Tomohiko Yamaguchi

Numerical Analysis for Heat and Mass Transfer of Granular Flow in a Duct by the Discrete Particle Simulation

The solid-gas or liquid-gas two phase flow has many industrial applications such as spray drying, pollution control, transport systems, fluidized beds, energy conversion and propulsion, material processing, and so on. Though the solid-gas multiphase flow has been studied experimentally and numerically, the transport phenomena have not been cleared due to its complexity, computational time and economical costs for the hardware. In this study the heat and mass transfer of solid-gas collision dominated flow is analyzed by the Discrete Particle Simulation (DPS), a kind of the Dispersed Element Method (DEM)[1]. This method describes the discrete phase and the continuous phase by Lagrange and Euler methods respectively, and has been used to simulate the multiphase flow of various geometrical systems. In order to analyze the thermal field we took account of the energy equation and heat conduction between colliding particles. The heat transfer rate is summation of conductive heat transfer and convective heat transfer. Furthermore, the fluid flow has a two dimensional velocity profile, because the void fractions are analyzed as two dimensions. But momentum space has not been resolved by the two dimensional simulation. We call this method, the quasi two-dimensional simulation in this paper. To obtain the temperature distribution of the continuous phase the energy equation is solved in addition to the momentum equations. We treated the interaction between continuous and discrete phases as one and two way couplings. The positions, the momentum and the temperature information of particles and the velocity and the temperature distribution of the fluid were obtained as functions of time from results of these numerical simulations. When the hot air that is suspending small glass particles flows in a duct from bottom up, we traced the particles and got the temperature distribution of fluid and compared with the former results of one-dimensional flow. At the beginning, the cooler particles decrease the fluid temperature near the bottom of the vessel. The temperature profile of the particles obtained by the one-dimensional simulation is as same as quasi two-dimensional simulation. After 0.5 second the particles cool the downstream air. At 1.2 second, particles do not decrease the air temperature because the temperatures of particles are close to the inlet temperature of the air.© 2009 ASME

Film Boiling Around a Vertical Cylinder With Top and Bottom Horizontal Surfaces

Saturated and subcooled film boiling heat transfer around a vertical finite-length silver cylinder with top and bottom horizontal surfaces has been investigated, experimentally and analytically, in terms of cooling curve, and the correlations of heat transfer were proposed in the present paper. Pool film boiling experiments were carried out by quenching method. Cooling curves are obtained for saturated water at atmospheric pressure. The heated cylinder is made of silver and 18 kinds of cylinder are tested in the ranges of the diameter from 8 to 100 mm and the length from 8 to 160 mm. For subcooled water, the experiments were carried out in the similar method to the case of saturated water. The ranges of the diameter and length of the cylinder are 32 to 50 mm and 16 to 64 mm, respectively. The degree of liquid subcooling ranges from 2 to 30 K. In order to predict the film boiling characteristics, the overall heat transfer rate from a cylinder with finite length was modelled by taking into account each convective heat transfer on the bottom, side and top surfaces of the vertical cylinder. Present correlation equations for heat transfer and the lower limit of film boiling are good agreement with the experimental data for saturated and subcooled water. The values of wall heat flux and temperature at the lower limit of film boiling are obtained as the point where the cooling rate has a minimum value on the cooling curve. For the case of saturated water, wall temperature at the lower limit of film boiling is about 136 K and irrespective of the configuration of a cylinder. For subcooled water, the correlation is proposed for the effect of liquid subcooling on wall temperature at the lower limit of film boiling.Copyright

Topics on Boiling: From Fundamentals to Applications

This chapter deals with the various topics on boiling with regard to aspects of the fundamentals and applications to introduce the development of each author’s research in recent decades. The first four sections investigate the physics of boiling as phase change phenomena, including thermodynamic phase equilibrium state (Section 6.1), molecular dynamics of phase change (Section 6.2), computational analysis of boiling in micro-nano scale (Section 6.3), and transient boiling under rapid heating (Section 6.4). Section 6.5 deals with two-phase distribution measurement using neuron radiography. The following three sections then examine a specific boiling regime during highly subcooled boiling, called microbubble emission boiling (MEB). Each section treats the overall characteristics of MEB (Section 6.6), the occurrence conditions of MEB (Section 6.7), and vapor collapses in subcooled liquid related to MEB (Section 6.8). The next four sections are devoted to heat transfer augmentation with various techniques: thermal spray coating (Section 6.9), porous media (Section 6.10), patterned wettability refinement (Section 6.11), and self-rewetting fluid (Section 6.12). The last seven sections describe topics on applications of boiling. Sections 6.13 and 6.14 introduce boiling research in steel industries. Sections 6.15 and 6.16 explore vapor explosion. Boiling of refrigerant is discussed with heat pump systems in Section 6.17 and with automobile air conditioners in Section 6.18. Boiling related to emergency cooling core systems is considered in Section 6.19.

