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Comparison between wire-mesh sensors and conductive needle-probes for measurements of two-phase flow parameters
Nuclear Engineering and Design 239 (2009) 1718–1724
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Comparison between wire-mesh sensors and conductive needle-probes for measurements of two-phase flow parameters A. Manera a,b,∗ , B. Ozar c , S. Paranjape c , M. Ishii c , H.-M. Prasser b,d a
Paul Scherrer Institute, 5232 Villigen, Switzerland Research Center Dresden Rossendorf, Dresden, Germany c Purdue University, West Lafayette, USA d ETH Zürich, Sonneggstrasse 3, 8092 Zürich, Switzerland b
a r t i c l e
i n f o
Article history: Received 3 December 2007 Received in revised form 2 May 2008 Accepted 12 June 2008
a b s t r a c t Measurements of two-phase flow parameters such as void-fraction, bubble velocities, and interfacial area density have been performed in an upwards air–water flow at atmospheric pressure by means of a four-tip needle-probe and a wire-mesh sensor. For the first time, a direct comparison between the two measuring techniques has been carried out. Both techniques are based on the measurement of the fluid conductivity. For void-fraction and velocity measurements, similarity exists between the two methodologies for signal analysis. A significantly different approach is followed, instead, for the estimation of the interfacial area concentration: while the evaluation based on the needle-probe signal is carried out by using projections of the gas–liquid interface velocity, the evaluation based on the wire-mesh signals consist in a full reconstruction of the bubbles interfaces. The comparison between the two techniques shows a good agreement. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since several years, multiple-tip electrical and optical probes are used for the measurement of interfacial area densities, and comparisons between the two techniques have been reported in the literature. In spite of their different origin (conductive vs. optical), the signals delivered by the two types of probe have similar structure, bounded by two signal levels identifying the liquid and the gas phase respectively. The two measurement techniques present the same type of shortcomings consisting in an indirect measurement of bubble size based on bubble chords (depending on the position at which the bubble is pinched by the probe and on the bubble orientation), and low bubble statistics since the probe is localized in a limited portion of the pipe cross-section. For both types of probe, the local interfacial area density is derived on the basis of velocity measurements according to Ishii’s derivation (Ishii, 1975). Lately, wire-mesh sensors were presented as an alternative to the widely used multiple-tip electrical or optical fiber probes to measure interfacial area concentration in vertical pipes (Prasser, 2007). A wire-mesh sensor delivers a three-dimensional matrix of
∗ Corresponding author at: Paul Scherrer Institute, 5232 Villigen, PSI, Switzerland. Tel.: +41 56 3102729; fax: +41 56 3102327. E-mail address: [email protected] (A. Manera). 0029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2008.06.015
local instantaneous gas fractions in the pipe cross-section where the sensor is installed, therefore allowing a three-dimensional reconstruction of each bubble crossing the sensor. In a previous work (Prasser, 2007), a method was presented to fully reconstruct the gas–liquid interface out of the wire-mesh signal, and therefore to estimate the interfacial area density based on the actual bubbles shape. In the present paper a comparison between conductive four-tip needle probes and wire-mesh sensors is presented for measurements of radial profiles of void-fraction, bubble velocity and interfacial area density in a vertical pipe. The experiments were performed at the Purdue University, in a 50.8-mm diameter pipe, for stationary developed flow. Bubbly- as well as slug-flow regimes have been analyzed. Since the two measuring techniques are based on different approaches, the comparison presented in this paper helps giving a better insight in the soundness of multiple-tips needle-probes and wire-mesh sensors for interfacial area measurements and in general for two-phase flow parameters.
2. Experimental set-up The experimental facility, built in the Thermal-Hydraulics Laboratory of the Purdue University, is designed for the investigation of adiabatic, vertical air–water flows. A scheme of the set-up is shown in Fig. 1. It consists of two test sections made of acrylic pipes
A. Manera et al. / Nuclear Engineering and Design 239 (2009) 1718–1724
Nomenclature ai Nb Tmeas u vb
interfacial area density (1/m) number of bubbles measurement time (s) fluid conductivity (S/m) bubble velocity (m/s)
Greek symbols ˛ void-fraction l distance between probe tips (m) tb bubble residence time (s)
with internal diameters of 2.54 and 5.08 cm respectively and a total length of 3.81 m. A two-phase flow mixture injection unit is present at the top and bottom ends of both pipes. The mixture injection unit uses a porous tube with a mean pore size of 10 m. The water flow is supplied by a 25 hp (∼18.6 kW) centrifugal pump. Its frequency driver is used for coarse adjustment of water flow rate, while ball
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valves are used for fine adjustments of the flow. Air is supplied by an air compressor, which provides a maximum pressure of 130 psi. A pressure regulator is provided to maintain a constant supply air pressure. The air flow rate is controlled by valves installed upstream of the air flow meters. By appropriate combination of the valves, co-current or counter-current, upward or downward flow can be achieved in the two test sections. After passing through the test section, the mixture eventually exits to a water tank, where air is separated from the mixture and exits to the atmosphere. The bubble generating section consists of a porous sparger tube through which the bubbles are generated. The sparger is surrounded by a tube forming an annular channel around it. The water flow rate is divided into two: primary and secondary flow. The secondary water flows in the annular channel shearing off the bubbles generated by the porous sparger tube. The secondary flow rate is used to control the bubble size at the inlet of the test section. Demineralized water is used for the experiments. Small amounts of morpholine and ammonium hydroxide are added to increase the electrical conductance, in order to achieve better signal-to-noise ratios for the needle-probe. Each test section is equipped with three ports, for holding probe mounts. For the
Fig. 1. Scheme of the experimental set-up.
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Table 1 Test matrix Test case
Jg (m/s)
Jl (m/s)
Case 1 Case 2 Case 3 Case 4
0.036 0.142 0.534 2.089
0.647 2.550 2.549 2.554
2.54 cm I.D. pipe the ports are located at z/D of 13, 68 and 133, while for the 5.08 cm I.D. pipe the ports are located at z/D of 7, 34 and 67 respectively. Here, z is defined as the distance of the port from the air injection. At each port, needle probes can be installed. The experiments discussed in the present paper have been performed with the 5.08-cm diameter test section, in upwards cocurrent flow configuration. The liquid and gas superficial velocities for the conditions analyzed in the present work are reported in Table 1. Fig. 2. Instrumentation set-up (not in scale).
