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IN THIS ISSUE

### Research Papers

J. Thermal Sci. Eng. Appl. 2016;9(2):021001-021001-10. doi:10.1115/1.4034849.

Combined with the use of renewable energy sources for its production, hydrogen represents a possible alternative gas turbine fuel for future low-emission power generation. Due to the difference in the physical properties of hydrogen compared to other fuels such as natural gas, well-established gas turbine combustion systems cannot be directly applied to dry low NOx (DLN) hydrogen combustion. The DLN micromix combustion of hydrogen has been under development for many years, since it has the promise to significantly reduce NOx emissions. This combustion principle for air-breathing engines is based on crossflow mixing of air and gaseous hydrogen. Air and hydrogen react in multiple miniaturized diffusion-type flames with an inherent safety against flashback and with low NOx emissions due to a very short residence time of the reactants in the flame region. The paper presents an advanced DLN micromix hydrogen application. The experimental and numerical study shows a combustor configuration with a significantly reduced number of enlarged fuel injectors with high-thermal power output at constant energy density. Larger fuel injectors reduce manufacturing costs, are more robust and less sensitive to fuel contamination and blockage in industrial environments. The experimental and numerical results confirm the successful application of high-energy injectors, while the DLN micromix characteristics of the design point, under part-load conditions, and under off-design operation are maintained. Atmospheric test rig data on NOx emissions, optical flame-structure, and combustor material temperatures are compared to numerical simulations and show good agreement. The impact of the applied scaling and design laws on the miniaturized micromix flamelets is particularly investigated numerically for the resulting flow field, the flame-structure, and NOx formation.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2016;9(2):021002-021002-8. doi:10.1115/1.4034915.

Use of oscillatory flow and phase-change material (PCM) microcapsules to enhance heat transport efficiency in micro/minichannels is among many new concepts and methodologies that have been proposed. In this paper, we propose a novel and simple heat spreader design concept that integrates the technologies of oscillating flow streaming and PCM microcapsules. Phenomenon of the flow streaming can be found in oscillating, zero-mean-velocity flows in many channel configurations. The pumpless bidirectional streaming flow can be generated by heating instability oscillation or by displacement of a lead zirconate titanate diaphragm. Discrepancy in velocity profiles between the forward and backward flows causes fluid and PCM microcapsules, suspended in the fluid near the walls, to drift toward one end while particles near the centerline move toward the other end. Flow streaming is a common mechanism in many biological systems but an innovative feature for heat transfer devices. We conducted preliminary work on scale analysis and computer simulations of suspended PCM microcapsules streaming in mini/microbifurcation networks. Computer simulated microcapsules distribution patterns are verified by visualization experiments reported in the literature. This work demonstrates that flow streaming with PCM microcapsule entrainment has the potential to be used as a cost-effective technology for a heat spreader design.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2016;9(2):021003-021003-7. doi:10.1115/1.4034963.

Mine Safety and Health Administration (MSHA) regulations require underground coal mines to install refuge alternatives (RAs). In the event of a disaster, RAs must be able to provide a breathable air environment for 96 h. The interior environment of an occupied RA, however, may become hot and humid during the 96 h due to miners' metabolic heat and carbon dioxide scrubbing system heat. The internal heat and humidity may result in miners suffering heat stress or even death. To investigate heat and humidity buildup with an occupied RA, the National Institute for Occupational Safety and Health (NIOSH) conducted testing on a training ten-person, tent-type RA in its Safety Research Coal Mine (SRCM) in a test area that was isolated from the mine ventilation system. The test results showed that the average measured air temperature within the RA increased by 11.4 °C (20.5 °F) and the relative humidity approached 90% RH. The test results were used to benchmark a thermal simulation model of the tested RA. The validated thermal simulation model predicted the average air temperature inside the RA at the end of 96 h to within 0.6 °C (1.1 °F) of the measured average air temperature.

