A model for the fluid flow and heat transfer in an electrohydrodynamically (EHD) augmented micro-heat pipe is presented utilizing a macroscopic approach. Coulomb and dielectrophoretic forces have ...been considered in the model. The coupled non-linear governing equations for the fluid flow, heat and mass transfer are developed based on the first principles and are solved numerically. The effects of Coulomb and dielectrophoretic forces have been studied together and the effects have been compared. The analytical expressions for the critical heat input and for the dry-out length have been obtained, which show that with an increase in the electric field intensity, the critical heat input increases and the dry-out length decreases. It is found that using EHD, the critical heat input can be increased by 100 times. The contribution of Coulomb force is observed stronger than that of dielectrophoretic force. Also, the critical heat input and the dry-out length have been successfully compared with the experimental results available in the literature. The general nature of the model and the associated parametric study will be useful to understand the EHD pumping in a micro-heat pipe.
In this paper, four simulated annealing based multiobjective algorithms—SMOSA, UMOSA, PSA and WMOSA have been used to solve multiobjective optimization of constrained problems with varying degree of ...complexity along with a new PDMOSA algorithm. PDMOSA algorithm uses a strategy of Pareto dominant based fitness in the acceptance criteria of simulated annealing and is improved. In all algorithms, the current solution explores its neighborhoods in a way similar to that of classical simulated annealing. The performance and computational cost for all algorithms have been studied. All algorithms are found to be quite robust with algorithmic parameters and are capable of generating a large number of well diversified Pareto-optimal solutions. The quality and diversification of Pareto-optimal solutions generated by all algorithms are found to be problem specific. The computational cost is least by WMOSA and is followed by PDMOSA. The algorithms are simple to formulate and require reasonable computational time. Hence, the simultaneous use of all algorithms is suggested to obtain a wider spectrum of efficient solutions.
We investigate effects of surface-tension gradients on the performance of a micro-grooved heat pipe in this work. The surface-tension gradient force is accounted for in the present model, and ...expressions for radius of curvature, liquid pressure, liquid velocity, and maximum heat throughput are found analytically using a regular perturbation technique. With a favorable surface-tension gradient, the liquid pressure drop across the heat pipe can be decreased by ∼90%, and the maximum heat throughput can be increased by ∼20%. In contrast, using an unfavorable surface-tension gradient, the liquid pressure drop increases by ∼150%, and the maximum heat throughput decreases by ∼15%. For the same values of the favorable and unfavorable surface-tension gradients, the unfavorable effect is more pronounced than the favorable one. The effects of the surface-tension gradients are found to be increasing with the corner angle of a polygonal heat pipe. Adverse effects of the surface-tension gradient could be due to the variations in the liquid temperature and/or surfactant concentration. Nevertheless, a favorable situation where the surface-tension gradient can facilitate the liquid flow in a heat pipe can also be obtained using a suitable surfactant, surface charge, etc., and then the performance of a micro heat pipe can be improved.
We here investigate drawing of multi-layered Newtonian and non-Newtonian fluid fibers, drawn under isothermal and non-isothermal conditions. We first develop one-dimensional equations governing mass, ...momentum, and energy balances and solve them numerically to obtain steady state draw root shape, velocity, and temperature profiles. These solutions are then used to perform linear stability analysis. For the case of isothermal draw, the system displays an oscillatory instability when the draw ratio (ratio of cross-sectional areas of fiber at the entrance and exit of the drawing) is higher than a critical draw ratio (highest stable draw ratio) of about 20.21. Investigation of stability behavior under non-isothermal draw conditions is performed by considering radiative heating and convective cooling. Employing only radiative heating enhances the critical draw ratio, and simultaneous heating and convective cooling increase the critical draw ratio even further. For the case of simultaneous heating and cooling, with increasing convective cooling strength, the critical draw ratio first increases, reaches a maximum, and then gradually decreases. However, with only convective cooling, the critical draw ratio decreases with an increase in convective cooling strength. We also find that the stabilizing effect of a non-isothermal operation can be enhanced by considering fluids with higher viscosity sensitivity to temperature, increasing the maximum temperature, and for sharper attenuation of the fiber cross-sectional area with length. For the case of isothermal drawing of non-Newtonian fluid fibers, the system has a higher critical draw ratio for shear thickening fluids (power-law exponent,
n>1). In contrast, the use of a shear thinning fluid (
n<1) reduces the critical draw ratio. Consideration of a non-isothermal operation of non-Newtonian fluid fibers reveals that the critical draw ratio is primarily determined by the non-Newtonian behavior rather than the non-isothermal drawing.
