MHD Effects Research Articles

Investigation of Thermophysical Properties of Nanofluids and Effects of MHD

Heat transfer through a fluid; It is an important parameter in many engineering fields such as heat exchangers, solar collectors, refrigerators, automobiles, cooling of electronic devices, power plants. The increasing demand for energy has made it necessary to use energy more efficiently. For this purpose, it is necessary to design compact, low-cost systems with high thermal conductivity and high-performance heat transfer fluids. Researchers for the use of nanofluids as advanced heat transfer fluid compared to conventional fluids used in heat transfer applications have been aimed at improving the thermophysical properties of nanofluids on magneto fields. Therefore, many parameters such as the size, concentration, shape, base fluid, pH effect and operating temperature of the nano particles used were examined. In this study, information is given about the studies in the literature related to magneto hydrodynamic and the effects of thermophysical properties of the nanofluids on heat and flow characteristics are exemplified by a numerical study. Keywords: Heat transfer, nanofluid, thermophysical properties, heat exchanger, CFD, MHD DOI : 10.7176/JETP/9-4-03 Publication date : April 30 th 2019

On both MHD and slip effect in micropolar hybrid nanofluid past a circular cylinder under stagnation point region

The physical model of the micropolar hybrid nanofluid flow with magnetohydrodynamic and thermal slip effects is considered. The flow of three-dimensional stagnation point of micropolar hybrid nanofluid past a circular cylinder is also taken into account. The flow model is controlled through partial differential equations. These equations are highly nonlinear in character and reduce to ordinary differential equations through applying the suitable similarity transformation. The reduced system is solved numerically using the shooting technique. The mutual effects are presented using graphs while comparative numerical results are presented in the tables. The diverse physical parts of the issue have been talked about. This study aims to investigate the solid nano-sized particle on the surface of a circular cylinder having sinusoidal variation. These nanosized particles are used to analyze the heat transfer improvement of micropolar hybrid nanofluid as compared to micropolar nanofluid.

Influence of Ohmic and ambipolar heating on the thermal structure of accretion discs

We investigate dynamics of accretion discs of young stars with fossil large-scale magnetic field. Our magneto-gas-dynamic (MHD) model of the accretion discs includes equations of Shakura and Sunyaev, induction equation, equations of thermal and collisional ionization. Induction equation takes into account Ohmic and magnetic ambipolar diffusion, magnetic buoyancy. We also consider the influence of Ohmic and ambipolar heating on thermal structure of the accretion discs. We analyse the influence of considered dissipative MHD effects on the temperature of the accretion discs around classical T Tauri star. The simulations show that Ohmic and ambipolar heating operate near the borders of the region with low ionization fraction (`dead' zone). Temperature grows by $\sim 1000$ K near the inner boundary of the `dead' zone, $r\approx (0.5-1)$ au, and by $\sim 100$ K near its outer boundary, $r\approx (30-50)$ au.

Computational Modelling on MHD Radiative Sisko Nanofluids Flow through a Nonlinearly Stretching Sheet

The boundary layer phenomena for Sisko-nano fluid flow is being observed with the effect of MHD and thermal radiation on a non-linear stretched surface. For developing a fundamental flow model, a boundary layer approximation is done, which represents time subservient momentum, concentration and energy equations. By taking the assistance of Compaq Visual Fortran, the fundamental equations are analysed by imposing a finite difference scheme explicitly. A stability and convergence study is also exhibited, and the ongoing investigation is found converged for Lewis number, Le >= 0.161 and Prandtl number, Pr >= 0.668. The impression of Sisko fluid parameter (A1, A2) along with diversified appropriate parameters is depicted in various flow fields. However, the developed visualisation of fluid flow is also depicted through streamlines and isotherms.

Effects MHD and Heat Generation on Mixed Convection Flow of Jeffrey Fluid in Microgravity Environment over an Inclined Stretching Sheet

In this paper, Jeffrey fluid is studied in a microgravity environment. Unsteady two-dimensional incompressible and laminar g-Jitter mixed convective boundary layer flow over an inclined stretching sheet is examined. Heat generation and Magnetohydrodynamic MHD effects are also considered. The governing boundary layer equations together with boundary conditions are converted into a non-similar arrangement using appropriate similarity conversions. The transformed system of equations is resolved mathematically by employing an implicit finite difference pattern through quasi-linearization method. Numerical results of temperature, velocity, local heat transfer, and local skin friction coefficient are computed and plotted graphically. It is found that local skin friction and local heat transfer coefficients increased for increasing Deborah number when the magnitude of the gravity modulation is unity. Assessment with previously published results showed an excellent agreement.

