Testing of a low-temperature liquid-metal MHD power system

A complete small-scale power conversion facility with a two-phase liquid-metal MHD generator has been built and tested. The working fluids were mercury and Refrigerant-113, and the working temperature was 80°C. The flow characteristics and the electrical current output have been measured and the efficiencies calculated. Because a complete system of this type has never before been built, it is significant that the facility has performed properly and produced net power for about 1000 h to date.     

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.     

Effect of nonideal conditions on the electrical performance of faraday MHD generators

The effect of nonideal conditions on the current flow in a MHD generator has been investigated by two-dimensional analysis in terms of internal resistance, Hall voltage and uniformity of current distribution. In particular, the effects of (a) temperature profile along the Faraday direction in the plasma, (b) current bunching on cathode surfaces due to phenomena like arc spots, and (c) electrical leakage between adjacent electrodes due to finite resistance of the insulator segments are investigated. the results show that cold boundary layers near metallic electrode surfaces tend to relieve current concentration. the optimum resistivity of wedge-shaped electrodes for obtaining uniform current distribution in the duct has been calculated for various temperature profiles. the results indicate that the internal resistance of a MHD generator strongly depends on the location and size of current bunched spots on cathodes. the fluctuations in the spot size and its location can lead to large fluctuations in power output. the present calculations reveal that the electrical performance of a MHD generator is not adversely affected by current leakage along insulator segments for small insulator conductivities.     

Hall field break down of seeded walls in MHD channel

The effect of seed on the break down of the insulating wall has been considered using a simple model. It has been shown that evaporation of the seed from the wall improves its electrical insulation.     

The effects of nonlinearities in the boundary layer characteristics on the fault currents in an MHD channel

An impulse excitation technique is used for the exact two-dimensional analysis of a short circuit between one pair of electrodes in a Faraday channel. The effects of the presence of slag layers and of arcs in the boundary layers on the fault current distributions have been included in the analysis. The results show that the presence of a slag layer tends to reduce the fault current while the arcs tend to increase it.     

Effects of ionizational non-equilibrium on the electron density across the non-equilibrium MHD channel

This paper gives the effects of ionizational non-equilibrium of charge carriers on the electron density across the channel of a non-equilibrium MHD generator. The steady and unsteady diffusion equations are solved numerically using an implicit finite difference scheme. It is found that as a result of ionizational non-equilibrium effects, the electron density reduces in the central region and increases near the wall region compared to its value obtained from Saha's equation. The ionizational non-equilibrium effects increase with increasing velocities.     

Bulk distribution of fault currents in a MHD faraday channel

A general impulse excitation technique is developed for exact two-dimensional analyses of electric faults. The exact impulse response function is derived and used to determine analytically the distribution of the fault current in the plasma core for the three specific cases: (1) short circuit of one load, (2) one load rejection, and (3) short circuit between two adjacent electrodes. The obtained fault currents are also presented in three dimensional plots for clarity. The results show that the faults hardly affect the core of the plasma, and that the perturbation currents flow in the thin boundary layers adjacent to the electrodes.     

垂直矩形窄通道启动过程壁温变化特性研究

以去离子水为介质,对竖直矩形窄通道在启动过程中壁面温度的变化规律进行了研究。将一次启动与二次启动时壁面温度的变化特性进行了对比,并改变入口温度,研究了入口温度对启动壁面温度的影响。结果表明：一次启动时壁面温度经过三个阶段,即急剧上升阶段、缓慢升高阶段、缓慢上升至最高壁面温度后又缓慢降至稳态饱和沸腾阶段;二次启动时产生的沸腾滞后现象较一次启动时明显;提高入口温度可增加饱和沸腾段,减少壁面温度的波动,在一定程度上可避免壁面温度过冲对换热器的损坏。     

Influence of undulating wall heat and mass flux on MHD natural convection boundary layer flow from a vertical wall

