Local Two-phase Flow Parameters Research Articles

Experimental investigation of local boron concentration in a 2 × 2 rod bundle channel under subcooled boiling flow

During the operation of pressured water reactors (PWRs), subcooled boiling occurring at the upper-span region of the fuel assembly causes migration of the boron concentration in the coolant. The distribution of boron concentration in the coolant is essential to predict the boron hideout within the Corrosion Related Unidentified Deposition (CRUD) and crud-induced power shift (CIPS). However, experimental data on local boron concentration of rod bundle geometries in subcooled boiling conditions are not available. In this paper, a vertical test channel is constructed to determine the distribution of the local boron concentration of a 2 × 2 rod bundle channel under subcooled boiling conditions. The bubble-boron concentration simultaneous measurement device is developed and calibrated to measure the local two-phase flow parameters and boron concentrations under the subcooled boiling conditions. The experimental results indicate that the boron concentration is highest near the narrowest gap and decreases toward the subchannel center or subchannel wall. The boron concentration increases linearly with void fraction in the central subchannel, while the boron concentration first increases rapidly and then slowly in the wall channel. The underlying mechanism for the boron concentration distribution is revealed by considering the boron enrichment on the bubble–liquid interface, diffusion of the boron concentration, boron mixing induced by bubble wake, and boron dilution caused by bubble condensation. On the basis of that, a boron equilibrium model considering the evaporation and condensation of bubbles is developed. The experimental data is used to support the development and validation of the boron equilibrium model. The comparison results show that the boron equilibrium model can predict the performance of the boron concentration in the rod bundle channel with an accuracy of 20 %.

Multi-dimensional characteristics of upward bubbly flows in a vertical large-size square channel

Gas-liquid two-phase flows frequently occur in various flow channels in engineering systems with heat and mass transport processes. The channel geometry affects thermo-fluid behavior generally. Extensive research is done on gas-liquid two-phase flows in circular and annular channels with a wealth of local two-phase flow measurement data. Square channels are used in various engineering disciplines, but there has only been limited experimental research on two-phase flows in square channels with local flow parameter measurements. For two-phase flow analyses in square channels, knowledge of the flow characteristics and model validation against some key parameters, such as the drift-flux parameters and the interfacial area concentration, is crucial. An experiment for upward bubbly flows was conducted in a large square channel with an inside cross-section of 0.136 × 0.136 m. A four-sensor optical probe and a hot-film anemometer were used to measure local two-phase flow parameters at 66 points in an octant symmetric triangular cross-section at three axial locations. The measured two-phase flow parameters were the void fraction, axial gas velocity, axial liquid velocity, interfacial area concentration, and bubble Sauter mean diameter. The flow characteristics through flow development were investigated based on the local measurement data. The local measurement data directly determined the drift-flux parameters through their definitions. The distribution parameters determined through experimentation could be reproduced by the existing correlation for large circular pipes indicating that the flow characteristics in large square channels may be similar to those in large circular pipes. Based on existing drift-flux correlations for large circular pipes, the void fractions in the current experiment could be fairly predicted. Furthermore, the interfacial area concentrations could be predicted by existing correlations with reasonable accuracy.

Effects of Pipe Inclination on Global Two-Phase Flow Parameters

Inclined two-phase flow geometries can be found in advanced nuclear reactor systems, such as the helical coil steam generators being considered for use in current integral steam generator designs. While this geometry includes inclination and centrifugal effects coupled together on two-phase flow, there have been limited studies to separate these effects to develop robust models. The majority of two-phase flow research is conducted on vertical channels, with recent work being conducted in a horizontal orientation and limited work in inclined pipes. In the current work, experiments are conducted in an adiabatic two-phase flow test facility to investigate the inclination effect on an air-water flow in straight pipes near atmospheric pressure. The pipe is made of clear acrylic with an inner diameter of 25.4 mm. The inclination of the flow loop can be adjusted in increments of 0.1 deg. Measurement capabilities are included to obtain local two-phase flow parameters such as void fraction, interfacial area concentration, bubble velocity, and Sauter-mean diameter using a local multisensor conductivity probe, local two-phase flow static pressure and pressure drop using a pressure transducer, and flow visualization using a high-speed video camera system. The experimental studies performed in the current work demonstrate how changes in inclination angle can affect the gas distribution flow regime transition and two-phase frictional pressure drop. Based on these experimental results, existing correlations for frictional pressure drop are evaluated, and the modified Lockhart-Martinelli correlation is found to predict the two-phase frictional pressure drop for inclined two-phase flows. This method agrees with experimental data within 7% on average.

