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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
J. Thermal Sci. Eng. Appl. 2018;11(2):021010-021010-14. doi:10.1115/1.4041804.

Data centers consume an extraordinary amount of electricity, and the rate of consumption is increasing at a rapid pace. Thus, energy efficiency in data center design is of substantial interest since it can have a significant impact on operating costs. The server cooling infrastructure is one area which is ripe for design innovation. Various designs have been considered for air-cooled data centers, and there is growing interest in liquid-cooled server designs. One potential liquid-cooled solution, which reduces the cost of cooling to less than 5% of the information technology (IT) energy use, is a chiller-less or warm water-cooled system, which removes the chiller from the design and lets the cooling water supply vary with changes in the outdoor ambient conditions. While this design has been proven to work effectively in some locations, environmental extremes prevent its more widespread implementation. In this paper, the design and analysis of a cold water storage system are shown to extend the applicability of chiller-less designs to a wider variety of environmental conditions. This can lead to both energy and economic savings for a wide variety of data center installations. A numerical model of a water storage system is developed, validated, and used to analyze the impact of a water storage tank system in a chiller-less data center design featuring outdoor wet cooling. The results show that during times of high wet bulb operating conditions, a water storage tank can be an effective method to significantly reduce chip operating temperatures for warm water-cooled systems by reducing operating temperatures 5–7 °C during the hottest part of the day. The overall system performance was evaluated using both an exergy analysis and a modified power usage effectiveness (PUE) metric defined for the water storage system. This unique situation also necessitates the development of a new exergy definition in order to properly capture the physics of the situation. The impacts of tank size, tank aspect ratio, fill percentage, and charging/discharging time on both the chip temperature and modified PUE are evaluated. It is determined that tank charging time must be carefully matched to environmental conditions in order to optimize impact. Interestingly, the water being stored is initially above ambient, but the overall system performance improves with lower water temperatures. Therefore, heat losses to ambient are found to beneficial to the overall system performance. The results of this analysis demonstrate that in application, data center operators will see a clear performance benefit if water storage systems are used in conjunction with warm water cooling. This application can be extended to data center failure scenarios and could also lead to downsizing of equipment and a clear economic benefit.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021011-021011-12. doi:10.1115/1.4041440.

Design optimization of a three-dimensional (3D) heat flow structure for power electronics gate drive circuit thermal management is described. Optimization methods are described in the creation of several structural concepts targeted toward simultaneous temperature reduction of multiple gate drive integrated circuit (IC) devices. Each heat flow path concept is intended for seamless integration based on power electronics packaging space constraints, while maintaining required electrical isolation. The design synthesis and fabrication of a select concept prototype is presented along with the development of an experimental test bench for thermal performance characterization. Experimental results indicate a significant 45 ∘C maximum temperature reduction for the gate drive IC devices in a laboratory environment, which translates to an estimated 41 °C maximum temperature reduction under high temperature (∼100 °C) ambient conditions. The technical approach and design strategy are applicable to future wide band-gap (WBG) electronics packaging applications, where enhanced 3D thermal routing is expected to be critical to maximizing volumetric power density.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021012-021012-10. doi:10.1115/1.4041635.

Effects of air/water mist flow on endwall heat transfer in a square channel were experimentally investigated using infrared thermography. The purpose was to study the detailed heat transfer contour variation caused by the generation of the liquid films. The surface was roughened with staggered partial pin-fin arrays to enhance flow mixing and liquid entrainment. Two streamwise spacings (Xp/d = 3 and 6) of the fin array were investigated. The gas Reynolds number ranged from 7900 to 24,000. The calculated droplet deposition velocity was comparable to the literature results and was not substantially affected by the gas Reynolds number or fin spacing. For the pin-fin array, heat transfer was dominated by the water accumulation and liquid film formation, which was dependent on the carrier gas flow rate and fin spacing. Furthermore, thick liquid fragments entrained between the fins substantially enhanced local convective heat transfer coefficients. The average heat transfer enhancement on the finned surfaces using mist flow was four times as high as the air flow. The pressure drop from the mist flow was two times as high as the air flow.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021013-021013-7. doi:10.1115/1.4041873.

In this article, magnetohydrodynamic (MHD) mixed convection in an exponentially stretchable surface saturated with viscous fluid has been studied. BVPh 2.0 is employed which is mathematica-based algorithm created on the basis of optimal homotopy analysis method (OHAM). Adequate transformations are utilized for the conversion of governing system into nonlinear ordinary differential system. Convergence of BVPh 2.0 results is demonstrated through tabular values of squared residual errors. Graphical analysis is executed for broad range of governing parameters. It has been revealed an increase in buoyancy leads to the growth of boundary layer width. Further results predict the heat infiltration into the fluid increases as Brownian motion and Biot number enlarges. Mathematically this work exhibits the potential of BVPh 2.0 for nonlinear differential systems.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021014-021014-19. doi:10.1115/1.4041937.

