Makoto Shibahara

Transient Heat Transfer for Forced Convection Flow of Helium Gas Over a Horizontal Plate

Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q0 exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.

Quantum molecular dynamics study of light-to-heat absorption mechanism in atomic systems

The quantum molecular dynamics method is applied to understand the fundamental mechanism of light-to-heat conversion in atomic systems under light irradiation. The light energy governs the time history of kinetic energy of the atomic system under light irradiation. Under infra-red light irradiation, the atomic fragments made by light irradiation have translational velocities parallel to the direction of light fluctuation. Under light irradiation at electron energy levels, the fragments tend to be isolated atoms having random translational velocities. The light electric field interacts with the atomic system through different terms of the molecular dynamics equations in the thermal conversion processes of light in the visible to infra-red range. Under infra-red light irradiation, the light interaction comes through fluctuations of effective dipole moment, whereas light irradiation at electron energy levels, it comes from changes in the potential energy between atoms associated with the electronic excitation.

Transient Heat Transfer From a Horizontal Plate in Forced Flow of Various Gases

In this research, to obtain fundamental experimental data of transient heat transfer and to clarify the transient heat transfer process at wide experimental conditions for the safety assessment of very high temperature reactor (VHTR), forced convection transient heat transfer coefficients were measured for Helium, Carbon dioxide, Argon and Nitrogen gases flowing over a horizontal plate due to exponentially increasing heat input. The platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as the test heater and heated by electric current. The heat generation rate was controlled and measured by a heat input control system, it was exponentially increased with a function of Q0 exp(t/τ). The periods (e-fold times) of heat generation rate, τ, ranged from 46 ms to 17 s, the gas flow velocities ranged from 1 to 10 m/s, the pressures ranged from 400 kPa to 800 kPa, and the gas temperatures ranged from 290 to 353 K. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The heat transfer coefficient increases with the increases in pressure and velocity, and it shows some dependence on temperature at the experimental range of this research. The dependence of transient heat transfer on the gas flow velocity becomes weaker when the period becomes very shorter. Effect of gas thermal physical properties on heat transfer was investigated, and helium gas shows higher heat transfer coefficients than those of other gases due to its higher thermal conductivity. Empirical correlations for quasi-steady-state heat transfer and transient one for various gases were obtained based on the experimental data.© 2011 ASME

Transient heat transfer for helium gas flowing over a horizontal cylinder in a narrow channel

ABSTRACT Transient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for the safety assessment of the transient heat transfer process in a very high temperature gas-cooled reactor (VHTR). Forced convection transient heat transfer for helium gas flowing over a cylinder in a narrow channel was experimentally studied at various periods of exponential increases in heat input. The test heater was mounted horizontally along the center part of a circular test channel with a diameter of 5 mm. The heat generation rates of the heater,, was increased with the exponential function, . By using a narrow channel, relatively high flow velocity was achieved, and experimental data at a high Reynolds number were obtained. According to the results, the surface temperature and the heat flux exponentially increased with time. It was clarified that the heat transfer coefficient approached the quasi-steady-state for a period of more than approximately 1 s, and it reached higher values for a period of less than approximately 1 s. The heat transfer coefficients present high dependence on the flow velocity of the helium gas and the heater diameter. Heat transfer correlations at the quasi-steady state and the transient state were obtained based on the experimental data.

Experimental and Numerical Investigation of Melting Process in PCM Storage

Thermal energy storage (TES) technologies have been developed using Phase Change Materials (PCM) at various power plants to utilize waste heat sources. The melting process of PCM has been investigated experimentally and numerically to construct a fundamental database of TES systems. D-Mannitol was selected as a PCM for medium TES systems in this study. The experimental apparatus consisted of the cartridge heater, thermocouples, test tube, acryl tube, vacuum pump, pressure indicator, volt slider and shunt resistance. The temperatures near the cartridge heater were measured by K-type thermocouples. The heat inputs were ranged from 10W to 15W. As a result, temperature of D-mannitol increased with time linearly under the solid state until the fusion temperature. When D-mannitol changed from the solid phase to the liquid phase, temperatures remained constantly due to the latent heat. Moreover, the numerical simulation was conducted using the commercial CFD code, ANSYS FLUENT. As a result of the numerical simulation, it was understood that the melting process was affected by the natural convection at the inner wall. As the heat flux of the cartridge heater input from the inner wall, the liquid fraction increased from the inner wall to the outer wall. The numerical result was compared with the experimental data. It was understood that the temperature of numerical simulation was approximately consistent with that of the experiment during the phase change process.Copyright

Heat Transfer Performance of Twisted Heat Exchanger for Carbon Dioxide Gas in a Dispersed Power Plant System Using Marine Biomass Resource