Experimental Investigation on Effects of Flow Orientation on Interfacial Structure of Air–Water Two-Phase Flow

This study focusses on investigating the effect of flow orientation on global and local two-phase flow parameters under two-phase flow conditions. Flow visualization, pressure measurement, and conductivity probe measurements are performed in a test facility made of 50.8 mm inner diameter acrylic pipes with flow visualization, pressure measurement, and conductivity probe measurement. Characteristics of flow regimes and interfacial structures, and frictional pressure drop prediction in the vertical upward and vertical downward two-phase flows are compared and analyzed. The results show that unique coring phenomenon and slug bubbles with off-centered noses appear in the vertical downward flow. The flow regime transition boundaries shift to the lower gas superficial velocities in the vertical downward flow compared to that in the vertical upward flow. Furthermore, the distribution and one-dimensional transport of the void fraction, interfacial area concentration, bubble velocity, bubble Sauter-mean diameter, and bubble frequency are acquired and compared to study the effect of flow orientation on interfacial structures. The interfacial structure and its development are found to be mainly affected by the lift force and the turbulence related bubble interactions. The frictional pressure drop is acquired and modeled by the Lockhart–Martinelli correlation. The frictional pressure drop is found to be larger in the vertical upward flow than that in the vertical downward flow. The recommended C value for both vertical upward and vertical downward flows are proposed.

Development of ANN structural optimization framework for data-driven prediction of local two-phase flow parameters

In this paper, a workflow of ANN-based model development for data-driven prediction of the local two-phase flow parameters has been proposed. To effectively construct an ANN-based model, a framework of ANN structural optimization was also developed based on Genetic Algorithm method and classical Trial-and-Error method. Two case studies with 638 data points of local void fraction (in subcooled boiling flow) and 3125 data points of local interfacial area concentration (in adiabatic air-water flow) of two-phase flow were investigated. The results clearly proved the effectiveness of optimization framework as well as the predictive capability of developed ANN-based models. Also, comparison results with CFD (computational fluid dynamics) showed that better agreements with experimental data can be obtained with well-trained ANN structure.

Reduction of skin friction and two-phase flow structure beneath wall in horizontal rectangular channel

From the viewpoint of the improvement of the air lubrication method for the ship's hull resistance reduction, an experimental study was conducted for two-phase flow in a horizontal rectangular channel with a length of 3.0 m, a gap of 0.05 m, and a width of 0.2 m, which simulates the external two-phase flow. The database on frictional drag and local two-phase flow parameters, including void fraction profile and gas chord length, was collected using a shear stress sensor and a double sensor probe. Measurement of two-phase flow structure was performed at three axial positions of z = 0.930, 1.43, and 1.93 m from the bubble injection port. The wall shear stress was obtained at z = 1.93 m. A total of 72 datasets are acquired a room temperature and at flow conditions of superficial liquid velocity <jf> ranging from 0.832 to 3.00 m/s and superficial gas velocity <jg> ranging from 0.0232 to 0.714 m/s. The effects of entry length, liquid and gas flow rates on the phase distribution characteristics beneath the wall were discussed. The bubble layer thickness was newly defined as the length scale. The bubble layer thickness was not so sensitive to the changes in the liquid flow rates and the entry length, and highly dependent on the gas flow rates in the present experimental conditions. The mean void fraction in the bubble layer was highly reliant on the liquid flow rate and decreased with the superficial liquid velocity. In contrast, the insignificant effect of gas flow rate on the mean void fraction was confirmed. A strong correlation between the drag reduction effect due to the air lubrication and the mean void fraction in the bubble layer was confirmed. The obtained data are expected to be used for the modeling of the interfacial area transport terms, development of the constitutive equations of the drift flux model, modeling of the drag reduction for the external two-phase flow and the benchmark tests of various CFD codes in the future study.