Three-dimensional Reynolds-averaged Navier–Stokes equations with shear stress transport turbulence model are used to analyze the film cooling effectiveness on a flat plate having single row of film hole involving cylindrical hole (CH) and laidback hole (LBH). The CH and LBH are inclined at 35 deg to the surface with a compound angle (β) orientation ranging from favorable to adverse inclination (i.e., β = 0–180 deg) and examined at high and low blowing ratios (M = 1.25 and 0.60). CH with an adverse compound angle of 135 deg gives the highest area-averaged film cooling effectiveness in comparison with LBH configuration. Also, CH β = 135 deg film hole shows a higher lateral coolant spread. Later, double jet film cooling (DJFC) concept is studied for this CH. In all the cases, the first hole compound angle is fixed as 135 deg, and the second hole angle is varied from 135 deg to 315 deg. At high blowing ratio, the dual jet cylindrical hole (DJCH) with β = 135 deg, 315 deg gives a higher area-averaged film cooling effectiveness by around 66.50% compared to baseline CH β = 0 deg. On comparing all CH, LBH, and DJCH cases, the highest area-averaged film cooling effectiveness is obtained by CH configuration with β = 135 deg. Hence, the CH with its adverse compound angle (β = 135 deg) orientation could be an appropriate film cooling configuration for gas turbine blade cooling.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021015-021015-9. doi:10.1115/1.4041793.

An entropy analysis and design optimization methodology is combined with airfoil shape optimization to demonstrate the impact of entropy generation on aerodynamics designs. In the work herein, the entropy generation rate is presented as an extra design objective along with lift-drag ratio, while the lift coefficient is the constraint. Model equation, which calculates the local entropy generation rate in turbulent flows, is derived by extending the Reynolds-averaging of entropy balance equation. The class-shape function transform (CST) parametric method is used to model the airfoil configuration and combine the radial basis functions (RBFs) based mesh deformation technique with flow solver to compute the quantities such as lift-drag ratio and entropy generation at the design condition. From the multi-objective solutions which represent the best trade-offs between the design objectives, one can select a set of airfoil shapes with a low relative energy cost and with improved aerodynamic performance. It can be concluded that the methodology of entropy generation analysis is an effective tool in the aerodynamic optimization design of airfoil shape with the capability of determining the amount of energy cost.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):021016-021016-7. doi:10.1115/1.4041684.

As an alternative treatment to hydrothermal carbonization, sewage sludge (SS) was subjected to dry carbonization at temperatures of 200–700 °C to produce pyrochar. The fuel properties of the obtained chars were characterized, and their combustibility was checked by thermal analysis method. The combustibility of the chars was evaluated considering the criteria such as the ignition index (Ci), burnout index (Cb), comprehensive combustibility index (S), and the burning stability index (DW). Although even low temperature treatments such as 200 °C and 300 °C did not improve the calorific value, some improvements took place in the combustion characteristics upon treatment.

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
J. Thermal Sci. Eng. Appl. 2018;11(2):024502-024502-6. doi:10.1115/1.4041594.

Determination of core body temperature (Tc), a measure of metabolic rate, in firefighters is needed to avoid heat-stress related injury in real time. The measurement of Tc is neither routine nor trivial. This research is significant as thermal model to determine Tc is still fraught with uncertainties and reliable experimental data for validation are rare. The objective of this study is to develop a human thermoregulatory model that uses the heart rate measurements to obtain Tc for firefighters using a 3D whole body model. The hypothesis is that the heart rate-derived computed Tc correlates with the measured Tc during firefighting activities. The transient thermal response of the human body was calculated by simultaneously solving the Pennes' bioheat and energy balance equations. The difference between experimental and numerical values of Tc was less than 2.6%. More importantly, a ± 10% alteration in heart rate was observed to have appreciable influence on Tc, resulting in a ± 1.2 °C change. A 10% increase in the heart rate causes a significant relative % increase (52%) in Tc, considering its allowable/safe limit of 39.5 °C. Routine acquisition of the heart rate data during firefighting scenario can be used to derive Tc of firefighters in real time using the proposed 3D whole body model.

Topics: Temperature , Heat , Stress
Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):024503-024503-4. doi:10.1115/1.4041636.

In this work, consideration is given to capillary convection on ferrofluids from the concentration gradient induced when a nonhomogeneous magnetic field is applied. It is known that mass transfer along an interface between two fluids can appear due to a gradient of the surface tension in the so-called Marangoni effect (or Gibbs–Marangoni effect). Because the surface tension is both thermal and concentration dependent, Marangoni convection can be induced by either a thermal or a concentration gradient, where in the former case, it is generally referred as thermocapillary convection. Now, it has been theoretically and experimentally demonstrated that a ferrofluid under the action of a non-homogeneous magnetic field can induce a concentration gradient of suspended magnetic nanoparticles, and also the effect of Fe3O4 nanoparticles on the surface tension has been measured. Therefore, by deductive reasoning and taking into account the above mentioned facts, it is permissible to infer ferrohydrodynamic capillary convection on magnetic fluids under the presence of a magnetic gradient field. Utilizing a simplified physical model, the phenomenon was investigated and it was found that ferrohydrodynamic-Marangoni convection could be induced with particle size in the range up to 10 nm, which is the range of magnetic fluids to escape magnetic agglomeration.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(2):024504-024504-7. doi:10.1115/1.4041682.

This study deals with the heat transfer characteristics of magnetohydrodynamic (MHD) flow of a third-grade fluid through parallel plates, subjected to a uniform wall heat flux, but of different magnitudes. The effect of viscous dissipation has been included for both heating and cooling of the fluid. The least square method (LSM) has been adopted for solving the nonlinear equations. The expressions for the velocity and temperature fields have been derived which, in turn, is utilized to evaluate the Nusselt number. The results indicate an increase in Nusselt number for higher values of the third-grade fluid parameter during heating and indicate a reverse trend for cooling. Nusselt number increases with an increase in Hartmann number during heating, whereas it decreases with increasing values of the Hartmann number while cooling the fluid.

Commentary by Dr. Valentin Fuster

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