Combustion of polypropylene (PP) in a high-temperature, low-oxygen oxidizer enriched with H2O and CO2 in stagnation-point flow was studied numerically to explore fundamental characteristics of polymer incineration under the condition of high-temperature air combustion (HiTAC). The detailed chemistry of propene as a decomposition gas was used for the calculation. Two typical gas radiation models, i.e., the optically thin model (OTM) and the statistical narrow-band (SNB) model, were employed to clarify the effect of gas radiation on PP combustion as well as the validity of the use of these models under HiTAC conditions because the H2O and CO2 included in the burnt gas recirculated in HiTAC furnaces are highly radiative species. Results showed that, under HiTAC conditions, calculations using OTM overpredicts regression rates compared with those using SNB, indicating that OTM is not suitable for use with polymer combustion under HiTAC conditions, while the differences of these gas radiation models were slight when ordinary air at high temperature was used as an oxidizer. It was also shown that when SNB was used, CO2 enrichment in the oxidizer hardly enhanced the regression because CO2 near the PP surface behaves as a barrier against the radiative heat flux. The most effective condition to increase the regression rate is to maintain a higher concentration of H2O in the high-temperature oxidizer so as to enhance the gas radiation to the PP surface.
This study presents an optimization methodology for finding the heaters setting that produce a desired heat flux and temperature distribution over a region of the enclosure surface, called the design surface. Radiation element method by ray emission model (REM2) was used to calculate the radiative heat flux on the design surface, which enable us to handle reflecting surfaces. Micro-genetic algorithm was used to minimize an objective function, which was expressed by the sum of square error between estimated and desired heat fluxes on the design surface. The novel features of this methodology were demonstrated by finding the optimal heaters setting of a three dimensional enclosure for three aspect ratio. The effects of refractory properties have been studied as well.
This paper describes the transient sorption characteristics of organic sorbent under the low temperature. Our previous research result has shown that the organic sorbent had a large amount of water vapor uptake ability, and was also excellent in processability and deterioration resistance. The measured results of sorption isotherm equilibrium revealed that the organic polymer sorbent was a kind of material which sorption ability depends on temperature. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were conducted under various conditions of the air flow velocity, the brine temperature and the air absolute humidity. It is found that the sorption rate of vapor is affected most greatly by the air absolute humidity. Finally the average mass transfer coefficient of the organic sorbent is non-dimensionalized as a function of Reynolds number, non-dimensional temperature and non-dimensional vapor pressure of saturation.
The dynamic sorption characteristics of organic sorbent materials have been studied by using fluidized bed with a cooling pipe. The organic powder type sorbent made from a bridged complex of sodium polyacrylate which is one of the sorption polymers is adopted in this study. Sorption rate of water vapor and the variation of temperature in the sorbent bed with time were measured under various conditions. As a result, sorption ratio increases and the completion time for the sorption process decreases by using a cooling pipe. Furthermore, the non-dimensional correlation equations were obtained for water vapor mass transfer under sorption process in terms of relevant non-dimensional parameters.
Computations have been carried out to evaluate heat transfer coefficients given by the single-blow method that is characterized by a transient and conjugate heat transfer problem between the fluid and the solid. Both heat conduction and convection equations are solved numerically to obtain the transient fluid and fin temperature distributions. Finite volume solutions indicate that the fin temperature distribution of the single-blow method varies with time and position and that the fluid temperature distribution of the single-blow method is close to that observed in the steady-state computation specified with the constant wall temperature condition. It is found that the local heat transfer coefficient resulting from the single-blow method is almost identical to that from the steady-state constant wall temperature computation and is nearly time-independent.
Microscopic structures of a liquid-vapor interface are investigated by molecular dynamics simulations. In the previous studies, we proposed the local and instantaneous definition of the interface at the molecular level, which can capture the thermal fluctuation of the interface. By using this definition, the layering structure of water molecules at the interface was found, in other words, the structurization phenomena of water at the molecular level were clearly seen as usually found at the liquid-solid interface. In this study, we investigated the liquid-vapor interface of Lenard-Jones fluid. The effect of well depth of L-J potential parameter on the structure was also studied. Although the structurization was found at the L-J fluid as well as water, characteristic of this structure was clearly different from that of water. We consider that the difference is ascribed to the intrinsic structure of liquid and associative trend of molecules. We also discussed the anisotropic characteristics of the molecular diffusion at the interface. The anisotropy of the translational diffusion at the interface of water is stronger than that of the L-J fluid.