MHD Effects in Continuous Spin Detonation

MHD Effects in Continuous Spin Detonation

Improved CMB anisotropy constraints on primordial magnetic fields from the post-recombination ionization history

We investigate the impact of a stochastic background of Primordial Magnetic Fields (PMF) generated before recombination on the ionization history of the Universe and on the Cosmic Microwave Background radiation (CMB). Pre-recombination PMFs are dissipated during recombination and reionization via decaying MHD turbulence and ambipolar diffusion. This modifies the local matter and electron temperatures and thus affects the ionization history and Thomson visibility function. We use this effect to constrain PMFs described by a spectrum of power-law type, extending our previous study (based on a scale-invariant spectrum) to arbitrary spectral index. We derive upper bounds on the integrated amplitude of PMFs due to the separate effect of ambipolar diffusion and MHD decaying turbulence and their combination. We show that ambipolar diffusion is relevant for $n_{\rm B}>0$ whereas for $n_{\rm B}<0$ MHD turbulence is more important. The bound marginalized over the spectral index on the integrated amplitude of PMFs with a sharp cut-off is $\sqrt{\langle B^2 \rangle}<0.83$ nG. We discuss the quantitative relevance of the assumptions on the damping mechanism and the comparison with previous bounds.

MHD Effects on Mixed Convective Nanofluid Flow with Viscous Dissipation in Surrounding Porous Medium

In existence of concerning magnetic field, heat together with mass transfer features on mixed convective copper-water nanofluid flow through inclined plate is investigated in surrounding porous medium together with viscous dissipation. A proper set of useful similarity transforms is considered as to transform the desired governing equations into a system as ordinary differential equations which are nonlinear. The transformed equations for nanofluid flow include interrelated boundary conditions which are resolved numerically applying Runge-Kutta integration process of sixth-order together with Nachtsheim and Swigert technique. The numerical consequences are compared together with literature which was published previously and acceptable comparisons are found. The influence of significant parameters like as magnetic parameter, angle for inclination, Eckert number, fluid suction parameter, nanoparticles volume fraction, Schmidt number and permeability parameter on concerning velocity, temperature along with concentration boundary layers remains examined and calculated. Numerical consequences are presented graphically. Moreover, the impact regarding these physical parameters for engineering significance in expressions of local skin friction coefficient in addition to local Nusselt together with Sherwood numbers is correspondingly examined.

MHD effect under the interaction of external magnetic field with electrolytic hydrogen and oxygen bubbles

MHD effect under the interaction of external magnetic field with electrolytic hydrogen and oxygen bubbles

Effect of joule heating and MHD on peristaltic blood flow of Eyring–Powell nanofluid in a non-uniform channel

This paper examines the effect of joule heating and MHD on peristaltic blood flow of a Eyring–Powell nanofluid in a non-uniform channel. The transport equations involve the combined effects of Brownian and Thermophoresis diffusion of nanoparticles. The mathematical modelling is carried out by utilizing long wavelength and low Reynolds number assumptions. The energy equation is modelled by taking joule heating effect. The resulting nonlinear system of partial differential equations is solved for the stream function, velocity, pressure gradient, concentration and temperature distributions with the help of the Homotopy Analysis Method. The effect of various physical parameters on the flow characteristics is shown and discussed with the help of graphs. Pressure gradient gives opposite behaviour with increasing values of Eyring–Powell fluid parameters A and B. Pressure gradient decreases by increasing Hartman number and joule heating. Nanoparticle concentration increases with increasing thermophoresis parameter, but the reverse trend is observed with the effect of Brownian motion parameter.

Enhanced thermal energy transport of a ferrofluid contained in a double-sided lid-driven square container due to adiabatic block in the presence of magnetic force

