0.25 m/s to

2 m/s and superficial gas velocity (jg) range of 0.1 m/s

to 6 m/s, at locations 7Dh upstream and downstream of

the entrance and exit to the spacer grid section,

respectively. These positions are selected so that the

visualizations correspond to the available conductivity

probe measurement locations. The videos were

independently viewed and classified by the

researchers into bubbly, cap-turbulent, slug, and

churn-turbulent flow regimes. Example images of

these flow regimes are shown in Fig. 4.

The flow conditions were then plotted on the jf-jg plane

to identify transition boundaries for the flow regimes

observed both before and after the grid. Fig. 5 shows

the resulting flow regime map for the facility. The

solid and dashed lines indicate the transition

boundaries identified before and after the grid

Four-Sensor

Conductivity Probe



54

respectively. The numbered dots indicate the flow

conditions that were selected for further investigation

with the local conductivity probe, which are discussed

in the next section.

0.1

1

0.1 1

j f[m

/s]

jg [m/s]

1 2 3 4

5 6 7 8

5

5

FIG. 5 FLOW REGIMES BEFORE AND AFTER THE SPACER GRID.

An impact on the flow regime transition is apparent in

Fig. 5. The transition boundaries between bubbly to

turbulent-cap and turbulent-cap to slug are observed

to shift to lower jg after the spacer grid. This indicates

that the grid is promoting increased bubble

coalescence near the transition boundaries. High speed

images demonstrating this effect for the turbulent-cap-

to-slug transition boundary are shown in Fig. 6.

In the transition between the slug and churn-turbulent

flow regimes, the grid is found to induce a breakup

effect on the flow structure, causing the flow to

resemble a slug regime immediately after the spacer

grid. The grid breaks up the larger gas voids as they

travel through the spacer grid, generating increased

amount of larger bubbles observed after the grid.

Smaller bubbles are generated when the larger bubbles

are forced to breakup upon entering the spacer grid.

Fig. 7 shows example images of this phenomenon.

(A) (B)

FIG. 6 IMAGES OF TWO-PHASE FLOW (A) BEFORE AND (B)

AFTER THE SPACER GRID INDICATING ENHANCED BUBBLE

COALESCENCE AT THE TURBULENT-CAP TO SLUG

BOUNDARY (jf = 1.00 m/s and jg,1= 0.87 m/s).

(A) (B)

FIG. 7 IMAGES OF TWO-PHASE FLOW (A) BEFORE AND (B)

AFTER THE SPACER GRID INDICATING BUBBLE BREAKUP AT

THE SLUG TO CHURN-TURBULENT BOUNDARY (jf = 1.00 m/s

and jg,1= 1.6 m/s).

Two-Phase Flow Experimental Conditions and

Measurement Locations

Table 3 shows the ‹jg1›1 and ‹jf› values for the two-

phase flow conditions considered in the present study.

The subscript 1 indicates that the jg values shown

correspond to the first measurement location 7Dh

upstream of the spacer grid. To study the effect of

increasing gas flow rate through different flow

regimes, gas flow rates were selected in each of the

identified flow regimes from Fig. 5.

(A) (B) (C) (D)

FIG. 4 IMAGES OF (A) BUBBLY, (B) TURBULENT-CAP, (C) SLUG, AND (D) CHURN-TURBULENT TWO-PHASE FLOW REGIMES.


55

TABLE 3 TWO-PHASE FLOW CONDITIONS INVESTIGATED

Run ‹jf› [m/s] ‹jg1›1 [m/s] Run ‹jf› [m/s] ‹jg›1 [m/s]

1 3.00 0.10 5 1.00 0.40

2 3.00 0.23 6 1.00 0.87

3 3.00 0.34 7 1.00 1.1

4 3.00 0.51 8 1.00 1.6

Fig. 8 shows the cross-sectional measurement mesh

that was used for conductivity probe measurements.

For a given axial location, local measurements were

performed at 64 locations throughout the cross-section

around the center rod. The measurement points were

selected to capture the variation of two-phase flow

parameters near the surface of the rod. Measurements

were performed at four axial locations: 7Dh upstream

and 7, 16, and 45Dh downstream of the spacer grid.

The last measurement location corresponds to the

location of the next spacer grid in a prototypic system.

(Hochreiter et al., 2010).