3. Instrumentation A four-tip conductivity needle-probe sensor is mounted 3.4 m (z/D = 67) downstream of the air injection. By means of a traversing mechanism (see Fig. 2), it is possible to change the radial position of the needle-probe in the cross-section. By performing measurements at different radial locations, radial profiles of time-averaged two-phase flow parameters can be measured. A sketch of the fourtip needle-probe is shown in Fig. 3. One of the probe tips (upstream tip) is longer than the others. In such a way a four-tip probes is equivalent to three double sensors, allowing the measurement of three components of the local interfacial velocity on the basis of the time delays between the upstream sensor and the other three tips respectively. A detailed analysis on the accuracy of needle-probe is given by Le Corre and coworkers (2003). A double-layer wire-mesh sensor is present 1.3 cm downstream of the needle-probe. The sensor consists of three layers of electrodes of 100-m diameter. The axial distance between two successive electrodes layers is 0.9 mm. The middle plane of the sensor (transmitter wires) consists of 24 electrodes, crossing the pipe cross-section at a distance of 2 mm from each other. These electrodes are rotated 90◦ with respect to the electrodes of the other
two planes (the so-called receiver wires. See Figs. 2 and 4 for a simplified scheme. Here only two electrode layers are indicated for simplicity). Two successive layers deliver a two-dimensional matrix of time-dependent instantaneous local fluid conductivity. Details on the measurement principle are given by Prasser and coworkers (1998). The use of three layers allows the measurement of bubble velocities by evaluating bubbles time-of-flight or by applying crosscorrelation techniques. Details on the accuracy of the wire-mesh sensor are given by Prasser and coworkers (1998, 2003). The measuring frequency of the needle probe was chosen between 10 and 30 kHz and the signal was recorded at each radial position for 180 s. Up to 14 radial positions have been measured for each experimental series, leading to a net measuring time of 42 min for each flow condition. A measuring frequency of 5000 frames/s was employed for the measurements with the wire-mesh sensor. A single measurement of 20 s was recorded for each flow condition. The wire-mesh signal was recorded with the needle-probe located as close as possible to the pipe wall, in order to minimize its effect on the wire-mesh measurement.
Fig. 3. Sketch of the 4-tip needle-probe.
A. Manera et al. / Nuclear Engineering and Design 239 (2009) 1718–1724
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time Tmeas
˛=
b
tb
Tmeas
∈ [0, 1]
(1)
The void-fraction measurement suffers from the employment of the voltage threshold (see Section 4), which yields a lower value of the bubble residence time tb and leads therefore to an underestimation of the time-averaged void-fraction. In order to overcome this drawback, a correction algorithm is applied for each bubble, which evaluates an average rising slope (dV/dt) of the squared signal in the proximity of the starting point of the voltage rise and increases the bubble residence time tb by adding a value equal to Vs∗ (dV /dt)−1 . To get a void-fraction radial profile, a series of separate measurements is carried out at the same flow conditions for different locations of the needle probe in the pipe cross-section. Fig. 4. Scheme of the wire-mesh sensor.
4. Signals preconditioning 4.1. Needle-probe A four-tip needle-probe generates four voltage signals, one for each tip of the probe. These signals indicate whether the given probe tip is at contact with gas or with liquid. For the evaluation of two-phase flow parameters, the signals are first filtered by means of a 5-points moving-median filter. In this way, high-frequency noise is filtered, while keeping enough time resolution for appropriate bubble velocity measurements. Then, the signal is normalized to cope with voltage-drifting and inter-channel interference. The base voltage of the liquid phase Vbase and the maximum voltage Vmax are adjourned every 8000 samples. In the normalized signal, the remaining noise due to voltage variation in the liquid phase is removed by setting a threshold level Vs . On the basis of experimental observation, this threshold level is generally set equal to 10% of the maximum voltage Vmax . Finally, the signal is shaped in the form of squared waves, where a voltage change represents a bubble interface (front or rear). Details on the signal pre-processing are given by Fu and Ishii (1999) and Fu (2001). 4.2. Wire-mesh sensor For the wire-mesh sensors, no pre-processing of the signal is needed. Before the experimental series, a short measurement (generally 1 s) has to be carried out, in order to measure the liquid conductivity in absence of air. In this way, a matrix uijk of liquid conductivities is obtained (i and j are the indexes for the spatial coordinates x and y in the pipe cross-section, while k stands for the time step index). The matrix uijk is averaged over the time-index k, to give the two-dimensional calibration matrix u0ij . This is used as reference liquid conductivity to convert the measured raw data of fluid conductivity in absolute values of void-fraction (Prasser et al., 1998). 5. Measurements of void-fraction 5.1. Needle-probe The void-fraction is calculated on the basis of the signal obtained by the upstream probe tip, after being normalized and transformed to a square signal, by applying a voltage threshold. The timeaveraged void-fraction is evaluated as the ratio between the sum of the residence time tb of each bubble, b, and the total measuring
5.2. Wire-mesh sensor For the wire-mesh sensor, a single measurement of 20 s is performed. The raw signal uijk is converted into void-fractions assuming a linear dependency of the local instantaneous voidfraction with respect to the flow conductivity (this assumption has been proved to be satisfactory in previous works by Prasser and coworkers, 1998, 2003; and Richter, 2001): ˛ijk =
uijk − uij,air u0ij − uij,air
(2)
where ˛ijk is the three-dimensional matrix of measured voidfractions, uij,air is the air conductivity, and u0ij is the reference liquid conductivity. The air conductivity is usually zero. Nevertheless, due to finite insulation impedances in the sensor and the cables, as well as an offset of the input cascades of the wire-mesh electronics, a calibration for an empty pipe is necessary and was performed in the experiments presented here. By averaging in time, a two-dimensional void-fraction distribution is obtained. For a direct comparison with the needle probe, the two-dimensional distribution is converted into a radial profile by averaging along circumferential slices (rings). In addition, an algorithm has been developed that recognizes single bubbles out of the three-dimensional void-fraction signal. A bubble-object is defined as a region of connected elements (i, j, k) containing gas phase surrounded by elements containing liquid phase. The algorithm uses a recursive scheme to identify the elements (i, j, k) belonging to the same bubble-object. Details can be found in Prasser et al. (2001) and Prasser and Bayer (2007). The bubble recognition algorithm allows to resolve two-phase flow parameters in terms of bubbles size classes. 5.3. Results In Figs. 5 and 6 the void fractions measured by means of the wire-mesh sensor and the needle probe are reported. As expected, for cases 1 and 2 the wall peaked profile typical for bubbly flow is found. Considering the very low level of absolute void-fraction (between ∼3% and 8%), a very good agreement is found between needle probe and wire-mesh sensor, the maximum difference between the two measurement technique being of less than 2% of absolute void-fraction. For cases 3 and 4 the void-fraction presents a central-peaked profile. Excellent agreement is found for case 3, while for case 4 a maximum deviation of about 13% (relative) is found between the two measuring techniques. For slug flows with air and water
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The local average bubble velocity is finally estimated, by dividing the sum of all bubble velocities vb by the total number of bubbles: v¯ b =
l 1 Nb b tb,front
(4)
A radial velocity profile is obtained by performing measurements at different locations of the needle-probe along a radius of the pipe cross-section and by repeating the estimation procedure for each measurement. 6.2. Wire-mesh sensor Three methodologies can be followed to estimate a radial profile of the gas velocity:
Fig. 5. Comparison between wire-mesh (solid lines) and needle-probe (circles) for void-fraction measurements.
