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The flow-pattern transition has been a challenging problem in two-phase flow system. We propose the terahertz time-domain spectroscopy (THz-TDS) to investigate the behavior underlying oil-water flow in rectangular horizontal pipes. The low water content (0.03-2.3%) in oil-water flow can be measured accurately and reliably from the relationship between THz peak amplitude and water volume fraction. In addition, we obtain the flow pattern transition boundaries in terms of flow rates. The critical flow rate Qc of the flow pattern transitions decreases from 0.32 m3/h to 0.18 m3/h when the corresponding water content increases from 0.03% to 2.3%. These properties render THz-TDS particularly powerful technology for investigating a horizontal oil-water two-phase flow system.
© 2015 Optical Society of America
1. Introduction
Oil and water two-phase flow in horizontal pipes is a common occurrence in the petroleum industry for long-distance transportation. The investigation of pattern transitions is very important in horizontal oil-water two-phase flow system due to the existence of various flow patterns and different mechanisms governing them [1–5 ], especially for the prediction of rheological behavior in the horizontal oil wells, measurement of flow parameters and optimization of industrial production process. Usually, the flow pattern of oil-water in horizontal pipe switches from stratified structure to dispersed structure with increasing flow rate and the oil-water interaction is also presented during this change.
On account of its significant importance in physical and chemical research fields, the horizontal oil-water two-phase flow has attracted a considerable research effort. The hydrodynamic characteristic of each flow pattern has been addressed and a flow pattern contains the shape and spatial distribution of the two-phase flow in the pipe [6–23 ]. Despite extensive efforts, the dynamics of various patterns in horizontal oil-water two-phase flow is still controversial. In particular, an ability to distinguish liquid-liquid flow behavior accurately is of fundamental importance. How to reveal the complex oil-water flow structure from experimental signals still represents a significant challenge.
The amplitude and phase composition of THz light transmitted or reflected from various materials can be obtained in terahertz time-domain spectroscopy (THz-TDS). THz-TDS has been used to study the dielectric properties of oil–water complexes [24]. THz radiation was also used to study the moisture content and distribution in various materials, such as paper, biomolecules, food wafers, plant leaves and crude oil [25–28 ]. THz-TDS has undergone a remarkable development in the last decade and provided us a powerful tool for investigating complex systems from different disciplines in fast and situ phase transitions.
In this letter we investigated the flow structures of oil-water two phase fluid systems with the low water contents (0.03%–2.3%) by THz-TDS. A correlation is presented for the prediction of the inversion point of an oil-water dispersion system. The formation and evolution of different oil-water flow structures are also discussed.
2. Experiments
Before the measurement, the conventional 0# diesel and tap water were mixed using a homogenizer for more than thirty minutes in the storage tanks [see Fig. 1 ] at room temperature and atmospheric outlet pressure. Then the oil-water mixtures were pumped into pipe and the flow rate from 0.1 to 0.36 m3/h was accurately measured by using fluid flowmeter. The input water volume fractions increased from 0.03% to 2.3%. The oil-water mixtures flowed along a three-meter long horizontal pipe from the entry point to the test section. A long flow distance provided sufficient entrance length to stabilize the flow. The THz-TDS measurement for different water content oil-water mixtures was carried out when the steady state reached in a certain flow condition.
Fig. 1 Experimental flow loop facility. The insert shows a schematic of the THz-TDS measurement process.
The amplitude and phase composition of THz-TDS can be used to investigate various materials by coherence measurements of THz light [28–34 ]. Previously, a typical THz-TDS system was used to measure the flow states [28,29 ]. Here, the focus diameter of the THz beam was about 1 mm. In order to reduce optical absorption by the curved (transparent) tube wall, a high-purity polyethylene pipe with a square cross section (10mm × 10mm) was chosen as the flow pipe and the pipe thickness was 1 mm. To minimize the influence of moisture, the measurements were performed at 20°C under dry nitrogen. The pipe was located in the focus of the two Si lens, as shown in Fig. 1, and perpendicular to the incident THz beam. The polyethylene pipe is transparent for visible light and has a low refractive index and absorption in THz range. Both the time-domain sample and reference spectra were obtained by testing the polyethylene pipe holding the mixtures and empty pipe, respectively.
3. Results and discussion
THz-TDS waveforms of diesel-water mixtures are shown in Fig. 2 with different water content (0.03%, 0.7%, 1%, 1.3%, 1.5%, 1.7%, 2% and 2.3%) at flow rate 0.28 m3/h. The THz signal decreases as the water content of the mixture increases. The peak signals of the transmitted THz pulses dropped from 67 mV to 19 mV. Due to the absorption characteristics of water in THz range [25,28 ], the water content in oil-water mixtures has great influence on the THz signal. Hence, THz-TDS can provide information on the water content of mixtures by using the amplitude and phase change. Generally, a phase shift relative to the reference pulse occurs for THz pulses transmitted through the empty cell and mixture which is from the time delay between their THz amplitude signals. The phase changes of THz-TDS can be used to obtain the optical parameters of samples, such as the refractive index and the extinction coefficient.
Fig. 2 THz-TDS for various water content of diesel oil-water mixtures at flow rate 0.28 m3/h. The insert shows the peak value of THz signal for different water content in oil-water mixtures.
