Boiling explosion phenomena are experimentally observed in various fields and are applied in macro- and nano-fields. Analysis of boiling explosion has been recently conducted from a non-equilibrium point of view to understand the experimental results under some special conditions such as linear quick heating, ultra high heat flux pulse heating, and liquid contact with high temperature surface. Analytical result is shown to agree well with those experimental results.
In this review, an overview of cracks and interfacial voids formed in membrane electrode assemblies in polymer electrolyte fuel cells is presented. State-of-the-art of findings on formation processes of the cracks and the voids are described together with recent advances on development of characterization and diagnostic tools. The drawbacks and potentials of the cracks and the interfacial voids in the MEAs are also discussed. The cracks and the voids possibly deteriorate cell performance due to less electrical contact in the MEAs. On the other hand, the cracks can enhance the liquid and the gas transport in the MEAs that is better for PEFCs. Finally, future perspectives and remaining challenges concerned with the cracks and the interfacial voids are addressed.
A numerical study is performed in order to investigate the effects of the inlet flow structure on the flow and heat transfer characteristics in the reattachment zones over open cavity. Indeed, two inlet flow configuration are tested ; a turbulent wall jet which has a particular structure with two sources of turbulence production (the first is due to the shear flow associated to the inner layer characterized by small scale and second one is of the free shear jet flow with large turbulence scales) and a free boundary layer flow .The inner region of these two flows is similar, but their external regions are extremely different. The separating and reattaching flow phenomena are of particular interest in engineering fields. The numerical results of the analyses the loca convective heat transfer. The Nusselt number is more important and decreases immediately downstream the reattachment under the wall jet inlet flow. This detail may be explained from a dynamical point of view; the turbulent energy is more important but in small area around the each reattachment zone. The local Nusselt number increases when the Reynolds numbers augments. The evolution of local Nusselt , depends on incoming flow configuration. For the two configuration of incoming flow (Boundary layer or Wall jet), the distribution of mean Nusselt number is correlated according with some problem parameters.
The spurious velocity around curved interface, arising from the calculation of the Poisson equation with staggered grids, is reduced in the free-energy-based two-phase flow lattice Boltzmann method (LBM) for large density ratios. It is found that the pressure calculation from the Poisson equation, using the successive over-relaxation method with staggered grids, would introduce anisotropic discretization errors and lead to deviations of its calculated value from the theoretical prediction. Moreover, the anisotropic pressure would induce a large magnitude of spurious velocity, which is the driving force for droplet shape deformation. By blending the velocity components in the discretization equations of the Poission equation from two types of staggered grids that separately make use of the velocity components in the orthogonal and the diagonal directions, the magnitude of the spurious velocity and the droplet deformation are diminshed. It is found that, by appropriate choice of the blending factor, the magnitude of the spurious velocity can be reduced to half of its original value, and the shape deformation and pressure deviation from the theoretical prediction can be minimized.
The atomization of the injection fuel in the diesel engine directly affects on the combustion process of engine, and as a result, it is the main factor that determines the operating performance of the engine. In this study, the breakup process of the droplets by the atomization of the injected was investigated, using the breakup models (Reitz&Diwakar, CAB, ETAB and TAB) included in the commercial numerical analysis program. In order to select the optimum model for the droplet breakup interpretation, the results of spray tip penetration, Sauter mean diameter (SMD) and momentum length obtained as each breakup model were compared with the experimental data. In the case of changing of the spray tip penetration as the research results, TAB model was measured the shorter compared to the experimental measurements, and other models showed the similarity with the experimental value. In the case of SMD analysis, the use of Reitz&Diwakar model showed the best results. In addition, the frequency of breakup which is the microscopic spray behavior characteristics considering the impact of the coefficient of contraction in the nozzle was investigated in order to determine the diameter of initial particle in the fuel injection necessary to the numerical analysis. As a result, it was found that there is suitable initial droplet diameter in numerical analysis, in the case of nozzle-hole diameter 0.12mm of this study, it was the most appropriate that the diameter of the initial particle for the numerical analysis is about 0.06mm.
A new form of the phenomenological theory of unsteady combustion of solid propellants is proposed. It is based on the explicit use of a single physical quantity - effective initial combustion temperature. The solution of the problem of combustion stability in the presence of heat loss from the side surface of fuel of cylindrical form is given. The heat loss reduces the region of combustion stability and can lead to extinction.
