Recently, hydrogen is paid the most attention all over the world, because the CO2 emission can be reduced by an increase of its usage. In our previous study, hydrogen has been added in the premixed methane-air flame. The OH chemiluminescence intensity and the burning velocity were experimentally obtained, using a Bunsen burner. For further study, we investigated OH concentration by using the OH-PLIF technique. Especially, the OH fluorescence intensity was measured by keeping the burning velocity constant. The premixed gas velocity of CH4-H2-air mixture was 1.3 m/s. For comparison, OH chemiluminescence intensity was measured. Results showed that even when the hydrogen ratio in the total fuel of methane and hydrogen was changed, the OH fluorescence intensity was almost constant. On the other hand, OH chemiluminescence intensity decreased monotonically. Conclusively, different from the OH chemiluminescence intensity, the OH fluorescence intensity can be a good marker to evaluate the burning velocity in the combustion of mixed fuel of hydrogen and methane.
In this study, we experimentally investigated soot and NOx in a diffusion flame on a coaxial burner. By applying a dielectric-barrier-discharge (DBD) plasma, we tried to reduce these emissions. When the DBD plasma was exposed to the air flow, the height of the luminous flame slightly decreased. By considering the fact that the flame height without the DBD plasma is proportional to the fuel flow rate, the DBD plasma surely affects the soot concentration of the luminous flame. Based on the LII image, the soot region identified by the LII signal corresponds to the luminous flame zone. This soot distribution is similar even when the DBD plasma is activated, but the soot concentration of the luminous flame zone is reduced. As the air flow rate increases, the reduction of the integrated LII signal by the DBD plasma is smaller. This could be because the plasma is only active at the exit of the air flow, and the effect of the plasma is relatively weaker when the air flow rate increases. As for NOx emission, it is confirmed that the EINOx without plasma increases by increasing the air flow rate, showing that the thermal NOx could increase. Overall, when the plasma is activated, the EINOx always increases at any air flow rate. That is, the simultaneous reduction of soot and NOx cannot be achieved.
The mixing process is used not only to homogenize the materials, but also to bring out better functionality by mixing different materials. In the mixing process, temperature control during mixing is important to prevent thermal deterioration of the material and control chemical reactions. In this study, a 3D model was used to numerically simulate the flow field around a single rotating rotor in a partially filled chamber to investigate the thermal flow characteristics of a highly viscous fluid. The liquids examined contained glycerin, a Newtonian fluid, and carboxymethyl cellulose (CMC) aqueous solution, a non-Newtonian fluid. A three-wing rotor was chosen as the mixing rotor and the shear heating of viscous fluid was modeled by heating the tip of the rotor wing with a heater. Obtained simulation results of the temperature distribution around the rotor was agreed well with the independently performed experimental results. Evaluation of thermal flow properties was performed using history particles. Also, in order to analyze the temperature field, the flow rate through the rotor tip was used to classify the effect on the flow field. As a result, it was found that there was a correlation between the shear-thinning property and the temperature non-uniformity, and the flow rate ratio between the passing flow rate and the circulating flow rate due to the shear-thinning property of the fluid affected the thermal diffusion of the high temperature part.
Two-dimensional axisymmetric numerical simulations were performed for a Bunsen flame to investigate the OH concentration and heat release rate of methane-hydrogen premixed flames. The OH production rate of the premixed flame increases as the hydrogen ratio in the fuel increases. Resultantly, the OH concentration in the premixed flame increases with the larger heat release rate, but the OH concentration in the diffusion flame remains almost unchanged. The higher the total equivalence ratio, the greater the change in OH concentration and the heat release rate. This could be due to the fact that most of the hydrogen in the fuel reacts in the premixed flame. Independent of the total equivalence ratio, the relationship between the maximum OH concentration in the premixed flame and the OH fluorescence intensity measured by the PLIF technique shows some similarity in both methane and methane-hydrogen flames. Since the same linearity between the maximum value of the heat release rate and the burning velocity is observed, it is derived that the heat release rate in the premixed flame can be estimated by the burning velocity.