Experiments have been conducted to measure the rise in temperature of hydrogen and vessel wall during filling of commercially available, practical tanks to 35 and 70 MPa. Three test vessels with volumes 205, 130 and 39 liters are investigated. The filling time ranges from 5 to 20 minutes. The heat transfer process is modeled using a one-dimensional unsteady heat conduction equation for the wall coupled with a flow and heat balance for the compressed gas. The model requires heat transfer coefficients between the hydrogen and the wall and the wall and surrounding air. Values of 500 W/(m2K) during filling, 250 W/(m2K) after filling for the inside wall and 4.5 W/(m2K) for the outside tank wall are tentatively assumed based on results from a previous study on a smaller vessel. The measured temperatures for the hydrogen gas and the wall are in good agreement with the calculations.
The purpose of this study is to elucidate flame propagation behavior under the application of uniform and non-uniform electric fields by using a constant volume vessel. Two electrodes are attached to the ceiling and the bottom of the combustion chamber and electric fields are applied in the direction of the chamber's vertical axis. A Nd:YAG laser is used to apply laser-induced breakdown for igniting the mixture at the center of the combustion chamber. A homogeneous propane-air mixture is supplied at three equivalence ratios of 0.7, 1.0 and 1.5 and ignited under atmospheric pressure and room temperature. Under a uniform electric field, the premixed flame rapidly propagates both upward and downward, forming a cylindrically shaped flame front. The maximum combustion pressure decreases with increasing input voltage because the flame front reaches the chamber wall rapidly and the heat loss to electrodes increases. However, the combustion duration is little affected by the input voltage. In a non-uniform electric field, the flame propagation velocity in the downward direction increases. Combustion is markedly enhanced when the input voltage is larger than 12 kV because a brush corona discharge occurs and intense turbulence is generated at the flame front. For both uniform and non-uniform electric fields, the horizontal flame velocity is almost the same.
The authors investigated the effect of the turbulence of an air current on droplet dispersion in a spray flame. Wire meshes of different mesh sizes were inserted behind a two-fluid type nozzle each time to vary the air current turbulence characteristics, and then methanol was sprayed to form a spray flame. Droplets in the spray flame were measured with a Phase Doppler Particle Analyzer (PDPA). In the no-reaction field, the turbulence characteristics were measured by using a hot wire anemometer to determine the influences of the turbulence on droplet dispersion in a flame. As the mesh size of the wire mesh was reduced in the investigation, the droplet dispersion was shown to reduce the volume of cluster and also the number density of the droplets. Measuring the turbulence statistics confirmed a decrease of the Stokes number in proportion to the mesh size of the wire mesh. Droplet flying trajectories through turbulence were calculated. According to the calculation results, inserting a finer wire mesh was found to move flying trajectories closer to the streamline of the vortex. When a finer wire mesh was inserted, droplets were drawn closer to the vortex and formed clusters near the vortex. Consequently, droplets were found to disintegrate and disperse under the great in fluences of a vortex.
NOx catalytic converter systems periodically require rich or stoichiometric operating conditions to reduce NOx. The HC (hydrocarbon) concentration in a diesel engine is typically so low that the HC is not sufficient for NOx conversion. It was proposed that a rich air fuel ratio in a diesel engine could be realized via post fuel injection or supplemental fuel injection into the exhaust gas. A new method that optimizes the control of an external HC injection to diesel exhaust pipes for HC type LNT (Lean NOx Trap) catalyst systems has been developed. The external injection has other benefits: it can be controlled independently without disturbing engine control, it can be adapted to various layouts for exhaust systems, it has no oil dilution problems, among other benefits. In this study, the concentration and amount of HC can be controlled via control of the external injection. This research investigated the spray behavior of hydrocarbons injected into the transparent exhaust pipe. From this experiment, we obtained useful information about the optimal injection conditions for the HC-LNT catalyst system with an MPI injector.
This paper reviews characteristics of heat transfer during quenching high temperature body using liquid jet impingement. The temperature of body is initially kept higher than the Leidenfrost temperature. The flow pattern dramatically changes with a decrease in the surface temperature. At a moment of jet impingement, the liquid is randomly splashed away. After that, it seems to change a cone shape splashed flow, in which liquid is confirmed to be contact with the surface in an impinging zone. Finally, the liquid contact area where occurs rigorous boiling starts moving radially. Under such flow configuration, how surface temperature and heat flux change with time and homogenous nucleation for vapor generation are discussed.
