For a practical thermoelectric generator (TEG) system, the performance is affected by many irreversible processes. This study evaluates the effects of multi-irreversibilities on TEG performance using exergy analysis. Based on the exergy analysis, two performance indexes, the energy efficiency and exergy efficiency, are used. A finite element scheme is employed to model the TEG system. The heat loss from the TEG to the environment and temperature-dependent material properties are considered. The results suggest that when the application of the small electrical current is considered, decreasing the hot-reservoir temperature or increasing the cold-reservoir temperature can improve the TEG exergy efficiency. Although heat loss slightly decreases the maximum energy and exergy efficiency, it can improve energy and exergy efficiencies for a case with large electrical current. On the other hand, when the Seebeck coefficient or thermal conductivity is temperature-dependent, both the maximum energy efficiency and exergy efficiency increase with increasing hot-reservoir temperature. However, the temperature-dependence of the electrical resistivity reduces the maximum exergy efficiency when the hot-reservoir temperature increases even though the maximum energy efficiency is increased.
Numerical simulations have been performed to study the film cooling of a flat plate that has an internal cooling passage perpendicular to the main flow. The goal is to understand the effect of the orientation of turbulence promoting ribs installed in the internal cooling passage on the adiabatic film cooling effectiveness in the external main flow downstream of the cooling-air injection. Detached Eddy Simulation is carried out for the flow characteristics for two rib orientations at two blowing ratios. The simulation results have revealed that the rib orientation affects significantly the temperature and flow structures downstream of the cooling hole and therefore affects the film-cooling performance on the surface. The revealed difference is due to the presence/absence of intense spiral motion at the cooling-hole outlet and such a difference originates from an interaction between the flow separation behind the inclined rib and the flow suction at the inlet of inclined cooling hole. The simulation results are exploited to draw a clear picture for the fluid-dynamical mechanisms responsible for the effect of the rib orientation.
Responses of conical premixed methane/air mixture flames with equivalence ratio oscillations were numerically investigated at three different oscillation frequency regimes; low, medium and high (10, 50 and 150 s-1) under a rich condition. One-step and two-step reaction mechanism have been investigated. The pre-exponential factors for the one-step and two-step reaction mechanism were calibrated at minimum and maximum equivalence ratio of this study, Ø= 1.1 and 1.5, by comparing the flame length of the numerical results with the experimental results. The two-step reaction mechanism predicted well the variation in the flame shape under a steady state condition with equivalence ratio variations while the one-step reaction mechanism failed to predict the variation in the flame shape especially under the near rich flammability limit condition. This is because of the CO formation inclusion in the two-step reaction mechanism. The effects of the equivalence ratio oscillation frequency towards flame dynamics were discussed. The amplitude of the flame tip movement attenuates following the attenuation of the equivalence ratio oscillation towards the downstream direction at high oscillation frequency. The dynamic response of the flame tip for low, medium and high oscillation frequency regimes shows interesting behavior. The quasi-steady manner of the flame tip movement was observed at the low frequency regime. In the medium and high frequency regime cases, we found that the attenuation of the flame tip motion is affected by the wrinkling of the flame surface in addition to the attenuation of the equivalence ratio oscillation amplitude. Moreover, an increase in the number of wrinkles as the equivalence ratio oscillation frequency is increased induces large attenuation of the flame tip oscillation amplitude. Furthermore, we found that the period of the equivalence ratio oscillation, T and the convective transport time, τ control the flame response in that T/τ, is a controlling parameter of the conical flame response in quasi-steady (T/τ ≥ 1.0) or unsteady (T/τ ≤ 1.0) manner.
An extremely lean burning engine has been expected to improve fuel consumption rate of engines. To achieve this, stable combustion should be realized for a wide range of operating conditions at air-fuel ratio over 40.0. In direct injection gasoline engines, cyclic variations of combustion derive from some main factors such as those of air flows and spray motions. In this report, we examine the influence of cyclic variations of "Air flows" and "Spray motions" on combustion instabilities, while other variation factors are fixed for each cycle of engine, by using our simulation model. The cyclic variations of stratified-charge turbulent combustion in direct injection gasoline engine can be simulated during ten continuous cycles based on the multi-level formation for the stochastic compressible Navier-Stokes equation and also a spray model. Computational results agree with cycle-averaged experimental data fairly well, while the cyclic variations computed are comparable to experimental ones reported by the other research group. Consequently, the present computational model will reveal an essential factor which generates the cyclic variations and will offer us an effective way to control combustion instabilities on very lean burning conditions.