Topics: Heat , Temperature , Sensors
Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2016;9(2):021004-021004-7. doi:10.1115/1.4034903.

This article reports on the experimental investigation of heat transfer to cocurrent air–water two-phase flow in a horizontal tube. The idea is to enhance heat transfer to the coolant liquid by air injection. Experiments were conducted for different air water ratios in constant temperature heated tube. Visual identification of flow regimes was supplemented. The effects of the liquid and gas superficial velocities and the flow regimes on the heat transfer coefficients were investigated. The results showed that the heat transfer coefficient generally increases with the increase of the injected air flow rate, and the enhancement is more significant at low water flow rates. A maximum value of the two-phase heat transfer coefficient was observed at the transition to wavy-annular flow as the air superficial Reynolds number increases for a fixed water flow rate. It was noticed that the Nusselt number increased about three times due to the injection of air at low water Reynolds number. Correlations for heat transfer by air–water two-phase flow were deduced in dimensionless form for different flow regimes.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021005-021005-9. doi:10.1115/1.4035291.

This paper presents the results of an experimental study of ionic wind heat transfer enhancement in internal rectangular channels. Ionic wind is a potential technique to enhance natural convection cooling noise-free and without using moving part and thus ensuring a high reliability and a long lifetime. The goal of the present study is twofold: first, the multiphysics numerical model of ionic wind developed in previous work is validated experimentally. Second, the potential of the heat sink concept combining a fin array with an ionic wind generator is demonstrated by building a technology demonstrator. The heat sink presented in this work dissipates 240 W on a baseplate geometry of 200 × 263 mm. It is shown that the baseplate temperature can be reduced from 100 °C under natural convection to 81 °C when the ionic wind generator is turned on.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021006-021006-13. doi:10.1115/1.4035290.

Film cooling performance of the antivortex (AV) hole has been well documented for a flat plate. The goal of this study is to evaluate the same over an airfoil at three different locations: leading edge suction and pressure surface and midchord suction surface. The airfoil is a scaled up first stage vane from GE E3 engine and is mounted on a low-speed linear cascade wind tunnel. Steady-state infrared (IR) technique was employed to measure the adiabatic film cooling effectiveness. The study has been divided into two parts: the initial part focuses on the performance of the antivortex tripod hole compared to the cylindrical (CY) hole on the leading edge. Effects of blowing ratio (BR) and density ratio (DR) on the performance of cooling holes are studied here. Results show that the tripod hole clearly provides higher film cooling effectiveness than the baseline cylindrical hole case with overall reduced coolant usage on the both pressure and suction sides of the airfoil. The second part of the study focuses on evaluating the performance on the midchord suction surface. While the hole designs studied in the first part were retained as baseline cases, two additional geometries were also tested. These include cylindrical and tripod holes with shaped (SH) exits. Film cooling effectiveness was found at four different blowing ratios. Results show that the tripod holes with and without shaped exits provide much higher film effectiveness than cylindrical and slightly higher effectiveness than shaped exit holes using 50% lesser cooling air while operating at the same blowing ratios. Effectiveness values up to 0.2–0.25 are seen 40-hole diameters downstream for the tripod hole configurations, thus providing cooling in the important trailing edge portion of the airfoil.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021007-021007-9. doi:10.1115/1.4035448.

The paper presents a novel study on film cooling effectiveness of a 3D flat plate with a multihole arrangement of mixed hole shapes. The film cooling arrangement consists of two rows of coolant holes, organized in a staggered pattern with an L/D (length to diameter ratio) of 10. The two rows consist of varied combinations of triangular and semi-elliptic shaped holes for the enhancement of film-cooling effectiveness. The results were obtained for a coolant to mainstream temperature ratio of 0.5 and a blowing ratio of 1.0. The computed flow temperature fields are presented in addition to the local two-dimensional streamwise and spanwise distribution of film cooling effectiveness. Validation of the results obtained from the turbulence model has been done with the experimental data of centerline film cooling effectiveness downstream of the cooling holes available in the open literature. The results showed the rapid merging of coolant jets emerging from front row of multiholes with the secondary staggered row of mixed holes. Due to the mainstream–coolant jet interaction, the strength of the counter rotating vortex pair was mitigated in the downstream region for certain arrangement of mixed hole shapes. The optimal hole combination with maximum overall effectiveness has been deduced from this study. The best configuration (M.R. VI) not only favored for the developed film, but also enhanced the averaged film cooling effectiveness to a large extent.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021008-021008-7. doi:10.1115/1.4035502.

