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Technical Brief  
Sameh A. Nada and Karem Elfeky
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036009
Effects of servers/racks locations, servers powers densities and servers loading configurations on the thermal performance of data center racks array are experimentally investigations using a scaled physical model simulating real data. Front and rear rack temperatures profiles, servers temperatures, performance indices SHI/RHI (supply/Return Heat Index) are used to evaluate the thermal management of the racks array. The results showed that (i) servers located in high level rack cabinet have the worst thermal performance, (ii) middle racks of the rack row showed optimum thermal performance and energy efficiency, and (iii) locating the servers of high power densities in the middle of the racks row improves the thermal performance and energy efficiency of the racks array.
TOPICS: Data centers, Thermal management, Temperature, Energy efficiency, Heat
research-article  
Roberto Maffulli and Li He
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036012
The present work is aimed to examine how the HTC and main 3D passage aerodynamic features may be affected by a non-adiabatic wall temperature condition. A systematic computational study has been firstly carried out for a 3D NGV passage. The impacts of wall temperature on the secondary flows, trailing edge shock waves and the passage flow capacity are discussed, underlining the connection and interactions between the wall temperature and the external aerodynamics of the 3D passage. The local errors in HTC in these 3D flow regions can be as high as 30-40\% if the wall temperature dependence is not corrected. The effort is then directed to a new 3-point non-linear correction method. The benefit of the 3-point method in reducing errors in HTC is clearly demonstrated. A further study illustrates that the new method also offers much enhanced robustness in the wall heat flux scaling, particularly relevant when the wall thermal condition is also shown to influence the laminar-turbulent transition exhibited by two well-established transition models adopted in the present work.
TOPICS: Aerodynamics, Turbine blades, Wall temperature, Heat transfer coefficients, Flow (Dynamics), Errors, Robustness, Turbulence, Shock waves, Nozzle guide vanes, Heat flux
Discussion  
Asterios Pantokratoras
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036013
The present comment concerns some doubtful results included in the above paper.
TOPICS: Fluid dynamics, Fluids, Magnetic fields, Engineering systems and industry applications
Technical Brief  
H. S. Bhaskaran, P. I. Ro, J. K. Park and K. R. Ramakrishnan
J. Thermal Sci. Eng. Appl   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.
TOPICS: Temperature, Compressors, Pistons, Pressure, Compression, Heat transfer, High pressure (Physics), Heat transfer coefficients, Computational methods
Technical Brief  
Marios M. Fyrillas, Sayat Ospanov and Ulmeken Kaibaldiyeva
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035971
We consider the heat transfer problems associated with a periodic array of triangular, longitudinal, axisymmetric and pin fins. The problems are modeled as a wall where the flat side is isothermal and the other side, which has extended surfaces/fins, is subjected to convection with a uniform heat transfer coefficient. Hence our analysis differs from the classical approach because (i) we consider multi-dimensional heat conduction and (ii) the wall, that the fins are attached, is included in the analysis. The latter results in a non-isothermal temperature distribution along the base of the fin. The Biot number (Bi=h~t/k) characterizing the heat transfer process is defined with respect to the thickness/radius of the fins (t). Numerical results demonstrate that the fins would enhance the heat transfer rate only if the Biot number is less than a critical value which, in general depends on the geometrical parameters, i.e. the thickness of the wall, the length of the fins and the period. For pin fins, similar to rectangular fins, the critical Biot is independent of the geometry and is approximately equal to 3.1. The physical argument is that, under strong convection, a thick fin introduces an additional resistance to heat conduction.
TOPICS: Fins, Heat transfer, Heat conduction, Convection, Geometry, Temperature distribution, Heat transfer coefficients
research-article  
Biplab Das, Asis Giri and Suman Debnath
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035970
A computational analysis of conjugate mixed convection heat transfer from shrouded vertical non-isothermal heat sink on a horizontal base is performed. The overall Nusselt number and the product of friction factor (f) and Reynolds number (Re) are found to vary significantly with the spacing of heat sink as well as with the clearance between shroud and heat sink. By increasing the fin conductance by 200%, an enhancement of Nusselt number is noted to be around 58%, while the same Nusselt number enhancement is 134% for isothermal fin, within the range of parametric studies. The fRe value for smaller fin spacing shows a maximum with clearances, while the same for higher fin spacing remains same or increase with clearances. Finally overall Nusselt number and friction factor are well correlated with the governing parameters of the problem.