Heat release rate is an important parameter that affects diesel engine characteristics. The behaviors of intake and exhaust systems are also important because these systems govern the fluid flow into engine cylinders. This study presents a diesel engine dynamics model based on the identification of the combustion heat release rate and intake and exhaust valve properties. Using the identification results, instantaneous engine speeds and cylinder pressures can be obtained by considering fuel injection period, air intake and exhaust properties, the in-cylinder combustion process, and crank rotational dynamics. The simulation results for the engine speed and in-cylinder pressure indicate good agreement with experimental data for different external loads and low and high engine speeds.
For predicting the void fraction distribution in subcooled flow boiling, it is necessary to measure the interfacial heat transfer coefficient—which determines the amount of condensation rate of vapor bubble—with high accuracy. In this study, the relationship among the bubble number distributions, void fraction, bubble collapse rate and phase change rate was clarified through measurement of the average interfacial heat transfer coefficient using image analysis under the condition of forced convectional subcooled flow boiling. The bubble number distributions were measured by translating bubble images observed by a high-speed camera. The bubble collapse rates were obtained by the process of bubble number. The obtained bubble collapse rates were possible to predict the decrease rate of the void fraction in low subcooling condition with high accuracy. The average heat transfer coefficient was shown to depend on the interfacial area concentration, via calculation of the average interfacial heat transfer using the obtained bubble number distribution. The developed experimental correlation of the interfacial heat transfer with the interfacial area concentration was compared with the measured value, and the interfacial heat transfer using the developed correlation equation corresponded to the measured one. A comparison with previous correlation equations for interfacial heat transfer coefficients and consideration of the bubble number distribution confirmed the applicability of the experimental correlation of the interfacial heat transfer coefficient to a boiling channel.
We present the numerical simulation, using the finite difference time domain (FDTD) method, of the radiative heat transfer between two thin SiC slabs. We aim to explore the ability of the FDTD method to reproduce the analytical results for the Surface Phonon Polariton (SPhP) assisted near field radiative energy transfer between two SiC slabs separated by a nano/micro-metric vacuum gap. In this regard, we describe the key challenges that must be addressed for simulating general near-field radiative energy transfer problems using the FDTD method. FDTD is a powerful technique for simulating the near-field radiative energy transfer because it allows simulating arbitrary shaped nano-structured bodies, like photonic crystals, for which an analytical solution is not readily obtained.
The multiple injection strategies have been widely employed in many new diesel applications, such as HCCI (Homogeneous Charge Compression Ignition) and LTC (Low Temperature Combustion), which are very effective in improving fuel-air mixture and entrainment. In this paper, using high injection pressure of common rail and high-speed camera system, the experimental studies on the spray characteristics of the single, double and triple injections are conducted. The spray average fuel/air equivalence ratio is employed to evaluate the overall effects of the fuel-air mixture, which is the ratio of the fuel/air mass ratio in the spray and the fuel/air stoichiometric mass ratio. It is shown that the spray average fuel/air equivalence ratio decreased with the increased injection pressure, indicating that increasing injection pressure could intensify fuel-air mixture. For double injections, spray tip penetrations and volumes increase with the increased injected volume percentage. It is also found that spray development rate of the second injection is higher than the first injection. Double injections lead to lower spray average fuel/air equivalence ratios than the single injections. For triple injections, the spray average fuel/air equivalence ratios are almost lower than stoichiometry. The triple injection of which the volume distribution of every injection time is evenly has the lowest spray average fuel/air equivalence ratio, and has the best effect of fuel-air mixture.
The effect of feed flow rate of hydrogen mixture on the H2 permeation for a flat sheet Pd/Ag membrane with stagnating flow at the upstream side was investigated experimentally and theoretically. A 77wt.% Pd/23wt.% Ag flat sheet membrane with 25µm thickness and 0.02m diameter was used. The permeation rate of H2 was investigated under various feed flow rates (1.489 × 10-5-2.976 × 10-4mol/s), for pressures of 0.20-0.30MPa and reference membrane temperatures of 523-723K. Experimental results demonstrated that when the feed flow rate is decreased, the H2 permeation rate decreases. This is supposed to be due to the phenomena of hydrogen concentration decrease at the membrane surface of the upstream side, as a result of the effect of H2 permeation itself. When a theoretical equation that takes into account the effect of H2 permeation is used, the H2 permeation mole flux can be predicted quantitatively by using the concentration of H2 of the feed mixture. This shows that the diffusive transport effect plays an important role as well as the convective transport effect when determining H2 concentration at the membrane surface. In addition, the normalization of the theoretical results shows that the trend of the decrease in the H2 permeation mole flux with respect to the feed mole flux follows the first order lag function, regardless of the inlet H2 partial pressures.
