J. Thermal Sci. Eng. Appl. 2019;11(1):010201-010201-3. doi:10.1115/1.4042209.

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Commentary by Dr. Valentin Fuster

Research Papers

J. Thermal Sci. Eng. Appl. 2018;11(1):011001-011001-11. doi:10.1115/1.4041196.

Based on the constructal theory concepts, an investigation is carried out to optimize circular multilayer microchannels embedded inside a rectangular heat sink with different numbers of layers and flow configurations. The lower surface of the heat sink is uniformly heated, while both pressure drop and length of the microchannel are fixed. Also, the volume of the heat sink is kept fixed for all studied cases, while the effect of solid volume fraction is examined. All the dimensions of microchannel heat sinks are optimized in a way that the maximum temperature of the microchannel heat sink is minimized. The results emphasize that using triple-layer microchannel heat sink under optimal conditions reduces the maximum temperature about 10.3 °C compared to the single-layer arrangement. Further, employing counter flow configuration in double-layer microchannel improves its thermal performance, while this effect is less pronounced in the triple-layer architecture. In addition, it is revealed that the optimal design can be achieved when the upper channels of a multilayer microchannel heat sink have bigger diameters than the lower ones. Finally, it is observed while using two layers of microchannels is an effective means for cooling improvement, invoking more layers is far less effective and hence is not recommended.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011002-011002-14. doi:10.1115/1.4040964.

The ignition, combustion, and emission behavior of crushed corn cob pellets of different shapes and sizes with a chemical binder (Epoxy1092) under certain operating conditions in a fixed-bed combustor were investigated in this study. Also, chemical kinetic parameters are determined by using thermogravimetric and differential thermogravimetric (TG/DTG) analysis data for both pellet and binder. It was found that the activation energy value is 129.82 kJ mol−1 for pellets, while the activation energy value is 109.62 kJ mol−1 for epoxy 1092. The surface and central pellet temperatures histories, the mass loss rates, conversion rate as well as a simple combustion ash analysis are recorded and analyzed. It was found that increasing the starting air temperature and air velocity and decreasing the size of pellet lead to a decrease in devolatilization time, time to reach maximum temperature, char combustion time, and an increase in the total combustion rate. Regarding to emissions; it was found that the CO2 content increased with increasing the starting air temperature and flow velocity and the maximum CO concentration reaches to 49 ppm at 9.6 ± 1.04% O2. The fouling, slagging indices, and ash viscosity were investigated. The corn cob pellets show a relatively high fouling inclination (FI) and a medium slagging inclination.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011003-011003-10. doi:10.1115/1.4040990.

The goal of this work is to predict the tool-chip interface temperature during cryogenic machining and determine the effectiveness of this cooling strategy. Knowledge of the tool-chip interface temperature is needed to conduct process planning: choosing a tool cooling geometry, cutting speed, and cryogen flow rate as well as predicting tool life and achievable material removal rate. A detailed explanation of the analytical heat transfer model is presented, which is a modified form of Loewen and Shaw's orthogonal cutting model, where a thermal resistance network is applied to represent the heat transfer mechanisms in, and out of, the cutting tool. An in-depth discussion of the temperature rise at the tool-chip interface during orthogonal machining of titanium alloy Ti–6Al–4V is presented. The effect of cutting speed, cryogen flow rate and quality, and cooling strategy are explored. The model is used to compare the effect of internal cryogenic cooling with external flood cooling using a water-based metalworking fluid or liquid nitrogen. A sensitivity analysis of the model is conducted and ranks the relative importance of various design parameters. The thermal conductivity of the cutting insert has the greatest influence on the predicted interface temperature. The low boiling temperature and phase change are what make internal cooling of a cutting insert with liquid nitrogen effective at reducing the tool-chip interface temperature. If the heat flowing into the tool, from the tool-chip interface, does not exceed the available latent heat in the cryogen, then this method is more effective than external flood cooling.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011004-011004-11. doi:10.1115/1.4040991.

