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In this work, a mean-line model with the row-by-row stacking scheme is developed to simulate the off-design operation of a multi-stage transonic axial compressor of an industrial gas turbine. A database of computational fluid dynamics (CFD) simulation results validated with the field data is generated at different rotating speeds and pressure ratios by using CFD simulation tools and available detail geometry of the studied compressor. The compressor characteristic parameters including inlet/exit blockage factor, deviation angle parameter, rotor temperature and pressure coefficients, and the pressure loss of the stator for all stages, plus variable inlet guide vane, are extracted from CFD simulation results. Based on analytical/empirical relations in the literature, a generic formulation for the characteristic parameters is defined as a function of airflow parameters such as inlet Mach number, inlet flow angle, and velocity ratio. Since the accuracy of this type of model depends on the formulated characteristic parameters, and due to the error propagation potential of the sequential stacking solutions, a multi-objective optimization program is developed to specify the coefficients and exponent values of the generic formulations. The functionality of characteristic curves versus airflow parameters for different compressor rows is comprehensively investigated. The comparison between the results of the mean-line model and CFD simulations shows the high accuracy of the model. The developed model can be used in applications such as condition monitoring of in-service gas turbines, studying the effects of rescheduling the variable stator vanes, and investigating the wet compression process.

Supersonic turbulent high-pressure jet flows, which are discharging in low-pressure quiescent ambient, are recognized as imperfectly expanded turbulent jet. Steady-state imperfectly expanded jet flow has been already studied analytically; however, the transient flow has not been thoroughly studied. In the present study, the transient imperfectly expanded jet flow with focus on fuel spray in combustion is investigated analytically employing two-step separation of variables method and Fourier-Bessel expansion. The results are validated using available experimental data. The effects of different parameters such as eddy viscosity and pressure ratio on the behavior of the jet are studied. Results show that increasing the eddy viscosity decreases the velocity magnitude and required time to reach fully developed jet. Increasing the pressure ratio almost linearly increases the required time to reach steady state. The density distribution which affects the combustion performance is reported for different axial and radial positions. In the transient region, tip penetration is obtained and validated with the experimental results in the literature, and the velocity profile at different times is presented. The simplicity and accuracy are key advantages of the developed method compared with the experimental and numerical methods. The analytical method proposed in the present research helps to understand the behavior of jet flows from transient to steady state condition without using expensive and time-consuming numerical or experimental techniques.

Nanoparticle halo mechanism is a stabilization method for microparticle suspensions. This study investigates suspension pH and nanoparticles–microparticles collision effects on the stabilization of...

We use an efficient molecular theory approach to study electrokinetic flow within a pH-responsive nanopore grafted with a polyelectrolyte (PE) brush. The flow rate, migration and convective conductance, electric potential and velocity fields, species distributions and the degree of ionization of the weak PE functional groups and nanopore selectivity are obtained and interpreted while considering pH-induced surface charges. The theory is generally based on writing the overall free energy of the system including the entropies arising from the conformations of flexible, excluded volume chains, the mixing of mobile species, electrostatic contribution, and the free energy due to the chemical acid–base equilibrium reactions. We demonstrate how, by controlling the bulk salt concentration, pH, surface grafting density, and PE drag coefficient, the flow inside the pore can be controlled. Generally, the flow rate gets enhanced upon decreasing pH, but the effect of salt concentration is more complex. As long as the pH is small (large), the flow rate decreases (increases) by increasing the salt concentration, while a nonmonotonic trend is evident at moderate pH values. We find that, when the PE drag coefficient is high (low), the flow rate decreases (increases) by increasing surface grafting density. For intermediate drag coefficients, the flow rate varies nonmonotonically with surface grafting density. It is observed that the convective ionic conductance obeys almost the same trend as the flow rate. It is also illustrated that the mean degree of ionization of the polymer chains and the migration ionic conductance enhance on increasing the background salt concentration, whereas the opposite is true for nanopore selectivity. However, when very low salt concentration is accompanied by a high pH value, there is a minimum in the nanopore selectivity. The present approach allows investigation of the application of PE-coated nanopores as smart nanovalves. © 2020 American Chemical Society, The Journal of Physical Chemistry C, 124 (34), ISSN:1932-7455, ISSN:1932-7447

Electrophoretic motion of hydrophobic particles has been scrutinized numerically in solid-state nanopores. The Poisson, Stokes, and Nernst–Planck equations are solved simultaneously, and the Newton–Raphson algorithm is used to compute the correct velocity at each point. For the hydrophobic surface characterization, the Navier-slip boundary condition with a wide range of slip lengths is applied to the nanoparticle's surface. The effects of the electric field intensity, the electrolyte concentration, and the particle's size on the electrophoretic velocity are examined. Then, the nanopore's size and surface charge density are manipulated to achieve the configuration for separating hydrophobic and hydrophilic particles based on their slip lengths. The results show that the hydrophobic and hydrophilic particles, under particular circumstances, would move in the opposite direction in a nanopore. Finally, the resistive pulses of the particles with various slip lengths are studied. The resistive pulse properties of the hydrophobic and the hydrophilic particles are completely distinguishable and show potential application for resistive pulse sensing as a tool for reckoning the particle's slip length.

