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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

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