Journal of Thermal Science and Technology
Online ISSN : 1880-5566
ISSN-L : 1880-5566
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  • Riku YAMASHITA, Satoshi WAKU, Daisuke MORI, Seiya UENO, Kimitoshi TANO ...
    2021 Volume 16 Issue 2 Pages JTST0014
    Published: 2021
    Released: January 02, 2021
    JOURNALS FREE ACCESS

    The prechamber combustion characteristics of a cogeneration system were studied using a rapid compression and expansion machine (RCEM) to improve the efficiency of cogeneration natural gas engines. The generated torch flames by the prechamber were used to investigate the effect of prechamber on the main combustion. In our previous study, we observed the correlation between the torch flame and the main flame, which is so-called “prechamber combustion.” In this study, the prechamber combustion characteristics of a gas engine were examined by visualization using a RCEM for methane and propane. In addition, the study mainly aimed to clarify the effects of the prechamber parameters, such as the equivalence ratio, initial pressure, and prechamber nozzle diameter, on the ignition and combustion in the main chamber for methane by comparing the combustion characteristics in the cases of methane and propane. Consequently, the ignitability of both fuels was improved by increasing the initial pressure and nozzle diameter. However, the effect of the equivalent ratio on the ignitability was different for the two fuels.

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  • Wei YUAN, Fengzhong SUN, Ruqing LIU, Xuehong CHEN, Ying LI
    2021 Volume 16 Issue 2 Pages JTST0015
    Published: 2021
    Released: January 02, 2021
    JOURNALS FREE ACCESS

    The water loss caused by evaporation heat dissipation is non-negligible for a natural draft counter-flow wet cooling tower (NDWCT), which is evidently influenced by many change factors. Based on Merkel method, the Merkel number is revised in consideration of the effect of water loss. Taking the NDWCTs equipped for 300MW and 600MW power plants as experimental objects, a mathematical model is established for predicting the evaporation loss of the NDWCT. The accuracy of the model is verified by experimental data, and the mathematical model for predicting evaporation loss is feasible. Then, the effect of change factors, including mass flow rate of circulating water and air parameters, on the tower evaporation loss is studied in the case of fixed the thermal load dissipated by the condenser. Results show that the evaporation loss varies little with the change of mass flow rate of circulating water. Therefore, Water conservation cannot be achieved by changing mass flow rate. With the increase of dry bulb temperature, the evaporation loss, as well as the rate of evaporation loss caused by unit temperature drop increases. As the relative air humidity increases, the NDWCT outlet water temperature increases, whereas the evaporation loss and the rate of evaporation loss caused by unit temperature difference decrease.

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  • Chun Loon CHA, Ho Yeon LEE, Chang Hyun YOU, Sang Soon HWANG
    2021 Volume 16 Issue 2 Pages JTST0016
    Published: 2021
    Released: January 02, 2021
    JOURNALS FREE ACCESS

    Moderate Intensity Low Oxygen Diluted (MILD) combustion has been investigated for a long time in order to minimize NOX emission still enhancing thermal efficiencies in the combustion equipment. Many researches about MILD combustion have been recently performed, but studies on MILD combustion of renewable fuel such as ethanol has been very scarce and particularly, specific information on the NOX production in ethanol-air MILD combustion has not been reported yet. In order to satisfy the condition for MILD combustion, internal recirculation is known to be essential in order to entrain the combustion products gas into air and fuel jets of combustion system. In this work, a series of numerical analysis with simplified opposed jet geometry have been done using the OPPDIF in Ansys program. Numerical analysis on how the recirculation ratio (KV) affects NOX emission in the ethanol-air combustion for MILD formation were carried out under the condition of various burnt gas dilution in reactant flow. The results show that the temperature was decreased by the increase of the recirculation ratio and the maximum heat release value became also low by the increase of the recirculation ratio. It was also found that the pyrolysis zone of the heat release was disappeared and the two heat release peaks are merged into one as combustion pattern is changed to MILD combustion mode.

