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In this paper, we address the morphology of nanoparticles in a base fluid as a variable to see how different shapes can be used to control thermal conductivity along with the types of nanoparticles. Hybrid and mono particle suspensions, including TiO2, Al2O3, and hybrid TiO2–Al2O3, have been used. Water and ethylene glycol are used as base fluids, and nanoparticles range from 0 to 4% in a volume fraction. The Nusselt number, average heat transfer, thermal resistance, pressure drop, and pumping power as a function of Reynolds number are studied for different nanofluids in microchannels. Overall performance is characterized by the analysis of pressure drop, pumping power, thermal resistance, and maximum power. Simulation results show that the thermal conductivity increases linearly with particle morphology. By adding 4% of HyNF (50% Al2O3-50% TiO2) with a morphology of n1 = 7 and n2 = 3, respectively, it results in a substantial 18.6% enhancement in thermal conductivity, compared to 12% and 10.04% enhancement for mono-nanofluids with Al2O3 and TiO2 nanoparticles, respectively. The type of base liquid has a negligible effect on the thermal conductivity of the same kind of nanoparticles. Nevertheless, the findings of this study provide valuable guidance for cooling microelectronic cooling components via nanofluid-based thermal management. [ABSTRACT FROM AUTHOR]

Continuous solar steam generation requires a photoabsorber with high capillarity or a hydrophilic surface, the capability of floating on water and low thermal conductivity. Hence, we propose an alternative interfacial solar steam substrate made of soil as a novel cost-effective photoabsorber. For this purpose, three soil types, sand (0.05–2 mm), silt (0.002–0.05 mm) and clay (less than 0.002 mm), were used and floated using a polystyrene foam to reduce heat losses. Our results reveal that a maximum evaporative efficiency of 80.01 ± 3.1% at 1 kW m−2 was achieved for the silt sample, whereas the corresponding value for water alone was 23.22 ± 1.4%. In a desalination test, the quality of obtained water from the process greatly exceeds the World Health Organization's drinking water standards. The hydrophilic properties of silt prevent the deposition of impurities on its surfaces, ensuring high durability. [ABSTRACT FROM AUTHOR]

The pressure of natural gas stream must be reduced in city gas station. The natural gas has to be preheated before pressure reduction takes place usually through throttling valves. In conventional city gas stations, the natural gas is preheated by indirect water bath heaters, which burn a large amount of the natural gas as fuel. In this study, the rejected heat from a supercritical carbon dioxide recompression cycle using solar energy is recovered for preheating the natural gas. The novel design of this system generates uniform electricity as well as preheats the natural gas in city gate station. The proposed system is simulated for Birjand city gas station as a case study, and a thorough techno-economic analysis is performed in Engineering Equation Solver for evaluating the system performance. The results of this study demonstrate that parabolic trough collectors with 25 rows are the most efficient solar system while the annual average of thermal and exergy efficiency of the system is 0.56 and 0.41, respectively. The exergetic analysis of the system shows that the highest average exergy destruction takes place in the throttling valve and the second highest in the solar collectors. Also, the total amount of fuel saving is estimated at 4.87 million cubic meters annually and the net power output is equal to 2.86 MW. From the economic point of view, the value of the payback period is estimated 4 years and, based on the net present value method, after 8 years, the initial investment could be returned. [ABSTRACT FROM AUTHOR]

Water vapor is vital both as an energy carrier and as an intermediary state for removing impurities from water. In nature, transpiration occurs when water is transported (against gravity) from the roots to the underside of leaves where it evaporates. Using this process, one large tree can pump and purify 400 L of water each day. Based on trunk cross-sectional area, this corresponds to a water flux range of ∼100–1000 kg/m2day, but based on evaporation area it only corresponds to a rate of ∼0.1 kg/m2day. Compared to industrial mechanisms of producing water vapor (i.e. typical thermal-driven systems have a flux of ∼4000 kg/m2day), natural wood has a relatively low flux. In an effort to boost the flux of sustainable, natural wood, we investigated wood surface modifications, laser carbonization and deposition of gold nanolayers, which achieved an instantaneous evaporation rate of ∼4 kg/m2h—under 3 kW/m2 light intensity, exceeding all previous studies of synthetic materials (including 3.8 kg/m2h reported by Zhou et al. in a 2016 Nature Photonics article) for solar steam generation applications. The cost analysis of different natural and synthetic material-based techniques for solar steam generation indicated that the carbonization and laser treatments are very cost-effective and even the gold coating was comparable to previously reported synthetic materials. Based on these results, we suggest that natural, surface-modified poplar wood could represent a viable alternative to synthetic materials for liquid/vapor separation. • Treated wood samples as evaporators were tested with a solar simulator (1–3 Suns). • Poplar wood sample of 5 mm length had the best capillary action. • Laser treatment was reported as a fast and accurate way of carbonizing wood surface. • Gold nanolayer coating and carbonized wood surfaces showed high solar absorption. • Combination method led to a record of 4.02 kg/m2 h evaporation rate at 3 Suns. [ABSTRACT FROM AUTHOR]

• Effect of nanofluids in solar steam and pure water generation was investigated. • The performance of MWCNT, SWCNT, GO and GNP nanomaterials was compared. • The sensible heat storage of nanofluids during the light on and off were studied. • High quality of distilled water was proved by seawater desalination using nanofluids. In the present study, the performance of water based-nanofluids containing four different types of carbon nanostructures for solar vapor generation has experimentally been investigated. The considered nanofluids are multi-walled carbon nanotube, single-walled carbon nanotube, graphene nanoplate and graphene oxide. Nanofluids with three nanoparticle mass concentrations of 0.001%, 0.002%, and 0.004% were prepared and their evaporation characteristics were investigated for solar radiation intensities in the range of 1.5 and 3.5 Suns. The results indicated that the multi-walled carbon nanotube nanofluid with a mass concentration of 0.004% provides the highest evaporation rate with the maximum total efficiency of 94% at 3.5 Suns. The total efficiencies for nanofluids containing the single-walled carbon nanotube, graphene nanoplate and graphene oxide at similar conditions and that of pure water, respectively, are about 91%, 90%, 81% and, 54%. Furthermore, it is observed that for the mass concentration of 0.004%, the evaporation rate with respect to water increases to about 72%, 63%, 58% and 36%, respectively, for the multi-walled carbon nanotube, the single-walled carbon nanotube, graphene nanoplate and graphene oxide nanofluids. As far as the sensible heat is concerned, graphene oxide shows the most considerable sensible heating efficiency at the mass concentration of 0.004% with the maximum fluid bulk temperature rise of 20.5 °C, which is 40% higher than that of water. Finally, it was found that increasing the radiation intensity from 1.5 to 3.5 Suns improves the evaporation rate; however, the sensible heat efficiency is more influenced such that the evaporation efficiency declines. The quality of the produced water from a seawater sample is also examined and it was shown that it is well below the limitations of the drinking water standards. [ABSTRACT FROM AUTHOR]

