Asymptotic analysis on premixed flames with high-temperature combustible mixtures was performed to elucidate the instability of flame fronts. Planar premixed flames with sufficiently low Mach numbers were treated, and a one-step irreversible chemical reaction with sufficiently high activation energy was assumed. Through the asymptotic analysis, we obtained the formula for the burning velocity, depending on the temperature of burned and unburned gases and on the mass fraction of fuel. To investigate the instability of high-temperature premixed flames under the constant-enthalpy conditions, we obtained the instability intensity which depended on the burning velocity and on the temperature ratio of burned and unburned gases, where the burned-gas temperature was constant. As the unburned-gas temperature became higher, the instability intensity became higher, except for premixed flames with low reaction exponents under the conditions of constant-density unburned gases. This was because that the instability intensity was determined by the competition between the increase of the burning velocity and the decrease of the temperature ratio.
Thermochemical conversion of sugarcane bagasse into bio-crude oils by fluidized-bed reactor has been taken into consideration in this study. The bagasse in particle form was pyrolyzed in an externally heated 7cm diameter and 37.5cm high fluidized-bed reactor with nitrogen as a carrier gas. The reactor chamber and gas-preheater were heated by means of a renewable energy biomass source cylindrical heater. At a reactor bed temperature of 450°C for a feed particle size of 420-600µm and at a gas flow rate of 30 l/min, an oil yield of 48wt% of dry feed was obtained. The pyrolysis process temperature was found to have influenced on the product yields. Characterization of the whole pyrolysis liquids obtained at optimum operating conditions has been carried out including physical properties, elemental analyses, GCV, FT-IR, and 1H NMR analysis. The results show that pyrolysis of sugarcane bagasse waste is a good option for producing bio-crude oils to be used as alternative to petroleum fuels and valuable chemical feedstocks.
An adjoint-based shape optimization method of heat exchangers, which takes into account the heat transfer performance with the pressure loss penalty, is proposed, and its effectiveness is examined through a series of numerical simulation. Undulated heat transfer surface is optimized under an isothermal heated condition based on the variational method with the first derivative of the cost function, which is determined by an adjoint analysis of momentum and heat transfer. When applied to a modeled heat-exchanger passage with a pair of oblique wavy walls, the present optimization method refines the duct shape so as to enhance the heat transfer while suppressing the flow separation. It is shown that the j/f factor is further increased by 4% from the best value of the initial obliquely wavy duct. The effects of the initial wave amplitude upon the shape evolution process are also investigated.
The spray from a multi-hole injector applied to direct injection spark ignition (DISI) engine was investigated. In order to understand the detailed structure of the transient spray near the nozzle a combined LDA/PDA system was used. PDA system was optimized in order to detect relative smaller droplets. Size-classified technique was used to get deep information about the spray characterizations near nozzle. The experiments were performed at 7 MPa of injection pressure. At early stage of spray the droplet velocity distribution in the centre of the spray showed high value. Smaller droplets under 15 µm showing followability to air flow, while larger droplets over 20 µm will have drag due to momentum decay. Droplets of 15 <D≤20 µm in diameter is criteria for follow/penetration near the nozzle. Near the nozzle, the atomization predominantly occurs at the centre region of the spray.
A submicroscale flow sensor has been developed that consists of a suspended hot film and carbon nanotube (CNT) fins. Flow measurement experiments, together with a theoretical model, revealed the advantages of the use of CNT fins. The suspended metal film reduces heat loss and the CNT fins enhance the heat transfer to the fluid flow. Herein, the working principle of the CNT fins is presented in detail, together with a description of the micro electro mechanical systems (MEMS) / nano electro mechanical systems (NEMS) techniques used to fabricate the sensor. The CNTs were deposited by a manipulation method that is based on dielectrophoresis.
