{'fraction_non_alphanumeric': 0.02454780361757106, 'fraction_numerical': 0.011686633779657035, 'mean_word_length': 4.470285897847735, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Low Cost True Monofiber Optical Probe for Local Void Fraction Measurements in Minichannels

Two phase flow inside minichannels is one of the most investigated research topic at present. The measurement of the flow rate parameters is fundamental to characterize the flow pattern and its evolution over time. This paper shows that an optical technique, well-known for large diameter pipes, can be applied to mini channels with a laminar mass flow rate. In particular, a Y-junction mono-fiber optic system with a chamfered tip probe has been built and tested. This method is applied to the local void fraction measurement in a copper capillary pipe with internal diameter of 2 mm and external diameter of 3.00 mm. Different probes have been developed and tested. The accuracy of the method depends on the size, the shape of the tip and on the tip distance from the pipe centre. Different distances and liquid flow rate have been tested. The two-phase flow pattern is also visualized and recorded by a high speed camera (FASTEC Troubleshooter 16000 fps) and post processed with an image analysis technique. A good agreement between the optical and the video signal has been observed.


Introduction
Two-phase flow prediction in minichannels is a basic very active research field at present, because of the increasing number of mini equipments in ground and space applications which involves capillary pipes. The measurement techniques of parameters as pressure, velocity, temperature, mass flow rate are well assessed at the moment even in capillary pipes. On the other hand the measurement of void fraction inside two-phase flow for minichannels is currently an unsolved problem. The simplest measurement technique of void fraction is the quick-closing valve method, still used at present. This method traps liquid in a tube to measure mass and liquid volume [1]. Until now there are several studies concerning void fraction measurements and involving different configurations and setups. They are largely applied to large pipes [2]- [4]. Maurus et al. [5] visually observed the void fraction in a subcooled flow of boiling water through a horizontal rectangular channel. A high-speed camera's top and side views of vapor shape helped to estimate the void fraction. Bowers and Hrnjak [6] visually observed the void fraction measurement of R-134a stratified flow through three different tube diameters, 7 al. [7] used image analysis to measure the void fraction of R-134a's condensing process while flowing through a minichannel ranging from 2.00 to 4.91 mm. Their results show that neither mass flux nor channel size had a significant effect on the void fraction of a wavy flow pattern.
The optical system based on the images acquisition has, however, a big limitation because it cannot be applied to opaque pipes. Among the void fraction measurement techniques, the employment of optical fiber probes seems to be one of the most interesting one, because of they can be applied to opaque pipes of different materials (metallic, plastic, glass), they are appropriate for a large number of working fluids and they can be used with a very high sampling rate [8]- [9]. This technique is based on the difference, in terms of refraction index, between the vapour and liquid phases where the tip of the probe is periodically immersed. Unfortunately the fiber optic probe is an intrusive measurement because the tip is immersed in the fluid and the sensitive surface, depending on the size of the probe, could be not negligible respect the cross sectional area of the pipe. In the past it is usually applied to large ducts measurements.
In addition optical fiber probe measures the local void fraction evolution over the time of the zone where the tip is immersed, so that a big number of probes, in different points of the cross section, must be used if the flow pattern must be characterised. However, in the case of two-phase flow pattern characterisation for mini-channels the number of probes can be drastically reduced down to one. The highly confined flow rate shows different flow patterns respect to the large channels case: patterns as slug/plug, annular and semi annular are the most frequently observed [8] [11]. In annular, semi-annular and slug/plug patterns the position of the probe tip inside the cross section and its distance from the pipe centre assumes lower importance in the flow pattern characterisation. This paper deals with the limitations and the accuracy of the application of a fiber optic technique to capillary pipes to detect the two-phase flow pattern over time. Capillary pipes are characterised by very low diameters, given by Bond number lower than 2. This configuration is typical of Pulsating Heat Pipes devices and other automotive heat exchangers. Different probes have been built and tested in a water/air loop. The void fraction experimentally detected by the optical probe inside a copper pipe with an internal diameter of 2 mm, has been compared with that measured with an image acquisition system, showing good agreement. Some preliminary qualitative studies on the influence of the probe tip position with respect to pipe centre have been presented.

