Analysis of Galinstan-Based Microgap Cooling Enhancement Using Structured Surfaces

Existing theories can predict separately the effects of hydrodynamic slip and ion-ion electrostatic correlations on diffusioosmosis. However, a predictive model for the coupled dynamics of hydrodynamic slip and electrostatic correlations in diffusioosmosis is lacking. In this work, we develop a mathematical model to compute the diffusioosmotic mobilities of valence-symmetric electrolytes in a charged parallel-plate channel. We employ the Navier slip condition to model the hydrodynamic slip at the channel walls and the modified Poisson equation to model ion-ion electrostatic correlations. We report two key findings arising from the competition between electrostatic correlations and hydrodynamic slip, which radically change diffusioosmosis. First, in a divalent electrolyte, a minute hydrodynamic slip defers the reversal in the direction of diffusioosmosis caused by electrostatic correlations to a higher concentration. Hydrodynamic slip can even eliminate the diffusioosmosis reversal in a monovalent electrolyte. Second, electrostatic correlations limit the change in the mobility due to hydrodynamic slip, by hindering the slip-enhanced ionic transport via surface charge overscreening. Electrostatic correlations can reduce the change in the mobility by ≈60% in a monovalent electrolyte, whereas the stronger electrostatic correlations in a divalent electrolyte can even reduce the change by an order of magnitude. The model developed from this work can be used to understand and predict diffusioosmosis in natural settings such as metamorphic transformation, in addition to that in applications such as colloidal species separation, nanoparticle drug delivery, and enhanced oil recovery.

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called "crossover phenomenon" for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.

On the thermal characteristics of a manifold microchannel heat sink subjected to nanofluid using two-phase flow simulation

The effects of surface roughness on fully developed laminar and transitional flow friction factors and heat transfer coefficients in horizontal circular tubes

In this article, the use of gas as a working fluid in the parabolic trough receiver (PTR) has been investigated numerically. The present study aims to determine which gas will work best as a heat transfer fluid (HTF) by analyzing the impact of several gases on the PTR’s performance. So, gases, such as carbon dioxide, helium, nitrogen, oxygen, hydrogen, methane, neon, and air, are considered in the present study, which flow through a thick-wall stainless steel pipe under a laminar flow regime. The temperature-dependent thermo-physical properties of the gas are considered. The finite difference method (FDM) with the harmonic mean technique is employed to solve this conjugate heat transfer problem. The current research findings are verified with those reported in the literature and are in excellent agreement. The grid-independent study is conducted to choose the optimal grid size. After that, the multi-objective function optimization technique and performance enhancement factor (PEF) is used to select the best suitable HTF, and it found that hydrogen gas is most appropriate based on these two criteria. Then, the influence of various parametric studies such as Reynolds number, solar flux, and receiver pipe thickness on the Nusselt number has been carried out. It is found that the Nusselt number increases with the Reynolds number while it decreases with solar flux and receiver pipe thickness. The significance of the current study may aid in choosing the appropriate gas as HTF in the parabolic trough solar collector (PTSC).

Exact analytical solutions for heat and mass transfer of MHD slip flow in nanofluids

This article describes molecular dynamics simulations of an ionic liquid (IL) confined between iron oxide surfaces under relatively high pressure and severe shearing, representative of a typical steel-steel lubricated contact. The simulations reveal the presence of hydrodynamic and thermal slip at the walls, despite the wetting nature of the fluid/wall interface. A crucial consequence of the temperature slip is the subsequent increase of the fluid temperature under shear, which modifies its effective rheology, resulting in saturation of the shear stress at high shear rates. Overall, this article provides a methodology for accurate modeling of tribological contacts lubricated by a nanometer-thick IL film. The results contribute to the debate on the saturation of the shear stress at high shear rates, and reveal the rich phenomenology arising in severe tribological conditions, departing from the traditional understanding of nanofluidic transport, mainly built in the linear response regime and standard thermodynamic conditions.

