Prediction of wall shear for horizontal annular air–water flow

In this study, non-intrusive pressure drop, liquid base film thickness distribution, and wave behavior measurements have been obtained for 206 horizontal annular two-phase (air–water) flow conditions in 8.8, 15.1, and 26.3 mm ID tubes. Wall shear was correlated to within 8% by a friction factor involving flow quality and gas Reynolds number. This correlation was found to perform better than those available in the literature, including film roughness correlations, two-phase multiplier methods, and pure data fits. Among published relations, the Müller-Steinhagen and Heck correlation was found to be the most accurate, while the Lockhart–Martinelli correlation can be modified to provide reasonable results. The gas friction velocity is found to be similar to the disturbance wave velocity, which suggests that waves are important sources of shear.     

Distribution of air–water annular flow in a header of a parallel flow heat exchanger

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as header mass flux, and quality are investigated, and the results are compared with previous 30 channel data. The flow at the header inlet is annular. For the downward flow configuration, water flow distribution is significantly affected by tube protrusion depth. For flush-mounted geometry, significant portion of water flows through frontal part of the header. As the protrusion depth increases, more water is forced to rear part of the header. The effect of header mass flux or quality is qualitatively the same as that of the protrusion depth. For the upward flow configuration, however, significant portion of water flows through rear part of the header. The effect of protrusion depth is the same as that of the downward flow. However, the effect of header mass flux or quality is opposite to the downward flow case. Compared with the previous 30 channel configuration, the present 10 channel configuration yields better flow distribution. Possible explanation is provided from flow visualization results.     

Capillary pressure of a continuous gas–oil–water flow in a horizontal micron capillary

Capillary pressure is one of the most important factors in characterizing the fluid behavior in a wide variety of processes with environmental and energy concerns. In this paper, Washburn equation is extended to describe the displacement kinetics of the continuous gas–oil–water flow in a single horizontal tube. Experimental investigations of the continuous two- and three-phase flows in capillaries with diameters of 2–10 μm were performed to study the effect of the capillary pressure on their fluid behavior. The results indicate that the total capillary pressure of a continuous three-phase flow is affected by the combined action of the two fluid interfaces presented: gas–liquid and liquid–liquid interfaces.     

Study of turbulent flow characteristics in a 180°-bend annular diffuser with blowoff

An investigation of turbulent flows occurring in a 180°-bend annular diffuser with an aperture in front of the bend is numerically and experimentally conducted. The blowoff mass flow rate and the inlet pressure have been systematically changed to study their influence on the diffuser performance. Results show that the pressure recovery coefficient increases with increasing blowoff mass flowrate and inlet pressure, but remains nearly constant if the inlet pressure is higher than about 10 bar. The numerical predictions are compared with the experimental data and excellent agreement is achieved. The results provide insight into the characteristics of flow in the diffuser and are useful for diffuser designs of engineering interest.     

Numerical study of a dense gas–solid jet flow

A new approach using both Eulerian and Lagrangian coordinates and taking account of inter-particle interactions has been developed for the study of gas–solid flows. A numerical algorithm is presented. Comparison with experimental results shows reasonably good agreement.     

Experimental study of the flow structure in a counter flow Ranque–Hilsch vortex tube

The mechanism of the temperature separation in a Ranque–Hilsch vortex tube has been investigated since the discovery of this phenomenon. In spite of being investigated by many researchers, no consensus has yet been reached regarding the mechanism’s hypothesis.This paper reports on a study in progress exploring the temperature separation in a counter-flow vortex tube. The effects of the geometrical parameters, including inlet nozzles, cold exit, hot exit and length of the tube, were investigated, which indicated the settings for the best performance of the vortex tube. Flow properties in the vortex tube were measured and used to understand the flow structure inside the tube. Accurate measurements of the three-dimensional velocity distribution in the tube were conducted. The results provided enough evidence that the flow in the tube consists of a forced vortex formed near the inlet gradually transforming to a free vortex at the hot end. Experimental results found in this research show the vortex transformation along the tube and support the hypothesis proposed in previous study.     