Heat advection by groundwater flow is known to improve the performance of ground heat exchangers (GHEs), but the effect of groundwater advection on performance is not yet fully understood. This numerical study examined how parameters related to groundwater flow, such as aquifer thickness, porosity, lithology, and groundwater flow velocity, affected the performance of a borehole GHE. Under a thin-aquifer condition (10 m, or 10% of the entire GHE length in this study), groundwater flow velocity had the greatest effect on heat flux. At a groundwater flow velocity of at least 10−4 m/s through a low-porosity aquifer filled with granite gravel with high thermal conductivity, the heat flux of a GHE was as much as 60% higher than that of a GHE in a setting without an aquifer. If the aquifer was as thick as 50 m, the high thermal conductivity of granite gravel doubled the heat flux of the GHE at a groundwater flow velocity of at least 10−5 m/s. Thus, not only groundwater flow velocity but also aquifer thickness and thermal conductivity were important factors. However, groundwater seldom flows at such high velocities, and porosity, gravel size and composition, and aquifer thickness vary regionally. Thus, in the design of ground source heat pump systems, it is not appropriate to assume a large groundwater effect.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021009-021009-6. doi:10.1115/1.4035500.

A heat pipe utilizes liquid–vapor phase change mechanism to efficiently transfer heat. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable, and remote/long-distance heat transfer for thermal management of modern avionics systems. Aiming at this goal, a 2 m long LHP is developed to transport heat from the avionics chassis to the remote heat rejection site. To reduce inner saturation pressure and ensure structural safety at high operating temperature, water is used as the operating fluid in LHP. Within the avionics chassis, conduction heat transfer is enhanced by sandwiching a PHP with two printed circuit boards (PCBs) and solder-bonding them. Each PHP/PCB assembly is 20 cm long and 12.5 cm wide, with electrical heaters mounted on both sides to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400 W. For all the three PHP/PCB assemblies set in the avionic chassis, heat source temperature is maintained below the required 150 °C even when heat dissipation is twice as high as the state-of-the-art (and coolant temperature is 50 °C). This combo heat transfer system reduces power consumption and increases reliability, enabling the avionic system operation in harsh environments.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021010-021010-12. doi:10.1115/1.4035892.

The effect of rotation on jet impingement cooling is experimentally investigated in this study. Pressurized cooling air is supplied to a smooth, square channel in the radial outward direction. To model leading edge impingement in a gas turbine, jets are formed from a single row of discrete holes. The cooling air from the first pass is expelled through the holes, with the jets impinging on a semi-circular, concave surface. The inlet Reynolds number varied from 10,000 to 40,000 in the square supply channel. The rotation number and buoyancy parameter varied from 0 to 1.4 and 0 to 6.6 near the inlet of the channel, and as coolant is extracted for jet impingement, the rotation and buoyancy numbers can exceed 10 and 500 near the end of the passage. The average jet Reynolds number varied from 6000 to 24,000, and the jet rotation number varied from 0 to 0.13. For all test cases, the jet-to-jet spacing (s/djet = 4), the jet-to-target surface spacing (l/djet = 3.2), and the impingement surface diameter-to-diameter (D/djet = 6.4) were held constant. A steady-state technique was implemented to determine regionally averaged Nusselt numbers on the leading and trailing surfaces inside the supply channel and three spanwise locations on the concave target surface. It was observed that in all rotating test cases, the Nusselt numbers deviated from those measured in a nonrotating channel. The degree of separation between the leading and trailing surface increased with increasing rotation number. Near the inlet of the channel, heat transfer was dominated by entrance effects, however moving downstream, the local rotation number increased, and the effect of rotation was more pronounced. The effect of rotation on the target surface was most clearly seen in the absence of crossflow. With pure jet impingement, the deflection of the impinging jet combined with the rotation-induced secondary flows offered increased mixing within the impingement cavity and enhanced heat transfer. In the presence of strong crossflow of the spent air, the same level of heat transfer is measured in both the stationary and rotating channels.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021011-021011-9. doi:10.1115/1.4035836.