The objective of the present study was to investigate the effect of the distance between fuel and oxidizer nozzles on NOx emissions from a laboratory-scale spray combustion furnace simulating an industrial high-temperature air combustion (HiTAC) furnace. The furnace employed in these trials was fueled with commercially obtainable kerosene in combination with highly preheated oxidizer gases. The oxidizer was pre-diluted by the addition of nitrogen in order to produce dilution levels equivalent to those in industrial HiTAC furnaces. The resulting NOx measurements indicate a trend opposite to that reported by previous studies, such that increasing the distance between the nozzles increases the NOx emissions, and a theory is advanced to explain this unusual observation. In a furnace within which the nozzle distance in the burner is small, the properties of the oxidizer supplied from the nozzle located near the spray nozzle greatly affect NOx emissions. The use of an oxidizer with a much lower preheat temperature and a lower O2 concentration induces flame lifting further downstream, which significantly reduces NOx emissions through the formation of an invisible flame exhibiting a uniform temperature distribution throughout the furnace. In contrast, in a furnace in which the burner has a large nozzle distance, a highly luminous flame is anchored near the spray nozzle exit due to ignition of fuel vapor by high-temperature burned gases recirculated to the lower section of the furnace. This flame represents a zone with high combustion intensity and generates significant NOx emissions. In such cases, the influence of the oxidizer properties is moderated due to the widely separated nozzle locations.
Dwindling energy resources and strong demand for better power sources as compared to conventional batteries have sparked research interest in micro power generation. The invention of state-of-the-art electronic devices requires more energy capacity, shorter charging period and light in weight, characteristics of which batteries lack. Therefore, in recent years micro power generation systems have been seen as a potential alternative to batteries owing to the obvious advantages that it has. It is essential to fully understand the underlying factors that affect the combustion stability in meso and micro-scale combustors. One of the popular methods to examine these factors is by performing numerical simulations. This paper demonstrates an axisymmetric two-dimensional steady state numerical simulation of propane-air combustion in meso-scale cylindrical tube combustors with concentric rings. The inner diameter of the tube is set to 3.5 mm and the wall thickness is specified to 0.7 mm. The concentric rings are placed between the unburned and burned gas region. The main function of these rings is to act as a flame holder where a stable flame can be easily established. The wall thermal conductivity in the unburned and burned gas region is varied from 1 W/m/K to 1000 W/m/K and the results in terms of gas, inner wall, outer wall surface temperature distribution, the blowout limits and combustion efficiency are analyzed and presented. In addition, the effect of the inlet velocity and the equivalence ratio is also investigated. The results show that the inlet velocity and equivalence ratio have significant impacts on the flame temperature, which in turn change the wall temperature distribution. Although the wall thermal conductivity has minimal effect on the flame temperature, both inner and outer wall surface temperature are greatly affected. Consequently, this variation of wall temperature contributes to the significant changes on the blowout limits. It is also shown that the combustion efficiency is influenced by the wall thermal conductivity of the combustors.
The computational efficiency of different composite schemes for the convection terms of the convection-diffusion equations is systematically studied in this paper. They are evaluated by the problems of lid-driven flow and natural convection in a square cavity, within the framework of single-grid and multigrid finite volume methods on collocated grids. For the convection schemes, the computational efficiencies of QUICK scheme and commonly used ten composite schemes (SMART, STOIC, MINMOD, etc.) are compared under the same calculation conditions. The influences of implementation strategy of composite schemes, grid number, Reynolds number and Rayleigh number on the computational efficiency are analyzed. It is found that the computational efficiency of composite schemes is close to that of QUICK scheme without extra calculation burden.
Imploding detonations can generate ultra-high pressures at their implosion centers. In this basic study, we measured the maximum pressure in an imploding detonation apparatus with a pre-combustion chamber. The aim was to investigate such ultra-high pressure states for industrial use; namely, as a microorganism treatment technique driven by underwater shock waves. The experimental apparatus is fitted with an exchangeable nozzle for altering the minimum converging radial distance. The variable experimental conditions are the inner diameter of the nozzle, the initial pressure, the equivalence ratio, and the concentration of the nitrogen diluent. We found that the maximum pressure increased with increasing inner diameter of the nozzle, because the duration time of the flame passing the nozzle increased. The maximum pressure was also an increasing function of the initial pressure and the equivalence ratio. High pressure was maintained up to nitrogen concentrations of approximately 65%. This result was attributed to detonation of the overdriven state and reduced propagation speed of the flame.
