This experimental study was conducted to heat gases using an arc driven by an external magnetic field. A dc arc was produced between a tungsten rod cathode and a cylindrical anode. Experimental observations revealed that the magnetically driven arc deformed unstably and rotated with no particular frequency. The arc voltage increased concomitantly with the increase of the magnetic flux density. Superheated steam was obtained using the rotating arc. The temperature increased with the increase of the imposed magnetic field. The arc heating efficiency was nearly 40-50%.
Burning velocity of spherically propagating turbulent flames kept increasing with flame propagation. On the other hand, turbulent burning velocity obtained from burner stabilized flame is constant for a given turbulence intensity. This difference of turbulent burning velocity characteristic was discussed by Abdel-Gayed and Bradley et al. For spherically propagating flames, not all turbulent eddies contribute to the turbulence that affect the turbulent flame. Small flames may be wrinkled only by small scale turbulent eddies. As the flame propagates, it becomes progressively wrinkled by the larger eddies. The turbulence which contributes to turbulent flame may increase with the increase in flame radius. This increase in turbulence may cause the increase in turbulent burning velocity. The effective turbulence intensity was proposed by Abdel-Gayed and Bradley et al. as the turbulence intensity which contributes effectively to turbulent flame. In this study, the turbulence characteristics which are necessary for obtaining the effective turbulence intensity were evaluated by Particle Image Velocimetry (PIV) measurement. Then turbulent burning velocity of iso-octane/air flame was investigated with effective turbulence intensity. The turbulent burning velocity increased with the increase in effective turbulence intensity. The increase in turbulent burning velocity may be caused by the increase in flame front area with increase in the effective turbulence intensity.
In this paper, we investigate the combined effects of suction/injection and asymmetric Navier slips on the entropy generation rate in a steady flow of an incompressible viscous fluid through a porous channel subjected to non-uniform temperature at the walls. The nonlinear model problem is tackled numerically using shooting quadrature. Both the velocity and temperature profiles are obtained and utilized to compute the entropy generation number. The effects of slip parameter, Brinkmann number, the Peclet number and suction/injection Reynolds number on the fluid velocity, temperature profile, skin friction, Nusselt number, entropy generation rate and Bejan number are depicted graphically and discussed quantitatively.
Contact-probe methods have been developed to measure thermal transport properties of solid materials. Although they have an advantage of being used for non-destructive in-situ measurement, it has a substantial problem that the measured results are influenced by thermal contact between the probe and the specimen. To overcome this problem, we proposed a new technique using a gel to eliminate the contact resistance. A unique feature of the method is that a thin film heater is fabricated on a substrate at the bottom of a shallow cavity, which provides a gel layer of the thickness that is almost the same as the cavity depth. When we use this sensor, we press it against a surface of a specimen with a gel in between. We named it "stamp sensor" from its supposed use and appearance. The thermal conductivity and the thermal diffusivity of a specimen and the thickness of the gel layer are determined simultaneously by comparing the measured temperature rise of the sensor with that obtained by numerical analysis. The objective of the present paper is to demonstrate the feasibility of the method by virtual experiments. The process to determine the thermal conductivity and the thermal diffusivity was examined using simulated experimental data that have been generated from the temperature rise of the sensor obtained by numerical analyses with given artificial scattering. Effects of the scattering of the data, the heating power and the heating time on the measurement error were also discussed.
A prototype thermophotovoltaic (TPV) generation system based on super-adiabatic combustion with reciprocating flow in porous media was developed experimentally. Stacked porous quartz glass plates in the system were used as an optical filter and for heat storage. The distributions of temperature and spectral radiation indicated that both energy recirculation and spectral control were realized with the quartz glass plates. In addition, electric power was obtained by introducing the spectral controlled radiation into GaSb cells. The measured output power was in close agreement with that estimated using a parallel ray-tracing model. Application of a one-dimensional configuration to this system is expected to result in a total fuel to electricity conversion efficiency that would reach 10%. Consequently, a spherical super-adiabatic combustion TPV system is proposed as an idealized one-dimensional system to achieve this.
A continuous circulating of sorbent powder for sorption and desorption process in two connected fluidized beds has been developed. Two fluidized beds are arranged next to each other and connected by spiral tubes. The experiments were carried out under the various conditions such as air velocity, desorption air temperature, and sorbent powder circulation rate. Sorption and desorption characteristics of sorbent powder of the organic sorption materials show that sorption and desorption performance promoted by increasing air velocity and desorption air temperature. It was found that the spiral revolution speed has optimal value on the humidification. Furthermore, the non-dimensional correlation equations were obtained for water vapor mass transfer under sorption and desorption process in terms of relevant non-dimensional parameters.
