Investigation on Variations of Void Fraction in a Subcooled Boiling Channel Under Vertical Forced Excitations

2021 ◽  
Vol 143 (6) ◽  
Author(s):  
Shao-Wen Chen ◽  
Wei-Cheng Lee ◽  
Yu-Hsien Chang ◽  
Ailing Ho ◽  
Jin-Der Lee ◽  
...  
Keyword(s):  
Heat Flux ◽  
Void Fraction ◽  
Experimental Tests ◽  
Dominant Frequency ◽  
Heat Fluxes ◽  
Fluid Temperature ◽  
Subcooled Boiling ◽  
Wall Superheat ◽  
Boiling Flow ◽  
Near Wall

Abstract Experimental tests were carried out to investigate the vertically forced excitation effects on the subcooled boiling flow. The heated circular channel with an inner diameter of 11.9 mm was operated with various heat fluxes (q″ ≈ 14.6–41.1 kW/m2) and inlet flow conditions (vin ≈ 0.21–0.42 m/s) under various vertical forced excitations (f ≈ 0–1.63 Hz), and the time variations of void fraction, near-wall fluid temperature and pressure were recorded during the tests. Fast Fourier transform (FFT) was applied to extract the dominant frequency from the transient signals, and the variations of averages and standard deviations of test data were obtained for analysis. Under lower heat flux, lower flow, and lower void conditions, the time-averaged void fraction may decrease under forced excitations, and the dominant frequencies of void variations were identical to those of forced excitations. However in higher heat flux and higher void conditions, the void fraction can slightly increase under forced excitations, but the excitation frequencies may not be clearly observed in the void FFT plots. In general, the transient and time-averaged void fraction can be affected by forced excitations, and the void variation trends are similar to those of near-wall fluid temperature, which implies the void variations may be related to the changes of thermal boundary layer thickness. Besides, the potential variations of void fraction were estimated by assuming changes of heat transfer coefficient and/or wall superheat, which appear similar trends to the observed void variations in the present tests.

A Mechanistic Model for Predicting Subcooled Boiling Flow

2003 ◽  
Author(s):  
G. H. Yeoh ◽  
J. Y. Tu
Keyword(s):  
Void Fraction ◽  
Liquid Velocity ◽  
Fluid Model ◽  
Mechanistic Model ◽  
Three Dimensional ◽  
Annular Channel ◽  
Heat Fluxes ◽  
Group Model ◽  
Subcooled Boiling ◽  
Boiling Flow

Population balance equations combined with a three-dimensional two-fluid model are employed to predict subcooled boiling flow at low pressure in a vertical annular channel. The MUSIG (MUltiple-SIze-Group) model implemented in CFX4.4 is extended to account for the wall nucleation and condensation in the subcooled boiling regime. Comparison of model predictions against local measurements is made for the void fraction, bubble Sauter diameter and gas and liquid velocities covering a range of different mass and heat fluxes and inlet subcoolings. Good agreement is achieved with the local radial void fraction, bubble Sauter diameter and liquid velocity profiles against measurements. However, significant weakness of the model is evidenced in the prediction of the vapor velocity. Work is in progress to circumvent the deficiency of the extended MUSIG model by the consideration of an algebraic slip model to account for bubble separation.

scholarly journals Investigation of the Turbulent Near Wall Flame Behavior for a Sidewall Quenching Burner by Means of a Large Eddy Simulation and Tabulated Chemistry

Fluids ◽  
2018 ◽  
Vol 3 (3) ◽  
pp. 65 ◽  
Author(s):  
Arne Heinrich ◽  
Guido Kuenne ◽  
Sebastian Ganter ◽  
Christian Hasse ◽  
Johannes Janicka
Keyword(s):  
Heat Flux ◽  
Large Eddy Simulation ◽  
Gas Turbines ◽  
Internal Combustion Engines ◽  
Heat Fluxes ◽  
Maximum Heat ◽  
Eddy Simulation ◽  
Tabulated Chemistry ◽  
Large Eddy ◽  
Near Wall