Two-group phase distribution characteristics for air-water flow in 5 × 5 vertical rod bundle channel with mixing vane spacer grids

The phase distribution in rod bundle channel was associated with the heat transfer coefficient, flow resistance and critical heat flux, which was of great significance for the design and safe operation of nuclear reactor and steam generator. In order to clarify the phase distribution characteristics and reveal the effect of mixing vane spacer grid (MVSG) in two-group air-water flow in rod bundle channel, the adiabatic upward air-water two-phase flow experiments were conducted in 5 × 5 rod bundles with MVSGs and the detailed local two-phase flow parameters were acquired with miniaturized four-sensor conductivity probe (MFSCP). Some interesting phenomena were reported: Group I and II void fraction profiles showed the core-peak features; Group I void fraction almost stayed at the region from 0.2 to 0.3 for two-group air-water flow at current flow conditions; the dissipation length of MVSG was larger than 32.1 L/D at current two-group flow conditions; and the influencing region of MVSG was divided into at least three regions along the axial direction with the flow development for current flow conditions.

Experimental characterization of boiling two-phase flow structures under BWR core operating conditions

Local two-phase flow parameters have been measured at the Westinghouse FRIGG facility in a test fuel bundle under Boiling Water Reactor (BWR) core operating conditions, over a wide range of flow and power (up to critical power). The measurements were performed at end of heated length using sapphire optical probes and Pitot tubes placed in various radial positions of the bundle. By using both instruments simultaneously, the void fraction, mixture velocity, dispersed phase (bubble or drop) characteristic lengths and volumetric interfacial area can be measured. In addition, the identification of two-phase flow regimes can be attempted. The measurements are of particular interest since there is very limited available data in the literature at high pressure relevant to BWR operation.Experimental measurements are reported in this paper covering a range of inlet flow and bundle power relevant to BWR operation, at a system pressure of 70 bar. The dynamic void fluctuations are used to establish two-phase flow regime maps, one based on global parameters (for 1-D applications) and the second based on local parameters (for CFD applications). However, it is discovered that transition flow regimes typically observed at low pressure (cap, slug, etc) are not similarly observed at high pressure. In addition, the measured characteristic lengths of the dispersed phase are reported in bubbly and annular two-phase flow regimes. In view of the results, recommendations are provided for further experimental work and modeling improvements.

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.

Measurement of local two-phase flow parameters of downward bubbly flow in mini pipes

In order to extend a precise database on local two-phase flow parameters in mini pipes, experiments were conducted for adiabatic gas–liquid bubbly flows flowing down in vertical mini pipes with inner diameters of 1.03, 3.00, and 5.00 mm. A stereo image-processing was applied to observe the phase distribution characteristics in pipe cross-section. The local flow parameters including profiles of void fraction, Sauter mean bubble diameter, and interfacial area concentration in pipe cross-section were obtained at three axial locations in the test pipes with various flow conditions: superficial gas velocity of 0.00508–0.0834 m/s and superficial liquid velocity of 0.208–3.00 m/s. The axial developments of the local flow parameters were discussed in detail based on the obtained data and the visual observation. It was confirmed that the core peak distributions were formed at low liquid flow rate conditions in which the buoyancy force dominated while the wall peak distributions were formed at high liquid flow rate conditions in which the body acceleration due to the frictional pressure gradient dominated. The result indicated the existence of lift force pushing the bubbles towards the pipe wall even in vertical downward flows. The database obtained through the present experiment is expected to be useful in modeling the interfacial area transport terms, the validation of the existing lift force models as well as the benchmarking of various CFD simulation codes.

