We investigate diffusion and mixing in a microchannel by using luminol chemiluminescence (CL) to estimate the local chemical reaction rate. The degree of mixing in micromixers is generally evaluated from the deviation of concentration profiles measured by fluorescence from a uniform concentration profile. The degree of mixing measured by this method is a macroscopic estimate, which is inappropriate for investigating diffusion and mixing in microchannels. In this study, the luminol CL reaction is used to visualize and quantitatively measure local diffusion and mixing at an interface between two liquids in a microchannel. Blue CL is observed where luminol reacts with hydrogen peroxide at the mixing layer. Diffusion and mixing in a microchannel are investigated by sequentially measuring the CL and fluorescence. The experimental results are compared with the results of a numerical simulation that involves solving transport equations including the chemical reaction term. By calibrating the CL intensity with the chemical reaction rate estimated by the numerical simulation, the local chemical reaction can be quantitatively estimated from the CL intensity profile.
Knocking intensity under the in-cylinder flow field was investigated by using a rapid compression machine (RCM). The nitrogen diluted and non-diluted fuel-air mixtures were employed for the examination of the combustion characteristics under the in-cylinder flow field. The behaviors of flame propagation and the spontaneous ignition in end gas were observed. The analyses of the in-cylinder flow field and the dependency of the knocking intensity with considering the volume fraction for flame propagation and the heat release rate of the spontaneous ignition in end gas were carried out. As a result, the flame propagation velocity increased with heightening the turbulent intensity. The change of the flame propagation velocity provided the change of the volume fraction for flame propagation. The knocking intensity depended on the volume fraction for flame propagation and it reached a peak at about 0.6 in the volume fraction, when the heat release rate due to the spontaneous ignition was high enough. This agreed with the numerical prediction qualitatively. The combustion due to the spontaneous ignition in end gas was prolonged by the broader temperature variation by turbulence when the spontaneous ignition in end gas was delayed or turbulence was strong. In these cases, the knocking intensity was lowered with the prolongation of the combustion in end gas. It was to be expected that the dependency on the volume fraction for flame propagation remained the same even when the heat release rate due to the spontaneous ignition was lowered, by the numerical simulation.
A performance analysis of a three stage axial flow compressor with part load consideration in gas turbine application regarding surge margin is presented in this paper. A three stage compressor was designed and numerical simulation as well as a modified stream line curvature analysis was applied to validate the design model. These two analyses verified the design which was based on a test case model and showed an acceptable accuracy. Performance was predicted by introducing a modified one dimensional method for the compressor. A typical gas turbine with specific components was considered to evaluate the effect of compressor early design parameters on its behavior at part loads. Four compressors were designed with change in reaction ratio and flow coefficient as the input data of the design process. The analysis showed that an increase of 8% and decrease of 5% in reaction ratio and flow coefficient respectively, compared with the base design model, would lead to getting away the running line from the surge line by an amount of 5% approximately and working in a safer margin as well as higher efficiency working range, so the energy conversion can be done efficiently.
Heat transfer coefficient and film-cooling performance have been studied numerically by using the detached eddy simulations. The detailed characteristics of heat transfer coefficient influenced by the orientation of internal turbulence promoting ribs are examined. The heat transfer coefficient and the adiabatic film cooling effectiveness reported previously are discussed with relation to the flow structures of the film-cooling air. Those simulation results are combined to evaluate the net heat flux reduction and the net surface temperature reduction. The effects of the rib orientation on the evaluated film-cooling performance are discussed based on the flow structures. The present results have demonstrated the significant effects of the rib orientation on the film-cooling performance of gas turbine blades.
Experimental and kinetic studies of the chemical role of CO2 in hydrocarbon reactions were conducted in a fuel-rich CH4 flat flame with air ratios varying from 0.60 to 0.74. Unburned hydrocarbons (CH4, C2H2, C2H4, and C2H6) in O2/CO2 combustion were found to be lower than those in air combustion. The differences in the CH4 oxidation characteristics between the air and O2/CO2 combustion were caused by the chemical role of CO2 in the reaction R1 (CO2 + H → CO + OH), R2 (CH2(S) + CO2 → CH2O + CO), and higher third body efficiencies of CO2 at an air ratio (λ) = 0.62 where the concentrations of reactants were high. The role of CO2 in R1, R2, and the higher third body efficiencies of CO2 decreased the rate of CH4 oxidation during the early stage of combustion, where O2 was present. Even though R2 did not directly compete with the main chain branching reaction R3 (H + O2 → H + OH) for H radicals, like R1 did, R2 changed the hydrocarbon reaction pathway, thereby decreasing the rate of R4 (CH3 + HO2 → CH3O + OH) which had negative sensitivity in CH4 oxidation. However, we found that R1 and R2 advance CH4 oxidation in the last stage of combustion where O2 was mostly consumed. This is attributed to the fact that the reactions R1 and R2 were able to advance without the presence of O2, and that R1 produced OH radicals that were active in hydrocarbon oxidation in the specific temperature range and R2 enhanced hydrocarbon oxidation when the rate of R4 was insignificant. Although R1 was the dominant reaction to reduce unburned hydrocarbons in the O2/CO2 combustion, the role of R2 was significant at λ = 0.62. Meanwhile, when the air ratio was 0.74 where concentrations of reactants were relatively low, the chemical role of CO2 is to only decrease the rate of CH4 oxidation due to the presence of an excessive amount of O2.
