Investigation of two-phase (oil-water) flow in coiled geometries using “Radioactive Particle Tracking-Time of Flight (RPT-TOF)” and “Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements

Chemical Engineering Science xxx (2017) xxx–xxx

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Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle Tracking-Time of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements Loveleen Sharma, K.D.P. Nigam, Shantanu Roy ⇑ Department of Chemical Engineering, Indian Institute of Technology-Delhi, New Delhi 110016, India

h i g h l i g h t s
Hydrodynamics investigation of the oil-water flow through a coiled tube geometry.
Measured time of flight and volume fraction using Radioactive Particle Tracking technique.
Scaling of dispersion behavior with varying Schmidt number has been quantified.
Developed flow regime map based on Peclet number and volume fraction measurements.

a r t i c l e

i n f o

Article history: Received 31 August 2016 Received in revised form 27 February 2017 Accepted 7 March 2017 Available online xxxx Keywords: Coiled flow Dean vortices Radiotracer RPT-TOF: time of flight RPT-VOF: volume fraction

a b s t r a c t In this work, the characteristics of two-phase oil-water flow through a coiled tube geometry have been investigated. Radial mixing, local volume fractions, and phase holdup, over a range of Reynolds numbers of either phase, have been presented. The specific novelty of this work has been the use of time-of-flight (TOF) and volume fraction (VOF) measurements for a single radioactive particle, marked in turn, to represent the flow in either of the phases. This use of the RPT-TOF and RPT-VOF to investigate two-phase oilwater flow has never been reported thus far. In our measurements, a c-ray emitting radiotracer was made neutrally buoyant with respect to each phase by turn, and subsequently its motion was monitored for its multiple trajectories through coiled geometry, via an array of strategically placed scintillation detectors (NaI(TI)). From the ‘‘sojourn times” of the tracer particle through the coiled tube, the characteristic exit age distribution (E(t)) and occurrence density distribution were obtained. The former information has related to the velocity field of either phase (RPT-TOF), while the latter information has related to the volume fraction field of either phase (RPT-VOF). Further analysis of this data has revealed interesting features, such as the onset of phase inversion and the distinction of flow regimes. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Coiled geometries are relevant in many industrial applications such as chemical reactors, heat exchangers, nuclear reactors and oil transportation systems, etc., owing to their compact structure and characteristic flow pattern produced by the action of centrifugal force on the fluid flowing through the coiled path. It is wellreported that in single phase flow when fluid flows through a coiled path, it undergoes a circulation pattern perpendicular to

⇑ Corresponding author.

its axis of flow. This is referred to as Dean circulation (Dean, 1927, 1928). This circulation pattern/secondary flow, which is caused by the centrifugal forces experienced by the fluid elements, causes the fluid to mix in the tangential direction (with respect to mean flow path). If chemical species are being transported in the fluid, this kind of circulatory flow results in diminished concentration gradients in the cross-sectional plane (shown schematically in Fig. 1). In turn, this reflects in better cross-sectional mixing and controlled axial mixing, which is desirable in most chemical and processing applications. Hence, in case the fluid flow through coiled length is compared to the straight length of identical volume (i.e., a comparison is

E-mail address: [email protected] (S. Roy). http://dx.doi.org/10.1016/j.ces.2017.03.021 0009-2509/Ó 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Nomenclature A dc dt D E(t) f h Ku L NDe NRe,o NRe,w NPe,o NPe,w DP Sk t t i;holdup t i;RPTTOF t i;ov erall: U Vi

radiotracer activity (Ci) diameter of coil (m) inside diameter of tube (m) effective dispersion coefficient (m2 /s) exit age distribution (s1) friction factor (–) pitch (m) kurtosis (–) tube length (m) dean number (–) Oil phase Reynolds number (–) water phase Reynolds number (–) oil phase Peclet number (–) water phase Peclet number (–) pressure drop (Pa) skewness (–) time (s) mean residence time calculated from holdup data (s) mean residence time calculated from RPT-TOF data (s) mean residence time based on overall flow rate (s) superficial velocity (m/s) volume collected of the respective phase (m3)

VC Qi

volume of coil (m3) volumetric flow rate of respective phase (m3/s)

Greek symbols ew water phase volume fraction (–) l viscosity (cP) l3 third moment about the mean (s3) l4 fourth moment about the mean (s4) q density (kg/m3) r00 surface tension (N/m) r2 variance (s2) r2h dimensionless variance (–) k curvature ratio (=dc/dt) ai holdup of the respective phase (–) Subscript i oil (o) or water (w) phase (–) o oil phase (–) w water phase (–)

Fig. 1. Formation of Dean circulation in a coiled tube cross section.

made on an equal mean residence time basis), a narrower residence time distribution and higher heat and mass transfer coefficient can be achieved (Sharma et al., 2017a, 2017b). Significant literature contributions have reported their applications for enhanced mixing, heat and mass transfer coefficient (Kumar et al., 2006a, 2006b; Vashisth et al., 2008a; Tohidi et al., 2015). These are mainly single phase applications, in which Dean circulation is better understood. Detailed investigations of multiphase flow through coiled configurations are rare. Amongst them, the significant contributions are on gas-liquid flows through coiled geometries include the work of Rippel et al. (1961), Saxena et al. (1990), Awwad et al. (1995), Xin et al. (1997), Saxena et al. (1996), Murai et al. (2006), Vashisth and Nigam (2007, 2008b, 2008c), Sharma et al. (2017a). Further, there are very few reports available on liquid-liquid (immiscible) flows through coiled geometries, notable amongst them being Chen and Guo (1999), Gelfgat et al. (2003), Mandal et al. (2011), Gürsel et al. (2016) and Kurt et al. (2016). Moreover, most of these available reports have focused on the applications of the coiled geometries rather than any detailed investigation of hydrodynamics. The only noteworthy experimental analysis was carried out by Chen and Guo (1999), in a work that investigates the hydrodynamics of