Effect of the Bottom and Top Configurations on Pool Film Boiling Around a Vertical Finite-Length Cylinder

The bottom configuration of a vertical finite-length cylinder is an important factor to examine the convective heat transfer by film boiling around a vertical finite-length cylinder, as the vapor generated under the bottom surface grows thicker during flowing upward along the vertical lateral surface and finally leaves the top surface as bubbles. In this study, four types of silver cylinder with a vertical lateral length equal to the diameter of 32mm were prepared for the possible combinations of bottom and top configurations: with a flat bottom and a flat top, with a flat bottom and a curved top, with a curved bottom and a flat top, and with a curved bottom and a curved top, where “flat” refers to “horizontal” and “curved” to “convex hemispherical”. Quenching experiments have been carried out for the test cylinders for saturated and subcooled water at atmospheric pressure. The initial temperature in the measurement is 600 °C. Boiling curves were obtained from the cooling curves measured using a K-type thermocouple inserted near the center on the axis of the test cylinder and the film boiling process was observed by still and high speed video cameras. The following results were obtained from the experiments using four types of test cylinder. 1. For saturated water, the test cylinders are entirely covered with a thick continuous vapor film, however, the effect of bottom configuration on film boiling heat transfer is appeared within 18% in terms of the wall heat flux averaged over the entire surface depending on the vapor fluid flow on the bottom and vertical lateral surfaces. 2. For the cylinders with a flat bottom surface, the wall heat flux averaged over the entire surface increases significantly with an increase in liquid sub cooling. This is attributed to that the convective heat transfer and the surface area ratio on the vertical lateral surface are predominant and govern the total heat transfer. 3. The effects of the cylinder top configurations on the film boiling heat transfer are small as the heat transfer on the top surface is small compared with that on the vertical lateral surface. 4. The differences between film boiling characteristics due to the bottom and top configurations are explained by examining the average heat transfer coefficient composed of the heat transfer coefficient and the surface area ratio on each surface. 5. The minimum wall superheat corresponding to the vapor-film-collapse is almost constant at 133K for four types of test cylinder in saturated water. In subcooled water, the minimum wall superheat for the cylinders with a flat bottom surface is larger than that for the cases with a convex hemispherical bottom surface.© 2011 ASME

Experiments on Flow Boiling Heat Transfer of Ammonia/Water Mixture Inside an Internally Spirally Grooved Horizontal Tube

Experiments were performed on the flow boiling of the zeotropic mixture of water-ammonia inside an internally spirally grooved horizontal steel tube with a 12mm average inner diameter. The experimental conditions were the mass fraction of ammonia: 0.95, 1.0 kg/kg, the mass velocity: 40 to 80 kg/(m2 s), the heat flux: 0 to 20 kW/m2 and the pressure: 0.7 MPa. The measured heat transfer coefficient reached its maximum as the quality approached about 0.6 but decreased abruptly as the quality increased. This sharp decrease as the quality increased beyond 0.6 may have been caused by mass diffusion resistance that increased the temperature locally at the vapor-liquid interface. The temperature increase at the vapor-liquid interface is discussed by analyzing the phase equilibrium characteristics and is explained in terms of the relationship between the bulk temperature and vapor quality. The heat transfer coefficients are also compared with those for pure ammonia.Copyright

Experiments and Analysis on Film Boiling Heat Transfer Around a Finite-Length Vertical Cylinder With a Convex Surface Swelling Downward

A method of predicting the overall heat transfer coefficient and the temperature at the lower limit of film boiling for a finite-length cylinder with flat top and bottom surfaces has been researched and proposed in a previous paper. This paper presents and compares an analysis in the case of a cylinder with a hemispherical bottom. The film boiling heat transfer around a vertical silver cylinder with a convex hemispherical bottom surface is investigated both experimentally and analytically in the present study. The obtained results are also compared and discussed with the authors’ previous results for a finite-length cylinder with flat top and bottom surfaces. Quenching experiments were performed using silver cylinders in saturated water. The diameter and length of the test cylinders are 32mm and 48mm, respectively. The test cylinder was heated up to about 600°C in an electric furnace and then cooled down in saturated quiescent water at atmospheric pressure. The resultant cooling and boiling curves and photographs of the film boiling phenomena are presented and discussed. The average heat transfer performance of the hemispherically bottomed cylinder is about 20% higher than that of the flat bottomed cylinder. The degree of wall superheating at the lower limit of film boiling is about 133K. The saturated film boiling heat transfer around the vertical finite-length cylinder with a convex hemispherical bottom was analyzed by taking into account the convective heat transfers from the bottom, side and top surfaces of the cylinder. The resulting analytical data correlated closely with the experimental data in the present study.Copyright

Numerical Analysis of Heat and Mass Transfer on Collision Dominated Particles Flow in a Vessel

Though the solid-gas multiphase flow has been studied experimentally and numerically, the transport phenomena have not been cleared due to its complexity, computational time required and economical costs for hardwares. In this study the heat and mass transfer of solid-gas collision dominated flow in a rectangular vessel is analyzed by the Discrete Particle Simulation (DPS), a kind of the Dispersed Element Methods (DEM)[1]. This method describes the discrete phase and continuous phase by the Lagrange and the Euler methods respectively, and has been used to simulate the multiphase flows of various geometrical systems. In order to analyze the thermal field we took account of the energy equation and heat conduction between colliding particles. We treated the continuous phase as a pseudo two dimensional flow, and the interaction between continuous and discrete phases as two way coupling. The positions, the momenta and the temperature information of particles and velocity and temperature distribution of fluid were obtained as functions of time from results of these numerical simulations. When the hot air flowed from bottom to top in the vessel of packed bed, we traced the particles and got the temperature distribution of fluid. The particles at the surface of the packed bed jumped first and made the void areas at the middle of vessel. We found the void areas that rise in the dispersed particles.Copyright