This paper extends the analysis of solute dispersion in electrohydrodynamic flows to the case of band broadening in polyelectrolyte-grafted (soft) capillaries by accounting for the effects of ion partitioning, irreversible catalytic reaction and pulsatile flow actuation. In the Debye–Hückel limit, we present the benchmark solutions of electric potential and velocity distribution pertinent to steady and oscillatory mixed electroosmotic–pressure-driven flows in soft capillaries. Afterwards, the mathematical models of band broadening based on the Taylor–Aris theory and generalized dispersion method are presented to investigate the late-time asymptotic state and all-time evolution of hydrodynamic dispersion, respectively. Also, to determine the heterogeneous dispersion behaviour of solute through all spatiotemporal stages and to relax the constraint of small zeta potentials, a full-scale numerical simulation of time-dependent solute transport in soft capillaries is presented by employing the second-order-accurate finite difference method. Then, by inspecting the dispersion of passive tracer particles in Poiseuille flows, we examine the accuracy of two analytical approaches against the simulation results of a custom-built numerical algorithm. Our findings from hydrodynamic dispersion in Poiseuille flows reveal that, compared to rigid capillaries, more time is required to approach the longitudinal normality and transverse uniformity of injected solute in soft capillaries. For the case of dispersion in mixed electrohydrodynamic flows, it is found that the characteristics of the soft interface, including the thickness, permittivity, fixed charge density and friction coefficient of the polymer coating layer, play a significant role in determining the Taylor diffusion coefficient, advection speed and dispersion rate of solutes in soft capillaries.

In this paper, a transient model is developed to simulate the start-up operation of an industrial twin-shaft gas turbine based on real geometry and control logic. The components’ characteristic curves of the studied gas turbine were generated with the aid of CFD simulation tools and according to the real geometry of each component. The control logic of the studied gas turbine was simplified and coupled with the developed engine performance model. Also, the gas turbine actuators including variable inlet guide vanes, compressor bleed valves, and fuel valves were simulated so that their concurrent effects on the gas turbine transient behavior can be captured precisely. Moreover, the air-cooled compressor turbine was modeled with a new approach. The integrated model was set up and then tuned with the field data in the base load condition. Next, the start-up operation of the gas turbine from zero speed to the base load condition was simulated and validated with the field data. The results show good agreement between the model outputs and field data. Regarding the model outputs, it can be used as the base code for a more comprehensive investigation of flexibility, operability, and the life concerns of this type of gas turbines.

Pulsating heat pipes (PHPs) are one of the new devices used for cooling in several applications such as electronic and aerospace systems. Their low cost, effectiveness at various conditions, being equipped for passive energy conversion, and well distribution of temperature compared to conventional heat pipes are among the reasons of their popularity. To investigate the effect of surface tension of the working fluid on the behavior of PHPs, a copper heat pipe is fabricated with inner and outer diameters of 2 mm and 4 mm, respectively. Five different concentrations of cetrimonium bromide (C-Tab) surfactant are dissolved in water and are tested with a filling ratio of 50% (± 1%). A piece of glass is placed in the adiabatic section to make the flow visualizations possible. Thermal resistance and flow visualization results are compared. Visualization of the flow shows that by increasing the surfactant concentration, annular and semi-annular regimes can be observed at lower powers. It is also detected that by increasing the surfactant concentration, thermal resistance will decrease, while the maximum heat flux is reduced. This can be explained by thinner film thickness in the evaporator. The lowest thermal resistance was detected to be 0.44 K/W for the 0.25 g L−1 C-Tab concentration at 25 W heat input power which shows a significant improve of 77.5% compared to pure water at the same power.

In this research heating, cooling and electrical demands of a residential tower are evaluated for Iran various weather conditions. For this purpose, several cities are selected as the representativ...

In this study the first and the second laws of thermodynamics are evaluated for a single effect absorption chiller. Entropy generation and COP are selected as the objective functions and their variations are studied by varying the generator temperature for various condensing temperatures. For this purpose, the enthalpy and the entropy data of the super-heated steam, saturated steam and the saturated water are formulated in the mathematical equations. Also to provide the required steam of generator, prime mover in the form of internal combustion engine is applied and its partial load conditions are analyzed. A residential tower is considered as the case study for selecting the proper prime mover capacity. The number of prime movers is estimated based on the required heating power of the generator steam for each partial load. It is seen, the entropy generation would be decreased rapidly by increasing the generator temperature and after a while, the entropy generation would be approximately constant. The optimum entropy generation for each condensing temperature and the generator temperature may be decreased by increasing the evaporator temperature. It is deduced that COP is increased rapidly by increasing the generator temperature but in the following, the COP will be constant. The maximum amount of COP is decreased by increasing the condensing temperature. Also the COP is increased by increasing evaporator temperature for a certain generator and condensing temperatures.