oil-water flow through coiled tubes by reporting the visual observation of flow regimes and overall pressure drop measurements. Indeed this information of flow regime is helpful, but to understand the effect of given flow patterns in each phase is very crucial, on which this paper sheds limited light. Thus, it is important to perform reproducible measurements in two-phase flow in coils, to capture the behavior of each phase (oil and water) under specified set of operating conditions. Even though the work of Chen and Guo (1999) provides valuable insights, one notes that photography-based investigations are restricted to the observing the wall flow, which is very limited information in any multiphase flow situation (because of radial segregation of phases). In the context of coiled flows, the situation is even more limiting, because the principal direction of flow is a coiled path which at any point is perpendicular to the plane of observation (of the eye or a camera placed around the coiled tube). The phenomena of phase segregation of flow through pipes (both, straight tubes and arguably, coiled tubes as well) lead to erroneous interpretations during visual observations. For coiled tubes, the curvature effect adds further complications. The current study is motivated by the need to investigate liquid-liquid (oil-water) flows through coiled geometries, without

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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resorting to a visual observation technique. In our method, we employ single radiotracer particle based experiments in each phase, which obviates the need for visual observation. We perform our investigations over a wide range of Reynolds numbers, by varying oil and water phase Reynolds numbers (NRe,o & NRe,w) from 5 to 700, and 1061 to 23,150, respectively. It is noteworthy that performing classical residence time distribution (RTD) tracer measurements is not feasible for the twophase oil-water system, since it is not trivial from a practical implementation standpoint to obtain to clean ‘‘mixing cup” measurements of each phase. This is because, at the exit plane, one always collects a two-phase mixture, which must be properly phase separated before any tracer detection can be done. This propagates error because of phase transfer of the tracer during the collection and detection process. An alternative to this is to do online measurements by ‘‘dipping” the probe into the twophase flow, but such measurements are very noisy when one is monitoring a tracer in one phase, owing to the intermittent presence of the second phase. Further, such measurements would neither be flow-averaged (mass weighted averaged), nor crosssectionally averaged. This latter challenge has never been satisfactorily resolved, even though attempts have been made to address this issue (e.g. Gupta et al., 2000). On the other hand, the competence of radiotracer techniques in probing multiphase flow system is well documented in the literature (Pant et al., 2001; Roy et al., 2001, 2005; Roy, 2017). Radiotracer techniques can be used even when we have a high volume fraction of the dispersed phase since the highly penetrating gamma radiation allows ‘‘optical access” into a flow field which is otherwise laid opaque by scattering of other larger wavelength radiation, including the visible range. In this work, experimental investigations have been conducted by tracking a single radioactive tracer particle (rendered neutrally buoyant with respect to the phase under interrogation). Thus, one tracer particle was prepared which is made neutrally buoyant with respect to the oil phase, while the other tracer particle has been prepared which is designed to be neutrally buoyant with respect to the water phase. Once flow conditions were fixed (i.e., the external flow rate of oil and water was fixed), one set of experiments were performed with the oil tracer particle (to track the oil phase). Another set of experiments were performed with the tracer particle that was made neutrally buoyant with respect to the water phase. The latter experiments tracked the behavior of the water phase. For either of these set of experiments, the traces of the respective tracer particle (tagged to either phase) are measured to give ‘‘time-of-flight” and occurrence density. Occurrence density of the tracer particles could be interpreted as the local volume fraction, as we explain later. To complement the information of ‘‘Radioactive Particle Tracking - Time of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking - Volume Fractions (RPT-VOF)” measurements; phase holdup is also measured globally. Put together, all these measurements indicate some interesting clues about phase inversion and flow regime transition, and eventually help us to provide a deeper insight for oil-water flows through coiled geometry. Most importantly, all measurements are made in a non-invasive manner without disturbing the flow in any manner.

2. Experimental The schematic of the experimental setup to perform Radioactive Particle Tracking - Time of Flight (RPT-TOF) measurements, Radioactive Particle Tracking - Volume Fraction (RPT-VOF) measurements and phase holdup measurements is shown in Fig. 2. A horizontally positioned coiled geometry made up from mild-steel tubing with 0.05 m internal diameter (dt), was used to conduct the experiments. A total tube length (L) of 20 m has been used to

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fabricate the helical coil, to result in a uniform curvature diameter (dc) of 0.48 m, with 14 coil turns, each of 0.06 m pitch (h). To mark out the volume of the coiled geometry, entrance and exit planes were defined precisely at the starting and ending point of the curved path (as shown in Fig. 2). Also, an entrance length of 1.5 m was provided before the entry plane to diminish flow fluctuations caused by bends and other fittings in the line arrangement. The two-phase (oil-water) experiments were performed using water and lubricating machine oil (density (q) = 880 kg/m3, viscosity (l) = 30 cP). The surface tension of the oil with water (r”) = 0.022 N/m at ambient temperature in the range of 38–41 °C. During two-phase (oil-water) flow through coiled geometry, the water flow rate was kept at a constant value throughout all the experiment, while a different oil flow rate was used in each experiment. A similar protocol was repeated for another set of water flow rates, and so on. The complete experimental matrix spans the oil and water phase Reynolds numbers (NRe,o & NRe,w) ranges from 5 to 700, and 1061 to 23,150 respectively (detailed conditions are mentioned in Table 1). The flow supply of two phases to coiled geometry was arranged via a T-connection at the entrance of coil, wherein a constant supply (through their respective rotameters), of oil and water was mixed before the entrance, as shown in Fig. 2. Once passing through coiled geometry the mixture of oil-water was sent to separation tank, where this mixture (oil-water) separates slowly by the action of gravity. The overflow (i.e. oil) from this separation tank was collected and checked for density and viscosity before use. Fig. 2 shows the schematic arrangement for measuring the time of flight (TOF) of single c-emitter radioactive tracer particle. For this, two collimated NaI/Tl scintillation detectors (D11 and D12) were placed at the entry and exit of the coiled geometry, as shown in Fig. 2. These detectors serve as the ‘‘gatekeepers” to the coil, and therefore can be called ‘‘sentry detectors.” The collimator slit (of dimensions 0.002 m  0.06 m) ensures the photon count detection along the specified entry and exit planes only. By using this arrangement, multiple entries and exits of the c-emitter radioactive tracer particle were monitored by following the response of the sentry detectors. Further, detectors D3 – D10 were arranged around the coil (Fig. 2): these are the coil detectors. On following the multiple visits of tracer particle, an estimate of occurrences of the tracer particle at a given crosssectional plane were made using the coil detectors, which in turn could be used to represent the occurrence density, and hence the phase volume fraction. As stated earlier, for each combination of oil and water flow rates, once the experiment was conducted with a tracer that was neutrally buoyant with the oil phase, and once it was done with a tracer that was neutrally buoyant with the water phase. Since the measurements are ergodic, this process allowed the investigation of each phase distinctly, as long as the overall flow rates and external conditions for the two-phase flow were held constant. For all the cases, the tracer particle was prepared by encapsulating a small quantity of radioactive scandium (Sc-46, having activity (A)  100 lCi) into a PVC sphere of 1.5–2.0 mm diameter, so that the effective density of the tracer particle thus prepared matched precisely with the density of the tagged phase. Thus, the tracer particle’s effective density matched with the oil density when oil phase was being tracked; and then matched the water density when water phase was being tracked. Depending upon the rendered density to the tracer particle, it is deemed to follow the fluid streamlines (either water or oil), as per the Basset-BoussinesqOseen (BBO) equation (Zhu and Fan, 1998). During its course through the coiled geometry (which traveling with either of the phases), precise entry and exit of the tracer particle are monitored while emitting c-emitter photons using two