Combustion will play a major part in fulfilling the world’s energy demand in the next 20 years. Therefore, it is necessary to understand the fundamentals of the flame–wall interaction (FWI), which takes place in internal combustion engines or gas turbines. The FWI can increase heat losses, increase pollutant formations and lowers efficiencies. In this work, a Large Eddy Simulation combined with a tabulated chemistry approach is used to investigate the transient near wall behavior of a turbulent premixed stoichiometric methane flame. This sidewall quenching configuration is based on an experimental burner with non-homogeneous turbulence and an actively cooled wall. The burner was used in a previous study for validation purposes. The transient behavior of the movement of the flame tip is analyzed by categorizing it into three different scenarios: an upstream, a downstream and a jump-like upstream movement. The distributions of the wall heat flux, the quenching distance or the detachment of the maximum heat flux and the quenching point are strongly dependent on this movement. The highest heat fluxes appear mostly at the jump-like movement because the flame behaves locally like a head-on quenching flame.

Radiation and Convection Heat Transfer in a Porous Bed

1968 ◽  
Vol 90 (1) ◽  
pp. 51-54 ◽  
Author(s):  
W. A. Beckman
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Collection Efficiency ◽  
Convection Heat Transfer ◽  
Heat Fluxes ◽  
Fluid Temperature ◽  
Radiation Heat ◽  
Radiation Heat Flux ◽  
Flowing Fluid ◽  
Porous Bed

The one-dimensional steady-state temperature distribution within an isotropic porous bed subjected to a collimated and/or diffuse radiation heat flux and a transparent flowing fluid has been determined by numerical methods. The porous bed was assumed to be nonscattering and to have a constant absorption coefficient. Part of the radiation absorbed by the porous bed is reradiated and the remainder is transferred to the fluid by convection. Due to the assumed finite volumetric heat transfer coefficient, the bed and fluid have different temperatures. A bed with an optical depth of six and with a normal incident collimated radiation heat flux was investigated in detail. The radiation incident on the bed at the fluid exit was assumed to originate from a black surface at the fluid exit temperature. The investigation covered the range of incident diffuse and collimated radiation heat fluxes expected in a nonconcentrating solar energy collector. The results are presented in terms of a bed collection efficiency from which the fluid temperature rise can be calculated.

Analysis of RELAP5 Drift-Flux Model for Vertical Subcooled Boiling Flow at Low Pressure Conditions

2000 ◽  
Author(s):  
Boštjan Končar ◽  
Ivo Kljenak ◽  
Borut Mavko
Keyword(s):  
Void Fraction ◽  
Drift Velocity ◽  
Bubbly Flow ◽  
Low Pressure ◽  
Subcooled Boiling ◽  
Drift Flux Model ◽  
Drift Flux ◽  
Boiling Flow ◽  
Flux Model ◽  
Relap5 Code

Abstract The RELAP5/MOD3.2.2 Gamma code was assessed against low pressure boiling flow experiments performed by Zeitoun and Shoukri (1997) in a vertical annulus. The predictions of subcooled boiling bubbly flow showed that the present version of the RELAP5 code underestimates the void fraction increase along the flow and strongly overestimates the vapor drift velocity. It is shown that in the calculations, a higher vapor drift velocity causes a lower interphase drag and may be a possible reason for underpredicted void fraction development. A modification is proposed, which introduces the replacement of the EPRI drift-flux formulation, which is currently incorporated in the RELAP5 code, with the Zuber-Findlay (1965) drift-flux model for the experimental low pressure conditions of the vertical bubbly flow regime. The improved experiment predictions with the modified RELAP5 code are presented and analysed.

