In this study, OH chemiluminescence of methane-hydrogen premixed flames formed on a Bunsen burner was investigated. As for the chemiluminescence intensity, we obtained the values at the edge of the flame and at the central axis of the flame, compared with the value calculated by the inverse Abel transform. It was found that the value at the central axis of the flame was less dependent on the measurement position than that at the edge of the flame, because the effect of integrated chemiluminescence intensity on the optical path was small. In addition, four filters with different center wavelengths were tested. Although the chemiluminescence intensity was changed by the filter, the dependence of the chemiluminescence intensity on the equivalence ratio was almost the same. For more discussion, the OH concentration and the heat release rate predicted by the numerical simulation of a one-dimensional methane-hydrogen flame were compared with the OH chemiluminescence and the burning velocity in experiments. It was revealed that, even when hydrogen was added by keeping the total equivalence ratio constant, the OH chemiluminescence intensity was proportional to the production rate of excited OH* produced by the reaction of CH and O₂ concentrations. Similarly, the OH chemiluminescence intensity was apparently related with the heat release rate.

In this study, the cooling capabilities of flow boiling heat transfer aided by electrohydrodynamic (EHD) force and diamond-electrodeposited boiling surface is investigated in a micro-slit channel (MSC). The MSC uses a two-phase flow cooling system, in which an electric field is applied to a dielectric liquid using a slit electrode. To reduce the wall temperature below 60 °C and promote cooling in electronic devices, a dielectric liquid with a saturation temperature of 15 °C HCFO-1224yd (AGC, AMOLEA, CF₃CF = CHCI) was selected as a working fluid. Moreover, the entire system was pressurized using nitrogen gas to suppress liquid flow instabilities due to the generation of cavitation at the saturated system pressure. To enhance boiling heat transfer, the surface was electrically deposited with fine diamond particles (mixture of particles with diameters 20 and 1.5 μm), and an electric field of −5 kV/mm was applied between the surface and slit electrode. The experiments were conducted under various system pressures (75-230 kPa), mass flow rates (1.67-5.00 g/s), and degrees of subcooling (5-15 K) to evaluate the heat transfer performance. The electric field was effective in increasing both the critical heat flux (CHF) and heat transfer coefficient (HTC). The high electric field enhanced the boiling heat transfer until the inflow liquid entirely evaporated. Increasing the mass flow rate was also effective in increasing the CHF and HTC at lower wall temperatures, resulting in a maximum of 101 W/cm² at 64 °C and 37 kW/m²·K at 52 °C, respectively. Increasing the system pressure improved the HTC but elevated the wall temperature. Subcooling was effective in increasing HTC. Increase in either pressure or subcooling did not change the CHF because the entire inflow liquid evaporated in the MSC chamber due to the electric field effect.

When the biomass gasifier is connected with a gas engine system directly, tar should be removed from the syngas to prevent the engine from breaking down. A downdraft packed bed gasifier has the advantage for low tar emission because the syngas passes through the char gasification zone downstream of the reactor, where tar compounds can be trapped and decomposed. Then, objective of this study is to confirm the tar decomposition behaviors inside the downdraft packed bed reactor. Woody biomass gasification experiments were carried out, using an auto-thermal downdraft packed bed gasifier. The reactor’s height and inner diameter were 1000mm and 100mm, respectively. Black pine pallets were continuously fed into the reactor from the top. The gasifying agent was air, which was introduced into the reactor at the air-fuel equivalent ratio of 0.49. The packed bed height was kept to be constant at 600mm. The reactor has eleven thermo-couples and eleven sampling ports at the wall along the flow direction. They were used for measurements of temperature profiles and gas compositions in the reactor. Micro-GC was used for the measurement of N₂, O₂, CO, CO₂ and H₂, and FIDGC was used for other hydrocarbons. In some ports among them, tar in syngas was also sampled via dichloromethane scrubbing in ice-bath, and analyzed for molecular weight distributions of tar compounds by TOF-MS. As a result, tar and larger hydrocarbons were confirmed to be generated in the upstream, and then decomposed downstream inside the downdraft reactor.

