Papers by Keyword: Two-Phase Flow

Abstract: The literature about various modifications to the vortex generators as passive techniques and some combined techniques was discussed numerically and experimentally. In general, all vortex generators have a good coefficient of thermal performance compared to a plain tube. Most vortex generators don’t consume any amount of energy to perform their function, so can be considered a good tool to enhance the thermal performance coefficient in industrial applications, but they have the disadvantage of high pressure drop. When using vortex generators in two phase flow accelerated the process of transition between phases pattern in addition to increasing heat transfer. The inserted twisted tapes have a low pressure drop and good heat transfer efficiency, resulting in a thermal performance factor of 1.6.In addition, the easier to manufacture where can be changed in size and shape in proportion to the place of use inside the tubes also having a low cost, and do not consume any amount of energy to perform its function, the hybrid methods can be used with it, such as a twisted tape with nonfluids, which had proven effective in thermal performance coefficients, but their disadvantages include the high cost and risk of dealing with these materials ,bubble injectors can also be used, despite the fact that they require energy to operate. So, the twisted tapes inserted can be considered a best tool to be used in enhancing the thermal performance coefficient in various Industrial fields.Based on the reviewed literature, it was determined that the following research is limited and could garner more attention in the future: multiple coaxial cross-twisted tapes with different perforations, and twisted tapes with variable cross-section, typical twisted tapes with regular wavy edge, hybrid methods such as nanofluids and twisted tape can be used in multiphase flow.

Abstract: Effective cooling of the electronic devices is of great importance to their performance. Of particular interest is when the size of the equipment has scaled down to micro level. Despite a continuous improvement over the years, there is only a handful information on gas-liquid flow in wavy microchannels. This paper presents an experimental study of the air-water two-phase flow in a wavy microchannels heatsink typically used for cooling of electronic devices. The heatsink was made from copper with 26 wavy channels. The width and the depth of each channel were 800 and 500 μm, respectively. Two cartridge heaters were used to generate a constant heat flux. Pressure and temperatures of the working fluid at the inlet and outlet of the test section were measured using a pressure transducer and thermocouples. To evaluate the benefit of using the wavy microchannels, experiment was performed using water as the working fluid. The results showed that the wavy microchannels could elevate the Nusselt number by up to 112% compared to that obtained from the straight microchannels. Experiments at various air and water flowrates in the wavy microchannels suggested that the advantages of using the air-water flow would be obvious at the liquid Reynolds number above 379.

Abstract: This work presents the development of a computational model for the simulation of an Oscillating Water Column device that converts wave flow into electrical energy. The device is placed into a wave channel and a Savonius turbine is inserted in the inlet/outlet duct of the converter. The modeling of the turbine is performed with a rotational moving mesh that simulates the turbine movement in stabilized operating conditions. This coupling provides the minimization of simplifying assumptions, addressing in a single problem the two phenomena inherent to the device approach: the two-phase, incompressible and turbulent flow of air and water in a wave channel containing the oscillating water column device and the incompressible and turbulent airflow passing through a rotational turbine. The computational model was verified/validated for a free stream turbulent flow over a Savonius turbine and verified for the case of wave flow over a converter without the inserted turbine. Results showed that the coupled model allowed obtaining not only available power but also mechanical power in the turbine. For the rotation imposed in the domain, the turbine did not affect the behavior of the wave flow that impinges on the chamber of the OWC device. An augmentation of the power coefficient of the turbine in comparison with turbines subjected to free stream flows was obtained, showing that the fairing of turbine can led to increased power takeoff.

Abstract: A two-phase flow is the simplest form of multi-phase flow. Two-phase flow has basic parameters, including flow behavior, flow pattern, void fraction, pressure gradient, or pressure drop. The development and application of fluid flow in small channels (mini and micro) are in the Micro Electromechanical System (MEMS). In general, the application of two-phase flow can be found in various applications in the industrial world, such as boilers, cores, and steam generators in nuclear reactors, petroleum transportation, electronic cooling, and different other types of chemical reactors. One of the most common challenges is void fractions and flow patterns in horizontal and vertical pipelines. The purpose of this study was to find out the significant difference in the characteristic value of fundamental parameters, primarily void fractions and flow patterns that occur in capillary pipes with a slope of 15o to horizontal. The fraction value and shape of this flow pattern are essential because it determines the pressure drop value, which means determining the level of danger that can occur if the pressure drop value reaches the extreme value. The fluid used a mixture of air-water and glycerin with 0, 10, 20, and 30% concentrations. To determine the void fraction is used digital image processing method with MATLAB R2014a application. A 1200 fps high-speed camera and 640 x 480-pixel resolution are used for video shooting. The experiment was conducted at superficial gas (JG) velocities of 0.025 - 66.3 m/s and superficial fluid velocities (JG) intervals of 0.033 – 4.935 m/s. The experiment results showed a significant difference between the void fraction value for the bubbly flow pattern and the churn flow pattern, which has not been widely discussed in previous studies. The analysis concluded that the higher the velocity of superficial gases, the void fraction value increases. This increase in the void fraction is why there is a significant change in flow patterns from bubble flow patterns to churn. In addition, it was also found that the increased viscosity of the fluid significantly affected changes in flow patterns, incredibly bubbly and plug, but its velocity decreased. It is also the result that the higher homogeneous void fraction (β) value affects the increase in the length of bubbly flow patterns and plugs.