Numerical Prediction of Flow Patterns in Bubble Pumps

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Ali Benhmidene ◽  
Bechir Chaouachi ◽  
Mahmoud Bourouis ◽  
Slimane Gabsi
Keyword(s):  
Heat Flux ◽  
Void Fraction ◽  
Liquid Velocity ◽  
Fluid Model ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
Tube Diameter ◽  
Tube Length ◽  
Velocity Versus ◽  
Bubble Pump

In the present study, the ammonia-water mixing flow in a bubble pump is numerically simulated. The flow patterns of a two-phase flow in a bubble pump were studied under different conditions of heat flux and tube diameter. A one-dimensional two-fluid model was developed under constant heat flux. This model was used to predict the variations in void fraction and liquid and vapor velocities throughout the tube. Then, the void fraction profile and the curve of liquid velocity versus vapor velocity were used to predict the flow patterns along the tube length. It was found that at heat fluxes below 15 kW m−2, bubbly, slug, and churn flows are the dominating regimes, and the length of these flow regimes depends on the tube diameter. For heat fluxes higher than 15 kW m−2, the bubble pump operates under the churn and annular regimes, and the bubble pump performance is improved when the tube diameter increases.

Inverse Design of Cooling of Electronic Chips Subject to Specified Hot Spot Temperature and Coolant Inlet Temperature

2015 ◽  
Author(s):  
Sohail R. Reddy ◽  
George S. Dulikravich
Keyword(s):  
Heat Flux ◽  
Hot Spot ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Heat Transfer Analysis ◽  
Maximum Temperature ◽  
Fluid Temperature ◽  
Cooling Fluid ◽  
Pin Fin ◽  
The Given

Most methods for designing electronics cooling schemes do not offer the information on what levels of heat fluxes are maximally possible to achieve with the given material, boundary and operating conditions. Here, we offer an answer to this inverse problem posed by the question below. Given a micro pin-fin array cooling with these constraints: - given maximum allowable temperature of the material, - given inlet cooling fluid temperature, - given total pressure loss (pumping power affordable), and - given overall thickness of the entire electronic component, find out the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is completely removed by the cooling fluid. This problem was solved using multi-objective constrained optimization and metamodeling for an array of micro pin-fins with circular, airfoil and symmetric convex cross sections that is removing all the heat inputted via uniform background heat flux and by a hot spot. The goal of this effort was to identify a cooling pin-fin shape and scheme that is able to push the maximum allowable heat flux as high as possible without the maximum temperature exceeding the specified limit for the given material. Conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto frontier of best trade-off solutions. The Pareto optimized configuration indicates the maximum physically possible heat fluxes for specified material and constraints.

Rewetting Velocity of High-Temperature Vertical-Narrow-Annular Flow Passages Under Counter-Current Flow Condition

2003 ◽  
Author(s):  
Yasuo Koizumi ◽  
Hiroyasu Ohtake ◽  
Masanori Tsukudo ◽  
Naoki Sakamoto
Keyword(s):  
Heat Flux ◽  
Annular Flow ◽  
Pool Boiling ◽  
Outer Wall ◽  
Heat Fluxes ◽  
Annular Gap ◽  
Wall Superheat ◽  
Peak Heat Flux ◽  
Inner Diameter ◽  
Counter Current

Quenching of a thin gap annular flow passage by gravitational liquid penetration was examined experimentally by using R-113. The outer wall was made of copper. The inner wall was made of copper or glass. The inner diameter of the outer wall of the annular flow passages was 40 or 41 mm and the annular gap clearance δ was 0.5, 1.0, 2.0 and 5.0 mm. The outer wall was heated initially up to 250 °C and also the inner wall was heated when the copper inner wall was used. The quenching was observed in δ ≥ 1.0 mm. When δ = 0.5 mm, the wall was just gradually cooled down. The relation between the wall superheat and the heat flux during quenching process was similar to the boiling curve of pool boiling. However, the peak heat flux as well as the heat flux in the film and the transition boiling was lower than those in the pool boiling. These heat fluxes became lower as the gap clearance became narrow. The rewetting velocity became slow as the gap clearance became narrow. The rewetting velocity seemed to have a unique relation for the Peclet number Pe = (ρSCSδSU/λS) and the Biot number Bi = hδs/λs ; Pe ∝ Bi which was the same as that of the Yamanouchi correlation. A decrease in the heat flux (the heat transfer coefficient) in the rewetting front region, which corresponds to the peak heat flux, results in a decrease in the rewetting velocity as the gap clearance becomes narrow.