The dispersion of a neutral solute band by electrokinetic flow in polyelectrolyte layer (PEL)-grafted rectangular/slit microchannels is theoretically studied. The flow is assumed to be both steady and fully developed and a first-order irreversible reaction is considered at the wall to account for probable surface adsorption of solutes. Considering low electric potentials, analytical solutions are obtained for electric potential, fluid velocity and solute concentration. Special solutions are also obtained for the case without wall adsorption. To track the dispersion properties of the solute band, the generalized dispersion model is adopted by considering the exchange, the convection and the dispersion coefficients. The solutions developed are validated by comparing the results with the predictions of finite-element-based numerical simulations. Even though the solutions can take any form of initial solute concentration into account, the results are presented by considering a solute band of rectangular shape. The results reveal that, while the short-term transport coefficients are strongly affected by the initial concentration profile, the long-term values are not dependent upon the initial conditions. In addition, it is shown that the mass transport coefficients are strong functions of the channel aspect ratio; hence, approximating a rectangular geometry by the space between two parallel plates may lead to considerable errors in the estimation of mass transport characteristics. This is particularly important for the dispersion coefficient for which the long-term values for a slit microchannel are quite different from those for a rectangular channel of very high aspect ratio. It is also illustrated that the exchange and convection coefficients increase on increasing the Damkohler number, whereas the opposite is true for the dispersion coefficient. The convection and dispersion coefficients are generally increasing functions of the PEL fixed charge density and the PEL thickness and decreasing functions of the PEL friction coefficient. Last but not least, a thicker electric double layer is found to provide a larger degree of solute dispersion, which is the opposite of that observed in a microchannel with bare walls.

In underground U-tube heat exchangers (boreholes) it is important to predict its heating performance to design and select the proper parameters such as length, diameter, material etc. to have an optimized borehole from the point of view of heat capacity and economical aspects. For this reason, having trusty equations is vital to foresee borehole heating performance and applying it in design issues. In this study a single vertical U-tube borehole with constant wall temperature is considered and analytical equations for temperature distribution in the surrounding ground around the borehole is evaluated based on one and two dimensional heat conduction respectively. The analytical equation is compared to experimental data for a borehole with 50 m depth in which warm water of 40 C is pumped into it a time period of 120 hours and the heat transfer rate per unit length is recorded. The comparison between analytical expression and experimental data shows a good agreement between them. Also the borehole entropy generation number is studied and the optimized parameters are evaluated to minimize it. It is concluded that for the considered borehole, entropy generation number is decreased by increasing its length and by decreasing the borehole radius and pipe outer radius.

Solid oxide fuel cells (SOFCs) are efficient and relatively small modular technologies for electric power generation with high-fuel flexibility. Despite such advantages their high-operating temperature (400°C–1000°C) and existence of electrochemical reactions could lead to various thermomehcanical stresses and faults such as reduction–oxidization reactions. Consequently the robust and reliable mechanical design of SOFCs is a daunting task. SOFC common geometries (tubular and planar) do not satisfy the requirements of portable applications such as fast start-up and high voltage. Hence different geometries like integrated planar (IP) or segmented in series (SIS), cone-shaped, flat tube (FT), honeycomb, and microtubular (MT) SOFCs, as well as SOFC stacks for higher power and voltage production were introduced by a number of researchers. These complicated designs can overcome some of the disadvantages associated with conventional SOFC designs to a achieve higher power density. In addition the start-up time of MT-SOFCs is lower than macro ones because of their high resistance to thermal shocks. However, more research is needed to overcome further problems related to the geometry of SOFCs. As fabrication methods play an essential role in the reliability and residual stresses in each device, different methods of fabrication for functional and supported layers are presented in this chapter. Dip coating, screen printing, and tape casting are common methods of large-scale fabrication. The chapter also explores the causes of SOFC failures. Delamination, the gap between electrode and electrolyte, redox, reduction–oxidation reactions of Ni-based anodes, and degradation change in the microstructure and materials of the cell and are the major problems facing SOFC cells and stacks. Thermal design and thermal stresses induced in SOFCs due to temperature gradient and mismatch of thermal expansion coefficients among their components are discussed with the aim of reviewing thermal modeling studies. Another prominent aspect of thermal analysis is the start-up and shut-down of the cell as well as thermal management, which are discussed in detail. The chapter concludes with summarizing the key observations and proposing future research directions.