Accelerated development in the field of electronics and integrated circuit technology further pushed the need for better heat dissipating devices with reduced component dimensions. In the design optimization of microchannel heat transfer systems, multiple objectives must be satisfied but correlations limit the satisfaction levels. End users define their preferences associated with the desired quality/quantity of each parameter and specify the priorities among each preference. In this paper, an optimization strategy based on the prioritized performances is developed to find the optimal design variables for the preferences in three different aspects namely: minimized thermal resistances, minimized pressure drop, and maximized heat flux. The preferences are often fuzzy and correlated but can be modeled mathematically using Gaussian membership functions with respect to different levels of user preferences. The overall performances are maximized to find the most favorable solution on the Pareto frontier. Two different types of single-phase liquid cooling (straight and U-shaped microchannel heat sinks) have been utilized as heat exchangers of electronic chips and made as practical examples for the proposed optimization strategy. The optimal design points vary with respect to the priorities of the preferences. The proposed methodology finds the most favored solution on the Pareto frontiers. It is novel to reveal that the chosen significant factors were maximized with results yielding to lower thermal resistance, lower pressure drop, and higher heat flux in the microchannel heat sink based on the design preferences with different priorities.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021012-021012-9. doi:10.1115/1.4035837.

Tremendous efforts had been given to ensure proper heat dissipation in electronics cooling but very seldom consider design robustness and user preferences in design principles of the heat-dissipating devices. Multi-objective optimization problems are one of the preferences elicitation tools that could be used and is highly visual on the costs and benefits associated in choosing different preferences. It would be better if a wider temperature range is offered for thermal management schemes and is made available if the user desires. It would also be sought upon if automatic determination of the user preference for a wider range of varying performance were available. In this paper, a liquid impinging heat exchanger with a thermoelectric module was chosen as the example of how this paradigmatic scheme was implemented using black box models. An orthogonal sampling method was applied with three parameters considered. The temperature at the interface between the chip surface and the liquid impinging thermoelectric cooler (LITEC) is taken as the desired response. A response surface was generated using Kriging method, after which, a multi-objective optimization problem was then formulated to include robust definition and user preference for energy efficiency. The optimal operation parameters of the inlet flow velocity and the thermoelectric (TE) chip control voltage were found for various levels of heat loading conditions and different considerations of design robustness and energy awareness.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021013-021013-9. doi:10.1115/1.4035838.

Although an annular stepped fin can produce better cooling effect in comparison to an annular disk fin, it is yet to be studied in detail. In the present work, one-dimensional heat transfer in a two-stepped rectangular cross-sectional annular fin with constant base temperature and variable thermal conductivity is modeled as a multi-objective optimization problem. Taking cross-sectional half-thicknesses and outer radii of the two fin steps as design variables, an attempt is made to obtain the efficient fin geometry primarily by simultaneously maximizing the heat transfer rate and minimizing the fin volume. For further assessment of the fin performance, three more objective functions are studied, which are minimization of the fin surface area and maximization of the fin efficiency and effectiveness. Evaluating the heat transfer rate through the hybrid spline difference method, the well-known multi-objective genetic algorithm, namely, nondominated sorting genetic algorithm II (NSGA-II), is employed for approximating the Pareto-optimal front containing a set of tradeoff solutions in terms of different combinations of the considered five objective functions. The Pareto-optimal sensitivity is also analyzed for studying the influences of the design variables on the objective functions. As an outcome, it can be concluded that the proposed procedure would give an open choice to designers to lead to a practical stepped fin configuration.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021014-021014-9. doi:10.1115/1.4035839.