FIG. 8 LOCAL CONDUCTIVITY PROBE MEASUREMENT POINTS

IN THE CHANNEL CROSS-SECTION

Repeatability and Benchmarking of Local Two-Phase

Flow Measurements

To confirm the repeatability of the experimental data,

measurements along the centerlines between the rods

were repeated throughout the study. Fig. 9 compares

void fraction measurements collected along the left

rod-gap centerline 7Dh before the spacer grid for two

different trials of Run 8. The trials were separate flow

start-ups using different conductivity probes. The

solid red line on the inset view of the channel cross-

section indicates the line along which the local

measurements were acquired. Referring to Fig. 8, the

void fraction data is plotted against the y-coordinate

nondimensionalized by the half-pitch (Phalf). The

negative and positive y/Phalf values refer to the front

and back of the channel, respectively. In each trial, the

flow condition was set up using the same start-up

procedure beginning with an empty test section. From

the repeatability tests, it is found that the

measurements are generally repeatable within

approximately ±10%. This provides confidence in the

ability to consistently replicate the experimental data.

0

0.2

0.4

0.6

0.8

1

-1 -0.5 0 0.5 1

α [

-]

y/Phalf [-]

Trial 1Trial 2

10% error bars on

FIG. 9 REPEATABILITY OF VOID FRACTION MEASUREMENTS

ALONG THE LEFT ROD-GAP CENTERLINE at L/Dh=-7 FOR RUN

8 (jf =1.00 m/s & jg,1= 1.6 m/s).

To benchmark the conductivity probe measurements,

two measures of the volumetric gas flux (i.e.

superficial gas velocity, ‹jg›z) at each axial location are

compared. One measure of ‹jg›z can be obtained by

area-averaging the product of the local void fraction

and bubble velocity measurements obtained with the

conductivity probe as:

‹jg›z = ‹αvg›z (2)

where the brackets ‹-› denote the area-average

operator. The second measure of ‹jg›z is obtained from

the inlet gas flow rate, Qg, and the pressure pz at a

given axial location. Qg and pz are measured by the

rotameters and the pressure transducer, respectively,

so they provide a global measurement, ‹jg›z,global. Thus,

comparing ‹αvg›z and ‹jg›z,global compares local and

global measurement techniques for obtaining ‹jg›z.

Based on the accuracies of the conductivity probe

(Kim et al., 2000; Wu and Ishii, 1999), pressure

transducer, and flow meters, measurements that

benchmark within approximately ±10% are considered

reliable for bubbly flows.

Fig. 10 compares the ‹αvg›z obtained from the

conductivity probe data to the value obtained from the

gas flow rate and pressure measurements, ‹jg›z,global.

Ideally, the values should be equal and fall on a 45°

line extending from the origin; ±10% deviation from

the ideal case is indicated by the dashed lines. The

majority of the measurements agree well within ±10%,

for the bubbly conditions. For non-bubbly flow

conditions, such as Runs 6-8, there are four data points

showing probe measurements that underestimate

those estimated by the pressure transducer and

volumetric gas flux. Considering that these points



56

represent data acquired near the inlet, the difference

may stem from inlet effects.

0.0

0.5

1.0

1.5

2.0

0.0 0.5 1.0 1.5 2.0

<αv

g>z

[m/s

]

<jg>z,global [m/s]

Run 1 Run 5

Run 2 Run 6

Run 3 Run 7

Run 4 Run 8

±10% lines

FIG. 10 VOLUMETRIC GAS FLUX BENCHMARK

Local Two-Phase Flow Parameters

A state-of-the-art four-sensor conductivity probe was

used to acquire measurements of local time-averaged

two-phase flow parameters in the test section.

Experimental data from Runs 2, 6, and 8 are presented

to discuss characteristic effects of the spacer grid on

the development of two-phase flow parameters.

Run 2 is representative of the bubbly conditions

considered at jf=3.0 m/s and 1.0 m/s. Fig. 11 presents

conductivity probe measurements for Run 2 of local

void fraction, interfacial area concentration, bubble

velocity, and Sauter-mean diameter along the left rod-

gap centerline at each axial location. ±10% error bars

are included on the measurements at -7Dh for

reference. A slightly skewed void fraction profile, Fig.

