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J. Thermal Sci. Eng. Appl. 2019;12(1):011001-011001-11. doi:10.1115/1.4042123.

World energy demand has increased immediately and is expected to continue to grow in the foreseeable future. Therefore, an overall change of energy consumption continuously from fossil fuels to renewable energy sources, and low service and maintenance price are the benefits of using renewable energies such as using wind turbines as an electricity generator. In this context, offshore wind power refers to the development of wind parks in bodies of water to produce electricity from wind. Better wind speeds are available offshore compared to on land, so offshore wind power's contribution in terms of electricity supplied is higher. However, these structures are very susceptible to degradation of their mechanical properties considering various hostile loads. The scope of this work is the study of the damage noticed in full-scale 48 m fiberglass composite blades for offshore wind turbine. In this paper, the most advanced features currently available in finite element (FE) abaqus/Implicit have been employed to simulate the response of blades for a sound knowledge of the mechanical behavior of the structures and then localize the susceptible sections.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011002-011002-6. doi:10.1115/1.4042152.

To improve air pollution, we must reduce soot particulates in vehicle exhaust gas, which are inevitably harmful to the environment as well as to human health. Many countries are setting new regulations of nanoscale particle emission. Then, a ceramic porous filter such as diesel particulate filters (DPFs) has been developed. However, as more particles are trapped within their wall pores, the pressure difference (drop) across the filter increases. Resultantly, this situation could worsen the fuel efficiency, simultaneously with less torque. Usually, the filter regeneration process for particle oxidation inside the filter should be periodically needed. Thus, a filter with lower pressure drop would be preferable. In the current stage, the responses of the pressure drop during both particle filtration and oxidation are not fully understood. This is because these are the small-scale processes, and we cannot observe the internal physical phenomenon experimentally. In this paper, focusing on the exhaust flow with soot particles, the filtration was numerically simulated by a so-called lattice Boltzmann method (LBM). Here, the time-variation of the filter-back pressure was evaluated, which is important for the transport phenomena in the porous filter. For comparison, the pressure drop during the filter regeneration was also simulated to show the different pressure response affected by the soot oxidation zone.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011003-011003-13. doi:10.1115/1.4041936.

The rapid rate of improvement in electronic devices has led to an increased demand for effective cooling techniques. The purpose of this study is to investigate the heat transfer characteristics of an aluminum metallic foam for use with an Intel core i7 processor. The metal foams used have a porosity of 0.91 and different permeabilities ranging from 10 pores per inch (PPI) to 40 PPI. The flow rate at the entrance of the porous cavity varied from 0.22 USGPM to 0.1 USGPM. The fluid consists of water with aluminum nanoparticles having a concentration from 0.1% to 0.5%. The heat fluxes applied at the bottom of the porous test cell vary from 13.25 W/cm2 to 8.34 W/cm2. It has been observed that nanofluid and forced convection improves heat extraction. These observations lead to the conclusion that heat enhancement is possible with nanofluid and it is enhanced further in the presence of a high flow rate. However, it was detected experimentally, verified numerically, and agreed upon by different researchers that higher heat extraction is found for a nanofluid concentration of 0.2%. This observation is independent of the porous permeability or applied heat flux. It has also been shown that heat enhancement in the presence of nanofluid is evident, when experimental results were compared to water.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011004-011004-7. doi:10.1115/1.4042153.

Nonuniform heat fluxes are commonly observed in thermo-electronic devices that require distinct thermal management strategies for effective heat dissipation and robust performance. The limited research available on nonuniform heat fluxes focus mostly on microchannel heat sinks while the fundamental component, i.e., a single microchannel, has received restricted attention. In this work, an experimental setup for the analysis of variable axial heat flux is used to study the heat transfer in a single microchannel with fully developed flow under the effect of different heat flux profiles. Initially, a hot spot at different locations, with a uniform background heat flux, is studied at different Reynolds numbers, while varying the maximum heat fluxes in order to compute the heat transfer in relation to its dependent variables. Measurements of temperature, pressure, and flow rates at a different locations and magnitudes of hot spot heat fluxes are presented, followed by a detailed analysis of heat transfer characteristics of a single microchannel under nonuniform heating. Results showed that upstream hotspots have lower tube temperatures compared to downstream ones with equal amounts of heat fluxes. This finding can be of importance in enhancing microchannel heat sinks effectiveness in reducing maximum wall temperatures for the same amount of heat released, by redistributing spatially fluxes in a descending profile.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011005-011005-11. doi:10.1115/1.4042353.

A common occurrence in many practical systems is that the desired result is known or given, but the conditions needed for achieving this result are not known. This situation leads to inverse problems, which are of particular interest in thermal processes. For instance, the temperature cycle to which a component must be subjected in order to obtain desired characteristics in a manufacturing system, such as heat treatment or plastic thermoforming, is prescribed. However, the necessary boundary and initial conditions are not known and must be determined by solving the inverse problem. Similarly, an inverse solution may be needed to complete a given physical problem by determining the unknown boundary conditions. Solutions thus obtained are not unique and optimization is generally needed to obtain results within a small region of uncertainty. This review paper discusses several inverse problems that arise in a variety of practical processes and presents some of the approaches that may be used to solve them and obtain acceptable and realistic results. Optimization methods that may be used for reducing the error are presented. A few examples are given to illustrate the applicability of these methods and the challenges that must be addressed in solving inverse problems. These examples include the heat treatment process, unknown wall temperature distribution in a furnace, and transport in a plume or jet involving the determination of the strength and location of the heat source by employing a few selected data points downstream. Optimization of the positioning of the data points is used to minimize the number of samples needed for accurate predictions.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011006-011006-8. doi:10.1115/1.4042861.

An improved model is presented for the formation of bainitic structures during isothermal heat treatment conditions. The model based on displacive mechanism consists of a new expression for the volume fraction of bainite as a function of time, incorporating a temperature and chemical composition-based expression for the number density of initial nucleation sites and limiting the volume fraction of bainite. The model has been validated with respect to experimental data of high- as well as low-carbon steels. It has been found that the isothermal transformation kinetics is well predicted for all steels.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;12(1):011007-011007-7. doi:10.1115/1.4043186.

A model for the quantum Brayton refrigerator that takes the harmonic oscillator system as the working substance is established. Expressions of cooling load, coefficient of performance (COP), and ecological function are derived. With numerical illustrations, the optimal ecological performance is investigated. At the same time, effects of heat leakage and quantum friction are also studied. For the case with the classical approximation, the optimal ecological performance, and effects of heat leakage and quantum friction are also investigated. For both general cases and the case with classical approximation, the results indicate that the ecological function has a maximum. The irreversible losses decrease the ecological performance, while having different effects on the optimal ecological performance. For the case with classical approximation, numerical calculation with friction coefficient μ = 0.02 and heat leakage coefficient Ce = 0.01 shows that the cooling load (RE) at the maximum ecological function is 6.23% smaller than the maximum cooling load (Rmax). The COP is also increased by 12.1%, and the exergy loss rate is decreased by 27.6%. Compared with the maximum COP state, the COP (ɛE) at the maximum ecological function is 0.55% smaller than the maximum COP (ɛmax) and that makes 7.63% increase in exergy loss rate, but also makes 6.17% increase in cooling load and 6.20% increase in exergy output rate.

Commentary by Dr. Valentin Fuster

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