TOPICS: Heat transfer, Mixed convection, Heat sinks, Friction, Reynolds number, Clearances (Engineering), Electrical conductance
research-article  
Andrew A. Wereszczak, J. Emily Cousineau, Kevin Bennion, Hsin Wang, Randy H. Wiles, Timothy B. Burress and Tong Wu
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035972
The apparent thermal conductivity of packed copper wire test specimens was measured parallel and perpendicular to the axis of the wire using laser flash, transient plane source, and transmittance test methods. Approximately 50% wire packing efficiency was produced in the specimens using either 670 or 925 µm diameter copper wires that both had an insulation coating thickness of 37 µm. The interstices were filled with a conventional varnish material and also contained some remnant porosity. The apparent thermal conductivity perpendicular to the wire axis was about 0.5 - 1 W/mK whereas it was over 200 W/mK in the parallel direction. The Kanzaki model and an FEA model were found to reasonably predict the apparent thermal conductivity perpendicular to the wires but thermal conductivity percolation from non-ideal wire-packing may result in their underestimation of it.
TOPICS: Copper, Wire, Anisotropy, Thermal conductivity, Packing (Shipments), Packings (Cushioning), Transients (Dynamics), Lasers, Coating processes, Coatings, Percolation theory, Finite element analysis, Insulation, Porosity
research-article  
M. Sheikholeslami and Houman B. Rokni
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035973
Steady nanofluid convective flow in a porous cavity is investigated. Darcy and KKL models are considered for porous media and nanofluid, respectively. The solutions of final equations are obtained by CVFEM. Effective parameters are number of undulations, CuO-water volume fraction, Hartmann and Rayleigh numbers for porous medium. A correlation for Nuave is presented. Results depicted that heat transfer improvement reduces with rise of buoyancy forces. Influence of adding nanoparticle augments with augment of Lorentz forces. Increasing Hartmann number leads to decrease temperature gradient.
TOPICS: Magnetohydrodynamics, Nanofluids, Water, Porous materials, Rayleigh number, Nanoparticles, Cavities, Temperature gradient, Buoyancy, Heat transfer, Flow (Dynamics)
research-article  
Liu Bin, Jianfei Song, Zhaodan Yang and Rachid Bennacer
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035936
In order to explore the effect of DC and AC magnetic fields on the biological (fruits and vegetables) phase transformation and ice crystal formation, we used carrots strips (0.5×0.5×1cm3) and put them at low temperature control panel. The samples were frozen under AC and DC magnetic field of 50Hz with different intensity, i.e. 0,4.6,9,18,36 and 72Gs. The ice crystals formation during the process of cell freezing was observed and recorded using optical microscope , and the beginning and ending time of the phase transformation with the corresponding temperatures were determined.The results show that the DC and AC magnetic field situation compared to non using magnetic field can decrease ice crystal volume and be more flocculent. Such change will reduce the cell membrane damage rate of. The increase of magnetic field intensity delay the phase change time and induces short phase transition duration, A reduction in the cells's lowest non-crystallization temperature was also observed. Such change in thermal dynamic process and size elementary freezing (rapid formation of small ice crystals) reduces the damage of fruit and vegetable and increase the product quality.
TOPICS: Crystals, Magnetic fields, Biomaterials, Ice, Phase transitions, Freezing, Crystallization, Temperature, Damage, Low temperature, Delays, Strips, Cell membranes, Control panels, Product quality, Optical microscopes
research-article  
Ayman, A.M. Bayomy and Ziad Saghir
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035937
Continuous improvements in electronic devices for high-performance computers have led to a need for new and more effective methods of chip cooling. The first purpose of this study was to investigate the heat transfer development and characteristics of aluminum foam heat sink subjected to steady water flow for electronics cooling (Intel core i7 processor). The second purpose was to implement a new type of water flow through the aluminum foam which is pulsating or oscillating flow in order to achieve more uniform temperature distribution over the electronic surfaces. The aluminum foam heat sink was subjected to a water flow covering the non-Darcy flow regime (297 to 1353 Reynolds numbers). The bottom side of the heat sink was heated with a heat flux between 8.5 and 13.8 W/cm2. The pulsating flow frequency was ranged from 0.04 to 0.1 Hz. In addition, in order to complement the experimental studies, a numerical model was developed using finite element method and compared with the experimental data. The results revealed that the thermal entry length of the fluid flow through metal foam (porous media) is much smaller than that for laminar internal flow through empty channel. The result also showed that the local surface temperature increases along with increasing the axial flow direction for steady water flow case. On the other hand, for pulsating flow, the local temperature distributions act as a convex profile with the maximum surface temperature at the center of the test section. In addition, it was observed that the pulsating water flow through the aluminum foam heat sink achieves enhancement by 14% in the average Nusselt number and by 73% in temperature uniformity over the surface compared with steady water flow case.