11(A), towards the front of the channel (-y/Phalf) is

observed at the measurement location upstream of the

spacer grid (-7Dh). This is attributed to inlet effects

characteristic of the present test facility. The void

fraction profile becomes more symmetric after the

spacer grid (7Dh), indicating that the grid induces a

redistribution of the bubbles within the channel,

reducing the inlet effect. Downstream of the grid

(45Dh), a wall-peaked void fraction profile is observed.

Peaking observed at the channel wall would not occur

in a rod bundle of infinite extent. However, a peak is

also observed in the center of the channel, indicating

peaking near the surface of the rods. Similar trends are

observed in the interfacial area concentration profiles,

Fig. 11(B), since the interfacial area concentration is

proportional to the void fraction in bubbly flows.

FIG. 11 CONDUCTIVITY PROBE MEASUREMENTS OF LOCAL

(A) α, (B) ai, (C) vg, AND (D) DSm ALONG THE LEFT ROD-GAP

CENTERLINE FOR RUN 2 (jf=3.00 m/s & jg,1 =0.23 m/s). ±10%

ERROR BARS SHOWN.


57

The bubble velocity, Fig. 11(C), and Sauter-mean

diameter, Fig. 11(D), centerline profiles do not vary

significantly between the measurement locations

immediately upstream and downstream of the spacer

grid. Considering that the spacer grid induces

additional pressure loss, this observation is

counterintuitive. At the measurement locations further

downstream, there is a noticeable increase in both vg

and DSm. The trend across the grid for vg and DSm may

be indicative of bubble disintegration due to the

spacer grid. The vg measurements closest to the

channel wall remain relatively similar at each axial

location with significant change towards the center,

and the DSm profile is relatively flat along the entire

centerline. The increased velocity gradient near the

channel wall further downstream may be responsible

for the wall-peaking phenomenon in the void fraction

and interfacial area concentration profiles.

The observed phenomena indicate two-characteristic

effects of the spacer grid in the bubbly flow conditions

investigated. The redistribution of the void fraction

after the spacer grid indicates increased mixing and

lateral motion of the bubbles. Also, spacer grid

induced bubble disintegration is indicated by the near

constant DSm across the grid.

The relatively dense measurement mesh allows for the

experimental data to be plotted as a surface

throughout the cross-section of the subchannel to help

better visualize the distribution of the parameters.

Three-dimensional surface plots using cubic

interpolation of the experimental data, assuming all

parameters go to zero at the surface of the rod and

channel walls, are generated in MATLAB. Figs. 12(A)

and (B) are plots of the measured local void fraction

and interfacial area concentration for Run 2. The black

region on the surface plots represents the rod location.

In Figs. 12(A) and (B), the redistribution effect of the

grid is readily observed, as the skewed void fraction

and interfacial area concentration profiles before the

grid are redistributed throughout the subchannel by

7Dh after the grid. 16Dh downstream of the grid,

additional redistribution is observed, leading to more

uniform profiles. Slight wall-peaking is beginning to

develop, as well; this is particularly clear near the

surface of the rod in the interfacial area concentration

profile. By 45Dh downstream of the grid, a distinct

wall-peaked profile at the channel walls and around

the surface of the rod is observed. A similar pattern of

the gas phase being redistributed and developing a

wall peak by 45Dh after the grid is observed for Runs

1,3, and 4. The turbulent-cap condition of Run 5 at the

lower superficial liquid flow rate is also redistributed

after the grid. However, it is wall-peaked before the

grid (-7Dh) and remains so up to 45Dh after the grid.

(A) (B)

FIG. 12 SURFACE PLOTS OF CONDUCTIVITY PROBE

MEASUREMENTS OF LOCAL (A) VOID FRACTION AND (B)

INTERFACIAL AREA CONCENTRATION ALONG THE TEST

SECTION FOR RUN 2 (jf=3.00 m/s & jg,1 =0.23 m/s).

With increasing gas flow rate, the flow transitions

away from bubbly and turbulent-cap flows to become

a slug flow. Slug flows contain intermittent slug

bubbles, large bullet-nosed bubbles that span across

subchannels. The slug bubbles are followed by a wake

region containing bubbly-like flow called liquid slugs.

Since different size bubbles have different transport

characteristics, the bubbles are categorized into two

groups in the conductivity probe signal processing

scheme. Group-I consists of smaller spherical and

distorted bubbles, and group-II consists of cap, slug,

and churn-turbulent bubbles (Kim et al., 2000).

