Thermodynamic acceleration of two-phase layered flows in channels with permeable walls of MGD-generators

When creating MHD (Magnetohydrodynamic) alternating current generators using liquid-metal working body, the latter is accelerated by piston method, in which the working body is a periodic structure of alternating zones of liquid metal (pistons) and zones of compressed gas, the latter is accelerated liquid-metal pistons. This raises the form stability problem for liquid metal pistons. Viscous frictional forces generated inside the pistons lead to the destruction of the pistons and significantly reduce the efficiency of the MHD unit. One solution is to use gas-permeable walls of the channels of MHD-generators, through the pores of which gas is injected. In this way, the piston flow is isolated from the side surface of the channel by the penetrating gas in the channel cavity. As a result, friction losses are drastically reduced. At the final values of the One solution is to use gas-permeable walls of the channels of MHD-generators, through the pores of which gas is injection coefficients, (? ≥ 0.03)the friction practically disappears. With the channel length determined by the coordinate of the maximum piston speed, 92% of the current marginal efficiency values can be achieved. The maximum efficiency of the runaway channel can be achieved by selecting the optimal value of the air injection coefficient. The operation of the devices commutating the injected gas must ensure that there is an injection in an area that is no more than twice the length of the piston.

The ultimate state of turbulent permeable-channel flow

Direct numerical simulations have been performed for heat and momentum transfer in internally heated turbulent shear flow with constant bulk mean velocity and temperature, $u_{b}$ and $\theta _{b}$ , between parallel, isothermal, no-slip and permeable walls. The wall-normal transpiration velocity on the walls $y=\pm h$ is assumed to be proportional to the local pressure fluctuations, i.e. $v=\pm \beta p/\rho$ (Jiménez et al., J. Fluid Mech., vol. 442, 2001, pp. 89–117). The temperature is supposed to be a passive scalar, and the Prandtl number is set to unity. Turbulent heat and momentum transfer in permeable-channel flow for the dimensionless permeability parameter $\beta u_b=0.5$ has been found to exhibit distinct states depending on the Reynolds number $Re_b=2h u_b/\nu$ . At $Re_{b}\lesssim 10^4$ , the classical Blasius law of the friction coefficient and its similarity to the Stanton number, $St\approx c_{f}\sim Re_{b}^{-1/4}$ , are observed, whereas at $Re_{b}\gtrsim 10^4$ , the so-called ultimate scaling, $St\sim Re_b^0$ and $c_{f}\sim Re_b^0$ , is found. The ultimate state is attributed to the appearance of large-scale intense spanwise rolls with the length scale of $O(h)$ arising from the Kelvin–Helmholtz type of shear-layer instability over the permeable walls. The large-scale rolls can induce large-amplitude velocity fluctuations of $O(u_b)$ as in free shear layers, so that the Taylor dissipation law $\epsilon \sim u_{b}^{3}/h$ (or equivalently $c_{f}\sim Re_b^0$ ) holds. In spite of strong turbulence promotion there is no flow separation, and thus large-amplitude temperature fluctuations of $O(\theta _b)$ can also be induced similarly. As a consequence, the ultimate heat transfer is achieved, i.e. a wall heat flux scales with $u_{b}\theta _{b}$ (or equivalently $St\sim Re_b^0$ ) independent of thermal diffusivity, although the heat transfer on the walls is dominated by thermal conduction.

Conjugate heat transfer numerical study of the ejector by means of SU2 solver

In this paper, the test supersonic ejector with conjugate heat transfer in solid bodies has been studied numerically. An extensive numerical campaign by means of open-source SU2 solver is performed to analyze the fluid dynamics of the ejector flowfield accounting for the heat conduction in solids. The fluid domain simulation is carried out by employing compressible RANS treatment whilst the heat distribution in solids is predicted by simultaneous solving the steady heat conduction equation. The working fluid is R245fa and all simulations are performed accounting for real gas properties of the refrigerant. Experimental data against numerical results comparison showed close agreement both in terms mass flow rates and static pressure distribution along the walls. Within the CFD trials, the most valuable flow parameters at a wall vicinity are compared: distribution across the boundary layer of the temperature and the turbulent kinetic energy specific dissipation rate, boundary layer displacement and momentum thicknesses. A comprehensive analysis of the simulation results cases with adiabatic walls against cases with heat permeable walls revealed the actual differences of the flow properties in the wall vicinity. However, the ejector performance has not changed noticeably while accounting for the heat conduction in solids.