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  • Noriyuki UNNO, Kazuhisa YUKI, Jun TANIGUCHI, Shin-ichi SATAKE
    2021 Volume 16 Issue 2 Pages JTST0017
    Published: 2021
    Released: January 06, 2021
    JOURNALS FREE ACCESS

    For boiling heat transfer, it is important to improve both the critical heat flux and heat transfer coefficient. In general, the heat transfer coefficient is improved by promoting the nucleation of boiling bubbles on the heating surface. However, this decreases the critical heat flux. To improve the heat transfer coefficient without decreasing the critical heat flux, we previously developed a technique using a boiling bubble resonator, which consists of a material attached close to the heating surface that vibrates in response to the growth and collapse of boiling bubbles. In this study, we used spacer plates to vary the gap height between the heating surface and boiling bubble resonator to maximize the boiling heat transfer. By optimizing the gap height, the wall superheat decreased by 7 and 25 K at 0.8 and 5.6 MW/m2, respectively. The maximum heat flux was 5.8 MW/m2 with the optimized gap height. In addition, we observed sound signals when the boiling bubble resonator was optimally vibrating. Moreover, jet flow from the gap appeared with the vibration of the boiling bubble resonator.

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  • Yoko SEKI, Ryuji KOBAYASHI, Wookyung KIM, Tomoyuki JOHZAKI, Takuma END ...
    2021 Volume 16 Issue 2 Pages JTST0018
    Published: 2021
    Released: January 13, 2021
    JOURNALS FREE ACCESS

    We experimentally investigated the promotion of deflagration-to-detonation transition (DDT) in hydrogen-air mixtures contained in a tube in which straight-shaped rods were installed as obstacles. In the experiments, the number of obstacle rods, their spacing, their blockage ratio, and the equivalence ratio of the hydrogen-air mixture were varied as the governing parameters. The obstacle rods had a spacing of 10 or 20 mm and a blockage ratio of 0.32, 0.41, or 0.51. As a result of an optimization of the obstacle-rod conditions, when fourteen rods, whose blockage ratio was 0.32, were installed in a tube with a spacing of 20 mm and with a hydrogen-air mixture with equivalence ratios from 0.8 to 1.2, the run-up distance to the DDT was shortened to approximately 20 times the tube diameter.

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  • Mohammed A. ALMESHAAL
    2021 Volume 16 Issue 2 Pages JTST0019
    Published: 2021
    Released: January 26, 2021
    JOURNALS FREE ACCESS

    A three-dimensional numerical study is conducted to investigate the double-diffusive natural convection inside a trapezoidal solar pond filled with water and salt solution and subjected to discrete heat boundary conditions. It investigates the effect of stored thermal extraction on the non-convective zone (NCZ) of the solar pond. The heat extraction from the middle region on the lower convective zone (LCZ) is modeled as a discrete thermal boundary condition in the NCZ-LCZ. The governing equations are developed using the three-dimensional potential-vorticity formulation and solved using the finite volume method. The numerical computations are performed for the following parameter ranges: Rayleigh number 103 ≤ Ra ≤ 105 and buoyancy ratio-10≤ N ≤0. Two different cases related to the thermal boundary conditions are investigated. The most important results that are drawn from this study are: (i) the non-uniform heat extraction from the LCZ increased due to vortex formation which disturbed the structure of the flow, and (ii) the three-dimensional effects become more pronounced that caused a decrease in the efficiency of the solar pond. Therefore, when extracting the stored heat from the LCZ layer, it is imperative to maintain the surface temperature as constant as possible and to avoid getting discrete heat surfaces.