The effect of PAG oil concentration on the heat transfer performance and pressure drop of a fin-tube heat exchanger using carbon dioxide(CO2) heat pump during the gas cooling process of CO2 was investigated by experiment. The experimental apparatus consisted of a gas cooler, a pre-heater, a sub-cooler, a mass flow meter, pumps and a measurement system. Experiments were conducted for various conditions of inlet temperature(110°C), mass flow rate of refrigerant(50, 55, 60, 65, 70 g/s) and PAG oil concentration(0 to 2.6 wt%). The heat transfer rate decreased with the increase of the oil concentration and the decrease of the inlet pressure. And the pressure drop increased with increases of the oil concentration and the mass flow rate of the refrigerant. The COP reduction by the deterioration of gas cooler performance with oil concentration was analyzed. When the inlet pressure of the gas cooler was 100 bar, the COP reduction was estimated to be 6% under 1 wt% of oil concentration.
Here we describe a new method to deliver membrane impermeable cryo-/lyo-protective agents (CPAs) into the cytosol of living cells via their electrofusion with Giant Unilamellar Vesicles (GUVs) containing large amounts of CPAs. Membrane electrofusion is commonly believed to be triggered by the irreversible electrical breakdown of the membrane at contact region induced by a DC field pulse. Therefore, analysis of the temporal changes of the membrane potential distribution in a cell-GUV pair is is necessary to achieve optimum electrofusion conditions with respect to the field pulse strength and duration. In this study, the GUV-Jurkat cell electrofusion rates under various pulse strength and length were measured. In addition, we calculated the transient membrane potentials in a deformed GUV-Jurkat pair during the electric field application by a finite element method (FEM)-electric field analysis. The relevant electric properties of both fusion partners reported previously were used for the analysis to validate the quantitative calculation results. Both experimental results and theoretical calculation suggest that; 1) GUV-Jurkat cell pairs should be stretched by AC field prior to electrofusion, 2) the whole membrane contact zone should undergo electrical breakdown at the same time to accomplish electrofusion, 3) after applying an electric field for around 1µsec, membrane potential in contact region becomes homogeneous, 4) after applying an electric field for around 10µsec, membrane potential is saturated, 5) the irreversible breakdown occurs at a membrane voltage of about 3V.
The effects of orientation of turbulence promoting ribs in the internal cooling air passage on the film cooling performance of outer surface of gas turbine blades have been studied experimentally. Both the adiabatic film cooling effectiveness and the heat transfer coefficient downstream of the film-cooling hole of a flat plate were measured for two different orientations of the turbulence promoting ribs. It is found that the adiabatic film cooling effectiveness and heat transfer coefficient on the external surface are considerably affected by the inversion of the rib orientation in the internal passage. The film cooling performance is evaluated in terms of the net heat flux reduction and the net surface temperature reduction, both of which are calculated from the measurement of adiabatic film cooling effectiveness and heat transfer coefficient. The evaluation of these quantities has clarified the effects of the internal cooling structure on the film cooling performance.
We investigated influences of SO2 concentration and relative humidity in fuel and air streams on anode and cathode poisoning behaviors under open circuit voltage and load operating conditions in polymer electrolyte membrane fuel cells (PEMFCs). The rate of cell voltage degradation increased with the increase in SO2 concentration and the normalized cell voltage drop curves were consistent with each other. Cell voltage at equilibrium with SO2 supply was inversely proportional to the concentration of supplied SO2, suggesting that adsorption of sulfur species on the catalyst was in equilibrium and was determined by the SO2 concentration in the electrode. Relative humidity significantly impacted electrode contamination. Electrochemical surface area (ECSA) measured by cyclic voltammetry (CV) revealed pronounced anode contamination under lower RH conditions and suppressed contamination under high RH conditions. Fuel cell load operation showed mitigated anode poisoning, suggesting that appropriate water management of the polymer electrolyte membrane fuel cell (PEMFC) mitigates electrode contamination.