A tubular flame burner equipped with a two-fluid nozzle for liquid precursor injection was developed and applied to the synthesis of fine particles. A tubular flame was established based on the axial spray of ethanol as a liquid fuel from the two-fluid nozzle. As the axial spray mixture from this nozzle merged with the tangential methane/air mixture from the tubular flame burner, a single flame front was established at the base of the burner. The flame structure was assessed by fabricating an optically accessible burner from quartz and OH*, CH* and C₂* emissions were investigated using a spectrometer and ICCD camera. The results show that a lifted flame was formed when the tubular flame was outside the flammable range, whereas a stabilized flame was obtained with the tubular flame in the flammable range. These results indicate that flame stability was primarily determined by the tubular flame. Optimal particle synthesis also required the tubular flame to be in the flammable range. Gas phase temperature measurements indicated that the main flame could be well-stabilized while maintaining a high-temperature environment by using a fuel-lean tubular flame, even in conjunction with the direct injection of liquid fuels. Using this flame system, both titania (TiO₂) and silica (SiO₂) nanoparticles could be synthesized.

Experimental investigations of burning characteristics of a tested specimen consisting of a polyethylene foam soaked in 100 wt% hydrogen peroxide are made. All experiments are carried out in a large volume chamber, which newly introduced in our previous work at an initial pressure range from p = 0.1 to p = 0.35 MPa in absolute with various fuel porosity range from ε = 0.6 to ε = 0.9, which corresponds to global equivalence ratios from φ = 0.51 to φ = 3.8. Temperature measurements using an R-type thermocouple embedded into the specimen are then conducted to investigate thermal structure (e.g., profiles of temperature and temperature gradients and burning surface temperature) of the burning specimen for deep understandings of the burning process. Following forced ignition at top surface of the specimen, steady successive-burning process is successfully observed for all conditions studied in this work. Burning rates (a rate at which the top surface moves downward) are measured by carefully tracking the top surface of the specimen by adopting an image processing software. Findings show that overall burning rates at the rate from 1 to 3.2 mm/s are obtained and influenced by pressure and the fuel porosity. Additionally, results of the direct temperature measurements reveal that the temperature gradient in the gas-phase layer near the top surface (burning surface) of the specimen increases as pressure increases, resulting in an increase in the overall burning rate. The top surface temperature and its pressure dependency, and a global activation energy at the top surface temperature of the specimen are experimentally measured accordingly.

Abnormal combustion such as pre-ignition and knocking is becoming one of the biggest problems in the latest gasoline engines that have a higher compression ratio and boosting for higher efficiency. An ion-current sensor integrated in an ignition system is used for accurately detecting knocking cycles. First, the problem for accurate knocking detection with an ion-current sensor was clarified in the test engine. The oscillation in the ion-current signal was observed in knocking cycles as is commonly known in the previous research. However, heavy oscillation in the ion-current signal can be observed occasionally even in the small knocking cycles. This phenomenon leads to the misdetection of knocking cycles with the conventional signal-processing method, which defines the oscillation intensity of the change amount in the ion-current signal as a knocking indicator. Second, to solve the problem mentioned above, a new signal-processing method is proposed on the basis of the thermal characteristics of ion-current signals. This method defines the oscillation intensity of the “normalized ion-signal change rate” as a knock indicator in order to suppress the effect of temperature dependency in ion-current signals. Finally, the proposed method was applied to an actual gasoline engine, and the knocking detection performance was evaluated. The method enabled the misdetection of the knocking cycles to be avoided and enhanced the correlation factor with knock intensity compared with the conventional method.

In this paper, the characteristics of hydrogen-propane-butane-air lean premixed flames on a flat flame burner were experimentally studied. To elucidate the shape and fluctuation of premixed flames, the cell width, RMS of fluctuation, peak frequency and reconstructed attractor were obtained from the direct photographic images and time series of light emission intensity. The cellular flames were observed at sufficiently low equivalence ratios; the flat flames were found under the other conditions. When the concentration of hydrogen was higher, the flat flames were formed at lower equivalence ratios. As the equivalence ratio became lower, the cell width and RMS increased in the range of cellular flames, which was due to high instability level. At low (high) equivalence ratios, moreover, the size of attractor was large (small) and the trajectory was unstable (quasi periodic). The experimental results revealed the instability characteristics of hydrogen-propane-butane-air lean premixed flames, and the obtained findings were applicable to the safe control of lean premixed flames.