Thermodynamic analysis of the isentropic and polytropic expansion profiles of typical working fluids was carried out in order to design a highly efficient displacement-type expander for a low-temperature Rankine cycle. First, expansion profiles were analyzed for three typical working fluids: HFC245fa, ammonia, and supercritical CO2. The hot-side temperature ranged from 60 ° to 120 °C, and the cold-side temperature was 10 °C. In the analysis, isentropic and polytropic expansion processes were assumed to behave thermodynamically. In the analysis results, we noted similarities among the expansion profiles for different hot-side temperatures. This similarity allowed us to introduce the unique concept of a variable mechanism for expansion profile fitting in displacement-type expanders. This variable expansion mechanism can be achieved by simply adjusting the position of the inlet and/or outlet port of the expander.
An electrothermal gun possesses a great potential to be an efficient source of pulsed plasma discharge for nanomaterials production or thermal plasma spray coatings. A plasma discharge by intense pulsed power is numerically studied utilizing time-dependent gas dynamics equations which are solved by FCT (flux-corrected transport) algorithm in two-dimensional domain of the interior capillary bore region and the external region of extended bore. Plasma conditions at the bore exit, mass ablation of polycarbonate bore wall, and degree of ionization are determined at different levels of transient arc current profile. As a way to controlling the plasma discharge, the extended bore at the capillary exit is considered and the flow pattern of pulsed plasma discharge in the extended bore exhibit complex shock structure from slightly to highly underexpanded jet depending on the level of arc current profiles. Flow instability of oscillating Mach disk is found at higher level of arc current profile cases.
This paper proposes a non-destructive technique using a thermophysical handy tester to inspect the deterioration of metallic products. In the early stage of fatigue in a metallic body, many micro-cracks appear on the surface of the stressed body. As fatigue progresses, these micro-cracks multiply and grow, while the apparent thermal conductivity of the surface layer spontaneously decreases. This phenomenon introduces the possibility of determining the progress of deterioration through in situ measurement of thermophysical properties. To estimate the degree of deterioration of materials, a dimensionless deterioration factor α is introduced. In order to corroborate this technique, several fatigue tests using carbon steels were conducted herein. Throughout the tests, apparent thermal conductivities and deterioration factors up to the limit of fatigue were periodically measured using a thermophysical handy tester. Furthermore, microscopic observations to investigate the evolution of micro-cracks were performed for stage-assessment during the period of fatigue tests. The results clearly demonstrate that this technique is useful for the non-destructive diagnosis of such deterioration.
Micromachined titanium thin film current collector, Ti-CC, that is developed in our previous work is used to investigate the concentration overpotential due to the liquid water filling in the through-holes of Ti-CC. The effects of the Ti-CC porosity, volumetric flow rate and the channel/rib width on i-V characteristics are examined. Previously proposed model for the limiting current density can describe the trend in experimental data. Comparison with the results of visualization experiment confirms the feasibility of the newly developed model for i-V relation at each water coverage ratio of the Ti-CC. The results quantitatively show the importance of concentration overpotential due to the water droplet and suggest possible improvement of the cell performance with proper liquid water management.
Numerical analysis of CH4/O2/H2O laminar premixed flame under various conditions of pressure, equivalence ratio and steam concentration was performed using GRI-Mech 3.0 and the mechanism proposed by Davis and Law, which consists of C1 to C6 hydrocarbons in addition to GRI-Mech 3.0. The pressure dependence of laminar burning velocity and flame structure under fuel-rich conditions was focused on. Effects of the formation of higher hydrocarbons under fuel-rich conditions were also clarified using the mechanism proposed by Davis and Law. Results showed that for extremely fuel-rich conditions, laminar burning velocity increases as pressure increases for both mechanisms. The increase of laminar burning velocity is caused by the shift of the oxidation pathway of CH3 radical from the C2 Route to the C1 Route. The formation of C3-C6 hydrocarbons has only a small effect on laminar burning velocity. Under fuel-rich conditions, super-adiabatic flame temperature (SAFT) occurs and its pressure dependency was clarified.
Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5Hz and 20Hz. The results were summarized as follows: (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.