For the various flow structures (flow patterns), the horizontal oil-water flow patterns can be classified into segregated flow and dispersed flow. The segregated flow includes a stratified flow pattern (ST) and a stratified flow with mixing at interface pattern (ST and MI), while the dispersed flow includes a dispersion of oil in water and water flow pattern (D O-in-W and W), a dispersion of water in oil and oil in water flow pattern (D W-in-O and D O-in-W), a dispersion of oil in water flow pattern (D O-in-W) and a dispersion of water in oil flow pattern (D W-in-O) [2]. Here 0# diesel fuel belongs to light oil (ρo/ρw = 0.85, μo/μw = 2.7 and εw = 0.03% where ρo and ρw are oil and water densities, μo and μw are oil and water viscosities as well as εw is water volume fraction.). The flow pattern of the diesel oil-water flow is a dispersion of water in oil flow pattern (D W-in-O). The water phase is dispersed as droplets in oil continuous phase for water content at a low level (εw < 3%) [2,6,17 ]. For different water content, the THz-TDS peak intensity of diesel-water mixtures shows the transition point at flow rates from 0.1 m3/h to 0.36 m3/h, as shown in Figs. 3(a)-3(h) . These flow rates of THz-TDS amplitude transition points are named critical flow rates Qc. Water content dependence of Qc is shown in Fig. 4 where Qc decreases from 0.32 m3/h to 0.18 m3/h with the water content εw increasing from 0.03% to 2.3%.
Fig. 3 (a)-(h) Peak of THz-TDS amplitude for different water content diesel oil-water mixtures with increasing flow rates from 0.1 m3/h to 0.36 m3/h.
Fig. 4 Critical flow rate Qc and Reynolds number Re for different water content diesel oil-water mixtures. The inserts show schematics of the flow patterns: laminar flow pattern (left image) and transition flow pattern (right image). The color of the symbol is corresponding to those of the measurements shown in Fig. 3.
In fluid mechanics, the Reynolds number (Re) is a dimensionless quantity that is used to predict similar flow patterns in different fluid flow situations. The Reynolds number can be used to determine dynamic similitude between two different cases of fluid flow. Moreover, the Reynolds number can be also used to characterize different flow regimes within a similar fluid. For flow in a pipe, the Reynolds number is generally defined as
where DH is the hydraulic diameter of the pipe (DH = 0.01 m), Q is the volumetric flow rate (m3/s), A is the pipe cross-sectional area (m2) and ν is the kinematic viscosity (m2/s). For oil-water two phase flows, ν is also defined as [35]where εo is oil volume fraction, μ o = 2.73 mPa·s, μw = 1.01 mPa·s, ρo = 852 kg/m3, and ρw = 998 kg/m3 [35–37 ]. Based on our experiment, the Qc decreases from 0.32 m3/h to 0.18 m3/h, correspondingly Re decreases from 2775 to 1586, with εw increasing from 0.03% to 2.3%.In general, laminar flow occurs when Re < 2300 and turbulent flow occurs when Re > 4000. In the interval between 2000 and 4000, laminar and turbulent flows are possible and called transition flows. The flow patterns also depend on other factors, such as pipe roughness and flow uniformity [36,37 ]. The transition Re between 2300 and 4000 is also called critical Reynolds number Rec and Rec is different for every geometry [37,38 ]. In fact, the matching Reynolds number is not on its own sufficient to guarantee similitude. Generally, the fluid flow is chaotic, thus the very small changes on shape and surface roughness can result in very different flows. As shown in Fig. 4, Re is strongly dependent on the water content, indicating that the viscosity is not the only factor to determine pattern transition. Distribution of the dispersed phase in the continuous phase, the shape and particle size of the dispersed phase and the contact form between dispersed phase and the tube wall may also affect flow pattern transition.
For Q < Qc, the flow pattern of oil-water mixtures in the pipe is a stratified flow pattern with oil continuous phase, thus the energy of shear stress is not strong enough to break water droplets and the movements of water phase comes along with oil continuous phase regularly. The energy of shear stress increases along with the increase of flow rate Q, but the increase of shear stress is limited. Whereas for Q ≥ Qc, the flow patterns become transition flows, the turbulence energy of the mixtures flow increases further and the interaction between the oil and water becomes enhanced. Under this condition, the turbulence energy is high enough to break water phase into smaller droplets above Qc and there exist more water drops in the pipe center than that near pipe wall. The flow of abundant water droplets may increase the water distribution of the detecting areas [see Fig. 5 ], which denotes the increase of THz wave absorption. In addition, the scattering effect may also be enhanced with decreasing size of water drops, which would be another factor contributing to the THz absorption. Therefore, the transition of the oil-water flow pattern may increase the absorption of THz radiation. It also indicates the flow pattern changes with different water contents in critical flow rates.
4. Conclusion
In summary, we use THz-TDS system to investigate the flow states in diesel oil-water two-phase flow. The experimental results suggest that THz-TDS can be used to detect the water content and the pattern transitions in the oil-water two-phase flow. The amplitude of the THz pulse is strongly correlated to the water content. Moreover, the change of oil-water two-phase flow pattern at low water content can be also measured by THz-TDS system. The critical flow rates of the flow pattern transitions point show a non-linear trend with the water content in the mixtures. Such model can be used to qualitatively and quantitatively characterize the molecular state of oil-water flow. Accordingly, this work provides a fast and in situ way to investigate the oil-water flow system in the process of crude oil storage and transportation.
Acknowledgments
This work was supported by the Specially Funded Program on National Key Scientific Instruments and Equipment Development (Grant No. 2012YQ140005) and the National Basic Research Program of China (Grant No. 2014CB744302).
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