Simulation of complex liquids in micropump

Complex liquids can be encountered in many applications of microdevices. In the present study, the performance of microscrew pump using complex liquid is investigated numerically. The microscrew pump operation depends on the surface sweep forces. It consists of a screw placed inside a microchannel. When the screw rotates, a net force is transferred to the fluid due to differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace. Three-dimensional complex liquid simulations of micropump were performed.The effect of screw pitch, thread, Reynolds number and pump load on the micropump performance has been studied. The simulations of complex liquids indicate that the highest bulk velocity is achieved with high thread depth at low Reynolds number. However, effective pumping is accomplished at low Reynolds number, high pressure load and high thread depths.     

Effect of Mean Pressure Levels on the Dynamic Response Characteristics of the Carotid Sinus Baroceptor Process in Dog

The purpose of this investigation was to delineate the effect of imposed mean pressure levels on the open-loop dynamic response characteristics of the carotid sinus baroceptors in dogs. The experimental design consisted of measuring the intrasinus pressure and the gross baroceptor nerve activity while forcing the isolated sinus with sinusoidal pulse pressures, with peak-to-peak amplitude of 50 mm Hg, superimposed on mean pressures of 75, 125, 175, and 225 mm Hg at frequencies of 0.5, 1, 2, 3, 4, 5, 7, 10, 15 and 20 Hz. With this forcing protocol, we were able to divide the traditional sigmoidal pressurenerve activity relationship into three piecewise linear segments whose input-output (transfer) functions could then be determined by conventional linear system analysis. We found that (a) at each mean pressure level, the transfer function relating nerve activity, N(s), to forcing pressure, P(s), was second order and of the form, N(s)/P(s) = ?(1 + ?s + ?s2), and (b) the coefficients ?, ?, and ? were all quadratic functions of the mean pressure level, P? Incorporating the equations for each coefficient as a function of mean pressure into the transfer function yielded the nonlinear differential equation, N(t) = k?(P?) [(P(t) - P?) + ?(P?) (dp(t)/dt) + ?(P?) (d2P(t)/dt2)], which describes the dynamic response of the carotid sinus baroceptor nerve, N(t), over the entire pressure range, P(t), studied.     

Impact of Artificial Compressibility on the Numerical Solution of Incompressible Nanofluid Flow

The numerical solution of compressible flows has become more prevalent than that of incompressible flows. With the help of the artificial compressibility approach, incompressible flows can be solved numerically using the same methods as compressible ones. The artificial compressibility scheme is thus widely used to numerically solve incompressible Navier-Stokes equations. Any numerical method highly depends on its accuracy and speed of convergence. Although the artificial compressibility approach is utilized in several numerical simulations, the effect of the compressibility factor on the accuracy of results and convergence speed has not been investigated for nanofluid flows in previous studies. Therefore, this paper assesses the effect of this factor on the convergence speed and accuracy of results for various types of thermo-flow. To improve the stability and convergence speed of time discretizations, the fifth-order Runge-Kutta method is applied. A computer program has been written in FORTRAN to solve the discretized equations in different Reynolds and Grashof numbers for various grids. The results demonstrate that the artificial compressibility factor has a noticeable effect on the accuracy and convergence rate of the simulation. The optimum artificial compressibility is found to be between 1 and 5. These findings can be utilized to enhance the performance of commercial numerical simulation tools, including ANSYS and COMSOL.     

Electrooxidation of 4-methylcatechol in the presence of barbituric acid derivatives

Electrooxidation of 4-methylcatechol (1) in the presence of 1,3-dimethylbarbituric acid (2a) and 1,3-diethylthiobarbituric acid (2b) as nucleophiles has been studied in detail by cyclic voltammetry and controlled-potential coulometry. The results indicate that 1 can be oxidized to its related o-benzoquinone (1a) and without conversion to its quinone methide tautomeric form, via an ECEC mechanism pathway, is converted to barbiturate derivatives (5a-b). The electrochemical synthesis of 5a-b have been successfully performed in one-pot in an undivided cell.     

Flow Enhancement of Medium-Viscosity Crude Oil

This study investigated the different alternatives to enhance the flowability of crude oil with medium viscosity. These alternatives include the addition of water into crude oil to form water-in-oil emulsion, the addition of light petroleum product, the addition of flow improver, and a preheating technique. Temperature range of 10-50°C, water concentration range of 0-50% by volume, flow improver concentration range of 0-5000 ppm, and kerosene concentration range of 0-50% by volume were investigated in the flowability enhancement study of crude oil with medium viscosity. The flowability enhancement in terms of viscosity reduction was investigated using RheoStress RS100 from Haake. A cone-plate sensor was used with a cone angle of °4, cone diameter of 35 mm, and 0.137-mm gap at the cone tip. The addition of kerosene to crude oil improves the flowability much better than any other investigated technique.     