Heat transfer and fluid flow in a single-rib mounting channel were investigated by directly solving Navier-Stokes and energy equations. Flow and thermal fields were considered to be fully developed at the inlet of the channel, and the simulation was made for spatial advancement of turbulent heat transfer. Keeping the frictional Reynolds number, Reτ0, at 150, the rib height ratio was changed in four steps from H/δ = 0.05 to H/δ = 0.4. Computational results were confirmed to be nearly independent of grid meshes. In addition, numerical accuracy was confirmed through close agreement between computed mean pressure and the experiment by Yao et al. (1995). The numerical results revealed that the highest value of the mean Nusslet number was as large as 1.3 times the smooth surface consuming the same pumping power, and the local enhancement of heat transfer was correlated with the turbulence increase near the rib front and the reattachment point. According to the Reynolds stress budgets for H/δ = 0.2, there were mechanisms to induce powerful fluctuations: (1) Streamwise fluctuation was increased through production by flow deceleration in the upstream of the rib; (2) Redistribution to wall-normal and spanwise fluctuations was fortified by the fluid splattering to the rib front. Therefore, excellent performance of heat transfer was concluded to occur due to flow structures, which induce the strong disturbance near the rib front triggering smooth transition of the separated shear layer.
In this study, effect of desiccant wheel, heat exchanger and cooling coil will be evaluated on decreasing the wet bulb temperature of entering air to cooling tower and decreasing the outlet cold water temperature. For this purpose, change effect of desiccant wheel parameters will be investigated on wet bulb temperature of outlet air from heat exchanger. After that, optimum parameters and minimum wet bulb temperature will be selected. Then, outlet cold water temperature will be achieved for various cooling coil surface temperature with definition of by pass factor and also by using optimum desiccant wheel parameters and entrance air wet bulb temperature to tower related to cooling coil surface temperature. To calculate wet bulb temperature, a mathematical model will be used that shows physical properties of air. After that a nomograph will be used to predict effect of decrease of entrance air wet bulb temperature on reducing the outlet water temperature and it will be done for several cities in Iran. At the end, an equation will be used to calculate required water to air mass flow rate for each outlet cold water temperature. With considering of known circulating water mass flow rate, required air for tower would be calculated and suitable desiccant wheel can be selected.
A single-phase, fully three-dimensional transient numerical simulation was performed to analyze the dynamic response of a proton exchange membrane fuel cell (PEMFC) with single serpentine flow channels. . In addition, the effects of the membrane and gas diffusion layer thickness on current density transient behavior were investigated using numerical simulation. An overshoot of current density is observed for all thicknesses of the membrane and gas diffusion layer at an abrupt change of operating voltage from 0.7 V to 0.5 V. The peak of the overshoot and the elapsed thickness time to reach to the steady state value increase with decreasing membrane thickness. It is thought that the thin membrane facilitates the transport of water and ions through the membrane, resulting in an increase in current density and humidification of the membrane. The elapsed time to reach steady state voltage become shorter and the peak of the overshoot decreases as the thickness of the gas diffusion layer decreases. We suggest that this occurs because a thick gas diffusion layer increases the distance between the current collector (as heat exchanger) and catalyst layer (as heat source), resulting in a low transport rate of heat generated by the electrochemical reaction at the catalyst layer.
A single-phase, fully three-dimensional simulation model for a proton exchange membrane(PEM) fuel cell was used to examine the interdigitated flow field with electrochemical reaction and ion, electron, and water transport(electro-osmotic drag flux and back diffusion flux) through the polymer membrane. The numerical results showed that the fuel cell with an interdigitated flow field resulted in better performance than a fuel cell with a conventional flow field due to its strong convective transport across gas diffusion layer(GDL). However, the pressure drop in an interdigitated flow field is much greater than in conventional flow field. To investigate the effect of relative humidity on the performance of a PEM fuel cell, the humidification condition was set to 100% at the anode flow field and was changed by 0-100% at the cathode flow field. Maximum power density was obtained for a 70% humidified condition at the cathode where the oxygen concentration is moderately high while maintaining high ion conductivity at the polymer membrane.