Special Issue

Technology of Combustion and Thermal Energy for Minimum Environmental Impact

- Utilization of Computer Simulation -

Technical Report

Numerical simulation in combustion chemistry

Kentaro TSUCHIYA (Combustion Engineering Div., Thermal Energy and Combustion Engineering Dept., NIRE)

Abstract
For chemical kinetics in combustion numerical simulation and sensitivity analysis are useful in experimental design and data analysis as well as model development and evaluation. This paper present an elementary knowledge of numerical simulation and sensitivity analysis in homogeneous gas phase chemical reactions. Also, this paper introduce CHEMKIN as a powerful tool for modeling chemistry and transport in flowing systems.

Key words : chemical kinetics, numerical simulation, sensitivity analysis, CHEMKIN

(Language:Japanese)

Technical Report

Thermodynamic Analysis of Process Systems

Takeshi HATANAKA (Advanced Combustion System Div., Thermal Energy and Combustion Engineering Dept., NIRE)

Abstract
This paper describes exergy changes from the viewpoint of energy transformation in a process system and methodologies for thermodynamic synthesis, estimate and analysis of a process system. It is important that these methodologies use both enthalpy changes related to the first law of the thermodynamics and entropy changes related to the second law, because these are necessary for making a quantitative analysis of the whole system in a unified manner. In a thermodynamic compass, a process is shown as a vector in the diagram. This diagram clarifies the thermodynamic characteristics of the process and is a powerful methodology for process synthesis of a novel system. An energy utilization diagram (EUD) graphically shows the exergy loss as the product of enthalpy change and energy level A, in which the former is the quantity of transformed energy and the latter represents its quality. The EUD methodology can easily clarify the points which should be improved and estimate the results of improvement on a quantitative basis. It is applicable to thermodynamic analysis of a process system. The procedure for system synthesis using graphical tools for designing a process system is described. These tools make it easy to use EUD methodology and develop a highly-efficient system.

Key words : Thermodynamic analysis, Exergy, Thermodynamic compass, Energy utilization diagram

(Language:Japanese)

Technical Report

Application of a General Purpose Flow Simulation Program as a Tool of Multi-Phase-Flow Simulation

Kotaro ENDOH (System safety Div., Safety Engineering Dept., NIRE)

Abstract
In these two decades, remarkable progress has been made in computer equipment regarding their capability to perform arithmetic operations much more quickly and cheaply. Computational Fluid Dynamics is becoming much more popular and is applied to a lot of engineering fields. Many flow calculation programs are on the market and some of them are available even on personal computers.

We use PHOENICS in Research Information Processing Station (RIPS) of our agency. The reason why we selected PHOENICS among some flow simulation cods is that it has features of multi-phase flow calculation from the beginning of its comming out. We have carried out the simulation of ventilation in the dome shaped underground space and dispersion phoenomena of spilled propane gas.

PHOENICS is one of the general purpose flow simulation program developed by Dr. D. B. Spalding and his team of CHAM Ltd. in 1978, and the first version of the program debutted in 1982. Several times of improvements have been carried out and its functions have been broadening year by year. In RIPS, PHOENICS was intstalled in 1988. The version 1.6.6 of it was adopted to RIPS in autumn 1995 when the menu function, one of the graphic user interface system, was added and it became easier for us to make input file for the PHOENICS pre-processor called SATELLITE. With the version updated to PHOENICS 2.2.1 this spring, the new feature GENTRA, which is a particle tracking program and calculates particle trajectories by the Lagrangian equations, becomes ready to use.

In this report, the outline of PHOENICS mainly about the functions of calculating multi-phase flows and its examples of application are explained.