In this paper, we aim to account for the partitioning of finite sized ions and electric double layer (EDL) overlapping effects on the electrostatics and hydrodynamics of soft nanofluidics by stablishing a modified Poisson-Boltzmann (MPB) equation enjoying mean field approach. The application of the present MPB equation enables us to describe the interaction between the steric effect and electrostatic repulsion of EDL ions due to permittivity difference of polyelectrolyte layer (PEL) and electrolyte solution. Utilizing the Debye-Huckel approximation pertinent to low surface potentials, we analytically derive the solutions of electric potential and velocity profiles of mixed electroosmotic and pressure driven flows in polyelectrolyte grafted nanotubes. As well, we theoretically predict the electrophoretic mobility of soft colloidal particles immersed in aqueous solutions of NaCl, KCl, NaMnO4 and LiIO3 in the presence of ion partitioning effect (IPE). For the case of larger surface potentials, the nonlinear governing equations of the problem are analyzed by finite difference method, and the accuracy of the numerical solutions are examined by verifying the results with existing experimental data. Moreover, we calculate three electrokinetically associated phenomena namely streaming potential, electroviscous effect and electro-chemo-mechanical energy conversion (ECMEC) efficiency through soft nanotube having constant surface potential/charge density. Then, we compare our results under the purview of the present MPB equation with those predicted by point-like charge modeling of ions. The essential findings of our investigation highlight the importance of accounting for IPE arising from the permittivity difference of PEL and bulk salt solution when high degrees of softness, charge density and thickness of the PEL are encountered in transport mechanism in soft nanofluidics. We also show that one of the most important consequences of IPE is several fold increase of electrophoretic mobility of soft particle and ECMEC efficiency of soft nanotube due to increasing the PEL-electrolyte permittivity difference.

Purpose The purpose of this paper is to investigate the natural convection behavior of nanofluids in an enclosure. The enclosure is a 3D capsule with curved boundaries filled with TiO2-water nanofluid. Design/methodology/approach In this paper, a multiple relaxation times lattice Boltzmann method (MRT-LBM) has been used. Two-component LBM has been conducted to consider the interaction forces between nanoparticles and the base fluid. Findings Results show that the enhanced Nusselt number (Nu*) increases with the increase in volume fraction of nanoparticles (ϕ) and Ra number and decrease of nanoparticle size (λ). Additionally, the findings indicate that increasing volume fraction beyond a certain value decreases Nu*. Originality/value This paper presents a MRT model of lattice Boltzmann in a 3D curved enclosure. A correlation is also presented based on the current results for Nu* depending on Ra number, volume fraction and size of nanoparticles. Furthermore, a comparison for the convergence rate and accuracy of this model and the SIMPLE algorithm is presented.

Pulsating heat pipe (PHP) is a type of wickless heat pipe that has a simple structure and an outstanding thermal performance. Nanofluid is a type of fluid in which nanoparticles are dispersed in a base fluid and have generally a better thermal conductivity in comparison with its base fluid. In this article, the performance of a nanofluid PHP is investigated. Graphene/water nanofluid with a concentration of 1 mg mL−1 and TiO2 (titania)/water nanofluid with a concentration of 10 mg mL−1 are used as the working fluids. To simultaneously investigate the thermal performance and flow regimes in the PHP, a one-turn copper PHP with a Pyrex glass attached to its adiabatic section is used. A one-turn Pyrex PHP is also used to fully visualize flow patterns in the PHP. Our results show that the material for the fabrication of a PHP and temperature of the working fluid are the most important parameters that affect the stability of a nanofluid in the PHP. The more stable nanofluid keeps its stability in the cupper PHP, while the less stable nanofluid starts to aggregate right after the injection to the cupper PHP. The more stable nanofluid has a better thermal performance than water, while the less stable nanofluid has a worse thermal performance than water. In the case of flow regimes, no significant differences are observed between the nanofluid PHP and the water PHP which is different from the previous observations. These results can help researchers to choose the best working fluid for PHPs.

Hybrid photovoltaic–thermal collectors (PVT) are cogeneration components that convert solar energy into both electricity and heat. Pulsating heat pipe (PHP) is a fast-responding, flexible and high-performance thermal-conducting device. The aim of this work is design and performance of a novel hybrid photovoltaic–thermal collector with pulsating heat pipe (PVTPHP) for improving the electrical efficiency, by reducing the photovoltaic panel’s temperature, as well as taking advantage of the thermal energy produced. An experimental setup of PVTPHP is constructed, and its operating parameters are measured. The measured parameters include solar radiation intensity, ambient temperature, filling ratio, inclination angle, PV module temperature, open-circuit voltage, short-circuit current, condenser inlet temperature, condenser outlet temperature, water flow rate, fill factor, electrical efficiency, heat delivery and combined efficiency. The results show that this new design has given a good thermal and electric performance compared to the traditional PV and PVT.

In recent years, increasing attention has been paid to the use of renewable energy sources in building sector. The ground source heat pumps are a promising alternative for heating and cooling applications. Despite the high coefficients of performance and significant environmental benefits, their performance may deteriorate over the long-term operation if the annual ground loads are not well balanced. An appropriate solution is the use of hybrid ground source heat pump system through which some portion of the thermal loads can be offset by supplemental components. The aim of this study is to assess the feasibility of a solar-assisted ground source heat pump system for heating a detached house in a cold climate of northwestern Iran. The economic and environmental aspects of the proposed hybrid system are compared with those of a ground source heat pump, an air source heat pump, and a natural gas space heater for a 20-year design life. The analyses show that despite having high initial cost, the hybrid system has the second lowest life cycle cost and the lowest carbon dioxide emissions.