TOPICS: Aluminum, Heat, Heat sinks, Pulsatile flow, Flow (Dynamics), Water, Temperature distribution, Temperature, Heat transfer, Cooling, Fluid dynamics, Metal foams, Porous materials, Computer simulation, Reynolds number, Finite element methods, Internal flow, Computers, Axial flow, Computer cooling, Heat flux, Temperature uniformity
research-article  
Eman Al-Sarairah, Bilal Al-Hasanat and Ahmed Amine Hachicha
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035938
In this paper, we provide a numerical study of the stability analysis of a planar premixed flame. The interaction of pref- erential diffusion and heat loss for a planar premixed flame is investigated using a thermal diffusive (constant density) model. The flame is studied as a function of three non- dimensional parameters namely, Damko ̈ hler number (ratio of diffusion time to chemical time), Lewis number (ratio of thermal to species diffusivity) and heat loss. Maximum of four steady state solutions are identified in some cases, two of which are stable. The behavior of the eigenvalues of the system, is also discussed. For low Lewis number the heat loss is playing a major role in stabilizing the flame for some moderately high values of Damk ̈o hler number. The results show the effect of increasing or decreasing Lewis number on adiabatic and non-adiabatic flames temperature and reaction rate as well as the range of heat loss at which flames can survive.
TOPICS: Stability, Diffusion (Physics), Flames, Heat losses, Steady state, Density, Eigenvalues, Temperature
research-article  
Seyedeh H. Mozaffari, Seshasai Srinivasan and M. Ziad Saghir
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035939
The objective of this paper is to investigate the behavior of two well-known boundary driven molecular dynamics (MD) approaches, namely, reverse non-equilibrium molecular dynamics (RNEMD) and heat exchange algorithm (HEX), as well as introducing a modified HEX model (MHEX) that is more accurate and computationally efficient to simulate mass and heat transfer mechanism. For this investigation, the following binary mixtures were considered: one equimolar mixture of argon (Ar)-krypton (Kr), one non-equimolar liquid mixture of hexane (nC6) and decane (nC10), and three non-equimolar mixtures of pentane (nC5) and decane. In estimating the Thermodiffusion factor in these mixtures using the three methods, it was found that consistent with the findings in the literature, RNEMD predictions have the largest error with respect to the experimental data. Whereas, the MHEX method proposed in this work is the most accurate, marginally outperforming the HEX method. Most importantly, the computational efficiency of MHEX method is the highest, about 7% faster than the HEX method. This makes it more suitable for integration with multi-scale computational models to simulate Thermodiffusion in a large system such as an oil reservoir.
TOPICS: Heat, Heat transfer, Equilibrium (Physics), Molecular dynamics, Algorithms, Errors, Liquid mixtures, Molecular dynamics methods, Hydrocarbon reservoirs
research-article  
H. Gurudath Nayak and Venkatarathnam Gadhiraju
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035940
Auto refrigerant cascade (ARC) refrigerators are used extensively in the semiconductor manufacturing industry to provide refrigeration in the temperature range of 80 to 150 K. The performance of the ARC refrigerator depends on the mixture composition, operating pressures etc. ARC refrigerators employ one or more liquid-vapor phase separators to separate the compressor lubricating oil and the condensed high boiling components and return to the compressor at an intermediate temperature to prevent freezing of the compressor lubricating oil and high boilers at low temperatures. However, dry-out of the phase separator can occur at some conditions. The phase separator dry-out phenomenon in ARC refrigerators has been studied experimentally with different mixtures and operating temperatures, the results of which are reported in this paper. The results of the studies show that the temperature difference between the streams at the cold end of the first heat exchanger can be used to reliably predict the dry-out of the phase separator.