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  • Nae-Hyun KIM
    2021 Volume 16 Issue 2 Pages JTST0020
    Published: 2021
    Released: January 30, 2021
    JOURNALS FREE ACCESS

    Conventional louver fins have an inherent problem of condensate drainage. In this study, a newly developed louver fin is introduced. The louver fin has an extension at leading edge. Dry and wet surface heat transfer and pressure drop characteristics of the heat exchanger made of the new fins were investigated, and results are compared with those having trailing edge extension fins or conventional fins. For conventional sample, significant differences between dry and wet j and f factors were observed. Dry j factors were 166% larger than wet j factors, and wet f factors were 68% larger than dry f factors. The discrepancies were significantly reduced for the samples having extensions, which suggests that extensions are effective in condensate control. The present sample having leading edge extension yielded higher dry j factors than the sample having trailing edge extension, probably due to proper allocation of louvers. Wet j factors of the two extended samples were approximately the same, which means that the condensate drainage is better for trailing edge extension than leading edge extension. Under dry condition, j/f1/3 of leading edge sample were 132 % and 59% higher than those of trailing edge extension and conventional sample respectively. However, under wet condition, j/f1/3 of leading edge extension sample were 87% and 205% higher than those of trailing edge extension and conventional sample. This confirms that the present leading edge extension sample shows better performance than trailing edge extension sample, both under dry and wet condition.

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  • Satoshi KADOWAKI, Thwe Thwe Aung, Taisei FURUYAMA, Kazumasa KAWATA, To ...
    2021 Volume 16 Issue 2 Pages JTST0021
    Published: 2021
    Released: February 09, 2021
    JOURNALS FREE ACCESS

    Effects of pressure and heat loss on the unstable motion of cellular-flame fronts in hydrogen-air lean premixed flames were numerically investigated. We adopted the reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, and the compressibility, viscosity, heat conduction, molecular diffusion and heat loss were taken into account. A sufficiently small disturbance was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the pressure became higher, the maximum growth rate increased and the unstable range widened. These were due mainly to the decrease of flame thickness. As the heat loss became larger, the former decreased and the latter narrowed, which were due mainly to the decrease of burning velocity. To investigate the characteristics of cellular-flame fronts, a disturbance with the critical wavelength was superimposed. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. The burning velocity of a cellular flame normalized by that of a planar flame increased as the pressure became higher and the heat loss became larger. This indicated that the pressure and heat loss affected strongly the unstable motion of cellular-flame fronts. The burning velocity of a cellular flame increased monotonically with an increase in the space size. This was attributed to long-wavelength components of disturbances. Moreover, we estimated the fractal dimension of flame fronts through the box counting method. As the pressure and heat loss increased, the fractal dimension became larger, which denoted that the flame shape became more complicated.

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  • Masaharu KOMIYAMA, Takafumi KUMAZAKI, Satoshi NISHIDA
    2021 Volume 16 Issue 2 Pages JTST0022
    Published: 2021
    Released: February 16, 2021
    JOURNALS FREE ACCESS

    Lean premixed combustion can be expected to reduce the high-temperature area and NOx emission. However, it has a risk of flashback due to flame propagation. It can be thought to modulate the gas swirl intensity to prevent flashback and to stabilize combustion. In this research, we developed time-variable-angle swirl vanes which had 56 mm of the inner diameter, 70 mm of the outer diameter and consisted of 36 vanes, each vane was directly connected to a stepping motor, and the swirl intensity could be changed periodically by keeping the constant air ratio and flow rate. We confirmed the periodical movement of the swirl vane angle using a high-speed camera. The flame position could be made reciprocating move upstream and downstream by the periodic change of the swirl vane angle. The upstream direction flow at the tip of the flame appeared when the flame moved in the upstream direction with increasing vane angle and the downstream direction flow appeared when that moved in the downstream direction with decreasing the vane angle. When the flame propagation was changed from upstream direction to downstream, the upstream direction flow near the flame tip was weakened. In advance of changing the flame propagation from upstream direction to downstream, the axial velocity near the flame tip changed from decreasing to increasing.