Key words : Computational fluid dynamics, PHOENICS, Two phase flow, Interphase-slip-algorithm, General tracking program, Algebraic slip model

(Language:Japanese)

Original paper

Analysis of Heat and Mass Transfer Mechanism across a Free Surface in a Turbulent Flow using Direct Numerical Simulation

Ryuichi NAGAOSA and Takayuki SAITO (Ocean Mechanics Div., Mining and Geotechnology Dept., NIRE)

Abstract
This paper describes a heat and mass transfer mechanism at a free surface in an open-channel flow using a direct numerical simulation of turbulence. These results show that anisotropy of both the velocity and vorticity fields are emphasized near the free surface. These anisotropic velocity and vorticity fields are brought about by a pressure-strain effect near the free surface. The thickness of the anisotropic vorticity layer is about one-third of the characteristic length scale for a velocity field. It means that the effect of the free surface drastically has an influence the vorticity structures in an open-channel flow.

Coherent structures (CS) are extracted from the DNS database, as an essential agent of the scalar transfer near the free surface. Two types of the CS are found in the database : quasi-streamwise CS and surface-attached CS. The quasi-streamwise CS actively contributes to the intercomponent energy transfer due to the pressure-strain effect. In addition, these vortical structures replace the fluid on the free surface with that inside the turbulence. Therefore, these structures directly contribute to the scalar transfer at the free surface as surface-renewal motions. However, a surface-attached CS do not have strong vertical motions : they only rotate on the free surface. As a result, they are not essential structure to the scalar transfer at the free surface.

Key words : Heat and Mass Transfer, Coherent Structures, Direct Numerical Simulation, Free Surface, Turbulence Statistics

(Language:Japanese)

Original paper

Numerical Simulation of Coal Combustion in Circulating Fluidized Beds

Hendrik SCHOENFELDER (Dept. of Chemical Engineering Technical University Hamburg-Harburg)
Yoshizo SUZUKI and Hiroyuki HATANO (Advanced Combustion System Div., hermal Energy and Combustion Engineering Dept., NIRE)

Abstract
In circulating fluidized bed coal combustors, the flow mechanism and combustion reactions are quite complex. In this study, a simulator for a catalytic reactor is modified for a circulating fluidized bed coal combustor. The parameters required for the simulator were obtained from the experiment using a quartz bench scale combustor. Based on the parameters obtained, the simulator was able to estimate not only the concentration profiles in the CFB riser but also the pollutant emission profiles.

Key words : Circulating fluidized bed, Coal combustor, Simulation, Pollutant emission

(Language:Japanese)

Original paper

Prediction of the Operational Performance of the Gaia Snow-Melting System by Numerical Simulation

Koji MORITA and Makoto TAGO (Geo-Energy Div., Mining and Geotechnology Dept., NIRE)

Abstract
The earth has thermal functions such as heat source and heat sink which can be utilized for space heating and/or cooling, snow-melting and the production of warm or hot water. These functions are available everywhere and these resources are abundant. Another one of the earth's thermal functions is heat storage body which is available when there is no ground water flow. The utilization of the earth's thermal functions should contribute to reductions in carbon dioxide emission and fossil fuel consumption.

The authors have developed the Gaia Snow-Melting System in cooperation with several private companies. This system consists of Downhole Coaxial Heat Exchangers (DCHEs) proposed by the authors, a heat pump and heating tubes buried in a pavement or roadbed. In this system, the thermal energy in the shallow earth up to between 100 to 200m in depth is used as the main heat source and solar energy stored over the summer is used as an auxiliary.

The first Gaia Snow-Melting System was constructed in Ninohe City, Iwate Prefecture, in 1995 and has been successfully implemented over successive winters and has demonstrated effectiveness in reducing the emission of carbon dioxide and the consumption of fossil fuels. Also, the major design specifications of the system such as the number of DCHEs and the capacities of the heat pump and circulation pumps have proved to be appropriate.

The authors have developed two numerical simulation codes for designing the system and predicting performance of the system. One is for analyzing temperature behavior in the pavement and roadbed surrounding the heating tubes, and the other is for predicting the operational behavior and performance of the system.

Assuming the construction of another Gaia Snow-Melting System for a roadway in Ninohe, the authors have investigated the operational behavior and performance of the system using these codes. In this study, the area covered by the system was assumed to be 500m², and the heat flux to be supplied to the road surface for melting snow was assumed to be 145W/m² based on meteorological data from Ninohe.

