Concentrated solar power (CSP) plants with parabolic trough technology require a solid absorber and a liquid to transfer heat. In recent years, some research works have developed new materials that can act as both a solar absorber and heat carrier. Higher efficiencies are achieved with such materials, which are called solar nanofluids. In solar nanofluids, nanoparticles absorb solar radiation. This absorption causes the temperatures of nanoparticles to increase and, therefore, the heat transfer fluid (HTF) temperature rises during an energy transfer process. Accordingly, solar to thermal conversion efficiencies up to 95% can be obtained. In this work, nanofluids based on two different thermal oils were prepared. One is composed of therminol 66 as the base fluid with diphenyl sulfone as the surfactant and carbon black nanoparticles. The second is based on silicone oil with butyl sulfone as the surfactant, and also carbon black nanoparticles. Both nanofluids were evaluated as potential solar nanofluids. For this purpose, they underwent ageing treatment, which consisted in heating up to 200 degrees C at a constant N-2 flow rate of 60 l/s for 60 h. The absorption spectrum was calculated by reflection, transmission spectrum and scattering phenomena. A spectrometer was used with an integrating sphere to measure the spectra within the 400-2000 nm range, which included infrared and visible ranges. Thermal conductivity was measured within the whole working temperature range of nanofluids and base fluids. Relevant thermal and optical measurements were taken before and after thermal ageing.

Engineers and scientist have a long tradition in trying to improve the thermophysical properties of convective heat carriers such as water and transformer oil. Technological developments of the last decades allow the dispersion of particle of sizes ranging between 10 and 100 nm in these liquids. In a large number of recent studies the resulting nanofluids have been reported to display anomalously high increase of convective heat transfer. The present study compiles experiments from five independent research teams investigating convective heat transfer in nanofluid flow in pipes, pipe with inserted twisted tape, annular counter flow heat exchanger, and coil and plate heat exchangers. The results of all these experiments unequivocally confirm that Newtonian nanofluid flow can be consistently characterized by employing Nusselt number correlations obtained for single-phase heat transfer liquids such as water when the correct thermophysical properties of the nanofluid are utilized. It is also shown that the heat transfer enhancement provided by nanofluids equals the increase in the thermal conductivity of the nanofluid as compared to the base fluid independent of the nanoparticle concentration or material. These results demonstrate that no anomalous phenomena are involved in thermal conduction and forced convection based heat transfer of nanofluids. The experiments are theoretically supported by a fundamental similarity analysis of nanoparticle motion in nanofluid flow.

The use of nanofluids (NFs) based on Solar Salt (SS) and nanoparticles (NPs), either as Thermal Energy Storage (TES) material or as Heat Transfer Fluid (HTF), is attracting great interest in recent years. Many authors [1,3] have reported important improvements on the thermophysical properties (specific heat capacity cp, thermal conductivity k) of NFs based on SS and ceramic NPs. These improvements would lead to important savings and better performance of TES facilities on new Concentrated Solar Power (CSP) plants due to lower quantities of material required and smaller storage tanks. To achieve these advantageous features in the final NFs, it is essential to avoid NP agglomeration during their preparation. Different synthesis procedures have been reported: mixing of solid NPs within a SS solution by means of ultrasounds [1-3], direct mixing of solid NPs and molten salt [4]. In this work, NFs based on SS and 1% by wt. of silica NPs were synthetized from a SS-water solution and a commercial water-silica NF called Ludox HS 30% (Sigma-Aldrich). The influence of the mixing water volume (MW) on the cp of NFs was evaluated. With this aim, the cp of these samples was measured by Differential Scanning Calorimetry (DSC) both in the solid and the liquid state. In addition, the distribution of sizes was measured during the whole preparation process by Dynamic Light Scattering (DLS). Further information about sizes and uniformity of the final NFs was obtained from Scanning Electron Microscopy (SEM) images. X-ray Diffraction (XRD) patterns of the SS and final NF were performed.

A nanofluid composed of Solar Salt (SS) and boehmite nanoparticles (A) in a concentration of 1% by weight, is proposed as thermal storage medium for Concentrated Solar Power (CSP) plants. A wide characterization of the raw materials has been done, focused on their thermal stability and the nanoparticle primary size and shape among other properties such as its specific heat and crystalline structure. Some features of the final nanofluids have been also investigated: thermal stability, nanoparticle sizes and their distribution and specific heat. The showed results confirm that these materials are thermally stable in the working temperature range both individually and combined. In addition, the synthesis procedure implemented is effective to keep the nanoparticle sizes in the nanometric range (<100 nm). These findings mean the first step to carry on research and characterization of this nanofluid.