The multiple-ion-probe method detects flames using a plurality of ion probes installed on the wall of a combustion chamber and by reconstructing the dynamic behavior of the flame front along the wall. Although this method is effective only close to the wall, it can indirectly aid in visualizing flame propagation behavior. Because this method has an extremely high time resolution, it can accurately measure high-speed phenomena such as knocking in a spark-ignition engine. This study aims to establish a method to automatically determine the characteristics of combustion for each mixture using the acquired data from a multiple-ion-probe measurement system; various propagating flames in combustible mixtures with different compositions were used. The combustible mixture was composed of methane or LPG as the fuel and argon or nitrogen as the diluent. A stoichiometric mixture of fuel and oxygen was diluted by changing the diluent ratio to prepare fourteen types of mixtures for investigation. From the obtained individual experimental data, twelve types of scalar features were extracted and compared with the experimental data. We compared the scalar features extracted from the output data of the multiple-ion probe with the different combustion modes obtained in the series of experiments; we found that a few of the features strongly responded to specific propagation states. Thus, we confirmed that by using the features, it is possible to determine the characteristics of the propagation states; such characteristics are difficult to determine merely from the flame surface shape reconstructed from the multiple-ion-probe data.
In 2021, more than 40 countries have agreed with "de-coal" statement at the COP26. The torrefied solid biofuels have come to attract attention as promising alternative fuels to coal in coal-fired power plants and industrial boilers. However, the energy property such as higher heating value (HHV) and energy yield of torrefied biomass, which is one of the most important fuel properties, has been evaluated by means of experiments for various biomass species so far. That is, the torrefaction conditions to produce torrefied solid biofuel with predetermined HHV have been provided only empirically. In this study, a generalized HHV estimation method applicable to any torrefied woody biomass is investigated based on pyrolysis kinetics of primary constituent polymers of woody biomass, cellulose, lignin and xylan. Analyses of pyrolysis kinetics of constituent polymers are conducted by assuming that the process in non-isothermal pyrolysis experiments is modeled as the single reaction. The HHVs of torrefied constituent polymers are obtained by isothermal torrefaction experiments, and the experimental correlations of HHV of torrefied constituent polymers are proposed as a function of solid mass yield. For a given woody biomass whose mass fractions of three constituent polymers are known, HHV of torrefied woody biomass produced with any torrefaction condition can be estimated by applying pyrolysis kinetic models and experimental correlations of HHV for three constituent polymers to the torrefaction process of woody biomass. From the comparison between estimated HHV of torrefied woody biomass and experimental data, it is concluded that the generalized HHV estimation method can be useful for evaluating HHV of torrefied woody biomass with about 10% accuracy, although the HHV is somewhat underestimated.
Heat transfer enhancement is essential in low Reynolds number flow because recent small heat exchangers require narrow flow passages. In the present study, we conducted direct numerical simulations of the channel flow to investigate the turbulence sustaining effect by feedback blowing and suction from the wall. The initial flow field corresponded to the fully developed turbulent channel flow, and the Reynolds number suddenly decreased at the beginning of the simulation. The results indicated that there are two approaches for the turbulence maintenance. If the amplitude of the blowing and suction is high, then the sensor detects the blowing and suction from the wall, and the velocity fluctuation corresponds to self-maintenance. Then, we obtained large heat transfer. However, the gain was small. If the amplitude is moderate, then the blowing from the wall to the low-speed streaky structures pushes up, and the suction attracts the high-speed streak toward the wall. The effect to increase the Reynolds shear stress and heat transfer results in the promotion of the turbulence and heat transfer with high gain.
An experiment study was conducted on the operational characteristics of the JEST-type loop thermosyphon when an initial liquid level of a working fluid was lowered. This thermosyphon was invented in 2012 by one of the authors with a jet explosion stream technology (JEST) for cooling high-heat-generation and high-heat-flux CPUs. The present experiment aims at lowering the height of the thermosyphon for rack-level thermal management in a datacenter. Hydrofluoroether (HFE)-7000 was used as the working fluid. In experiment, an evaporator section of the thermosyphon was heated with a heating block while a condenser section was water-cooled using a thermostatic bath. Temporal changes in temperatures of the thermosyphon were obtained with thermocouples. Moreover, the circulation flow rate of the working fluid in the thermosyphon was obtained with a simple measurement method. The initial liquid level of the working fluid was changed as 166, 268, 368 mm while the height of the thermosyphon was 1200 mm. Experimental results are shown regarding the effect of the initial liquid level on the circulation flow rates of the vapor and liquid phases of the working fluid as well as the heat transfer coefficient at the evaporator section. An additional experiment was also conducted when the height of the thermosyphon was lowered from 1200 mm to 480 mm. It was confirmed that the thermal performance of the thermosyphon decreased when the initial liquid level was lowered; however, the thermal performance was recovered by lowering the height of the thermosyphon. Therefore, the JEST-type loop thermosyphon can be applied to the rack-level thermal management in a datacenter.