This article contains computational results for mixed convective energy flow in cobalt-based ferrofluid enclosed in a two-sided lid-driven container providing heat from the left vertical moving boundary under MHD effects influenced by a source of heat generation/absorption when a square adiabatic block of different aspect ratios is located in the center of a square container. The governing equations describing the heat transfer and fluid flow are exposed to the penalty method first and after that reduced equations are simplified by the Galerkin technique. The governing flow parameters are a concentration of ferroparticles ( $ 0.0 < \phi < 0.1$ ), Reynolds number ( $ 50 < Re < 200$ ), Hartmann number ( $ 0 < Ha < 100$ ), Richardson number ( $ 0.1 < Ri < 100$ ) and heat source/sink coefficient ( $ -10 < Q < 10$ ). The solutions show that the enhancements in heat transport occur due to the presence of the block up to a certain block size. Streamlines recirculation cells suppressed and augmentation in heat transfer are remarkably high because of the influence of an adiabatic block. The results also show that the patterns of energy and fluid flow are significantly dependent upon the concentration of nanoscale solid ferromagnetic particles, heat generation/absorption coefficient, Richardson, Reynolds and Hartman numbers. The obtained results are expressed in terms of streamlines, isotherms, local and overall energy flow rates.

Investigation of MHD effect on nanofluid heat transfer in microchannels

An incompressible preconditioned lattice Boltzmann method (IPLBM) is proposed to investigate the fluid flow and heat transfer characteristics of nanofluid in microchannel with hydrophilic or superhydrophobic walls and partially under the influence of transverse magnetic field as well as a heat flux. The modified IPLBM is shown to overcome the velocity inaccuracy in developing regime under partial magnetic field with respect to standard LBM. Then, the method is utilized to resolve the velocity and temperature fields at Re = 100 and various volume fractions of nanoparticles (0 ≤ φ ≤ 0.2%), Hartmann numbers (0 ≤ Ha ≤ 30) and slip coefficients (0 ≤ B ≤ 0.1). Superhydrophobic walls are shown to reduce the wall shear stress at B = 0.1 of up to 38.4, 58.5 and 70%, respectively, for Ha = 0, 15 and 30. Ignoring the temperature jump in modeling overestimates the Nusselt number with an error that culminates at B = 0.1 and φ = 0.2% to 19.6, 22.7 and 25%, respectively, for Ha = 0, 15 and 30. It is concluded that with magnetic field presence and realistic temperature jump, the surface material of superhydrophobic walls should be chosen properly to avoid inevitable and uncontrolled reduction in heat transfer, such that the highest hydrophobicity is not always the best choice. Reasonable agreements are achieved by comparing our results with credible analytic and numerical solutions and also with an experimental study.

English

English

Gravitoturbulent dynamos in astrophysical discs

The origin of large-scale and coherent magnetic fields in astrophysical discs is an important and long standing problem. Researchers commonly appeal to a turbulent dynamo, sustained by the magneto-rotational instability (MRI), to supply the large-scale field. But research over the last decade in particular has demonstrated that various non-ideal MHD effects can impede or extinguish the MRI, especially in protoplanetary disks. In this paper we propose a new scenario, by which the magnetic field is generated and sustained via the gravitational instability (GI). We use 3D stratified shearing box simulations to characterise the dynamo and find that it works at low magnetic Reynolds number (from unity to ~100) for a wide range of cooling times and boundary conditions. The process is kinematic, with a relatively fast growth rate ($< 0.1 \Omega$), and it shares some properties of mean field dynamos. The magnetic field is generated via the combination of differential rotation and spiral density waves, the latter providing compressible horizontal motions and large-scale vertical rolls. At greater magnetic Reynolds numbers the build up of large-scale field is diminished and instead small-scale structures emerge from the breakdown of twisted flux ropes. We propose that GI may be key to the dynamo engine not only in young protoplanetary discs but also in some AGN and galaxies.

MHD stagnation-point flow and heat transfer of a nanofluid over a stretching/shrinking sheet with melting, convective heat transfer and second-order slip

PurposeThe purpose of this paper is to study the effects of MHD, suction, second-order slip and melting on the stagnation-point and heat transfer of a nanofluid past a stretching/shrinking sheet.Design/methodology/approachUsing appropriate variables, the governing partial differential equations were transformed into ordinary (similarity) differential equations, which are then solved numerically using the function bvp4c from Matlab.FindingsIt is found that dual (upper and lower branch) solutions exist for some values of the governing parameters. From the stability analysis, it is found that the upper branch solution is stable, while the lower branch solution is unstable. The sample velocity, temperature and concentration profiles along both solution branches are graphically presented.Originality/valueThe results of the paper are new and original with many practical applications of nanofluids in the modern industry.