Numerical modeling of three-dimensional two-phase gas–liquid flow in the flow field plate of a PEM electrolysis cell

Numerical simulations were performed for three-dimensional two-phase water/oxygen flow in the flow field plate at the anode side of a PEM electrolysis cell. The mixture model was used to simulate two phases for the purpose of examining flow features in the flow field plate in order to effectively guide the design of electrolysis cells. The water flow rate was maintained as a constant of 260 mL/min, while the flow rate of oxygen generation was assumed to change from 0 to 14 mg/s. The obtained results including the velocity, pressure, and volume fraction distributions are presented and discussed. It is found that the obtained results for single-phase flow cases cannot be linearly extrapolated into the two-phase flow cases. The irregular velocity profile (locally low velocity magnitude near the exit port section) is not observed when the flow rate of oxygen generation is relatively low. As the mass flow rate of oxygen generation increases, reverse flow develops inside the flow channels.     

Study of the combustion efficiency of polymers using a pyrolysis–combustion flow calorimeter

The combustion efficiency of various polymeric materials was studied using a pyrolysis–combustion flow calorimeter (PCFC). Decreasing the combustion temperature in a PCFC leads to partial combustion and lower heat release rates. Combustion efficiency versus combustion temperature was modeled using a phenomenological equation and model parameters were related to the chemical structures of eight pure polymers. The flame inhibition effect was evaluated for two classical approaches in flame retardancy by plotting the combustion efficiency versus the combustion temperature. In the first one (the reactive approach), polystyrenes with different chemical groups substituted on the aromatic ring were studied. In the second one (the additive approach), three well-known flame retardants were incorporated into an ABS matrix: ammonium polyphosphate, tetrabromobisphenol A (TBBA), and a TBBA/antimony trioxide system. Results confirm the flame inhibition effect of halogenated compounds in both approaches. Finally, a correlation between peaks of heat release rate (pHRR) in a cone calorimeter and in a PCFC was attempted. Predicting pHRR in a cone calorimeter using a PCFC appears possible when no barrier effect is expected, if PCFC tests are carried out at a precise combustion temperature, for which the combustion efficiencies in both tests are the same.     

Numerical analysis and optimization of a spectrum splitting concentration photovoltaic–thermoelectric hybrid system

This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb₃ solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb₃ hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h = 3000–4500 W/m² K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb₃ hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.     

Numerical simulation of turbulent flow in a Ranque–Hilsch vortex tube

The present work is about numerical simulations of the internal flow in a commercial model of a Ranque–Hilsch vortex tube (RHVT) operating in jet impingement. Simulation of the turbulent, compressible, high swirling flow was performed by both RANS and LES techniques. The effect of different turbulence closure models have been tested in RANS simulations using a first order closure RNG k–ε and, for the first time in this kind of flow, a second order RSM (Reynolds Stress differential Model) closure. RANS computations have been executed on an axis-symmetric two-dimensional mesh and results have been compared with LES ones, obtained over a three-dimensional computational grid. Smagorinsky sub-grid scale model was used in LES. All the calculations were performed using FLUENT^? 6.3.26. The use of a common code for the different simulations allowed the comparison of the performances of the different techniques and turbulence models, avoiding the introduction of other variables.In all the simulations performed, consistency with the real commercial vortex tube model in jet impingement operation has been followed by substituting an axial hot computational exit to the usual radial one. Comparison of the results between RANS simulations performed on both a traditional radial hot outlet computational domain and one with an axial hot outlet, demonstrates the suitability of the computational model adopted in this work, closer to the real geometry of the device, particularly in RANS RSM simulations. Results in different sections of the tube show significant differences in the velocity profiles, temperature profiles and secondary vortex structures, varying turbulence model.The accurate numerical simulation of the flow in a RHVT, resulting in an improved prediction capability of the kinematic and thermal properties of outgoing jets, could allow a correct estimation of the cooling performance of this device in jet impingement operation.     

