Guest Editorial

J. Thermal Sci. Eng. Appl. 2017;9(3):030301-030301-1. doi:10.1115/1.4036156.

The ICTEA conference series was inspired by the need to help provide an opportunity for professional development of scientists and engineers in the Middle East, including the Gulf region and North Africa. The need for such development persists, despite the strong commitment of regional governments for improving undergraduate education and for building research capabilities in institutions of higher learning. Until recently, attracting highly motivated academic staff to advance research agendas and to make significant contributions to GDP growth was not among the top priorities. But, thanks to the foresight of regional leaders, higher education in this part of the world is starting to change. However, the fact remains that highly skilled scientists and engineers in the region who are dedicated to research often must seek work abroad in academic and research institutions in order to develop themselves professionally.

Commentary by Dr. Valentin Fuster

Research Papers

J. Thermal Sci. Eng. Appl. 2017;9(3):031001-031001-6. doi:10.1115/1.4035922.

We investigate numerically the effect of heat loss and strain rate on the premixed flame edges encountered in a two-dimensional counterflow configuration for Lewis number higher than one. Under nonadiabatic conditions, multiple flame edges and multiple 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 Damkohler number, which is inversely proportional to the strain rate. A combustion wave is modeled by 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 Dam number into flame edges with the fact that the strongly burning flame is the dominant.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031002-031002-8. 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 0–100%. 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 (CF), boiler efficiency (BE), 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 (DFGLs) 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 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031003-031003-9. doi:10.1115/1.4035924.

Damage to plasma facing components (PFC) 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 critical heat flux (CHF) conditions are predicted showing good agreement with both published numerical and experimental data.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031004-031004-6. 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 64,000. Apart from recording the 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, the 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031005-031005-6. doi:10.1115/1.4035936.

In order to explore the effect of direct current (DC) and alternating current (AC) magnetic field (MF) on the biological (fruits and vegetables) phase transformation and ice crystal formation, we used carrot strips (0.5 × 0.5 × 1 cm3) and put them at low temperature control panel. The samples were frozen under AC and DC MF of 50 Hz with different intensities, i.e., 0, 0.46, 0.9, 1.8, 3.6, and 7.2 mT. The ice crystals formation during the process of cell freezing was observed and recorded using the 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 MF situations compared to non-MF can decrease ice crystal volume and be more flocculent. The changes will reduce the cell membrane damage rate. The increase of magnetic field intensity delays the phase change time and leads to a shorter phase transition duration, a reduction in the cells’s lowest noncrystallization temperature is also observed. Such changes in thermal dynamic process and size elementary freezing (rapid formation of small ice crystals) reduce the damage to the quality of fruits and vegetables.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031006-031006-20. 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 laminar flow regime (297–1353 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031007-031007-7. doi:10.1115/1.4035925.

Frost on heat exchanger fin surfaces increases the thermal resistance and blocks the air flow passages, which reduce the system energy efficiency. Therefore, investigations of frost formation especially simulations of frosting on the heat exchanger surfaces are essential for designing heat exchangers that operate with frosting. In this paper, the frost growth and densification processes on fin-and-tube heat exchanger surfaces are numerically investigated using a mass transfer model implemented as a user-defined function (UDF) in fluent. The model predicts the frost distributions on the heat exchanger surfaces, the temperature distributions, and the air flow pressure drop. The results show that the frost is thicker and the frost density is higher on the fin surfaces on the windward side near the tubes, while the frost is thinner and the density is lower near the inlet. Very little frost appears in the tube wake region. Frost on the fin-and-tube heat exchanger surfaces restricts the airflow and about doubles the pressure drop after frosting for 50 min. The simulated frost distributions and pressure drops are in good agreement with experimental data, which means that the frosting model can be used to predict frost layer growth on heat exchanger surfaces and the resulting airflow resistance.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031008-031008-13. 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, 120–350 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031009-031009-6. doi:10.1115/1.4035943.

The increase in the number of data centers 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 center room environment, provide substantial challenges in thermal manageability. Three operational data centers 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 centers and to identify the thermal issues.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031010-031010-6. 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 preferential diffusion and heat loss for a planar premixed flame is investigated using a thermodiffusive (constant density) model. The flame is studied as a function of three nondimensional parameters, namely, Damköhler number (ratio of diffusion time to chemical time), Lewis number (ratio of thermal to species diffusivity), and heat loss. A maximum of four solutions are identified in some cases, two of which are stable. The behavior of the eigenvalues of the linearized system of stabilty is also discussed. For low Lewis number, the heat loss plays a major role in stabilizing the flame for some moderately high values of Damköhler number. The results show the effect of increasing or decreasing Lewis number on adiabatic and nonadiabatic flames temperature and reaction rate as well as the range of heat loss at which flames can survive.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031011-031011-9. 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 nonequilibrium 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 the mass and heat transfer mechanism. For this investigation, the following binary mixtures were considered: one equimolar mixture of argon (Ar) and krypton (Kr), one nonequimolar liquid mixture of hexane (nC6) and decane (nC10), and three nonequimolar 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 multiscale computational models to simulate thermodiffusion in a large system such as an oil reservoir.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031012-031012-6. 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 80–150 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031013-031013-6. doi:10.1115/1.4035941.

The effect of the electric field on laminar nonpremixed counterflow 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.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;9(3):031014-031014-11. doi:10.1115/1.4036438.

This study was conducted to investigate the effect of cavitation on liquid jet atomization characteristics in nozzles of different length to diameter (L/D) ratios. For this purpose, a spray test facility with an ambient pressure chamber was constructed, and sprays were recorded using a high-speed camera for a wide range of conditions, which provided complete characterization of the orifice flow fields and the emerging jet. Collapse length measurements are provided and indicate the complex nature of the nozzle flow. Extensive discharge coefficient measurements for each nozzle are also presented. Finally, the influence of the length to diameter ratio on cavitation and subsequently on the spray structure is also addressed.

Commentary by Dr. Valentin Fuster

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