The paper presents improvements in simulated annealing based multiobjective algorithms and the modifications in the metrics methods used for the performance measure in multiobjective optimization. ...Simulated annealing (SA) based multiobjective algorithms have been made self-stopping as they do not require a total number of iterations. The performance metrics have been improved such that they can be used for the performance measure of a multiobjective algorithm and two multiobjective algorithms without a known true Pareto set. Pareto dominant based multiobjective simulated annealing with self-stopping criterion (PDMOSA-I) has been compared with existing Suppapitnarm multiobjective simulated annealing (SMOSA) in detail. The computational cost with PDMOSA-I is less than that with SMOSA. With the help of metrics methods, it has been found that the quality, extent and diversification of a Pareto set of solutions produced by PDMOSA-I is better than that produced by SMOSA.
We investigate instabilities that arise when the free surface of a liquid covered with an insoluble surfactant is vertically vibrated and inertial effects are negligible. In the absence of ...surfactants, the inertialess Newtonian system is found to be stable, in contrast to the case where inertia is present. Linear stability analysis and Floquet theory are applied to calculate the critical vibration amplitude needed to excite the instability and the corresponding wavenumber. A previously reported long-wavelength instability is found to persist to finite wavelengths, and the connection between the long-wavelength and finite-wavelength theories is explored in detail. The instability mechanism is also probed and requires the Marangoni flows to be sufficiently strong and in the appropriate phase with respect to the gravity modulation. For viscoelastic liquids, we find that instability can arise even in the absence of surfactants and inertia. Mathieu equations describing this are derived and these show that elasticity introduces an effective inertia into the system.
The adsorption of isolated charged dendrimers onto oppositely charged flat surfaces is studied in this work using Brownian dynamics simulations. The dendrimer is modeled as a freely jointed bead−rod ...chain in which excluded-volume interactions are modeled by a repulsive Lennard-Jones potential and bead−bead and bead−surface electrostatic interactions are described by screened Coulombic potentials. Adsorption behavior is studied as a function of inverse screening length, dendrimer generation, and dendrimer charge distribution. Adsorbed dendrimers adopt a disclike conformation in which they flatten in the direction normal to the surface and expand in the direction parallel to the surface. As the inverse screening length increases, the dendrimer expands in the normal direction and contracts in the parallel direction, adopting a conformation that is more stretched in the normal direction. When the inverse screening length becomes sufficiently large, the dendrimer desorbs and adopts a spherelike conformation. Bead density profiles show that adsorbed dendrimers form a two-layer structure, with one layer corresponding to adsorbed beads and a second, less dense layer corresponding to beads one rod length away from the surface. They also reveal how the distribution of monomers within the dendrimer and near the surface can be tailored by changing various problem parameters. The results presented here are expected to be helpful in providing qualitative guidance for dendrimer design in various applications.
The paper proposes a new simulated annealing (SA) based multiobjective optimization algorithm, called orthogonal simulated annealing (OSA) algorithm in this work. The OSA algorithm incorporates an ...orthogonal experiment design (OED) with a simulated annealing based multiobjective algorithm aiming to provide an efficient multiobjective algorithm. OED involves several experiments based on an orthogonal table and a fractional factorial analysis to extract intelligently the best combination of decision vectors making the classical SA to explore search space effectively, to enhance convergence, and to improve quality of solutions in the Pareto set. These benefits have been tested by comparing the performance of OSA with one state-of-the-art multiobjective evolutionary algorithm (NSGA2) and one classical simulated annealing based multiobjective algorithm (CMOSA) considering multiobjective problems of varying degrees of complexity. The obtained Pareto sets by these three algorithms have been tested using standard methods like measure
C, hypervolume comparison, etc. Simulation results show that the performance of and CPU time required by these algorithms are problem dependent, and with some problems, the OSA algorithm outperforms the other two algorithms. In particular, the comparison between OSA and CMOSA suggests that around 70% times OSA outperforms CMOSA and obtains a well diversified set of solutions. In addition, with some problems, OSA captures the Pareto fronts where CMOSA fails. Therefore, the development of OSA is noteworthy, and it provides an additional tool to solve multiobjective optimization problems.
