MHD Channel Research Articles

Arc mode molten seed electrodes for MHD power generation

Experiments have been conducted in an electric furnace at atmospheric pressure with the object of investigating the seed arcing characteristics and electrical conductivities of seed materials, and ceramic consumption rates by seed arcing. Many important facts have been clarified from these experiments, namely that the arcing behaviors of molten seeds resemble closely those of mercury in a mercury rectifier, the cathode voltage drop of K 2CO3 seed is close to that obtained for the copper electrode in an MHD channel, the molten seed has an electrical conductivity comparable to those of advanced ceramic electrodes, and so on. These results show that so-called mode molten seed electrodes are highly promising for practical application. The arc mode molten seed cathodes for commercial MHD generating channels are discussed.

Analysis of power takeoff in diagonal conducting wall channels

An analysis of diagonal-conducting-wall MHD channels leads to design principles for the power takeoff and good quantitative agreement with experimental data. The effect of power takeoff schemes on the output power level is investigated analytically. It is found that it is possible to design the electrical circuit of the channel with passive elements in the takeoff region, and, achieve a fairly even load current distribution under changing load conditions without loss of power output.

Effect of non-uniform hall parameter on the electrode voltage drop in Faraday-type combustion mhd generators

Following a simplified approach, we derive an expression for the gas-dynamic voltage drop in a finitely segmented Faraday-type combustion MHD generator, taking into account the non-uniform Hall parameter across the channel. Combining the electrical sheath voltage drop, discussed briefly, with the gas-dynamic voltage drop, the effect of a non-uniform Hall parameter on the electrode voltage drop is studied using the theoretical and experimental input parameters of the Indian MHD channel test. The condition for the validity of the usual assumption of uniform Hall parameter across the channel is pointed out. Analysis of the measured electrode voltage drop predicts the real gas conductivity in the core to be in the range of 60 to 75 per cent of the theoretically calculated core conductivity.

MHD発電における電極の腐食とエロージョン

MHD発電における電極の腐食とエロージョン

MHD oscillatory flow with heat transfer in a flat plate channel

An analysis of the oscillatory MHD channel flow of an electrically conducting, viscous, incompressible fluid under a transverse magnetic field is presented. The transient velocity and the transient temperature are exhibited graphically, whereas the numerical values of the amplitude, phase of the skin-friction and the first and second harmonics of the rate of heat transfer are displayed in tabular form. Also discussed are the effects of M (the Hartmann number), e (the loading parameter), ω* (the frequency), and E (the Eckert number).

Heat Transfer Scaling Laws for MHD Channels and Diffusers

A quasi-three-dimensional flow model is used to study the convective and radiative heat transfer characteristics of coal-fired MHD channels and diffusers. The radiative contributions of carbon dioxide, water vapor, potassium atoms, and slag particles are all included in the model. The scale-up of heat transfer with the size of MHD channels and diffusers is characterized as an aid to extrapolation of experimental heat transfer measurements. The scale-up of radiation with the channel or diffuser size, centerline temperature, wall temperature, boundary layer thickness, gas pressure, and average size and number density of slag particles is investigated. Generalized plots of gas emissivity and absorptivity are provided for use in estimating radiative heat transfer in MHD channels and diffusers.

Two-Dimensional Performance Analysis and Design of MHD Channels

A two-dimensional model for MHD channel design and analysis has been developed for three different modes of operation: velocity, Mach number, and pressure. Given the distribution of any of these three parameters along the channel, the channel aspect ratio, and the channel operating conditions, the MHD channel geometry can be predicted. The developed two-dimensional design model avoids unnecessary assumptions for surface losses and boundary layer voltage drops that are required in one-dimensional calculations and, thus, can yield a better prediction of MHD channel geometry and performance. The subject model includes a simplified treatment for possible arcing near the electrode walls. A one-dimensional model for slag flow along the channel walls is also incorporated. The effects of wall temperature and slag carry-over on channel performance are discussed.