Hydrogen can be used not only as a fuel, but also for the chemical processing. One of the applications is as a synthesis gas in the gas-to-liquid (GTL) process which produces liquid fuel from a gaseous one and has tremendous potential for the future energy industry. Steam methane reforming (SMR) is one of the major methods of hydrogen production. To optimize SMR, pressure dependence is important since the Fischer-Tropsch (FT) process, which is the oil production process in GTL, favors high pressure. In this study, using the rate expressions of SMR, which are obtained by experiment using a nickel-based catalyst supported on a metal plate in a rectangular channel, two-dimensional (2-D) simulation was performed at four different pressures ranging from 0.1 to 0.4 MPa in addition to one-dimensional (1-D) simulation. The reaction rates deduced from the experimental results on the basis of pseudo-bulk reaction naturally fit the 1-D simulation. The more precise 2-D simulation, however, inevitably needs some modification to reasonably predict the phenomena on the basis of the data from the pseudo-bulk reaction. As a result, the discrepancy between the results could be fixed by adjusting the reaction rate for the related pressure, and the adjusted factors show consistency between 1-D and 2-D calculations. Finally, the proper rate equations were estimated.
Combustion oscillation of a self excited combustion-acoustic phenomenon occurs inside the gas turbine combustor. In general, excessive pressure fluctuation of combustion oscillation may impair the gas turbine engine operation and may result in hardware damages. Therefore, combustion oscillation has been one of the problems of the gas turbine development. In this paper, for predicting combustion-acoustic instability on the designing stage, we have developed one-dimensional linear instability analysis method and verified the analysis accuracy by premixed combustor laboratory tests. The phase lag between combustion dynamics and acoustic system is considered to affect whether combustion acts to destabilize the whole system. Heat release fluctuation is considered to be influenced from fuel/air ratio fluctuation, which generates at fuel nozzle position and flows to combustion region. Its advection delay time is one of the important parameters in the instability. Further, even if heat source itself is steady, the heat release can fluctuate because gas flow rate fluctuates acoustically, and as results, combustion-acoustic instable can occur. At this mechanism, the flame position and the reaction delay time have important roles. By theoretical discussions and laboratory verifications, we conclude that analysis models of this paper have captured the basic framework needed to predict combustion-acoustic instability.
Ethylene plays an important role in regenerative-cooling scramjet engine, but its combustion characteristics have not yet been studied thoroughly. This paper reports the design of a square flat flame burner used for chemiluminescence measurement technique and the concentration of OH* and CH* of laminar premixed ethylene/air flame at low pressure. An approach of obtaining quantitative concentration through both PLIF experiments and CHEMKIN simulations is proposed. Results show that the maximum concentration is about 108 cm-3 and increases with elevated pressure. Although the maximum concentration depends much on equivalence ratio and pressure, the ratio of the maximum CH*/OH* concentration is independent of pressure (10kPa∼50kPa). The results are prerequisite for employing chemiluminescence technique in ethylene combustion process.
In the actual design circumstances, the radiation effect on the cooling performance of the extended surfaces must be taken into consideration. The central goal of this paper is to design the constructal architecture of a T-Y shaped assembly of fins by incorporating the thermal radiation with convection heat transfer. Our motivation in selecting a T-Y shaped assembly of fins is that the so-called assembly is of more cooling performance among the other shapes according to the literature. The temperature field and the peak temperature (hot spot) are calculated from a numerical approach based on finite element method. The peak temperature is minimized by manipulating the geometry of the assembly regarding the constraints of space and materials. A comparative study is executed among the present work and the published work. The comparison revealed that although the optimized geometry is approximately independent of the heat transfer mechanism, the hot spot temperature can be affected when the finned surfaces are exposed to thermal radiation in addition to convection. This issue is more pronounced when the emissivity of the finned surfaces is higher in comparison with the convection heat transfer coefficient.
Burning velocity just after spontaneous ignition has been examined not only experimentally but also theoretically, as related to a Self-propagating High-temperature Synthesis (SHS) process, for a Ti-Al system. After varying the mixture ratio, the degree of dilution, and the compact and particle sizes, the spontaneous ignition temperature, which is determined from the inflection-point of the temporal variations of the surface temperature, was measured. The burning velocity, which is defined as the normal component to the flame surface, has also been measured. It was found that the burning velocity just after spontaneous ignition first increases, and then decreases with an increase in the mixture ratio, which is due to an increase and a decrease respectively, in the heat of combustion. The burning velocity also decreases with an increase in the degree of dilution, which is due to a decrease in the heat of combustion. In addition, the burning velocity increases with increasing size ratio, defined as the ratio of compact and particle sizes, as a result of an increase in the reaction surface per unit spatial volume in the compacted mixture. Experimental comparisons with theoretical results have also been conducted and a fair degree of agreement has been demonstrated, indicating that the formulation used has captured the essential features of SHS flame propagation as it passes through the compacted mixture. Since this kind of particle size effect, which is specifically relevant to flame propagation after spontaneous ignition, has not yet been captured in previous studies, its elucidation can be considered both notable and useful, especially while manipulating the combustion process during materials synthesis.
