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Rotating Vortex Rope (RVR) resulting from flow instabilities inside draft tube affects the hydraulic turbine’s efficiency and wear. Among the passive and active RVR mitigation methods, the water injection method has been widely investigated lately. The present research focuses on the dynamics of RVR mitigation using the water injection method and the optimization of exergy loss during the water injection. The numerical simulations are performed using a reduced turbine geometry consisting of one stay vane, two guide vanes, one runner passage by applying periodic boundary conditions at side boundaries, and a complete draft tube. The pressure fluctuations, velocity field, and the RVR structure are well captured using the Shear Stress Transpo
In this study, the asphaltene deposition and the fouling layer growth are modeled using the multi-fluid approach. The crude oil is considered as a multi-species fluid which the asphaltene species can deposit on the heat transfer surfaces due to the volume reaction. The effective mechanisms of asphaltene particle transport are the molecular and eddy diffusions. Therefore the species modeling approach is utilized for particles transport by diffusion processes in the carrier phase. To model the fouling layer growth without considering an initial thickness for the deposited layer, extra terms are added to the momentum equations of primary and secondary phases. The results of the numerical model are validated against available experimental data
The closed-circuit cooling tower is described as the combination of both wet and dry cooling towers that hot water passes through the bundle of tubes as in the dry cooling towers and surrounding air passes around them in a forced or natural regimes. Thus, secondary water circulates as an open cycle and is sprayed on the bundle of tubes to preserve the tower cooling process. In the present research, the operation of a model of the closed-circuit wet cooling tower has been investigated numerically and experimentally. The effects of environmental condition on process water temperature, sprayed water temperature and air temperature have been evaluated, and the mass and heat transfer coefficients on the surface of hot water tubes have been calcu
Closed circuit cooling tower is described as the combination of both wet and dry cooling tower that hot water passes through the bundle of tubes as in the dry cooling towers and surrounding air passes around them in a forced or natural regimes. Thus, secondary water circulates as an open cycle and is sprayed on the bundle of tubes to preserve the tower cooling process. In the current study, operation of a model of closed circuit wet cooling tower is investigated numerically and experimentally. Effects of environmental condition on process water temperature, sprayed water temperature and air temperature are evaluated, and the mass and heat transfer coefficients on the surface of hot water tubes are calculated. According to these results, sur
Accumulation of ice particles inside the pipes of an ice slurry refrigeration system is a problem that should be avoided to ensure the efficient performance of the system. In the present research, the swirling flow of ice slurry is numerically investigated using the Eulerian-Eulerian model coupled with the kinetic theory of granular flow. The swirling flow is generated by twisted tapes inserted into a smooth pipe. The results show that by swirling the flow, the ice particle diameter may be increased up to about three times larger than the typical particle diameter used in the ice slurry, without particle accumulation. Moreover, for the swirling flow of ice slurry with the ice particle diameter of 0.3 mm, the rate of mass transfer increase
In this paper, a parametric study of anode channel clogging of a proton exchange membrane fuel cell (PEMFC) is presented using the numerical volume of fluid method. The droplet size distribution in the present research is considered close to reality. Therefore, three droplets with different sizes are initially placed on the anode channel wall. The droplet sizes are obtained based on the distribution of the saturated liquid water along the whole channel under a specific working condition and the amount of condensed water in the channel, using the PEMFC code. Finally, the dynamic behavior of the droplets is investigated and compared to that of a single droplet with the same total volume. Numerical results show that, in the case of three dropl
Radiant cooling panels are believed to be of considerable energy saving potentials and can be employed as new cooling devices in HVAC systems. Even though the popularity of radiant cooling systems grows, particularly for cooling and heating commercial buildings, the condensation problem limits their application. The present research is conducted to introduce and evaluate a new method to control the condensation problem in the Ceiling Radiant Cooling Panels (CRCP). A dehumidification coil is integrated with CRCP in order to regulate the water condensation and remove the latent heat load. To evaluate the capability of the new model, the performance of the system was numerically investigated in a sample office, for different wall insulation an
