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[en] Floating LNG bunkering offshore terminal (FLBT) has been adopted as a floating unit by the marine industry equipped with the natural gas liquefaction plant on the ship's deck with the practical interest in controlling the plume emission discharged from the ship. The downwash of emitted plumes has adverse consequences on the ship’s engine intake and ventilation system followed by the interference of the smoke with helicopter operations. Owing to this fact, understanding of the plume behavior is considered to be so significant in the aspect of ship design and thereby, flow visualization techniques assist to study the plume path ensuring the safety of the personnel and functioning system. The unignited flare gas emitted from the tower invites heat transfer due to the temperature difference between the atmospheric wind and the ship’s exhaust, which follows the examination of mixing enhancement of fluid mediums as well as the mitigation of plume concentration depending upon wind speeds. The parametric study is intended to investigate the plume dispersion pattern around a ship based on the flare motion and bending angle for light and strong wind speeds. It is observed that the plume gas rises higher and disperses over the larger area with wider streamline separation due to the effect of buoyancy for light wind speeds. On the other hand, the motion of flare gas is found to be narrow for strong winds restricting vertical movements due to dominant inertial forces than gravity pull. Thus, the current numerical investigation facilitates in understanding the configuration and plume distribution by the variation in streamline behavior with contours plots. In this work, the calculated results are analyzed systematically in pretty realistic conditions and simple measures are obtained, which will be applied to the preliminary design of plume stack depending on the ship’s deck arrangement.
[en] In this research, a series of experiments have been performed to study the thermal resistance of an oscillating heat pipe equipped with cooling tower. The effects of filling ratio and input heating power on the thermal resistance of the heat pipe and temperatures of different sections of evaporator and condenser of the heat pipe are investigated and discussed. All tests are taken for input heating power and filling ratio in the ranges of 20–200 W and 10–60%, respectively. A correlation for the thermal resistance is presented, which the effects of input heating power and filling ratio are taken into account in this correlation. The results showed that the heat pipe with filling ratio of 40% and input heating power of 160 W has the minimum value of thermal resistance among all cases considered in this research. Moreover, the thermal resistance decreases about 86% as the input heating power increases in the range of 20–120 W, while this reduction is only 23% by increasing the input heating power in the range of 160–200 W.
[en] Extended surfaces represent one of practical approaches to enhance heat transfer. Based on the laws of conductive and convective heat transfer, an increase in the area across which the object is in contact with the fluid can increase heat transfer. Due to its special structure, porous media can be seen as suitable alternatives for extended surface applications. On this basis, this research investigates the effect of connection type of sintered porous fins on heat transfer and pressure drop in the fluid flow. Connection model of four- and six-contact sintered balls of constant dimensions was evaluated by means of CFD simulation in this research. To describe the problem further, surface analysis on the reference cube is presented. The results indicate that the six-contact model has more porosity than the four-contact in reference cube by 29.45%. It was further found that the six-contact model tends to increase convective heat transfer by 33%. Results of surface analysis show that the main reasons for the difference in heat transfer between the four- and six-contact models are porosity and the angle at which balls are arranged with another.
[en] In this study, the impingement of a conductive droplet on a hot wall under an electric field in the Leidenfrost regime is simulated. Apart from electrostatic equations, the governing equations are conservation equations of mass, momentum, and energy in the incompressible case. The level set method is used for interface tracking. For the appropriate application of discontinuities at the interface, the ghost fluid method is adopted. First, a sessile droplet on a superheated surface under an electric field is simulated. Simulation results are validated against the experiments. Under an electric field, an increase in the heat flux dissipated from the surface is observed for a sessile droplet. In the next step, droplet impact on a hot surface in the range of low Weber numbers in the presence of an electric field is simulated. According to the results, the droplet spreading radius and contact time increase with electric field strength. In addition, applying an electric field increases the heat transfer rate and total heat removal from the surface. If the potential difference between the droplet and the surface exceeds a specific value, the Leidenfrost state is suppressed. The threshold potential difference for Leidenfrost suppression decreases with Weber number and increases with surface superheat.