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Fig. 2. Schematic of experimental setup. Table 1 Experimental conditions. Single phase

NRe,o (–)

Two phase

NRe,w (–) 1061 1700 2986 5060 7997 10,500 15,888 23,150

5

8

13

20

32

50

80

140

252

415

700

NRe,o (–) 5 5 5 5 5 5 5 5

8 8 8 8 8 8 8 8

13 13 13 13 13 13 13 13

20 20 20 20 20 20 20 20

32 32 32 32 32 32 32 32

50 50 50 50 50 50 50 50

80 80 80 80 80 80 80 80

140 140 140 140 140 140 140 140

252 252 252 252 252 252 252 252

415 415 415 415 415 415 415 415

700 700 700 700 700 700 700 700

collimated ‘‘sentry” scintillation detectors (D11 and D12), positioned at the entry and exit planes. By tracking the entry and exit of tracer particle at a fixed frequency of 50 Hz, one obtains the residence time for one trajectory. For this, data acquisition electronic hardware system (MIDASÒ, manufactured by Electronic Enterprises India Pvt. Ltd., Mumbai, India), made according to NIM bin or NIM crate standards, was employed. On tracing the multiple visits of tracer particle through coiled geometry, a histogram of ‘‘times of residence” through these multiple sojourns was interpreted by classifying the time spent by multiple trajectories into different time intervals. The fraction of trajectories in each time interval with respect to a large number of trajectories followed is used to represent the normalized residence time distribution (E (t)) of the respective phases. In addition to these measurements,

the multiple trails of the tracer particle through coiled geometry were followed by using the eight scintillations ‘‘coil” detectors (D3–D10) at a fixed frequency of 50 Hz (using MIDASÒ). The instantaneous positions (r, h, z) for each this trail were reconstructed by using an appropriate reconstruction algorithm (Roy et al., 2002). Furthermore, by discretizing the entire coil domain into finite cells of equal volume, the number of tracer positions lying within each of such cells were calculated to present the occurrence density of the tracer particle positions at any given cross-sectional plane. The coil is a flow system in which both phases continuously flow in and out. The tracer particles (for either phase) have to be periodically recycled back into the coil in order for them to tag the many possible realizations of the two-phase flow. In order to

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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track multiple trajectories of either of the tracer particles, each representing a typical fluid element of the respective phase in the multiphase mixture flowing through the tube, an automatic circulation scheme was designed and installed (details are shown in Fig. 2); wherein a synchronization between four solenoid valves (S1, S2, S3 & S4), two electrically actuated ball valves (B1 & B2) and two ‘‘flow controlling collimated detectors” D1 and D2 was used to direct the flow. This tracer particle recycling arrangement could recirculate the tracer particle periodically without disturbing the system steady-state flow condition, in a way similar to the procedure put forth in the previous contribution of the authors (Sharma et al., 2017a). The nomenclature used to indicate each valve, pump and detector are kept same as that was mentioned by Sharma et al. (2017a) to avoid confusion with understanding of recirculation procedure. Details are omitted here in the interest of length of this manuscript, but it suffices to say that the arrangement could successfully ‘‘trap” either of the tracer particles in the external loop (Fig. 2), and return it with fidelity and no mechanical damage to the inlet of the coil. This way, many trajectories of either of the tracer particles could be collected, and the data could be consolidated. The experimental study is performed first by examining the single phase flow through coiled geometry; wherein only oil flow and only water flow were analyzed with the RPT-TOF measurements. The necessity of this important step was to establish beyond doubt the ability of the respective tracer particles to follow with fidelity the single-phase oil flow, and then the single-phase water flow. Once this was established, subsequently the problem of twophase flow (oil and water mixture) through coiled geometry was addressed. The phase holdup data was independently measured under each flow condition. Fig. 2 shows the schematic of the quick shut-off valves at the coil entry and exit to perform the classical holdup measurements. The information from each measurement was then used to explain the important fact of phase inversion and flow regimes. 3. Results and discussion 3.1. Experimental protocol and representative data By using eight ‘‘coil” detectors (D3–D10, judiciously positioned around the coil structure), and two collimated ‘‘sentry” detectors (D11–D12) (as shown in Fig. 2), the multiple trajectories of tracer particle through coiled geometry were recorded. These coupled measurements helped us to determine the volume fraction (VOF)