at ambient temperature, by comparison with an ultra-fast X-ray tomograph, it has been found that the wire-mesh slightly underestimate the void-fraction due to the formation of a thin water film on the sensors wires (Prasser et al., 2003), the effect disappearing with increasing liquid temperature (Manera, 2003). The underestimation of the needle-probe for case 4 in Fig. 6 is presumably due to the effect of the voltage threshold discussed in Section 4 which is needed for the preconditioning of the signal. 6. Measurements of gas velocity 6.1. Needle-probe The needle-probe estimates the velocity of each bubble touching the probe, by measuring the time of flight of each single bubble on the basis of the bubble front. This is done by looking at the timedelay between the signals measured by the upstream probe (probe ‘0’ in Fig. 3) and one of the downstream sensors (probe ‘1’ in Fig. 3). Knowing the axial distance l between the two probes, an estimate for the velocity vb of a given bubble is obtained: vb =
l tb,front
(I) the velocity is computed separately for each single bubble by estimating the time of flight of the given bubble on the basis of the bubble front. The computed velocity vb is assigned to the bubble center of mass. A two-dimensional velocity profile is calculated by weighting the obtained velocities with the voidfraction carried by each given bubble; (II) the void-fraction signal ˛ijk is cross-correlated with the corresponding signal of the upper wire-mesh layer, in order to obtain a two-dimensional distribution of most-probable timeof-flight, which are then converted into velocities, knowing the axial distance between the two measurement planes; (III) the void-fraction signal ˛ijk assigned to a given bubble by the bubble recognition algorithm is correlated with the voidfraction measured by the upper measuring layer, in order to estimate the bubble time-of-flight and therefore its velocity. A parabolic fit around the correlation maximum is carried out to achieve a better accuracy on the time-of-flight estimation and thus on the bubble velocity. The two-dimensional velocity distribution is then evaluated in the same way as in method (I). This method is used in the present work. The two-dimensional velocity distribution obtained with one of the three methods described above is converted into a radial profile in the same way as for the void-fraction. 6.3. Results
(3) The comparison between the velocity measurements is presented in Fig. 7. The maximum deviation between the two measuring techniques is about 15%. This deviation, however, occurs for case 4 where relatively high velocities are measured and for which the error due to the sample time discretization is high. Overall, satisfactory agreement is found. 7. Measurements of interfacial area 7.1. Needle-probe According to the theoretical derivation by Ishii (1975), the local time-averaged interfacial area density can be expressed by the following relation: a¯ i =
Fig. 6. Comparison between wire-mesh (solid lines) and needle-probe (circles) for void-fraction measurements.
1 1 T j vi ni
(5) j
where j denotes the jth interface passing through a local point during the observation time interval T, being vi and ni the bubble interfacial velocity and the unit surface normal vector of the jth interface respectively. Based on Eq. (5), Kataoka and coworkers
A. Manera et al. / Nuclear Engineering and Design 239 (2009) 1718–1724
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Fig. 8. Bubble boundary interpolated on the measuring wire-mesh grid.
Fig. 7. Comparison between wire-mesh (solid lines) and needle-probe (circles) for gas velocity measurements.
(1989) have derived a formulation in order to derive the local timeaveraged interfacial area density from the signal of a four-tip probe: a¯ i (x0 , y0 , z0 ) = 2N
1
vi cos ϕ
(6)
where 2N is the total number of interfaces (i.e. N is the total number of bubbles). In order to apply Eq. (6), it is necessary to measure three separate components of the gas–liquid interface. This is achieved by computing the time-of-flight of a given bubble from the upstream sensor to the three downstream tips. In such a way, a velocity vector can be assigned to each bubble and the interfacial area density is then estimated on the basis of Eq. (6). The contribution to the interfacial area cannot be computed for those bubbles which do not touch all four tips of the needle-probe (missing bubbles), unless the bubble is assumed to be spherical (Kim et al., 2002; Le Corre and Ishii, 2002). Since no assumption is made on the bubbles shape, the contribution to the interfacial area density due to the missing bubbles is lost. In order to overcome this problem, a simple recovery method consists in assigning to the missing bubbles an average interfacial area computed on the basis of the effective bubbles (Kim et al., 2000). The effective bubbles are those that with their interface intersect all four tips of the probe so that their interface normal velocity and hence the interfacial area concentration can be determined.
