Takeo S. Saitoh

Bench mark solutions to natural convection heat transfer problem around a horizontal circular cylinder

Abstract High-accuracy bench mark solutions are presented for the natural convection flow around a horizontal circular cylinder with uniform surface temperature. In the past two decades, several approximate and numerical solutions to this problem have been reported in the literature. However, owing to limitations of (i) computer running time (CPU time) and (ii) resolution of the solution methodology, no exact bench mark solution has been presented to date. A large computational domain is inevitably necessitated in the case of small Rayleigh numbers. A thin boundary layer forms when the Rayleigh number increases, thereby requiring high resolution in the vicinity of the cylinder surface. Both of these features make it difficult to obtain exact bench mark solutions. These difficulties were overcome by adopting a high-accuracy fourthorder finite difference method and a coordinate transformation technique. The present bench mark solutions are accurate to at least three decimal places for small Rayleigh numbers (e.g. Ra = 103 and 104) and will be quite useful as standard comparison solutions to which many numerical solutions can be compared. Further, typical isotherms, streamlines, vorticities, local Nusselt numbers, tangential and radial velocities and temperature distributions were clarified in detail.

Unsteady Behavior of Diffusion Flames and Premixed Flames for Counter Flow Geometry

Abstract Unsteady analysis was carried out for both diffusion flames and premixed flames stabilized in the counter flow stagnation region when the velocity normal to the stagnation plane pulsates in a sinusoidal motion around its steady state value with small amplitude. Fluctuating and time mean components are investigated by adopting Arrhenius second order reaction rate model, assuming that the flow is inviscid incompressible and Lewis number unity. Furthermore, the theoretical results are compared with the experiment performed under the conditions involved in theoretical analysis.

Melting of unfixed material in spherical capsule with non-isothermal wall

Abstract Close-contact melting within a spherical capsule is investigated both numerically and analytically. A complete mathematical model is solved numerically by utilizing the boundary fixing method. The approximate approach developed by Bareiss and Beer for the horizontal cylinder is applied to constructing an approximate mathematical model of contact melting in a spherical capsule with a non-isothermal wall. The main characteristic scales and dimensionless parameters which describe the principal features of the melting process are found. Due to the presence of the small parameter in governing equations the perturbation method is implemented. As a result, simple analytical solutions were found which describe close-contact melting inside the capsule with a non-isothermal wall and account for the streamwise convection in the molten layer. The extensive validation of the analytical solution, and its comparison with the numerical results, gives the proof of accuracy of the analytical solutions with estimated error of 10–15%. This conclusion is of crucial importance for evaluating the real latent heat thermal energy storage systems which contain thousands of capsules, since the simple closed-form solutions for a single capsule, used in the mathematical modeling of such kind of complex systems, significantly reduces the cost of numerical computations.

An experimental study of droplet ignition characteristics near the ignitable limit

Abstract The present paper reports the experimental results for ignition characterisitcs of a fuel droplet near the ignitable limit. It is found experimentally that the ignition time increases as the droplet diameter decreases at the region near the ignition limits, and that, over the broad range, ignition occurs more rapidly as the diameter increases if the initial droplet temperature is high. The reason why such pehnomena occur is examined. A comparison between the results of Faeth and Olson and the present ones is made and the discrepancies discussed. The effect of initial droplet temperature on ignition is examined. The general trends agree with the results of computation obtained previously.

Effect of Thermal Radiation on Transient Combustion of a Fuel Droplet

The effect of radiation heat transfer on transient combustion of a fuel droplet with a finite rate of chemical reaction and variable properties has been studied under the assumption of spherical symmetry. Evaporation curves, transient variation of flame location, temperature profiles, and the ratio of the flame to droplet radius were compared to previously published results without the radiation effect. It was found that the radiation reduces by at least 25% the maximum flame temperature. Furthermore, the present results were compared to the experimental data of several researchers. As a consequence, it was shown that the reason for the previous discrepancy between the theory and experiment was attributed to the radiation.