[en] In this study, a heat transfer simulation of three types of vacuum insulation panels that can be mechanically fastened with two external panels was conducted. The linear thermal transmittance of the thermal bridge was calculated and the design of superiority in terms of insulation was derived. Results of heat transfer analysis indicated that when the length of the thermal bridge was linearly increased by two times to 0.039, 0.083, and 0.1656 m, the linear thermal transmittance of the thermal bridge was 0.0476, 0.2113, and 0.2663 W/m·K, respectively. In addition, the linear thermal transmittance did not increase linearly in proportion to the length. The linear thermal transmittance was calculated by substituting the data obtained from the two-dimensional heat transfer analysis into the formula given in the ISO 10211 standard. A comparison of the values of the linear thermal transmittance of the joints of the three cases verified that Case 1 had the best thermal bridge design because it exhibited the smallest linear thermal transmittance.
[en] This article scrutinizes the features of viscous dissipation in the stagnation point flow past through a linearly stretched Riga wall by implementing Cattaneo-Christov heat flux model. Viscous dissipation is carried out in Cattaneo-Christov diffusion analysis for the first time in this letter. As a result of Cattaneo-Christov model, some extra terms of viscous dissipation are appeared in the energy equation. These extra terms of viscous dissipation are missing in the literature. On the utilization of suitable transformations, the equations governing the problem are reduced under the boundary layer approximation into the non-linear and dimensionless ordinary differential equations. Convergent approach is utilized to solve the dimensionless governing equations. The solution thus acquired is used to highlight the effects of emerging parameters on velocity distribution and fluidʼs temperature through the graphs. Features of the drag force (or skin friction co-efficient) are graphically interpreted. It is noticed that the presence of modified Hartman number helps to reduce the fluidʼs temperature but enhances the velocity profile. Further an enlargement in the value of thermal time relaxation parameter helps to decrease the temperature distribution. (paper)
[en] The uniformity principle of temperature difference field is very useful in heat exchanger analyses and optimizations. In this paper, we analyze some other heat transfer optimization problems in the thermal management system of spacecrafts, including the cooling of thermal components, the one-stream series-wound heat exchanger network, the volume-to-point heat conduction problem, and the radiative heat transfer optimization problem, and have found that the uniformity principle of temperature difference field also holds. When the design objectives under the given constraints are achieved, the distributions of the temperature difference fields are uniform. The principle reflects the characteristic of the distribution of potential in the heat transfer optimization problems. It is also shown that the principle is consistent with the entransy theory. Therefore, although the principle is intuitive and phenomenological, the entransy theory can be the physical basis of the principle. (paper)
[en] Array impinging jets have been applied to combustors, vanes and blades of gas turbines that require high cooling performance. However, the jet is deflected toward the downstream direction owing to the influence of the crossflow, which decreases the cooling performance. In this study, a corrugated structure is proposed to increase the space between the jet holes by securing the space for the spent air. The average Reynolds number of each jet hole is 10,000 and numerical simulations are conducted by ANSYS-CFX 17.2. The modified corrugated structure that has an additional plate-making secondary channel for the spent air shows a higher Nusselt number than the corrugated structure. Furthermore, a greater distance of impinging and jet plate (h/d) shows a higher Nusselt number at the stagnation region owing to the development of the jet flow. Therefore, the modified corrugated structure with a smaller distance of impinging and jet plate (h/d) shows greater heat transfer than the corrugated structure.
[en] Enhancement techniques based on artificial roughness are used in numerous applications of heat exchangers. Heat exchange devices are essential components in complex engineering systems, such as industrial energy generation and energy conversion. In this study, a short helical plate was used to enhance heat transfer in a horizontal circular annulus tube. Experiments and CFD were performed to study the heat transfer effect of the helical plate, and Teflon and brass materials were used for the helical plate. Uniform heat flux was considered, and particle image velocimetry (PIV) was used for comparisons. The Nusselt number profiles increased steeply around the Teflon helical plate and decreased suddenly to 1.0 along the test section.
[en] The osmotically driven flow across a semipermeable membrane under a constant static pressure difference is revisited with reference to previous reports on reverse osmosis. The osmosis due to the local solute concentration difference across the membrane induces solvent seeping flow, which advects solutes in a layer in contact with the membrane and reduces the osmosis itself as a result of nonlinear feedback. A few mathematical techniques for obtaining approximate solutions of the seeping flow and the layer thickness, such as that for inverse problems used in the field of heat transfer, are presented with an emphasis on nonlinear boundary conditions and the time-dependent solvent flow rate. We determined that the layer on the membrane forms rapidly and spontaneously through osmosis at a timescale of and that the layer thickness increases with no upper limit in an infinite time interval. Based on the obtained solution, we also discuss the thermodynamic output work in an irreversible process, which is extracted from the seeping flow as an osmotic engine. (paper)