Ryozo Echigo

Superadiabatic combustion in a porous medium

Abstract Superadiabatic combustion with reciprocating flow in a porous medium has been investigated through a numerical calculation. In this system, a combustible gas with an extremely low heat content flows into the porous medium, where the flow direction reverses regularly. By the reciprocating flow system, the combustion gas enthalpy is effectively regenerated into an enthalpy increase in the combustible gas through the porous medium, which provides heat storage. The results have revealed that the flame temperature in the porous medium is 13 times higher than the theoretical one of the ordinary flame in free space. The heating value of the combustible gas is about 65 kJ m−3, which is equivalent to a temperature increase of only 50 K.

Experimental study on combustion in porous media with a reciprocating flow system

Abstract The results of an experimental study on the behavior of a reciprocating superadiabatic combustion system, the attainable flammable limits, and the low NOx and CO emissions which can be achieved with this system, are reported. Attention was focused on the influence of the dominating parameters, i.e., flow velocity, half cycle, and equivalence ratio, on the formation of temperature profiles in the porous body. Hereby the flammable limit was extended to the extremely low equivalence ratio of 0.026. In addition, the influence of the parameters on the levels of NOx and CO emissions was experimentally clarified and the emission levels are discussed with regard to the obtained temperature profiles. Extremely low NOx emissions of less than one ppm were obtained for all conditions investigated. The CO emissions were strongly dependent on the flow velocity and the type of porous medium used. For favorable conditions, low CO emissions in the order of ppm were also obtained.

Mathematical model of self-sustaining combustion in inert porous medium with phase change under complex heat transfer

Abstract The phenomenon of self-sustaining combustion of a gaseous mixture in inert high porous medium with prior vaporization of liquid droplets is studied by means of a numerical simulation. The complex heat transfer includes convective, conductive and radiative heat transfer between three phases: gas, solid and liquid. Evaporation and different modes of convective heat transfer between liquid, gaseous and solid phases are considered in detail. In particular, the behavior of particles which collide against and deposit on the skeleton of porous medium is taken into account, as well as the dependence of the heat transfer coefficient between liquid and solid on the skeleton superheat.

Turbulence structure and heat transfer of a two-dimensional impinging jet with gas-solid suspensions

Abstract The heat transfer mechanism of a two-dimensional impinging jet with gas-solid suspensions has been investigated through flow measurements using laser-Doppler anemometry. The most striking feature of the flow is the presence of particles rebounding from the impingement plate and of the gas-phase reverse flow caused by those particles. As a result, the turbulent intensity normal to the plate increases markedly near the stagnation point. However, in the wall jet region where the gas-solid interaction is relatively weak, the turbulence structure undergoes only a slight change. Heat transfer experiments in which the loading ratio is varied from 0 to 0.8 have been conducted. Around the stagnation point, the Nusselt number reaches 2.7 times as great as that of the single-phase flow, and the heat transfer enhancement ascribed to the drastic change in the turbulence structure.

Transient characteristics of combined conduction, convection and radiation heat transfer in porous media

Abstract This paper presents transient heat transfer characteristics of an effective energy conversion between high-temperature gas enthalpy and thermal radiation by means of porous media. A theoretical analysis is conducted for the one-dimensional system where conduction, convection and radiation take place simultaneously. The porous medium is assumed to be a homogeneous continuum which absorbs and emits thermal radiation. The coupled energy equations for gas and porous media are solved numerically. To confirm the validity of the analysis, experiments which strictly correspond to the analytical model are performed. The predicted results agree well with those of the experiments.

A new concept of porous thermoelectric module using a reciprocating flow for cooling/heating system (numerical analysis for heating system)

The paper presents the conceptual design of a novel thermoelectric cooler and/or heater utilizing the heat transfer effect due to forced convection. A porous thermoelectric converter combined with a reciprocating flow system in which the flow direction of air passing through the element is reversed after regular intervals is proposed. This flow system in effect makes the thermal conductivity insignificant and contributes toward the achievement of a highly efficient cooler and/or heater. A one-dimensional numerical analysis is performed to examine the detailed characteristics of the porous thermoelectric heater by systematically varying the relevant thermo-fluid parameters. In the calculation, for a fixed ambient temperature of 27/spl deg/C, dependences of the flow velocity, material porosity, and input power on the system performance are clarified. Moreover, a series of computation is carried out in order to obtain the systems COP.

Experimental study on the pressure drop and the entry length of the gas-solid suspension flow in a circular tube

Abstract This paper presents an experimental study on the flow characteristics of a dilute gas-solid suspension medium in a circular tube, wherein the inlet entry length and the frictional pressure drop are measured systematically. In particular, the effects of particle size on the frictional pressure-drop are examined in some detail. The spherical copper particles, whose average diameter ranges from about 45 to 170 μm, are used for the dispersed medium and the ranges of gas Reynolds number and solid loading ratio are up to 50,000 and 5 respectively. The experimental results show that the entry lengths are feasible to be correlated briefly to the apparent Reynolds number of suspension flow and also that the reduction of the frictional pressure drop is observed at the lower loading ratio for the smallest size particles.

Radiative heat transfer by flowing multiphase medium—part II. An analysis on heat transfer of laminar flow in an entrance region of circular tube

Abstract An analysis has been performed on the heat transfer with thermal radiation by flowing gaseous suspensions of solid and/or liquid fine particles in an inlet section of circular tube. The examination of the results on temperature profiles of both phases and heat transfer parameters illustrates that the multiphase medium is pertinent to heat transfer at high and extremely high temperatures because of the absorption behavior of the dispersed phase for thermal radiation and the results are summarized for wide ranges of parameters such as loading ratio, heat transfer characteristics between two phases, optical thickness of a duct, interaction parameter of conduction with radiation, etc. The interactions between the convection to fluid and the radiation are. thereafter, examined in some details.

Numerical analysis of electromagnetic wave in a partially loaded microwave applicator

Abstract A two-dimensional finite difference time domain method was employed to investigate the electromagnetic field in a microwave applicator filled partially with a dielectric material, operating in the dominant TE 10 mode at a frequency of 2.45 GHz. The power distributions developed in the applicator and inside a lossy material are calculated. Results show correlations between the power absorption ratio and physical parameters, e.g. the position of the dielectric in the applicator and permittivities of the dielectric, which are dominant in the heating process. In particular, the energy absorption ratio strongly depends on the position of the dielectrics, which shows a sharp maximum when the dielectric is located around the middle of the applicator. In addition, power absorption ratios are calculated according to the definition newly proposed in this study, and they are compared with those obtained experimentally.

EXPERIMENTAL STUDY ON FORCED CONVECTIVE HEAT TRANSFER OF FLOWING GASEOUS SOLID SUSPENSION AT HIGH TEMPERATURE

Abstract The results of an experimental study of the forced convective heat transfer to helium-graphite suspension at high temperatures up to 1173K are presented. Entering gas Reynolds number ranges from 1.0 x 10 4 to 2.0 x 10 4 and the particle loading ratio reaches about 4. The ratio of the Nusselt number of the suspension to that of gas alone increases considerably in a range of high loading ratios as the wall temperature increases. Subsequently, two kinds of turbulence promoters (200 and 400 mm pitch twisted tapes) are inserted in the flowing gaseous solid suspensions to make use of the large inertia forces of particles. The current results show that the local heat fluxes with use of the tapes increase significantly with the rise in the wall temperature owing to the radiative effect of the particulate phase.