Run 6 is located in the transition region between

turbulent-cap and slug flows. Fig. 13 shows the local

total, group-I, and group-II void fraction

measurements for Run 6 along the left rod-gap

centerline at each axial location. The total void fraction

is equal to the sum of the group void fractions. The

total void fraction profile, Fig. 13(A), is initially



58

skewed at -7Dh and becomes more uniform

downstream of the spacer grid, indicating a

redistribution effect of the grid similar to the bubbly

flows. Figs. 13(B) and (C) show the group-I and group-

II void fractions, respectively. The group-I void

fraction is distributed relatively uniformly throughout

the channel, peaking near the walls. The group-II void

profile is highly skewed at -7Dh, which is attributed to

inlet effects, but becomes more symmetric as the flow

develops along the test section. Thus, the group-II

bubbles are the primary source of the asymmetry

observed in the total void fraction profile before the

spacer grid and the continuation downstream. It is

also observed that immediately downstream of the

grid (7Dh) there is a significant decrease in the group-I

void fraction with an increase in group-II void

fraction. The flow visualizations shown in Fig. 6 for

this condition support this result with the increased

number of slug bubbles downstream of the grid. Such

a dramatic exchange between the group void fractions

is one indication that bubble coalescence is enhanced

by the spacer grid in the transition from turbulent-cap

to slug flows.

Fig. 14 shows the local total, group-I, and group-II

interfacial area concentration measurements for Run 6

along the left rod-gap centerline. In contrast the total

void fraction for this condition, the total interfacial

area concentration, Fig. 14(A), has a more uniform

profile across the cross-section, with slight wall

peaking. Unlike bubbly flows where bubbles are

dispersed and approximately spherical, the total

interfacial area concentration is not directly

proportional to the void fraction in higher gas flow

rate conditions owing to the wider range of bubble

sizes and shapes. Noting the different scales in Figs.

14(B) and (C) that show the group-I and group-II

interfacial area concentrations, respectively, it is

apparent that the group-I interfacial area

concentration contributes to the majority of the total

interfacial area concentration in this condition. In view

of the more uniform group-I void fraction and

interfacial area concentration profiles, this explains

why the total interfacial area concentration is more

uniform in comparison to the total void fraction. The

group-I interfacial area concentration decreases

dramatically across the spacer grid, similar to the

group-I void fraction. A decrease in the interfacial area

concentration also indicates bubble coalescence. The

trends in the local void fraction and interfacial area

concentration profiles are also supported by the

development of the area-averaged parameters along

the test section (Wheeler et al., 2014).

FIG. 13 CONDUCTIVITY PROBE MEASUREMENTS OF (A) TOTAL, (B) GROUP-I, AND (C) GROUP-II VOID FRACTION FOR RUN 6 (jf=1.00

m/s & jg,1 =0.87 m/s). ±10% ERROR BARS SHOWN.

FIG. 14 CONDUCTIVITY PROBE MEASUREMENTS OF (A) TOTAL, (B), GROUP-I, AND (C) GROUP-II INTERFACIAL AREA

CONCENTRATION FOR RUN 6 (jf=1.00 m/s & jg,1 =0.87 m/s). ±10% ERROR BARS SHOWN.


59

The next flow condition considered, Run 8, is located

in the transition from the slug flow regime into the

churn-turbulent regime. Fig. 15 shows local total,

group-I, and group-II void fraction measurements for

Run 8 along the left rod-gap centerline at each axial

location. The corresponding interfacial area

concentration measurements for are shown in Fig 16.

The total void fraction, Fig. 15(A), shows an increasing

trend along the test section. The total interfacial area

concentration, Fig 16(A), increases across the grid, but

it then decreases as the flow continues downstream.

The group void fractions are not observed to change

significantly across the grid. However, the group-I

interfacial area concentration, Fig. 16(B) is observed to

increase across the grid, while the group-II interfacial

area concentration, Fig. 16(C), remains relatively

unchanged. The decreasing group interfacial area

concentrations farther downstream (16Dh and 45Dh) is

an indicator of this flow condition’s natural tendency

to coalescence in the absence of flow obstructions. The

increasing trend of interfacial area concentration

across the grid is indicative of the grid breaking up the

bubbles, offsetting the natural tendency of the flow.