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and time of flight (TOF) measurements of the respective phases. Actually, this coupled information is rich enough to also decipher the precise velocity field inside the coil. However that would be a separate topic to deal with, and in this communication, we wish to restrict the discussion to the time of flight (TOF) and volume fraction (VOF) measurements. Algorithms for position reconstruction of tracer particle follow the procedure of Radioactive Particle Tracking (RPT), which have been well reported in the literature (see for example, Larachi et al. (1994, 1995), Roy et al. (2002, 2005), Upadhyay et al. (2013)). For accomplishing this, the instantaneous photon counts recorded at all the ‘‘coil detectors” were used to reconstruct the exact location of the tracer particle during its sojourn through the coiled tube. By assembling all the reconstructed positions of tracer particle, the ‘‘occurrence distribution” is obtained. Naturally, occurrence distribution is an assemblage of points in the domain (in this case the coil) where the tracer particle occurs at successive time points (depending on the frequency if data acquisition). Fig. 3a (I) shows the typical result of reconstructed tracer particle occurrence density for the oil, as the tracer particle followed the oil phase faithfully (owing to rendered oil density to it). In this figure, a certain cross section of the coil geometry (0° position within the fifth coil turn) is shown, as in the inset schematic. As the tracer particle moves inside the coil, it serves as the marker for the given phase. Thus, during its multiple trajectories in the coil through any given cross-section, it would have visited the region where the given phase (oil phase) was present, and thus yielded an occurrence in that region. Since the multiple trajectories are practically uncorrelated from each other, the occurrence density distribution (which is a spatial distribution), serves to mimic unbiased Monte Carlo sampling of the phase volume (the phase that is being tracked by the tracer particle). This is the crux of the RPT-VOF (volume fraction through RPT) method that is proposed. Thus, the higher occurrence density of tracer particle at any cross-sectional plane of the coiled geometry represents the presence of given phase there and vice versa. This is shown in Fig. 3a (I). In order to report volume fraction (VOF) from such data, a normalization procedure was followed for visual clarity (as shown in Fig. 3a (II)). Similar procedure was followed for each phase (oil and water), by turn, to measure volume fraction under every flow condition. It is noteworthy that owing to the variation of centrifugal, gravitational and buoyancy forces, the variation of volume fraction along a coil turn was evident. For illustration the volume fraction measurement shown in Fig. 3a is continued over a turn of the coiled geometry, as shown in Fig. 3b. From Fig. 3b it can

Fig. 3. (a) Reconstructed tracer particle occurrence density (I), and volume fraction (II); (b) variation in the oil volume fraction within one coil turn at water phase velocity = 0.039 m/s and oil phase velocity = 0.025 m/s; (c) photon count time series obtained from two collimated entry and exit detectors (D11–D12); (d) moving averaged photon count data time series for one trajectory; (e) typical exit age distribution curve (E(t)).

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Fig. 3 (continued)

be seen that at azimuthal positions of h = 90°, 180° and 270°, the oil phase is largely occupying the inner wall. While at azimuthal positions (h) = 0°, the oil phase has shifted towards the outer wall, owing to its lighter density with respect to the water phase. It is important to note that the overall pattern of the volume fractions remain same for particular flow condition, which implies that the overall flow behavior remains same irrespective of the phase distribution within. Moreover, the volume fractions remain same for same azimuthal positions (h) in each coil turn. In the subsequent sections, the phase distributions at the same cross-sectional plane are shown in order to compare different flow conditions. The photon counts data for the multiple entries and exits of the tracer particle from coiled geometry (as shown in Fig. 3c) was analyzed for the time of flight (TOF) measurements (RPTTOF). The successive set of maxima for entry and exit detector (‘‘sentry detectors”) in the counts time series are corresponding to the closest position of the tracer particle to the detector that aligned itself to the slit of the detector (i.e. defined entry and exit planes). Thus, the time corresponding to each maximum represents the time of entry/exit of the tracer particle from coiled geometry depending upon the series for which it is obtained. On zooming into one such photon counts time series for entry and exit sentry detectors, one observes sharp spikes in the counts data series. The sharp spikes in the photon count time series are the result of Poisson statistics of the counting events (shown by solid line in zoomed view of entry and exit data in Fig. 3c). The presence of such sharp spikes in the count series may be prone to the erroneous measurement of the peaks, and hence in isolating the corresponding time of entry and exit of the tracer particle in the coiled geometry. In order to avoid such ambiguity in defining clear maxima in count series, a moving average of the data was taken (shown by the hollow line in zoomed view of entry and exit data Fig. 3c). On observing the moving average photon count data time series, one observes a mild bias in the location of the peaks and also a reduced magnitude. This is brought in by taking the moving average, which by the way is brought into a minor degree in all filtering, including moving average. One can however easily show that such alteration does not affect

Fig. 4. (a) Comparison of RPT-TOF measurement from single phase oil and water flow through coiled geometries; (b) correlation for dispersion of fluid flow through coiled geometries.

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the analysis because the bias equally affects all peaks and shifts the signal to the right by an equal amount, so that the time interval between successive peaks (and hence the time of residence of the tracer particle), is not affected in any way. The result of the moving averaged photon count data time series for one such trajectory is shown in Fig. 3d, which also represents the residence time of single trajectory thus measured. Similarly, on measuring the residence time for all such trajectories, the distribution of residence times was calculated to give residence time density function (E(t)), as reported in standard texts in reaction engineering (Levenspiel, 1999; Nauman and Buffam, 1983; Wen and Fan, 1975). The typical results of E(t)-curve obtained on following 1050 independent trajectories (a typical number) of the tracer particle (neutrally buoyant with oil phase) is shown in Fig. 3e. It is found that this measurement satisfies the overall material balance and hence ‘‘central volume principle”, very well (within 5% of error). Similar results were obtained for the entire range of flow conditions (single as well as two phase) explored in the present study, and indeed has been reported for air-water flows in our earlier work (Sharma et al., 2017a). The total number of independent particle trajectories required to estimate RTD or characteristic exit age distribution (E(t)) curve of any flow system were estimated on the basis of complete convergence of residence time density function, E(t). The convergence of residence time density function, E(t), was tested by calculating the mean (t, which is also the first moment of the RTD), normalized central variance (r2h , related to the second moment of the RTD), skewness (Sk, related to the third moment of the RTD), and kurtosis (Ku, related to the fourth moment of the RTD), of the E(t)-curve (Eqs. (1-5)). In

other words, the choice of the number of trajectories that are tracked (which is linked to the total time span of an experimental run), is determined by ensuring that these moments are invariant with consideration of a further number of trajectories. Once the converged E(t)-curve have been extracted for each flow condition, an estimation of Peclet number (NPe) was made (shown in Eq. (6) below), to quantify the extent of radial mixing based on the length (L) of the coiled geometry (total tube length elapsed from entry to exit plane).