Combustion oscillation of a self-excited combustion-acoustic phenomenon occurs inside the gas turbine combustor. In general, excessive pressure fluctuation of combustion oscillation may impair the gas turbine engine operation and may result in hardware damages. Hence, combustion oscillation has been one of the problems of the gas turbine development. For predicting the combustion-acoustic instability on the designing stage, we have developed one-dimensional linear instability analysis method and verified the analysis accuracy by premixed combustor laboratory tests. According to the prior efforts, the basic framework of combustion-acoustic system has been captured, and then, we have known that not only precise analysis method but also high precise model parameters are needed for accurate analysis. So in this paper, to achieve this, three-dimensional combustion-acoustic analysis method with finite element model which can simulate precisely the geometry and combustion state of an actual gas turbine combustor is developed. Further, for the calculation efficiency, the approximation calculation method for evaluating the complex eigenvalues of finite element model is proposed and its accuracy is validated at simple examples.
A simple mathematical model has been proposed for combustion of solid fuel deposited over an inert porous medium. The thermal nonequilibrium between the porous medium and gas is incorporated in the model considering two energy equations with different density, thermal conductivity and specific heat capacity for the porous medium and gas. A simple parameter “deposition of fuel” is newly introduced instead of fuel mass fraction to simplify the model. We employ large activation energy asymptotic to investigate the moving speed of the burning front of solid fuel for opposed flow combustion where the burning front (defined as the location of heterogeneous combustion takes place) propagates opposite to the direction of oxidizer (gas) flow velocity. During the analysis we consider all transport terms in the energy equations which have not been accomplished so far for heterogeneous combustion in an inert porous medium. We have successfully obtained an implicit analytical expression of the moving speed of the burning front including the other pertinent parameters such as deposition of solid fuel, porosity of the porous medium, thermal conductivity of the porous medium, gas flow velocity and the heat exchange coefficient between the porous medium and gas. The effects of variation of these parameters are discussed in terms of the moving speed of the burning front. Analytical solutions are found to be satisfactory in certain range of parameters based on the comparison with numerical solution provided by us. Additionally, the validity of the analytical result is also convinced by the past work done by Fatehi and Kaviany .
The objectives of this study were to establish a theoretical model of the phase change heat transfer in a microchannel and to investigate the possibility of cooling under higher heat flux than the critical heat flux for nucleate boiling. A bubble expansion model including transient variation of the liquid film thickness was proposed. The heat transfer coefficient and wall superheat were calculated by proposed model. The validity of this model was evaluated by using cited experimental data. It was confirmed that the bubble length derived by this model was in good agreement with that data. In order to evaluate the heat transfer regime in a microchannel, the minimum wall superheat to initiate nucleate boiling was introduced, and the conditions to achieve perfect evaporative heat transfer in a microchannel under the critical heat flux were derived. The diameter of the microchannel where perfect evaporative heat transfer occurs is called the “boiling limit diameter”. The boiling limit diameters of various fluids were calculated. The results clarified the influence of the surface tension on the phase change heat transfer in a microchannel. Furthermore, this result also indicated the possibility of high heat flux cooling with perfect evaporative heat transfer in a microchannel.
This paper reports the effect of temperature distribution on tube rupture at the pulverized coal fired thermal power plant. A computational model was applied to a 150 MWe boiler burning high-ash, medium-volatile coal. The radial and axial flame temperature distribution in the boiler was simulated using CFD code FLUENT. Flame temperatures were measured at points close to wall in some boiler levels and compared with computational fluid dynamics (CFD) solutions. CFD analysis showed that the flame shaped in the centre of the boiler and the temperature decreased gradually towards the boiler walls. The coal and ash composition was analyzed, and tube thickness was measured. Analysis showed that the ash and chlorine content of the coal, and SiO2 content of the ash were very high. Deposits on tubes can occur overheating, fouling and slagging which lead to tube ruptures. The abrasive effect of the ash accelerated thinning of the tubes and caused them to rupture.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)