In this paper, a two-component Lattice Boltzmann Method (LBM) has been utilized to simulate the natural convection of TiO2-water nanofluid in a curved geometry. The main purpose of this research is to study the effect of nanoparticle size and also boundary conditions on the thermal characteristics of the nanofluid. Furthermore, the effect of Rayleigh number (Ra) and volume fraction of nanoparticles (ϕ) on the average Nusselt number (Nuave) have been investigated. Two different thermal boundary conditions, namely adiabatic and constant temperature, have been considered in the current work for the curved boundaries. The Rayleigh number varies from 103 to 109. Four different sizes, namely 10, 25, 65 and 90 nm, have been chosen for nanoparticles. Results show that the augmentation of nanoparticle size has a deteriorating effect on the effectiveness of nanoparticles while increasing Ra number and ϕ improves Nusselt number (Nu). It is also shown that the boundary conditions change the enhanced Nusselt number (Nu*). The effect is shown to be due to the variation of nanoparticles motion dictated by the boundary conditions. Furthermore, results reveal that increasing volume fraction increases viscosity which at higher values of ϕ might reduce Nu*.

The performance of a vertical ground source heat pump system (GSHPS) largely depends on the fluid temperature leaving the borehole heat exchanger (BHE) that may be affected by the short-term behavior of the BHE. Although considerable research has been carried out to analyze the short-term transient response of the BHEs, few studies have investigated its impact on dynamic simulation of GSHPS. Therefore, this paper presents a numerical approach based on a transient BHE model to evaluate the performance of a residential GSHPS over short and long timescales. The numerical results are compared with the results of EnergyPlus software. It is shown that the proposed model can appropriately predict the dynamic behavior of the system. Moreover, effect of borehole thermal capacity on the performance of the GSHPS is investigated in comparison with a quasi-steady state model. It is found that including the borehole thermal capacity substantially affects the design borehole length. Using the transient model instead of the quasi-steady state model leads to a 16% reduction in the required borehole length.

Multi-stream Plate-fin heat exchangers (MSPFHEs) are among the most efficient heat exchangers in energy-based industries. They should be carefully designed to have maximum effectiveness and to avoid wasting energy. Flow maldistribution could significantly degrade the performance of heat exchangers. This study presents a novel modeling framework to capture the thermally-induced maldistribution in two-phase MSPFHEs. The developed model is used to show how thermally-induced flow maldistribution affects the performance of the heat exchanger, and some possible modifications are investigated to reduce its consequences. Our case study results show that thermally-induced maldistribution decreases the total heat transfer by 10.8%. Heat leakage helps to induce less flow near the leakage region, so it can be managed to overcome the initial thermally-induced flow. Moreover, thermally-induced maldistribution affects a limited region along the heat exchanger height direction. So, thermal performance deterioration decreases with increasing the number of layers for each stream. Nevertheless, increasing the heat exchanger length increases the degraded region in the heat exchanger. Thus, increasing the core length is not a good way to counteract the thermal performance deterioration caused by thermally-induced maldistribution. It is also found that an optimum layer arrangement could help to eliminate the thermally-induced maldistribution in the heat exchanger.

Nanofluids-dispersions of nanometer-sized particles in a liquid medium-have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local viscosity in the immediate vicinity of a metallic nanoparticle for a wide range of solid-liquid interaction strength, where a nanolayer of thickness 1 nm is observed as a result of the interaction with the nanoparticle. The viscosity of the nanolayer, which is found to be higher than its corresponding bulk value, is directly dependent on the solid-liquid interaction strength. We discuss the origin of this viscosity enhancement and show that the liquid density increment alone cannot explain the values of the viscosity observed. Rather, we suggest that the solid-like structure of the distribution of the liquid atoms in the vicinity of the nanoparticle contributes to the nanolayer viscosity enhancement. Finally, we observe a failure of the Stokes-Einstein relation between viscosity and diffusion close to the wall, depending on the liquid-solid interaction strength, which we rationalize in terms of the hydrodynamic slip.

In the present work, a micro combustor for thermophotovoltaic (TPV) devices is proposed which is included a U-shaped microtube in a box with a secondary fluid in space between U-shaped microtube and box walls. By utilizing the three-dimensional CFD model, combustion characteristics of the premixed lean hydrogen-air mixture in the present micro combustor are studied numerically with detailed chemistry and transport taking into account heat transfer through the wall. The results show that the establishment of secondary flows and better preheating in the curved tubes is caused the flammability limits to be at least four times in comparison with straight tubes. Combustion characteristics are studied for different parameters, namely inlet velocity, wall thermal conductivity, heat loss conditions, tube curvature and number of U-shaped tubes. By increasing inlet velocity up to 16 m/s, flame front moves toward downstream, the maximum temperature is increased and rate of reactions is intensified. The highest efficiency for thermophotovoltaic devices is calculated at 4 m/s inlet velocity. A wall thermal conductivity of 3 W/m K creates a better condition for flame stability. It is shown that the secondary fluid with higher thermal diffusivity could improve combustion characteristics. Energy conversion efficiency ( η Q ), emitter efficiency ( η Rad ) and total energy conversion efficiency of the TPV ( η total ) is utilized to investigate the simulated cases for the present geometry. This type of micro combustor is well fitted for thermophotovoltaic applications.