Numerical solutions for conjugate heat transfer of a hydro-dynamically fully developed, thermally developing, steady, incompressible laminar gas flow in a microtube with uniform wall heat flux boundary condition are presented. The mathematical model takes into account effects of rarefaction, viscous dissipation, flow work, shear work, and axial conduction in both the wall and the fluid. The effect of the tube wall thickness, the wall-to-fluid thermal conductivity ratio, as well as other factors on heat transfer parameters is investigated, and comparisons with the case of zero wall thickness are presented as appropriate. The results illustrate the significance of heat conduction in the tube wall on convective heat transfer and disclose the significant deviation from those with no conjugated effects. Increasing the wall thickness lowers the local Nusselt number. Increasing the wall-to-fluid thermal conductivity ratio also results in lower Nusselt number. In relatively long and thick microtubes with high wall-to-fluid thermal conductivity ratio, the local Nusselt number exhibits minimum values in the entrance regions and at the end sections due to axial conduction effects. The analysis presented also demonstrate the significance of rarefaction, shear work, axial conduction, as well as the combined viscous dissipation and flow work effects on heat transfer parameters in a microtube gas flow. The combined flow work and viscous dissipation effects on heat transfer parameters are significant and result in a reduction in the Nusselt number. The shear work lowers the Nusselt number when heat is added to the fluid.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011005-011005-7. doi:10.1115/1.4040993.

With today's computing technology, research on soot particles using simulation works has become more preferable as a supplementary to the existing experimental methods. The objective of this study is to investigate the effect of different engine load conditions to in-cylinder soot particles formation. This is to clarify the relationship between soot mass fraction (SMF) and size distribution. The first section of the study is conducted by computational analysis using a detailed kinetics soot model, particulate size mimic (PSM), which is based on the concept of the discrete sectional method. The analysis is carried out within closed-cycle combustion environment which is from the inlet valve closing (IVC) to the exhaust valve opening (EVO). The next section is conducted by experimental work deliberately for validation purpose. The total soot mass obtained from the computational work during EVO is comparable to the calculated value by less than 13% error for all of the experimental cases. The soot size distribution measurement indicates that exhaust out particles are dominantly in the dual-mode size range, <10 nm and 11–30 nm. The relationship between the soot mass and size distribution demonstrates that soot mass fraction does not completely rely on soot size distribution as well as particle size range. In most of the cases, particles with the moderate size range (11–60 nm) hold the highest mass fraction during EVO. On the whole, this paper provides significant information that contributes key knowledge to indicate that soot mass fraction is not entirely dependent on soot size distribution as well as particle size range.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011006-011006-8. doi:10.1115/1.4040992.

This paper addresses whether synergistic interaction or additive behavior govern the co-combustion characteristics of lignite and biochars produced from hybrid poplar (HP), ash tree (AT), and rhododendron (RH). The biochars were blended with lignite and the burning behavior was investigated by thermal analysis. Upon carbonization, fundamental change occurred in the burning mechanisms of biomass from homogeneous to heterogeneous reactions. Blending the lignite with biochars led to improvement in the calorific value and reductions in the ash yield. Carbonization limited the high reactivity of biomass, and the reactivities of biochars became closer to the lignite's reactivity, consequently they burned in accord without segregation.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011007-011007-11. doi:10.1115/1.4041341.

A 3D transient numerical model of a ductile iron ladle has been developed to predict the fluid flow and temperature drop during the holding and teeming. The volume of fluid (VOF) multiphase model has been employed to track the interface between the liquid metal and the air. The SST k-ω model has been applied to model the turbulence due to natural convection in the ladle. The temperature evaluation in the refractory lining walls during preheating and teeming is shown. Appropriate boundary conditions are applied for natural convection and radiation to surroundings from all the outer steel surfaces as well as from the top glass wool cover. The heat loss due to radiation from the liquid metal surface to the surrounding walls is also considered in the present model by applying an energy sink term to the cells at the interface. The numerical results of the 780 kg ladle have been compared with the measured temperature drop of the metal using an S-type thermocouple for two ladle cycles and the difference between the measured and predicted temperature at the end of two cycles is 3 °C. Decreasing the ladle capacity to 650 kg for pouring the same amount of metal increased the temperature drop by 11 °C due to increase in surface area to melt volume ratio. Also increasing the refractory thickness for 650 kg ladle increased the temperature drop by 4 °C due to the heat accumulation in the ladle during the cyclic transient heat transfer process.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011008-011008-11. doi:10.1115/1.4041342.

Effects of an upstream combustor wall on turbine nozzle endwall film cooling performance are numerically examined in a linear cascade in this paper. Film cooling is by two rows of cooling holes at 20% of the axial chord length upstream of the vane leading edge (LE) plane. The combustor walls are modeled as flat plates with square trailing edges (TE) positioned upstream of the endwall film cooling holes. A combustor wall is in line with the LE of every second vane. The influence of the combustor wall, when shifted in the axial and tangential directions, is investigated to determine effects on passage endwall cooling for three representative film cooling blowing ratios. The results show how shed vortices from the combustor wall greatly alter the flow field near the cooling holes and inside the vane passage. Film cooling distribution patterns, particularly in the entry region and along the pressure side of the passage, are affected. The combustor wall leads to an imbalance in film cooling distribution over the endwalls for adjacent vane passages. Results show a larger effect of tangential shift of the combustor wall on endwall cooling effectiveness than the effect of an equal axial shift. The study provides guidance regarding design of combustor-to-turbine transition ducts.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011009-011009-9. doi:10.1115/1.4041344.