Z Mean residence timeðtÞ :

t¼

1

tEðtÞdt

ð1Þ

0

Varianceðr2 Þ :

r2 ¼

Z

1

Dimensionless varianceðr2h Þ : SkewnessðSkÞ Sk ¼ KurtosisðKuÞ Ku ¼

Peclet number

2

ðt  tÞ EðtÞdt

ð2Þ

0

r2h ¼

r2 ðtÞ

l3 r3

ð3Þ

ð4Þ

l4 r4

  UL NPe ¼ D

2

ð5Þ

r2h ¼ 2



  2 D D 2 ½1  eUL=D  UL UL ð6Þ

Fig. 5. Water and oil phase volume fractions for coil cross sectional plane located at 90° position within the fifth coil turn by using water and oil phase velocities from: (a) 0.025–0.118 m/s and 0.004–0.064 m/s; (b) 0.025–0.118 m/s and 0.064–0.558 m/s; (c) 0.186–0.541 m/s and 0.004–0.064 m/s; (d) 0.186–0.541 m/s and 0.064–0.558 m/s.

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Fig. 5 (continued)

In the following discussion, the results from single phase (oil only) and two phase (oil-water) RPT-TOF are discussed in detail to present the characteristic radial mixing behavior of each phase. In addition to RPT-TOF measurements, RPT-VOF, and holdup measurements are also discussed to explain the hydrodynamics of twophase (oil-water) flow through coiled geometry. 3.2. Single phase flows through coiled tube At first, the extent of radial mixing induced by the flow of oil through the coiled geometry is presented. Therefore, before examining the combined behavior of oil and water phase flow through coiled geometry, pure oil phase flow was examined over for the range of 5 < N Re;o < 700. The extent of axial dispersion under each flow condition was measured by calculating the Peclet number (NPe). Fig. 4a shows the increasing magnitude of oil phase Peclet numbers (NPe,o) with increasing oil phase Reynolds number (NRe). This happens because, with increasing flow velocity, the strength of secondary flow field increases, that offers enhanced crosssectional mixing and hence intensified Peclet numbers (NPe). Moreover, the results from current measurements were readily compared with those reported by previous contributions of the authors using water flow through coiled geometry (Sharma et al., 2017a, 2017b), wherein a well-accepted match between RTD tracer measurements and RPT-TOF measurements has been reported. Indeed from such a comparison, it can be seen that owing to the higher Schmidt number (NSc) for oil phase in contrast to the water phase, oil phase offers a higher axial dispersion at an equivalent Reynolds number.

Furthermore, in order to quantify the change in axial dispersion brought in by fluids with different Schmidt numbers (NSc), the data from oil phase RTD measurements (this study) and water phase RTD measurements (Sharma et al., 2017a, 2017b) has been analyzed in terms of intensity of axial dispersion. On analyzing such data for coiled geometries, it has been concluded that the intensity of axial dispersion (D=Udt ) scales with the following functional relationship (Eqs. (7a) or (7b)).

D 1 / N  1=3 Udt pRffiffie ðN Sc Þ

ð7aÞ

D 1 / Udt ðNDe ÞðNSc Þ1=3

ð7bÞ

k

where NDe is Dean number. The Dean number (NDe) for coils is analogous to Reynolds number (NRe) for straight tubes, and it incorpopffiffiffi rates the effect of curvature ratio (k) in its definition (N De ¼ N Re = k). The aptness of the proposed correlation was checked by plotting data from different literature contributions (Sharma et al., 2017a, 2017b; Saxena and Nigam, 1984; Trivedi and Vasudeva, 1975), as shown in Fig. 4b. It is evident from the figure that all experimental measurements, employing different coil dimensions (dc, dc, L and k) and different fluid properties (oil and water), are well-represented by a single curve using proposed correlation (mentioned by Eq. (7)). On comparing the proposed correlation (Eq. (7)) with one for straight tubes (ðN Re ÞðN Sc Þ, reported in standard texts like Levenspiel (1999)), it can be postulated that the effect of viscous forces on the extent of axial dispersion (D=Udt )

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

L. Sharma et al. / Chemical Engineering Science xxx (2017) xxx–xxx

is weakened in the former case. Further, the correlation between the intensity of axial dispersion of straight tubes can be deduced from Eq. (7) on multiplying it by a correction factor of ðkÞ1=2  ðN Sc Þ2=3 . Once the extent of dispersion offered by oil flow coiled geometry has been investigated and thus an effectual tracing of two phases has been ensured, the two-phase (oil-water) flow through coiled geometry were analyzed to explore the change in dispersion behavior by adding water phase into it.

3.3. Two-phase flow through coiled tubes The liquid-liquid (oil-water) two-phase flows have been explored using straight tubes (Trallero et al., 1997; Ghorai et al., 2005; Vielma et al., 2008). Whereas industries today are utilizing the coiled geometry in a variety of applications, including two phase, still very limited literature exists on oil-water (two-phase) flows through coiled tubes. In order to have a comparative basis with scant literature in this field, almost identical properties for oil as well as water phase were used, as reported by Chen and Guo (1999). The volume fraction and axial dispersion behavior of each phase (oil and water separately) have been measured noninvasively using the radioactive tracer particle, without being constrained in any way to the flow regime and corresponding holdup variations. As discussed earlier, each experimental flow condition (combination of oil and water flow rates), was repeated to first track water phase only and then oil phase only.