Fiber Optic void fraction measurements
The fiber optic void fraction measurement is based on the physical principle that a light beam ray passing through a transparent surface can be refracted with different angles according to medium in contact with the surface. The basic relationship between governing refraction at the interface between two media is given by the Snell's law: where n 1 and n 2 are the refraction indexes of the two media and θ 1 and θ 2 are and the incidence angles of the incoming and outcoming light beam, respectively. It is interesting to note that as θ 1 decreases, θ 2 tends to 0 and the intensity of the light beam, that crosses the interface, decreases to 0, therefore the light beam is reflected back. The value of θ 1 , named θ TIR and corresponding to the total reflection, is given by the limit of θ 2 that tends to 0, so that cosθ 2 ≈ 1 = cos .
(2) Eq (2) shows that if θ 1 ≤ θ TIR the beam inside the fiber core is totally reflected, and it is completely contained inside it. This phenomenon allows optical fibers to correctly work, because the refraction index of the core is higher than the refraction index of the external cladding and, therefore, the beam isconfined inside the core channel. In this method the material of the probe immersed in the fluid assumes great importance in the feasibility of the measurement. One of the most applied probe in large pipes in the past is the U-shaped probe. This type of probe tip is characterized by a single optical fiber bended with a relative small curvature radius. The U-turn of the fiber is then inserted at the measurement point. This configuration is the simplest because it needs no fiber junctions or splitter, as there is only one fiber where the beam can pass. However this technique presents several disadvantages: the fiber optic must be sufficient flexible to be curved, and the probe assumes large dimensions. It cannot be applied to capillary pipes. Another probe tip is named "Joined-tip". It is characterized by a welded probe composed of two optical fibers polished and fused or joined together by means of a suitable glue or resin. Thismanufacturing process is shown in [12] . The tip is fixed by means of a suitable epoxy resin. This probe also has large dimensions, and it cannot be applied to capillary pipes. The true monofiber optical probe is characterized by a single fiber section of variable length. The light beams, both the one coming from the laser source and going to the detector, pass through the same core channel and are then splitted by means of a Y-Junction.
In the past this technique has been used by Carrica et al. [13] to characterise the void fraction in a pool boiling process. Carrica et al. state that the tip of the true monofiber probe must be opportunely chamfered to obtain a back signal as the light beam impacts on the fiber external surface. An angle of 45° is advised. Figure 1 shows the different behaviour of the chamfered tip in case of total reflection or refraction. The beam (not necessarily parallel to the fiber axis) incident on the tip surface (interface between the two media) is partially reflected and partially refracted. The quantity of reflected light is higher when the tip is in contact with gas phase than with liquid phase.  The Y-junction is the most critical part of the optical system. It conveys the light beams, coming from the source, into the monofiber probe and splits the light coming back from the probe to the photodetector. It operates as a circulator. The incorrect alignment of the fiber causes high light losses, therefore and the signal going back to the photodetector could be insufficient or very noisy. The basic scheme was proposed by Carrica et al. [13] and is shown in Figure 3. The two fibers coming from the light source and going back to photodetector are collimated with the aid of a 20x microscope. The fibers are then placed and glued in a stainless steel micropipe. Then the two tips are cut and polished with polish paper. The same polishing procedure is used to manufacture the larger diameter fiber used for the chamfered probe. The surface finish is checked by means of a microscope , and a first signal check is performed in order to enhance fiber alignment. A bicomponent resin, used for connectorizing fibers, is placed on the junction in order to fix the components and reduce losses. The probe is made of a 400 µm fiber and the tip is -chamfered at an angle of 45°. The fiber has not been stripped, and there was the need to remove part of the coating. The polishing procedure is the same as the junction. At first the tip has been cut to length and polished with 1000K paper, then finished with 2000K paper. During this procedure, it has been constantly checked by means of a microscope. Due to the fact the tip must fit into a minichannel, the chamfering has been extended to the section before the tip, obtaining a conical shape. An enlarged (20X) photo of the tip probe is shown in Figure 4a.
The tip probe has been inserted inside the void fraction measurement cell, that consists of a copper T junction between the copper pipe (OD=3 mm, ID = 2mm) and a stainless steel pipe (OD=1 mm, ID=900 µm). The Y junction has been placed over a micro-regulated chassis that allows the regulation of the position of the tip inside the pipe with an accuracy of ±0.05 mm.
The enlarged photo of the probe with the tip positioned at the pipe centre, see Figure 4b, shows that the probe is highly intrusive, and a big portion of the cross sectional area is occupied by the probe. The ratio between the surface occupied by the probe and the entire cross sectional area, versus the position of its tip along the pipe radius, is shown in Figure 5. The signal coming from the photo detector has been acquired by an acquisition board Arduino ATmega328 . It has 6 analogue inputs, and features the Atmega16U2 (Atmega8U2 up to version R2) programmed as a USB-to-serial converter. The clock speed is 16 MHz and the acquisition sampling rate is 40 Hz.