FIG. 15 CONDUCTIVITY PROBE MEASUREMENTS OF (A) TOTAL, (B) GROUP-I, AND (C) GROUP-II VOID FRACTION FOR RUN 8 (jf=1.00

m/s & jg,atm =1.6 m/s). ±10% ERROR BARS SHOWN ON.

FIG. 16 CONDUCTIVITY PROBE MEASUREMENTS OF (A) TOTAL, (B), GROUP-I, AND (C) GROUP-II INTERFACIAL AREA

CONCENTRATION FOR RUN 8 (jf=1.00 m/s & jg,1 =1.6 m/s). ±10% ERROR BARS SHOWN.

FIG. 17 (A) GROUP-I AND (B) GROUP-II SAUTER-MEAN DIAMETERS FOR RUN 8 (jf=1.00 m/s & jg,1 =1.6 m/s). ±10% ERROR BARS SHOWN.



60

The group DSm provide additional insight to what is

occurring in this flow condition. The group-I DSm, Fig.

17(A), decreases across the grid and stays relativity

constant downstream. Group-II maintains relativity

constant across the grid even though there is an

additional pressure drop, which should lead to an

increase. These trends collectively are indicative a

breakup effect that results in the generation of group-I

bubbles and smaller group-II bubbles from group-II

bubbles as they travel through the spacer grid. This is

supported by the flow visualizations for this

condition, Fig. 7, where the wake regions following

the large bubbles were observed to become denser

after the grid, indicating the generation of numerous

smaller bubbles. These observations are also

supported in the trends of the area-averaged

experimental data discussed by Wheeler et al. (2014).

Single-Phase Liquid Velocity Measurements

Single-phase axial liquid velocity measurements were

performed using an LDA to study the development of

the turbulence structure across the grid. Mean local

axial velocities and axial turbulence intensities were

investigated. At a given axial elevation, local

measurements were performed at 89 positions within

the cross-section around the center rod on a mesh

similar to that used for the conductivity probe

measurements. The measurements were performed at

three axial locations along the test section: -3Dh before

the spacer grid and 3Dh and 40Dh after the spacer grid.

To ensure statistical reliability of measurements, a

minimum of 5000 samples are measured at each local

point, and the burst signal validation was maintained

above 93%.

Two single-phase liquid flow conditions were chosen

such that the bulk liquid velocities, Ubulk, corresponded

to the superficial liquid velocities used during the two-

phase experiments. The flow conditions and

corresponding channel Reynolds numbers are

provided in Table 4.

TABLE 4 SINGLE-PHASE LIQUID FLOW CONDITIONS FOR LDA

Run Ubulk [m/s] Ref [-]

i 3.00 6.68x104

ii 1.00 2.23x104

In view of presenting the experimental results, the

local mean axial velocity (U) is normalized by the bulk

velocity Ubulk. Fig. 18 shows the contour plots for

normalized liquid velocity measurements taken in the

test section. The black semicircle region represents the

location of the rod. The dashed semicircles at 3Dh after

the spacer grid show the location and approximate

geometry of the two most downstream features of the

spacer grid, the dimples. The dots note the locations of

the actual measurements of the flow. The gray section

represents the area near the channel wall that was not

measured in the current experiments focusing on the

distribution of the flow around the center rod. The

axial velocity in the test section is decreased in the

areas directly above the spacer grid features and

increases in the azimuthal gap between them. Far

downstream of the spacer grid, the velocity contours

return to a similar flow structure found before the

spacer grid.

FIG. 18 CONTOURS OF MEAN AXIAL LIQUID VELOCITY

MEASUREMENTS AT THREE AXIAL LOCATIONS FOR RUN I

(Ubulk=3.00 m/s).

The LDA system used can also obtain the turbulence

intensity. The turbulence intensity (I) is defined as the

root-mean-square velocity variation divided by the

mean axial liquid velocity:

'RMS

uI

U (3)

Figure 19 shows contour plots of the turbulence

intensity at axial locations 3Dh before the spacer grid,

3Dh and 40Dh after the spacer grid. In general, the


61

spacer grid creates a rise in turbulence intensity

despite the absence of swirl vanes. The two most

downstream dimples appear to have the most

significant impact on axial turbulence intensity; these

are located in the front of the channel and in the left

rod gap. The most prominent turbulence intensity

peak is found at the azimuthal midpoint between

these two spacer grid features. Another region of high

turbulence intensity appears to the right of the front

dimple. Axial turbulence intensity is not found to

increase directly above the grid features, and is in fact

lower above the left dimple than on the right side of

the channel at this axial location. Far downstream of

the spacer grid the turbulence intensity returns to a

similar flow structure found before the spacer grid.