In this paper, an alternative model is presented to simultaneously achieve thermodynamic consistency, leading to high-density ratios, and surface tension adjustment capability based on the pseudo-potential lattice Boltzmann model (LBM). These features are achieved by adding two relatively simple terms, each with an adjustable coefficient, selected from the Taylor expansion of the original interaction force, one for achieving and maintaining thermodynamic consistency, and the other for adjusting the surface tension independent of the density ratio. The model also takes advantage of being independent of using SRT, MRT, 2D or 3D lattice models. The capability of the model is evaluated and verified by performing several static and dynamic benchmark test cases. First, the coexistence densities extracted for a flat interface from the model are compared with those of the Maxwell equal area rule in a wide temperature range. Results show that the thermodynamic consistency is well achieved and the coexistence densities are independent of the surface tension strength as well. Next, the well-known Laplace law for a droplet is evaluated and satisfied with the model. In addition, a wide range of surface tension values is achievable at a fixed temperature by adjusting the surface tension adjustment coefficient. After that, the ellipsoidal droplet oscillation is simulated with a good agreement between the analytical and numerical results for the oscillation period. The spurious velocities around the droplet are then evaluated and shown to be reasonable, and comparatively low even at a relatively high-density ratio around of 2400. It was found that the magnitude of these velocities can be further reduced, especially at high-density ratios, by reducing the liquid to gas dynamic viscosity ratio or equivalently increasing the gas to liquid kinematic viscosity ratio. Next, the stability, and the fascinating phenomenon of symmetric bifurcation of a rotating planar droplet are simulated and formation of two- and four-lobed shapes along with rigid rotation is presented depending on the value of the surface tension. Finally, the droplet impact on a liquid film is simulated at a relatively high-density ratio around of 720, and Reynolds number of 1440. However, the upper limit of physical parameters for simulating this phenomenon is found to be a density ratio of 720 and Re and We numbers of 6600 and 275, respectively. Effects of the surface tension strength are shown as well. The crown spread radius from the model agrees well with the analytical solution reported in the literature. [ABSTRACT FROM AUTHOR]

In the present paper, the performance of the graphene oxide in the solar steam generation has experimentally been examined. For this purpose, a setup was built for measuring the evaporation rate, which consists of a solar simulator with a xenon lamp as a radiation source, a pyranometer for light intensity measuring, and a sensory system for measuring the temperature and the mass. Then, the nanofluid with three nanoparticle mass concentrations of 0.001, 0.002, and 0.004% was prepared and exposed to the light intensity of 3.5 Suns (3.5 kW m−2). Finally, the effects of the light intensity variations on the solar steam generation were studied at the steady and transition conditions. The results showed that the examined carbon nanostructure is efficiently capable of direct solar energy harvesting, such that a maximum total efficiency of 78.9% at 3.5 Suns can be obtained, while the corresponding value for the case of pure water is about 54%. Furthermore, it was found that increasing the light intensity from 1.5 to 3.5 Suns enhances the evaporation flux rate, yet, reduces the evaporation efficiency. [ABSTRACT FROM AUTHOR]

In this paper, a numerical scheme has been developed to examine the effective parameters on thermal management of distribution transformers and subsequently to optimize their cooling systems. In this regard, the response surface methodology (RSM) was used as the optimization method to minimize the hotspot temperature in the transformer as the response factor. A comprehensive three-dimensional computational scheme was employed considering the detailed geometrical specifications of an actual 200kVA distribution transformer to obtain the temperature field and the hotspot temperature. The accuracy of the numerical model was established via comparing the numerical results with the measured temperatures of a running transformer. The thermal variations of the thermo-physical properties of the transformer oil are determined experimentally and incorporated in the numerical modeling. A comprehensive parametric study among seven evidently effective parameters has been performed to identify the most effective parameters on the thermal performance of the transformer. It was found that fin height, length, and spacing are the more influential parameters among the examined parameters, which are also considered as the input variables in the optimization procedure. According to the RSM, the effects of the variations of these input variables in pre-specified ranges on the response, which is the hotspot temperature, are examined through the suggested runs by RSM. The results indicated that the hotspot temperature is more influenced by the fin height as compared to the fin length and spacing. Furthermore, the hotspot temperature decreases with the increase in fin height and length, while decreases as fin spacing increases. In addition, a correlation for the variations of the hotspot temperature as a function of the fin height ( H ), length ( L ), and spacing ( S ) is suggested using RSM. The significant finding is that the proposed optimum transformer configuration ( H  = 0.9 m, L  = 0.08 m and S  = 0.036 m) leads to the hotspot temperature reduction of about 16 °C as compared to the actual transformer geometry, which greatly affects the transformer life expectancy and safer performance. [ABSTRACT FROM AUTHOR]

Highlights • Adsorption cooling system driven by the exhaust gas for A/C application is modeled. • Optimum fin spacing of longitudinal and annular finned tube adsorber beds is determined. • Heat exchangers with annular fins provide higher cooling power than longitudinal fins. • Adsorption cooling system can decrease fuel consumption and greenhouse emissions. • Heat and mass transport in porous beds of adsorbent particles is investigated. Abstract Adsorption cooling systems (ACS) are considered as potential alternatives to traditional vapor compression air conditioning systems in heavy-duty vehicles. In ACS, adsorber bed heat exchangers (ABHEx) replace compressors and their appropriate design directly impacts the ACS performance. During operation, ABHEx undergo a large temperature swing to derive a refrigerant in ACS and their response time affects the dynamic behavior of ACS. In this study, a detailed three dimensional non-equilibrium model is developed to study the effects of heat and mass transfer in annular and longitudinal finned tube adsorber beds filled with zeolite-13x particles. The effects of fin height and spacing are studied on the system operating parameters to identify an optimum fin geometry. The simulation results show that a decrease in fin spacing leads to a decrease in the coefficient of performance (COP) and an increase in the specific cooling power (SCP), and no optimum value is observed for them in a specific fin spacing. However, variations of the total cooling power (TCP) maximize at a certain fin spacing. For longitudinal finned tube ABHEx, the optimum averaged fin spacing shifts from 5.4 to 6.8 mm for the adsorber beds with 10, 15, and 20 mm fin heights, while the optimum fin spacing of annular finned tube ABHEx changes from 5.0 to 6.4 mm. Furthermore, the results show that under similar dimensions and operating conditions, an ACS with annular finned tube ABHEx provides a 10% higher total cooling power than that with a longitudinal finned tube ABHEx at the optimum fin spacing. Using the ACS with optimized ABHEx in a truck would annually save about 370 L of fuel consumption and decreases greenhouse emissions by up to 738 kg CO 2e. [ABSTRACT FROM AUTHOR]

In the present study, a double multiple relaxation time lattice Boltzmann method (MRT-LBM) simulation is performed to analyze the performance of an active micromixer with rotationally oscillating stirrer in high Peclet number flows. Almost all previous LBM studies consider the stirrer as a zero-thickness flat plate, while in the present problem, the stirrer has been considered as a rectangular cylinder with different aspect ratios. Calculations are performed for the important dimensionless parameters of the problem including the oscillation amplitude ( K ) and aspect ratio ( AR ) in the range of 0.1 – 1 , while the Strouhal number ( St ) varies in the range of 0.5–2 for Re = 1 and Pe  = 10,000. The aim was to evaluate the effects of these parameters on the mixing efficiency in search for the optimal micromixer design. Results indicated that the dimensionless mixing efficiency at the microchannel outlet ( ε out ) increases smoothly with the increase of K at relatively large aspect ratios. Furthermore, at a given amplitude, the ε out is slightly influenced by the aspect ratio variations, especially at lower values of K, while it is strongly affected by the Strouhal number (St). Moreover, the costs associated with the mixing process is evaluated with the parameter mixing energy cost ( mec ) defined as the required cyclic average input power to the obtained averaged mixing at the channel exit. It was found that mec is much higher at the St  = 2 as compared to its lower values for all AR and K values. Also, increasing the AR increases the mec for almost all K and St . In general, it can be concluded that 0.1, 1.1, 1 for the ε out and 0.1, 0.5, 1 for mec are determined as the optimum values of AR , St and K, respectively. [ABSTRACT FROM AUTHOR]

