Peyman Taheri

Couette and Poiseuille microflows : Analytical solutions for regularized 13-moment equations

The regularized 13-moment equations for rarefied gas flows are considered for planar microchannel flows. The governing equations and corresponding kinetic boundary conditions are partly linearized, such that analytical solutions become feasible. The nonlinear terms include contributions of the shear stress and shear rate, which describe the coupling between velocity and temperature fields. Solutions for Couette and force-driven Poiseuille flows show good agreement with direct simulation Monte Carlo data. Typical rarefaction effects, e.g., heat flux parallel to the wall and the characteristic dip in the temperature profile in Poiseuille flow, are reproduced accurately. Furthermore, boundary effects such as velocity slip, temperature jump, and Knudsen boundary layers are predicted correctly.

An extended macroscopic transport model for rarefied gas flows in long capillaries with circular cross section

Pressure-driven and thermally driven rarefied gas flows in long capillaries with circular cross sections are investigated. For both Poiseuille and thermal transpiration flows, a unified theoretical approach is presented based on the linear form of regularized 13-moment (R13) equations. The captured nonequilibrium effects in the processes are compared to available kinetic solutions, and the shortcomings of classical hydrodynamics, i.e., the Navier–Stokes–Fourier equations, are highlighted. Breakdown of Onsager’s symmetry is proposed as a criterion to determine the range of applicability of extended macroscopic models. Based on Onsager’s reciprocity relation it is shown that linearized R13 equations provide agreement with kinetic data for moderate Knudsen numbers, Kn≤0.25. Two-way flow pattern and thermomolecular pressure difference in simultaneous pressure-driven and temperature-driven flows are analyzed. Moreover, second-order boundary conditions for velocity slip and temperature jump are derived for the ...

Streaming current magnetic fields in a charged nanopore

Magnetic fields induced by currents created in pressure driven flows inside a solid-state charged nanopore were modeled by numerically solving a system of steady state continuum partial differential equations, i.e., Poisson, Nernst-Planck, Ampere and Navier-Stokes equations (PNPANS). This analysis was based on non-dimensional transport governing equations that were scaled using Debye length as the characteristic length scale, and applied to a finite length cylindrical nano-channel. The comparison of numerical and analytical studies shows an excellent agreement and verified the magnetic fields density both inside and outside the nanopore. The radially non-uniform currents resulted in highly non-uniform magnetic fields within the nanopore that decay as 1/r outside the nanopore. It is worth noting that for either streaming currents or streaming potential cases, the maximum magnetic field occurred inside the pore in the vicinity of nanopore wall, as opposed to a cylindrical conductor that carries a steady electric current where the maximum magnetic fields occur at the perimeter of conductor. Based on these results, it is suggested and envisaged that non-invasive external magnetic fields readouts generated by streaming/ionic currents may be viewed as secondary electronic signatures of biomolecules to complement and enhance current DNA nanopore sequencing techniques.

A circuit-based approach for electro-thermal modeling of lithium-ion batteries

A two-dimensional electro-thermal model has been developed to provide a tool that can be used to gain a better understanding of dynamic behaviour of lithium-ion (Li-ion) batteries. The model incorporates an equivalent circuit model (ECM) and a coupled electro-thermal model to simulate the non-uniform heat generation rate, the temperature distribution, and the voltage response of the Li-ion cell across variety of operating conditions. The ECM is comprised of two resistance-capacitance (RC) networks, one sires resistor, and one voltage source that are all function of state-of-charge (SOC) and temperature (T). Hybrid pulse power characterization (HPPC) test is applied to extract the ECM parameters at different ambient temperature. The simulations results of the cell model under constant current discharge tests show a good agreement with the experimental data at different environmental temperature. The developed model can be employed for battery thermal management system (BTMS) and battery management system (BMS) design with a light computational demand.

Thermal Transpiration Flow in Annular Microchannels

Thermal transpiration flows of rarefied gases in annular channels are considered, where the driving force for the flow is a temperature gradient applied in the channel walls. The influence of gas rarefaction, aspect ratio of the annulus, and surface accommodation coefficient on mass and heat transfer in the process are investigated. For this, the linearized Navier–Stokes–Fourier (NSF) and regularized 13-moment (R13) equations are solved analytically, and a closed-form expression for Knudsen boundary layers is obtained. The results are compared to available solutions of the Boltzmann equation to highlight the advantages of the R13 over the NSF equations in describing rarefaction effects in this particular thermally-driven flow. Through comparisons with kinetic data it is shown that R13 equations are valid for moderate Knudsen numbers, i.e., Kn < 0.5, where NSF equations fail to describe the flow fields properly.Copyright

Poiseuille Flow of Rarefied Gases Between Concentric Cylinders

Miniaturized devices are used currently in many engineering applications. Nonetheless, despite much progress in their fabrication, the fundamental understanding of fluid flow and heat transfer on the microscale is still not satisfactory. In this study, rarefaction effects in pressure-driven gas flows in annular microchannels are investigated. The influence of Knudsen number, aspect ratio of the annulus, and surface accommodation coefficient on wall friction, mass flow rate, and thermal energy flow rate is discussed. For this, the linearized Navier–Stokes–Fourier (NSF) and regularized 13-moment (R13) equations are solved analytically. The results are compared to available solutions of the Boltzmann equation to highlight the advantages of the R13 over the NSF equations in describing rarefaction effects in the process. Moreover, in order to improve the accuracy of the NSF system a second-order slip boundary condition is proposed.Copyright

EFFECTS OF ELECTRICAL CONTACT RESISTANCE ON EXTERNAL ENERGY LOSSES IN LITHIUM-ION BATTERY PACKS FOR HYBRID AND ELECTRIC VEHICLES

Lithium-ion (Li-ion) batteries are favored in hybrid-electric vehicles and electric vehicles for their outstanding power characteristics. In this paper the energy loss due to electrical contact resistance (ECR) at the interface of electrodes and currentcollector bars in Li-ion battery assemblies is investigated for the first time. ECR is a direct result of contact surface imperfections and acts as an ohmic resistance at the electrode-collector joints. ECR is measured at electrode connections of a sample Li-ion battery, and a straightforward analysis is presented to evaluate the relevant energy loss. Through the experiments, it is observed that ECR is an important issue in energy management of Li-ion batteries. Effects of surface imperfection, contact pressure, joint type, collector bar material, and interfacial materials on ECR are highlighted. The obtained data show that in the considered battery, the energy loss due to ECR can be as high as 20% of the total energy flow in and out of the battery under normal operating conditions. However, ECR loss can be reduced to 6% when proper joint pressure and/or surface treatment are used. A poor connection at the electrode-collector interface can lead to a significant battery energy loss as heat generated at the interface. At sever conditions, heat generation due to ECR might cause serious safety issues, thermal runaway, sparks, and even melting of the electrodes.

Analytical and Numerical Solutions of Boundary Value Problems for the Regularized 13 Moment Equations

Classical hydrodynamics—the laws of Navier‐Stokes and Fourier—fails in the description of processes in rarefied gases. For not too large Knudsen numbers, extended macroscopic models offer an alternative to the solution of the Boltzmann equations. Anlytical and numerical solutions show that the regularized 13 moment equations can capture all important linear and non‐linear rarefaction effects with good accuracy.