Abstract
Numerical simulation of fluid flow and heat transfer processes has been an important vehicle for scientific researches and applications. This annual special issue focuses on the up-to-date progresses in this field. We received active submissions from China (including Taiwan), Japan, Republic of Korea, Brazil, Malaysia, Pakistan, South Africa, Iran, and Tunisia and finally accepted 19 research articles to be published in the special issue after the strict peer-review processing. The topics cover three aspects: numerical methods for fluid flow and heat transfer problems, fluid flow in engineering, and heat transfer. They will be introduced as follows.
For numerical methods for fluid flow and heat transfer problems, two articles have been published. In the article “Benchmark Solutions for Two-Dimensional Fluid Flow and Heat Transfer Problems in Irregular Regions Using Multigrid Method” by Li et al., the authors numerically studied the benchmark solutions by multigrid method for two-dimensional fluid flow and heat transfer problems in the irregular regions under the body-fitted coordinate system to overcome the shortcomings such as small number of computational grids, simple computational domain, small characteristic number, and lack of mixed convection benchmark solution. The results verified the reliability of the present method and proved it to be more accurate than the benchmark solutions in the literature. In the article “An Overall Ablation Model of EPDM Based on Porous Characteristics in Char Layer” by Liu et al., the numerical model for overall ablation of ethylene propylene diene monomer (EPDM) material, including the particle erosion and thermo-chemical ablation process, was presented. The particle erosion model has been validated by different ablation cases; the errors between calculated ablation rates and test ablative rates are <6%, and the calculated structures of the char layers agree well with the test images.
For fluid flow in engineering, nine articles have been published. In the article “Numerical Simulation of Dynamic Flow Characteristics in a Centrifugal Water Pump with Three-Vanned Diffuser” by Shuai et al., the characteristics of the rotating turbulent flow in a centrifugal water pump with three asymmetrical diffusers were studied through numerical simulations. The authors concluded that the existence of the three asymmetrical diffusers has damping effect on the pressure fluctuation amplitude (PFA) and energy amplitude of pressure fluctuations in the diffuser domain dramatically, which indicates that the diffusers can effectively control the hydraulic exciting vibration in the pump. In the article “Investigation into the Flow Details of Runner Region in a Pump Turbine at Off-Design Conditions” by Gong et al., the authors analyzed the reason of S-shaped characteristic in pump turbine via numerical simulations of a pump turbine’s internal flow field. They concluded that the development of vortex and backflow under large guide vane opening shows hysteresis quality with respect to that under small guide vane openings. The S-shaped characteristic can be reduced in the period of designing a turbine. The designer can avoid the turbine operating in the S-type section by controlling the motion principle of guide vane when the working condition of the turbine is adjusted. In the article “Numerical Investigation of Degas Performance on Impeller of Medium-Consistency Pump” by Ye et al., a modified Herschel–Bulkley model and the Eulerian gas–liquid two-phase flow model were utilized to approximately represent the behaviors of the medium-consistency pulp suspension in papermaking industry. It was found that r/R = 0.53 is the best position for degas hole as the gas volume content Cvp at the pump outlet is less than 0.1% and Cvap at the vacuum inlet is 9.8%, while the output head is at maximum. In the article “Development of a Bi-Directional Electrohydrodynamic Pump—Parametric Study with Numerical Simulation and Flow Visualization” by Suh et al., the authors demonstrated a parametric study that there is an optimum distance between two large grounded electrodes for producing a maximum pumping velocity at the diameter of two small electrodes fixed at 0.3 mm. A general account is also given of the basic ideas of electrode arrangement for the enhancement of pumping. In the article “Numerical Investigations of Hydrodynamic Performance of Hydrofoils with Leading-Edge Protuberances” by Cai et al., the hydrodynamic characteristics of two modified hydrofoils with leading-edge protuberances were numerically investigated and compared to the baseline foil. The streamwise vortices, which are similar to those induced by a delta wing, are considered as creating nonlinear lift, thus improving the post-stall performance. The results implied that a big enough spanwise width is needed to gain a better description of the flow patterns around the leading-edge protuberance. In the article “An Industry-Scale Modelling for the Turbulence Agglomeration of Fine Particles” by Xu et al., the authors developed a mixed Eulerian–Lagrangian model for the modeling of turbulence-induced agglomeration of fine particles/droplets. Benchmark solutions of the turbulent collision frequencies were carried out and compared with the results provided by the well-characterized experiment by Duru et al. Detailed information on the growth of particles mean diameter and the temporally evolving particle distribution was obtained. In the article “Entropy-Based Detached-Eddy Simulation (SDES) of the Airwake over a Simple Frigate Shape (SFS)” by Zhao et al., computational fluid dynamics simulations of a generic simple frigate shape ship airwake were performed using the SDES method. The mean velocity and turbulence data were compared to show that the SDES model could accurately predict levels of turbulence in the airwake. Determination of this complex flow can assist in the definition of safe ship–helicopter operating limits and future ship design. In the article “Simulation of Cooling System Using Internal Air Flow with Bending Flow Passages” by Youn et al., the cooling performance is predicted using a numerical simulation model and examined by the experiment when the flow rate is within 30 L/min (ANR). The cooling system using a flow passage with four turns is the optimal system for practical use. In the article “Numerical Investigation on the Flow, Combustion and NOx Emission Characteristics in a 660 MWe Tangential Firing Ultra-Supercritical Boiler” by Sun et al., a three-dimensional numerical simulation was carried out to study the pulverized-coal combustion process in a tangentially fired ultra-supercritical boiler. The good agreements with the predicted results and the measurements imply that the adopted simulation models are appropriate to predict the coal combustion and NOx emissions. The results indicate that more steady coal combustion is achieved with the novel air staging.