This paper proposed radiative characteristics' expressions for media containing randomly oriented fibers in space. In deriving these simple radiative characteristics' expressions, the fibrous medium effective extinction coefficient is defined to match with the one of large particle obtained by combining geometric optics and Fraunhofer diffraction theory. Fibrous media radiative characteristics are then derived as an average over all incident radiation angles of single fiber radiative characteristics. Theoretical hemispherical reflectance and normal transmittance predictions using the proposed fibrous media radiative characteristics based on the Mie theory agreed well with literature experiments. Therefore, media containing fiber randomly oriented in space can be scaled to a suitable equivalent media such that scattering mechanisms behave similarly to that occurring in a participating media containing spherical particles. Numerical investigations show that a theoretical model which assumes Henyey–Greenstein (HG) scattering phase function can conveniently be used for the estimation of equivalent fibrous media radiative characteristics using hemispherical reflectance measurements. On the other hand, the estimated equivalent fibrous media radiative characteristics from hemispherical measurements and using a two-flux model with isotropic scaling radiative characteristics may be subjected to serious errors in the case of semitransparent media for which the absorption is significant.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021015-021015-9. doi:10.1115/1.4035733.

This paper concerns with calculation of heat transfer and pressure drop in a mixed-convection nanofluid flow on a permeable inclined flat plate. Solution of governing boundary layer equations is presented for some values of injection/suction parameter (f0), surface angle (γ), Galileo number (Ga), mixed-convection parameter (λ), volume fraction (φ), and type of nanoparticles. The numerical outcomes are presented in terms of average skin friction coefficient (Cf) and Nusselt number (Nu). The results indicate that adding nanoparticles to the base fluid enhances both average friction factor and Nusselt number for a wide range of other effective parameters. We found that for a nanofluid with φ = 0.6, injection from the wall (f0 = −0.2) offers an enhancement of 30% in Cf than the base fluid, while this growth is about 35% for the same case with wall suction (f0 = 0.2). However, increasing the wall suction will linearly raise the heat transfer rate from the surface, similar for all range of nanoparticles volume fraction. The computations also showed that by changing the surface angle from horizontal state to 60 deg, the friction factor becomes 2.4 times by average for all φ's, while 25% increase yields in Nusselt number for the same case. For assisting flow, there is a favorable pressure gradient due to the buoyancy forces, which results in larger Cf and Nu than in opposing flows. We can also see that for all φ values, enhancing Ga/Re2 parameter from 0 to 0.005 makes the friction factor 4.5 times, while causes 50% increase in heat transfer coefficient. Finally, we realized that among the studied nanoparticles, the maximum influence on the friction and heat transfer belongs to copper nanoparticles.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):021016-021016-8. doi:10.1115/1.4035811.

Numerical study of jet impingement cooling of a corrugated surface with water–SiO2 nanofluid of different nanoparticle shapes was performed. The bottom wall is corrugated and kept at constant surface temperature, while the jet emerges from a rectangular slot with cold uniform temperature. The finite volume method is utilized to solve the governing equations. The effects of Reynolds number (between 100 and 500), corrugation amplitude (between 0 and 0.3), corrugation frequency (between 0 and 20), nanoparticle volume fraction (between 0 and 0.04), and nanoparticle shapes (spherical, blade, brick, and cylindrical) on the fluid flow and heat transfer characteristics were studied. Stagnation point and average Nusselt number enhance with Reynolds number and solid particle volume fraction for both flat and corrugated surface configurations. An optimal value for the corrugation amplitude and frequency was found to maximize the average heat transfer at the highest value of Reynolds number. Among various nanoparticle shapes, cylindrical ones perform the best heat transfer characteristics in terms of stagnation and average Nusselt number values. At the highest solid volume concentration of the nanoparticles, heat transfer values are higher for a corrugated surface when compared to a flat surface case.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Thermal Sci. Eng. Appl. 2017;9(2):024501-024501-8. doi:10.1115/1.4035449.