The major results obtained in this study are as follows :

For designing the system, 18^oC and 15% are recommended as the average temperature of the heating medium and as the heat loss ratio from the heating tubes downward, respectively. In this case, 171W/m² must be supplied to the pavement and 85.5kW to the heating tubes.

Taking energy utilization efficiency and economics into account, 50liter/min/DCHE and 8^oC are recommended as the flow rate of the heat extraction medium and as the difference between delivery and return temperatures of the heating medium from/to the heat pump, respectively.

Five DCHEs, 160m in length, and a heat pump of 30kW_e are required for the system.

Average coefficients of performance (COP) over a snow melting season are estimated to be 4.6 for the heat pump and 4.4 for the system. The seasonal performance factor (SPF) is estimated to be 4.1.

In the case of the assumed system in this study, the COP of the heat pump and the system, and SPF decrease at rates of 0.07/^oC, 0.06/^oC and 0.05/^oC, respectively, with increases in heating medium temperatures.

Key words : Snow-Melting, Numerical Simulation, Geothermal Energy, Downhole Coaxial Heat Exchanger, Heat Pump

(Language:Japanese)

Original paper

The development of Pulverized Coal Combustion Simulator

H. TOMINAGA (Coal Research Lab., Energy Development Dept., Idemitsu Kosan Co.,Ltd.)
M. HARADA and T. ANDO(Technical Development Dept., Center For Coal Utilization, Japan)
Y. SUZUKI (Advanced Combustion System Div., Thermal Energy and Combustion Engineering Dept. NIRE)

Abstract
A pulverized coal combustion simulator has been developed to predict carbon-burnout and NOx emissions. Coal combustion model of modern computational fluid dynamics codes "FLUENT" was modified, and the modified model was verified by using 6[kg/hr] experimental pulverized coal combustion furnace. Combustion calculations were performed with a conventional coal combustion model and a modified one, and calculated results were compared with measured data. As a result, the validity of the proposed model in predicting carbon-burnout and temperature profiles was confirmed by the data from an experimental furnace.

Similar studies were carried out on a commercial boiler. As a result, we found that the proposed model can predict carbon-burnout and temperature profile with good accuracy.

Key words : Coal combustion, Boiler, Simulation

(Language:Japanese)

Original paper

Change in concentration distribution and equivalent rate constant with flow velocity in a boundary layer around a catalyst of non-uniform surface activity

Junko KONNO (Combustion Engineering Div., Thermal Energy and Combustion Engineering Dept., NIRE)

Abstract
Reaction on a solid catalyst in a flow reactor is often modeled as a combination process of diffusion to the external surface of porous catalyst, diffusion into the pore, and the surface reaction on the active points throughout the catalyst. Diffusion to the external surface of the catalyst is conventionally modeled as diffusion to a uniform external surface ; ignored is potential concentration distribution along an external surface.

We propose a model for mass transfer in a laminar film around a solid, non-permeable catalyst whose catalytic activity of external surface is nonuniform. We modeled the external surface of the catalyst as a plane consisting of two groups of patches placed regularly ; one group of patches has higher reaction rate constant than the other group. Dimensionless equations for mass transfer, including diffusion, convection and reaction, are given using dimensionless parameters, Damköèlar number (i=1,2;>) and Pecréô number Pe. Concentration distribution and consumption rate of reactant is calculated. Also calculated is equivalent rate constant _eq, a rate constant which gives the same consumption rate on a uniform activity surface with that on the surface in the model. Without flow, concentration along the surface is not uniform when 1 and .Under the above condition, concentration distribution changes with flow velocity in the range of 10¹<Pe<10⁶. Nonuniformity of concentration along the surface disappears at high Pe, resulting in an increase in _eq..Ratio of _eq at high Pe limit with that at low Pe limit is about 12 at most when patches are distributed in strips, and about 2 at most when patches are distributed in islands.

Key words : Reaction rate, Film model, Solid catalyst, Mass transfer

(Language:Japanese)