A simplified model of gas–liquid two‐phase flow pattern transition

An experiment of upward gas–liquid two‐phase flow was conducted in an air–water isothermal system under atmospheric pressure. The differential pressure was measured at the fully developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high‐speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one‐dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas‐component advection based on the spatiotemporal void fraction behaviors by considering the gas compressibility, the wake, and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple‐like, rectangular, distorted rectangular, and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns corresponded well to the experimentally observed flow patterns. The transitions among void wave patterns agree well with the Mishima–Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 445–461, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20029     

Response compensation of fine‐wire thermocouples and its application to multidimensional measurement of a fluctuating temperature field

A novel sensor technique is proposed to visualize readily the spatiotemporal behavior of a fluctuating temperature field of air flow. Two temperature probes were fabricated: a rod‐type probe consisting of 24 two‐thermocouple sensors set in line, and a plane‐type probe with 64 two‐thermocouple sensors arrayed on a two‐dimensional grid (8 × 8 points). Each two‐thermocouple sensor used in these probes was composed of two fine‐wire thermocouples (type K) of unequal diameters of 25 and 51 μm, respectively. An adaptive response‐compensation scheme was applied to accurately reconstruct rapidly‐changing airflow temperatures. The plane‐type probe enables visualization of fluctuating temperature fields of an artificially disturbed hot‐air jet and of the adequate capture of spatiotemporal behavior of a rapid circular motion of a hot‐air jet blown out from a hair dryer. A time‐constant estimation scheme was proposed to estimate instantaneous time‐constants which serve as a basis for a real‐time response compensation technique for multidimensional temperature measurement. In addition, by scanning a temperature field with the rod‐type probe, the temperature distribution can be reconstructed in one‐dimensional space and time. This quasi two‐dimensional visualization can become a prototype of a “scanner” for fluid temperature fields. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20355     

Low‐cost dye‐sensitized solar cells with ball‐milled tellurium‐doped graphene as counter electrodes and a natural sensitizer dye

This paper presents a facile and economic development of dye‐sensitized solar cells using a nonprecious counter electrode made from ball‐milled tellurium‐doped graphene (Te‐Gr) and a natural sensitizer extracted from Calotropis gigantea leaves. The prepared materials were characterized using various techniques, such as Raman spectroscopy, X‐ray diffraction (XRD), atomic force microscopy (AFM), impedance spectroscopy, and scanning electron microscopy with built‐in energy‐dispersive X‐ray spectroscopy (SEM with EDS). The electrochemical activity of the produced counter electrodes and the impedance of the fabricated cells were examined and discussed to devise plans for future enhancement of cell performance. A clear pattern of improvement was found when using cost‐effective Te‐Gr relative to the costly platinum counter electrodes, especially when compared with cells employing another natural sensitizer. The results show approximately 51% enhancement over chlorophyll‐based cells made from spinach, where the added advantage in our approach is the utilization of an abundant plant extract with little nutritional appeal.     

All ambient environment‐based perovskite film fabrication for photovoltaic applications

Low‐temperature solution process‐able perovskite solar cells are highly desirable for future photovoltaics. Chemical root was utilized to synthesize and optimize mixed halide‐based MAPbIBr₂ light absorber perovskites on electron transport layer of TiO₂ nanoparticles in ambient atmosphere. For the first time all synthesis work was performed in an ambient environment and observe material behavioral characteristics. To accurately control the film morphology, one‐step deposition technique was applied to investigate material's optoelectronic behavior. The role of the perovskite structure, physical, and optical properties in planner device architecture was studied through ultraviolet visible, X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscope characterization techniques to confirm a band gap of 1.76 eV with cubic crystalline structure having a particle size of 12.5–13.0 nm, which is highly suitable for perovskite solar cells.     