A numerical simulation is performed to analyze the performance of viscous micropumps with elliptic and rectangular rotors with variable aspect ratios by lattice Boltzmann method (LBM). The influences of three effective geometrical parameters of viscous micropump namely rotor aspect ratio ( AR ) , microchannel height ( S ) and rotor eccentricity (ɛ) in the ranges of 0.2–1, 1.5–3.5 and 0.1–0.9, respectively, are investigated on the cross sectional instantaneous dimensionless averaged velocity ( U ¯ ) and the instantaneous area averaged total entropy generation ( S ¯ F ) as well as their time-averaged values ( U ave  and  S F, ave ). A five-level central composite design (CCD), which is one of the most important response surface method (RSM) schemes, is adopted for the analysis of main and interaction effects as well as the optimization of these parameters. Results show that the U ave increases with the increase of AR and ɛ in all aspect ratios while decreases as S increases at high aspect ratios for both rotors. However, this trend is reversed at low aspect ratios. Furthermore, results indicate that S F, ave increases with the increase of all three geometrical parameters for both rotors. Finally, a numerical optimization is employed to simultaneously optimize both responses of U ave and S F, ave for elliptic and rectangular rotors. Overall desirability values of 0.701 and 0.758 are obtained for elliptic and rectangular rotors, respectively. For elliptic rotor, the obtained optimal values of geometrical parameters are 0.884, 1.5 and 0.744, respectively, for aspect ratio, microchannel height, and eccentricity, while the respective values for rectangular rotor are 1, 1.5 and 0.694. [ABSTRACT FROM AUTHOR]

A three dimensional numerical scheme has been developed to examine geometrical configuration effects on the performance of a single bed adsorption chiller with a trapezoidal aluminum finned flat-tube heat exchanger (FFT HEx). A mathematical distributed model along with the linear driving force (LDF) model and Darcy’s law have been considered to take into account the effects of heat and both intra/inter-grain mass transfer resistances. The developed numerical scheme has also been validated experimentally by using a composite sorbent SWS-1L and water as a working pair. Additionally, rectangular beds with identical working conditions and bed dimensions as the tested trapezoidal beds have been examined in similar details to identify, which type of the fin geometry provides a superior performance. It was found that in this particular HEx, the heat transfer resistance is mainly influenced by both of the fin pitch and height, while the inter-grain mass transfer resistance is independently controlled by the bed length. This fact makes the role of the fin pitch and fin height almost interchangeable with respect to the cycle time and specific cooling power ( SCP ) especially in rectangular beds. However, the coefficient of performance ( COP ) is more influenced by the fin height than the fin pitch. In addition, higher SCP can be achieved at smaller bed dimensions at the expense of lower COP . Moreover, it was found that using rectangular bed is more appropriate, since its SCP is either the same or higher than its corresponding trapezoidal bed especially at shorter bed lengths, while COP remains almost the same for both bed types in all considered ranges of bed dimensions. Finally, a bed designing procedure has been proposed for proper designing of effective adsorption HExs based on the performed parametric studies. [ABSTRACT FROM AUTHOR]

This paper presents a multi-objective optimization model for estimating the cooling capacity of refrigeration in a trigeneration system for both grid on and off operating modes. In trigeneration systems, the cooling demand can be provided by single effect absorption and compression chillers (as the refrigeration system). Genetic Algorithm and LINMAP selection procedure are used for obtaining the final optimum point considering four-E analysis (energy, exergy, economy and emission) with the help of net future worth benefit difference (NFWBD) and global exergy efficiency objective functions. It was found that the NFWBD and exergy efficiency with cooling capacity of 1800 and 5600 kW for absorption and compression chillers, respectively, are 330 × 10 6 $ and 41.9% for grid on mode, while they reduce to 232 × 10 6 $ and 32.7% for grid off mode. Sensitivity analysis in grid on mode shows that the increase in the number of equipment (gas engine and chillers) leads to the decrease in both NFWBD and exergy efficiency, while selecting two similar absorption chillers (with cooling capacity of 900 kW), has payback period of 5.1 years, and provides the maximum NFWBD (321 × 10 6 $) and exergy efficiency (40.02%). [ABSTRACT FROM AUTHOR]

• Synergistics effects of Fe 3 O 4 nanoparticles and Au nanolayer in solar evaporation. • Solar steam fluxes of 3.76 kg m−2 h−1 by Fe 3 O 4 /Au photoabsorber under 3 sun. • Reduction of brackish well water's salinity to below the drinking water standards. For the first time, we investigated the synergistic effect of Fe 3 O 4 nanoparticles (NPs) and Au nanolayer in interfacial solar steam generation (ISSG) of well water. Herein, single- or double- layer photoabsorbers were prepared by coating poplar wood with different concentrations of Fe 3 O 4 NPs and/or various thicknesses of Au nanolayer. The maximum evaporation efficiency for the single-layer photoabsorber composed of Fe 3 O 4 was 80% under 3 sun (1 sun=1 kW m−2) illumination. The most efficient single-layer photoabsorber coated by Au nanolayer resulted in the evaporation efficiency of 83.2% under 3 sun. The double-layer photoabsorber of Fe 3 O 4 over Au nanolayer (Fe 3 O 4 /Au) and Au layer over Fe 3 O 4 (Au/Fe 3 O 4) were prepared. The Fe 3 O 4 /Au reached the evaporation efficiency of 88.7% and the evaporation rate of 3.76 kg m−2 h−1 under 3 sun due to the synergistic effects of surface plasmon effect in Au NPs as well as black color and visible light absorbing ability of Fe 3 O 4 NPs. [Display omitted] [ABSTRACT FROM AUTHOR]

Blood non-Newtonian behavior on low-density lipoproteins (LDL) accumulation is analyzed numerically, while fluid-multilayered arteries are adopted for nonstenotic and 30%-60% symmetrical stenosed models. Present model considers non-Newtonian effects inside the lumen and within arterial layers simultaneously, which has not been examined in previous studies. Navier-Stokes equations are solved along with the mass transport convection-diffusion equations and Darcy's model for species transport inside the luminal flow and across wall layers, respectively. Carreau model for the luminal flow and the modified Darcy equation for the power-law fluid within arterial layers are employed to model blood rheological characteristics, appropriately. Results indicate that in large arteries with relatively high Reynolds number Newtonian model estimates LDL concentration patterns well enough, however, this model seriously incompetent for regions with low WSS. Moreover, Newtonian model for plasma underestimates LDL concentration especially on luminal surface and across arterial wall. Therefore, applying non-Newtonian model seems essential for reaching to a more accurate estimation of LDL distribution in the artery. Finally, blood flow inside constricted arteries demonstrates that LDL concentration patterns along the stenoses inside the luminal flow and across arterial layers are strongly influenced as compared to the nonstenotic arteries. Additionally, among four stenosis severity grades, 40% stenosis is prone to more LDL accumulation along the post-stenotic regions. [ABSTRACT FROM AUTHOR]