We investigate the effect of soil type and moisture on the operation of a ground source heat pump (GSHP) system in supplying the energy needs of a greenhouse in Karaj, Alborz province, Iran, in terms of the required length of ground heat exchanger, the working hours, the electricity consumption, as well as the coefficient of performance (COP) of heat pumps. In order to predict the capacity of heat pumps, we use the numerical heat transfer model of Noorollahi et al. (2016, “Numerical Modeling and Economic Analysis of a Ground Source Heat Pump for Supplying Energy for a Greenhouse in Alborz Province, Iran,” J. Cleaner Prod., 131, pp. 145–154) in which the governing equations of heat transfer in the ground heat exchanger are numerically solved through a novel finite difference method. Thermal properties of various soil types, namely sandy soil, sand, silty loam, and silty clay, with three different levels of moisture content referred to as dry, damp, and saturated, are considered as the main inputs for the computer code. The simulations indicate that when moisture is increased from dampness to saturation, the annual working hours of heat pumps decrease by 1.1%, 5.1%, 6.1%, and 4.6%, and their annual electricity consumption is reduced by 2.2%, 10.6%, 12.6%, and 9.7% for sandy soil, sand, silty loam, and silty clay, respectively. Moreover, the average COP of heat pumps increase by 0.9%, 4.0%, 5.2%, and 3.7% in heating mode and 2.4%, 13.0%, 16.5%, and 11.7% in cooling mode for the mentioned soils, respectively.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011010-011010-8. doi:10.1115/1.4041345.

In this study, losses analysis at bushing regions of a transformer covers is done using finite difference method (FDM), considering that FDM being more flexible to deal with the nonlinear constitutive law and easier to be implemented than finite element (FE) and analytical methods. The analysis is performed based on a 2-level adaptive mesh solution of Maxwell equations and Ohm law at the cross section area in the axial symmetry page of a steel disk, taking account the nonlinear magnetic permeability of the steel. The losses density obtained, as a heat source, is imported into an alternating direction implicit (ADI) approach of heat conduction equation. Therefore, a finite difference (FD) solution algorithm for magneto-thermal analysis on cover plate is obtained by combination of adaptive mesh refinement and ADI-FDM, which improves the accuracy and decreases the computational time without losing accuracy. The reliability of the proposed technique is confirmed by experimental and FE method (FEM) results, considering the temperature distribution of the cover. The comparison of the results with those obtained from FEM and experiments shows the efficiency and capability of the method.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011011-011011-7. doi:10.1115/1.4041347.

Heat exchangers are used widely in many fields, and various kinds of exchanger have been developed according to the requirement of the practical applications. Recently, heat exchangers that are highly efficient or compact have become more desirable from the viewpoint of energy conservation, and several new types have been developed, such as a compact fin tube type and a double tube type having an inner pipe with a special geometry. In this study, the flow and heat transfer characteristics of a petal-shaped double tube with a large wetted perimeter of six and five petals and five shallow petals and the effect of tube shape on the heat transfer and heat transfer efficiency were examined experimentally. The heat transfer of the double tube with a petal-shaped inner tube was increased because of the large wetted perimeter, but the pressure loss by friction increased. The optimal shape of the petal-shaped double tube with a high heat transfer performance and the greatest efficiency is discussed.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011012-011012-11. doi:10.1115/1.4041346.

The present work demonstrates the use of manifold microchannel technology in conjunction with conventional macrogeometries to achieve superior performance compared to traditional heat exchangers. A novel tubular manifold heat exchanger is designed using three-dimensional (3D) printed manifold and conventional double enhanced tube. The experiments are performed using water as the working fluid and the manifold side heat transfer coefficient up to 9538 Wm−2K−1 with a low flowrate of 4.25 lpm is achieved with as low pressure drop as 323 Pa. A comparison with respect to thermal hydraulic performance of the results with a plate heat exchanger shows clear advantage of the proposed exchanger. Overall, microscale heat transfer characteristics are obtained by using relatively simple and economical fabrication techniques.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011013-011013-10. doi:10.1115/1.4041349.