A model for entropy generation in stagnation‐point flow of non‐Newtonian Jeffrey,Maxwell, and Oldroyd‐B nanofluids

We present a generalized model to describe the flow of three non‐Newtonian nanofluids, namely, Jeffrey, Maxwell, and Oldroyd‐B nanofluids. Using this model, we study entropy generation and heat transfer in laminar nanofluid boundary‐layer stagnation‐point flow. The flow is subject to an external magnetic field. The conventional energy equation is modified by the incorporation of nanoparticle Brownian motion and thermophoresis effects. A hydrodynamic slip velocity is used in the initial condition as a component of the stretching velocity. The system of nonlinear equations is solved numerically using three different methods, a spectral relaxation method, spectral quasilinearization method, and the spectral local linearization method, first to determine the most accurate of these methods, and second as a measure to validate the numerical simulations. The residual errors for each method are presented. The numerical results show that the spectral relaxation method is the most accurate of the three methods, and this method is used subsequently to solve the transport equations and thus to determine the empirical impact of the physical parameters on the fluid properties and entropy generation.     

Fabrication of novel hybrid composite La2‐xCoxCuO4 electrode for high performance supercapacitors

Hybrid composites La_2‐xCo_xCuO₄ (x = 0, 0.1, 0.2, and 0.3) are prepared using one‐step simple hydrothermal route as electrodes for supercapacitors. The effect of varying cobalt content on morphological, structural, and electrochemical properties has been explored using X‐ray diffraction, scanning electron microscopy, and cyclic voltammetry, respectively. The structural parameters obtained by X‐ray diffraction showed tetragonal phase of hybrid composite without any evident impurity phases. The analysis of morphological properties suggested a strong correlation with electrochemical properties, for instance, a relationship between fabric porous structures and electrochemically active sites for redox reactions and intercalation/de‐intercalation processes. The hybrid composite electrodes demonstrated high specific capacitance of the order of 1304 F/g at 10 mV/s scan rate and exhibited decreasing trend on increasing scan rate. Hybrid composites were also tested for their ability as an electrode of high performance supercapacitors in different aqueous electrolytes, i. e, KOH, H₂SO₄, and Na₂SO₄ to optimize the best compatible electrolyte. The composite electrode material showed excellent cyclic stability and 98% capacitance retention for 1 A/g after 2000 cycles. The remarkable performance of hybrid composite electrode entails its potential for commercial applications of supercapacitors.     

Performance enhancement of direct methanol fuel cell using multi‐zone narrow flow fields

New serpentine and spiral flow field configurations were developed to enhance the performance of direct methanol fuel cells (DMFCs). The new configurations are based on two primary concepts, namely, narrowing the flow field and partitioning the total active area of the fuel cell. Three flow channel heights of 0.8, 0.4, and 0.2 mm were investigated in serpentine and spiral flow fields. The main active area is considered a single zone and is partitioned into two‐ and four‐zone designs while maintaining the total inlet mass flow rate of the reactant and oxidant. To determine the performance parameters of the newly proposed designs, a three‐dimensional single‐phase isothermal model was developed, numerically simulated, and validated through experimental measurements. The findings of the current study indicate that a serpentine flow field configuration with a channel height of 0.2 mm and two zones attains an enhancement of the net power density of 37% compared to a conventional single‐zone design with a flow channel height of 0.8 mm. Similarly, for a spiral flow field design, the maximum net power density increased by 26% using a two‐zone configuration with a channel height of 0.2 mm, in comparison to the conventional design of a single‐zone and a flow channel height of 0.8 mm. The newly developed designs utilize the lower height of the flow fields to decrease the dimensions of the fuel cell stacks and reduce the material costs required.     

