Modeling of Unsteady Natural Convection for Double-Pipe in a Partially Cooled Enclosure

In this article, a model is developed for unsteady natural convection heat transfer and fluid flow in a partially cooled enclosure with a hollow cylinder through it. The right vertical wall of the enclosure is cooled partially. The location of the partial cooling is set up in three different configurations; namely, bottom (P ₁), middle (P ₂), and top (P ₃). A hollow cylinder is located at the middle of the enclosure to simulate a double-pipe heat exchanger. Three values of Grashof number are applied in this work, i.e., 10⁴, 10⁵ and 10⁶, and three lengths of the cooler, i.e., 0.2, 0.4 and 0.6. Finite element method was utilized to solve the unsteady dimensionless conservation equations of mass, momentum and energy. It is found that the length and location of cooler does not have a significant effect on the natural convection for the case of the low Grashof number. The maximum heat transfer rate is reached when the cooler is located at the middle of the vertical wall.     

Unsteady analysis of natural convection in a carbon nanotube-water filled cavity with an inclined heater

ABSTRACT

A finite element solution has been performed in this work to solve unsteady governing equations of natural convection in a carbon nanotube–water-filled cavity with inclined heater. The temperature of ceiling and left vertical walls is lower than that of the heater while the other walls are adiabatic. The main governing parameters are nanofluid volume fraction and Rayleigh number (Ra). It is found that the heat transfer rate shows different trends based on Rayleigh number and it increases with increase in nanoparticle volume fraction. It has been estimated that average Nusselt number (Nu) is dependent onRa through power regression models with strong positive linear correlation between ln (Nu) and ln (Ra). In particular, for the maximum time, when the solid volume fraction is varied from 0 to 0.1 the dependence between average Nu and linear Ra, on a logarithmic scale, is very high.     

SIMULATION OF TURBULENT FLOW IN IRREGULAR GEOMETRIES USING A CONTROL-VOLUME FINITE-ELEMENT METHOD

A modified error indicator and a locally implicit scheme with anisotropic dissipation model on quadrilateral-triangular mesh are developed to study the supersonic turbulent flow over a backward-facing step. In the Cartesian coordinate system, the unsteady Favre-averaged Navier-Stokes equations with a low-Reynolds-number k –ε turbulence model are solved. The modified error indicator, in which the unified magnitude of substantial derivative of pressure and unified magnitude of substantial derivative of vorticity magnitude are incorporated, is applied to treat the new node spacing of mesh remeshing. To assess the present approach, the transsonic turbulent flow around an NACA 0012 airfoil is performed. According to the high-resolution result on the adaptive mesh, the structure of the back-step corner vortex, expansion wave, and oblique shock wave are distinctly captured.     

Experimental Investigation of the Defrosting Process on Domestic Refrigerator Finned Tube Evaporators

Frost formation is an important problem for household refrigerator and air conditioning equipment manufacturers. When frost accumulates on the evaporator surface, it acts as a thermal insulator and reduces heat flow. Therefore, frost negatively affects evaporator performance. The purpose of this study is to decrease energy consumption and increase the efficiency of the defrosting process. In the first part of the experiment, frost formation on a no-frost refrigerator evaporator at real operating conditions was investigated. The ambient temperature was maintained at a constant 23°C. It was observed that when the evaporation temperature reaches –35°C, the frost formation on the evaporator exhibits a rather dense structure that is unlike the needle-type structure observed at higher temperatures. In the second part of the experiment, the defrosting process was observed with an endoscopic camera, and the initial melting points were investigated. The experiment revealed that although the heater density is higher on the lower rows, the frost on the higher rows of the evaporator melts faster. On the theoretical side, we prepared an analytical model that calculates the melting time of the frost on the fin. The experimental and theoretical results are within 5%.     

Thermo-electrical properties in Pb-Sb hypereutectic alloy

Pb-17 wt% Sb hypereutectic alloy was directionally solidified upward with three different growth rates (8 μm/s, 18 μm/s, 42 μm/s, 166 μm/s and 497 μm/s) at a constant temperature gradient (3.6 K/mm) in the Bridgman-type growth apparatus. The dependence of characteristic microstructure parameters such as primary dendrite arm spacing (λ₁) and secondary dendrite arm spacing (λ₂) on the growth rate (V) were determined by using a linear regression analysis. Variations of electrical resistivity (ρ) for cast samples for temperatures in the range of 300–550 K were measured by using a standard dc four-point probe technique. The variations of thermal conductivity with temperature for the same alloys were determined from the WiedemannFranz and SmithPalmer equations by using the measured values of electrical resistivity. The enthalpy of fusion for the same alloys were determined by means of a differential scanning calorimeter from a heating trace during the transformation from solid to liquid. The corrosion resistance was analyzed by Tafel extrapolation and linear polarization resistance methods conducted in a 5% sodium chloride solution in contact with air maintained at room temperature. Effects of the growth rate on the electrical resistivity and corrosion resistance were also investigated.     