The effects of geometrical arrangement on the heat transfer and pressure drop characteristics in compact louvered fin-and-tube heat exchangers were studied experimentally and numerically along with $ε−NTU$ method. Different geometrical parameters including louver angle, louver pitch, louver number, the nonlouvered inlet and exit fin length, and redirection of fluid flow are considered to determine their effects on the flow field. The study is performed for different louver angles varying from $θL=12$ to $60$ deg, and optimal heat transfer rate is obtained at louver angle of $θL=28deg$. Also, it is found that increasing the louver number, $NL$, on the fin surface enhances the heat transfer performance. It is shown that the average Nusselt number is increased as the louver pitch is decreased and its optimum value is obtained at . However, comparing to the effect of louver number, the louver pitch has a small effect on the performance of the heat exchanger. Additionally, the optimum values of nonlouvered inlet and exit fin length and redirection length of fin are obtained with different flow conditions.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):024502-024502-12. doi:10.1115/1.4035810.

In this paper, the effect of magnetic field on natural convection of Al2O3/water nanofluid in an enclosure containing twin protruding heat sources placed on top and bottom walls arranged in-line and staggered manner is presented. For this purpose, coupled equations governing fluid flow and heat transfer are solved in Cartesian framework using streamline upwind/Petrov–Galerkin (SUPG) finite element method. Numerical computations are performed to predict the fluid flow, heat transfer, and entropy generation for a wide range of Hartmann number (0.0 $≤$ Ha $≤$ 100.0), Rayleigh number ($103≤Ra≤106$), and nanoparticle volume fraction ($0.0≤ϕ≤0.1$). The simulated results indicate that, for both in-line and staggered arrangement, the entropy generation due to heat transfer is significant along isothermal surfaces, whereas entropy generation due to fluid friction is higher at no-slip walls and along the regions of contact between adjacent recirculation cells. For both in-line and staggered arrangement, increase in global total entropy generation and average Nusselt number along top and bottom heat sources is obtained with decreasing Ha and increasing Ra. Furthermore, for both in-line and staggered arrangement, variation in global total entropy generation and average Nusselt number along top and bottom heat sources with increasing nanoparticle volume fraction, depend on both Ha and Ra.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(2):024503-024503-13. doi:10.1115/1.4035969.

This paper analyses a novel heat transfer enhancement technique that can be used in compressors to limit the temperature rise during compression. This technique is based on the injection of external high-pressure gas into the chamber during the compression process. The impact of different factors on the effectiveness of this technique has been studied using experimental and computational methods. In the first set of trials, the location and angle of injection of the external air was varied. It was observed that the heat transfer coefficient governing the heat transfer rate from the chamber varied greatly with change in location and angle of injection. In the second set of experiments, the source pressure of the injected gas was varied from 100.66 kPa to 551.58 kPa. It was observed that the temperature rise of air in the chamber was reduced with an increase in source pressure. Additionally, the increase in chamber pressure was steeper in the higher source pressure cases. In the third set of experiments, the injection profile of the injected gas was varied. This parameter did not greatly impact the effectiveness of external gas injection. In the last set of experiments, the time of initiation of injection was varied. Earlier injection had a positive impact on reducing the temperature rise in the chamber. However, the pressure in the chamber was seen to increase more rapidly in the runs with early injection. Considering that these factors could have a positive/negative impact on the temperature and pressure in the chamber (work required for compression), it may be required to optimize the injection of external high-pressure gas depending on the application.

Commentary by Dr. Valentin Fuster