Low-density lipoprotein (LDL), which is recognized as bad cholesterol, typically has been regarded as a main cause of atherosclerosis. LDL infiltration across arterial wall and subsequent formation of Ox-LDL could lead to atherogenesis. In the present study, combined effects of non-Newtonian fluid behavior and fluid–structure interaction (FSI) on LDL mass transfer inside an artery and through its multilayer arterial wall are examined numerically. Navier–Stokes equations for the blood flow inside the lumen and modified Darcy’s model for the power-law fluid through the porous arterial wall are coupled with the equations of mass transfer to describe LDL distributions in various segments of the artery. In addition, the arterial wall is considered as a heterogeneous permeable elastic medium. Thus, elastodynamics equation is invoked to examine effects of different wall elasticity on LDL distribution in the artery. Findings suggest that non-Newtonian behavior of filtrated plasma within the wall enhances LDL accumulation meaningfully. Moreover, results demonstrate that at high blood pressure and due to the wall elasticity, endothelium pores expand, which cause significant variations on endothelium physiological properties in a way that lead to higher LDL accumulation. Additionally, results describe that under hypertension, by increasing angular strain, endothelial junctions especially at leaky sites expand more dramatic for the high elastic model, which in turn causes higher LDL accumulation across the intima layer and elevates atherogenesis risk. [ABSTRACT FROM AUTHOR]

Motivated by the growing interest in electroosmotic flows (EOFs) as a reliable no moving parts strategy to control fluid motion in various micro fluidics, two-dimensional (2D) steady-state EOFs in the presence of a charged mid-plate have been numerically investigated. A variation of lattice Boltzmann models is used to recover all governing equations, where the Nernst-Planck equation is employed to model the internal electrical field. Results are validated by comparing the velocity profiles in the 2D uniform EOFs against their analytical solutions. The numerical results show that the velocity profile and volumetric flow rate are strongly influenced by the ratio of the electrical double layer (EDL) thickness to the channel height. In addition, the effects of positioning and thickness of the charged mid-plate on the electric field distribution, velocity profile and flow rate are examined in detail. [ABSTRACT FROM AUTHOR]

Rupture of the abdominal aortic aneurysm (AAA) is the result of the relatively complex interaction of blood hemodynamics and material behavior of arterial walls. In the present study, the cumulative effects of physiological parameters such as the directional growth, arterial wall properties (isotropy and anisotropy), iliac bifurcation and arterial wall thickness on prediction of wall stress in fully coupled fluid-structure interaction (FSI) analysis of five idealized AAA models have been investigated. In particular, the numerical model considers the heterogeneity of arterial wall and the iliac bifurcation, which allows the study of the geometric asymmetry due to the growth of the aneurysm into different directions. Results demonstrate that the blood pulsatile nature is responsible for emerging a time-dependent recirculation zone inside the aneurysm, which directly affects the stress distribution in aneurismal wall. Therefore, aneurysm deviation from the arterial axis, especially, in the lateral direction increases the wall stress in a relatively nonlinear fashion. Among the models analyzed in this investigation, the anisotropic material model that considers the wall thickness variations, greatly affects the wall stress values, while the stress distributions are less affected as compared to the uniform wall thickness models. In this regard, it is confirmed that wall stress predictions are more influenced by the appropriate structural model than the geometrical considerations such as the level of asymmetry and its curvature, growth direction and its extent. [ABSTRACT FROM AUTHOR]

Natural convection gaseous slip flows in open-ended vertical parallel-plate microchannels with symmetric wall heat fluxes are numerically investigated. A second-order model, including thermal creep effects, is considered for velocity slip and temperature jump boundary conditions with variable thermophysical properties. Simulations are performed for wide range of Rayleigh numbers from 5 × 10− 6to 5 × 10− 3in the continuum to slip flow regime. The developing and fully developed solutions are examined by solving the Navier–Stokes and energy equations using a control volume technique. It is found that the second-order effects reduce the temperature jump and the slip velocity, whereas thermal creep strongly increases the slip velocity in both developing and fully developed regions. Moreover, the rarefaction effects increase the flow and heat transfer rates considerably, while decreasing the maximum gas temperature and friction coefficient as compared to the continuum limit. It was also shown that the axial temperature variations of the gas layer adjacent to the wall in the modeling of the thermal creep are of paramount importance and neglecting these variations, which is common in literature, leads to unphysical velocity and temperature distributions. [ABSTRACT FROM PUBLISHER]

In the present work, a new Lattice Boltzmann model has been developed to study the electroosmotic flows in a 2-D flat microchannel. The governing equations are presented in the continuum model, while a set of equivalent equations in Lattice Boltzmann model is introduced and solved numerically. In particular, the Poisson and the Nernst-Planck (NP) equations are solved by two new lattice evolution methods. In the analysis of electroosmotic flows, when the convective effects are not negligible or the electric double layers have overlap, the NP equations must be employed to determine the ionic distribution throughout the microchannel. The results of this new model have been validated by available analytical and numerical results. The model has also been used to examine the electroosmotic flows in a heterogeneous flat microchannel. Furthermore, the predictions of the NP model are compared with those of the Poisson Boltzmann (PB) equation to identify the applicability of the PB equations for such flows, which can also serve as validations for the present model. [ABSTRACT FROM AUTHOR]

The effects of thermal creep on the development of gaseous fluid flow and heat transfer in rectangular microchannels with constant wall temperature are investigated in the slip-flow regime. Thermal creep arises from tangential temperature gradients, which may be significant in the entrance region of channels, and affects the velocity and temperature fields particularly in low Reynolds number flows. In the present work, the Navier--Stokes and energy equations coupled with velocity-slip and temperature-jump conditions applied at the channel walls are solved numerically using a control-volume technique. Despite the constant wall tempera-ture, tangential temperature gradients form in the gas layer adjacent to the wall due to the temperature-jump condition. The effects of slip/jump and thermal creep on the flow patterns and parameters are studied in detail for a wide range of channel aspect ratios and, Knudsen and Reynolds numbers. Furthermore, the effects of variable properties on velocity-slip and, friction and heat transfer coefficients are also examined. [ABSTRACT FROM AUTHOR]

In the current study, the DSMC method is utilized to obtain the entropy, entropy generation and the local gradient length Knudsen number in the rarefied flows. Two particular geometries, cavity and flat plate, are considered to study the departure from equilibrium state in the presence of sudden expansion/contraction, bend in the velocity profile, boundary flow and shock waves. The entire slip regime is considered to investigate small and large nonequilibrium effects on the entropy and entropy productions. Our investigation reveals that the distribution of entropy in the rarefied flow is very similar to the temperature contour. The entropy generation distribution in the micro cavity indicates that the two top corners are the regions around which departure from equilibrium state takes place. The study of entropy generation over the flat plate reveals that the entropy production is maximized along the shock wave. Moreover, increasing the rarefaction effects thickens the nonequilibrium shock wave. We also observed that increasing the nonequilibrium effects reduces the level of entropy generation in the rarefied flow. As the flow density decreases in the nonequilibrium regime, the level of shear stress and heat flux reduces, which subsequently lower the level of entropy generation in the rarefied flows. Furthermore, it was found that although the level of entropy generation in the flow reduces as the Knudsen number increases, the boundaries of the maximum entropy production region extends under large rarefaction effects. [ABSTRACT FROM AUTHOR]