In this paper the heat transfer performance of Al2O3, SiO2 and multi-walled CNTs (MWCNTs) in a closed loop were investigated. Heat transfer coefficient and pressure drop were measured in a horizontal thermal insulated test-section. Special care was taken in the loop calibration and the estimation of measurement uncertainties. The results show that the Gnielinski correlation can be used to predict the turbulent heat transfer coefficient as long as the proper experimental values for the thermophysical properties of each nanofluid are used. Also, the Colebrook-White correlation for the friction factor showed good agreement with the experimental results for pressure drop. The nanofluids showed an increased heat transfer coefficient with respect to that of water on a constant Reynolds number basis, but a reduced performance when compared on a constant pumping power basis. Note that some authors treat CNTs as nanofluids because one of their dimensions is in the nanoscale, but strictly speaking this dimension is not the one that is actually contributing to the changes in the thermal properties of the mixture. We include them in our analysis to demonstrate that this kind of suspension also follows conventional correlations. (C) 2015 Elsevier Ltd. All rights reserved.

Submerged gaseous jets injected into stagnant water are commonly found in many industrial processes and engineering applications, like underwater propulsion, metallurgical and chemical processes, and nuclear industry.The high air-water density ratio and the aggressiveness of the pool discharge process result in very complicated flow structures, which are essentially unsteady and turbulent. Consequently, it leads to a challenging issue to measure the different parameters involved in this process.Round turbulent air jets submerged in stagnant water have been studied experimentally in this paper. To achieve this objective a water pool with an air injector has been built and particle image velocimetry visualization techniques (PIV) have been employed to capture images of the submerged jet throughout its spreading. From these images one of the most important variables that characterizes submerged jets the velocity of the entrained droplets, was determined, finding that the function which best fits the entrained droplet velocity distribution is a decreasing exponential function. In addition, a correlation that relates the initial submerged gaseous jet properties, via the gas Reynolds number, with the entrained droplet velocities was developed, in this case via the entrained droplet Reynolds number. (C) 2016 Elsevier Inc. All rights reserved.

The integration of computer-assisted learning and simulation tools into engineering studies has become very important for the development and optimization of industrial processes. The increasing trend of this integration makes it convenient the introduction of these techniques in the early stages of the learning process. However, the use of simulation tools is usually complex as the simulation of industrial processes requires a deep understanding of the physical phenomena involved. Also, the classical methodology that is being actually used is based on a strong theoretical introduction concerning the mathematical methods that are used by the simulation programs, as well as a good background in linear programming and Computer-Aid Design. As a consequence, the learning of these tools is limited to very specific courses, usually addressed to PhD students or post-graduate code developers. This work describes the experience and methodology used in a set of courses focused on the development of students technical capabilities through the use of simulation techniques, mainly Computer Fluid Dynamics. This methodology is proposed for undergraduate and graduate levels, and with course-lengths limited by the ETCS credits. As the students come from very different fields (mechanical engineering, chemical engineering, and architecture), the examples proposed in the classes and the concepts involved were kept as general as possible so that they could serve as a tool to develop their basic engineering capabilities. Finally, we also analyze the pros and cons of using either commercial packages (like ANSYS or STAR-CD) or open-source libraries (as OpenFoam), giving some recommendations for the implementation of any of these two approaches.

Nanofluids based on water with halloysite (Hal) nanotubes were prepared and characterized in order to evaluate its suitability to be used as a heat transfer fluid. A characterization of the Hal powder nanoparticles was performed by means of SEM, TEM, WAXS, FTIR and TGA so that chemical composition, size and shape were determined. Stability of nanofluids was analyzed by means of zeta potential and light transmission measurements. Thermal conductivity, specific heat and viscosity of nanofluids prepared at different solid contents (0.5, 1, 3 and 5% volume fraction) and temperatures (40, 60 and 80°C) were obtained in order to optimize the Prandtl number. The nanofluids exhibited a good performance for its application as heat transfer fluids, with low Prandtl numbers compared to other commonly used nanofluids. High thermal conductivity enhancement with moderate viscosity and good stability results was obtained for the Hal nanofluid.

Experimental work was carried out to study the effects of temperature variation in bubbly, bubbly to slug transition. Experiments were carried out in an upward air-water flow configuration. Four sensor conductivity probes and LDA techniques was used together for the measurement of bubble parameters. The aim of this paper is to provide a bubble parameter experimental database using four-sensor conductivity probes and LDA technique for upward air-water flow at different temperatures and also show transition effect in different temperatures under the boiling point.