The interpolation is carried out at the rectangular grid span out by the electrode wires of the sensor. To avoid equivocal situations, each of the square meshes is subdivided in a pair of triangles and an interpolation is also performed at the diagonal. Since there are two possibilities to define a diagonal, the one is taken, which results in the smaller length of the bubble contour within the mesh. This has proven to supply best results (Prasser, 2007). In meshes, at all four corners of which the gas fraction is either above or below the threshold, a contour is not found and the corresponding contribution to the contour length is zero. The second step is carried out in cubical control elements formed by the square meshes extended in the axial direction (orthogonal to the measuring plane) by the distance the flow travels in the period between two sampling instants. The axial extension of the control cuboids is defined as the product of the sampling period times the bubble velocity. In this step, the area of each surface element of the bubble is calculated that unites the linear bubble boundaries found in the measuring plane in two successive sampling instants. This is done in an approximate way assuming a plain surface. The bubble surface is then the sum of the surface elements found in this way. In parallel, the part of the area of the rectangular mesh that belongs to the bubble is calculated as well. The total area occupied by the reconstructed bubble, multiplied by the effective height of the control cuboids, supplies the volume of the reconstructed bub-
7.2. Wire-mesh sensor For the wire-mesh sensor, the interfacial area is calculated by fully reconstructing the gas–liquid interface (Prasser, 2007). Before the reconstruction of the gas–liquid interface is performed, the bubble recognition algorithm is executed. After this step, each element of the distribution of local instantaneous gas fraction delivered by the sensor is assigned to an individual bubble, using a bubble identification number. This allows the reconstruction of the gas–liquid interface for each bubble individually by taking into account only those gas fractions which belong to the currently analyzed bubble. The full reconstruction is performed in two steps. In the first step, the contour of the bubble is found within the measuring plane for each time step. It is defined as a polygon uniting all points at which the interpolated instantaneous gas fraction is equal to a threshold that characterizes the boundary of the bubble (see Fig. 8).
Fig. 9. Comparison between wire-mesh (solid line) and needle-probe (circles) for interfacial area density.
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(48.3 mm vs. 50.4 mm of present measurements). The comparison between Fu’s needle-probe measurement and the wire-mesh is reported in Fig. 11, where the interfacial area density is shown as function of the normalized pipe radius. Excellent agreement is found, which provides a good confidence also on measurements reproducibility. 8. Conclusions
Fig. 10. Case 4. Interfacial area density classified for bubbles diameter. Comparison between wire-mesh (solid lines) and needle-probe (circles).
Good agreement has been found between four-tip conductivity needle-probes and wire-mesh sensors. Additional measurements in the slug and in the churn flow-regime should be performed to extend the range of validation of the two measurements techniques. In general, smoother profiles are obtained with the wire-mesh sensor due to the higher number of measuring locations. In addition, since the wire-mesh occupies the entire pipe cross-section no bubble is missed and a better statistics can be obtained. Due to the very short time required to achieve a good measurement statistics (20 s measurement periods were used in the present work), the wire-mesh is a very good candidate to achieve a full mapping of the interfacial area density, which is needed for the further development and the assessment of transport equations for the interfacial area density. It has also to be pointed out that a full threedimensional reconstruction of the gas bubbles can be achieved with the wire-mesh sensor, allowing a sound separation of the twophase flow parameters according to bubble classes, based on the actual dimension of the gas bubbles. The needle-probe, on the other hand, is less intrusive and yields therefore fewer disturbances in the downstream flow. References
Fig. 11. Case 3. Indirect comparison between wire-mesh sensor and needle probe (measurement by Fu, 2001).
ble. This is important, because the gas fraction threshold has to be adapted for each bubble individually. Finally, the interfacial area can be transformed into an area density by relating it to the total volume of the two-phase flow. 7.3. Results The measurements of the interfacial area density are presented in Fig. 9. Again, very good agreement is found. A maximum deviation of less than 12% is observed for case 4. This deviation however occurs again in the center of the pipe, where the velocity is high and the uncertainty due to the sampling frequency is not negligible. In Fig. 10 the interfacial area density measured for case 4 is reported separately for small and large bubbles (10-mm diameter is used as separation between the two bubble classes). It is seen that the deviation between needle-probe and wire-mesh is mainly due to the large bubbles in the central part of the pipe, where the needle-probe measurement presents a certain scatter. A measurement in the same flow conditions as case 3 has been performed by Fu (2001) on a pipe with a slight smaller diameter
Fu, X., Ishii, M., 1999. User’s guide for 4PT100 advanced four-point probe signal processor. PU/NE-99-14, Purdue University, School of Nuclear Engineering. Fu, X., 2001. Interfacial area measurement and transport modeling in air–water twophase flow. Ph.D. Thesis, Purdue University, School of Nuclear Engineering. Ishii, M., 1975. Thermo-fluid Dynamic Theory of Two-phase Flow. Eyrolles, Paris. Kataoka, I., Ishii, M., Serizawa, A., 1989. Local formulation and measurements of interfacial area concentration in two-phase flow. Int. J. Multiphase Flow 12, 505–529. Kim, S., Fu, X.Y., Wang, X., Ishii, M., 2000. Development of the miniaturized foursensor conductivity probe and the signal processing scheme. Int. J. Heat Mass Transfer 43, 4101–4118. Kim, S., Ishii, M., Wu, Q., McCreary, D., Beus, S.G., 2002. Interfacial structures of confined air–water two-phase bubbly flow. Exp. Thermal Fluid Sci. 26, 461–472. Le Corre, Ishii, 2002. Numerical evaluation and correction method for multisensor measurement techniques in two-phase bubbly flow. Nucl. Eng. Des. 216, 221–238. Le Corre, J.-M., Hervieu, E., Ishii, M., Delhaye, J.-M., 2003. Benchmarking and improvements of measurement techniques for local-time-averaged two-phase flow parameters. Exp. Fluids 35, 448–458. Manera, A., 2003. Experimental and analytical investigations on flashing-induced instabilities in natural circulation two-phase systems. Ph.D. Thesis, Delft University of Technology, The Netherlands. Prasser, H.-M., Böttcher, A., Zschau, J., 1998. A new electrode tomograph for gas–liquid flows. Flow Meas. Instrum. 9, 111–119. Prasser, H.-M., Scholz, D., Zippe, C., 2001. Bubble size measurement using wire-mesh sensors. Flow Meas. Instrum. 12, 299–312. Prasser, H.-M., Misawa, M., Tiseanu, I., 2003. Comparison between wire-mesh sensor and ultra-fast X-ray tomograph for an air–water flow in a vertical pipe. Flow Meas. Instrum. 16, 73–86. Prasser, 2007. Evolution of interfacial area concentration in a vertical air–water flow measured by wire-mesh sensors. Nucl. Eng. Des. 237, 1608–1617. Prasser, H.-M., Bayer, M., 2007. Bubble recognition algorithms for the processing of wire-mesh sensors data. In: Proceedings of the International Conference on Multiphase Flows, ICMF-2007, July 9–13, Leipzig, Germany. Richter, S., 2001. Study on multidimensional characteristics of developing bubbly flow in a rectangular channel. Ph.D. Thesis, Tokyo University of Technology.