Current study expounds an unsteady magnetohydrodynamic natural convective flow along a vertical wall in presence of variable transverse magnetic field. Small amplitude undulation in wall heat flux and wall mass flux are imposed at the vertical wall to generate the boundary layer flow. The flow governing equations are divided into sets of steady and unsteady equations and then transformed into the similarity and nonsimilarity equations, respectively, by introducing stream function formulations. The sets of nonsimilarity equations are solved numerically by using three different techniques, namely, perturbation solution technique, asymptotic solution technique and implicit finite difference technique applied, respectively, for lower, higher, and all frequencies (ξ). Results are illustrated in connection with the amplitude and phase angles of shear stress, wall temperature, and concentration against the frequency (ξ) for wide ranges of physically significant parameters. Likening of the results obtained by above mentioned numerical methods are presented in every figure and table. Results reveal that the amplitude of undulating shear stress and wall temperature dwindle and the amplitude of wall concentration increases due to increment in Prandtl number (Pr). Besides, on incrementing Schmidt number (Sc) the amplitude of undulating shear stress and wall concentration dwindle and the amplitude of wall temperature increases. Results also reveal that on incrementing magnetic parameter (M) the amplitude of transient shear stress dwindles while the amplitude of transient wall temperature and concentration increase.     

Integral transform analysis of MHD flow and heat transfer in parallel-plates channels

A hybrid solution through the so-called Generalized Integral Transform Technique (GITT) is obtained for the MHD flow and heat transfer of a Newtonian fluid in parallel-plates channels. A simple mathematical formulation for the problem is adopted, which evidences both the transient regime flow sustainable only by a constant pressure gradient; and the steady state situation that considers both a constant pressure gradient and a movement of the upper plate, as well as the action of an inflow and outflow perpendicular to the porous plates. Results for the velocity and temperature fields are computed within the governing parameters, namely, pressure gradient, suction velocity, upper plate velocity and Hartmann numbers, for typical situations. A convergence analysis is also performed showing the consistency of the results. In addition, the present results are confronted with those previously reported ones in the literature showing excellent agreements.     

Radiation and ionizational nonequilibrium effects on the electron density in a nonequilibrium MHD generator duct

This paper presents an investigation of the radiation and ionizational nonequilibrium effects of charge carriers on the electron density across a nonequilibrium MHD channel. Using the generated and empirical velocity and temperature profiles, the steady and unsteady diffusion equations are solved numerically using an implicit finite difference technique. It is found that, as a result of the radiation effects during ionizational nonequilibrium processes, the electron density is more, in comparison with its equilibrium value, in the boundary layer region, whereas it is less than its equilibrium value in the core.     

Evaluation of electrode voltage drop in Faraday-type MHD generators

We have considered finitely segmented, Faraday-type and combustion gas driven MHD generators. Accounting for the finite electrode segmentation effect and the nonuniformity of gas in the channel, we have derived an expression for the gas dynamic drop for a turbulent boundary layer. Combining the electrical sheath drop, discussed in brief, with the gas dynamic drop, we have evaluated the electrode voltage drop for two experimental runs of MHD generators. The computed results are compared and found to be in good agreement with other evaluations.     

Comparative study of electrode voltage drops in MHD generators with and without ash in the combustion products

For estimation of the influence of slag layer on the electrode voltage drop, we have considered two identical open-cycle MHD generators with and without ash in the combustion products. Using the simple model of a slag layer with ohmic condution of current, we have evaluated the electrode voltage drop in a finitely segmented Faraday-type MHD generator with slagging electrodes. The electrode voltage drops in the presence of slag at different values of slag/plasma surface temperatures with and without slag for various values of Hall parameter and electrode wall temperature have been compared. It has been found that the presence of slag layer over the electrode surface with high slag/plasma surface temperature is more advantageous for lower electrode wall temperature and higher Hall parameter.     

