Hideo Yoshida

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.

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.

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

This 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 high efficient cooler and/or heater. For the first phase, a one-dimensional numerical analysis is performed to examine the detailed characteristics of the device by systematically varying the relevant thermo-fluid parameters. In particular, the flow velocity and the porosity of the thermoelectric elements are the most important parameters which directly affect the system performance. For example in a porous thermoelectric cooler, the lowest temperature of air is approximately -20/spl deg/C for an ambient temperature of 27/spl deg/C, which is attained with a flow velocity u=0.35 m/s, a material porosity /spl epsiv/=0.5, and 2.5 cm thick thermoelectric elements. As a notable feature, the temperature of the cooling section of the system varies considerably with the velocity, and it attains a minimum at u=0.35 m/s. Subsequently, on the basis of the proposed cooling system, an extended concept aimed to realize commercial coolers and/or heaters is briefly discussed.

Thermoelectric conversion analysis in a counter‐flow heat exchanger

Numerical computations are performed to study the effects of the relevant parameters on the thermoelectrical conversion installed in a counter‐flow heat exchanger. Results are reported for the temperature profiles in the channel as well as for the electrical power generated P and system efficiency ηsystem. These quantities show a strong dependence on a dimensionless parameter Rsk, consisting of an aspect ratio and a thermal conductivity ratio, and on the electric resistance ratio m. Principal results are summarized as follows: under the condition Rsk∼20 and m=1∼2, P and ηsystem reach maxima. As Re increases, P and ηsystem increase except for values of Rsk<20 where their values can decrease depending on the m parameter. Heat transfer enhancement produces an increase on ηsystem but its effects on P are negligible. Increasing L results in an increase on ηsystem that it expected to reach a maximum asymptotic value and also an increase on P, but it is noticed a trend in which further increase of L will produce a decrease on P.Numerical computations are performed to study the effects of the relevant parameters on the thermoelectrical conversion installed in a counter‐flow heat exchanger. Results are reported for the temperature profiles in the channel as well as for the electrical power generated P and system efficiency ηsystem. These quantities show a strong dependence on a dimensionless parameter Rsk, consisting of an aspect ratio and a thermal conductivity ratio, and on the electric resistance ratio m. Principal results are summarized as follows: under the condition Rsk∼20 and m=1∼2, P and ηsystem reach maxima. As Re increases, P and ηsystem increase except for values of Rsk<20 where their values can decrease depending on the m parameter. Heat transfer enhancement produces an increase on ηsystem but its effects on P are negligible. Increasing L results in an increase on ηsystem that it expected to reach a maximum asymptotic value and also an increase on P, but it is noticed a trend in which further increase of L will produce...

Analytical study on flame stabilization in reciprocating combustion in porous media with high thermal conductivity

A unique feature of reciprocating combustion in porous media is the systems capability to sustain combustion of extremely dilute fuel gases and to maintain steep temperature gradients in the inlet and outlet area of the porous media. However, for porous media with a high thermal conductivity, which are considered for application in thermoelectric conversion, the temperature gradients decrease; without a steep temperature gradient, reaction occurs in a wide region of the porous medium and it becomes difficult to establish a stable flame position. To counter this problem, a new arrangement of the porous media with a center space is proposed that stabilizes flame position for a wide operating range. Additionally, the new system largely extends the flammable range and reduces the specific energy input needed to sustain combustion. Overall, the effectiveness to generate a temperature difference in porous media with high thermal conductivity could be improved by the new system. Besides flame stabilization, an additional interesting feature of introducing a center space is the possibility of fuel injection into it, which leads to increased maximum temperatures (i.e., to a heat release at a higher energy level). This presents a great advantage over conventional reciprocating combustion in porous media where the maximum temperature is limited by the temperature of self-ignition for the premixed combustible gases.

Experimental and computational study of turbulent heat transfer characteristics in serrated channel flow

A two-dimensional (2-D) channel with a serrated wall is proposed as a device for heat transfer augmentation. Measurements of the flow velocity in the whole field as well as of the heat transfer coefficient along the wall are undertaken for two different channel heights. The same flow fields are calculated using a second-moment closure and the standard k-e model. The flow field containing successive separation and reattachment partly resembles the conventional backward-facing step flow, although the extremely high turbulence level found in the present configuration indicates a promising performance of the serrated channel as a heat transfer promoter.

Fine-tube heat exchanger woven with threads

Abstract A new type of compact heat exchanger is suggested, and the fundamental heat transfer performance is studied chiefly from experimental aspects. The design concept is based on the new principles for heat transfer enhancement, namely ‘reducing the size of heat transfer surfaces’ and ‘arranging a couple of turbulence promoters so as to cause drastic change in the turbulence structure’. To satisfy such requirements, the heat exchanger consists of fine tubes (o.d. ~ 1 mm) and woven threads (diameter ~ 0.3 mm) ; the latter play a combined role of both turbulence promoter and fin. The maximum heat transfer coefficient obtained in the experiment is 14-fold larger than that around a cylinder without threads. Owing to the combination of the woven threads and the very small characteristic length, the heat transfer coefficient per unit projected area is about 5 × 10 3 W m −2 K −1 , and that per unit volume reaches 3× 10 6 W m −3 K −1 .