[en] A cavitating venturi in the auxiliary feed water system of a nuclear power plant is a flow control device that limits the maximum flow rate in critical conditions. In this research, a numerical study was conducted to predict the characteristics of the cavitating flow in a venturi with various geometrical configurations. Assuming axisymmetric geometry and a steady state, equations of the flow field were solved numerically. The mixture model for the multi-phase flow and the standard k-ε turbulence model were adopted. The primary diffuser angle was varied to perform detailed parametric study. The flow and pressure variations, shape of the cavitation bubble, and corresponding flow control characteristics were summarized. Finally, the calculation results were analyzed to provide a physical basis for the design limits proposed in a previous patent. It is expected that the results of this study will be useful information for the future design of an optimized venturi

[en] Among tubular heat exchangers, fin tube types are the most widely used in refrigeration and air-conditioning equipment. Efforts to enhance the performance of these heat exchangers included variations in the fin shape from a plain fin to a slit and louver type. In the context of heat transfer augmentation, the performance of vortex generators has also been investigated. Delta winglet vortex generators have recently attracted research interest, partly due to experimental data showing that their addition to fin-tube heat exchangers considerably reduces pressure loss at heat transfer capacity of nearly the same level. The efficiency of the delta winglet vortex generators widely varies depending on their size and shape, as well as the locations where they are implemented. In this paper, the flow field around delta winglet vortex generators in a common flow up arrangement was analyzed in terms of flow characteristics and heat transfer using computational fluid dynamics methods. Flow mixing due to vortices and delayed separation due to acceleration influence the overall fin performance. The fin with delta winglet vortex generators exhibited a pressure loss lower than that of a plain fin, and the heat transfer performance was enhanced at high air velocity or Reynolds number

[en] In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.

[en] Film flows applied to shell-and-tube heat exchangers in various industrial fields have been studied for a long time. One boundary of the film flow interfaces with a fixed wall, and the other boundary interfaces with a gaseous region. Thus, the flows become so unstable that wavy behaviors are generated on free surfaces as the film Reynolds number increases. First, high-amplitude solitary waves are detected in a low Reynolds number laminar region; then, the waves transit to a low-amplitude, high frequency ripple in a turbulent region. Film thickness is the most significant factor governing heat transfer. Since the wave accompanied in the film flow results in temporal and spatial variations in film thickness, it can be of importance for numerically predicting the film's wavy behavior. In this study, various turbulent models are applied for predicting low-amplitude ripple flows in turbulent regions. The results are compared with existing experimental results, and finally, the applied turbulent models are appraised in from the viewpoint of wavy behaviors

[en] Today, computational fluid dynamics (CFD) is widely used in industry because of the availability of high performance computers. However, full-scale analysis poses problems owing to the limited resources and time. In this study, the performance and optimal size of a heat exchanger were calculated using the effectiveness-number of transfer units (ε-NTU) method and a database of characteristics heat exchanger. Information about the geometry and performance of various heat exchangers is collected, and the performance of the heat exchanger is calculated under the given operating conditions. To determine the optimal size of the heat exchanger, a Genetic Algorithm (GA) is used, and MATLAB and REFPROP are used for the calculation

[en] Diverse cross-corrugated surface geometries were considered to estimate the sensitivity of four variants of k-ε turbulence models (Low Reynolds, standard, RNG and realizable models). The cross-corrugated surfaces considered in this study are a conventional sinusoidal shape and two different asymmetric shapes. The numerical simulations using the steady incompressible Reynolds-averaged Navier Stokes (RANS) equations were carried out to obtain the steady solutions of the flow and thermal fields in the unitary cell of the heat exchanger matrix. In addition, the experimental test for the measurement of local convective heat transfer coefficients on the heat transfer surfaces was performed by means of the Transient liquid crystal (TLC) technique in order to compare the numerical results with the measured data. The features on detailed flow structure and corresponding heat transfer in the unitary cell of the matrix type heat exchanger are compared and analyzed against four different turbulence models considered in this study.

[en] This study presents an investigation of the flow and heat transfer characteristics in a Sinusoidal wavy circular tube (SWCT). We solved numerically the conservation equations of continuity, momentum and energy which govern the steady, three-dimensional, and incompressible fluid flow and the heat transfer in the SWCT. The friction factor, heat transfer rate, and goodness factors of the SWCT were taken into consideration as performance metrics. The present paper also discusses the effect of the geometry parameters, such as the peak amplitude and wavelength, on the distribution of the local pressure coefficient, fluid flow, wall vorticity and local Nusselt number in the sinusoidal wavy circular tube. The simulations were performed for the different Reynolds numbers of 100, 1000 and 10000. The calculated results were evaluated in terms of the friction factor and Colburn j factor, and their overall performance was estimated using the area and volume goodness factors.

[en] Liquid film flows are classified into waveless laminar, wavy laminar, and turbulent flows depending on the Reynolds number or the flow stability. Since the wavy motions of the film flows are so intricate and nonlinear, studies on them have largely been experimental. Most numerical approaches have been limited to the waveless flow regime. The various free surface of tracking schemes adopted for this problem were used to more accurately estimate the average film thickness, rather than to capture the unsteady wavy motion. In this study, the wavy motions in laminar wavy liquid film flows with Reynolds numbers of 200.1000 were simulated with various numerical schemes based on the volume of fluid (VOF) method for interface tracking. The results from each numerical scheme were compared with the experimental results in terms of the average film thickness, the wave velocity, and the wave amplitude

[en] We investigated the flow and heat transfer characteristics in a Twisted elliptic tube (TET). The effects of geometry parameters such as the aspect ratio and number of rotations in the TET were analyzed comparatively using three-dimensional (3-D) numerical simulation. We also solved numerically the conservation equations of continuity, momentum, and energy in the TET. Fully developed flow in the TET was modeled using the realizable k-ε turbulence model and steady incompressible Reynolds-averaged Navier-Stokes (RANS) equations. The simulation was performed for Reynolds numbers of 100, 1000 and 10000. The pressure drop and the heat transfer of the TET were assessed in terms of the Darcy friction factor and Colburn j-factor, and overall performance was evaluated using the area and volume goodness factors

[en] The inherent two-phase heat transport of heat pipes (HPs) is progressively being examined for potential uses. These thermal devices are affected by many operating factors, prompting this study to investigate the effects of different types of wicks and working conditions on the time-dependent thermal behavior. Primarily, the effects of different wick performances were investigated under various operating conditions. The resulting surface temperatures depicted in the time to steady performance and the dry-out behavior revealed the conditions to improve the HPs design. The thermal resistance decreased from 0.6 K/W (at 25 W) to 0.05 K/W (at 200 W) by increasing the HP diameter from 6 to 10 mm; these values are relative to those of copper rods, which decrease from 2.70 K/W (at 25 W) to 0.40 K/W (at 200 W). Non-linear and linear temperature responses were recorded when the HPs diameter and length were varied. Compared to conventional mesh and groove wicks, the composite groove-sintered, mesh-sintered, and groove-mesh wicks recorded lower thermal resistance with distinctively faster startup times, lower startup temperatures, better temperature uniformity and less dynamic instability. Tilting the HPs relative to the horizontal position lessens failure tendencies. Usually, dynamic responses are typically first-order under the conditions studied. Hence, proper sizing of HPs and correct wick selection can improve their performance