Nonsimilar solution of a boundary layer flow of a Reiner–Philippoff fluid with nonlinear thermal convection

The thermodynamics modeling of a Reiner–Philippoff-type fluid is essential because it is a complex fluid with three distinct probable modifications. This fluid model can be modified to describe a shear-thinning, Newtonian, or shear-thickening fluid under varied viscoelastic conditions. This study constructs a mathematical model that describes a boundary layer flow of a Reiner–Philippoff fluid with nonlinear radiative heat flux and temperature- and concentration-induced buoyancy force. The dynamical model follows the usual conservation laws and is reduced through a nonsimilar group of transformations. The resulting equations are solved using a spectral-based local linearization method, and the accuracy of the numerical results is validated through the grid dependence and convergence tests. Detailed analyses of the effects of specific thermophysical parameters are presented through tables and graphs. The study reveals, among other results, that the buoyancy force, solute and thermal expansion coefficients, and thermal radiation increase the overall wall drag, heat, and mass fluxes. Furthermore, the study shows that amplifying the space and temperature-dependent heat source parameters allows fluid particles to lose their cohesive force and, consequently, maximize flow and heat transfer.     

Magnetohydrodynamic free convection flow of water at 4°C through a porous medium

MHD forced and free convection flow of water at 4°C through a porous medium in the presence of a uniform transverse magnetic field is considered. A family of solutions of the coupled non-linear equations is presented using shooting numerical techniques for two point boundary value problems. Velocity and temperature profiles are shown for different values of the parameters Ha²/Re, Gr/Re² and K.     

Influence of the particle–turbulence modulation modelling in the simulation of a non-isothermal gas–solid flow

A gas–solid suspension upward flowing in a heated vertical pipe has been simulated numerically using both Eulerian–Eulerian and Eulerian–Lagrangian approaches. Particular attention has been paid to the influence of the modelling of the particle–turbulence interactions. A model based on a source-term formulation derived from a study by Crowe (Int. J. Multiphase Flow 26 (5) (2000) 719) allows predicting turbulence enhancement due to a strong particle influence in the core of the pipe flow. Calculations of suspension Nusselt numbers, characterizing the heat transfer between the pipe wall and the flow, have therefore been performed, with a satisfactory level of accuracy, compared with available experimental data. Some numerical difficulty remains however, especially due to the near-wall layer interactions which seem very difficult to simulate.     

Steady revolving flow and heat transfer of a non-Newtonian Reiner–Rivlin fluid

The steady revolving flow and heat transfer of a non-Newtonian Reiner–Rivlin fluid is studied. The momentum equation gives rise to a highly nonlinear boundary value problem. Attempt has been made to study the properties of the solution of the momentum equation analytically before proceeding for numerical solution. The effects of non-Newtonian fluid characteristic on the velocity and temperature fields have been discussed in detail and shown graphically.     

Phase-field modeling of bubble growth and flow in a Hele–Shaw cell

A phase-field model is presented for gas bubble growth and flow in a supersaturated liquid inside a Hele–Shaw cell. The flows in the gas and liquid are solved using a two-phase diffuse interface model that accounts for surface tension, interfacial mass transfer, and density and viscosity differences between the phases. This model is coupled with a phase-field equation for interface motion and a diffuse interface conservation equation for gas species transport. The model is first validated for a planar gas layer and a spherical bubble growing in a supersaturated liquid. It is shown that the results converge to the solution of the corresponding sharp interface model in the thin-interface limit. The model capabilities are demonstrated in several examples involving the growth and flow of multiple bubbles, including bubble coarsening and coalescence.     

