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J. Thermal Sci. Eng. Appl. 2018;11(2):021001-021001-10. doi:10.1115/1.4041492.

The scope for the heat transfer enhancement in the tubular heat exchanger is high due to its unique property of having two separate convective heat transfer coefficients. The variation of diameter and annular space has a direct effect on the value of convective heat transfer coefficients due to their inverse relation. Thus, the strong emphasis must be given on the influence of diameter and annular space on the thermal characteristics of the tubular heat exchanger. In this numerical analysis, peculiarities in the improvement of the performance parameters are studied with the variation in the value of inlet velocities of the fluids (cold and hot), inner pipe diameter, and annular space for the combination of dimensional range such as miniscale and microscale range. The inner tube diameter is observed to be sensitive to the improvement in the performance parameter. The growth in the performance parameter of the tubular micro heat exchanger is found to be higher when both the values of diameter and annular space are in the microscale range.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021002-021002-9. doi:10.1115/1.4041491.

A novel stainless steel fiber sintered felt (SSFSF) with honeycombed channels (SSFSFHC) is a promising support for catalytic combustion of the volatile organic compounds (VOCs). The SSFSFHC consists of stainless steel fiber, three-dimensionally reticulated porous structures, and interconnected honeycombed channels. The equivalent thermal conductivity (ETC) of the SSFSFHC is tested. It is found that the ETC of the SSFSFHC increases with the hot side temperature increasing but decreases with the porosity increasing and channel occupied area ratio increasing. The ETC of the SSFSFHC changes little with channel diameter increasing. The heat transfer model of the SSFSFHC is considered as parallel/series combinations of relevant thermal resistances. In order to estimate the ETC of the SSFSFHC, the correlation of the ETC of the SSFSF is derived. The expressions of the axial temperature under different porosities are deduced when eliminating the radial heat transfer between the channel section and the SSFSF section. The relationships of the transferred heats and the corresponding resistances along the radial direction are obtained by assuming that the radial heat transfer can be simplified as a serial of heat resistances located between the channels and the SSFSF.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021003-021003-13. doi:10.1115/1.4041597.

Gear drives are widely used in mechanical driving devices, and the heating problem of gear has been paid much attention. The tooth surface temperature field of spur/helical gear is compared and thermal characteristic of spur/helical gear is studied in this paper. The calculation formula of frictional heat flux and convective heat transfer coefficient, which considers different surfaces of gear tooth, is derived. The frictional heat flux of the helical gear is different from that of the spur gear, and the calculation method is different. The finite element parametric model for thermal analysis is built and it realizes the automatic parametric modeling, loading, and generation of temperature field by ANSYS parametric design language (APDL) program. The influence of different parameters on gear temperature rise is analyzed and the distribution of the three-dimensional (3D) temperature field of spur/helical is obtained. The simulation analysis and experiment are compared to validate the accuracy of thermal analysis results. The research result reveals the distribution law of the 3D temperature field of spur/helical gear transmission at different working parameters. It provides theoretical guidance for gear antiscuffing capability and gear optimization design.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021004-021004-11. doi:10.1115/1.4041348.

The vortex tube is a mechanical device with no moving parts that can separate a compressed gas into a hot and a cold stream. Pressurized gas is injected tangentially into a swirl chamber and accelerated to a high rate of rotation. This gas motion creates a cold core and a hot shell. In certain engineering applications such as gas drilling, the use of a high flow-rate air with high pressure and low temperature can improve process efficiency. In these applications, demand for the cold air stream as high as 40 kg/s is not uncommon. In this paper, the use of a vortex tube bundle for generating this large flow-rate of the cold air stream is proposed and evaluated, using numerical simulations. A single commercially available vortex tube can only produce a cold air stream up to 0.008 kg/s. Thus, it will take 5000 such vortex tubes to reach the required flow rate of 40 kg/s. Space limitation, as well as assembly difficulty, makes such an approach unrealistic. The objective of this work is to design a custom-made vortex tube so that a minimum number of such tubes can be used to meet the performance requirement posted by these applications. In this study, computational fluid dynamics (CFD) is used to analyze the flow field, temperature field, and pressure field, and to optimize the vortex tube parameters so that a specific set of desired output can be achieved to meet the application requirements.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021005-021005-16. doi:10.1115/1.4041493.

In this paper, the problem of air cooling and temperature nonuniformity at the cell and pack level is addressed. Passive techniques are developed by integrating jet inlets and vortex generators (VGs) in a simple battery pack with the goal to achieve an effective cooling, and the desired temperature uniformity at the cell and pack level to less than 5 °C, without an increase in the required mass flow and power requirements. Moreover, various configurations of the developed techniques are assessed and compared. In order to achieve the objectives, computational fluid dynamics (CFD) is used to conduct numerical studies on the battery packs. The results concluded that by adding both the delta winglet (DW) vortex generator arrays and jet inlet arrays in the same configuration, improvements in temperature reduction and uniformity can be achieved. The results showed that the maximum temperature of the battery pack was reduced by ∼6% and the temperature uniformity at the pack level was increased by 24%. Additionally, a ∼37% improvement in the temperature uniformity at cell level was achieved.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021006-021006-11. doi:10.1115/1.4041439.