The spray drying process is used in many industrial applications to produce powders with different characteristics. Drying models based on the reaction engineering approach (REA) have been found promising due to its simplicity and high accuracy at different drying conditions. In this work single droplets of silica-water nanofluids were dried in an acoustic levitator under different experimental conditions of initial solid mass fraction (0.02 < YS < 0.20), pH value (2 < pH < 10), salt concentration (0 M < [NaCl] < 0.05 M), air temperature (80oC < T < 120oC), and initial droplet volume (0.3 μl < V0 < 0.8 μl). The drying curves (X=f(t)) were experimentally obtained for each test conducted and the REA model was used to model the experimental data. The grains were collected and observed by SEM in order to measure the thickness of the shell formed. Finally, the packing of particles inside the droplet was checked to be constant and equal to the random close packing. This packing can be obtained from the modelling of the viscosity data to the Quemada equation. The final diameter and the packing fraction were used to calculate the shell thickness. Experimental results from the SEM micrographs and theoretical results show a good agreement. Introduction In many industrial applications such as ceramics, food products, detergents and pharmaceuticals, the spray drying process is used to produce powders with different characteristics. The main objective of the researchers in this field is to know the effect of the variables that influence the drying process on the final properties to be able to control them and therefore to produce granules of desired characteristics suitable for each application. Depending on the final application of these grains and their later handling and processing, different microstructures can be required. That makes of great importance to know the influence of all the variables involved in the drying process on the crust formation, the packing of the particles, the hollowness of the final grain, etc. The drying behaviour of a liquid-solid suspension droplet can be divided into two stages [1-3]. In the first stage, also known as constant rate period, particles move towards the inner part of the droplet to minimize its surface energy [4, 5]. The surface energy of the liquid-vapor interface is lower than the solid-vapor one so, particles move allowing the droplet surface to be fully wetted and the liquid to evaporate at the droplet surface. As a consequence, the mass transfer rate equals that of an equally sized pure liquid droplet. The diameter, d, decreases following the d-relation and the maximum drying rate value is achieved. At the critical moisture content, the entire droplet surface cannot longer be maintained saturated by moisture migration and the second drying period begins. In this period, called falling rate period, a shell is formed at the droplet surface and the evaporation occurs through the pores of the shell. The drying rate continues to decrease as the thickness of the shell increases. If the particles have been able to move or they have had enough time to diffuse inside the droplet before the shell formation, a compact grain will be formed. However, if the particles cannot easily move due to the suspension characteristics or the drying is so fast that there is no time for the particles to diffuse, they will remain in the outer part of the droplet and the external shell will be formed before. As a result, the grains will have a hollow morphology and a higher diameter. Authors have found the drying models based on reaction engineering approach (REA) promising due to its simplicity and high accuracy at different drying conditions [6-12]. The main theory of the model considers the drying as an activation process in which an energy barrier has to be overcome for moisture removal to occur. In the mathematical expression of the model, the vapor concentration gradient is considered the driving force for the drying. The water removal process is represented by an activation energy which value is zero when the solid is fully covered by water, and increases as the moisture content decreases, due to the difficult of water removal * Corresponding author: bolivar@emc.uji.es 12 ICLASS 2012 Abbreviated Paper Title Abbreviated Paper Title Abbreviated Paper Title Abbreviated Paper Title 2 for low contents. However, the model uses a correlation for the activation energy that depends on the material and drying conditions, and has to be determined experimentally. Drying models predicting the drying kinetics of single droplets can be used to relate the final powder properties (such as the final grain diameter, mean porosity, compacity, morphology, microstructure, etc.), with the spray dryer design and process parameters. From the drying kinetics the critical moisture content can be determined and the final grain diameter can be calculated [13]. If the packing fraction of the particles inside the shell is known, the degree of hollowness can be predicted. The packing fraction of the particles in the shell is constant and closer to the maximum packing that particles can achieve, i.e. the random close packing [14, 15]. The maximum packing fraction that the nanoparticles system can achieve (RCP) corresponds to the fluidity limit, φm. Below this value the nanofluid behaves like a liquid while above the fluidity limit the nanofluid has the properties of a solid. The graphical representation of viscosity versus the volume fraction allows obtaining the fluidity limit, concentration at which the viscosity takes infinite value. Krieger and Dougherty, and Quemada [16, 17] proposed equations to model the viscosity of numerous suspensions with particles different in nature, that allows the knowledge of the fluidity limit, and hence the random close packing, for each particulate system. In this work single droplets of silica-water nanofluids were dried in an acoustic levitator under different experimental conditions of initial solid mass fraction, YS, pH value, salt concentration, [NaCl], air temperature, T, and initial droplet volume, V0. The drying curves (X=f(t)) were experimentally obtained for each test conducted and the REA model was used to model the experimental data. The fluidity limit and the maximum packing fraction were obtained for this system using the Quemada equation to model the viscosity data. The packing fraction and the final diameter were used to calculate the shell thickness. The dried grains were collected to observe the internal microstructure by means of scattering electron microscopy and the shell thickness was measured and compared to that previously calculated. Materials and Experimental Methods All the experiments performed in this study were carried out with silica-water nanofluids. Commercial fumed silica (Aerosil 200 with primary units of 12 nm) provided by Degussa was purchased in dry powder. The isoelectric point (IEP) was established at pH = 2 from the measurement of the zeta potential at different pH values using a Zetasizer Nano ZS (Malvern Instruments). Nanofluids with different particle concentrations were prepared by adding distilled water containing different concentration of NaCl salt to the defined amounts of nanoparticles. The dispersion was performed using an ultrasonic probe (UP400s from Hielscher Company). Initially, the mixture of nanoparticles with the water or salt solution is submitted to a 3-minute sonication treatment. After this, the pH of the nanofluid is modified adding HCl or NaOH solutions. Finally, to ensure a correct dispersion of all the components, the nanofluids are submitted to a second sonication treatment during 2 minutes. The viscosity of the nanofluids was obtained at 25oC by conducting tests under steady state conditions using a Bohlin CVO-120 rheometer (Malvern Instruments Ltd., UK). A double gap (DG 40/50) device composed of two concentric cylinders suitable for low viscosity suspensions was used. Before each test, a pre-treatment, in which the samples are submitted to a constant shear stress, was applied to the nanofluids for 30 seconds to ensure similar starting conditions for all of them. Single droplets were dried in an ultrasonic levitator modified in order to work at high temperature conditions. The experimental set-up was described elsewhere [18]. The entire system is composed of an ultrasonic levitator in which the chamber that contains the droplet is heated by electric heaters. This chamber is isolated from the one that contains the piezoelectric transducer which is cooled by forced convection to ensure temperatures lower than 60 oC and avoid damages in the transducer. An optical system consisting of a CMOS camera and back-light illumination system are used to record and measure the droplet cross-sectional area and vertical position of the droplet during the drying process. Finally, a gas conditioning system is used to control the temperature, flow rate and relative humidity of the air inside the levitator tube. The images of the drying process recorded with the camera were processed with Matlab. Thus, the equivalent diameter and position of the droplet during all the drying stages were obtained. The instantaneous mass transfer rate for the first drying period (mL,1) was calculated from the decrease of the droplet diameter. This mass transfer rate is defined as, ICLASS 2012, 12 Triennial International Conference on Liquid Atomization and Spray Systems, Heidelberg, Germany, September 2‐6, 2012 3 2 1 3 2 3 1 1 , 6 t t d d m L L − − − = ρ π (1) where ρL is the liquid density and the sub-indexes 1 and 2 denote two consecutive instants during the drying process. The mass transfer rate of the second drying period (mL,2) is calculated using the position of the droplet inside the acoustic field. If this position is assumed to be dependent on the droplet density, the mass transfer rate during this period can be calculated as, 21 22 21 21 21 2 , ) ( y

Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt 100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt 1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 10(3)Re<10(5)). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.

Water based nanofluids of SiO2 and Al2O3 were characterized regarding its later use in heat transfer applications. Well dispersed nanofluids were prepared at three different volume fractions (0.5% v., 1% v., 5% v.) and for each one the experimental measurements were carried out at three different temperatures (40 degrees C, 60 degrees C, 80 degrees C). Commercial nanofluids acquired in liquid state were compared with nanofluids prepared by dispersing dry powder in water. The thermal conductivity, the viscosity and the specific heat of all the prepared nanofluids were measured in order to calculate the Mouromtseff number and establish the optimal conditions. Finally, the stability of the nanofluids was also studied through the evolution of the amount of light scattered by the sample during a period of time.

The flow structure of gas–liquid two-phase flow in vertical annulus channel has been investigated. The inner and outer diameters of the annular channel were 19.1 and 38.1mm, respectively. The total height of the test section was 4.37m. Nineteen inlet flow conditions were selected, which cover bubbly, cap-slug, and churn-turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration (IAC), and bubble interface velocity, were measured at nine radial positions within the gap of the annulus at z/Dh=230 of the test section. The flow regimes of the flow conditions, which were based on visual observations, were compared with several flow regime maps. In addition, the local measurements were used to calculate distribution parameter, C0 in drift-flux model, and area-averaged IAC. A new correlation of C0 was proposed based on the experimentally obtained C0 values. This correlation was tested in the drift-flux model successfully along with Ishii's drift velocity correlations. The area-averaged IAC values were compared with the most widely used models. The advantages and drawbacks of these models were highlighted.