Experimental study on local interfacial parameters in upward air-water bubbly flow in a vertical 6 × 6 rod bundle

This paper presents a complete database of the local two-phase flow parameters for upward adiabatic air-water two-phase flows in a vertical 6 × 6 rod bundle flow channel. A total of 16 flow conditions were selected and investigated at 16 measuring points in the octant triangular measuring region of a cross section at the position with height-to-diameter ratio of 149 (z/DH = 149) in the rod bundle experiment. The local void fraction, interfacial area concentration (IAC), bubble diameter and bubble velocity vector were measured by using the four-sensor optical probe measuring technique. In the measured cross-sectional distributions of local void fraction and IAC, both core-peaking and wall-peaking distribution shapes were observed and the distribution pattern trended to change from the core-peaking shape to the wall-peaking shape with the increase of superficial liquid velocity (〈jf〉) and the decrease of superficial gas velocity (〈jg〉). The radial distributions of local bubble diameter are nearly flat, which explains the similarity of the radial local void fraction and IAC distributions. The measured local bubble diameters increase with the increase of 〈jg〉 and the decrease of 〈jf〉. The bubbles move along the rod bundle flow channel with their velocity component in the main flow direction (vgz) showing a typical radial core-peaking profile and their velocity components in the cross section showing a significant movement from the central region to the channel box wall region especially under high superficial liquid velocities. The cross-sectional area-averaged results of the local parameters were obtained by a measuring-point-based graphical integration. The obtained area-averaged void fraction and IAC are compared with the predictions of the drift-flux model with its available 2 frequently-used distribution parameter and drift velocity correlations and the 2 recently-developed IAC correlations respectively. The applicability of the 2 drift-flux correlations and the 2 IAC correlations to the rod bundle flow channel is evaluated and analyzed.

환형채널 내 저압 미포화비등 유동장의 국소 이상유동 변수 측정

환형채널 내 저압 미포화비등 유동장의 국소 이상유동 변수 측정

Integration of conductivity probe with optical and x-ray imaging systems for local air–water two-phase flow measurement

Various techniques have been developed in the past to measure the different parameters in two-phase flow. Among them, the multi-sensor conductivity probe is one of the most commonly used techniques because of its good overall performance. Recently, more advanced techniques, such as high-speed optical imaging and fast x-ray densitometry, have matured and become easily accessible. The main objective of this work is to compare and integrate these different techniques, to achieve a more accurate and complete measurement of two-phase flow. In this study, a double-sensor conductivity probe is used to measure the local time-averaged parameters along the radial direction of a 2.54 cm ID round pipe. An x-ray densitometry system is used to measure the chordal averaged void fraction and gas velocity. A high-speed camera system is employed to provide a visualization of the two-phase flow structure and obtain the line-averaged gas velocity. The unique advantages and main uncertainties of these three techniques are analyzed by considering their measuring principles and possible issues in practical measurements. From this, a method is developed to integrate the data obtained by the optical and x-ray systems into the probe signal processing. Better accuracy can be achieved using this method for various local two-phase flow parameters, including the void fraction, bubble velocity and superficial gas velocity, compared to the original probe measurements.

Phase distribution characteristics of bubbly flow in 5 × 5 vertical rod bundles with mixing vane spacer grids

In order to predict the thermal–hydraulic parameters in nuclear reactor accurately, it is essential to reveal the phase distribution characteristics, the mechanisms of phase interactions and the effects of the mixing vane spacer grid (MVSG) in rod bundles, based on which the drift-flux model and interfacial area transport equation could be developed further. The profiles of local two phase flow parameters could give an insight into these phenomena. Therefore, the adiabatic air–water upward two-phase flow experiments are performed to measure local parameters in 5 × 5 rod bundles with mixing vane spacer grids (MVSGs) under different bubbly flow conditions. The miniaturized four-sensor conductivity probe (MFSCP) has been developed to measure local two-phase flow parameters, such as void fraction, interfacial area concentration (IAC), bubble velocity and bubble chord length. The detail local parameters are measured and analyzed in the bubbly flow. Typical wall peaks for void fraction are identified and the bubble velocity profiles have a reverse phase position with void fraction profiles. The mixing vane spacer grid (MVSG) plays an important role in phase distribution in rod bundles: the skewed peaks of void fraction is associated with the direction of mixing vanes; a larger value of area-averaged void fraction is induced at the downstream of MVSG; compared with other factors, the MVSG plays a leading role in interfacial area transportation. Moreover, the dissipation length of MVSG is less than 19.4 L/D for current bubbly flow conditions.