MHD and Slip Boundary Condition Effects on Forced Convection Flow through a Microchannel Including Viscous Dissipation

MHD and Slip Boundary Condition Effects on Forced Convection Flow through a Microchannel Including Viscous Dissipation

Analysis of Non Constant Viscosity on Avertical Oscillating Cylinder with Mhd Effects

A numerical analysis was carried out on convective flow past a vertical oscillating cylinder with non-constant viscosity under the influence of magnetic field effects. The governing equations related to the flow problem are unsteady, nonlinear coupled partial differential equations. The same was transformed into a dimensionless form and the final form of equations was solved numerically. The approximate values of velocity, temperature, skin friction and Nusselt number are obtained. The viscosity considered as a function of temperature.

Kinematics of neutral and ionzied gas in the candidate protostar with efficient magnetic braking B335

AbstractAmbipolar diffusion can cause a velocity drift between ions and neutrals. This is one of the non-ideal MHD effects proposed to enable the formation of large Keplerian disks with sizes of tens of au (Zhao et al. 2018). To observationally study ambipolar diffusion in collapsing protostellar envelopes, we analyzed the ALMA H13CO+ (3–2) and C18O (2–1) data of the protostar B335, which is a candidate source with efficient magnetic braking (Yen et al. 2015). We constructed kinematical models to fit the velocity structures observed in H13CO+ and C18O. With our kinematical models, the infalling velocities in H13CO+ and C18O are both measured to be 0.85 ± 0.2 km s−1 at a radius of 100 au, suggesting that the velocity drift between the ionized and neutral gas is at most 0.3 km s−1 at a radius of 100 au in B335. The Hall parameter for H13CO+ is estimated to be ≫1 on a 100 au scale in B335, so that H13CO+ is expected to be attached to the magnetic field. Our non-detection or upper limit of the velocity drift between the ionized and neutral gas could suggest that the magnetic field remains rather well coupled to the bulk neutral material on a 100 au scale in B335, and that any significant field-matter decoupling, if present, likely occurs only on a smaller scale, leading to an accumulation of magnetic flux and thus efficient magnetic braking in the inner envelope in B335.

The impact of non-ideal effects on the circumstellar disk evolution and their observational signatures

AbstractIt has been recognized that non-ideal MHD effects (Ohmic diffusion, Hall effect, ambipolar diffusion) play crucial roles for the circumstellar disk formation and evolution. Ohmic and ambipolar diffusion decouple the gas and the magnetic field, and significantly reduces the magnetic torque in the disk, which enables the formation of the circumstellar disk (e.g., Tsukamoto et al. 2015b). They set an upper limit to the magnetic field strength of ∼ 0.1 G around the disk (Masson et al. 2016). The Hall effect notably changes the magnetic torques in the envelope around the disk, and strengthens or weakens the magnetic braking depending on the relative orientation of magnetic field and angular momentum. This suggests that the bimodal evolution of the disk size possibly occurs in the early disk evolutionary phase (Tsukamoto et al. 2015a, Tsukamoto et al. 2017). Hall effect and ambipolar diffusion imprint the possibly observable characteristic velocity structures in the envelope of Class 0/I YSOs. Hall effect forms a counter-rotating envelope around the disk. Our simulations show that counter rotating envelope has the size of 100–1000 au and a recent observation actually infers such a structure (Takakuwaet al. 2018). Ambipolar diffusion causes the significant ion-neutral drift in the envelopes. Our simulations show that the drift velocity of ion could become 100-1000 ms–1.

Numerical investigation of MHD effects on nanofluid heat transfer in a baffled U-shaped enclosure using lattice Boltzmann method

Lattice Boltzmann method (LBM) was carried out to investigate the effects of magnetic field and nanofluid on the natural convection heat transfer in a baffled U-shaped enclosure. The combination of different specifications of the baffle, LBM, nanofluid and magnetic field is the main innovation in the present study. In order to consider the effect of Brownian motion on the thermal conductivity, Koo–Kleinstreuer–Li model is used to define thermal conductivity and viscosity of nanofluid. Effects of Rayleigh number, Hartmann number, nanoparticle volume fraction, height and position of the baffle on the fluid flow and heat transfer characteristics have been examined. It was found that raising the Rayleigh number and nanoparticle solid volume fraction leads to increase the average Nusselt number irrespective of the position of the hot obstacle. However, the heat transfer rate is suppressed by the magnetic field. The heat transfer enhancement by introducing nanofluid decreases as increasing Rayleigh number, but it increases as increasing the Hartmann number. Moreover, the maximum heat transfer rate was observed when the enclosure equipped with a baffle with (s, h) = (0.2, 0.3) or (0.4, 0.3).