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Comparison between wire-mesh sensors and conductive needle-probes for measurements of two-phase flow parameters A. Manera a,b,∗ , B. Ozar c , S. Paranjape c , M. Ishii c , H.-M. Prasser b,d a
Paul Scherrer Institute, 5232 Villigen, Switzerland Research Center Dresden Rossendorf, Dresden, Germany c Purdue University, West Lafayette, USA d ETH Zürich, Sonneggstrasse 3, 8092 Zürich, Switzerland b
a r t i c l e
i n f o
Article history: Received 3 December 2007 Received in revised form 2 May 2008 Accepted 12 June 2008
a b s t r a c t Measurements of two-phase flow parameters such as void-fraction, bubble velocities, and interfacial area density have been performed in an upwards air–water flow at atmospheric pressure by means of a four-tip needle-probe and a wire-mesh sensor. For the first time, a direct comparison between the two measuring techniques has been carried out. Both techniques are based on the measurement of the fluid conductivity. For void-fraction and velocity measurements, similarity exists between the two methodologies for signal analysis. A significantly different approach is followed, instead, for the estimation of the interfacial area concentration: while the evaluation based on the needle-probe signal is carried out by using projections of the gas–liquid interface velocity, the evaluation based on the wire-mesh signals consist in a full reconstruction of the bubbles interfaces. The comparison between the two techniques shows a good agreement. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since several years, multiple-tip electrical and optical probes are used for the measurement of interfacial area densities, and comparisons between the two techniques have been reported in the literature. In spite of their different origin (conductive vs. optical), the signals delivered by the two types of probe have similar structure, bounded by two signal levels identifying the liquid and the gas phase respectively. The two measurement techniques present the same type of shortcomings consisting in an indirect measurement of bubble size based on bubble chords (depending on the position at which the bubble is pinched by the probe and on the bubble orientation), and low bubble statistics since the probe is localized in a limited portion of the pipe cross-section. For both types of probe, the local interfacial area density is derived on the basis of velocity measurements according to Ishii’s derivation (Ishii, 1975). Lately, wire-mesh sensors were presented as an alternative to the widely used multiple-tip electrical or optical fiber probes to measure interfacial area concentration in vertical pipes (Prasser, 2007). A wire-mesh sensor delivers a three-dimensional matrix of
∗ Corresponding author at: Paul Scherrer Institute, 5232 Villigen, PSI, Switzerland. Tel.: +41 56 3102729; fax: +41 56 3102327. E-mail address: [email protected] (A. Manera). 0029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2008.06.015
local instantaneous gas fractions in the pipe cross-section where the sensor is installed, therefore allowing a three-dimensional reconstruction of each bubble crossing the sensor. In a previous work (Prasser, 2007), a method was presented to fully reconstruct the gas–liquid interface out of the wire-mesh signal, and therefore to estimate the interfacial area density based on the actual bubbles shape. In the present paper a comparison between conductive four-tip needle probes and wire-mesh sensors is presented for measurements of radial profiles of void-fraction, bubble velocity and interfacial area density in a vertical pipe. The experiments were performed at the Purdue University, in a 50.8-mm diameter pipe, for stationary developed flow. Bubbly- as well as slug-flow regimes have been analyzed. Since the two measuring techniques are based on different approaches, the comparison presented in this paper helps giving a better insight in the soundness of multiple-tips needle-probes and wire-mesh sensors for interfacial area measurements and in general for two-phase flow parameters.
2. Experimental set-up The experimental facility, built in the Thermal-Hydraulics Laboratory of the Purdue University, is designed for the investigation of adiabatic, vertical air–water flows. A scheme of the set-up is shown in Fig. 1. It consists of two test sections made of acrylic pipes
A. Manera et al. / Nuclear Engineering and Design 239 (2009) 1718–1724
Nomenclature ai Nb Tmeas u vb
interfacial area density (1/m) number of bubbles measurement time (s) fluid conductivity (S/m) bubble velocity (m/s)
Greek symbols ˛ void-fraction l distance between probe tips (m) tb bubble residence time (s)
with internal diameters of 2.54 and 5.08 cm respectively and a total length of 3.81 m. A two-phase flow mixture injection unit is present at the top and bottom ends of both pipes. The mixture injection unit uses a porous tube with a mean pore size of 10 m. The water flow is supplied by a 25 hp (∼18.6 kW) centrifugal pump. Its frequency driver is used for coarse adjustment of water flow rate, while ball
1719
valves are used for fine adjustments of the flow. Air is supplied by an air compressor, which provides a maximum pressure of 130 psi. A pressure regulator is provided to maintain a constant supply air pressure. The air flow rate is controlled by valves installed upstream of the air flow meters. By appropriate combination of the valves, co-current or counter-current, upward or downward flow can be achieved in the two test sections. After passing through the test section, the mixture eventually exits to a water tank, where air is separated from the mixture and exits to the atmosphere. The bubble generating section consists of a porous sparger tube through which the bubbles are generated. The sparger is surrounded by a tube forming an annular channel around it. The water flow rate is divided into two: primary and secondary flow. The secondary water flows in the annular channel shearing off the bubbles generated by the porous sparger tube. The secondary flow rate is used to control the bubble size at the inlet of the test section. Demineralized water is used for the experiments. Small amounts of morpholine and ammonium hydroxide are added to increase the electrical conductance, in order to achieve better signal-to-noise ratios for the needle-probe. Each test section is equipped with three ports, for holding probe mounts. For the
Fig. 1. Scheme of the experimental set-up.
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Table 1 Test matrix Test case
Jg (m/s)
Jl (m/s)
Case 1 Case 2 Case 3 Case 4
0.036 0.142 0.534 2.089
0.647 2.550 2.549 2.554
2.54 cm I.D. pipe the ports are located at z/D of 13, 68 and 133, while for the 5.08 cm I.D. pipe the ports are located at z/D of 7, 34 and 67 respectively. Here, z is defined as the distance of the port from the air injection. At each port, needle probes can be installed. The experiments discussed in the present paper have been performed with the 5.08-cm diameter test section, in upwards cocurrent flow configuration. The liquid and gas superficial velocities for the conditions analyzed in the present work are reported in Table 1. Fig. 2. Instrumentation set-up (not in scale).