Natural convection flow of Cu–Water nanofluid in horizontal cylindrical annuli with inner triangular cylinder using lattice Boltzmann method

In this paper the lattice Boltzmann method is used to investigate the effect of nanoparticles on natural convection heat transfer in two-dimensional horizontal annulus. The study consists of an annular-shape enclosure, which is created between a heated triangular inner cylinder and a circular outer cylinder. The inner and outer surface temperatures were set as hot (T_h) and cold temperatures (T_c), respectively and assumed to be isotherms. The effect of nanoparticle volume fraction to the enhancement of heat transfer was examined at different Rayleigh numbers. Furthermore, the effect of vertical, horizontal, and diagonal eccentricities at various locations is examined at Ra = 10⁴. The result is presented in the form of streamlines, isotherms, and local and average Nusselt number. Results show that the Nusselt number and the maximum stream functions increase by augmentation of solid volume fraction. Average Nusselt number increases when the inner cylinder moves downward, but it decreases, when the location of inner cylinder changes horizontally.     

Numerical solution of the parabolic multicomponent convection–diffusion mass transfer equations by a splitting method

The splitting method used in a previous study for the numerical solution of mass transfer equations in ternary systems is generalized to mixtures with n-components. The diffusion coefficients are considered constant. Theoretical results about the stability of the method are presented, as well as numerical simulations for mixtures with n?=?4, 5, and 6. The numerical experiments confirmed the theoretical results and show good numerical performances. Moreover, multicomponent diffusion effects without an imposed concentration gradient are investigated for mixtures with n?=?4, 5, and 6 components.     

Effect of iron on Ni–Mo–Fe composite as a low-cost bifunctional electrocatalyst for overall water splitting

By increasing demand for hydrogen and oxygen gas for energy and industrial applications, designing a cheap, high-efficiency, and bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) seems necessary. For this purpose Ni–Mo–Fe as a bifunctional electrocatalyst was synthesized by one-step electrodeposition. From this electrocatalyst with optimal composition and current density, a small overpotential of 65, 161 mV for delivering 10, 100 mA/cm² on HER in alkaline media was achieved. As-fabricated electrode exhibited 344,408 mV for delivering 10, 100 mA/cm² in OER. Furthermore, this electrocatalyst shows high stability and negligible degradation in overpotential for HER and OER under long term stability tests in alkaline media. The notable function of As-fabricated Ni–Mo–Fe is due to the synergism effect between Ni, Mo, and Fe element and binder-free structure. Owing to the high-performance and high-stability of Ni–Mo–Fe electrocatalyst under Hydrogen and Oxygen evolution reactions is a candidate for industrial uses in the alkaline electrolyzer.     

The effect of a horizontal pressure gradient on the onset of a Darcy–Bénard convection in thermal non-equilibrium conditions

In this paper there is studied the effect of a horizontal pressure gradient on the onset of Darcy–Bénard convection in a fluid-saturated porous layer heated from below, when the fluid and solid phases are not in local equilibrium. In the context of a linearized stability analysis, the problem is transformed into an eigenvalue equation. The problem, when cast in dimensionless form, contains three parameters (the pressure gradient, the porosity-scaled conductivity ratio and the scaled inter-phase heat transfer coefficient). This problem is solved numerically by using two methods: Galerkin approach and the numerical solver dsolve from Maple and comparisons between these methods are performed. Critical values of Rayleigh number, wave number and frequency are obtained for various values of the problem parameters.     

Two-dimensional Darcy–Forchheimer flow of a dusty hybrid nanofluid over a stretching sheet with viscous dissipation

The main objective of the present examination is to design a stable mathematical model of a two-phase dusty hybrid nanofluid flow over a stretching sheet with heat transfer in a porous medium, and the Darcy–Forchheimer flow is taken into account with viscous dissipation and melting effect. The equations of motion are reduced to nonlinear ordinary differential equations by considering suitable similarity variables. These dimensionless expressions are solved by a well-known numerical technique known as Runge–Kutta–Fehlberg fourth–fifth order method. The behavioral study and analysis of the velocity and thermal profile in dual phases (fluid phase and dust phase) for diverse values of parameters are estimated using graphs and tables. The result outcome reveals that the velocity gradient declines in the fluid phase and increases in the dust phase for a rise in values of the velocity interaction parameter. Also, the velocity gradients of the both phases diminish for increasing values of the porosity parameter. Furthermore, it is determined that the increase in the value of melting parameter leads to a decline in the thermal gradient of both phases.