Study of the behavior of ephemeral waves in gas–liquid two‐phase flow: I. Determination of sub‐wave‐veins

Time‐spatial measurements of liquid holdup distributions along the axis of a tube were carried out over the length of 1325 mm in upward gas–liquid two‐phase flow. In order to clarify the characteristics of the behavior of ephemeral waves, a method of determining sub‐wave‐veins, that is, the traces of ephemeral waves on the time‐spatial behavior charts of the interface, was developed. This method was applied to the flow conditions in huge wave flow and annular flow regimes, and the sub‐wave‐veins in these flow regimes were successfully determined. Time‐spatial behavior charts of the interface with determined sub‐wave‐veins were systematically presented and the characteristics of sub‐wave‐veins were discussed. Close inspection of the behavior of sub‐wave‐veins reveals that there are two types of ephemeral waves: one has a shorter life span and the other has a longer life span during which absorption and discharge of small ephemeral waves occurs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 114–125, 2001     

Numerical study of two‐phase flow of liquid helium in a vertical converging‐diverging nozzle

The fundamental characteristics of the two‐dimensional gas‐liquid two‐phase flow of liquid helium through a vertical converging‐diverging duct near the lambda point are numerically investigated to realize the further development and high performance of new multiphase superfluid cooling systems. First, the governing equations of the two‐phase flow of liquid helium based on the unsteady thermal nonequilibrium multifluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two‐dimensional structure of the gas‐liquid two‐phase flow of liquid helium though vertical converging‐diverging nozzle is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid‐ to vapor‐phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 432–448, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20071     

Comparing rotor plane induction determined from full‐scale measurements and CFD simulations

This paper investigates the flow field in the rotor plane of a full‐scale operating wind turbine using full‐scale light detection and ranging (LiDAR) measurements for the first time. Comparison of the measured flow field with results from large eddy simulations (LES) combined with an actuator line approach is also presented for in‐depth study of the induction field in the rotor plane. The measurements include data from two synchronized LiDAR systems—one scanning the undisturbed upstream inflow field and one measuring in the rotor plane. The standard deviation of the mean of velocity time series are and presented as a measure of reliability. The method for calculating the axial velocity based on the line‐of‐sight velocity is explained and the uncertainty of such method is presented. The process of calculating the yaw misalignment is described. The time‐averaged and phase‐averaged axial velocity and induction factors are presented relative to radius and azimuth, and the general behavior is described relative to the flow regimes around the blades, tower and nacelle. Simulations and measurements are compared with special emphasis on the flow structures in the vicinity of the individual rotor blades. A convincing agreement between measurements and simulations is demonstrated. The uncertainties originated from the imprecise positions and angles of the measurement instruments are shown. The uncertainties are limited to the middle parts of the blades between 15 m to 25 m from the root. In addition, longer selected time series show smaller uncertainties. This proves the reliability of the application of the methodology for even longer time series.     

Design considerations of jet pumps with supersonic two‐phase flow and shocks for refrigeration and thermal management applications

This paper describes results of a design‐oriented model for two‐phase jet pumps and ejectors for refrigeration and thermal management aerospace and terrestrial applications. The primary motivation for the work is the development of a reliable but simple design methodology that captures the important flow physics while being sufficiently fast in order to reduce the design cycle time. This work is particularly relevant to the flow boiling test facility under development at the NASA Marshall Space Flight Center as it allows rapid design optimization of the jet pump as well as the integrated system. The results presented in the paper show optimal geometric area ratio as well as system state point information as a function of the inlet states and entrainment ratio. Qualitative agreement with single‐phase ejector performance is predicted, lending additional confidence in the results. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical computation on Marangoni convective flow of two‐phase MHD dusty nanofluids under Brownian motion and thermophoresis effects