The aim of this paper is to simulate numerically the air flow induced by a walking visitor and its eff ects on the contaminant transport and ventilation system e ffectiveness. To this end, the following models will be used in this study: the Lagrangian Discrete Random Walk (DRW) model to trace the motion of BCPs, the dynamic mesh method to simulate the visitor movement, and the Reynolds Averaged Navier-Stokes (RANS) model to solve the air flow. The validation results of the numerical method are in full agreement with the available experimental data in the literature. The fi ndings of the present study indicate that the visitor's movement has remarkable e ect on the basic air flow, and the increase of the visitor moving speed can decrease the risk of infection in the AIIR. It is also found that the concentration of BCPs in the back of visitor exceeds10 cfu/m3, and the small distance between the patient and visitor has a negative impact on increasing the BCPs infection of the patient in AIIR. At the same time, it is observed that the effect of walking speed on the ventilation e ffectiveness index is not remarkable.

This paper presents an analysis of the combined electro-osmotic and pressure-driven axial flows of viscoelastic fluids in a rectangular microchannel with arbitrary aspect ratios. The rheological behavior of the fluid is described by the complete form of Phan-Thien–Tanner (PTT) model with the Gordon–Schowalter convected derivative which covers the upper convected Maxwell, Johnson–Segalman and FENE-P models. Our numerical simulation is based on the computation of 2D Poisson–Boltzmann, Cauchy momentum and PTT constitutive equations. The solution of these governing nonlinear coupled set of equations is obtained by using the second-order central finite difference method in a non-uniform grid system and is verified against 1D analytical solution of the velocity profile with less than 0.06% relative error. Also, a parametric study is carried out to investigate the effect of channel aspect ratio (width to height), wall zeta potential and the Debye–Huckel parameter on 2D velocity profile, volumetric flow rate and the Poiseuille number in the mixed EO/PD flows of viscoelastic fluids with different Weissenberg numbers. Our results show that, for low channel aspect ratios, the previous 1D analytical models underestimate the velocity profile at the channel half-width centerline in the case of favorable pressure gradients and overestimate it in the case of adverse pressure gradients. The results reveal that the inapplicability of the Debye–Huckel approximation at high zeta potentials is more significant for higher Weissenberg number fluids. Also, it is found that, under the specified values of electrokinetic parameters, there is a threshold for velocity scale ratio in which the Poiseuille number is approximately independent of channel aspect ratio.

Pulsating heat pipes (PHPs) are among the best solutions for the electronics cooling due to their low cost, effectiveness and being passive. Experiments to study the effective factors on heat transfer performance have been designed and as a result, improvement of ferrofluid PHP performance has been achieved. Two different heat pipes made of copper and glass were prepared to investigate the behavior of magnetic nanofluids. In order to find the best condition for heat transfer performance, different concentrations of nanofluid with a filling ratio of 50% were tested in 3 different cases of magnetic field. The results indicated that the ferrofluid is more stable in the glass PHP. It also shows that the presence of magnetic field in the copper PHP has the best outcome while in the glass PHP, the absence of magnetic field results better. It was detected that using a more concentrated ferrofluid causes a better performance in the copper PHP only when the magnetic field is applied, while in the glass PHP, increasing the concentration of ferrofluid improves the performance of the PHP in all conditions of applying the magnetic field.

Aquifers are underground porous formations containing water. Confined aquifers are surrounded by impermeable layers on top and bottom, called cap rocks and bed rocks. A confined aquifer with a very low groundwater flow velocity was considered to meet the annual cooling and heating energy requirements of a residential building complex in Tehran, Iran. Four different alternatives of aquifer thermal energy storage (ATES) were employed to meet the heating/cooling demands of the buildings. These alternatives were: using ATES for cooling alone, for heating alone by coupling with flat plate solar collectors and for cooling and heating by coupling with a heat pump. For the economic evaluation of the alternatives, a life cycle cost analysis was employed. For the environmental evaluation, Ret Screen software was employed. For the three considered operational alternatives, using ATES for cooling alone had the minimum payback period time of 2.41 year and the life cycle cost of 16000$. In the environmental consideration of the three alternatives, coupling of ATES with heat pump for cooling and heating had the minimum CO2 generation, corresponding to 359 tons/year.

A numerical model is developed to simulate the borehole heat exchanger both in the short and long time. In this regard, the computational domain is divided into the inside and outside borehole regions. A two-dimensional finite volume method is implemented in a cylindrical coordinate system for modeling of the outside borehole. Also, a thermal resistance-capacity model is presented for the borehole cross section. This model is extended to take into account the fluid transport through the U-tube and the temperature variation of the borehole components with depth. The governing equations of the two regions are solved iteratively in each time step. The proposed model is verified with the previously reported numerical, experimental and analytical results. Furthermore, the ability of the model in predicting the short-time response is evaluated in comparison with a three-dimensional computational fluid dynamics (CFD) model with a fine grid. The results show that the proposed model has a good performance in the prediction of the thermal response of the borehole in a wide time interval from 1 min to over 10 years. Moreover, the effects of time step size and number of capacity nodes on the results are investigated.