Advanced energy-efficient house (harbeman house) with solar thermal, photovoltaic, and sky radiation energies (experimental results)

Abstract An energy-independent residential house (‘HARBEMAN house’; Harmony BEtween Man And Nature), incorporating sky radiation cooling, solar thermal, and photovoltaic energies was built in Sendai, Japan during July, 1996. This paper reports monitored results of this house since September 1996 to date. The paper also presents simulation results for the HARBEMAN house and its results compared with the annual experimental data. The HARBEMAN house, which meets almost all the energy demands, including space heating and cooling, domestic hot water, electricity generated by photovoltaic cell and rainwater for standard Japanese homes. Sky radiation cooling, solar thermal/photovoltaic (PV), and underground coolness as well as rainwater and waste heat are utilized in combination. Annual variations of water temperature in the underground main tank, heating/cooling/domestic hot water demands, collected and emitted heats by the solar collector and sky radiator have been monitored.

Transient Combustion of a Fuel Droplet with Finite Rate of Chemical Reaction

Abstract A numerical analysis for the transient combustion of a single fuel droplet with finite rate of chemical reaction has been performed under the assumptions of spherical symmetry and constant transport properties. Evaporation curves and transient variation of the flame location, temperature profiles, and the ratio of flame to droplet radius were obtained in detail. Moreover, the effects of finiteness of chemical reaction were revealed. It was found that there exists a critical droplet diameter beneath which no stable combustion can be realized. The present results support those of earlier analytical works of Waldman, which were obtained by virtue of an asymptotic expansion method.

An investigation of the diffusion flame around a porous cylinder under conditions of natural convection

Abstract A numerical analysis was performed for the natural convective diffusion flame around a porous horizontal cylinder from which fuel gas was ejected into a quiescent oxidant atmosphere. Two dimensionless parameters were utilized to describe the characteristics of the present flame. As a consequence, the phenomena of thermal quenching, which arises when the gas ejection velocity diminishes, and flame extinction, which is caused by chemical limitation, were predicted. Moreover, various features like flame structure, compressibility effect, heat-transfer characteristics, and geometric difference were clarified in detail.

A pilot solar water disinfecting system: Performance analysis and testing

Abstract In most countries, contaminated water is the major cause of most water-borne diseases. Disinfection of water may be accomplished by a number of different physical–chemical treatments including direct application of thermal energy, chemical and filtration techniques. Solar energy also can be used effectively in this field because inactivation of microorganisms is done either by heating water to a disinfecting temperature or by exposing to ultraviolet solar radiation. A pilot solar system for disinfecting contaminated water is designed, constructed and tested. Investigations are carried out to evaluate the performance of a wooden hot box solar facility as a solar disinfectant. Experimental data show that solar energy is viable for the disinfection process. A solar radiation model is presented and compared with the experimental data. A mathematical model of the solar disinfectant is also presented. The governing equations are solved numerically via the fourth-order Runge–Kutta method. The effects of environmental conditions (ambient temperature, wind speed, solar radiation, etc.) on the performance of the solar disinfectant are examined. Results showed that the system is affected by ambient temperature, wind speed, ultraviolet solar radiation intensity, the turbidity of the water, the quantity of water exposed, the contact area between the transparent water container in the solar disinfectant and the absorber plate as well as the geometrical parameters of the system. It is pointed out that for partially cloudy conditions with a low ambient temperature and high wind speeds, the thermal efficiency of the solar disinfectant is at a minimum. The use of solar energy for the disinfection process will increase the productivity of the system while completely eliminating the coliform group bacteria at the same time.

An energy-independent house combining solar thermal and sky radiation energies

Abstract A unique energy-independent house incorporating solar thermal, underground coolness, and sky radiation cooling energies is presented. An experiment on the long-term thermal energy storage (TES) performance was carried out for a model solar house at Tohoku University, and its characteristics, such as heat loss from the reservoir and long-term variation of the water temperature are reported. An experiment on the long-term sky radiation cooling was also performed and the radiative cooling performance of the sky radiator is reported. It was shown that the proposed system provides almost all the heating, cooling, and domestic hot water (DHW) needs for a standard Japanese home.