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  • Manabu SAITO, Keisuke KOMADA, Daisaku SAKAGUCHI, Hironobu UEKI
    2021 Volume 16 Issue 2 Pages JTST0023
    Published: 2021
    Released: February 16, 2021
    JOURNALS FREE ACCESS

    Targeting the evaluation of the universal droplet breakup process, measurements were made on diesel fuel sprays injected from two solenoid type injectors with different specifications. The velocity and size of the spray droplets were measured using a laser 2-focus velocimeter (L2F). The velocity of small droplets that follow the flow was taken as the air velocity. The Weber number was evaluated using the velocity of the droplets relative to air as the representative velocity. Since the spray expands in a conical shape with the nozzle hole at its apex, the measurement points were placed on a straight line from the apex, which is the estimated flight direction of droplets. The change in droplet size in the flight direction was considered to be due to secondary breakup, and the rate of decrease in droplet size during this process was evaluated. It was confirmed that the velocity and size of the droplets inside the spray injected from the two injectors varied over time, and the spatial distribution of the Weber number and the rate of droplet size decrease in the middle of the injection period was non-uniform and different. It was found that there is a positive correlation between the Weber number and the rate of droplet size decrease for both sprays, and that the relationship is nearly identical despite the fact that the characteristics are different between the two sprays. The secondary breakup process was shown to be independent of the injection conditions such as the diameter and number of nozzle holes.

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  • Kitti NILPUENG, Somchai WONGWISES
    2021 Volume 16 Issue 2 Pages JTST0024
    Published: 2021
    Released: March 09, 2021
    JOURNALS FREE ACCESS

    The concept of heat transfer enhancement in a plate fin heat sink (PFHS) using twisted tape is presented. The airflow behavior in the flow channel and heat sink performance are investigated at the Reynolds numbers between 2000 and 5000. The twisted tape and perforated twisted tape with twist ratio between 2.5 and 3.5 are equipped between the fins of the PFHS. For perforated twisted tape, the holes are drilled along the twisted tape length with the ratio of perforation diameter to twisted taped width (d/W) between 0.2 and 0.6. The heat transfer coefficient and pressure drop are enhanced by decreasing the twist ratio and increasing the Reynolds number and the d/W ratio between 0.2 and 0.4. However, they are dropped when the d/W ratio higher than 0.4. The highest thermal performance factors of the plate fin heat sink equipped with twisted tape (PFHSTT) and the plate fin heat sink equipped with perforated twisted tape (PFHSPTT) are 1.28 and 1.33, respectively. The correlations of the Nusselt number and friction factor related to twist ratio and d/W ratio are generated and proposed for designing and selecting in the future.

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  • Dhanesh CHANDRASEKARAN, Venkatesan JAYARAMAN, Suseel Jai Krishnan SASI ...
    2021 Volume 16 Issue 2 Pages JTST0025
    Published: 2021
    Released: March 12, 2021
    JOURNALS FREE ACCESS

    This research article intends to discuss on the role and effects of dispersing solution combustion derived magnesia nanoflakes (~17 nm) within the biodiesel-diesel blends and pure diesel termed as nanofuels, in order to investigate the functional and pollutant emissions of a single-cylinder, electrically loaded, water-cooled diesel engine. The fuels focussed in this study are a blend of palm oil biodiesel and regular diesel dispersed with 50 ppm magnesia nanoflakes, and a pure diesel dispersed with 50 ppm magnesia nanoflakes. These fuels are compared with regular diesel which is considered as the base reference fuel, as well as with the biodiesel-diesel blend. From the experimental measurements, we inferred that the fuel density, viscous nature, and calorific value enhanced with the addition of nanoflakes. As for the engine performance attributes, the brake specific fuel consumption (BSFC) is lessened by 3.08% and 2.88% for particle dispersed biodiesel-diesel blend and particle dispersed diesel, respectively, whereas the brake thermal efficiency (BTE) enhances by 5.04% for particle dispersed biodiesel-diesel blend and 2.74% for particle dispersed diesel. With reference to emission, the unburnt hydrocarbon (UHC), white damp (CO), particulate exhaust or smoke, and the nitrogen oxides (NOx) are reduced by 9.51%, 18.71%, 13.64%, and 5.63%, respectively for particle dispersed biodiesel-diesel blend and 10.35%, 16.54%, 13.64%, 19.47%, and 4.70%, respectively for particle dispersed diesel.

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