For heat transfer, eight articles have been published. In the article “Analysis of Heat Exchanger Network for Temperature Fluctuation” by Jin et al., quantitative influence of temperature fluctuation in the network was carried out in order to examine the transmission character of temperature fluctuation in the system, according to the concept of temperature-change sensitivity coefficient. The information was obtained for improving the design quality of heat exchanger network. In the article “Unsteady Natural Convection Heat Transfer in a Nanofluid-Filled Square Cavity with Various Heat Source Conditions” by Nguyen et al., unsteady natural convection in a nanofluid-filled square enclosure cavity with a heat source at the bottom was numerically studied using a characteristic-based split (CBS) scheme in the finite element method (FEM). The authors found that the maximum temperature and the average Nusselt number started to oscillate, which is a typical characteristics of high Rayleigh number flow, at higher Rayleigh number in nanofluid than in pure water. In the article “Numerical Study on Condensation Heat Transfer of Trapezoid Grooved Surfaces” by Qi et al., heat transfer of filmwise condensation on a trapezoid groove has been studied in the present dissertation by the numerical method. The trapezoid grooved surface can enhance the heat transfer by at least 1.5 times compared with the smooth surface. The heat flux through the grooved surface with α = 60° was lower than the grooved surface with α = 75°when ΔT < 15 K, while the opposite conclusion was obtained when ΔT > 20 K. The thermal resistance of groove with α = 60° was lower but the liquid-discharged ability was better than that of groove with α = 75°. The present analyses can be used in the parameter design and heat transfer calculation of trapezoid grooved surfaces. In the article “Heat Transfer Capability Simulation of High Temperature Heat Pipe in Supersonic Vehicle Leading Edge Applications” by Liu et al., a numerical method was proposed to determine the heat transfer capability of the high-temperature heat pipe and the stagnation temperature with supersonic vehicle leading edge applications. The stagnation temperature was higher and the backward surface temperature was lower due to the thermal contact resistance of the composite structure. The effect of the thermal contact resistance should also be considered in the detailed design stage. In the article “Numerical Investigation of CO2 Emission and Thermal Stability of a Convective and Radiative Stockpile of Reactive Material in a Cylindrical Pipe” by Lebelo et al., the authors investigated the combined effects of emission of CO2 and O2 depletion on thermal stability in a long cylindrical pipe of combustible reactive material. Reaction processes that increase both the temperature and carbon dioxide emission profiles also increase the depletion of oxygen. The adoption of mathematical model to simulate this complex phenomenon helps to provide deep insight into this intricate problem in a cheaper and safer way since it may involve thermal runaways through exothermic chemical reaction. In the article “Heat Transfer Characteristics of Circular and Elliptic Cylinders in Cross-Flow” by Park et al., the authors compared heat transfer characteristics of circular and elliptic cylinders in cross flow. Numerical investigation is carried out to study the convective heat transfer over single cylinder and two-row cylinder banks along with experiments to verify the numerical results. The results indicated that elliptic cylinder has significantly lower friction factor compared to circular cylinder, while elliptic cylinder has only 9.3% lower Nusselt number than circular cylinder does. The elliptic cylinder has useful features for future development of efficient heat exchangers. In the article “Numerical Study of Heat Transfer Enhancement by Rectangular Winglet Vortex Generator Pair in a Channel” by Zhang et al., numerical study of heat transfer enhancement with rectangular winglet pair (RWP) mounted on the bottom wall of the channel is carried out. The best heat transfer performance of the studied physical model can be obtained when the attack angle of RWPs is 29°. In the article “Thermal-Hydraulic Analysis Under PLOFA Hypothesis of a Plate-Type Fuel Surrounded by Two Water Channels Using Relap5 Code” by Iliuk et al., the analysis of a single plate-type fuel surrounded by two water channels shows the capabilities of the RELAP5 code to perform transient analysis and to simulate a PLOFA severe accident. The results show that the critical heat flux was detected in the central region along the axial direction of the plate when the right water channel was blocked. In the real scenario, this could be the cause of the melting of the core.