9

3.3.1. RPT-VOF measurements On accomplishing two-phase (oil-water) RPT-VOF measurements, the results were analyzed for the particular cross section (located at 90° position within the fifth coil turn) for the sake of comparison. On varying the oil and water phase Reynolds numbers (NRe,o & NRe,w) from 5 to 700 and 1061 to 23,150, four distinctive patterns for the water and oil phase volume fractions were obtained, as shown in Fig. 5a–d. It is worth mentioning from Fig. 5a–d that in spite of conducting distinct measurements on water and oil phase (note that the oil tracer experiments were different experiments from the water tracer experiments), the volume fractions are showing a complementary pattern, almost mirroring each other at corresponding locations in the coil. On varying the water and oil phase velocities from 0.025 m/s to 0.118 m/s and 0.004 m/s to 0.064 m/s respectively, the volume fractions shown in Fig. 5a were obtained for each phase. Within this range, the water and oil phase were lying into two distinct layers. However, owing to the effect of centrifugal forces, the fluids towards the inner wall was being pushed outwards. The second characteristic pattern for the volume fraction (as shown in Fig. 5b) was applicable for water and oil phase velocities in the range of 0.025 m/s to 0.118 m/s and 0.064 m/ s to 0.558 m/s respectively. Over this range, the water was occupying the core of the pipe whereas the oil was moving along the periphery. The third typical pattern of the volume fractions (shown in Fig. 5c) was appeared for the water and oil phase velocities from 0.186 m/s to 0.541 m/s and 0.004 m/s to 0.064 m/s respectively. Within this pattern, the oil and water

Fig. 6. Water and oil phase volume fractions for coil cross sectional plane located at 180° position within the fifth coil turn by using water and oil phase velocities from: (a) 0.025–0.118 m/s and 0.004–0.064 m/s; (b) 0.025–0.118 m/s and 0.064–0.558 m/s; (c) 0.186–0.541 m/s and 0.004–0.064 m/s; (d) 0.186–0.541 m/s and 0.064–0.558 m/s.

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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L. Sharma et al. / Chemical Engineering Science xxx (2017) xxx–xxx

Fig. 6 (continued)

phases were occupying two distinct layers, with little turbulences in the oil phase. The fourth pattern of the volume fraction (shown in Fig. 5d) was lying in the range of 0.186 m/s to 0.541 m/s and 0.064 m/s to 0.558 m/s of water and oil phase velocities, respectively. In this pattern, the oil and water phases were completely intermingled. In order to assure the validity of these four distinctive patterns of water and oil phase volume fractions, a similar analysis was performed for the cross-sectional plane at 180° position (within the fifth coil turn). Fig. 6a–d illustrate the patterns for this location, which completely satisfies the existential ranges mentioned by using the results for a plane at 90° position (Fig. 5a–d). It is important to mention that within each of the four ranges (mentioned above), a small variation in the fractional area occupied by given phase takes place owing to changing volume fractions. 3.3.2. RPT-TOF measurements On conducting RPT-TOF measurement under each flow, viz. by varying the oil phase velocity at a fixed water phase velocity and vice versa, a significant variation in mean residence times   t i;RPTTOF was observed for both phases in contrast to their mean   residence time based on overall flow rate VQCi ¼ ti;ov erall . This variation in mean residence time was caused due to changing volume occupancy (or holdup) of the each phase with varying flow fractions. Therefore, an estimation of the mean residence time was also made by measuring overall holdup for each phase for an alternative validation. The method of trapping the contained dispersed phase was done by blocking the inlet and outlet of the coiled

geometry using quick shut-off valves (as shown in Fig. 2). Subsequently, the trapped fluids were blown out of the geometry into a graduated cylinder and allow to settle until the separation of collected fluids into two phases (oil and water) took place. As the coiled tube was already wet by the flowing phases, the negligible fraction of either of the phases was retained by the walls of the tube. The holdup of each phase was expressed as the ratio of the amount collected in the respective phase to total volume held in the geometry (as per Eq. (8)). From the respective phase holdup measurements (ai ) and the value of ti;ov erall: (adjusted as per the respective overall flow rate), an estimation of mean residence time (ti;holdup ) was made by using Eq. (9). Fig. 7a and b represent parity charts between the measured mean residence time from RPT-TOF experiments (t i;RPTTOF ) and holdup data (ti;holdup ), for water and oil phase respectively. These figures suggest a good consistency between the two measurements, very well within experimental error. Thus, the discrepancy between the ti;RPTTOF and ti;ov erall has been captured through holdup measurements.

Liquid holdupðai Þ :

ai ¼

Vi VC

ð8Þ

Liquid mean residence time for two  phase flow : ti;holdup ¼ ai t i;ov erall:

ð9Þ

Moreover, on performing a careful examination of the oil and water phase holdup data, some interesting observations of varying flow behavior were made. At water volume fraction (ew ) of approx. 0.17, which nearly matches to the phase inversion (the dispersed phase changes into continuous and vice versa) boundary proposed

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

L. Sharma et al. / Chemical Engineering Science xxx (2017) xxx–xxx

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Fig. 7. Parity plot for mean residence time measured from RPT-TOF and holdup data for: (a) water phase; (b) oil phase.

Fig. 9. Peclet number (NPe) variation with changing water fraction for two-phase (oil-water) flows through helical coil: (a) water phase; (b) oil phase.