3. Instrumentation A four-tip conductivity needle-probe sensor is mounted 3.4 m (z/D = 67) downstream of the air injection. By means of a traversing mechanism (see Fig. 2), it is possible to change the radial position of the needle-probe in the cross-section. By performing measurements at different radial locations, radial profiles of time-averaged two-phase flow parameters can be measured. A sketch of the fourtip needle-probe is shown in Fig. 3. One of the probe tips (upstream tip) is longer than the others. In such a way a four-tip probes is equivalent to three double sensors, allowing the measurement of three components of the local interfacial velocity on the basis of the time delays between the upstream sensor and the other three tips respectively. A detailed analysis on the accuracy of needle-probe is given by Le Corre and coworkers (2003). A double-layer wire-mesh sensor is present 1.3 cm downstream of the needle-probe. The sensor consists of three layers of electrodes of 100-m diameter. The axial distance between two successive electrodes layers is 0.9 mm. The middle plane of the sensor (transmitter wires) consists of 24 electrodes, crossing the pipe cross-section at a distance of 2 mm from each other. These electrodes are rotated 90◦ with respect to the electrodes of the other
two planes (the so-called receiver wires. See Figs. 2 and 4 for a simplified scheme. Here only two electrode layers are indicated for simplicity). Two successive layers deliver a two-dimensional matrix of time-dependent instantaneous local fluid conductivity. Details on the measurement principle are given by Prasser and coworkers (1998). The use of three layers allows the measurement of bubble velocities by evaluating bubbles time-of-flight or by applying crosscorrelation techniques. Details on the accuracy of the wire-mesh sensor are given by Prasser and coworkers (1998, 2003). The measuring frequency of the needle probe was chosen between 10 and 30 kHz and the signal was recorded at each radial position for 180 s. Up to 14 radial positions have been measured for each experimental series, leading to a net measuring time of 42 min for each flow condition. A measuring frequency of 5000 frames/s was employed for the measurements with the wire-mesh sensor. A single measurement of 20 s was recorded for each flow condition. The wire-mesh signal was recorded with the needle-probe located as close as possible to the pipe wall, in order to minimize its effect on the wire-mesh measurement.
Fig. 3. Sketch of the 4-tip needle-probe.
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time Tmeas
˛=
b
tb
Tmeas
∈ [0, 1]
(1)
The void-fraction measurement suffers from the employment of the voltage threshold (see Section 4), which yields a lower value of the bubble residence time tb and leads therefore to an underestimation of the time-averaged void-fraction. In order to overcome this drawback, a correction algorithm is applied for each bubble, which evaluates an average rising slope (dV/dt) of the squared signal in the proximity of the starting point of the voltage rise and increases the bubble residence time tb by adding a value equal to Vs∗ (dV /dt)−1 . To get a void-fraction radial profile, a series of separate measurements is carried out at the same flow conditions for different locations of the needle probe in the pipe cross-section. Fig. 4. Scheme of the wire-mesh sensor.
4. Signals preconditioning 4.1. Needle-probe A four-tip needle-probe generates four voltage signals, one for each tip of the probe. These signals indicate whether the given probe tip is at contact with gas or with liquid. For the evaluation of two-phase flow parameters, the signals are first filtered by means of a 5-points moving-median filter. In this way, high-frequency noise is filtered, while keeping enough time resolution for appropriate bubble velocity measurements. Then, the signal is normalized to cope with voltage-drifting and inter-channel interference. The base voltage of the liquid phase Vbase and the maximum voltage Vmax are adjourned every 8000 samples. In the normalized signal, the remaining noise due to voltage variation in the liquid phase is removed by setting a threshold level Vs . On the basis of experimental observation, this threshold level is generally set equal to 10% of the maximum voltage Vmax . Finally, the signal is shaped in the form of squared waves, where a voltage change represents a bubble interface (front or rear). Details on the signal pre-processing are given by Fu and Ishii (1999) and Fu (2001). 4.2. Wire-mesh sensor For the wire-mesh sensors, no pre-processing of the signal is needed. Before the experimental series, a short measurement (generally 1 s) has to be carried out, in order to measure the liquid conductivity in absence of air. In this way, a matrix uijk of liquid conductivities is obtained (i and j are the indexes for the spatial coordinates x and y in the pipe cross-section, while k stands for the time step index). The matrix uijk is averaged over the time-index k, to give the two-dimensional calibration matrix u0ij . This is used as reference liquid conductivity to convert the measured raw data of fluid conductivity in absolute values of void-fraction (Prasser et al., 1998). 5. Measurements of void-fraction 5.1. Needle-probe The void-fraction is calculated on the basis of the signal obtained by the upstream probe tip, after being normalized and transformed to a square signal, by applying a voltage threshold. The timeaveraged void-fraction is evaluated as the ratio between the sum of the residence time tb of each bubble, b, and the total measuring
5.2. Wire-mesh sensor For the wire-mesh sensor, a single measurement of 20 s is performed. The raw signal uijk is converted into void-fractions assuming a linear dependency of the local instantaneous voidfraction with respect to the flow conductivity (this assumption has been proved to be satisfactory in previous works by Prasser and coworkers, 1998, 2003; and Richter, 2001): ˛ijk =
uijk − uij,air u0ij − uij,air
(2)
where ˛ijk is the three-dimensional matrix of measured voidfractions, uij,air is the air conductivity, and u0ij is the reference liquid conductivity. The air conductivity is usually zero. Nevertheless, due to finite insulation impedances in the sensor and the cables, as well as an offset of the input cascades of the wire-mesh electronics, a calibration for an empty pipe is necessary and was performed in the experiments presented here. By averaging in time, a two-dimensional void-fraction distribution is obtained. For a direct comparison with the needle probe, the two-dimensional distribution is converted into a radial profile by averaging along circumferential slices (rings). In addition, an algorithm has been developed that recognizes single bubbles out of the three-dimensional void-fraction signal. A bubble-object is defined as a region of connected elements (i, j, k) containing gas phase surrounded by elements containing liquid phase. The algorithm uses a recursive scheme to identify the elements (i, j, k) belonging to the same bubble-object. Details can be found in Prasser et al. (2001) and Prasser and Bayer (2007). The bubble recognition algorithm allows to resolve two-phase flow parameters in terms of bubbles size classes. 5.3. Results In Figs. 5 and 6 the void fractions measured by means of the wire-mesh sensor and the needle probe are reported. As expected, for cases 1 and 2 the wall peaked profile typical for bubbly flow is found. Considering the very low level of absolute void-fraction (between ∼3% and 8%), a very good agreement is found between needle probe and wire-mesh sensor, the maximum difference between the two measurement technique being of less than 2% of absolute void-fraction. For cases 3 and 4 the void-fraction presents a central-peaked profile. Excellent agreement is found for case 3, while for case 4 a maximum deviation of about 13% (relative) is found between the two measuring techniques. For slug flows with air and water
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The local average bubble velocity is finally estimated, by dividing the sum of all bubble velocities vb by the total number of bubbles: v¯ b =
l 1 Nb b tb,front
(4)
A radial velocity profile is obtained by performing measurements at different locations of the needle-probe along a radius of the pipe cross-section and by repeating the estimation procedure for each measurement. 6.2. Wire-mesh sensor Three methodologies can be followed to estimate a radial profile of the gas velocity:
Fig. 5. Comparison between wire-mesh (solid lines) and needle-probe (circles) for void-fraction measurements.
