Passive Control of Two-Phase Flow Thermal Instabilities in a Vertical Tube Evaporator

2016 ◽  
Vol 8 (4) ◽  
Author(s):  
Ghazali Mebarki ◽  
Samir Rahal
Keyword(s):  
Heat Transfer ◽  
Void Fraction ◽  
Passive Control ◽  
Two Phase Flow ◽  
Tube Wall ◽  
Vertical Tube ◽  
Control Technique ◽  
Control Element ◽  
Phase Flow ◽  
Two Phase

Passive heat transfer techniques are considered to be one of the most important means to enhance heat transfer in heat exchangers that allow also reducing their size and manufacturing cost. Moreover, this passive technique can also be used to control the thermal instabilities caused by the two-phase flow in the evaporators. The thermal instabilities are undesirable because they can lead to a tube failure. For this purpose, a numerical study of the two-phase flow with evaporation in a vertical tube has been performed in this work. The volume of fluid (VOF) multiphase flow method has been used to model the water vapor–liquid two-phase flow in the tube. A phase-change model, for which source terms have been added in the continuity and energy equation, has been used to model the vaporization. The numerical simulation procedure was validated by comparing the obtained results with those given in the literature. The passive control technique used here is a ring element with square cross section, acting as a vortex generator, which is attached to the tube wall at various positions along the tube. Instabilities of temperature and void fraction at the tube wall have been analyzed using fast Fourier transforms (FFTs). The results show that the attachment of the control element has a significant influence on the value and distribution of the void fraction. Higher positions of the control element along the tube allow reducing the magnitude of void fraction oscillations.

Analogy Between Heat Transfer and Fluid Friction of Gas-Liquid Two-Phase Flow in Minichannel

2003 ◽  
Author(s):  
Masuo Kaji ◽  
Toru Sawai ◽  
Koji Mori
Keyword(s):  
Heat Transfer ◽  
Void Fraction ◽  
Two Phase Flow ◽  
Experimental Results ◽  
Stainless Steel Tube ◽  
Good Prediction ◽  
Phase Flow ◽  
Two Phase ◽  
Fluid Friction ◽  
Drift Flux Model

To investigate the analogy between heat transfer and fluid friction of two-phase flow in minichannel, heat transfer coefficient, pressure drop and void fraction were simultaneously measured for vertical upward heated air-water two-phase flow in a 2.01 mm I.D. stainless steel tube. Void fraction was slightly different from Smith’s correlation and drift flux model, but the latter gave fairly good prediction when α>0.8. Frictional pressure losses were greater than the experimental results of Mishima-Hibiki for a 2.05 mm I.D. Pyrex glass tube and the correlation of Chisholm-Laird. Heat transfer coefficients agreed with our previous experimental results for a 8.03 mm I.D. tube. A theoretical calculation was performed for an annular liquid film flow model using experimental values of wall shear stress and liquid holdup, and satisfactory results were obtained except when the liquid flow rate was very low.

scholarly journals Application of the 2-D Trefftz Method for Identification of Flow Boiling Heat Transfer Coefficient in a Rectangular MiniChannel

Energies ◽  
2020 ◽  
Vol 13 (15) ◽  
pp. 3973
Author(s):  
Mirosław Grabowski ◽  
Sylwia Hożejowska ◽  
Beata Maciejewska ◽  
Krzysztof Płaczkowski ◽  
Mieczysław E. Poniewski
Keyword(s):  
Heat Transfer ◽  
Void Fraction ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Flow Structures ◽  
Outlet Temperature ◽  
Phase Flow ◽  
Two Phase ◽  
Trefftz Method

The study presents the experimental and numeric heat transfer investigations in flow boiling of water through an asymmetrically heated, rectangular and horizontal minichannel, with transparent side walls. A dedicated system was designed to record images of two-phase flow structures using a high-speed video camera with a synchronous movement system. The images were analyzed with Matlab 2019a scripts for determination of the void fraction for each pattern of two-phase flow structures observed. The experimental data measured during the experimental runs included inlet and outlet temperature, temperature at three internal points of the heater body, volume flux of the flowing water, inlet pressure, pressure drop, current and the voltage drop in the heater power supply. The flows were investigated at Reynolds number characteristic of laminar flow. The mathematical model assumed the heat transfer process in the measurement module to be steady-state with temperature independent thermal properties of solids and flowing fluid. The defined two inverse heat transfer problems were solved with the Trefftz method with two sets of T- functions. Graphs were used to represent: the boiling curves, the local void fraction values, the boiling heat transfer coefficients and the errors of both of them for selected mass fluxes and heat fluxes.

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