Aerosol transportation in horizontal channels with gravitational effects

Thermographic flow visualization is a contactless, non-invasive technique to visualize the boundary layer flow on wind turbine rotor blades, to assess the aerodynamic condition and consequently the efficiency of the entire wind turbine. In applications on wind turbines in operation, the distinguishability between the laminar and turbulent flow regime cannot be easily increased artificially and solely depends on the energy input from the sun. State-of-the-art image processing methods are able to increase the contrast slightly but are not able to reduce systematic gradients in the image or need excessive a priori knowledge. In order to cope with a low-contrast measurement condition and to increase the distinguishability between the flow regimes, an enhanced image processing by means of the feature extraction method, principal component analysis, is introduced. The image processing is applied to an image series of thermographic flow visualizations of a steady flow situation in a wind tunnel experiment on a cylinder and DU96W180 airfoil measurement object without artificially increasing the thermal contrast between the flow regimes. The resulting feature images, based on the temporal temperature fluctuations in the images, are evaluated with regard to the global distinguishability between the laminar and turbulent flow regime as well as the achievable measurement error of an automatic localization of the local flow transition between the flow regimes. By applying the principal component analysis, systematic temperature gradients within the flow regimes as well as image artefacts such as reflections are reduced, leading to an increased contrast-to-noise ratio by a factor of 7.5. Additionally, the gradient between the laminar and turbulent flow regime is increased, leading to a minimal measurement error of the laminar-turbulent transition localization. The systematic error was reduced by 4% and the random error by 5.3% of the chord length. As a result, the principal component analysis is proven to be a valuable complementary tool to the classical image processing method in flow visualizations. After noise-reducing methods such as the temporal averaging and subsequent assessment of the spatial expansion of the boundary layer flow surface, the PCA is able to increase the laminar-turbulent flow regime distinguishability and reduce the systematic and random error of the flow transition localization in applications where no artificial increase in the contrast is possible. The enhancement of contrast increases the independence from the amount of solar energy input required for a flow evaluation, and the reduced errors of the flow transition localization enables a more precise assessment of the aerodynamic condition of the rotor blade.

Crude oil samples were collected from three-buried gathering lines of Cambay Basin and were characterized in Drilling Cementation and Stimulation (DCS) Research Center, PDPU for their hydrodynamic (Viscosity, Density) and thermal (Specific heat, thermal conductivity) properties along with solubility curve (WAT, pour point and precipitation curve) at Gujarat Energy Research Management Institute (GERMI), PDPU. Theoretical wax mass fluxes for four available wax deposition models were computed using the characterized properties of crude oil samples. Field wax mass fluxes were computed based on field data obtained from the gathering lines and were compared with the theoretical wax mass fluxes. The field data for both 2 km and 5 km long 8 inch ID gathering lines indicated that the crude oil in these pipelines was flowing under laminar flow regime, while for 25km 8 inch ID gathering line it indicated a turbulent flow condition. This leads us to conclude that the presumption of only turbulent flow condition prevails is a myth; both laminar and turbulent flow conditions can occur. Furthermore, a wax mass flux analysis revealed that field wax mass fluxes for gathering lines with laminar flow regime (Re = 200 to 2300) is in good agreement with Film Mass Transfer (FMT) model, while the predictions from Equilibrium Model (EM) and Matzain models are in good agreement with the field data of gathering line with turbulent flow condition (Re = 5400). These agreements are because, for laminar flow case Chilton-Colburn analogy (no-precipitation case) is satisfied which means no precipitation occurs in the mass transfer boundary layer and large amount of wax is deposited radially in the pipe. While, for turbulent flow conditions, the concentration profile is dependent on temperature profile and complete precipitation or instantaneous precipitation occurs in the mass transfer boundary layer. In our study, we made every attempt to obtain reliable field data for comparison with the existing models. The four unique field cases presented here would be useful to researchers in validating their models. In addition, it made us realize that wax deposition is not only an offshore flow assurance problem but also a major flow assurance problem for onshore operations.

The motion of quantized vortices is studied using a vibrating wire in superfluid 4He. A vortex filtering method provides a superfluid practically free of remanent vortices in which the vibration of a wire cannot generate turbulence. Vortex lines are produced by cooling through the superfluid transition and remain forming bridges between a wire and a surrounding wall. Bridged remanent vortices increase the resonance frequency of a vibrating wire: the rate of an increase due to the remanent vortices is constant in a laminar flow regime and steeply increases in a turbulent flow regime with increasing wire velocity. These results suggest that oscillation of the bridged vortices provides a linear contribution to the wire vibration in the laminar flow regime, until instability occurs in the oscillation of the vortices, causing turbulence.

Reduction of fluid forces and heat transfer on a square cylinder in a laminar flow regime using a control plate

An energy perspective of pore scale simulation and experimental evidence of fluid flow in a rough conduit

Numerical investigation of effective parameters in convective heat transfer of nanofluids flowing under a laminar flow regime

The authors deal with the formulation and solution of a boundary value problem describing the influence of natural convection on forced convection during diffusion to a sphere in a laminar flow regime, the orientation of the natural convection being opposite to that of the forced one. The results of numerical solution are discussed and compared with the case where the orientation of both types of convection is the same. There is a good qualitative agreement with illustrative physical concepts referring both to hydrodynamics and concentrations.