Prediction and optimization of water-cooling performance of a pin fin heat sink with variable pin diameters along a flow direction is attempted. We have developed a simple analyze procedure for pin fin heat sink that enables to evaluate the row-by-row performance when diameters of fins are morphed along a flow direction. Validity of the procedure is confirmed for uniform and non-uniform pin diameter cases. The Taguchi method is applied to an optimization method. Optimal design among the considered cases, in which the thermal resistance is minimum, corresponds to the configuration where the pin diameter equals 3 mm constructed from copper. This holds for constant inlet velocity and constant pumping power cases. Optimum structure of heat sink considering simultaneous parameters (minimum thermal resistance, pressure drop and mass) occurs when the heat sink is built from aluminum with a pin diameter of 2 mm. These three parameters influence the design of the heat sink. For instance, when the thermal resistance and the pressure drop are significant, optimum structure is a “wing like shape” with larger pin diameter near the central rows and copper as material. However, when the thermal resistance and the heat sink mass are of interest, the optimum structure associated with the larger pin diameter constructed from aluminum. Also, when we consider the pressure drop and heat sink mass, the optimum design is the one where the pin diameter is 1 mm built from aluminum.

The dew point evaporative air conditioning system has gradually become an attractive research area in recent years because of its low energy consumption and environmental friendly advantage. How to improve the system efficiency is the focus of current research. This paper presents the theoretical performance of the dew point evaporation model with or without perforations. The heat and mass transfer model of the dew point evaporative cooling system was established and solved. It was found that the optimal supply air ratio was 0.7-0.8 and the value of the channel height was 4mm-5mm according to the evaluation parameter of the cooling capacity. Compared with other parameters, the supply air ratio has a more significant effect on the performance of the perforated cooling system. Single perforation has the best cooling system performance with supply air ratio of 0.5-0.7. Two perforations is the best for supply air ratio 0.3-0.4, while three perforations is more appropriate when supply air ratio is 0.2.

Various surface modifications controlling wettability and wickability have effectively enhanced the critical heat flux (CHF) in saturated pool boiling. Among them, this paper focuses on the CHF enhancement using a honeycomb porous plate (HPP). The HPP, which is commercially available, was generally used to filter exhaust gases from combustion engines, and has micron-order pores and millimeter-order holes which is called as a “cell”. Once an HPP was installed on the heated surface, the CHF in saturated pool boiling of water was improved more than approximately three times compared with a bare surface. The enhancement may be caused by: (1) the liquid supply due to capillarity, (2) liquid flowing down through the cells of an HPP due to gravity onto the heated surface, and (3) the release of vapor generated through the cells. However, the liquid supply process to heated surface due to (1) and (2) has not been clarified yet. Therefore, it is necessary to elucidate the detailed liquid supply mechanism for further CHF enhancement. In the present paper, two separate sets of experiments have been designed to investigate the liquid supply effect to the heated surface independently, namely, (1) automatic liquid supply due to capillary action by the porous part and (2) bulk liquid flowing down through the cells of an HPP. In summary, the measured values from the experiment extracting the liquid supply due to capillarity were in good agreement with the proposed capillary limit model. Moreover, for the high heat flux region (more than 3.5 MW/m²), the liquid supply due to the capillary force is dominant in enhancing the CHF. It was concluded that the keys to further CHF enhancement were the promotion of gas-liquid circulation based on the capillary limit model and improvement of wickability of the heated surface.

Reason: The article authored by Suazlan MT AZNAM, Naru MARUOKA, Ryosuke IMAI, Shoji MORI and titled “Process of liquid supply to heated surface by a honeycomb porous plate for critical heat flux enhancement”, Journal of Thermal Science and Technology Vol.16, No.3 (2021), has been found by a third party to be a duplicate with the following previously published paper, and the authors retracted the paper in accordance with the Editorial Committee's suggestion.

Naru MARUOKA, Shoji MORI, Kunito OKUYAMA, The effect of liquid supply due to capillary action on a critical heat flux enhancement using a honeycomb porous plate in a saturated pool boiling Transactions of the JSME (in Japanese), Vol.82, No.840(2016)

https://www.jstage.jst.go.jp/article/transjsme/82/840/82_16-00106/_article/-char/ja

The Japan Society of Mechanical Engineers

Journal of Thermal Science and Technology Editorial Committee