The current theoretical study describes the Marangoni thermal convective flow of magnetohydrodynamic dusty nanofluids along a wavy vertical surface. The two‐phase mathematical model is developed under the influence of thermal radiation and exponentially varying space‐dependent heat source. Pure and hybrid nanoparticles together with dust particle suspension in the base fluid are taken into consideration to characterize the behavior of the flow. Brownian motion and thermophoresis mechanisms are considered, since it enhances the convection features of dusty nanofluid. Appropriate transformations are adopted to modify the flow governing equations and boundary conditions to dimensionless form. The forward finite difference scheme is implemented to illustrate the resultant coupled partial differential equations. The Newton quasi‐linearization technique is utilized to reduce the nonlinear system into a linear form, which is solved thereafter by Thomas algorithm. The responses of velocity, temperature, concentration, friction factor, and heat and mass transfer rate profiles with various governing parameters are discussed and portrayed graphically. The study evidences that the radiation and space‐dependent heat generating parameters strengthen the temperature distribution. Also, the heat transfer rate appreciably rises with the increment in Marangoni convection. The solution methodology and accuracy of the model is validated by generating the earlier outcomes for nonradiating nanofluid flow without heat source/sink.     

Magnetized squeezed flow of time‐dependent Prandtl‐Eyring fluid past a sensor surface

The present numerical investigation describes the influence of a transverse magnetic field on the heat and mass transfer characteristics of time‐dependent squeezed flow of Prandtl‐Eyring fluid past a horizontal sensor surface. The current physical problem is modeled based on the considered flow configuration. Also, the present problem is analyzed under the influence of Lorentz forces, to explore the impact of a magnetic field on the flow behaviour. The considered physical problem in the present study gives highly nonlinear coupled time‐dependent, two‐dimensional partial differential equations. The governing flow equations are reduced to the system of nonlinear ordinary differential equations by imposing the suitable similarity transformations on the laws of motion. Due to the inadequacy in the analytical methods, the present problem is solved by using the Runge‐Kutta fourth order integration scheme with shooting method. The flow and heat transfer behaviour of various control parameters are studied and presented in terms of graphs and tables. From the current investigation it is noticed that, the increasing magnetic parameter enhances the velocity field and diminishes the temperature profile in the flow region. Also, the magnifying permeable velocity parameter decreases the temperature field. The present similarity solutions are found to be in good agreement with previously published results.     

Flow and Heat Transfer Characteristics of Two‐Phase Fluid in Inclined Pipes on a Rotation Platform

A rotating platform was used to create dynamic load, and the mixture air–water two‐phase flow and boiling steam–water two‐phase flow were obtained in an inclined test pipe. By changing the parameters, such as inclination of the test pipe, rotational speed, inlet temperature, flow rate, and so on, the experiments for two‐phase flow in the pipe at inclination of 0°, 45°, and 66° were conducted, respectively. The effects of acceleration and inclination on their flow and heat transfer characteristics were investigated. The two‐phase flow patterns in inclined pipes under rotation conditions were caught with a video camera. The images show that the impact mixed flow and churn flow were found in this research. The results show that the acceleration and pipe inclination significantly influence the flow characteristic and heat transfer of the two‐phase pipe flow. As the directions of the dynamic load and the gravity are opposite to the flow direction, the greater the dynamic load and inclination, the higher the pressure drop and the heat emission, and the lower the flow rate, the void fraction, and the fluid temperature. Therefore, the dynamic load and gravity will improve the flow resistance, enhance heat emission and reduce the heat gained by the fluid.     

Numerical simulation of steam–water two‐phase flow under dynamic load

A separated‐phase physical model for steam–water two‐phase flow on a rotating platform was developed. The mesh generation for a horizontal pipe was conducted, and the finite volume method was used to discretize the equations. Equations were solved with the SIMPLE algorithm after setting the initial and boundary conditions. Predicted results were compared with experimental data, and they agreed well with each other. The results showed that the fluid outlet pressure and pressure drop in the test section increased with increasing dynamic load. However, the effective heat transferred to the fluid decreased with the increase of dynamic load. The developed model can be used to simulate the gas–liquid two‐phase flow under different gravity or rotary conditions.     