at ambient temperature, by comparison with an ultra-fast X-ray tomograph, it has been found that the wire-mesh slightly underestimate the void-fraction due to the formation of a thin water film on the sensors wires (Prasser et al., 2003), the effect disappearing with increasing liquid temperature (Manera, 2003). The underestimation of the needle-probe for case 4 in Fig. 6 is presumably due to the effect of the voltage threshold discussed in Section 4 which is needed for the preconditioning of the signal. 6. Measurements of gas velocity 6.1. Needle-probe The needle-probe estimates the velocity of each bubble touching the probe, by measuring the time of flight of each single bubble on the basis of the bubble front. This is done by looking at the timedelay between the signals measured by the upstream probe (probe ‘0’ in Fig. 3) and one of the downstream sensors (probe ‘1’ in Fig. 3). Knowing the axial distance l between the two probes, an estimate for the velocity vb of a given bubble is obtained: vb =
l tb,front
(I) the velocity is computed separately for each single bubble by estimating the time of flight of the given bubble on the basis of the bubble front. The computed velocity vb is assigned to the bubble center of mass. A two-dimensional velocity profile is calculated by weighting the obtained velocities with the voidfraction carried by each given bubble; (II) the void-fraction signal ˛ijk is cross-correlated with the corresponding signal of the upper wire-mesh layer, in order to obtain a two-dimensional distribution of most-probable timeof-flight, which are then converted into velocities, knowing the axial distance between the two measurement planes; (III) the void-fraction signal ˛ijk assigned to a given bubble by the bubble recognition algorithm is correlated with the voidfraction measured by the upper measuring layer, in order to estimate the bubble time-of-flight and therefore its velocity. A parabolic fit around the correlation maximum is carried out to achieve a better accuracy on the time-of-flight estimation and thus on the bubble velocity. The two-dimensional velocity distribution is then evaluated in the same way as in method (I). This method is used in the present work. The two-dimensional velocity distribution obtained with one of the three methods described above is converted into a radial profile in the same way as for the void-fraction. 6.3. Results
(3) The comparison between the velocity measurements is presented in Fig. 7. The maximum deviation between the two measuring techniques is about 15%. This deviation, however, occurs for case 4 where relatively high velocities are measured and for which the error due to the sample time discretization is high. Overall, satisfactory agreement is found. 7. Measurements of interfacial area 7.1. Needle-probe According to the theoretical derivation by Ishii (1975), the local time-averaged interfacial area density can be expressed by the following relation: a¯ i =
Fig. 6. Comparison between wire-mesh (solid lines) and needle-probe (circles) for void-fraction measurements.
1 1 T j vi ni
(5) j
where j denotes the jth interface passing through a local point during the observation time interval T, being vi and ni the bubble interfacial velocity and the unit surface normal vector of the jth interface respectively. Based on Eq. (5), Kataoka and coworkers
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Fig. 8. Bubble boundary interpolated on the measuring wire-mesh grid.
Fig. 7. Comparison between wire-mesh (solid lines) and needle-probe (circles) for gas velocity measurements.
(1989) have derived a formulation in order to derive the local timeaveraged interfacial area density from the signal of a four-tip probe: a¯ i (x0 , y0 , z0 ) = 2N
1
vi cos ϕ
(6)
where 2N is the total number of interfaces (i.e. N is the total number of bubbles). In order to apply Eq. (6), it is necessary to measure three separate components of the gas–liquid interface. This is achieved by computing the time-of-flight of a given bubble from the upstream sensor to the three downstream tips. In such a way, a velocity vector can be assigned to each bubble and the interfacial area density is then estimated on the basis of Eq. (6). The contribution to the interfacial area cannot be computed for those bubbles which do not touch all four tips of the needle-probe (missing bubbles), unless the bubble is assumed to be spherical (Kim et al., 2002; Le Corre and Ishii, 2002). Since no assumption is made on the bubbles shape, the contribution to the interfacial area density due to the missing bubbles is lost. In order to overcome this problem, a simple recovery method consists in assigning to the missing bubbles an average interfacial area computed on the basis of the effective bubbles (Kim et al., 2000). The effective bubbles are those that with their interface intersect all four tips of the probe so that their interface normal velocity and hence the interfacial area concentration can be determined.