Bubble Diameter Effects on CFD Simulation for Subcooled Boiling Flow in Rolling Circular Tube

2017 ◽  
Author(s):  
Sheng Xu ◽  
Liang-ming Pan ◽  
Yu Yang ◽  
Dewen Yuan ◽  
Jianjun Xu ◽  
...  
Keyword(s):  
Experimental Data ◽  
Void Fraction ◽  
Bubble Diameter ◽  
Low Pressure ◽  
Subcooled Boiling ◽  
Rolling Condition ◽  
Mean Diameter ◽  
Calculation Results ◽  
Boiling Flow ◽  
Fluctuation Amplitude

Eulerian two-fluid model coupled with wall boiling model was employed to calculate the three dimensional flow field and local parameter distribution with different bubble diameter models in circular tube under static and rolling condition. The wall boiling model utilized in this study was validated by Bartolomei experiment data, and a good agreement can be obtained. The calculation results of local void fraction are compared with experiment data to verify the accuracy of the numerical calculation for subcooled boiling flow under rolling condition. The Zeitoun bubble mean diameter model which the most recommended correlation for bubble diameter under low pressure and several fixed bubble diameters are applied to simulate the same condition in low pressure. These results are compared, include the distribution of void fraction, velocity distribution and radial flow induced by rolling motion. A good agreement with the experimental data has been achieved when Zeition bubble mean diameter and 2 mm fixed bubble diameter are used to describe vapor diameter in static condition. The local void fraction fluctuation has the same period with the rolling motion, and the fluctuation amplitude increases with the increase of rolling amplitude and rolling frequency. The difference shown in rolling condition between calculation results and experimental data demonstrates that better agreement with the experimental data has been achieved in the near-wall region about local void fraction which has bigger fluctuation amplitude. Higher void fraction has gotten using Zeition bubble mean diameter model to describe bubble diameter in subcooled boiling flow, tiny difference has showed in temperature, velocity and radial velocity in different bubble diameter model. Accurate vapor diameter model or method to describe vapor diameter coupled with suitable interphase force model is needed in rolling condition under low pressure to fit the calculation of subcooled boiling better under rolling condition.

Pool Boiling in Saturated and Subcooled FC-72 Dielectric Fluid From a Porous Graphite Surface

2004 ◽  
Author(s):  
Mohamed S. El-Genk ◽  
Jack L. Parker
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Porous Coating ◽  
Dielectric Fluid ◽  
Subcooled Boiling ◽  
Porous Graphite ◽  
Rate Of Increase ◽  
Boiling Incipience

Experiments are conducted that investigated pool boiling of FC-72 liquid at saturation and 10, 20, and 30 K subcooling on porous graphite and smooth copper surfaces measuring 10 × 10 mm. The nucleate boiling heat flux, Critical Heat Flux (CHF), and surface superheats at boiling incipience are compared. Theses heat fluxes are also compared with those of other investigators for smooth copper and silicon, etched SiO2, surfaces and micro-porous coating. No temperature excursion at boiling incipience on the porous graphite that occurred at a surface superheats of < 1.0 K. Conversely, the temperature excursions of 24.0 K and 12.4–17.8 K are measured at incipient boiling in saturation and subcooled boiling on copper. Nucleate boiling heat fluxes on porous graphite are significantly higher and corresponding surface superheats are much smaller than on copper. CHF on porous graphite (27.3, 39.6, 49.0, and 57.1 W/cm2 in saturation and 10 K, 20 K, and 30 K subcooled boiling, respectively) are 61.5%–207% higher than those on copper (16.9, 19.5, 23.6, and 28.0 W/cm2, respectively). The surface superheats at CHF on the porous graphite of 11.5 K in saturation and 17–20 K in subcooled boiling are significantly lower that those on copper (25 K and 26–28 K, respectively). In addition, the rate of increase of CHF on porous graphite with increased subcooling is ~ 125% higher than that on copper.

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