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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

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