This review covers several problems in thermal engineering which require consideration in nano-scale or at molecular level. One example in nanofluidics is “nano bubbles”; molecular simulation reveals that surface tension and vapor pressure of a spherical bubble hardly depend on its size and that the classical Young-Laplace equation is applicable even to a bubble as small as several nano meters. Combined with CFD schemes, molecular simulation can also treat oscillating dynamics of nanobubbles. Another example comes from solid-state physics, i.e., thermophysical properties of nano-scale elements. As the thickness of solid thin film decreases to “phonon mean free path”, the apparent thermal conductivity becomes smaller, the mechanism of which molecular simulation can explain.
A predictive model for describing the gas enthalpy-radiation conversion process in an open-cellular porous plate is proposed. The ability of the model in predicting temperature distributions within porous converters is examined in comparison with available experimental data. Extensive numerical simulations for commercially available porous plates made of Ni-Cr and partially-stabilized ZrO2 are performed by varying several system parameters such as porous plate thickness, inlet gas temperature and mass flow rate in order to evaluate decreases in gas temperature across the porous plates and the conversion efficiency defined by a ratio of recaptured radiation to incoming energy and also to give design tips on this kind of porous converter.
In the present paper, a system is proposed for improving the performance of steam power plant with air-cooled condenser during peak loads. In this system, the power plant comprises two steam turbines, and the air-cooled condenser is replaced by two condensers. The first one is air-cooled (dry) and used for condensing the exhaust steam of the first turbine, while the second is water-cooled and serves to condense the steam outlet of the second turbine. The warm cooling water exiting the wet condenser is pumped to a cooling storage container, where it is cooled and re-circulated to the wet condenser. Cooling is produced by a refrigeration machine driven by the extra electric power generated by the two turbines during the time of the off-peak-loads (low electricity rates). Simple energy analyses have been developed to predict the energy characteristics of this system. The results of this paper showed that the proposed system leads to improving the plant power output at peak-loads. About 6, 16, 24 and 33% increase in generated plant power can be achieved at peak-loads (high electricity rates) when the ambient temperature is 20, 30, 40 and 50°C respectively, and the whole steam exiting both turbines is cooled in a wet condenser to a design temperature of 20°C. The results showed also that choice of the capacity of each turbine is essentially affected by the quality of the refrigeration machine and ambient temperature.
This experimental study measures the detailed heat transfer distributions over the wavy fin channel with twin flow exits that simulates an elongated flow passage in a fin array. Heat transfer results acquired from the wavy and flat fin channels are compared. Impacts of Reynolds number (Re) and the undulant surface on heat transfer distributions over the wavy fin surface in the Re range of 500-11000 are examined. The spatially averaged Nusselt numbers (Nuav) over the flat and wavy fin surfaces are analyzed with the empirical Nuav correlations derived. Within the Re range tested, the lack of traversing flow momentum over the undulant fin surface prohibits the overall heat transfer elevations from the flat fin levels. Increments of cooling power available from the wavy fin surface due to the surface enlargement only develop at the test conditions of Re>6000 for the present test configuration.
This paper describes thermal performance of a small loop heat pipe device in an atmospheric condition. A comprehensive test program including start-up, power step up, power cycle and low power tests was performed. The effects of gravity on start-up and heat transport capability were also evaluated. An analytical model for the loop was developed to predict and evaluate the steady state operating performance. The test results demonstrated the robustness of the LHP. The analytical results showed good agreement with the test results except at the low power region. The feasibility of loop temperature control through compensation chamber temperature control was also experimentally demonstrated.
Systematic numerical simulations are carried out for a steady-state, laminar and constant-property air flow through a passage of a compact fin-tube heat exchanger specified with a constant wall temperature distribution and a uniform inlet velocity profile. A pair of delta winglet-type vortex generators (VGs) is punched out of the plain fin surface near the tube as a heat transfer enhancement device. A variety of VG configurations are investigated and their effects on the thermal-hydraulic (heat transfer) performance of the compact fin-tube heat exchanger are presented in terms of the Colburn factor j and the friction factor f for a specific Reynolds number (Re=400). In addition, the highest net enhancement in the thermal-hydraulic performance, defined as the ratio of the heat transfer enhancement to the pressure loss penalty, is sought. It is found that a moderate degree of the upstream shifting and the spanwise shifting of VG toward the tube contribute to the net enhancement. Nearly 9% of the net enhancement is achieved with an optimum configuration investigated presently.