Due to the increment of energy demand, the current requirement for optimum design of a trigeneration system has become more imperative than ever. The present work deals with the simulation of the demand energy and optimization of a trigeneration system for a hotel building in Mashhad using GAMS software where four objective functions including energy, exergy, environment, and economy have been considered. The results indicate that the improvement of the objective functions was 30.88%, 30.90%, 30.76%, and 17.19%, respectively, compared to the traditional system. Lastly, advanced exergy analysis was performed on the trigeneration system to determine the amount and location of endogenous, exogenous, avoidable, and unavoidable irreversibility rates. The results elucidate that the gas engine has the highest potential in the efficiency improvement, followed by heat exchangers set, electrical chiller, absorption chiller, and gas boiler. [ABSTRACT FROM AUTHOR]

Abstract: A transient three-dimensional numerical scheme considering both inter-particle and intra-particle mass transfer resistances is developed to examine the performance of silica gel/water adsorption chiller. The developed model is validated against the available experimental data in literature. The effects of bed contact resistance and variable thermal conductivity are examined and moreover, it is shown that modeling part of the bed including just three fins can well predict the performance of entire bed with 50 fins. Respecting to all mentioned considerations, a comparative study is performed to identify the appropriate fin shape and configuration for a given working condition of an adsorption chiller. Accordingly, effects of fin height and spacing on SCP and COP are investigated for beds with annular and square plate fins. Finally, a design procedure is proposed to configure the adsorbent bed specifications for a given application assuming that SCP and COP are specified parameters based on practical considerations. [Copyright &y& Elsevier]

Fluid flow and heat transfer in the entrance region of rectangular microchannels of various aspect ratios are numerically investigated in the slip-flow regime with particular attention to thermal creep effects. Uniform inlet velocity and temperature profiles are prescribed in microchannels with constant wall temperature. An adiabatic section is also employed at the inlet of the channel in order to prevent unrealistically large axial temperature gradients due to the prescribed uniform inlet temperature as well as upstream diffusion associated with low Reynolds number flows. A control-volume technique is used to solve the Navier-Stokes and energy equations which are accompanied with appropriate velocity slip and temperature jump boundary conditions at the walls. Despite the constant wall temperature, axial and peripheral temperature gradients form in the gas layer adjacent to the wall due to temperature jump. The simultaneous effects of velocity slip, temperature jump and thermal creep on the flow and thermal patterns along with the key flow parameters are examined in detail for a wide range of cross-sectional aspect ratios, and Knudsen and Reynolds numbers. Present results indicate that thermal creep effects influence the flow field and the temperature distribution significantly in the early section of the channel. [ABSTRACT FROM AUTHOR]

A numerical investigation has been performed to identify the rarefaction effects on the flow structure of an isolated micron-sized spherical particle. An isothermal sphere in the slip flow regime 10 ≤ Kn ≤ 10 at intermediate Reynolds numbers (1 ≤ Re ≤ 50) is considered. The Navier-Stokes equations are solved by a control volume technique in conjunction with the velocity slip boundary condition. It was found that the wake region can shrink considerably as the Knudsen number increases. Furthermore, the skin friction and pressure drag coefficients decrease as the Knudsen number increases due to the reduction in normal velocity gradients and shrinkage of the wake region, respectively. Engineering correlations for predicting the total drag coefficient in the slip flow regime are presented. [ABSTRACT FROM AUTHOR]

Exhaust Gas Recirculation (EGR) method has already shown its benefits on controlling NOx emissions in internal combustion engines. An important issue associated with this method is homogeneous cylinder-to-cylinder distribution of the recirculating gas. Any maldistribution leads to power reduction and increase of other pollutants, which are strictly limited by recent emission laws. In addition to EGR, these limitations force the engine manufacturers to recycle blowby gases into the cylinders as homogeneous as possible. Since geometrical parameters and injection locations of EGR/blowby have substantial effects on homogenous cylinder-to-cylinder distribution of EGR/blowby gases, any developments in identifying the injecting locations with the least EGR/blowby maldistribution are of great practical importance. The existing experimental and numerical methods for evaluating the EGR/blowby maldistribution are based on the injection of air with different temperature or of different gas (mostly CO2) from the main air stream. However, these methods are time consuming due to the large number of possible injection locations. It has been shown that the most uniform distribution cannot be obtained by just a single injection point; therefore, the study of simultaneous injection points becomes inevitable. Clearly, such a study is practically impossible with the present methods. In this research a new method based on particle tracking is proposed, which greatly reduces the time and effort to find the injection locations with the least maldistribution, especially when multiple injections are considered. [ABSTRACT FROM AUTHOR]

The conjugate heat transfer problem of food freezing inside a cavity was numerically investigated. A vegetable sponge has been considered as a food block freezing inside a square freezing chamber due to natural convection. The need for specifying the block surface convective heat transfer coefficients was eliminated by solving the surrounding cooling fluid and, therefore, a comprehensive understanding on heat transfer and airflow during freezing can be achieved. The 2-D unsteady Navier-Stokes and energy equations were solved using a finite volume method with the semi-implicit SIMPLE algorithm. Thermophysical properties of food block components were considered to be dependent on temperature, as well as moisture and ice content, which has been rarely considered in food freezing studies. The specific heat capacity method was employed to model the freezing process. The Krischer model was adopted for predicting the thermal conductivity of the food, which indicates that the model works with reasonable accuracy for humid porous foods. The mechanism of natural convection in the cavity was carefully studied and the effects of different parameters on the freezing time were examined. The food freezing curves were investigated for various Rayleigh numbers in the range of 104 ≤ Ra ≤ 106, with different area ratios of A = 1/16, 1/9, and 1/4, and food initial water contents of X tw  = 0.48, 0.58, and 0.68. It was concluded that increasing the Rayleigh number reduces the freezing time. On the other hand, area ratio and initial water content of the food were proved to extend the freezing time. [ABSTRACT FROM AUTHOR]

Abstract: A transient two-dimensional numerical model of combined heat and mass transfer in the adsorbent bed of a silica gel/water adsorption chiller is presented for a cylindrical bed with annular fins. The bed transient transport behaviors are discussed for all cycle phases. Complex flow patterns are found for isosteric heating and cooling phases due to the simultaneous occurrence of the adsorption and desorption processes. Furthermore, the effects of bed configurations such as fin spacing, bed height, and particle size on the performance of the system are examined. Results show that fins in general reduce the COP of the system; however, for a given total cooling power, the bed size can be reduced dramatically, when fins are employed at the cost of slightly lower COP. It is also shown that there is an optimum particle diameter associated with each bed configuration that maximizes the COP, TCP, and SCP of the system. [Copyright &y& Elsevier]

Abstract: This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al2O3, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications. [Copyright &y& Elsevier]