We numerically investigate the effect of thermocapillary convection on melting and deforming processes of Phase Change Material (PCM) subjected to local heating. Mass, momentum and energy conservation equations are solved in a 2-D system based on a fixed grid by means of a finite volume method. The Volume of Fluid (VOF) method and the Enthalpy-Porosity method are applied to model the deformable liquid-gas interface and the melting processes, respectively. Thermocapillary effect is accounted in the momentum equation as one of additional external force (related to surface tension force and it is controlled by temperature dependency of surface tension coefficient). In this study, temperature dependency of surface tension coefficient is given by linear function of temperature, which is considered as numerical parameter to examine the thermocapillary effect. Results successfully show that the thermocapillary convection under normal gravity environment appears clearly only at the early stage of melting at which the substantial deformation of PCM is not observed. Once the gravity drag starts to deform the molten PCM, thermocapillary convection seems to be masked, revealing that the gravity-induced deformation is dominant factor of internal motion of molten PCM. In zero-gravity, on the other hand, thermocapillary convection is continuously observed, ensuring that it dominates the internal motion so as to enhance the heat flux toward the un-melted solid. Importantly, even in zero-gravity case, the total effect of thermocapillary convection on melting is not so significant, at most 5% in the present system.
Geological sequestration of carbon dioxide (CO2) is an effective method to achieve a substantial reduction in CO2 emissions to the atmosphere at a relatively low cost; however, the migration process of CO2 in deep saline aquifers has not been clarified. In order to evaluate the storage site and assess the CO2 leakage risks and storage costs, fundamental visualization and study of immiscible two-phase flow in sandstone are required. This study observed the behavior of liquid CO2 injected into water-saturated Berea sandstone by using microfocus X-ray computed tomography with high spatial resolution. The three-dimensional CO2 distribution in the sample was clearly reconstructed. The bedding structure of the rock strongly determined the CO2 permeation process, and a strong correlation was seen between the local porosity of the sample and the CO2 saturation. The real time CO2 permeation process was also observed in the transparent images that showed the CO2 gradually permeating the rock in the axial direction with increased saturation in the higher porosity beddings. The effects of the sandstone micro-heterogeneity on the behavior of the injected liquid CO2 are discussed.
This study presents a method that can be employed to obtain the reaction rate and be widely applied to the simulation of steam methane reforming in a channel with a catalyst on the wall. Prior to the numerical simulation, an experiment was performed in an annular channel, in which the catalyst was deposited on the outer surface of the inner tube. In addition to varying the temperature and composition of the reactants, the velocity was also varied in order to investigate the range where the reaction-limiting condition occurs. Next, the results from the bulk concentration of the products were interpreted to determine the reaction rate of the surface catalyst. The reaction rate was properly introduced into the numerical simulation in two dimensions, although this method for obtaining the reaction rate is generally applied to a field that is supposed to be one-dimensional as a packed catalyst. On the other hand, a discrepancy between the experimental and simulation results was observed at a low velocity. The reason for this was investigated by considering the concentration and profile of the reactants. As a result, we concluded that it was necessary to pay close attention in a case where the amount of the reaction varied with the velocity, because the actual phenomena were controlled by other mechanisms that were not introduced into the numerical simulation.
A lean premixed surface flame has many advantages including low CO(Carbon Monoxide) and NOx(Nitrogen Oxide) emission and applicability of a small combustion volume leading to compact design. These advantages make it applicable to burner for condensing boilers with high thermal efficiency. Moreover recent severe regulation of global warming gas favored a lean premixed surface flame in development of a condensing boiler burner. This study focused on emission characteristics of lean premixed flame and the effect of flow distribution on flame stability of a surface flame in a cylindrical porous metal plate burner. For conceptual design of surface flame burner, the numerical calculation of a flow pattern inside the burner was performed and the calculated data were used for design of the burner system including the baffle plate and flame holder. The results show that the surface and stable premixed flame can be generated by implementing the proper baffle plate and flame holder. The surface cylindrical flame mode is changed into green flame, yellow radiation flame, blue flame and blow off with decreasing equivalence ratio. The blue flame has a wide stability region in the stability curve and showed the lowest CO and NOx emission at low equivalence ratio. And CO decreased as the mixture ratio became leaner but NOx showed almost the same emission level. For stability of a surface cylindrical flame, it was found to be very important to select the proper distribution of holes in a baffle plate and install the flame holder to prevent blow off at the rim of the cylindrical burner. NOx was measured below 6 ppm (0% oxygen base) from equivalence ratios 0.706 to 0.769 through the proper design of baffle plate and flame holder. CO which is a very important emission index in residential gas boiler was observed below 49.1ppm under the same equivalence ratio range.