Understanding how the individual structures impact

the turbulence intensity will enable methods to be

developed that increase the mixing in the flow

through the spacer grid.

FIG. 19 CONTOURS OF AXIAL TURBULENCE INTENSITY

MEASUREMENTS AT THREE AXIAL LOCATIONS FOR RUN I

(Ubulk=3.00 m/s).

Conclusions

The current study performed separate-effects

experiments on the effects of a spacer grid on the

hydrodynamics of single- and two-phase flows. The

investigation was conducted through the use of a

scaled 1x3 rod bundle, designed to allow for detailed

local measurements around the simulant fuel rods. As

a first step, a flow visualization study was performed

over a wide range of two-phase flow conditions to

study the effects of the grid on flow regime transition.

Although the transition boundaries developed from

the visualizations in the present study are specific to

the current facility and likely cannot be used in

general, they do provide insight into the possible

effects of spacer grids on flow regime transition. These

include enhanced bubble coalescence in the transition

from bubbly to slug flows, as well as bubble breakup

in the transition from slug to churn-turbulent flows.

Based on the flow visualization study, eight two-phase

flow conditions were chosen to investigate the effects

of the spacer grid on local two-phase flow parameters

in regions of flow regime transition using a four-

sensor conductivity probe. Detailed local

measurements of two-phase flow parameters

including the void fraction, interfacial area

concentration, bubble velocity, and bubble Sauter-

mean diameter were performed with the conductivity

probe throughout the cross-section of a representative

subchannel at four axial locations, yielding a database

of over 2000 local measurement points. The local

conductivity probe data demonstrates the increased

mixing effect induced by the spacer grid. The local

void fraction in bubbly flows demonstrated the effect

of the mixing creating a redistribution effect following

the grid. For the conditions on the transition boundary

of turbulent-cap to slug flow, the decrease in group-I

void fraction and increase in group-II void fraction

indicates a coalescence effect. This coalesce effect is

also observed globally in the shift in the flow regimes.

The experimental data on transition boundary of slug

to churn-turbulent flows, shows smaller group-I DSm,

and nearly constant group-II DSm across the grid. This

occurrence indicates that the group-II bubbles are

breaking up into smaller group-I and group-II upon

exiting the spacer grid, thus increasing the interfacial

area concentration.

In addition to the investigations in two-phase flow

conditions, single-phase liquid axial velocity

measurements were performed using a 1-D LDA to

investigate the change of the turbulence structure

induced by the spacer grid. The individual structures

of the grid are found to influence the axial turbulence,

downstream of the grid. Specifically, the turbulence

intensity is observed to increase immediately

downstream of the dimple structures in the spacer



62

grid. Understanding how these characteristic features

of the spacer grid impact the distribution of flow is

critical in understanding and improving the mixing

effects.

Nomenclature

Latin

ai Interfacial area concentration [1/m]

Dh Hydraulic diameter [cm]

Drod Rod diameter [cm]

DSm Bubble Sauter-mean diameter [mm]

I Axial turbulence intensity [-]

jk kth-phase superficial velocity [m/s]

L/Dh Non-dimensional axial distance [-]

P Rod pitch [cm]

Phalf Half rod pitch, P/2, [cm]

pz Gauge pressure [kPa]

Qg Inlet gas flow rate [scfh]

Reb Bubble Reynolds number [-]

Ref Channel liquid Reynolds number [-]

U Local mean axial liquid velocity [m/s]

Ubulk Bulk single-phase liquid velocity [m/s]

'RMSu Local root-mean square liquid velocity [m/s]

vg Bubble velocity [m/s]

Web Bubble Weber number [-]

Greek

α Void fraction [-] Generic parameter

]R Scaling ratio [-]

Subscripts and Superscripts

atm Atmospheric pressure condition

b Bubble

ch Subchannel

exp Experimental system

f Liquid phase

g Gas phase

I Group-I quantity

II Group-II quantity

ref Reference system

SG Spacer Grid

sph Spherical

total Total quantity

Mathematical Operators

Area-average, 1

AdA

A

Void-weighted area-average,

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Documents

Experimental Investigation on the Effects of a Spacer Grid on Single- and Two-Phase Flows