Experimental study on cold start performance of PEMFC based on parallel flow channels

In this study, experimental methods are used to study the effect of assembly torque and channel size on cold start performance. Through the analysis of pressure distribution, fuel cell voltage, cathode gas inlet and outlet pressure drop and high frequency impedance, it is considered that, under the experimental conditions of this study, the fuel cell assembly torque is 2 N m, which has better cold start performance than the assembly torque of 1 N m and 4 N m. In addition, Under the condition of 2 N m assembly torque, the effect of four different flow channel sizes on the pressure distribution and cold start performance of the fuel cell was studied. It is found that when the width of channel is 0.5 mm, the depth of channel is 0.3 mm, the cold start performance is best.     

Analysis of heat fluxes in the electrode walls of a combustion-driven MHD generator

A qualitative analysis is presented to explain the difference in the measured heat fluxes in the anode and cathode in the arc mode of current transfer of a combustion-driven MHD generator. It is shown that various surface phenomena that take place near the wall, like Debye voltage, thermionic work function and electron enthalpy, approximately account for the asymmetry in anode and cathode heat fluxes under the same current condition.     

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.     

Assessment of methods to reduce the energy consumption of food cold stores

Energy is a major cost in the operation of food cold stores. Work has shown that considerable energy savings can be achieved in cold stores. Results from 38 cold store audits carried out across Europe are presented.Substantial savings could be achieved if operation of cold storage facilities were optimised in terms of heat loads on the rooms and the operation of the refrigeration system. Many improvements identified were low in cost (improved door protection, defrost optimisation, control settings and repairs). In large stores (>100 m³) most improvements identified were cost effective and had short payback times, whereas in small stores there were fewer energy saving options that had realistic payback times. The potential for large energy savings of at minimum 8% and at maximum 72% were identified by optimising usage of stores, repairing current equipment and by retrofitting of energy efficient equipment. Often these improvements had short payback times of less than 1 year.In each facility the options to reduce energy consumption varied. This indicated that to fully identify the maximum energy savings, recommendations need to be specific to a particular plant. General recommendations cannot fully exploit the energy savings available and therefore to maximise energy savings it is essential to monitor and analyse data from each facility.     

Numerical analysis of entropy generation in mixed-convection MHD flow in vertical channel

A numerical analysis is performed of the entropy generation within a combined forced and free convective magnetohydrodynamic (MHD) flow in a parallel-plate vertical channel. The MHD flow is assumed to be steady state, laminar and fully developed. The analysis takes account of the effects of both Joule heating and viscous dissipation. The nonlinear governing equations for the velocity and temperature fields are solved using the differential transformation method (D.T.M.). It is shown that the numerical results are in good agreement with the analytical solutions. The numerical values of the velocity and temperature are used to derive the corresponding entropy generation number (N_s) and Bejan number (B_e) within the vertical channel under asymmetric heating conditions. The results show that the minimum entropy generation number and the maximum Bejan number occur near the centerline of the channel. Overall, the results confirm that the differential transformation method provides an accurate and computationally-efficient means of analyzing the nonlinear governing equations of the velocity and temperature fields for MHD flow.     

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.     

Experimental study of vertical ground-source heat pump performance evaluation for cold climate in Turkey

Heat pump systems are recognized to be outstanding heating, cooling and water heating systems. They provide high levels of comfort as well as offering significant reductions in electrical energy use. In addition, they have very low levels of maintenance requirements and are environmentally attractive. The purpose of this study is to evaluate the experimentally performance and energy analysis of vertical ground-source heat pump (GSHP) for winter climatic condition of Erzurum, Turkey. For this aim, an experimental analysis was performed on GSHP system made up in the Energy Laboratory in the campus of Ataturk University. The experimental apparatus consisted of a ground heat exchanger, the depth of which was 53 m, a liquid-to-liquid vapor compression heat pump, water circulating pumps and other measurement and control equipments. Tests were performed under laboratory conditions for space heating, in which experimental results were obtained during January–May within the heating season of 2007. The experimentally obtained results were used to calculate the heat pump coefficient of performance (COP) and the system performance (COPs). The COP and COPs were found to be in the range of 2.43–3.55 and 2.07–3.04, respectively. This study also shows that the system proposed could be used for residential heating in the province of Erzurum which is one of the coldest climate region of Turkey.