Gasification has played an important role in the development of clean coal technology. To seek appropriate operations for synthesis gas (syngas) formation, the present study developed an optimization analysis procedure of the gasification process in an entrained-flow gasifier through the application of the Taguchi method in conjunction with a simulation method. The effects of the wall temperature, O/F ratio, feed type and pressure on the performance of cold gas efficiency (CGE) were investigated. An orthogonal array was used to arrange the CFD experimental plan for the above factors. Analysis of the signal-to-noise ratio (S/N ratio) was used to evaluate the calculation results. Results suggest that the optimum conditions are a wall temperature of 1500 K, an O/F ratio of 0.6, coal feed type and a gasifier pressure of 3 MPa. The influence strength order of each control condition is feed type>O/F ratio>wall temperature>pressure. The value of the S/N ratio for the optimum case is 13.40, which is the highest value compared to other cases. Findings show that the Taguchi method is able to investigate the gasification process well and can therefore be applied in future studies conducted in various fields.
The three-dimensional, transient, isothermal, gas-flow behavior in the serpentine channel and porous gas-diffusion layer (GDL) of a polymer electrolyte membrane fuel cell was investigated numerically. Decreasing the pitch length of the channel increases the pressure drop. In the serpentine channel, reactant gas is distributed from one part of the channel to another through the GDL by cross flow induced by a pressure differential between adjacent channels. The amount of cross-flow, quantified in terms of the volume mass flux through the GDL under the rib and between two channels, is controlled predominantly by the thickness of the GDL, the pitch length of the channel, and the permeability. The permeability of the GDL has marked effects on the pressure drop and on cross flow. The cross flow rate through the GDL increases when decreasing the pitch length of a serpentine channel. Cross flow reduces the pressure gradient in the channel, whereas bends improve the pressure gradient. The pressure gradient in the channel increased with decreasing cross flow.
Aim of this study is to propose a numerical model and to calculate ultrashort pulse laser light propagation by using Finite-Difference Time Domain (FDTD). “Ultrashort” pulse here is defined so that the Slowly Varying Envelope Approximation (SVEA) breaks in Beam Propagation Method (BPM) and corresponds to the pulse width of typically several tens of femtoseconds or shorter. In this case, FDTD must be used instead of BPM. In our FDTD-based model, nonlinear absorption and the effect of laser-induced plasma are newly considered unlike previously reported FDTD models. In this paper, we examine the results of FDTD-based calculation by comparing with the results of BPM-based calculation in short pulse cases where the SVEA approximation is valid. Furthermore, we calculate ultrashort pulse laser propagation and the results can describe the essential features in ultrahigh power regime.
The inherent irreversibility in a variable viscosity hydromagnetic generalized Couette flow with suction/injection at the walls has been investigated theoretically. Using a fourth-order Runge-Kutta-Fehlberg integration scheme together with shooting technique, the model equations for momentum and energy balance are tackled numerically. The velocity and the temperature profiles are obtained and are utilized to compute the skin friction coefficient, Nusselt number, entropy generation rate and the Bejan number. The results are presented graphically and discussed quantitatively for several values of thermophysical parameters controlling the flow regime. Our results reveal that a decrease in fluid viscosity enhances dominant effect of heat transfer irreversibility and the imposition of magnetic field damping the entropy generation rate in the flow system.
As oil prices increase and regulations on emissions are tightened, demands for improved engine performance such as fuel economy and exhaust emission have increased accordingly. Therefore, various new technologies have been developed to meet such demands. Among them, cooling system is spotlighted because it has great effect on fuel economy, but there has been little work on cooling systems. In this study, we measured friction losses of engine parts according to engine oil temperature (using the strip-down method) and also obtained optimized oil temperature which has the minimum friction losses. In additions, we analyzed the correlation between engine oil temperature and coolant temperature. Based on these results, we determine the optimal range of engine oil temperature and investigate the effect of engine speed and indicated mean effective pressure on fuel consumption and exhaust emission characteristics. Finally, we found that although the fuel consumption was improved in the selected oil temperature range, the characteristics of exhaust emission were not improved under the conditions.