A one-dimensional model of plug flow pneumatic conveying

A one-dimensional model of pulse or plug flow is proposed. This type of flow is observed at high mass ratios of solids to gas and requires gas velocities orders of magnitude less than dilute phase systems. Plug stability criteria are examined by considering the axial interparticle stresses within single plugs and the effect of the radial transmission of these stresses on wall friction. Establishment of the fluid pressure gradient within the plug and the existence of a settled layer of solids in the interplug space are shown to be important requirements. These concepts are used in the formulation of an empirical pressure drop equation which is used in the correlation of experimental data. Comparison of predicted and experimental values is reasonable for the air-sand system considered. The model should be of value in the planning and interpretation of future experimental studies.     

Numerical study of hydrogen‐enriched methane–air combustion under ultra‐lean conditions

The demand for gas turbines that accept a variety of fuels has continuously increased over the last decade. Understanding the effects of varying fuel compositions on combustion characteristics and emissions is critical to designing fuel‐flexible combustors. In this study, the combustion characteristics and emissions of methane and hydrogen‐enriched methane were both experimentally and numerically investigated under ultra‐lean conditions (Ø ≤ 0.5). This study was performed using global mechanisms with a one‐step mechanism by Westbrook and Dryer and a two‐step mechanism with an irreversible and reversible CO/CO₂ step (2sCM1 and 2sCM2). Results show that the 2sCM2 mechanism under‐predicted the temperature, major species, and NO_x by more than 100% under ultra‐lean conditions; thus, we proposed a modified‐2sCM2 mechanism to better simulate the combustion characteristics. The mechanisms of Westbrook, 2sCM1, and modified 2sCM2 predicted the temperature and the CO₂ emission with an average deviation of about 5% from the experimental values. Westbrook and 2sCM1, however, over‐predicted the NO_x emission by approximately 81% and 152%, respectively, as compared with an average under‐prediction of 11% by the modified‐2sCM2 mechanism. The numerical results using the proposed modified‐2sCM2 mechanism shows that the presence of hydrogen in the fuel mixture inhibits the oxidation of methane that led to the formation of unburned hydrocarbons in the flame. We also showed that for any given fuel compositions of H₂/CH₄, there is an optimum equivalence ratio at which the pollutant emissions (CO and NO_x) from the combustor are minimal. Zero CO and 5 ppm NO_x emissions were observed at the optimal equivalence ratio of 0.45 for a fuel mixture containing 30% H₂. The present study provides a basis for ultra‐lean combustion toward achieving zero emissions from a fuel‐flexible combustor. Copyright © 2016 John Wiley & Sons, Ltd.     

A numerical comparison among different water-based hybrid nanofluids on their influences on natural convection heat transfer in a triangular solar collector for different tilt angles

Natural convection inside a triangular solar collector is investigated numerically for different nanofluids and hybrid nanofluids in this study. The individual effects of Al₂O₃–water, carbon nanotubes (CNT)–water, and Cu–water nanofluids are observed for different solid volume fractions of nanoparticles (0%–10%). Three types of hybrid nanofluids are prepared using different ratios of Al₂O₃, CNT, and Cu nanoparticles in water. A comparison is made varying the Rayleigh numbers within laminar range (10³–10⁶) for different tilt angles (0°, 30°, 60°, and 90°) of the solar collector. The inclined surface of the triangular solar collector is isothermally cold and the bottom wall (absorber plate) is isothermally hot, whereas the vertical wall with respect to the absorber plate is considered adiabatic. Average Nusselt numbers along the hot wall for different parameters are observed. Streamlines and isotherm contours are also plotted for different cases. Dimensionless governing Navier–Stokes and thermal energy conservation equations are solved by Galerkin weighted residual finite element method. Better convective heat transfer is found for higher Rayleigh number, solid volume fraction, and tilt angle. In the case of hybrid nanofluid, increasing the percentage of the nanoparticle that gives better heat transfer performance individually results in enhancing natural convection heat transfer inside the enclosure.     

Probabilistic analysis of a two-unit warm standby system subject to hardware and human error failures

Reliability analysis of a two-unit warm standby redundant system under hardware and human error failures is studied. The regenerative point technique is used. Various measures of reliability of the system are derived assuming all time distributions are general.     

Effect of Vehicle and Drug Concentration on transdermal delivery of Dihydroergotamine using excised Animal Skin

Dihydroergotamine (DHE) is widely used in the treatment, of migraine as I.M. injection of 1.0 mg/ml. Absorption of DHE averaged 23% when taken orally and the drug is subjected to extensive first-pass effect. The physicochemical and pharmacokinetic characteristics of DHE such as small dose, low M.W. and extensive hepatic metabolism, suggests that this drug is a possible candidate for trans-dermal delivery.

The purpose of this study was to investigate the effect of vehicle and dose variation on the percutaneous absorption of DHE. In-vitro diffusion studies were conducted utilizing improved Franz diffusion cells. The rabbit skin obtained from the dorsal area was employed as a barrier membrane.