A molecular tagging technique using the spark tracing method has been applied to measure velocity distributions in sub-millimeter-scale gas flows, formed as air jet flows through a sub-millimeter channel. Spark lines are generated by air ionization when applying high voltage due to the electrical discharge phenomena. The velocities measured using the displacement of spark lines were 10-30% smaller than those using the theoretical equation in a rectangular channel. In order to identify the cause of the measurement error, the relationship between the ionized air regions and the gas flow velocities was investigated by numerical simulation. The simulation revealed that a spark line goes through the pathway with the minimum electric resistance, and that the velocities from the theoretical equation agreed with the spark line velocities when the spark line width is assumed to be zero. Using this result, we propose a new velocity correction technique using the relationship between the spark line width and the measured velocity. The velocities from the experiments with the suggested correction agreed well with those from the theoretical equation. Furthermore, the corrected spark tracing method was applied to a mixing air jet flow field with different temperatures through two channels.
The double-diffusive convection in a porous medium due to the opposing heat and mass fluxes on the vertical walls is solved analytically. In the former analysis, we investigated only when ω < π, the parameter arising from a combination among the density stratification and the buoyancy effects. However, it is shown in the present research that a solution is also possible when ω > π. The Sherwood number Sh is shown to decrease monotonically with an increase in the buoyancy ratio N when ω > π, and Sh approaches 1 when N is 1. We define Nmin as the minimum value of N when Ω is imaginary and ω = π. Nmin increases with an increase in Rc. However, Nmin approaches a constant as Le increases. Furthermore, although the convection pattern is mainly temperature-driven, concentration-driven convection cells also exist under certain.
The melting of the ice plate with the transient double effects of the temperature and concentration was studied theoretically. In the present study, our main attention was paid to the early stage of melting, when the abrupt changes of the temperature and concentration distributions occur. The results of the present theoretical study were summarized as follows. (1) It was found that the approximate similar solutions can be obtained under the certain assumptions of the velocity component, and we derived the simple formula to evaluate the melting mass of the ice plate. (2) The present formula can evaluate the melting mass of the horizontal ice plate more correctly than the case of the vertical ice plate.
Porous media burners in comparison with free flame burners have major benefits such as higher thermal efficiency, stable flame in a wider range of stoichiometric ratios and feed flow rates, capability of using low calorific fuels and low production of pollutants. In the present study, premixed and laminar combustion of hydrogen in a solid matrix with spongy lattice is simulated. The axisymmetric solid matrix is considered to be inert, isotropic and homogenous in the unsteady simulations. The burner consists of a divergent inlet followed by a constant area section. A multi-step chemical kinetics is implemented. Heat exchange between the solid and gas phases is simulated using an experimental correlation for volumetric convective heat transfer coefficient, and the diffusion approximation is used to simulate the radiation mechanism inside the solid matrix. All physical properties of the gas mixture are considered as functions of local temperature and mixture composition. The governing equations are discreted and solved by the control volume scheme and SIMPLE algorithm. The effects of certain parameters such as flow rate and physical properties of the solid matrix on the thermal/stability performance of the burner are analyzed. Increasing the feed flow rate causes upstream movement of the flame front and increase in the flame temperature and pollutant formation. The flammability limits are obtained in the range of stoichiometric ratios between 0.5 and 1.2, where the widest belongs to a stoichiometric mixture.
Analytes separated by capillary zone electrophoresis are governed by ion migration. The study proposed a hybrid finite element and particle-in-cell method to simulate ion migration in the cross-channel capillary zone electrophoresis system. Different to the traditional numerical method, this method can be used to handle the migration of differently charged ions for complex geometry and boundary conditions. The research focuses on the arc effect on the migration phenomena of the positive and negative ions in the injection and separation process. The results indicate that arc effect can affect obviously the migration phenomena of analytes. In the injection process, the charged ions migrate not only faster into the separation channel but also more far away from the injection channel with increasing the corner arc radius. In the separation process, the zones of charged ions are wider and the time to completely separate is longer with increasing the corner arc radius. Consequently, as corner arc radius is larger, more time is needed to inject and separate charged ions. These findings have significant implications for design and control of capillary zone electrophoresis systems.
A three dimensional, single-phase, isothermal study of a polymer electrolyte fuel cell (PEFC) was conducted to investigate the mechanism of convective flow effect on the performance of fuel cell. Convective flow was caused by inlet velocity and cross flow through the GDL. To clarify the mechanism of cross flow effect on the performance of PEFC, it was induced in the parallel flow field, whereas cross flow inherently appear in the serpentine flow fields. In addition, to isolate the contribution of cross flow on the performance, the simulation was executed with a low current density where oxygen transport resistance was stronger than proton/electron transport resistance. Gas channels with a smaller pitch length produced a more uniform current density than those with a larger pitch length. With increase of inlet velocity the performance increases for any values of pitch length. Cross flow through the GDL caused by the differential pressure between adjacent channels had significantly enhanced the local current density of a PEFC and simultaneously increased the degree of non-uniformity in the current density. The cross flow can increase the performance of fuel cell by reducing the oxygen transport resistance. Therefore, it is possible to overcome the oxygen transport limitation by inducing cross flow.
In binary systems of hydrogen and hydrocarbons, the fluid-phase thermodynamic behavior is unique in having the divergence of the critical curves to a high pressure region. The thermodynamic properties of the binary systems including hydrogen with methane, ethane, propane, and carbon dioxide were calculated from a Peng-Robinson equation of state (PR EOS). The mixing parameter of the present EOS has a functional form of temperature generalized by the critical temperatures of the hydrocarbons and carbon dioxide. Based on the corresponding states principle, the coefficients of the parameter were determined with a non-linear least squares fitting to the experimental critical points of the mixtures. The developed PR EOS shows good agreement with the experimental data of not only the critical points but also the phase equilibria. In the hydrogen binary systems, retrograde condensation is expected. The volumetric and enthalpy changes in this process were simulated for a hydrogen + carbon dioxide mixture of 0.55 mole fraction using the PR EOS at 270 K.