Effects of internal pressure and inlet velocity disturbances of air and fuel droplets on spray combustion field are investigated by means of two-dimensional direct numerical simulation (DNS). n-decane (C10H22) is used as liquid spray fuel, and the evaporating droplets' motions are tracked by a Lagrangian manner. The pressure perturbation is captured by employing a pressure-based semi-implicit algorithm for compressible flows. The frequency and magnitude of the inlet velocity disturbances are set at 800 Hz and up to 50 %, respectively, and the internal pressure is set at 0.1 or 0.5 MPa. The results show that the pressure perturbation in the spray combustion field is enhanced by combustion reaction and increase in internal pressure. In addition, the inlet velocity disturbances do not change the frequency indicating the peak of power spectra of pressure perturbation, but decrease the intensity of pressure perturbation for the cases of 0.5 MPa.
The paper deals with determination of thermal diffusivity of liquid flowing in a pipe based on inverse problem. The hybrid method was developed to improve the inverse solution. The proposed method is a combination of the artificial neural network and inverse solution algorithm. The artificial neural network is adopted for making a good initial-guessed value for the inverse solution. Numerical examples are included to demonstrate the performance of the proposed method. In comparison with the simple inverse solution, the proposed hybrid method is efficiently converged to yield good estimates.
The effect of subcooling and length of hydrophobic-spot periphery on nucleate pool boiling heat transfer from TiO2-coated surface with and without PTFE (polyetatafluoroethylene) hydrophobic circle spots at intermediate heat flux has been examined. The experiments are performed with liquid subcooling ranging from 0-20 K and heat transfer block used were TiO2-coated copper block with a PTFE hydrophobic circle spot with various diameters with total area of PTFE being constant. Bubble nucleation and behavior were observed by using high-speed camera. The results showed that the heat transfer performance of surfaces with PTFE hydrophobic circle spot is better than superhydrophilic surface in overall condition. Furthermore, the heat transfer performance decreases under subcooled condition for all surfaces. Increase in peripheral length of hydrophobic-spot enhances the heat transfer performance.
In this study, the authors numerically investigate the frequency response on the three-dimensional thermal convection in a cubic cavity heated below in the gravitational field, concerning flow characteristics such as flow structure and a global quantity the spatially-averaged kinetic energy K. The authors assume incompressible fluid with a Prandtl number Pr = 7.1 (water) and with a Rayleigh number Ra = 1.0×104 or 4.0×104. The direction of a forced sinusoidal oscillation is parallel to the terrestrial gravity. The authors especially focus upon the influences of both the forced-oscillation amplitude η and frequency ω in non-dimensional forms, whose test ranges are 1.5 ≤ η ≤ 15 and 10 ≤ ω ≤ 103. The obtained results are as follows. For Ra = 4.0×104, as well as Ra = 1.0×104 (Tanigawa et al., 2009), we can observe the optimum frequency ωKmax where the amplitude of K attains the maximum, each η. And, for both Ra's, ωKmax becomes the minimum at η = 1.5 - 2.0. Especially for Ra = 4.0×104, ωKmax is affected by the initial conditions. For both Ra's, the maximum of the K amplitude uniquely exists at ω= ωKmax each η, when η < 1.5. On the other hand, we can observe not single but plural peak frequencies with locally-maximum K's each η, when η ≥ 1.5. It is confirmed that such plural frequencies are related with the appearances of various flow structures such as S1, S2, S4, S5, S6 and S8. Especially for Ra = 4.0×104, this relation is also affected by the initial conditions. In addition, the details of a new flow structure S8 is reported.
The effects of unburned-gas temperature and heat loss on the diffusive-thermal instability of premixed flames were studied by two-dimensional unsteady calculations of reactive flows, based on the diffusive-thermal model equation, under the conditions of constant temperature jump through flame fronts. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened at Lewis numbers smaller than unity, which was due mainly to the increase of the burning velocity of a planar flame. As for the growth rate and unstable range normalized by the burning velocity of a planar flame, the former decreased and the latter narrowed. This was due to the reduction of Zeldovich numbers. In addition, the normalized growth rate increased and the normalized unstable range widened when the heat loss was taken into account. This indicated that the heat loss had a pronounced influence on the diffusive-thermal instability of premixed flames with high unburned-gas temperature. Furthermore, the cellular shape of flame fronts formed owing to diffusive-thermal instability. The normalized burning velocity of a cellular flame decreased as the unburned-gas temperature became higher, and increased when the heat loss was taken into account. Compared with high-temperature premixed flames where the adiabatic flame temperature was constant, the normalized level of instability intensity was low. This was because of small Zeldovich numbers.