Wall leakage effect on nonuniformities in diagonal MHD channels

The effect of wall leakage, symmetric as well as asymmetric, is examined in the linear and nonlinear regimes. It is found that: the range of wavelength in the nonuniformities is reduced and in the asymmetric case splits into two separate passbands; depending on the wavelength, forward waves can propagate both upstream and downstream, and backward waves only upstream; the growth rate is minimal when the leakage at the cathode wall is four times that of the anode wall; and in all cases, a resistance introduced in the diagonal bars reduces the growth rate by a constant. Nomenclature h - width of the channel h' =/*tan0 H = electrode segmentation pitch Ie (k) =k mode component of the fluctuating current Ie (x) = total fluctuating current in the diagonal bars 70 = background dc current k = wave number Se = externally connected resistance in the diagonal bars x = longitudinal coordinate y = transverse coordinate 6 = angle of the diagonal link

Heat and Mass Transfer in MHD Channels

Accurate estimates of convective and radiative heat transfer in the magnetohydrodynamic channel are provided by simultaneousl y solving the radiation transport equation and the quasi-three-dimensional gasdynamic equations. Carbon dioxide, water vapor, and potassium atoms are considered as the principal constituents participating in gas radiation. Calculations performed for a base load-size channel indicate that heat transfer by gas radiation almost equals that by convection for smooth walls, and amounts to three-fourths as much as the convective heat transfer for rough walls. The evolution of slag particles by homogeneous nucleation and condensation is also investigated. The slag particle size spectrum is computed as a function of the ash carryover rate from the combustor to channel. The average particle size is found to increase and the total number density to decrease with ash carryover rate.

Wetting Layer and Void Fraction Nonunif ormity in a Liquid-Metal MHD Generator

THE question of the effect of a liquid layer on the walls of an MHD channel in the case of uniform void fraction distribution in the flow core was first considered several years ago. * More recently an analytic solution for high Hartmann numbers was obtained,2 which led to the conclusion that this layer does not have a significant effect on the efficiency of large generators. This paper postulates an analytic model which makes it possible to estimate the effect of a void fraction nonuniformity, in the presence of the wetting layer on the walls, on the generator performance. Contents Statement of the Problem The flow of a conducting fluid in the channel of an MHD generator with nonconducting walls z= ±a and conducting walls y= ±d/2 is considered. It is assumed that d>a and the channel length L>a. The fluid is taken to be a homogeneous liquid-gas mixture. The assumption of * 'homogeneity makes it possible to treat the mixture as a single-phase fluid, the properties of which are a function of void fraction a. It is also assumed that a. can vary in the magnetic field direction and a (z) is a known function. Let us introduce the nondimensional quantities (designated by an asterisk) using the following expressions V=VQU* (1)

Power Takeoff Analysis for Diagonally Connected MHD Channels

The electrical loading of the power take-off region of diagonally connected MHD channels is investigated by a two-dimensional model. The study examines the loading schemes typical of those proposed for the U-25 and U-25 Bypass channels. The model is applicable for the following four cases: (1) connection with diodes only, (2) connection with diodes and equal resistors, (3) connection with diodes and variable resistances to obtain a given current distribution, and (4) connection with diodes and variable resistors under changing load. The analysis is applicable for the power take-off regions of single or multiple-output systems. The general behaviors of the current and the potential distributions in all four cases are discussed. The analytical results are in good agreement with the experimental data. It is found possible to design the electrical circuit of the channel in the take-off region so as to achieve a fairly even load current output under changing total load current.

Approximate three-dimensional electric solution for a frame-type MHD generator

An approximate technique to determine the potential distribution and integral performance characteristics of a frame-type MHD channel is presented. In addition to including the effect of the conducting sideframes, the model considers the effects of finite segmentation and the effects of variable plasma conductivity, Hall parameter, and velocity. In its simplest form, the approximate technique effects results for the threedimensional potential distribution by solution of an ordinary differential equation. The accuracy of the technique is demonstrated by comparison with results of more detailed calculations and by comparison with experimental results. These comparisons show good agreement both for the integrated generator performance and for the current distribution to the electrode frame. In contrast to the detailed calculation procedure, the approximate technique has modest solution times and can be incorporated in a MHD generator design and prediction code.