This paper is to propose a basic concept of marine renewable energy power plant system as a dispersed one, which is composed of a marine biomass plantation and a micro gas turbine. In this system, high-efficiency compact heat exchanger becomes necessary for the limit of the marine plant space. The author has already reported about a steady and transient heat transfer process for CO2 flowing over a horizontal plate under wide experimental conditions assuming a plate-type heat exchanger. For the heat transfer enhancement of the heat exchanger, the twisted plates were inserted in the tube and parallel plates. In the experiment, the overall heat transfer coefficients of the heat exchanger for carbon dioxide gas (CO2) are measured to construct a fundamental database for the proposed marine renewable energy system. Moreover, the three-dimensional analysis of the twisted heat exchanger has been conducted using the commercial CFD code, CFD2000. The twisted plate with a thickness of 0.3 mm is inserted in a tube which inner diameter is 7 mm. The gas flow velocities are ranged from 2.5 to 7.18 m/s for the inlet gas temperature of 323K. In the experiment, the overall heat transfer coefficient increases as the gas flow velocity increases. In the numerical simulation, the fluid structure in the tube has been changed caused by the twisted plate. The flow velocity near the twisted plate increases due to the blockage of the flow-pass. The temperature distribution was affected by the helically twisting fluid motion.Copyright

ICONE19-43534 Numerical Simulation of Dynamic Flow Structure and Thermal Stratification Phenomena in LMFBR

The three-dimensional analysis of thermal stratification in the upper plenum of MONJU is conducted using the commercial CFD code, FLUENT ver. 12.0. Since the temperature gradient near the thermal stratification interface would cause thermal stress in the reactor components, it is important to understand the characteristics of thermal stratification. As the result of numerical analysis, it is understood that the interface of thermal stratification is influenced by the flow pattern in the upper plenum of MONJU. After the jet from the core outlet is impinged on the upper core structure, the hot fluid flows obliquely upward to the inner barrel under the steady-state condition. On the other hand, the jet from core outlet flows to the lower part of the upper plenum, and then cold fluid flows through the flow holes under the transient condition. Hence, the flow structure has changed from the steady-state condition as the flow rate and temperature of the core outlet decrease due to the turbine trip. It is considered that the flow path of flow holes comes to govern under the transient condition, since the hot sodium acts as the plug due to the buoyancy.

Enhanced Heat Transfer for Various Gases Flowing Over a Twisted Heater Due to Exponentially Increasing Heat Input

Forced convection transient heat transfer coefficients were measured for various gases (helium, nitrogen, argon and carbon dioxide gas) flowing over a twisted heater due to exponentially increasing heat input (Q0 exp(t/τ)). The platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as the test heater. It was twisted at the center of the heater with an angle of 45 and 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q0 exp(t/τ). The gas flow velocities ranged from 1 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate ranged from 45 ms to 17 s. The surface temperature difference and heat flux increase exponentially as the heat generation rate increases with exponential function. The heat transfer coefficients for twisted heater were compared with those of a plate heater. They are 13 ∼ 28% higher than those of the plate one. The geometric effect (twisted effect) of heater in this study shows an enhancement on the heat transfer coefficient. This is because the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. And also, it was understood that heat transfer coefficient increase with the angle of twisted heater due to swirl motion and raised turbulence intensity. Empirical correlations for quasi-steady-state heat transfer and transient one were obtained based on the experimental data.Copyright

Effect of Heater Configurations on Transient Heat Transfer for Various Gases Flowing Over a Twisted Heater

Forced convection transient heat transfer coefficients were measured for helium gas and carbon dioxide gas flowing over a twisted heater due to exponentially increasing heat input (Q0 exp(t/τ)). The twisted platinum plate with a thickness of 0.1 mm was used as test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q0 exp(t/τ). The gas flow velocities ranged from 1 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increase exponentially as the heat generation rate increases with the exponential function. Transient heat transfer coefficients increase with increasing gas flow velocity. The geometric effect of twisted heater in this study shows an enhancement on the heat transfer coefficient. Empirical correlation for quasi-steady-state heat transfer was obtained based on the experimental data. The data for heat transfer coefficient were compared with those reported in authors’ previous paper.Copyright

Transient Forced Convection Heat Transfer Due to Exponentially Increasing Heat Input for Helium Gas Flowing on a Narrow Plate

Steady and transient forced convection transient heat transfer due to exponentially increasing heat input to a heater is important as a database for safety assessment of the transient heat transfer process not only in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity but also in high heat flux gas cooling devices such as a gas turbine and a rocket engine. In this research, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input (Q0 exp(t/τ)) to a horizontal narrow plate was numerically solved based on a turbulent flow model. The platinum plate with a length of 50 mm was used as test heater. The velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, τ, ranged from 46 ms to 8.6 s. The values of numerical solutions for surface temperature and heat flux were compared and discussed with authors’ experimental values. It was obtained that the surface temperature difference and heat flux increase exponentially as the heat generation rate increases with the exponential function. Then the temperature within the boundary layer also increases with the increase of the surface temperature. It is understood that the gradient of the temperature distribution near the wall of the plate is higher at a higher surface temperature difference. The values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities. And also, heat transfer coefficients at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities. They agree within 15% at various periods and velocities.Copyright