Fig. 8. Holdup vs. water fraction data using two phase (oil-water) flow through coiled geometry.

by Chen and Guo (1999), one observes a dip in the holdup (as shown in Fig. 8). However owing to limited experimental data, a firm conclusion on phase inversion was not made by Chen and Guo (1999). To provide a further confirmation of this fact, the data from two phase (oil-water) RPT-TOF measurements was analyzed for the respective Peclet numbers (NPe,o and NPe,w). On plotting these values over variable water volume fraction (ew ) (as shown in Fig. 9a and b), using low water phase velocities (i.e. conditions that

are providing lower water fractions, ew ), one can clearly observe a sudden discontinuity in the magnitude of Peclet number (NPe) at ew = 0.17 (approx.). Thus, a confirmation on the observed onset of phase inversion is made from our experiments. Furthermore, the trends of oil phase and water phase Peclet number (NPe,o and NPe,w) were analyzed over the entire range of oil and water phase velocities (5 6 N Re;o 6 700 and 1061 6 N Re;w 6 23; 150) to present the mixing behavior of each phase, as shown in Fig. 10a and b. In these figures, the stems are provided for each marker (representing the magnitude of Peclet number) to understand their trend, clearly. The sharp variations in the trends of Peclet number (NPe) with varying flow conditions (as shown in Fig. 10a and b) signify the varying contacting pattern between two phases. For more clarity the results from Fig. 10a and b are projected on a 2-D plane by converting the water velocity axis into legends, as shown in Fig. 11a and b. On perceiving these trends, the maps of Peclet number (NPe) (for oil and water phase) with varying oil and water phase velocity have been divided into four regions (as shown in Fig. 11a and b). From Fig. 11a and b, it can be observed that both phases (water and oil) have experienced a similar demarcation despite of analyzing their trends independently. It is noteworthy that the demarcation lines are plotted to define the range of water and oil phase velocities associated

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Fig. 10. Peclet number (NPe) variation with changing water and oil phase velocity for two-phase (oil-water) flows through helical coil: (a) water phase; (b) oil phase.

with the particular trend of Peclet number (for oil and water phase). Or in other words, the demarcation lines are not a function of oil velocity and Peclet number (for respective phase) per se. A close look at the ranges of water and oil phase velocities delineated in Fig. 11a and b, and described through RPT-VOF measurements (described in Figs. 5a–d and 6a–d), suggest a well-accepted match. Thus in the following discussion, the flow behavior/mixing pattern within each of these regions has been explained by combining the outcomes from two measurements, viz. RPT-TOF (for mixing measurements), and RPT-VOF (for phase volume fraction measurements through the fractional occurrence density). Fig. 11a, shows the increasing magnitude of NPe,w with increasing water phase velocity, this attributes to increasing strength of secondary flow profiles with increasing water velocity that offers enhanced cross-sectional mixing. However, with varying oil phase velocity, the water phase Peclet number (NPe,w) shows changing trends, by which different flow regions have been demarcated. For region I (as shown in Fig. 11a or proposed stratified flow regime

by Chen and Guo (1999)), it can be observed that NPe,w decreases as we increase the oil phase velocity using fixed water phase velocity. This happens because the flow is being stratified in this region (i.e. two distinct phases moving with the relatively smooth interface, as shown in Figs. 5a and 6a). An increasing oil phase velocity results into depleting the faster-moving water fraction and driving it to come under the influence of slower moving oil phase, thus lowering its velocity. This lowering of phase velocity results into higher axial dispersion due to decreased strength of secondary flow. It is important to note that this region continues till the flow rates of two phases become comparable, once the oil phase velocity becomes higher the existing trend terminates. In Region II (observed as annular oil flow from Figs. 5b and 6b and Chen and Guo (1999)), relatively higher velocity and volume fraction of the oil phase in contrast to water phase gives an increasing trend of NPe,w with increasing oil phase velocity. We expect this to be happening because, in this region, water was being traveled at the core and faster-moving layer of oil at the annulus (as shown in Figs. 5b and 6b) that makes the condition like moving wall for

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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Fig. 11. Peclet number (NPe) variation with changing oil phase velocity for two-phase (oil-water) flows through helical coil: (a) water phase; (b) oil phase.

the fluid at the center, and thus gives higher NPe,w. Moreover, owing to the low volume fraction of the water phase in this region, it was easily being carried away by faster moving oil phase to give higher resultant velocity to the fluid phase which in turn enhanced radial mixing due to the influence of curved path. For Region III or oil dispersed stratified flow regime as per Chen and Guo (1999) and depicted in Figs. 5c and 6c, an increase in NPe,w is described with increasing oil phase velocity. This happens because in this region water phase velocity is relatively higher than oil phase velocity that makes slower moving oil to disperse into different size droplet by high shear to give a higher cross-sectional mixing by disturbing the oil-water interface by small disturbances. Furthermore, on increasing the oil phase velocity, more turbulence in oil phase has been produced which will cause higher momentum exchange between the phases and thus gives more cross-sectional mixing to the water phase and resulted into higher NPe,w. In Region IV, mentioned as oil dispersed flow by Chen and Guo (1999), and similar is shown by Figs. 5d and 6d, a sharp increase in the NPe,w is observed with increasing oil phase velocity. This happens because the higher velocity of both (oil and water) phases results into mixed flow pattern which has further been knocked by coiled flow

path to sustain such profile and hence higher cross-sectional mixing. Fig. 11b shows some distinct trends for NPe,o with their increasing oil phase velocities at lower water phase velocities, where oil phase Peclet number (NPe,o) decreases with increasing oil phase velocity. However, on observing the complete picture of the oil phase Peclet number (NPe,o) with varying oil phase velocity, similar regime demarcation is implemented (as presented in Fig. 11a). In Region I (mentioned as stratified flow regime by Chen and Guo (1999)), it has been found that oil phase Peclet number (NPe,o) results in a lower magnitude with increasing water as well as oil phase velocities. This appears owing to low velocities of both phases that makes fluids to move in two separate layers (shown in Figs. 5a and 6a). Increasing the velocity of water phase makes the oil phase in the vicinity of water phase to move at faster velocity as compared to rest of the oil phase, and hence gives more axial dispersion (or lower cross-sectional mixing/NPe,o) is seen. Also, on increasing oil phase velocity at fixed water phase velocity, the extent of radial mixing in oil phase decreases. This happens owing to the appreciably low velocity of the oil phase in this region; that results in developing axial laminar velocity profile than developing

