Journal of Thermal Science and Technology
Online ISSN : 1880-5566
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Papers
  • Daniel KRISTANTO, Thananchai LEEPHAKPREEDA
    2018 Volume 13 Issue 2 Pages JTST0020
    Published: 2018
    Released: July 09, 2018
    JOURNALS FREE ACCESS

    Dynamic prediction of air conditions within a car cabin is significant to technology design and usage for thermal comfort, air quality, and energy consumption of vehicles. In this paper, theoretical models are completely derived from mass and energy balances of moist air and carbon dioxide from an outdoor environment via vehicle envelope to car cabin. However, it is demanding to determine genuine modeling hypothesis and/or physical properties of car cabins for accurate prediction. Regardless of those requirements, artificial neural networks can be applied as universal models, which are derived from numbers of input/output data from real dynamic systems. Without any experimental efforts, the input/output data is generated from the theoretical models under various conditions. A few input/output data from experiments are combined for making prediction close to actual behaviors. With bilateral analyses, the artificial neural networks are trained effectively to simulate dynamic behaviors of air, moisture, and carbon dioxide within a car cabin. This viability of the proposed methodology is confirmed by that the averaged coefficients of determination to the perfect prediction under parking and driving conditions of a sedan car are R2=0.9394, and R2=0.9314, respectively. The numbers of experimental data for training are 3.5% of total numbers of training data.

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Short Paper
  • Satoshi KADOWAKI, Kota KOHATSU, Kazuki UEHARA, Toshiyuki KATSUMI, Thwe ...
    2018 Volume 13 Issue 2 Pages JTST0021
    Published: 2018
    Released: July 24, 2018
    JOURNALS FREE ACCESS

    Oxygen combustion of CH4-CO2 mixture in a small-scale counterflow burner was studied. A diffusion flame was stabilized in the gap between opposed ports ejecting fuel and oxygen gases, and its thickness which depended on the gap distance between burner ports, inner diameter of burner tubes, flow rate of gases and fuel gas component was measured. The gas flow rates were varied such that the apparent equivalence ratio became constant at unity, while the gap distance was varied in the range from 1 mm to less. It was shown that the flame thickness decreased monotonically as the gap distance decreased and that the diffusion flame became thinner when the methane concentration in fuel gas became lower. Increased flame thickness was observed at large gas flow rate, and a diffusion flame was found in smaller gap distance at larger inner diameter. From these data, the relation between the flame thickness and the flame stretch rate was summarized, showing that the flame thickness decreased as the flame stretch became stronger, i.e. the thickness varied inversely with the square root of stretch rate. To elucidate the dependence of the flame thickness on the flame stretch rate, the flame thickness normalized by the average velocity of oxygen gas and the inner diameter was introduced. It was confirmed that the normalized flame thickness depended only on the flame stretch rate.

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Papers
  • Maroua BEN NASR, Mohamed Mehdi BASSEM, Yuki UEDA, Amenallah GUIZANI
    2018 Volume 13 Issue 2 Pages JTST0022
    Published: 2018
    Released: July 24, 2018
    JOURNALS FREE ACCESS

    A thermoacoustic heat pump heater is a device having the ability to produce heat from acoustic power. The heat pump investigated was composed of an acoustic driver, a branched tube, and a looped tube. A porous component composed of many stacked steel screen meshes, called regenerator, was installed inside the looped tube. When an acoustic wave is supplied to the looped tube, filled with nitrogen gas at 0.5 MPa, heat pumping spontaneously occurs inside the regenerator. A numerical code was developed and tested. Then, the coefficient of performance was calculated for different temperatures by optimizing the regenerator flow channel radius and position inside the looped tube. It was found that the temperature affects considerably the regenerator optimum parameters.

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  • Hiroya MAMORI, Masanari FUJIMURA, Shotaro UDAGAWA, Kaoru IWAMOTO, Akir ...
    2018 Volume 13 Issue 2 Pages JTST0023
    Published: 2018
    Released: July 26, 2018
    JOURNALS FREE ACCESS

    Direct numerical simulations of thermal turbulent boundary layer flows over a wavy wall surface are performed to investigate the effect of a wavelength on drag coefficients and heat transfer performance. The Reynolds number based on an inlet boundary layer thickness is set to be 2820 and the Prandtl number is set to be Pr = 0.71 and 2.0. The wavy wall surface is homogeneous in the spanwise direction and the wave amplitude is fixed at 2a+ = 20. The six wavelength cases of λ/2a = 7.5 ~ 45 are examined. As the wavelength decreases, the skin-friction drag decreases and the pressure drag and heat transfer increase. The total drag peaks at λ/2a = 12.5 and the flow separation occurs at λ/2a < 15. In the separation region, the backward flow transfers the heat and results in a negative correlation coefficient between the velocity and temperature of R (utT ) at the bottom of the wavy wall. Spindle-shaped spots of the Nusselt number are also observed on the upslope of the wavy wall.