At high values of interfacial electrokinetic potentials, the effects of finite ionic sizes are consequential and must be taken in the purview. This paper addresses the non-trivial implications of steric effects on thermal transport of an electroosmotic-pressure driven flow within a microchannel under an imposed heat flux. The ion size is introduced into the modified Poisson-Boltzmann equation by the steric factor, which allows considering the ions as point charges or finite sizes. The non-linear, non-dimensional form of the electrokinetic potential, momentum, and energy equations are numerically solved in a spatial scheme discretized into the non-equally spaced elements. Our main point here is that the intricate physical interplay among zeta potential, pressure gradient, and steric effects has a considerable impact on the temperature distribution. Results indicated that by decreasing the surface cooling, the effect of ion finite size is significantly intensified while other factors become of peripheral importance. Also, unfavourable pressure gradients tend to mask the effect of additional heat generated by increasing the channel aspect ratio. Finally, adverse pressure gradients generated higher velocity gradients at the surface, results in a higher convective cooling and consequently an improved thermal dissipation. [Display omitted] • At low surface cooling, the role of ion size on heat rejection intensifies. • Negative pressure wipes out extra heat arisen from increasing the channel size. • Shear-thinning fluid velocity is largely altered by ionic size in adverse pressure. • S v does not affect the temperature of shear-thinning fluid in adverse pressures. [ABSTRACT FROM AUTHOR]

Abstract: Simultaneously developing velocity and temperature fields in the slip-flow regime are investigated numerically in trapezoidal microchannels with constant wall temperatures. A wide range of channel aspect ratios (0.25⩽ α ⩽2) and side angles (30°⩽ ϕ ⩽90°) are considered in the Reynolds number range 0.1⩽ Re ⩽10. A control-volume based numerical method is used to solve the Navier–Stokes and energy equations with velocity-slip and temperature-jump at the walls. As characterized by the Knudsen number (Kn ⩽0.1), the effects of rarefaction on the key flow features are examined in detail. Major reductions in the friction and heat transfer coefficients are observed in the entrance region due to large amounts of velocity-slip and temperature-jump. In the fully developed region, the friction coefficient decreases strongly both with increasing Kn and aspect ratio but has a weaker dependence on the side angle. The heat transfer coefficient also decreases strongly with increasing rarefaction and aspect ratio; however, as the aspect ratio increases, its sensitivity to Kn decreases. Practical engineering correlations are also provided for fully developed flow friction and heat transfer coefficients. [Copyright &y& Elsevier]

Simultaneously developing three-dimensional laminar flow and heat transfer in the entrance region of trapezoidal channels have been investigated using numerical methods in the Reynolds number range from 10 to 1000. The principal and secondary velocity fields, the temperature field, and all associated heat and momentum exchange parameters have been examined. The present results for the fully developed flow region of the channels compare well with the available literature. In the entrance region, it is observed that the axial velocity profiles develop overshoots near the walls and particularly at the channel corners. It is shown that boundary-layer type of approximations, which lead to Reynolds-number-independent Poiseuille and Nusselt numbers, can be used for Reynolds numbers over 50 and after a few hydraulic diameters from the channel inlet. It is also shown that hydrodynamic entrance lengths calculated with methods based on fully developed flow data are grossly in error. New correlations are proposed for the entrance length, and for the friction and heat transfer coefficients. [ABSTRACT FROM AUTHOR]

• Developing a numerical model to analyze twisted micromixer flow fields. • Deep parametric study on a twisted micromixer to optimize mixing performance. • Enhancing a twisted micromixer efficiency by introducing the eccentricity ratio. • Superior performance of rectangular cross-section over the square in the micromixer. Microfluidic mixers with twisted geometries show promise for high mixing efficiency, especially at elevated flow rates. However, there is a lack of understanding regarding the optimal geometrical parameters to enhance mixing across various Reynolds numbers. This study aimed to explore the influence of pitch number, cross-section geometry, and eccentricity ratio on the performance of the twisted micromixer. The flow field in these micromixers was solved numerically and the mixing index and pressure drop were calculated for Reynolds numbers of 1 to 400. Twisted micromixers with rectangular cross-sections and aspect ratios of 0.5 and 2 outperformed those with square cross-sections by up to 34% in mixing efficiency, exceeding 90% efficiency for Reynolds numbers from 1 to 400. Moreover, the eccentricity ratio (ER) was studied for the first time in twisted micromixers in this study and demonstrated a critical role in improving the mixing performance. For low to intermediate Reynolds numbers, the highest ER of 0.75 showed the best performance, while for high Reynolds numbers, an ER of 0.5 was optimal. These insights offer valuable direction for designing high-performance micromixers for advanced microfluidic systems. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, a new cost-effective method has been developed to utilize solar energy for steam generation. For the first time, the delignified wood was chosen as a self-regenerating substrate coated with Fe/Pd bimetallic nanoparticles (NPs) as either single or double-layer solar photoabsorbers. The results revealed that in the case of single-layer, the optimum wood thickness, the thickness at which maximum evaporation rate and minimum heat loss occur, is 10 and 5 mm for the deionized water and seawater, respectively. Under the radiation of 3 sun (3 kW m−2), the evaporation efficiency was enhanced up to 103% using the wood with a thickness of 5 mm coated with Fe/Pd NPs compared to the case in which evaporation happens from the seawater surface without any absorber on the surface (conventional case). In order to increase the evaporation rate, a double-layer substrate was utilized. The lower layer was composed of either wood or polystyrene foam with a thickness of 5 mm while the upper layer was composed of wood with 5 mm thickness coated with Fe/Pd NPs. The results showed 126% and 167% enhancement in evaporation efficiency for Fe/Pd W5W5 (wood with a thickness of 5 mm coated with nanoparticles in the upper layer, and wood with a thickness of 5 mm in the lower layer) and Fe/Pd W5F5 (wood with a thickness of 5 mm coated with nanoparticles in the upper layer, and foam with a thickness of 5 mm in the lower layer), respectively, compared to that of the seawater. Moreover, the economic analysis indicated that using two-layer photoabsorber (Fe/Pd W5F5) is an affordable technique for solar steam generation compared to some previously reported methods in the literature. Unlabelled Image • Different types of the wood were delignified to increase porosity and decrease density. • Delignified wood was chosen as a self-regenerating substrate to solar steam generation. • Delignified popular wood was coated with Fe/Pd nanoparticles as an absorber. • Various thickness of coated wood was tested with seawater. • Double-layer substrates wood/foam (W/F) showed high solar evaporation (84.54%). [ABSTRACT FROM AUTHOR]