Design and fabrication of solar collectors with high performance of energy efficiency to convert solar energy to utility energy is vitally important. This article reports the results obtained from design, construction and investigation of the performance of a Combined Multi-Purpose Vacuum Solar Collector (CMPVSC). This collector consists of three sections: the vacuum section, the liquid section and the air section. In the present collector, it is capable of transferring heat to two flows (liquid and air) simultaneously and separate with the possibility of multipurpose applications. The CMPVSC is compared with the existing individual collectors and the effects of different parameters on the efficiency of this collector are examined. Experimental data indicate that high temperature and high performance with a 43% reduction in cost can be obtained using CMPVSC compared to two individual collectors. To increase the efficiency of the collector, triangular and rectangular channels in the air section have been used. The vacuum part is implemented to reduce heat losses. The effect of water inlet temperature, air flow rate, shape of air channel and vacuum part on the heat delivery by air and water have been investigated. Furthermore, as a matter of comparison of CMPVSC with the individual collector, there is a chance of obtaining highest temperature and efficiency with minimum cost and space requirements.

The performance of an infinitely segmented magnetohydrodynamic generator is analyzed using the second law of thermodynamics entropy generation criterion. The exact analytical solution of the velocity and temperature fields are provided by applying the modified Hartmann flow model, taking into account the occurrence of the Hall effect in the considered generator. Contributions of heat transfer, fluid friction, and ohmic dissipation to the destruction of useful available work are found, and the nature of irreversibilities in the considered generator is determined. In addition, the electrical isotropic efficiency scheme is used to evaluate the generator performance. Finally, the implication of the Hall parameter, Hartmann number, and load factor for the entropy generation and the generator performance are studied and the optimal operating conditions are determined. The results show that the heat transfer has the smallest contribution to the entropy generation compared to that of the friction and ohmic dissipation. The application of the Hall effect on the system showed an appreciable augmentation of entropy generation rate which is along with what the logic implies. A parametric study is conducted and its results provide the generated entropy and also efficiency diagrams which show the influence of the Hall effect on the considered generator.

In the present work the spray characteristics of bio-ethanol and its blends have been experimentally and theoretically investigated. To have a comprehensive study, the effects of ambient condition and injection pressure on the spray of different blends have been considered. Macroscopic and microscopic characteristics of spray such as tip penetration length, cone angle, projected area, volume, Sauter Mean Diameter (SMD), and Ohnesorge number are investigated precisely. Besides, air entrainment and atomization analysis have been carried out to improve mixture formation process. Using curve fitting and least squares method, theoretical correlations have been suggested in such a way to predict experimental results with the accuracy of 9.9%. To have a good estimation for the calculated parameters, uncertainty analysis has been performed. The results demonstrate enhancing the injection pressure or decreasing the ambient pressure, improve the atomization characteristics of spray. Moreover outcomes of this study indicate, spray tip penetration is enhanced by increasing the injection pressure or bio-ethanol percentage in the blend, while spray cone angle showing opposite behavior.

Macroscopic and microscopic characteristics of spray of a Modified Bio-diesel Fuel (MBF), applying direct injection system have been explored and compared with those of conventional diesel fuel. MBF is a new combination of bio-diesel, molasses bio-ethanol, and water, which has been kept as a single-phase bio-fuel, employing an emulsifier. Lower emissions and production costs, higher oxygen content and cetane number are the key advantages of the MBF to be replaced by conventional fossil fuels in internal combustion engines. Applying atomization model, the spray atomization properties such as Ohnesorge number and Sauter Mean Diameter (SMD) have been investigated. Air entrainment analysis has been studied employing quasi-steady jet theory. Furthermore, considering physics of atomization of droplets and employing dimensional analysis, a new non-dimensional number namely Atomization Index (AI) has been developed. AI number, which is the ratio of square of inertia forces to product of viscous and surface tension forces, can predict the atomization level of the spray. Results show that increasing AI number decreases SMD and improves atomization behavior of the spray as well. Since outcomes of Ohnesorge and AI numbers verify each other, and AI number can clarify the relative atomization level of different cases without drawing a diagram, AI as a proper number can be applied in further studies.