In the field of additive manufacturing process, laser cladding is widely considered due to its cost effectiveness, small localized heat generation, and full fusion to metals. Introducing nanoparticles with cladding metals produces metal matrix nanocomposites, which in turn improves the material characteristics of the clad layer. The governing equations that control the fluid flow are standard incompressible Navier–Stokes and heat diffusion equation, whereas the Euler–Lagrange approach has been considered for particle tracking. The mathematical formulation for solidification is adopted based on enthalpy porosity method. Liquid titanium has been considered as the initial condition where particle distribution has been assumed uniform throughout the geometry. A numerical model implemented in a commercial software based on control volume method has been developed, which allows to simulate the fluid flow during solidification as well as tracking nanoparticles during this process. A detailed parametric study has been conducted by changing the Marangoni number, convection heat transfer coefficient, constant temperature below the melting point of titanium, and insulated boundary conditions to analyze the behavior of the nanoparticle movement. The influence of increase in Marangoni number results in a higher concentration of nanoparticles in some portions of the geometry and lack of nanoparticles in rest of the geometry. The high concentration of nanoparticles decreases with a decrease in Marangoni number. Furthermore, an increase in the rate of solidification time limits the nanoparticle movement from its original position which results in different distribution patterns with respect to the solidification time.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011014-011014-10. doi:10.1115/1.4041438.

Vortex generators (VG) are widely used in multifunctional heat exchangers/reactors for augmenting the heat transfer from fin plates to the working fluid. In this study, numerical simulations for longitudinal VGs are performed for both laminar and turbulent flow regimes. The shear-stress transport (SST) κω model is used for modeling turbulence. Inclination angle for a new streamlined VG configuration called inclined projected winglet pair (IPWP) was varied to study the effect of this angle on the heat transfer enhancement and pressure drop. Response surface methodology (RSM) was used to deduce the inclination angle effects on heat transfer, pressure drop, and vorticity from both local and global points of view. Such study highlights the optimization for this VG configuration for better heat transfer intensification, based on thermal enhancement factor (TEF). Finally, it is found that the VG with inclination angle ranging from 30 deg to 35 deg exhibits the best global performance compared to other inclination angles. This type of studies is important for the enhancement of the thermal performance of heat exchangers and static mixers in various engineering applications.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011015-011015-9. doi:10.1115/1.4041495.

The dry-low-NOx (DLN) micromix combustion technology has been developed originally as a low emission alternative for industrial gas turbine combustors fueled with hydrogen. Currently, the ongoing research process targets flexible fuel operation with hydrogen and syngas fuel. The nonpremixed combustion process features jet-in-crossflow-mixing of fuel and oxidizer and combustion through multiple miniaturized flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. The paper presents the results of a numerical and experimental combustor test campaign. It is conducted as part of an integration study for a dual-fuel (H2 and H2/CO 90/10 vol %) micromix (MMX) combustion chamber prototype for application under full scale, pressurized gas turbine conditions in the auxiliary power unit Honeywell Garrett GTCP 36-300. In the presented experimental studies, the integration-optimized dual-fuel MMX combustor geometry is tested at atmospheric pressure over a range of gas turbine operating conditions with hydrogen and syngas fuel. The experimental investigations are supported by numerical combustion and flow simulations. For validation, the results of experimental exhaust gas analyses are applied. Despite the significantly differing fuel characteristics between pure hydrogen and hydrogen-rich syngas, the evaluated dual-fuel MMX prototype shows a significant low NOx performance and high combustion efficiency. The combustor features an increased energy density that benefits manufacturing complexity and costs.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011016-011016-11. doi:10.1115/1.4041437.

Based on four-thermal-resistance-capacitance network within a borehole, an improved thermal-resistance-capacitance model (TRCM), which takes into account the effect of nonuniform temperature distribution along the borehole perimeter, is proposed for vertical single U-tube ground heat exchanger. For a given geometric and physical parameters of ground heat exchanger, the numerical simulations of the conventional TRCM based on three-thermal-resistance-capacitance network within borehole, the improved TRCM based on four-thermal-resistance-capacitance network within borehole and three-dimensional (3D) finite volume computational fluid dynamics (CFD) model by using fluent software were conducted, respectively. Through the comprehensive comparisons of simulation results between these above-mentioned three models for vertical single U-tube ground heat exchanger, it could be concluded that the proposed improved TRCM could not only provide relatively high accurate results, but also remarkably decrease the solving time as compared to the benchmark 3D finite volume CFD model. Since the proposed TRCM has better performance than the one based on three-thermal-resistance-capacitance network within borehole and 3D finite volume CFD model, a new reliable and feasible TRCM for vertical single U-tube ground heat exchanger could be available for the design and optimization of ground heat exchanger, the data interpretation of thermal response test (TRT) and other applications of ground heat exchanger in real industrial engineering.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011017-011017-5. doi:10.1115/1.4041343.