EFFECTS OF THERMAL RADIATION AND BUOYANCY FORCE ON TRANSIENT HARTMANN FLOW IN A CHANNEL WITH PERMEABLE WALLS

The combined effects of thermal radiation, buoyancy force and variable heat source on an unsteady MHD flow of a conducting fluid through a porous walled channel is theoretically investigated. Base on some simplified assumptions, the model partial differential equations are obtained and tackled analytically using variable separable technique. Numerical solutions depicting the impact of various embedded thermophysical parameters on the fluid velocity and temperature profiles, skin friction and Nusselt number are displayed graphically and quantitatively discussed. An escalation in both skin friction and heat transfer rate is observed with a rise in fluid injection–suction at the channel walls.

Computational analysis of heat and mass transfer in a micropolar fluid flow through a porous medium between permeable channel walls

The present work numerically investigates the mass and heat transport flow of micropolar fluid in a channel having permeable walls. The appropriate boundary layer approximations are used to convert the system of flow model equations in ODEs, which are then numerically treated with the quasi-linearization method along with finite difference discretization. This technique creates an efficient way to solve the complex dynamical system of equations. A numerical data comparison is presented which assures the accuracy of our code. The outcomes of various problem parameters are portrayed via the graphs and tables. The concentration and temperature accelerate with the impacts of the Peclet numbers for the diffusion of mass and heat, respectively. It is also found that the porosity of the medium has a substantial effect on the skin friction but low effect on the heat and mass transfer rates. Our results may be beneficial in lubrication, foams and aerogels, micro emulsions, micro machines, polymer blends, alloys, etc.

Free convective flow of Hamilton-Crosser model gold-water nanofluid through a channel with permeable moving walls

Background: The present manuscript analyses the influence of buoyant forces of a conducting time-dependent nanofluid flow through porous moving walls. The medium is also filled with porous materials. In addition to that, uniform heat source and absorption parameters are considered that affect the nanofluid model. Introduction: The model is based on the thermophysical properties of Hamilton-Crosser's nanofluid model, in which a gold nanoparticle is submerged into the base fluid water. Before simulation is obtained by a numerical method, suitable transformation is used to convert nonlinear coupled PDEs to ODEs. Method: Runge-Kutta fourth-order scheme is applied successfully for the first-order ODEs in conjunction with the shooting technique. Result: Computations for the coefficients of rate constants are presented through graphs, along with the behavior of several physical parameters augmented the flow phenomena. Conclusion: The present investigation may be compatible with the applications of biotechnology. It is seen that, inclusion of volume concentration the fluid velocity enhances near the middle layer of the channel and retards near the permeable walls. Also, augmented values of the Reynolds number and both cooling and heating of the wall increases the rate of shear stress.

Modeling Heat Transfer Enhancement of Ferrofluid (Fe3O4–H2O) Flow in a Microchannel Filled with a Porous Medium

Heat transfer characteristics and hydrodynamical properties of ferrofluid through microchannels with non-uniform permeable walls temperature and filled with porous media plays an important role in modern microfluidic applications, such as solar collectors, nuclear reactors, micro-electro-chemical cell transport, micro heat exchanging, microchip cooling, and electronic equipment. Therefore, this paper presents the investigation of ferrofluid (Fe3O4-H2O) heat transfer characteristics as well as hydrodynamical properties in a permeable microchannel with non-uniform permeable walls. The semi-discretization finite difference method is utilized to solve the highly non-linear partial differential equations that govern the momentum and energy equations. Accordingly, the numerical outcomes reveal that the ferrofluid velocity and temperature profiles indicate a rising trend as the pressure gradient parameter, the variable viscosity parameter, the Darcy number, the Eckert number, and Prandtl number increase. The Reynolds number, which is a suction/injection parameter, shows a contrary influence on the ferrofluid velocity and temperature whereas nanoparticles volume fraction and the Forchheimer constant show a decreasing effect on the ferrofluid velocity and temperature. The outcomes also depict that the coefficient of skin friction at the cold wall of the microchannel is larger for higher values of the nanoparticles volume fraction, the variable viscosity parameter, the Darcy number, and the Eckert number. Besides, the coefficient of skin friction at the hot wall rises with the Darcy number, and the Prandtl number. Furthermore, the heat transfer rate at both cold and hot walls of the microchannel increases as the variable viscosity parameter, the Darcy number, the Eckert number, and the Prandtl number increase. The nanoparticles volume fraction and Darcy number show a retarding effect on the heat transfer rate at both walls of the microchannel.

Application of the Three-Parameter Differential Model of Turbulence for Solving Problems of Flow and Heat Transfer in Channels of Variable Cross-Section. Part 1

A description of the method of numerical study in the approximation of a narrow channel of the problems of flow and heat transfer in flat and circular channels of variable cross-section using a differential three-parameter model of shear turbulence is presented. The main results of numerous studies using the proposed method are described, one of the goals of which was to substantiate the possibility of using the narrow channel approximation. This review study is carried out in two parts. In the first part the results of studies of mixed convection in vertical pipes under conditions of stable and unstable stratification, as well as flows in channels with permeable walls in the presence of blowing or suction on the wall, are presented.