One of the key solutions to improve engine performance and reduce exhaust emissions of internal combustion engines is direct injection of bio-fuels. A new modified bio-ethanol is produced to be substituted by fossil fuels in gasoline direct injection engines. The key advantages of modified bio-ethanol fuel as an alternative fuel are higher octane number and oxygen content, a long-chain hydro-carbon fuel, and lower emissions compared to fossil fuels. In the present study spray properties of a modified bio-ethanol and its atomization behaviors have been studied experimentally and theoretically. Based on atomization physics of droplets dimensional analysis has been performed to develop a new non-dimensional number namely atomization index. This number determines the atomization level of the spray. Applying quasi-steady jet theory, air entrainment and fuel-air mixing studies have been performed. The spray atomization behaviors such as atomization index number, Ohnesorge number, and Sauter mean diameter have been investigated employing atomization model. The influences of injection and ambient conditions on spray properties of different blends of modified bio-ethanol and gasoline fuels have been investigated performing high-speed visualization technique. Results indicate that decreasing the difference of injection and ambient pressures increases spray cone angle and projected area, and decreases spray tip penetration length. As expected, increasing injection pressure improves atomization behaviors of the spray. Increasing percentage of modified bio-ethanol in the blend, increases spray tip penetration and decreases the projected area as well.

Norouzak oil seeds that are the source of bio-diesel (Norouzak) fuel grow mostly in Gonabad and Kashmar located in Khorasan, north east of Iran. Spray characteristics of Norouzak bio-diesel have been experimentally and theoretically studied in this work. Norouzak fuel is added to the conventional diesel fuel, and mixture formations of several blends have been investigated and compared with each other. Moreover, by varying ambient and injection conditions, behaviors of spray of different blends have been explored. Microscopic and macroscopic properties of spray such as Sauter Mean Diameter (SMD), Ohnesorge number, spray tip penetration, spray cone angle, spray projected area and volume have been examined. To promote mixture formation of the aforementioned sprays, atomization and air entrainment analysis have been performed as well. Furthermore, one can predict experimental results with accuracy of 12% applying suggested theoretical correlations and using error analysis, precision of calculated char...

Hydrodynamically fully developed flow of power-law fluids under combined action of electroosmotic and pressure gradient forces in rectangular microreactors is analyzed considering heterogeneous catalytic reactions. The Poisson-Boltzmann, Cauchy momentum, and concentration equations are considered in two dimensions and after being dimensionless are numerically solved applying a finite difference algorithm. Variation of axial concentration gradient, and axial and horizontal mass diffusions are taken into account as well. To accomplish a more general analysis, the velocity distribution is obtained by solving continuity and Cauchy momentum equations and is not considered as an average axial velocity for solving the concentration equation. Profiles of mean concentration of reactants are analyzed as functions of operating variables. Results reveal that mean concentration increases with increasing Peclet number and dimensionless Debye-Huckel parameter. It is also a decreasing function of Damkohler number. Presence of pressure gradient will affect the decreasing trend of mean concentration through the channel. Shear-thinning fluids, due to their nature, are most strongly influenced by pressure gradient and Newtonian and shear-thickening fluids are less affected, respectively. Favorable pressure gradient will decelerate mean concentration rate of decrease and adverse pressure gradient acts the opposite. Moreover, a larger aspect ratio leads to slower descending rate of mean concentration in purely electroosmotic and pressure-assisted flows while in a pressure-opposed flow, the behavior might change depending on the strength of pressure gradient and the value of channel aspect ratio.

We perform a 3D numerical modeling of reaction-diffusion dynamics in a Y-shaped microreactor, considering a fully developed combined electroosmotic and pressure-driven flow. The governing equations, based on a second-order irreversible reaction, are solved invoking a finite-volume approach for a non-uniform grid system. We demonstrate that the reaction is highly position dependent: more production is observed adjacent to the horizontal walls for a favorable pressure gradient, whereas both the wall and centerline are the regions of highest production when a back pressure is applied. We further show that, to achieve the maximum production rate, the EDL should be thick enough, the pressure gradient should be unfavorable, and both the diffusiveties and the inlet concentrations of the reacting components should be identical. Although the same is true for the production efficiency, defined to be the ratio of the average production concentration to the inlet concentration of the limiting component, there is a main difference: when the inlet concentrations are the same the efficiency is minimal. Moreover, the concentration pick is inclined toward the component with either less diffusivity or less inlet concentration; the inclination is magnified when either the Debye length increases or an unfavorable pressure gradient is employed.

In this research, the airflow pattern and particle dispersion in a contaminated full-scale cleanroom are investigated numerically using both Eulerian and Lagrangian approaches. Three different supply diffuser configurations namely (1) central, (2) horizontal and (3) vertical and three different exhaust grille configurations namely (4) vertical symmetric, (5) asymmetric and (6) horizontal symmetric are selected for the analysis. The presented results reveal that the supply/exhaust openings arrangement has a significant influence on the particulate contaminant dispersion in the cleanrooms. The comparison of the above different supply diffuser configurations shows that the vertical and horizontal supply diffuser configurations lead to a more escaped particles and less deposited particles on the room walls and installed working table in the room than the central one. It is also found that for different exhaust grille arrangements, the performance of the ventilation system in the horizontal symmetric case is higher than asymmetric and vertical symmetric ones. At the same time, it is observed that the variations of contaminant concentration in both Eulerian and Lagrangian approaches are almost the same. The results of the velocity and particle concentration show good agreement with the available experimental data in the literature.