Measurement of void fraction in flow boiling of ZnO–water nanofluids using image processing technique
Introduction
The void fraction describes the part of the channel which is occupied by the vapor phase at any instant. Prediction of void fraction along with the direction of flow in subcooled flow boiling is very important for nuclear reactor safety during loss of coolant accident. The subcooled region can also be responsible for developing flow instabilities in a boiling water reactor (BWR) because of strong coupling between the void fraction and the reactivity (neutron moderation) in the reactor core. The loss of coolant in nuclear reactor increases the void fraction, which further decreases the reactivity in reactor core and reactor power. In a BWR, void fraction in the coolant flow affects nuclear reactivity and thermal-hydraulic characteristics of the reactor by altering coolant flow, pressure, temperature and density (Inoue et al., 1995).
Nanofluids, colloidal dispersions of nanoparticles in base fluids (Choi, 1995) have shown to be promising for applicability to nuclear reactor safety systems as a reactor coolant or as a coolant for emergency core cooling system or as a coolant for in-vessel retention of the molten core during severe accidents (Lerch, 2008, Buongiorno et al., 2008, Buongiorno et al., 2009). The promise of nanofluids as a coolant stems from their superior heat transfer properties with respect to the base fluids. They have a slight enhancement over base fluids in conductive heat transfer (Das et al., 2003), convective heat transfer (He et al., 2007), boiling heat transfer (Wen and Ding, 2005), but most importantly, a great enhancement (up to 200%) of critical heat flux (CHF) (You et al., 2003). It is mainly due to deposition of nanoparticles on the heat transfer surface, which results in the improvement of wettability and re-wetting characteristics.
Void fraction can be measured by either experimental or calculated by analytical methods which are represented in Table 1, Table 2, respectively. Experimental methods of void fraction can be classified as intrusive or non-intrusive. Intrusive methods such as monofiber optical technique and electrical resistive probe measurements adversely affect the flow field which leads to imperfect identification of the liquid interface. Non-intrusive methods like radiation techniques and impedance measurement techniques do not affect flow field and yield mean cross-sectional void fraction but dry angle and shape of the interface between gas and liquid cannot be found out. Real time neutron radiography (RTNR) and X-ray computed tomography (X-CT) were proved to be more sophisticated methods of finding void fraction. The RTNR technique allows determination of lateral void fraction with respect to time. However both the systems require enhanced radiation protection systems. A lead shielding has to be used in the case of X-rays and water concrete in the case of neutrons. Hence these methods are not suitable for flow boiling experiments unless appropriate security measures are taken. As the above methods have one or more demerits, one cannot completely relay on them.
Maurus et al., 2002, Maurus et al., 2004 used the optical technique to determine void fraction during subcooled flow boiling of water in a horizontal rectangular channel and used Matlab software for analyzing the images to study the effect of mass flux and heat flux on void fraction at atmospheric pressure, but the test section of this study was not suitable for acquiring images throughout the length of tube due to little optical access. Wojtan et al. (2005) developed an optical technique to determine the void fraction during flow boiling of water, which is non-intrusive method and allows measurement of void fraction through a glass tube in a cross-sectional view perpendicular to the flow. In this technique a CCD camera was used to record the illuminated laser sheet images of liquid–vapor interface in the cross-section of the tube and void fraction was calculated as the ratio of the number of pixels corresponding to the vapor phase and the total pixels corresponding to the total cross-sectional area of the tube. Puli and Rajvanshi (2012) determined the void fraction during subcooled flow boiling of water under different pressures, heat fluxes and mass fluxes by optical techniques using the annulus test section similar to the test section used by Wojtan et al. (2005). Images of boiling process were captured using high-speed camera and analyzed with IMAQ image processing software. Study showed that void fraction decreased with the increase in pressure and mass flux, and increased with the heat flux and heated length. Brutin et al. (2013) also used the optical technique for measuring the void fraction during flow boiling in rectangular minichannels. The flow visualization was performed using a Photron Ultima 1024 Fast-Cam and a program was developed using Matlab software to determine the void fraction in their study.