The interpolation is carried out at the rectangular grid span out by the electrode wires of the sensor. To avoid equivocal situations, each of the square meshes is subdivided in a pair of triangles and an interpolation is also performed at the diagonal. Since there are two possibilities to define a diagonal, the one is taken, which results in the smaller length of the bubble contour within the mesh. This has proven to supply best results (Prasser, 2007). In meshes, at all four corners of which the gas fraction is either above or below the threshold, a contour is not found and the corresponding contribution to the contour length is zero. The second step is carried out in cubical control elements formed by the square meshes extended in the axial direction (orthogonal to the measuring plane) by the distance the flow travels in the period between two sampling instants. The axial extension of the control cuboids is defined as the product of the sampling period times the bubble velocity. In this step, the area of each surface element of the bubble is calculated that unites the linear bubble boundaries found in the measuring plane in two successive sampling instants. This is done in an approximate way assuming a plain surface. The bubble surface is then the sum of the surface elements found in this way. In parallel, the part of the area of the rectangular mesh that belongs to the bubble is calculated as well. The total area occupied by the reconstructed bubble, multiplied by the effective height of the control cuboids, supplies the volume of the reconstructed bub-
7.2. Wire-mesh sensor For the wire-mesh sensor, the interfacial area is calculated by fully reconstructing the gas–liquid interface (Prasser, 2007). Before the reconstruction of the gas–liquid interface is performed, the bubble recognition algorithm is executed. After this step, each element of the distribution of local instantaneous gas fraction delivered by the sensor is assigned to an individual bubble, using a bubble identification number. This allows the reconstruction of the gas–liquid interface for each bubble individually by taking into account only those gas fractions which belong to the currently analyzed bubble. The full reconstruction is performed in two steps. In the first step, the contour of the bubble is found within the measuring plane for each time step. It is defined as a polygon uniting all points at which the interpolated instantaneous gas fraction is equal to a threshold that characterizes the boundary of the bubble (see Fig. 8).
Fig. 9. Comparison between wire-mesh (solid line) and needle-probe (circles) for interfacial area density.
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(48.3 mm vs. 50.4 mm of present measurements). The comparison between Fu’s needle-probe measurement and the wire-mesh is reported in Fig. 11, where the interfacial area density is shown as function of the normalized pipe radius. Excellent agreement is found, which provides a good confidence also on measurements reproducibility. 8. Conclusions
Fig. 10. Case 4. Interfacial area density classified for bubbles diameter. Comparison between wire-mesh (solid lines) and needle-probe (circles).
Good agreement has been found between four-tip conductivity needle-probes and wire-mesh sensors. Additional measurements in the slug and in the churn flow-regime should be performed to extend the range of validation of the two measurements techniques. In general, smoother profiles are obtained with the wire-mesh sensor due to the higher number of measuring locations. In addition, since the wire-mesh occupies the entire pipe cross-section no bubble is missed and a better statistics can be obtained. Due to the very short time required to achieve a good measurement statistics (20 s measurement periods were used in the present work), the wire-mesh is a very good candidate to achieve a full mapping of the interfacial area density, which is needed for the further development and the assessment of transport equations for the interfacial area density. It has also to be pointed out that a full threedimensional reconstruction of the gas bubbles can be achieved with the wire-mesh sensor, allowing a sound separation of the twophase flow parameters according to bubble classes, based on the actual dimension of the gas bubbles. The needle-probe, on the other hand, is less intrusive and yields therefore fewer disturbances in the downstream flow. References
Fig. 11. Case 3. Indirect comparison between wire-mesh sensor and needle probe (measurement by Fu, 2001).
ble. This is important, because the gas fraction threshold has to be adapted for each bubble individually. Finally, the interfacial area can be transformed into an area density by relating it to the total volume of the two-phase flow. 7.3. Results The measurements of the interfacial area density are presented in Fig. 9. Again, very good agreement is found. A maximum deviation of less than 12% is observed for case 4. This deviation however occurs again in the center of the pipe, where the velocity is high and the uncertainty due to the sampling frequency is not negligible. In Fig. 10 the interfacial area density measured for case 4 is reported separately for small and large bubbles (10-mm diameter is used as separation between the two bubble classes). It is seen that the deviation between needle-probe and wire-mesh is mainly due to the large bubbles in the central part of the pipe, where the needle-probe measurement presents a certain scatter. A measurement in the same flow conditions as case 3 has been performed by Fu (2001) on a pipe with a slight smaller diameter
Fu, X., Ishii, M., 1999. User’s guide for 4PT100 advanced four-point probe signal processor. PU/NE-99-14, Purdue University, School of Nuclear Engineering. Fu, X., 2001. Interfacial area measurement and transport modeling in air–water twophase flow. Ph.D. Thesis, Purdue University, School of Nuclear Engineering. Ishii, M., 1975. Thermo-fluid Dynamic Theory of Two-phase Flow. Eyrolles, Paris. Kataoka, I., Ishii, M., Serizawa, A., 1989. Local formulation and measurements of interfacial area concentration in two-phase flow. Int. J. Multiphase Flow 12, 505–529. Kim, S., Fu, X.Y., Wang, X., Ishii, M., 2000. Development of the miniaturized foursensor conductivity probe and the signal processing scheme. Int. J. Heat Mass Transfer 43, 4101–4118. Kim, S., Ishii, M., Wu, Q., McCreary, D., Beus, S.G., 2002. Interfacial structures of confined air–water two-phase bubbly flow. Exp. Thermal Fluid Sci. 26, 461–472. Le Corre, Ishii, 2002. Numerical evaluation and correction method for multisensor measurement techniques in two-phase bubbly flow. Nucl. Eng. Des. 216, 221–238. Le Corre, J.-M., Hervieu, E., Ishii, M., Delhaye, J.-M., 2003. Benchmarking and improvements of measurement techniques for local-time-averaged two-phase flow parameters. Exp. Fluids 35, 448–458. Manera, A., 2003. Experimental and analytical investigations on flashing-induced instabilities in natural circulation two-phase systems. Ph.D. Thesis, Delft University of Technology, The Netherlands. Prasser, H.-M., Böttcher, A., Zschau, J., 1998. A new electrode tomograph for gas–liquid flows. Flow Meas. Instrum. 9, 111–119. Prasser, H.-M., Scholz, D., Zippe, C., 2001. Bubble size measurement using wire-mesh sensors. Flow Meas. Instrum. 12, 299–312. Prasser, H.-M., Misawa, M., Tiseanu, I., 2003. Comparison between wire-mesh sensor and ultra-fast X-ray tomograph for an air–water flow in a vertical pipe. Flow Meas. Instrum. 16, 73–86. Prasser, 2007. Evolution of interfacial area concentration in a vertical air–water flow measured by wire-mesh sensors. Nucl. Eng. Des. 237, 1608–1617. Prasser, H.-M., Bayer, M., 2007. Bubble recognition algorithms for the processing of wire-mesh sensors data. In: Proceedings of the International Conference on Multiphase Flows, ICMF-2007, July 9–13, Leipzig, Germany. Richter, S., 2001. Study on multidimensional characteristics of developing bubbly flow in a rectangular channel. Ph.D. Thesis, Tokyo University of Technology.