Abstract: The investigation on the interaction between solid and fluid under combined convective flow has been carried out mathematically. The Jeffrey fluid model is taken as the fluid phase and the model is being embedded with the dust particles (solid phase). This two-phase model is constructed by introducing the fluid-particles interaction forces in the momentum equations of the fluid and dust phases, respectively. The natural and forced convections together with the aligned magnetic field are considered on the fluid flow. Also, the Newtonian heating as thermal boundary condition is induced on the vertical stretching sheet. In order to reduce the complexity of the model, the governing equations are transformed from partial differential equation into ordinary differential equation via suitable similarity transformation. The solutions are obtained in terms of velocity and temperature profiles for the fluid and particles phases respectively whereby the Keller-box method is utilized to obtain the desired outcomes. The influences of several significant physical parameters are visualized graphically to clarify the flow and heat transfer characteristic for both phases. The investigation found that the fluid’s velocity is affected by the presence of the dust particles which led to decelerate the fluid transference. The present flow model is able to be compared with the single-phase fluid cases if the fluid-particle interaction parameter is ignored.

Abstract: Mathematical model has been analyzed on MHD convective two-phase flow in a divergent channel with viscous dissipation. The effects of velocity and temperature slip are considered. The relevant governing equations are non-dimensionalised with the help of appropriate transformations and then solved numerically. The present results overlooked with existing results and found in an excellent agreement. Effects of emerging parameters on the flow are discussed and demonstrated graphically. Graphical aid is also used to present the variations in skin friction and Nusselt number. It is found that an increase in velocity slip, the velocities of fluid as well as particle phases are increased. An increase in temperature slip, the temperature of fluid phase gets decreased. KEY WORDS:Particulate suspension, two-phase flow, diverging channel,Slip effects

Abstract: The transition of one flow regime into another is a very common phenomena in pipeline networks, which can be potentially hazardous for the structural integrity of the pipeline. Literature review showed that there is almost no reported detail investigation of transitional flow whereby the fluid constituents change from one regime to another especially slug transition. Most of the open research papers focused on slug flow regime and its liquid holdup in horizontal pipelines and channels have been carried out on experimental test rigs. The objective of this study is to explore oil-gasoil vapor slug transition and its liquid holdup in a 3.15 inch diameter horizontal straight pipe. The abrupt change in gas/liquid velocities, which causes transition of flow patterns is analyzed using incompressible Volume of Fluid (VOF) method, along with Piecewise Linear Interface Construction (PLIC) technique to capture the sharp front of segregated gas-liquid interface. Slug liquid holdup derived from the present numerical model is compared to existing experimental correlations in the literature.

Abstract: By changing air to water flow rates of a two-phase flow through a minichannel we identified aggregation and partitioning of air bubbles and slugs of various sizes. It was found that for some flowconditions air bubbles formed periodic patterns. The identification of the spatio-temporal behaviourswas performed with a laser transitivity sensor and confirmed by a digital camera. We used the Hurstexponent to distinguish instabilities in air slugs, their breakups and aggregations. In addition, we performed noise level estimation.

Abstract: Oil transport is used mainly by pipeline networks to transport oil from refineries and distributions points to the consumers. This is the main way to transport oils especially in areas of difficult access, ensuring efficiency, lowest cost and safety. In the chemical and petroleum industry it is possible to observe the presence of leak in the pipes, which has stimulated the development of reliable techniques for the rapid and accurate detection of leaks along the pipe in order to eliminate or minimize loss and environmental damage. In this context, this study aims to evaluate the effect of the numerical presence of leakage of two-phase flow (oil-water) pipe connections using the commercial software ANSYS CFX. The results from the fields of pressure, velocity and volume fraction are presented and assessed for illustrating the effect of the presence of the leak in the dynamic flow in the pipe with a curved connection.

Abstract: The subject of this report is creation of three-dimensional thermal hydraulic model of horizontal steam generator for Dukovany nuclear power plant. A procedure is presented for simulation and analysis of secondary side of PGV-440 steam generator for nominal and increased reactor power. A two-fluid approach is applied for modeling physical processes inside the steam generator. Physical models were implemented in ANSYS Fluent CFD environment using User Defined Functions (UDFs). Results from this thermal hydraulic numerical model can be used for various other subsequent nuclear power plant operations and safety analysis.