Measurements of local two-phase flow parameters in fuel bundle under BWR operating conditions

Abstract The Westinghouse FRIGG facility, in Vasteras/Sweden, is dedicated to the measurement of critical power, stability and pressure drop in fuel rod bundles under BWR operating conditions (steady-state and transient). The facility is particularly relevant to test modern BWR fuel designs which typically have complex features, such as part-length rods and mixing vanes that make the flow heterogeneous and challenging to accurately simulate (e.g. using sub-channel analysis codes or CFD tools). In order to support the validation of advanced thermal-hydraulics codes for detailed BWR fuel assembly simulation, new local instrumentation techniques have been tested at the FRIGG facility for the measurement of two-phase dynamic pressure (Pitot tubes) and high time resolution phase detection (optical sensor). The optical sensors were custom-made by RBI Instrumentation for the FRIGG facility and optimized for annular two-phase flow (drop/steam) under BWR operating conditions. This new instrumentation was successfully tested and allows the first-time measurement, under BWR operating conditions, of relevant two-phase flow parameters such as the local void fraction in the steam core, the local drop/steam velocity, the volumetric interfacial area, the drop collision frequency and the assessment of drop size distribution during BWR steady-state and transient operations.

Characterization of horizontal air–water two-phase flow

This paper presents experimental studies performed to characterize horizontal air–water two-phase flow in a round pipe with an inner diameter of 3.81cm. A detailed flow visualization study is performed using a high-speed video camera in a wide range of two-phase flow conditions to verify previous flow regime maps. Two-phase flows are classified into bubbly, plug, slug, stratified, stratified-wavy, and annular flow regimes. While the transition boundaries identified in the present study compare well with the existing ones (Mandhane et al., 1974) in general, some discrepancies are observed for bubbly-to-plug/slug, and plug-to-slug transition boundaries. Based on the new transition boundaries, three additional test conditions are determined in horizontal bubbly flow to extend the database by Talley et al. (2015a). Various local two-phase flow parameters including void fraction, interfacial area concentration, bubble velocity, and bubble Sauter mean diameter are obtained. The effects of increasing gas flow rate on void fraction, bubble Sauter mean diameter, and bubble velocity are discussed. Bubbles begin to coalesce near the gas–liquid layer instead of in the highly packed region when gas flow rate increases. Using all the current experimental data, two-phase frictional pressure loss analysis is performed using the Lockhart–Martinelli method. It is found that the coefficient C=24 yields the best agreement with the data with the minimum average difference. Moreover, drift flux analysis is performed to predict void-weighted area-averaged bubble velocity and area-averaged void fraction. Based on the current database, functional relations for ≪vg≫ vs. <j> and <α> vs. <jg>/<j> have been studied. It is found that ≪vg≫ – <j> method predicts the void-weighted area-averaged bubble velocity and area-averaged void fraction better compared to <α> – <jg>/<j> method.

Interfacial area transport across a 90° vertical-upward elbow in air–water bubbly two-phase flow

This study develops a one-group interfacial area transport equation (IATE) for vertical-upward-to-horizontal air–water bubbly two-phase flows through a 90° elbow with a non-dimensional centerline radius of curvature of three. In order to develop the model, an extensive database is established by acquiring local two-phase flow parameters using a four-sensor conductivity probe upstream and downstream of the elbow. The data show there exist three characteristic regions in void distribution, including a bimodal-to-bimodal region, a bimodal-to-single-peaked region, and a developed horizontal flow region with void accumulated at the top of the pipe cross-section. Using the database, the preliminary dissipation length model developed by Yadav et al. (2014b) is improved by including the transition region near the exit of the elbow in addition to the dissipation region. To close the IATE model, the bubble velocity advection term and bubble interaction terms in the IATE are correlated with the parameter characterizing the “elbow-strength”. The two-phase pressure drop across the elbow is modeled using the modified Lockhart–Martinelli correlation which takes into account the minor loss effect. The closed IATE model is implemented to predict interfacial area transport in vertical-upward-to-horizontal two-phase flow. It is found that the developed model is capable of predicting interfacial area concentration with an average percent difference of less than ±6%.