Synthesis of W-3 wt% Mn-2 wt% VC composites by high energy milling and sintering

A W-3 wt% Mn matrix alloy reinforced with 2 wt% of 24 h pre-milled VC particles were synthesized via high energy milling for 3 h, 6 h, 12 h, 24 h and then sintered at 1300 °C. The W-3 wt% Mn alloy was also prepared by high energy milling of a powder blend of W-3 wt% Mn for 6 h. Effects of milling duration as well as Mn and VC addition on the microstructural and mechanical properties of the W-3 wt% Mn-2 wt% VC composite powders and sintered samples were investigated. Microstructural characterizations of the composite powders and sintered samples were carried out via SEM and XRD analyses. Density measurements and hardness measurements of the sintered samples were also carried out. While the relative density increased from 92.4% for the sintered W-3 wt% Mn matrix to 97.8% for the sintered W-3 wt% Mn-2 wt% VC composite milled for 24 h, there was an increase of ～85% in hardness values for the W-3 wt% Mn-2 wt% VC samples milled for 24 h.     

On the Volta potential measured by SKPFM – fundamental and practical aspects with relevance to corrosion science

ABSTRACT

The Volta potential is an electron-sensitive parameter and describes the thermodynamic propensity of a metal to take part in electrochemical reactions. It has found widespread acceptance among corrosion researchers due to its connection to the corrosion potential and its easy measurability in local scale, being often used to study localised corrosion phenomena and micro-galvanic activities. The principle object of this paper is to provide a comprehensive, fundamental insight into the meaning of the Volta potential and to define a polarity convention of measured potentials by the scanning Kelvin probe force microscopy (SKPFM) in order to assess local nobilities in microstructures. Conditions to relate the Volta potential with the mixed-potential theory are discussed and a possible connection to corrosion phenomena explained. The limitations of the Volta potential as well as the SKPFM technique are also aimed to be explained, with some practical information to maximise the output of high quality data.     

Mixed model disassembly line balancing problem with fuzzy goals

The collection of used products is the driving force of remanufacturing systems and enterprises can gain significant economic, technical and social benefits from recycling. All products are disassembled up to some level in remanufacturing systems. The best way to disassemble returned products is valid by a well-balanced disassembly line. In this paper, a mixed integer programming (MIP) model is proposed for a mixed model disassembly line balancing (MMDLB) problem with multiple conflicting objectives: (1) minimising the cycle time, (2) minimising the number of disassembly workstations and (3) providing balanced workload per workstation. In most real world MMDLB problems, the targeted goals of decision makers are frequently imprecise or fuzzy because some information may be incomplete and/or unavailable over the planning horizon. This study is the first in the literature to offer the binary fuzzy goal programming (BFGP) and the fuzzy multi-objective programming (FMOP) approaches for the MMDLB problem in order to take into account the vague aspirations of decision makers. An illustrative example based on two industrial products is presented to demonstrate the validity of the proposed models and to compare the performances of the BFGP and the FMOP approaches.     

Joint decisions on inventory replenishment and emission reduction investment under different emission regulations

Carbon emission regulation policies have emerged as mechanisms to control firms’ carbon emissions. To meet regulatory requirements, firms can make changes in their production planning decisions or invest in green technologies. In this study, we analyse a retailer’s joint decisions on inventory replenishment and carbon emission reduction investment under three carbon emission regulation policies. Particularly, we extend the economic order quantity model to consider carbon emissions reduction investment availability under carbon cap, tax and cap-and-trade policies. We analytically show that carbon emission reduction investment opportunities, additional to reducing emissions as per regulations, further reduce carbon emissions while reducing costs. We also provide an analytical comparison between various investment opportunities and compare different carbon emission regulation policies in terms of costs and emissions. We document the results of a numerical study to further illustrate the effects of investment availability and regulation parameters.