An experimental study on the interfacial area transport (IAT) of vertical, upward, air–water two-phase flows in an annulus channel has been conducted. The inner and outer diameters of the annular channel were 19.1mm and 38.1mm, respectively. Nineteen inlet flow conditions were selected, which cover bubbly, cap–slug, and churn–turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration (IAC), and bubble interface velocity, were measured at nine radial positions for the three axial locations (z/DH=52, 149 and 230). The radial and axial evolutions of local flow structure were interpreted in terms of bubble coalescence and breakup. The measured data can be used for the development of the bubble coalescence/breakup models for the IAT model and some closure models for computational fluid dynamics.

An upward isothermal co-current air-water flow in a vertical pipe (52mm inner diameter) has been experimental investigated. Local measurements of void fraction, interfacial area concentration (IAC), interfacial velocity, and Sauter mean diameter were made using a double-sensor conductivity probe. Liquid velocity and turbulence intensity were measured using laser Doppler anemometry (LDA). Different air-water flow configurations was investigated for a liquid flow rate ranging from 0.491 to 0.981m/s and a void fraction up to 15%. For each two-phase flow configuration 25 radial positions and 3 axial locations were measured by the conductivity probe methodology, and several radial profiles were measured with LDA at different axial positions.

The planar laser-induced exciplex fluorescence (PLIEF) technique has been used to visualize and measure the fuel/air concentration fields in both liquid and vapour phases of direct injection (DI) diesel sprays. Experiments have been performed in a hot rig, which provides a thermodynamic environment realistic for current DI diesel engines, using a blend of 90 per cent hexadecane, 9 per cent α-methyl-naphthalene, and 1 per cent N,N,N',N'-tetramethyl- p-phenylenediamine to separate spectrally the fluorescence signal from the liquid and vapour phases of the diesel. In this paper, measurements have been performed for three injection pressures and three combustion chamber density values. The results on penetration and fuel concentration in liquid and vapour phases are analysed, providing evidence of the similarities between diesel sprays and gaseous turbulent jets.

On-line identification of flow regimes is important in two-phase flow because hydrodynamics and adequate operation of multiphase systems are highly dependent on the flow pattern. This work describes the application of an artificial neural network (ANN) to process the signals measured by a conductivity probe and classify them into their corresponding flow regimes. Experiments were performed in an adiabatic air–water upward two-phase flow rig. Some statistical parameters of the cumulative probability density functions (CPDF) of the bubble chord length were used as the inputs to the ANN. Different ANN configurations were evaluated to optimize the characteristics that best suit the specific ANN application. The results demonstrate good agreement with the visual flow map identification, even for reduced temporal conductivity signals.

The Planar Laser Induced Exciplex Fluorescence (PLIEF) technique is widely used to visualize and measure the fuel concentration fields in both liquid and vapor phases of DI Diesel sprays. However, the real limitations of the PLIEF technique in Diesel sprays and the accuracy of the results obtained are still a source of controversy. In this work, a complete methodology for maximum penetration and fuel concentration measurements in evaporating conditions in Diesel sprays has been developed and the reliability of the results obtained has been investigated. The methodology includes new procedures for measuring both liquid and vapor phases, adapting, when necessary, correlations available in the literature for calibration.

The accuracy of the measurement of the void fraction in bubbly flows using an optical probe is investigated. Experiments were performed in tap water with ellipsoidal-shaped air bubbles with equivalent diameters and velocities in the range of 2.8–5.2 mm and 0.22–0.28 m∕s. Comparison of charge coupled devices (CCD) images of dynamic bubble piercing events with optical probe signals shows that for piercing in the area around the bubble side, the so-called low-level criterion gives the best agreement with the actual gas-liquid transition for the undisturbed bubble. In addition, residence time underestimation due to a partial blinding effect is observed in the outer regions of the bubble. Residence times of the probe inside the bubble are obtained from the probe signal and from CCD images of the undisturbed bubble. These are compared to study the relevance of various probe-bubble interaction effects. The crawling effect is found to play an important role. For perpendicular piercing, the experiment shows that in the central area of the bubble deceleration effects induced by the probe lead to local overestimation of residence times. In the outer region of the bubble, large-scale deformation leads to local underestimation of residence times. The larger cross-sectional area associated with the underestimation leads to a net underestimation of the total bubble volume. For nonperpendicular piercing, the probe inclination is found to generate an additional drifting effect, creating an additional source of underestimation.