In order to improve the performance and stability of anodes for solid oxide fuel cells (SOFCs) under low hydrogen concentration conditions, SrZr0.95Y0.05O3-α (SZY) proton conductor particles were incorporated into the conventional Ni/YSZ anode. Power generation experiments were conducted using the electrolyte-supported single cells with conventional Ni/YSZ and Ni/YSZ-5%SZY anodes. Enhanced output power density under low hydrogen partial pressure conditions was achieved with the modified anode. This enhancement is ascribed to a decline in the anode overpotential of the Ni/YSZ-5%SZY anode, estimated from impedance spectra, compared to that of the conventional Ni/YSZ anode. In addition, the amount of hydrogen adsorption on Ni, YSZ, and SZY was measured using thermal desorption spectroscopy (TDS). This revealed that the proton conductor SZY could adsorb a large amount of hydrogen compared with YSZ. Consequently, the SZY proton conductor may play an important role in the supply/storage of adsorbed hydrogen, or in increasing the oxidation resistance of Ni under low hydrogen concentration conditions.
A new approach to the simulation of a horizontal type ground heat exchanger is proposed to result in a better accuracy and at the same time a reduced computational effort. These results come from the concentration of the computational effort at the locations with the largest temperature and moisture gradients, i.e. the pipe-soil interface. Thus, the model takes into account coupled heat and moisture transfer in the unsaturated soil, allowing for more accurate predictions of the soil thermal response to the heat fluxes induced by the operation of the ground heat exchanger. This in turn allows for a more accurate prediction of the soil temperature field and the circulating fluid temperature profile. And also, the performance of a single pipe carrying warm fluid buried in a medium wet sand is described in this paper. The new coupled differential equations have been solved using the finite element method as a spatial discretization technique coupled with a finite difference relationship to describe the transient behavior by a numerical code. The time varying soil moisture concentrations and temperatures are graphically presented. As a result, it can be achieved that the soil moisture profiles develop at a slower rate than the temperature distribution functions.
The low heat conductivity of the packed bed is a key issue in thermal engineering fields. It is also one of the dominant factors behind realizing a fuel cell vehicle society because it prevents quick recharging of hydrogen due to its exothermic reaction. In this study, we examined the enhancement in the effective thermal conductivity of the packed bed with the use of single-walled carbon nanotubes, which were directly synthesized on the particles of the packed bed. We employed two kinds of particles for the packed bed; one was alumina of 10 µm in diameter, which is comparable to usually used in actual system and the other was zeolite, which was used for synthesizing high quality carbon nanotube. Consequently, 10 vol. % of carbon nanotube enhanced three times of effective thermal conductivity.
This study investigates the combustion, performance and emission characteristics of a diesel engine which is fuelled with sardine oil methyl ester and diesel in a diesel engine with cerium oxide as nano particle. A single cylinder four stroke diesel engine was used for the experiments at various load and constant speed. This study deals with an innovative method to improve the efficiency by reducing the fuel consumption and improving the combustion using nano particles. The objective of this study is to analyze the possible effects of adding nano particles with diesel and SOME. The nano particles used for this test is 25ppm. The present work mainly focuses on comparing the nano particles with diesel and biodiesel to improve the performance of compression ignition engine. The tests revealed that nano particles can be used as additive in diesel and biodiesel to improve complete combustion of the fuel and increase the exhaust emissions significantly.
Membrane-based total heat exchanger is a device to recover both sensible heat and moisture from exhaust air stream from a building. Heat and mass transfer intensification has been undertaken by using a structure of cross-corrugated triangular ducts. Conjugate heat under laminar flow regime in this total heat exchanger are investigated. Contrary to the traditional methods of assuming a uniform temperature or a uniform heat flux boundary condition, in this study, the real boundary conditions on the exchanger surfaces are obtained by the numerical solution of the coupled equations that govern the transfer of momentum and energy in the two air streams and in the membrane materials. The naturally formed heat boundary conditions are then used to calculate the cyclic mean Nusselt numbers along the exchanger ducts. The data are compared with those results under uniform temperature and uniform heat flux boundary conditions. The heat transfer will change with different thermal wall boundary conditions. Synergy principle is applied to reveal the Nusselt number variation with various thermal wall boundary conditions.
