The fuel cell micro-grid is expected as a distributed power supply with little environmental impact. However, if a micro-grid is installed in an urban area, a generation efficiency of less than 21% on an all-year basis is expected. Generally, in planning an electric power network using a micro-grid, all the target buildings are connected and electric power is supplied. In this paper, a micro-grid is divided into multiple and each is optimized for the purpose of maximization of power generation efficiency. In the cooperation management of a micro-grid, large fluctuations in load, or increases and decreases in a building, can be followed with a grid using a system-interconnection device. The system proposed in this paper obtained results with high generation efficiency (from 21.1% to 27.6%) compared with the central system (generation efficiency is 20.6% to 24.8%) of a fuel cell micro-grid.
Pressure and velocity are generally discontinuous at the solidification front when the solid domain is deformed. This discontinuity, which has been ignored in most conventional solidification models for continuous casting processes, could cause significant numerical error around the solidification front in numerical simulations. To overcome error, a solidification front-tracking model in which the liquid-solid interface is discontinuous for pressure and velocity was developed. Assuming a sharp solidification front at the solidification front, the liquid and the solid domains are weakly coupled for momentum transport equations. The present model was proved to be more effective in cases of higher solidification rate such as in twin roll rapid cooling casting.
A two-dimensional numerical simulation of heat storage behavior by natural convection in rectangular enclosures heated from below has been conducted with fluidity slurry composed of micro-encapsulated fine phase-change-material (PCM) and water. Both heat storage and heat transfer characteristics were discussed with micro-encapsulated PCM slurry, which exhibited the pseudoplastic non-Newtonian fluid behavior and a peak value in the specific heat capacity with latent heat in phase temperature range. This paper also compared their characteristics between the same microcapsule slurries with and without phase change of PCM, and its superior performance for heat storage to the PCM microcapsule slurry in the phase-change temperature range was clarified. The effects of the heating wall temperature TH, the PCM mass concentration Cm of the slurry and the height H of the enclosure on heat storage were revealed, respectively.
Use of thin metal wire structures as a new type of extended heat transfer surface is proposed. As one of the most basic shapes of such wire structures, heat transfer performance of spring shaped fins is experimentally investigated under relatively low Reynolds number conditions. The averaged heat transfer coefficient is evaluated by a single-blow method while the pressure drop is measured at a steady state flow condition. The effects of the geometric parameters such as the wire diameter, the spring pitch and the pitch ratio were systematically examined and the obtained data were compared with that of a conventional offset fin, which is commercially available. It was found that the geometric parameters of the spring fins and the arrangement of spring fins in the test section affect their heat transfer performance. Some types of spring fins showed better heat transfer performance than a conventional offset fin, when they are evaluated in terms of the total heat transfer at a constant pumping power.
In this paper, the lattice Boltzmann method (LBM) is applied to simulation of natural convection in porous media using Brinkman-Forchheimer equation. The Brinkman-Forchheimer equation is recovered from a kinetic equation for the density distribution function that has a forcing term and the equilibrium distribution function including the porosity. The temperature equation which neglects the compression work done by the pressure and the viscous heat dissipation is calculated by a kinetic equation for thermal energy distribution function. The velocity and temperature profiles of the LBM shows good agreement with those of the finite difference method (FDM) for the Poiseuille flow filled with a porous medium and with the analytical solutions for the porous plate problem. The stream lines and isothermal patterns show that the LB model is able to keep the same accuracy with the FDM. For various values of Darcy and Rayleigh numbers, and of porosities, the solutions of the LBM are compared with those of earlier studies. The numerical experiment shows excellent agreement for the Brinkman-extended Darcy model and for the Brinkman-Forchheimer model. This paper leads to the conclusion that the LBM can simulate natural convection in porous media in both Darcy and non-Darcy region at the representative elementary volume scale.
The results of electro — thermal analysis, which is widely known as hydrodynamic model, are strongly dependent on the mesh size of model. However, the theory and method of accurate mesh size have not investigated. In this paper, we presented the calculation errors caused by the mesh size by using several mesh size models. The calculation results show that the mesh size for lateral direction, i.e. direction from the source electrode to the drain electrode in MOSFET, does not strongly affect the calculated characteristics of MOSFET. On the other hand, the calculation results strongly depend on the mesh size for vertical direction, i.e. direction from the gate oxide to the bottom surface in MOSFET. Here, we proposed the mesh zoning method for electro — thermal analysis, which was derived from the theory of the semiconductor physics. The calculation results with our mesh zoning method showed good accuracy compared to the results of the fine mesh. Since the mesh zoning means the reduction of mesh number, as a result, our mesh zoning method could reduce the calculation time by at least 30 times.
Reduction in fuel cell capacity linked to a fuel cell network system is considered. When the power demand of the whole network is small, some of the electric power generated by the fuel cell is supplied to a water electrolysis device, and hydrogen and oxygen gases are generated. Both gases are compressed with each compressor and they are stored in cylinders. When the electric demand of the whole network is large, both gases are supplied to the network, and fuel cells are operated by these hydrogen and oxygen gases. Furthermore, an optimization plan is made to minimize the quantity of heat release of the hot water piping that connects each building. Such an energy network is analyzed assuming connection of individual houses, a hospital, a hotel, a convenience store, an office building, and a factory. Consequently, compared with the conventional system, a reduction of 46% of fuel cell capacity is expected.
In this paper, we propose a lattice Boltzmann model for simulation of two-phase flows pertinent to thermal nonideal fluids in two dimensions. This LBM has a modified pseudo-potential so that it recovers a full set of hydrodynamic equations for two-phase flows through the Chapman-Enskog expansion procedure. Numerical measurements of thermal conductivity and of surface tension agree well with theoretical predictions. Simulations of bubble rising and of pool boiling with heat transfer are carried out. They demonstrate that the model is applicable to two-phase flows with thermal effects. Using finite difference Lattice Boltzmann method ensures numerical stability of the scheme.
Heat transfer of single-walled carbon nanotubes (SWNTs) in practical situations is investigated using molecular dynamics (MD) simulations. Attenuation of the expected high thermal conductivity was simulated by mixing 13C isotope impurities to SWNTs or binding two SWNTs with different chirality with a junction structure in between. The heat transfer through the junction can be expressed with the thermal boundary conductance by considering a virtual boundary at the junction. The lateral heat conduction was compared with the thermal boundary conductance at the interfaces between an SWNT and surrounding materials. By applying the lumped capacity method on the non-stationary molecular dynamics simulations, the thermal boundary conductance of an SWNT bundle and an SWNT confining water were calculated. Finally, some conventional properties were estimated to characterize the anisotropic heat conduction.
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Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)