Heat transfer in a MHD channel with uniform wall heat flux — effects of Hall and ion slip currents

The effects of the Hall and the ion slip currents on forced convective heat transfer in the thermal entrance region of a magnetohydrodynamic channel have been analysed by solving the energy equation as an eigenvalue problem. Both the generator and the accelerator modes are discussed. It is found that the heat transfer rates are reduced due to these currents.

Convective Heat Transfer in MHD Channels and Its Influence on Channel Performance

The limitations of the integral boundary-layer methods and the potential of the differential boundary-layer method in analyzing MHD channel flows are assessed. By showing the sensitivity of results from the integral method to the choice of parameters, the importance of prescribing accurate boundary-layer profiles and wall heat flux in the integral method is emphasized. A mixing-length-type turbulence model for flow on rough walls is developed and validated by comparison with experimental data. The turbulence model is used in a quasi-threedimensional boundary-layer model to evaluate the influence of wall roughness and pressure gradients on the flow characteristics and performance of MHD channels. The behaviors of skin friction and Stanton number calculated from the analytical model are found to differ considerably from the empirical correlations valid for non-MHD flows without pressure gradients. Nomenclature A, B = parameters in mixing length model B = magnetic field Cf = skin friction E = electric field H = total enthalpy IL = current load

High Interaction Subsonic MHD Channel Operation

Analytical and experimental investigations were conducted into the subsonic operation of MHD generator channels. Experiments were performed with a combustion-driven MHD channel in which transitions from supersonic flow to subsonic flow and vice versa were caused by varying the magnetic field strength. Subsonic operation under various conditions was verified by increases in combustor pressure with increases in magnetic field. Experimentally measured operating parameters were compared with predictions. Satisfactory agreement was generally found.

Effects of external circuit on heat transfer in MHD channel flow of rarefied gas

An exact analysis of the heat transfer in MHD fully-developed channel flow of rarefied gas has been presented on taking into account the external circuit. Closed from solutions have been derived for temperature and Nusselt number. It has been observed that the temperature increases with increasing λ (rarefaction parameter) and Γ (temperature jump coefficient). The Nusselt number increases with increasing I sc ∗ (> 0) and decreases with decreasing I sc ∗ (< 0).

End effects with a slowly varying magnetic field in a MHD channel with segmented electrodes

The end effects phenomenon is investigated in a MHD channel with finitely segmented electrode walls when the applied magnetic field decays as a sinusoidal function at the entrance and is constant inside the channel. the governing equations to determine the electric potential are solved numerically by the successive overrelaxation method. the normal current distribution and the electrical efficiency are calculated for various types of electrode spacing. It is found that the leakage currents are more for the case when the electrode length is greater than the insulator length, and the end losses are less.

Quasi-Three-Dimensional Modeling of Flow in Mhd Channels

Quasi-Three-Dimensional Modeling of Flow in Mhd Channels

End effects in a MHD channel with diverging electrode walls

End effects phenomena in a Faraday type generator with diverging electrode walls for two types of velocity profiles—one with a source velocity and the other with a fully developed velocity—are discussed. The electric potential is determined numerically using the successive overrelaxation method in polar coordinates. It is found that the viscous forces increase the end losses and create current concentrations on the electrodes even at far distances from the entrance.

Preliminary design for the power take-off of singly-loaded magnetohydrodynamic channels

A one-dimensional analysis of diagonal-conducting-wall MHD channels leads to design principles for the power take-off and good qualitative agreement with experimental data. Analytical refinements which account for two- and three-dimensional effects enhance the accuracy of performance predictions.