Non-contacting face seals are applied in highly efficient devices in which safety and reliability are required the most. Meeting the requirements is possible due to some detailed research and analyses of physical processes related to the flow of the medium (fluid) through the radial gap, to the heat transfer in the sealing knot or to the thermoelastic strains which are undergone by the wear rings. Establishing and developing mathematical models which describe the above mentioned phenomena allow more and more detailed results of analyses to be achieved. This paper presents the results of the comparative studies for two different mathematical models that describe the same phenomenon, i.e. heat transfer in non-contacting face seals (type: face to face). The results of the research allow to determine which solution provides some more detailed glimpse of the described physical phenomenon and might be applicable to non-contacting face seals.
A new multi-dimensional flamelet generated manifolds(MFM) approach is proposed based on solving multi-dimensional flamelet equation set in mixture fraction Z and normalized flamelet progress variable c coordinate, to capture both non-premixed and premixed combustion characteristics in partially premixed flames. Local scalar dissipation rates appeared as coefficients in multi-dimensional flamelet equation set have been modeled based on the analysis of local combustion regime during multi-dimensional flamelet calculation. Simulation results of counter-flow laminar partially premixed flames suggests that this new MFM approach can reproduce both non-premixed and premixed flame structure accurately, with computational efforts greatly reduced compared to former MFM approach.
In recent years, microorganisms contained in ship ballast water have been identified as vectors for the destruction of marine ecosystems. To control such microorganisms, various techniques are being developed and tested. As part of this effort, we propose a technique that uses underwater shock waves driven by a gas-imploding detonation. Previously, we have reported characteristics of pressure generation at the implosion center of an imploding detonation. Subsequently, we installed a nozzle at the implosion center of an underwater shock wave generating apparatus and measured how characteristics of underwater shock waves responded to changes in operating conditions. Operating conditions were manipulated by changing the inner diameter of the nozzle, the initial pressure, the equivalence ratio, and the nitrogen concentration in the premixed gas. We examined how the maximum pressure, rise time, half-width time, and shock energy of underwater shock waves affected the mortality rate of microorganisms (Artemia salina). We found that when the inner diameter of the nozzle was increased, the rise time and half-width time decreased, and the maximum pressure and shock energy increased. When the initial pressure and equivalence ratio were increased, the maximum pressure and shock energy increased, the rise time became shorter, and the half-width time became longer. When the nitrogen concentration was increased, the rise time became longer, and the half-width time became shorter. The mortality rate of Artemia salina was increased by shortening the rise time of the pressure waves while increasing the maximum pressure, the half-width time, and the shock energy of the underwater shock waves.
In this paper, the thermoelastic interactions in an orthotropic unbounded body containing a cylindrical cavity are studied. This problem is solved by using the Green and Naghdi's (GN) generalized thermoelasticity model. The thermal material characteristic of the GN theory is taken as linear function of temperature. The surface of the cylinder is constrained and subjected to an exponentially decaying pulse boundary heat flux. The Laplace transform is used to remove the time dependency from the governing field equations. Finally, the transformed equations are inverted by the numerical inversion of the Laplace transform. Numerical results are shown graphically to estimate the effect of the thermal material coefficient and time of the pulse heat parameters. The distributions of all the studied felids in the space-time domain are also investigated.
The present paper reported the effects of electric fields produced by the mesh, ring and needle electrodes on outwardly propagating premixed flames in a constant chamber. The input voltages were 0 kV, -5 kV, -10 kV, -12 kV, and the excess air ratios of methane-air mixture were 0.8, 1.0 and 1.2, respectively. The electric field distributions developed by the three electrode configurations all affected the flame propagation in the horizontal direction rather than in the vertical direction. The electric field strength for the mesh electrode in the region between the high-voltage electrodes was the largest, and that for the ring and needle electrode decreased in turn. Under the application of the electric field, the flame front was stretched remarkably and the flame speed was accelerated significantly as the input voltage increased. For the three electrode geometries, the increase of the flame speed for the mesh electrode was the largest, followed by that for the ring and needle electrodes. For various excess air ratios, the increase of the flame speed at lean mixture was the largest, that at rich mixture the less large and that at stoichiometric mixture the smallest. By analyzing the effects of the three electrode geometries on pressure data, it was seen that the applied electric field could markedly enhance the combustion pressure growing up, and the timing of the peak pressure was advanced with the increase of the input voltage. The promotion effect on the combustion is most pronounced for the mesh electrode, while that for the ring and needle electrodes decreased in turn. For the various excess air ratios, the electric field most significantly enhanced the combustion pressure growing up at lean mixture.