Nucleate boiling is an important part of the pool boiling phenomenon which occurs in various processes involving heat transfer, such as, steam production, chemical processes, etc. The inclination of the heated surface, where bubbles nucleates, significantly affects the bubble growth dynamics as well as the heat transfer rate from the microlayer underneath the bubble during the nucleate boiling. In this study, the effect of the surface inclination on the bubble growth and detachment during the nucleate boiling is investigated numerically. For this purpose, the proposed model of Lay and Dhir for the microlayer is modified to include the effect of the inclination of the heated surface. The resulting equations are solved numerically, and the ef
In this paper, water droplet behavior and its removal process in the anode gas flow channel (GFC) of a proton exchange membrane fuel cell (PEMFC) with partial blockage are numerically investigated using a three-dimensional volume of fluid model. The governing equations are solved using the finite volume method. The effects of the droplet emergence location (, blockage ratio (, longitudinal ratio (, contact angle ( and current density on the water management characteristics, such as droplet shape, removal time, instantaneous and time-averaged pressure drop and water coverage ratio (WCR), are studied. Researchers have already considered the blocking effect on the PEMFC performance without considering the water droplet movement, but the resul
Original Research The main disadvantage of natural draft dry cooling towers is the influence of atmospheric conditions such as ambient temperature and wind speed on the thermal performance. Wind disrupts the natural flow of air inside the tower creating vortices at the back and inside the tower that disrupts the air flow structure. When the wind blows, increasing the velocity of inlet air through the front louvers causes the air to pass through the behind louvers rather than outlet opening. The negative effect of this phenomenon reduces the cooling performance and consequently reduces the turbine production power in power plants. A good solution to this problem is to adjust the Louvers angle correctly. Therefore, in the present study, the t
This research presents a numerical investigation of two-phase flow during the expulsion of entrapped air in a non-confined pipe. A modified version of the volume of fluid (VOF) approach is employed considering the effect of compressibility in the liquid. A modification is introduced to the original approach relating the density changes in the liquid to the pressure changes using the fluid bulk modulus. The bulk modulus is also modified to consider the pipe elasticity, the air bubble entrainment and the two-phase flow regime in a pipe. Fluid–structure interaction (FSI) code is developed and used to calculate the motion of the downstream orifice wall during the impact of the water column on the pipe end wall. The numerical results of the pr
Today, increasing the efficiency and optimization of energy systems in terms of economic and environmental conditions is of particular importance. So far, several methods have been proposed to increase the heat transfer in thermal systems, including the use of nanofluids and types of fluid flow turbulators. In this research, the application of both nanofluid and twisted tape to improve the heat transfer coefficient were numerically investigated. Different turbulence models were used to simulate fluid turbulence. The results showed that increasing the nanoparticle volume fraction, reducing the twisting ratio, and increasing the Reynolds number resulted in an increase in heat transfer. By reducing the twisting ratio from 15 to 5, the heat tra
Nucleate boiling is an important part of pool boiling process. Heat transfer from the microlayer plays a considerable role in heat transfer to the fluid. The axisymmetric assumption of the microlayer for a horizontal surface needs to be evaluated for an inclined one. In this study, the effect of surface orientation on the microlayer thickness and the heat transfer rate are investigated numerically. The governing equations are simplified employing scaling analysis. The results for the microlayer thickness, the heat flux and the total heat transfer rate for the heated surface are obtained and presented. The asymmetry of the microlayer increases as the surface inclination angle varies from horizontal to vertical. Even though, the driving force
Nucleate boiling is an important part of pool boiling process. Heat transfer from the microlayer plays a considerable role in heat transfer to the fluid. The axisymmetric assumption of the microlayer for a horizontal surface needs to be evaluated for an inclined one. In this study, the effect of surface orientation on the microlayer thickness and the heat transfer rate are investigated numerically. The governing equations are simplified employing scaling analysis. The results for the microlayer thickness, the heat flux and the total heat transfer rate for the heated surface are obtained and presented. The asymmetry of the microlayer increases as the surface inclination angle varies from horizontal to vertical. Even though, the driving force
In this research, the results of comparative analysis of a single fuel droplet evaporation models are presented. Three well-known evaporation models including Spalding, Borman-Johnson and Abramzon-Sirignano models are analyzed using Computational Fluid Dynamic (CFD). The original Spalding model is extended to consider the effects of the Stefan flow, unsteady vaporization, and variable properties. The evaporation models are validated using already existing experimental data. Numerical results show that the Spalding model overestimates the temperature of the droplet surface in comparison with the other two models, although some modifications were made in the aforementioned model. Our final evaluation concludes that Abramzon-Sirignano model pr