This empirical study investigates the influence of initial relative humidity (RH) variations (35 % and 95 %) on key fire behavior parameters, specifically focusing on the fully developed stage, occurrences of flashover, and the temperature of the upper gas layer. Wooden cribs weighing 10 kg–40 kg are used as fuel in a compartment with three openings to establish different natural ventilation conditions. The experimental findings reveal that increasing the initial RH within the chamber leads to a delay in the onset of the fire growth phase. Furthermore, it induces two noticeable effects on the fully developed fire stage: a reduction in its duration by up to 50 % and a delay in its initiation by at least 30 s. While heightened initial RH does not prevent flashover, it effectively postpones its occurrence by a minimum of 40 s. Experiments with maximum fuel loads demonstrate negligible effects of increased RH on the maximum temperature, even under varying ventilation conditions. Conversely, lower fuel loads exhibit a significant decline in temperature with rising humidity, notably from 626 °C to 474 °C. The quantitative and qualitative insights derived from this study have considerable potential to inform the development of more effective fire suppression strategies in enclosed compartments. • Augmenting the initial relative humidity (RH) not only retards the initiation of the wood crib fire growth phase by an average duration of 30–60 s but also, while not precluding flashover, significantly defers its onset by an average of 45 s. • Higher initial RH reduces the fully developed stage duration by up to 50 %, delaying its initiation by at least 30 s. • Implementing humidity in firefighting strategies can effectively delay fire stages, offering crucial time for extinguishment in multi-compartment scenarios. [ABSTRACT FROM AUTHOR]

In this paper for the first time, a Lattice Boltzmann Simulation is performed to analyze the simultaneous effects of a hot rotating elliptic cylinder and the magnetic field on the mixed convection flow in a square enclosure. Complicated flow patterns and isotherm plots are found and analyzed in the concentric annulus between the internal elliptic cylinder and the outer square enclosure. Results indicate that increasing the Reynolds number, instantaneous averaged Nusselt number of the enclosure and its oscillation amplitude increase, while decrease with increasing the Hartmann number especially at its lower values. Furthermore, response surface method is adopted to find the optimal location of the elliptic cylinder. Response surface optimization results reveal that the average Nusselt number shows a decreasing-increasing trend with increasing both nondimensional parameters of cylinder center (Xc, Yc). Finally, the optimal location of the elliptic cylinder for the maximum heat transfer rate is obtained as Xc = 0.65 and Yc = 0.35. Moreover, a comparative study is performed to evaluate the heat transfer effects of the elliptical cylinder rotation as compared to circular cylinder. It was found that the elliptical cylinder rotation has a significant effect on the heat transfer enhancement, especially at high values of Re and Ha. As an example, the heat transfer rate for the elliptical cylinder at Re = 200 is increased by 13% and 34% as compared to the circular cylinder at Ha = 50 and 100, respectively. [ABSTRACT FROM AUTHOR]

Magnetic drug delivery as a potential method to treat diseases such as cancer tumors has attracted the attention of many researchers. One of the problems in conventional and ineffective therapies is the spread of drug in the circulatory system. The method of magnetic drug delivery aims at directing the drug to the localized area of disease by using a magnetic field. Considering the effects of parameters such as non-Newtonian viscosity, oscillatory input, motion and the presence of atherosclerosis, the present study examines the magnetic drug delivery, which is under the influence of magnetic field, in a 90-degree bent in three situations of without atherosclerosis, a moderate atherosclerosis of 45% and severe atherosclerosis of 75% in two states of fixed vessel and moving vessel arising from the expansion and contraction of the heart. The magnetic field is in the range of 0 ≤ B ≤ 1.7 T, and the diameter of magnetic particles varies from 1 to 6 μm. The comparison of particle absorption percentage for different atherosclerosis revealed that the presence of atherosclerosis increases the value of particle absorption percentage. The results of comparison between fixed vessel and moving vessel indicate that the percentage of particle absorption is higher when the vessel is moving. In the presence of magnetic field, the maximum value of absorption percentage is 76.93% for the moving vessel with 75% atherosclerosis and the particles with the diameter of 6 μm and this value is equal to 75.65% when the vessel is considered fixed. Regardless of the size of particle, this value is approximately 15.8% and 3% for the moving vessel and the fixed vessel, respectively, when no magnetic force is applied to the vessel. [ABSTRACT FROM AUTHOR]

In this paper, the melting properties such as the thermal energy storage capacity and phase-change temperatures of paraffin are investigated. The paraffin is also modified as nano-PCM by dispersing small fractions (0.2% and 0.5% mass fractions) of highly conductive nanoparticles. The material used in this study has a melting temperature of 22 °C, making it suitable for domestic building applications. Additionally, the thermal conductivity and heating rate of the paraffin and paraffin with multiwall carbon nanotubes (MWCNTs) are studied experimentally. The results show that the paraffin with 0.2% and 0.5% mass fractions of MWCNTs could increase the thermal conductivity by 35% and 49% with respect to pure paraffin. The maximum thermal conductivity of 0.203 W m−1 K−1 is observed for the paraffin with 0.5% mass fraction of MWCNTs. Additionally, the heat of fusion slightly reduced by increasing the mass fraction of MWCNTs. The result implies that the materials used in this study could be used for building applications. [ABSTRACT FROM AUTHOR]

In this paper, an alternative approach to implement initial and boundary conditions in the lattice Boltzmann method is presented. The main idea is to approximate the nonequilibrium component of distribution functions as a third-order power series in the lattice velocities and formulate a procedure to determine boundary node distributions by using fluid variables, consistent with such an expansion. The velocity shift associated with the body force effects is included in this scheme, along with an approximation to determine the mass density in complex geometries. Different strategies based on the present scheme are developed to implement velocity and pressure conditions for arbitrarily shaped boundaries, using the D2Q9, D3Q15, D3Q19 and D3Q27 lattices, in two and three space dimensions, respectively. The proposed treatment is tested against several well-established problems, showing second-order spatial accuracy and often improved behavior as compared to various existing methods, with no appreciable computational overhead. [ABSTRACT FROM AUTHOR]

• A measure for deciding about proper adsrobent bed modeling is introduced. • The measure is a dimensionless parameter (C) obtained from the equations scale up analysis. • The parameter C is calculated based on the input parameter of the cooling system design. • A numerical investigation is performed to evaluate the C parameter. In order to investigate the effects of various parameters on the adsorption chiller performance, the adsorbent bed must be modeled with the appropriate governing equations and assumptions. One of the assumptions, which greatly influences the modeling complexity and computational cost is the bed uniform pressure assumption, which relaxes the need for solving the momentum equations governing the motion of the refrigerant vapor through the porous bed. However, in some bed conditions the bed pressure distribution should be calculated and the inter-particle resistance cannot be neglected, which in turn requires the determination of the bed velocity field. It is of great practical importance to know in advance, whether the uniform pressure approach is appropriate for the modeling of the problem in hand. In this study, a dimensionless parameter based on the scale up analysis of the governing equations is introduced, which can be calculated based on the input data of the problem to decide about the appropriate bed pressure assumption prior to start the modeling process. [ABSTRACT FROM AUTHOR]

Elasto-inertial microfluidics has drawn significant attention in recent years due to its enhanced capabilities compared to pure inertial systems in control of small microparticles. Previous investigations have focused mainly on the applications of elasto-inertial sorting, rather than studying its fundamentals. This is because of the complexity of simulation and analysis, due to the presence of viscoelastic force. There have been some investigative efforts on the mechanisms of elasto-inertial focusing in straight channels; however, these studies were limited to simple rectangular channels and neglected the effects of geometry and flow rates on focusing positions. Herein, for the first time, we experimentally and numerically explore the effects of elasticity accompanying channel cross-sectional geometry and sample flow rates on the focusing phenomenon in elasto-inertial systems. The results reveal that increasing the aspect ratio weakens the elastic force more than inertial force, causing a transition from one focusing position to two. In addition, they show that increasing the angle of a channel corner causes the elastic force to push the particles more efficiently toward the center over a larger area of the channel cross section. Following on from this, we proposed a new complex straight channel which demonstrates a tighter focusing band compared to other channel geometries. Finally, we focused Saccharomyces cerevisiae cells (3–5 μm) in the complex channel to showcase its capability in focusing small-size particles. We believe that this research work improves the understanding of focusing mechanisms in viscoelastic solutions and provides useful insights into the design of elasto-inertial microfluidic devices. [ABSTRACT FROM AUTHOR]