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  • Hirotaka HATA, Shumpei WADA, Tatsuya YAMADA, Koichi HIRATA, Takashi YA ...
    2018 Volume 13 Issue 2 Pages JTST0024
    Published: 2018
    Released: July 27, 2018
    JOURNALS FREE ACCESS

    Electrical vehicles equipped with lithium-ion batteries (LiBs) have been increasing in popularity on the market. LiBs have high energy density and high electric current; however, their lifetimes and performance are known to be strongly influenced by temperature rise due to heat generation, and thermal runaway may occur when the battery temperature exceeds 80°C. Hence, the development of LiB thermal-management technology is essential. In this study, an A4-sized LiB was short circuited in a prototype cooling system with phase-change material (PCM) and heat pipes (HPs), and the performance of the cooling system was evaluated. To compare the cooling performances, four experimental conditions were adopted: a combination of PCM and HP; PCM only; HP only; and not using the cooling system. In addition, a simulation was conducted under the experimental conditions using a scale model of the cooling system. Thus, we confirmed that the temperature increase of the LiB, especially up to 80°C, was extended by the effects of PCM. The combination of PCM and HP suppressed the temperature of LiB to be about 80°C.

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  • Yutaka TABE, Satoshi SAKAIDA, Takemi CHIKAHISA
    2018 Volume 13 Issue 2 Pages JTST0025
    Published: 2018
    Released: July 31, 2018
    JOURNALS FREE ACCESS

    In polymer electrolyte fuel cells (PEFCs), a gas diffusion layer (GDL) is a critical component to prevent flooding and to improve the cell efficiency under high current density operation. To advance the experimental method for evaluation of liquid water transport in GDL, this study proposes a method, termed scale model experiments. In this method, enlarged GDL structures are reproduced by a 3D printer, and simulated water behavior is observed with similarity conditions satisfying the flow in the GDL. The lattice Boltzmann simulation is applied to the enlarged model experiments and identifies dominant forces in the water dynamics. First, simulations are conducted to compare flow patterns at two different scales of GDL, the actual scale and 313 times enlarged structures, with combinations of two immiscible fluids. The results suggest that fluid behavior can be considered similar at the different scales, when the Capillary number is low enough for the flow to be dominated by capillary forces, at Ca = 3.0×10-3, with the additional condition of negligible buoyancy. Next, experiments with two types of 313 times enlarged GDL structures are conducted with silicone oil and water of similar densities, and the flows are compared to the simulation results. These suggest that the water transport in the GDL can be successfully reproduced by the enlarged model experiments. As similarity conditions, the Weber number must be kept below the order of 10-1 to suppress inertial forces as well as the Capillary number must be below the order of 10-3 for smaller viscous forces. Careful attention must be paid to the viscous forces of the two fluids in a relatively-uniform structure GDL. Finally, an experiment of water transport in the GDL with a channel is demonstrated, showing the effectiveness of the proposed experimental approach for designing GDL structures tolerant to flooding.

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  • Weilin DONG, Suofang WANG, Pengfei LI
    2018 Volume 13 Issue 2 Pages JTST0026
    Published: 2018
    Released: July 31, 2018
    JOURNALS FREE ACCESS

    In order to meet the needs of designing for high performance secondary air system in the turbine, a new turbine bite engineering rim seal of type-A and the related test rig were designed and set up. The numerical investigation and the experimental verification of the leakage characteristic of rim seal were carried out based on k - ω SST turbulence model. The key feature of the engineering rim seal of type-A, a bite type, was extracted, hence the other three kinds of rim seal named type-B, type-C and type-D were proposed. The sealing mechanism of bite rim seal was revealed from two aspects of the leakage characteristic and the seal effectiveness. The results show that: the numerical results of the k-ω SST turbulence model agree well with the experimental ones, and the maximum error is less than 4.2% in the range of this paper. The leakage of turbine bite rim seal of type-C and type-D are less than that of conventional rim seal (type-B). Compared with type-C, the radial inward shoulder structure of type-D reduces the leakage in the cavity, however, it enhances the pumping effect of the rotor. The average sealing efficiency of type-C and type-D are increased by 8.9% and 8.82% compared with that of conventional rim seal of type-B, respectively. The effects of entrainment and diffusion of the vortex in the sealing channel are one of the important reasons for the ingestion. There are two kinds of vortex structure named recirculation zone vortex (RZV) and rotationally induced vortex (RIV). The RZV will be rapidly replaced by the RIV with the rotation speed increasing at a low flow rate of the hot gas. However, the RZV will be relatively stable while the hot gas mass flow rate is increased.

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