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021

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L. Sharma et al. / Chemical Engineering Science xxx (2017) xxx–xxx

secondary flow patterns with increasing velocity magnitude (owing to higher viscous effects) and hence provide reduced radial mixing or NPe,o. A similar trend of NPe,o are observed for lower water phase velocities in region II, where annular oil flow regime was proclaimed by Chen and Guo (1999) and by RPT-VOF measurements (shown in Fig. 5b, Fig. 6b). However, at higher water phase velocities (in region II), the NPe,o increases with increasing oil phase velocity. This may be caused by the action of increasing water phase velocity that tends to disturb axial laminar velocity profile and give rise to NPe,o with increasing oil phase velocity. In Region III (termed as oil-dispersed stratified flow regime by Chen and Guo (1999) and shown in Figs. 5c and 6c), the NPe,o increases with increasing oil phase and water phase velocities. This happens because, in this region, the continuous oil phase has been dispersed into small bubbles for which an increasing velocity brings more intense mixing and hence NPe,o. Similarly in Region IV, NPe,o increases with increasing oil phase and water phase velocities. This happens because of the well-mixed flow pattern therein (characterized as oil dispersed flow regime by Chen and Guo (1999), and Figs. 5d and 6d show a similar trend), that becomes more intense with increasing phase velocities.

4. Summary and conclusions Investigation on oil-water (two-phase, liquid-liquid) flow through horizontally placed coiled geometry has been presented on employing RPT-TOF (mixing indices) and RPT-VOF (volume fraction or phase holdup), measurements in the range of 5 6 N Re;o 6 700 and 1061 6 N Re;w 6 23; 150. The TOF and VOF based on RPT (referred to as RPT-TOF and RPT-VOF, respectively), results in the measurement of exit age distribution (E(t)) and occurrence density distribution respectively, without offering any limitation of noise that is being produced by the presence of the second phase. Thus provides an accurate extent of radial mixing (regarding NPe from E(t)) and volume fraction (from occurrence density of tracer particle) for each phase (oil and water) under given flow conditions. The procedure to accomplish these investigations starts by analyzing the single phase, oil, flow through coiled geometry in the range of 5 6 N Re;o 6 700. The outcome from this analysis are compared with available literature on single phase, water flow, through coiled geometry; that has established a unique correlation to understand the dispersion behavior of different fluids through coiled geometries. This establishes, beyond doubt, the efficacy of the method. Furthermore, information on the two-phase, oilwater, flow has been explored by conducting two contemporary measurements based on RPT viz. RPT-TOF and RPT-VOF in the range of 5 6 N Re;o 6 700 and 1061 6 N Re;w 6 23; 150. From the RPT-VOF measurements, four distinct patterns of the phase distribution are obtained, that characterizes the flow behavior therein. Likewise from RPT-TOF measurements, different trends of water and oil phase Peclet number (NPe,w, NPe,o) are established over variable water and oil phase velocities. These are evolving trends of volume fractions (for water and oil phase) with phase flow rates, and Peclet number (NPe.w and NPe.o) are used to delineated different flow regions. Our findings are positioned against the flow regime information reported by Chen and Guo (1999). Together with the phase holdup information, our mixing behavior (characterized by the respective phase Peclet numbers), are used to delinate the different flow regimes in two-phase coiled flow. There are many applications in which oil and water are made to flow together. With the greater incentive for process intensification, there is a greater incentive towards adopting coils as potential process intensification devices for reactions and separations. We

expect this work to be invaluable in such contexts, as it is the first full-length analysis of oil-water flow through coiled geometries. We expect the follow-up work in this field to build on what we have presented, including investigations into the local velocity fields in coiled structures. Indeed, our own forthcoming submission will relate to this important topic. References Awwad, A., Xin, R.C., Dong, Z.F., Ebadian, M.A., Soliman, H.M., 1995. Measurement and correlation of the pressure drop in air-water two-phase flow in horizontal helicoidal pipes. Int. J. Multiphase Flow 21, 607–619. Chen, X., Guo, L., 1999. Flow patterns and pressure drop in oil-air-water three-phase flow through helically coiled tubes. Int. J. Multiphase Flow 25, 1053–1072. Dean, W.R., 1927. Note on the motion of fluid in a curved pipe. Phil. Mag. 4, 208– 223. Dean, W.R., 1928. The stream -line motion of fluid in a curved pipe. Phil. Mag. 5, 673–695. Gelfgat, A.Y., Yarin, A.L., Bar-Yoseph, P.Z., 2003. 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Vashisth, S., Nigam, K.D.P., 2008c. Liquid-phase residence time distribution for twophase flow in coiled flow inverter. Ind. Eng. Chem. Res. 47, 3630–3638. Vielma, M.A., Atmaca, S., Sarica, C., Zhang, H.Q., 2008. Characterization of oil/water flows in horizontal pipes. SPE Projects Facil. Constr. 3 (04), 1–21. Wen, C.Y., Fan, L.T., 1975. Models for Flow Systems and Chemical Reactors. Marcel Dekker, New York. Xin, R.C., Awwad, A., Dong, Z.F., Ebadin, M.A., 1997. An experimental study of single phase and two-phase flow pressure drop in annular helicoidal pipes. Int. J. Heat Fluid Flow 18, 482–488. Zhu, C., Fan, Liang-Shi, 1998. In: Johnson, Richard W. (Ed.), The Handbook of Fluid Dynamics, ‘‘Chapter 18 – Multiphase flow: Gas/Solid”.

Please cite this article in press as: Sharma, L., et al. Investigation of two-phase (oil-water) flow in coiled geometries using ‘‘Radioactive Particle TrackingTime of Flight (RPT-TOF)” and ‘‘Radioactive Particle Tracking-Volume Fraction (RPT-VOF)” measurements. Chem. Eng. Sci. (2017), http://dx.doi.org/ 10.1016/j.ces.2017.03.021