Numerical Study of Water-air Ejector using Mixture and Two-phase Models

A computational fluid dynamics (CFD) simulation of laminar convection of Al 2 O 3–water bio-nanofluids in a circular tube under constant wall temperature conditions was conducted, employing a single-phase model and three different two-phase models (volume of fluid (VOF), mixture and Eulerian). The steady-state, three-dimensional flow conservation equations were discretised using the finite volume method (FVM). Several parameters such as temperature, flow field, skin friction and heat transfer coefficient were computed. The computations showed that CFD predictions with the three different two-phase models are essentially the same. The CFD simulations also demonstrated that single-phase and two-phase models yield the same results for fluid flow but different results for thermal fields. The two-phase models, however, achieved better correlation with experimental measurements. The simulations further showed that heat transfer coefficient distinctly increases with increasing nanofluid particle concentration. The physical properties of the base fluid were considered to be temperature-dependent, while those of the solid particles were constant. Grid independence tests were also included. The simulations have applications in novel biomedical flow processing systems.

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A comparative analysis on the single-phase and two-phase mixture models for calculation of ferrofluid convective heat transfer in the presence of a magnetic field: a numerical study

International Journal of Numerical Methods for Heat &amp Fluid Flow ◽

10.1108/hff-05-2021-0358 ◽

2021 ◽

Vol ahead-of-print (ahead-of-print) ◽

Author(s):

Hamed Jafari ◽

Mohammad Goharkhah ◽

Alireza Mahdavi Nejad

Keyword(s):

Magnetic Field ◽

Heat Transfer ◽

Convective Heat Transfer ◽

Convective Heat ◽

Single Phase ◽

Numerical Study ◽

Phase Method ◽

Two Phase ◽

Content Type ◽

Phase Models

Purpose This paper aims to analyze the accuracy of the single and two-phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field. The findings of current study are compared with previous single-phase numerical results and experimental data. Accordingly, the effect of various parameters including nanoparticles concentration, Reynolds number and magnetic field strength on the performance of the single and two-phase models are evaluated. Design/methodology/approach A two-phase mixture numerical study is carried out to investigate the influence of four U-shaped electromagnets on the hydrodynamic and thermal characteristics of Fe3O4/Water ferrofluid flowing inside a heated channel. Findings It is observed that the applied external magnetic field signifies the convective heat transfer from the channel surface, despite local reduction at a few locations. The maximum heat transfer enhancement is predicted as 23% and 25% using single and two-phase models, respectively. The difference between the results of the two models is mainly attributed to the slip velocity effect which is accounted for in the two-phase model. The magnetic field gradient leads to a significant increase in the slip velocity which in turn causes a slight difference in velocity and temperature profiles obtained by the single and two-phase models in the magnetic field region. According to percentage error calculation, the two-phase method is generally more accurate than the single-phase method. However, the percentage error of both models improves by decreasing either magnetic field intensity or Reynolds number. Originality/value For the first time in the literature, to the best of the authors’ knowledge, the current work analyzes the accuracy of the single and two phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field.

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Numerical Study of Void Fraction and Critical Heat Flux in Subcooled Two Phase Flow Boiling in Boiler Tube

Heat Transfer Research ◽

10.1615/heattransres.2018021156 ◽

2018 ◽

Author(s):

Vimal Sain ◽

Sandip Kumar Saha

Keyword(s):

Heat Flux ◽

Critical Heat Flux ◽

Void Fraction ◽

Two Phase Flow ◽

Numerical Study ◽

Flow Boiling ◽

Phase Flow ◽

Boiler Tube ◽

Two Phase

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Numerical Study of a Cooling System Using Phase Change of a Refrigerant in a Thermosyphon

Energies ◽

10.3390/en14123634 ◽

2021 ◽

Vol 14 (12) ◽

pp. 3634

Author(s):

Grzegorz Czerwiński ◽

Jerzy Wołoszyn

Keyword(s):

Mass Transfer ◽

Phase Change ◽

Heat Dissipation ◽

Cooling System ◽

Numerical Study ◽

Relaxation Parameter ◽

Phase Changes ◽

Transfer Time ◽

Two Phase ◽

Time Relaxation

With the increasing trend toward the miniaturization of electronic devices, the issue of heat dissipation becomes essential. The use of phase changes in a two-phase closed thermosyphon (TPCT) enables a significant reduction in the heat generated even at high temperatures. In this paper, we propose a modification of the evaporation–condensation model implemented in ANSYS Fluent. The modification was to manipulate the value of the mass transfer time relaxation parameter for evaporation and condensation. The developed model in the form of a UDF script allowed the introduction of additional source equations, and the obtained solution is compared with the results available in the literature. The variable value of the mass transfer time relaxation parameter during condensation rc depending on the density of the liquid and vapour phase was taken into account in the calculations. However, compared to previous numerical studies, more accurate modelling of the phase change phenomenon of the medium in the thermosyphon was possible by adopting a mass transfer time relaxation parameter during evaporation re = 1. The assumption of ten-fold higher values resulted in overestimated temperature values in all sections of the thermosyphon. Hence, the coefficient re should be selected individually depending on the case under study. A too large value may cause difficulties in obtaining the convergence of solutions, which, in the case of numerical grids with many elements (especially three-dimensional), significantly increases the computation time.

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Numerical Study on an Interface Compression Method for the Volume of Fluid Approach

Fluids ◽

10.3390/fluids6020080 ◽

2021 ◽

Vol 6 (2) ◽

pp. 80

Author(s):

Yuria Okagaki ◽

Taisuke Yonomoto ◽

Masahiro Ishigaki ◽

Yoshiyasu Hirose

Keyword(s):

Linear Theory ◽

Volume Fraction ◽

Numerical Study ◽

Volume Of Fluid ◽

Interfacial Wave ◽

Validation Process ◽

Two Phase ◽

Numerical Diffusion ◽

Mesh Resolution ◽

Water Reactors

Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in the convection term of the volume fraction equation. Thus, in this study, we focused on an interface compression (IC) method for such a VOF approach; this technique prevents numerical diffusion issues and maintains boundedness and conservation with negative diffusion. First, on a sufficiently high mesh resolution and without the IC method, the validation process was considered by comparing the amplitude growth of the interfacial wave between a two-dimensional gas sheet and a quiescent liquid using the linear theory. The disturbance growth rates were consistent with the linear theory, and the validation process was considered appropriate. Then, this validation process confirmed the effects of the IC method on numerical diffusion, and we derived the optimum value of the IC coefficient, which is the parameter that controls the numerical diffusion.

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