TOPICS: Cascades (Fluid dynamics), Refrigerants, Temperature, Compressors, Lubricating oils, Operating temperature, Integrated circuit fabrication, Freezing, Vapors, Boilers, Boiling, Heat exchangers, Low temperature, Refrigeration
research-article  
Ahmad M. Younus AL-Naeemy, Abdul-Rahman D. Farraj, Dimitrios C. Kyritsis and Ashraf N. Al-Khateeb
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035941
The effect of the electric field on laminar non-premixed counter-flow propane flames was analyzed computationally. The computations were conducted using ANSYS-FLUENT platform associated with a detailed kinetic mechanism. The mechanism was supplemented with a set of three reactions accounting for the consumption/production of three chemi-ions. It was established that the position of the flame could be only controlled through altering the intensity of the applied electric field. The effect of the applied electric field was included within the reactive flow equations via introducing two distinct terms: a body force term that accounts for the electric field effects on the momentum of the reactive mixture, and an extra diffusion term that accounts for the mobility charged species; namely ambipolar diffusion. This study clearly shows that electric force provides a potential for controlling the location of propane flames without affecting their structure.
TOPICS: Flow (Dynamics), Flames, Electric fields, Diffusion (Physics), Mechanical admittance, Accounting, Ions, Chemically reactive flow, Computation, Momentum
research-article  
Peter J. Rodgers, Valerie Eveloy, Antoine Diana, Ismail Darawsheh and Fahad Almaskari
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035942
The thermal, mechanical and morphological characteristics of three selected commercially-available, injection-moldable, high thermal conductivity (20 - 32 W/m.K), polyimide 66 (PA66) polymer composites from two vendors are characterized for possible heat exchange applications in electronic equipment. The fillers are found to consist of 10 μm diameter, 120 to 350 μm long fibers, made of carbon in two composites, and a hybrid combination of essentially carbon, oxygen, and silicon in the third composite. Fiber weight loading ranges from 63% to 69%. The hybrid, high-length fiber reinforced material overall displays superior mechanical properties (i.e., ultimate tensile, flexural and impact strengths, and flexural modulus) compared with the other two carbon-filled composites. For the hybrid-filled and one carbon-filled material (both having a thermal conductivity of 20 W/m.K), good agreement between mechanical property measurements and corresponding vendor data is obtained. For the material having the highest vendor-specified thermal conductivity (i.e., 32 W/m.K) and weight filler fraction (i.e., 69%), mechanical properties are up to 37% lower than corresponding vendor data. The heat transfer rates of parallel plate, cross-flow air-water heat exchanger prototypes made of the three PA66 materials are comparable to that of an aluminum prototype having the same geometry. Based on the combined heat transfer and mechanical property characterization results, the hybrid, long-fiber filled PA66 polymer composite appears to have the best combination of mechanical and heat transfer characteristics, for potential use in electronics heat exchange applications.
TOPICS: Heat, Heat transfer, Polymer composites, Thermal conductivity, Electronic systems, Carbon, Fibers, Mechanical properties, Composite materials, Weight (Mass), Fillers (Materials), Engineering prototypes, Aluminum, Electronic equipment, Heat exchangers, Geometry, Oxygen, Silicon, Water, Electronics, Cross-flow, Impact strength
research-article  
Babak Fakhim, Srinarayana Nagarathinam, Steve W. Armfield and Masud Behnia
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035943
The increase in the number of data centres in the last decade, combined with higher power density racks, has led to a significant increase in the associated total electricity consumption, which is compounded by cooling inefficiencies. Issues, such as hot air recirculation in the data centre room environment, provide substantial challenges in thermal manageability. Three operational data centres have been studied to identify the cooling issues. Field measurements of temperature were obtained and were compared to numerical simulations to evaluate the overall thermal behavior of the data centres and to identify the thermal issues.
TOPICS: Computational fluid dynamics, Data centers, Thermal management, Cooling, Computer simulation, Temperature, Power density
research-article  
Eman Al-Sarairah, Chaouki Ghenai and Ahmed Amine Hachicha
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035922
Two dimensional numerical study of the effect of heat loss and strain rate on premixed counterflow flame edges for high Lewis number (Le > 1) is investigated. Finite element method was adopted to solve the numerical model. Under non adiabatic conditions, multiple flame edges and multi- ple propagation speeds (positive and negative) are discussed. Different regions of multiple propagation speeds have been revealed ranging from two to four depending on the value of the heat loss parameter and Damk ̈ hler number which is inversely proportional to the strain rate. Modeling a com- bustion wave connecting a strongly burning flame on one side of the burner to a weakly burning flame on the other side. These combustion waves are changing with increasing Damk ̈ hler number into flame edges with the fact that the strongly burning flame is the dominant.