This paper presents the experimental and theoretical analysis of a micro heat exchanger designed for the waste heat recovery from a high concentration photovoltaic (HCPV) system. A test bench was built to analyze the thermal behavior of a heat exchanger targeted to work in a similar condition of an existing HCPV panel. A high power heater was encapsulated inside a copper cartridge, covered by thermal insulation, leading to dissipated heat fluxes around 0.6 MW/m2, representative of the heat flux over the solar cell within the HCPV module. The experimental campaign employed water as the coolant fluid and was performed for three different mass flow rates. An infrared camera was used to nonintrusively measure the temperature field over the micro heat exchanger external surface, while thermocouples were placed at the contact between the heat exchanger and the heater, and at the water inlet and outlet ports. In the theoretical analysis, a hybrid numerical–analytical treatment is implemented, combining the numerical simulation through the comsolmultiphysics finite elements code for the micro heat exchanger, and the analytical solution of a lumped-differential formulation for the electrical heater cartridge, offering a substantial computational cost reduction. Such computational simulations of the three-dimensional conjugated heat transfer problem were critically compared to the experimental results and also permitted to inspect the adequacy of a theoretical correlation based on a simplified prescribed heat flux model without conjugation effects. It has been concluded that the conjugated heat transfer problem modeling should be adopted in future design and optimization tasks. The analysis demonstrates the enhanced heat transfer achieved by the microthermal system and confirms the potential in reusing the recovered heat from HCPV systems in a secondary process.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021007-021007-7. doi:10.1115/1.4041441.

Nanofluids are suspensions of nanosized particles in any base fluid that show significant enhancement of their heat transfer properties at modest nanoparticle concentrations. Due to enhanced thermal properties at low nanoparticle concentration, it is a potential candidate for utilization in nuclear heat transfer applications. In the last decade, there have been few studies which indicate possible advantages of using nanofluids as a coolant in nuclear reactors during normal as well as accidental conditions. In continuation with these studies, the utilization of nanofluids as a viable candidate for emergency core cooling in nuclear reactors is explored in this paper by carrying out experiments in a scaled facility. The experiments carried out mainly focus on quenching behavior of a simulated nuclear fuel rod bundle by using 1% Alumina nanofluid as a coolant in emergency core cooling system (ECCS). In addition, its performance is compared with water. In the experiments, nuclear decay heat (from 1.5% to 2.6% reactor full power) is simulated through electrical heating. The present experiments show that, from heat transfer point of view, alumina nanofluids have a definite advantage over water as coolant for ECCS. Additionally, to assess the suitability of using nanofluids in reactors, their stability was investigated in radiation field. Our tests showed good stability even after very high dose of radiation, indicating the feasibility of their possible use in nuclear reactor heat transfer systems.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021008-021008-10. doi:10.1115/1.4041596.

Fabric drying is an energy-intensive process, which generally involves blowing hot dry air across tumbling wet fabric to facilitate evaporation and moisture removal. Most of the energy supplied is used to overcome the enthalpy of vaporization for water. Although this process tends to be inefficient, it is fairly simple and forms the basis for the majority of existing clothes dryer technology today. To address the relatively low efficiency, a new method of drying called “direct contact ultrasonic fabric drying” is proposed. The process involves using high-frequency vibration introduced by piezoelectric transducers, which are in contact with wet fabric. The vibration is used to extract water droplets from the fabric mechanically. In this study, a total of 24 individual transducers are used in a module to dry a 142 cm2 sized fabric. The performance characterization of this single module has enabled successful scale-up of the system to a midscale prototype dryer, which can be used to ultrasonically dry clothing-sized fabric (∼750 cm2). The first-generation ultrasonic fabric dryer fabricated uses as little as 17% of the energy needed by traditional evaporation-based drying techniques. In addition to experimental data, this paper presents the results of a kinetic and scaling analysis that provides some important insights into ultrasonic drying.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021009-021009-9. doi:10.1115/1.4041595.

The Li-ion battery operation life is strongly dependent on the operating temperature and the temperature variation that occurs within each individual cell. Liquid-cooling is very effective in removing substantial amounts of heat with relatively low flow rates. On the other hand, air-cooling is simpler, lighter, and easier to maintain. However, for achieving similar cooling performance, a much higher volumetric air flow rate is required due to its lower heat capacity. This paper describes the fundamental differences between air-cooling and liquid-cooling applications in terms of basic flow and heat transfer parameters for Li-ion battery packs in terms of QITD (inlet temperature difference). For air-cooling concepts with high QITD, one must focus on heat transfer devices with relatively high heat transfer coefficients (100–150 W/m2/K) at air flow rates of 300–400 m3/h, low flow induced noise, and low-pressure drops. This can be achieved by using turbulators, such as delta winglets. The results show that the design concepts based on delta winglets can achieve QITD of greater than 150 W/K.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Thermal Sci. Eng. Appl. 2018;11(2):024501-024501-10. doi:10.1115/1.4041598.

In this study, numerical simulations are conducted to investigate the effects of pin fin and dimple shape on the flow structure and heat transfer characteristics in a rectangular channel. The studied shapes for dimple and pin fin are circular, spanwise-elliptical, and streamwise-elliptical, respectively. The flow structure, friction factor, and heat transfer performance are obtained and analyzed with Reynolds number ranging from 10,000 to 50,000. Channel with circular pin fin and dimple is chosen as the Baseline. Channels with spanwise-elliptical pin fins have the best heat transfer augmentation, while also accompanied with the largest friction factor. Spanwise-elliptical pin fin generates the strongest horseshoe vortex which is responsible for the best heat transfer augmentation. Besides, channels with streamwise-elliptical pin fins show the worst heat transfer augmentation and the smallest friction factors. Dimple plays an important role in improving the heat transfer. Spanwise-elliptical dimple yields the best heat transfer augmentation which is attributed to the strongest counter-rotating vortex, while streamwise-elliptical dimple shows the worst heat transfer enhancement.

Commentary by Dr. Valentin Fuster

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