Video acquisition system
The void fraction has been detected also by acquiring a video with a high speed videocamera (FASTEC Troubleshooter HR, up to 16000 fps @ 1280x32), and post processing the collected images. The video section consists of a glass pipe (ID=2 mm) and a video camera, externally triggered to synchronize the video capture with the signal coming from the photo-detector. The back light behind the glass pipe is a white LED Source 5,5 W DC12V.It has a good light diffusion, a constant background colour and reduced reflection noise effects. The video has been taken at 125 fps with a resolution of 1280x32 pixels. The images have been post-processed as shown in Figure 6.

Optical probe and video capture void fraction measurement
In order to c to understand the reliability of the fiber probe when applied to minichannels, a test has been carried out at constant water mass flow, pushing a measured volume of air, at the same temperature, into the loop, in order to obtain a two phase flow pattern. In accordance with Ong et al. [10] the water mass flow rate is 2.5 ml/s and the volume of air pushed inside the loop has been calculated to obtain a slug flow. Three different vapour slugs have been visualized. Figure 7 shows the comparison between the optical signal with a tip probe placed at the pipe centre, with that resulting from image acquisition.

Figure 7: Comparison between the void fraction measured by the video capture and fiber probe
According to the Snell's law and taking into account that the refraction index n 1 of the silica core is higher than the refraction index of liquid/gas, it is possible to define the critical total reflection angles of the two phases. By considering water (n water = 1.33) and air (n air = 1) and silica core (n silica = 1,46) the critical angles are 0.42 rad (24°) for water and 0.72 rad (41.2°) for air. For this reason the quantity of light which is reflected back to the light detector is greater when the probe tip is in contact with air. Figure 7 shows with great evidence three slugs passing through the probe. The local void fraction measured by the optical probe and that obtained by image processing show a good agreement and errors lower than 12%. A good repeatability has been observed.

Preliminary test on the influence of the tip position
A preliminary experimental activity has been carried out in order to understand to what extent the optical probe can be considered intrusive in flow pattern detection. Three different tests at different liquid flow rates (2.8, 5 and 8.3 ml/s) and two radial positions of the tip probe (r=0 and 5 mm) have been tested. The first position (r=0 mm) corresponds to the tip placed at the pipe centre with 82% of the cross sectional area left free by the probe, the second position is with the tip placed at r=5 mm (free surface of 92%). A visualization of the different flow patterns at different regimes is shown in Figure 8.
The images in figure qualitatively shows that the optical probe modifies the two-phase pattern distribution mostly at low flow rates. Figure (a) shows the end of a slug of vapour for a flow rate of 2.8 ml/s. It possible to see as the probe breaks the slug backward meniscus, and several following microbubbles can observed. This problem is evident at high mass flow rates and even when the tip is placed at r=5 mm, even if 92% of the surface is free in this case However it is possible to note that at high flow rates and at r=0.5 mm both the advancing (Figures 8c,  g, i) and the receeding one menisci, (Figures 8c, g, i, h) tend to be less disturbed than in the case with probe at r=0 mm.  Another interesting qualitative tendency, observable as the probe is receeded toward the inner surface, regards the pattern of the slug. For low flow rates ( Figure 8b) the flow pattern is higly disturbed by the fiber probe, but this disturbance becomes very low as the probe is placed at r=5 mm (Figure 8f, j) . Figure 9 and 10 show the voltage measured by the photo detector vs. time as two-phase flow is passing through the probe, at r=0 mm and flow rate of 2.8 ml/s, and at r=0.5 mm and flow rate of 8.3 ml/s, respectively. It is interesting to note as the probe is able to detect the flow pattern in both cases. Figure 10 shows a greater number of peaks, because the the slugs are shorter than the in the other case. The comparison between the probe signal and the void fraction calculated by the image processing has however confirmed a good agremeent also in these two cases.

Conclusions
This paper deals with the limitations and the accuracy of the application of a fiber optic technique to detect the flow pattern over time in capillary pipes. Among the void fraction measurement techniques, the fiber optic probes can be applied to opaque pipes of different materials (metallic, plastic, glass), they can be used with a large number of working fluids with a very high sampling rate. Different probes have been built and tested in a water/air loop. The void fraction detected by the optical probe has been compared with that measured with an image acquisition system, and a good agreement has been observed (errors lower than 12%). Unfortunately the fiber optic probe is an intrusive measurement: some preliminary qualitative studies on the influence of the probe tip position with respect to pipe centre have been presented. Experimental tests have confirmed that this probe highly disturbs the flow pattern by breaking the advancing and receeding menisci in slug flow. As expected, this disturbance increases when the probe is more deeply inserted along the tube diameter. Further studies will be aimed to optimize the radial position of the probe, in order to detect the void fraction making as low as possible the disturbances and modifications of the flow pattern.