This note is concerned with the evaluation of the unknown diffusion coefficient in a steady-state heat conduction problem. The proposed method is iterative and, starting with an initial guess, updates the assumed value at every iteration. The updating stage is achieved by generating a set of functions that satisfy some of the required boundary conditions. The correction to the assumed value is then computed by imposing the remaining boundary conditions. Numerical examples are used to study the applicability of this method.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011018-011018-7. doi:10.1115/1.4041442.

In recent years, a primary concern in the development of electronic technology is high heat dissipation of power devices. The advantages of unique thermal physical properties of graphite foam raise up the possibility of developing pool boiling system with better heat transfer efficiency. A compact thermosyphon was developed with graphite foam insertions to explore how different parameters affect boiling performance. Heater wall temperature, superheat, departure frequency of bubbles, and thermal resistance of the system were analyzed. The results indicated that the boiling performance is affected significantly by thermal conductivity and pore diameter of graphite foam. A proposed heat transfer empirical correlation reflecting the relations between graphite foam micro structures and pool boiling performance of Novec7100 was developed in this paper.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011019-011019-13. doi:10.1115/1.4041266.

In the current study, a numerical analysis was performed for the heat transfer over the surface of nozzle guide vanes (NGVs) using three-dimensional computational fluid dynamics (CFD) models. The investigation has taken place in two stages: the baseline nonfilm-cooled NGV and the film-cooled NGV. A finite volume based commercial code was used to build and analyze the CFD models. The investigated annular cascade has no heat transfer measurements available; hence in order to validate the CFD models against experimental data, two standalone studies were carried out on the NASA C3X vanes, one on the nonfilm-cooled C3X vane and the other on the film-cooled C3X vane. Different modeling parameters were investigated including turbulence models in order to obtain good agreement with the C3X experimental data; the same parameters were used afterward to model the industrial NGVs.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;11(1):011020-011020-13. doi:10.1115/1.4040989.

This paper presents an experimental study on frost formation on a plate under natural convection conditions. Frost thickness, mass, density, and surface roughness are measured during each test. Frost thickness and roughness are measured using an image processing technique. The effect of operating conditions (temperature of the plate, and temperature and relative humidity of the air) on the properties of frost is investigated. Frost surface roughness is quantified using two parameters: the average roughness and the skewness of the roughness, which can describe the frost layer shape. The surface roughness of the frost layer is considerably higher than the roughness of a flat plate, which should be considered in frosting studies. In this paper, it is shown that frost surface roughness is related to the frost layer shape, porosity and density. It is also found that the plate temperature affects the frost surface roughness significantly; as the plate temperature decreases, the frost layer has a high average roughness and negative skewness, which correspond to a highly porous, low density frost layer. Increasing the air humidity and air temperature affects the average surface roughness slightly but not skewness of the frost surface.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Thermal Sci. Eng. Appl. 2018;11(1):014501-014501-3. doi:10.1115/1.4041436.

Opto-microfluidic methods have advantages for manufacturing complex shapes or structures of micro particles/hydrogels. Most of these microfluidic devices are made of polydimethylsiloxane (PDMS) by soft lithography because of its flexibility of designing and manufacturing. However, PDMS scatters ultraviolet (UV) light, which polymerizes the photocrosslinkable materials at undesirable locations and clogs the microfluidic devices. A fluorescent dye has previously been employed to absorb the scattered UV light and shift its wavelength to effectively solve this issue. However, this method is limited due to the cost of the materials (tens of dollars per microchip), the time consumed on synthesizing the fluorescent material and verifying its quality (two to three days). More importantly, significant expertise on material synthesis and characterization is required for users of the opto-microfluidic technique. The cost of preliminary testing on multiple iterations of different microfluidic chip designs would also be excessive. Alternatively, with a delicate microchannel design, we simply inserted aluminum foil strips (AFS) inside the PDMS device to block the scattered UV light. By using this method, the UV light was limited to the exposure region so that the opto-microfluidic device could consistently generate microgels longer than 6 h. This is a nearly cost- and labor-free method to solve this issue.

Commentary by Dr. Valentin Fuster


Commentary by Dr. Valentin Fuster

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