Inlet effects on vertical-downward air–water two-phase flow

This paper focuses on investigating the geometric effects of inlets on global and local two-phase flow parameters in vertical-downward air–water two-phase flow. Flow visualization, frictional pressure loss analysis, and local experiments are performed in a test facility constructed from 50.8mm inner diameter acrylic pipes. Three types of inlets of interest are studied: (1) two-phase flow injector without a flow straightener (Type A), (2) two-phase flow injector with a flow straightener (Type B), and (3) injection through a horizontal-to-vertical-downward 90° vertical elbow (Type C). A detailed flow visualization study is performed to characterize flow regimes including bubbly, slug, churn-turbulent, and annular flow. Flow regime maps for each inlet are developed and compared to identify the effects of each inlet. Frictional pressure loss analysis shows that the Lockhart–Martinelli method is capable of correlating the frictional loss data acquired for Type B and Type C inlets with a coefficient value of C=25, but additional data may be needed to model the Type A inlet. Local two-phase flow parameters measured by a four-sensor conductivity probe in four bubbly and near bubbly flow conditions are analyzed. It is observed that vertical-downward two-phase flow has a characteristic center-peaked void profile as opposed to a wall-peaked profile as seen in vertical-upward flow. Furthermore, it is shown that the Type A inlet results in the most pronounced center-peaked void fraction profile, due to the coring phenomenon. Type B and Type C inlets provide a more uniform distribution of the void fraction profile with a reduced coring effect.

Experiments in Cap-Bubbly Two-Phase Flows for Two-Group IATE Development

In the two-group interfacial area transport equation (IATE) used to calculate the interfacial area concentration (ai), bubbles are categorized into two groups. Namely, group-I consists of spherical/distorted bubbles, and group-II consists of cap/slug/churn-turbulent bubbles. Robust models for the major bubble interaction mechanisms that cause the transition from purely one-group to two-group flows are essential to the dynamic closure of the two-fluid model with the two-group IATE. Therefore, the present study seeks to establish an experimental database in cap-bubbly flows that highlights this transition to support model development. A four-sensor conductivity probe is used to obtain measurements of local time-averaged two-phase flow parameters, including the void fraction and ai, in vertical-upward air-water two-phase flows in a 5.08-cm pipe. Four flow conditions are investigated at ⟨jf⟩ = 2 m/s with increasing ⟨jg⟩ to study the generation and growth of group-II bubbles. Characteristic features of the local void fraction and ai distributions are discussed. Additionally, axial development of area-averaged void fraction and ai that is indicative of exchange between the bubble groups is presented.

Experiments on the Effects of a Spacer Grid in Air-Water Two-Phase Flow

Understanding the effects of spacer grids on the coolant flow through a nuclear reactor core is required for best-estimate design and analysis of the plant. The impact of a spacer grid on two-phase flows is of particular importance because the geometric effects of the grid can alter the two-phase flow structure and, consequently, the mass, momentum, and energy transfer characteristics. Therefore, a scaled separate-effects test facility is constructed to investigate the effects of a spacer grid on the hydrodynamics of air-water two-phase flow through a rod bundle. The test facility is scaled to maintain hydrodynamic and geometric similarity to single- and two-phase flows in a conventional pressurized water reactor and to facilitate detailed local measurements of two-phase flow parameters around the simulant fuel rods with a four-sensor conductivity probe. This paper presents measurements of local time-averaged two-phase flow parameters acquired upstream and downstream of the spacer grid with the conductivity probe in a representative subchannel of a 1×3 rod bundle for eight flow conditions. Characteristic features of the development of the two-phase flow parameters along the test section are discussed.