TOPICS: Flames, Heat losses, Combustion, Waves, Finite element methods, Modeling, Computer simulation
research-article  
Chaouki Ghenai and Ahmed Amine Hachicha
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035926
This paper presents results on the performance of 10 MW biomass-fired steam power plant. The main objective is to test the performance of the power plant using different type of biomass fuels: bagasse, corn stover, forest residues, and urban wood residues. The biomass fuel was mixed with sub-bituminous coal with fractions of 0 to 100%. The effect of excess combustion air, flue gas temperature, and the parasitic loads on the power plant performance was investigated. The output results from the heat and mass balance analysis include the monthly and annual electrical power generated, capacity factor, boiler efficiency, thermal efficiency and gross and net heat rate. The results show a slightly decrease (1.7%) of the annual energy production when the biomass fractions increase from 6% to 100% but a substantial decrease of the CO2 equivalent emissions. A decrease of the excess combustion air from 25% to 5% will increase the boiler and thermal efficiencies and the annual energy output by 2%. This is mainly due to the reduction of the dry flue gas losses with the reduction of the excess combustion air. A reduction of the parasitic loads from 10% to 2% will increase the power plant performance by 9%. This is can be achieved by using more efficient pumps, fans and conveyors in the power plant. A reduction of the flue gas temperature from 480 F to 360 F increases the power plant performance by 4.4% due to the reduction of the dry flue gas losses.
TOPICS: Biomass, Thermal power stations, Power stations, Flue gases, Combustion, Fuels, Heat, Temperature, Boilers, Stress, Thermal efficiency, Wood products, Emissions, Conveyor systems, Pumps, Carbon dioxide, Cities, Coal, Energy generation, Fans, Electricity (Physics), Bagasse
research-article  
Pankaj Kumar, Shamit Bakshi and Dhiman Chatterjee
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035923
Cavitation behind a circular cylinder is studied with the aid of highly time resolved images at a constant Reynolds number of 64000. Apart from recording overall cavitation activity behind the cylinder, the study also delves into the dynamics of individual cavities. The length of cavity scales with cavitation number and this scaling is similar to the existing results obtained in flow regimes different from that presented here. Dynamics of individual cavities show distinct phases of cavity formation, growth and collapse. At lower cavitation numbers cavity collapse was followed by a rebounce. Variation of area normalized by the length of cavity shows self similarity in the growth phase of cavities for different cavitation numbers. Thus cavity length is the suitable length scale for dynamics of cavities, at least for the growth phase. The cavity lifetime scales inversely with the square of cavitation number. Dynamics of individual small cavity captured at higher frame rates was found to be similar to that of an isolated bubble. In this case, a rapid collapse follow a more gradual expansion phase, unlike that shown by larger cavities.
TOPICS: Cavitation, Circular cylinders, Cross-flow, Cavities, Dynamics (Mechanics), Collapse, Cylinders, Flow (Dynamics), Reynolds number, Bubbles
research-article  
Filippo Genco and Giacinto Genco
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4035924
Damage to plasma facing components due to high intense energy deposition during tokamak plasma instabilities, is still considered one of the most serious and unresolved problem for the fusion reactors. Key plasma facing components as the divertor and the entire first wall during off-normal operations, are generally subjected to high rate of deposition of energy, neutrons and radiation leading generally to structural catastrophic failures including burnout of coolant tubes. The use of alumina nanofluids applied to future fusion reactors is proposed to, at least, mitigate some of the problems described providing better thermal performance during off-normal events. A 1D heat transfer model using the characteristics of alumina nanoparticles dispersed in common water is presented. Heat transfer of alumina nanofluid is modeled. Results obtained are critically compared with other well-known computer packages and experiments used to predict the coolant heat removal capabilities during longer quasi-steady state plasma instabilities events. Enhancements produced by the use of alumina nanoparticles are evident. Comparisons with both pure water and swirl tape inserts are carried out and CHF conditions are predicted showing good agreement with both published numerical and experimental data.
TOPICS: Fusion reactors, Plasmas (Ionized gases), Nanofluids, Modeling, Water, Nanoparticles, Coolants, Heat transfer, Neutrons, Radiation (Physics), Tokamaks, Heat, Critical heat flux, Damage, Heart failure, Computers, Failure

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