Highlights • Geometry and zeta potential effects on mixing performance and flow rate of EOFs are investigated. • A series of 2D convergent/divergent micro-channels with different zeta potential patterns are examined. • EOFs are modeled using the Nernst-Planck equation to include flow effects on ionic distribution. • Flow parameters are optimized using RSM to improve the mixing efficiency and flow rate. Abstract Effects of zeta potential distribution and geometrical specifications on flow rate and mixing performance of electroosmotic flows in straight, converging, diverging, and converging-diverging microchannels are investigated by lattice Boltzmann method. Modified Navier-Stokes equations along with the Nernst–Planck equation for ionic distributions are solved and validated against the available electroosmotic flow solutions. The geometrical effects on flow patterns and mixing efficiencies in the presence of patched zeta potential distributions are examined in search for the simultaneously enhanced mixing performance and mass flow rate. Numerical results indicate that converging channel leads to a sizable increase in mixing efficiencies, while at the same time decreases the flow rate. In contrast, diverging channels increase the flow rate, where the mixing performance deteriorates. Thus, it is expected to achieve a balance between the mixing efficiency and mass flow rate using converging-diverging geometries. In this regard, the response surface methodology (RSM) is employed to maximize the mixing efficiency and flow rate of the converging-diverging microchannel. According to numerical results, the optimum parameters predicted by RSM are in excellent agreement with full numerical simulations. [ABSTRACT FROM AUTHOR]

Transformer is a critical component of electrical power systems, and its failure can disrupt the power supply. Additionally, the high investment cost of transformers emphasizes the importance of maintenance and improved service life. The transformer losses cause increased temperature in various parts of the equipment, particularly in the hot-spot temperature, which leads to insulation aging and decreases the service life of the transformer. This study investigates passive cooling techniques for an oil-immersed transformer by experimenting with four methods: shield (T-shield), photovoltaic panel canopy and cooling fan (T-PV/Fan), two heat pipes (T-Heat Pipe) and nanofluid (T-Nano Fluid) in a laboratory transformer with a 1750-W power. The bulk and hot-spot temperature of the oil tank are compared, and the results demonstrate that all techniques improve thermal management and expand the service life of the transformer. The photovoltaic panel and cooling fan (T-PV/Fan) have the highest effect and are expected to prolong the existing system's lifespan up to 48 years based on the IEC 60076 standard. [ABSTRACT FROM AUTHOR]

In this paper, a single-component multiphase pseudo-potential lattice Boltzmann model is developed to study droplet impact onto a thin liquid film in 2D coordinates. The model has certain capabilities which make it applicable to cases with large density ratios as high as 1000, low viscosities, and tunable values of surface tension independent of the density ratio. The coexistence values (densities) from the numerical model are compared to those of the analytical solution of the Maxwell construction. For further validation of the model, the simulation results for pressure difference at the droplet interface for various radii are compared to those of the theoretical solution based on the Laplace law. A test on an oscillating droplet is also performed for two different surface tension values to evaluate the ability of the model in simulating dynamic cases such as droplet impact. Next, the model is used to study a droplet impact onto a thin liquid film. It was found that the propagation rate of non-dimensional crown radius obeys the power law reported in the literature. Finally, effects of the surface tension (characterized as Weber number), the gas kinematic viscosity and the density ratio of liquid to gas on a droplet impact onto a thin liquid film are investigated. Results show that increasing the Weber number leads to more crown height causing more instability of the rim, and eventually generation of satellite droplets from the crown tip. Results of changing the gas kinematic viscosity revealed that at high density ratios such as 1000 selected in this paper, when the kinematic viscosity ratio of the gas to liquid is high (above 150), no satellite droplets are produced during the considered time. For a case with a low density ratio of 10, the raised liquid from the surface is not permitted to move freely in radial and vertical directions leading to the formation of an inward liquid turn on top of the raised liquid sheet. Therefore, the crown-shape liquid rise formed at high density ratios is not observed in a case of low density ratio. [ABSTRACT FROM AUTHOR]

One of the innovative methods of improving heat transfer characteristics of heat exchangers in solar systems is applying nanofluids as the heat transfer media. In this study, laminar convective heat transfer of water-based TiO 2 nanofluid flowing through a uniformly heated tube has been investigated via experiments and numerical modeling. The thermal conductivity and dynamic viscosity of the prepared nanofluids have also been measured and modeled at different temperatures and nanoparticle concentrations. Based on the results, a maximum enhancement of 21% in average heat transfer coefficient has been obtained using TiO 2 /water nanofluids. For the numerical section, the single-phase model was compared with the common two-phase numerical approaches. The numerical investigation indicated that the predicted heat transfer coefficients using single-phase and common two-phase approaches, even based on experimental thermophysical properties of nanofluids, underestimate and overestimate the experimental data, respectively. Therefore, some modifications are implemented to the common two-phase model in order to obtain more accurate predictions of the heat transfer characteristics of nanofluids. This modified model investigated the effects of particle concentration, particle diameter, and particle and basefluid material on the heat transfer rate at different Reynolds numbers. The results indicated that the convective heat transfer coefficient increases with an increase in nanoparticle concentration and flow Reynolds number, while particle size has an inverse effect. The obtained results can be very useful to the investigation of the potential application of nanofluid-based solar collectors. [ABSTRACT FROM AUTHOR]

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO 2 /ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO 2 nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles. [ABSTRACT FROM AUTHOR]

Direct simulation Monte Carlo (DSMC) method in low Knudsen rarefied flows at micro/ nanoscales remains a big challenge for researchers due to large computational require-ments. In this article, the application of the simplified Bernoulli-trials (SBT)ldual grid collision scheme is extended for solving low Knudsenllow speed and low Knudsen/high gradient rarefied micro/nanoflows. The main advantage of the SBT algorithm is to pro-vide accurate calculations using much smaller number of particles per cell, i.e., (N) «2, which is quite beneficial for near continuum DSMC simulations where the requirement of fine meshes faces the simulation with serious memory restrictions. Comparing the results of the SBT/dual grid scheme with the no time counter (NTC) scheme and majorant fre-quency scheme (MFS), it is shown that the SBT/dual grid scheme could successfully pre-dict the thermal pattern and hydrodynamics field as well as surface parameters such as velocity slip, temperature jump and wall heat fluxes. Therefore, we present SBT/dual grid algorithm as a suitable alternative of the standard collision schemes in the DSMC method for typical micro/nanoflows solution. Nonlinear flux-corrected transport (FCT) algorithm is also employed as a filter to extract the smooth solution from the noisy DSMC calcula-tion for low speed/low Knudsen number DSMC calculations. [ABSTRACT FROM AUTHOR]