While void fraction in boiling of water has been studied broadly, data for void fraction during boiling of nanofluids, which is the situation of interest for design of many industrial devices and nuclear reactor safety systems, are very scarce. Lerch (2008) measured the near-surface void fraction and macrolayer thickness during pool boiling of water and silica based nanofluid using optical probe and found lower void fraction in case of nanofluid as compared to water at the comparable heat fluxes. No systematic study on the void fraction measurement during flow boiling of nanofluids is available in the reported literature.
Objective of the present work is to determine void fraction in subcooled flow boiling of water and ZnO–water nanofluids by optical techniques for annulus test section, which is similar test section used by Wojtan et al. (2005), which has complete access to get images throughout the heated length of heater rod and study the effect of nanoparticle concentration, heat flux, flow rate and axial location of heater rod on void fraction.
Section snippets
Preparation of nanofluid
Preparation of nanofuids is an important step for changing heat transfer performance of conventional base fluids. Nanofluid does not simply refer to a liquid–solid mixture but nanofluid should have even suspension, stable suspension, durable suspension, minimal agglomeration of particles and no chemical change of the fluid. In general, following methods are used for preparation of effective suspensions such as changing the pH value of suspension, using surface activators and/or dispersants, and
Experimental setup
The schematic diagram of the experimental test setup is shown in Fig. 1. The closed loop test facility mainly consists of ultrasonic vibration mixer, storage reservoir, circulating pump, flowmeter, electrically heated horizontal annular test section, condenser and heat exchanger. The working fluid (water or nanofluid) is pumped from the reservoir to the test section via turbine type flow meter. The mixture of working fluid and steam coming out of the test section is passed through a condenser
High speed visualization and image analysis
The use of high-speed visualization and image processing technique is imperative due to its natural consequence of non-intrusiveness but also the advent of rapid advancement in use of software and hardware. The analysis of subcooled flow boiling phenomenon is not an exception for using this optical technique and highly recommended to study very short-time phenomenon of bubble formation, growth and condensation. Information about bubble behavior during nanofluid flow boiling may provide valuable
Heater surface characterization and bubble behavior analysis
After flow boiling experiments, heating surfaces were characterized by scanning electron microscope (SEM). SEM (make-JEOL, model-JSM-6510) images of heater surface after flow boiling experiment with water and nanofluid (0.01 vol.%) are shown in Fig. 5, which show the deposition of ZnO nanoparticles onto the heating surface in the form of very thin coated layer. Increase in heater surface temperature during heating increases the Van Der Waals attractive force between the surface and nanoparticles
Results and discussion
In order to evaluate the reliability and accuracy of void fraction measurements, void fraction values obtained from experiment with water are compared with Steiner (1993) version of the Rouhani–Axelsson drift flux model for horizontal tubes for the cross-sectional void fraction. This void fraction model is easy to apply and gives the void fraction as an explicit function of total mass flux and buoyancy and hence appreciated by most of the researchers (e.g. Kattan et al., 1998, Wojtan et al.,
Conclusions
Experimental study was carried out on subcooled flow boiling of ZnO–water nanofluids at constant inlet pressure and subcooling under various flow rates and heat fluxes. Bubble images of flow boiling process were taken by high speed camera and analyzed with Labview IMAQ Vision Builder 6.1 image processing software. Parametric effects of variable heat flux, flow rate, heated length and particle volume fraction of ZnO on void fraction were investigated and compared with water and the following
Acknowledgments
The financial help given by “Board of Research in Nuclear Science (BRNS)”, Department of Atomic Energy, India (sanction no. 2009/36/95-BRNS/3234) is highly appreciated by authors. Authors are also grateful to late Prof. A.K. Rajvanshi, Malaviya National Institute of Technology Jaipur for his kind guidance.
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