The behavior of chemical and electrochemical Ag deposition on FeNi‐mesh cathodes in Al‐air battery

In this study, FeNi/Ag cathode was made for aluminium air battery. For this purpose, FeNi‐mesh electrode surfaces were treated in two different ways (chemical and electrochemical deposition), with noble metal silver having a high catalytic activity. The optimum time for both methods was determined. The electrochemical properties were determined using cyclic voltammogram (CV), electrochemical impedance spectroscopy (EIS), and current‐potential (i‐t) curves. The battery performance tests were performed, and surface characteristics of the electrodes were determined with a scanning electron microscope an X‐ray diffraction method (SEM‐EDS and XRD). This study shows that FeNi mesh electrodes chemically and electrochemically deposited with silver are alternative for oxygen‐reducing cathodes for aluminium air battery because they are cheap, practical, and effective. The electrochemical (FeNi/Ag‐ED) deposition is better than the chemical (FeNi/Ag‐CD) deposition in terms of both economical and higher battery potential.     

Synthesis of templated carbons starting from clay and clay‐derived zeolites for hydrogen storage applications

Clay and its recrystallized zeolitic derivatives were used in this study as templating agents for carbon nanostructured materials. The conventional nanocasting process that involves impregnation with furfural alcohol and subsequent chemical vapour deposition was followed. Several techniques such as X‐ray diffraction (XRD), scanning electron microscopy (SEM), thermo‐gravimetric analysis (TGA) and surface area analysis were used to characterize the parent templating materials including the resulting nanocasted carbons. The study demonstrated that there is greater potential for the use of value‐added clays rather than their pristine form and hence presents a cost‐effective alternative for producing carbonaceous materials with more attractive properties for hydrogen storage applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Compact boiling refrigerant‐type cooling unit using multi‐stacked radiator cores for automobiles

We have developed a new closed two‐phase loop thermosyphon with multi‐stacked radiator cores and a new flow controller which forms refrigerant circulation. The features are: (1) more compact and lighter weight than heat pipe cooling units, (2) cooling performance can be easily adjusted by the number of radiator cores according to heat dissipation quantity. © 2000 Scripta Technica, Heat Trans Asian Res, 29(8): 634–647, 2000     

Heat transfer prediction of air‐to‐refrigerant two‐phase flow boiling of alternatives to HCFC‐22 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of some alternative refrigerants to HCFC‐22, on air/refrigerant horizontal enhanced surface tubing, is presented. Correlations have been proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R‐22; such as R‐507, R‐404A, R‐407C, R‐410A and R‐408A in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was found that the correlations were applicable to the entire heat and mass flux, investigated in the present study, for the proposed blends under question. The deviation between the experimental and predicted values for the heat transfer coefficient and pressure drop were less than ±20, and ±35 per cent, respectively, for the majority of data. Copyright © 2000 John Wiley & Sons, Ltd.     

Pt/graphite catalyst for hydrogen generation by HI decomposition reaction in S–I thermochemical cycle

Hydrogen is an attractive energy carrier for future because of various reasons. Therefore its large scale production is the need of the hour. One of the ways to achieve this is sulfur iodine thermochemical cycle and HI decomposition reaction is one of the three reactions constituting the cycle. Pt/graphite catalysts with different loading of platinum were prepared by impregnating colloidal graphite with hexachloroplatinic acid solution followed by reduction under N₂ flow. The catalysts prepared have been characterized by X‐ray diffraction, Raman, scanning electron microscopy, X‐ray photoelectron spectroscopy and Brunauer–Emmett–Teller surface area. These catalysts have been employed for liquid phase HI decomposition under different conditions. To evaluate the stability of this catalyst against noble metal leaching under the reaction conditions, the eluent was analyzed by using ICP‐OES. Platinum loaded catalysts (0.5%, 1% and 2%) show 8.4%, 17.5% and 23.4% conversion respectively. From the present study we conclude that Pt/graphite is a suitable and stable catalyst for liquid phase HI decomposition reaction. Copyright © 2015 John Wiley & Sons, Ltd.