CHF mechanism in flow boiling from a short heated wall—I. Examination of near-wall conditions with the aid of photomicrography and high-speed video imaging

1993 ◽  
Vol 36 (10) ◽  
pp. 2511-2526 ◽  
Author(s):  
J.E. Galloway ◽  
I. Mudawar
Keyword(s):  
High Speed ◽  
Flow Boiling ◽  
Heated Wall ◽  
Video Imaging ◽  
High Speed Video ◽  
Near Wall

Photographic Study of Orientation Effects on Near-Saturated Flow Boiling

2002 ◽  
Author(s):  
Hui Zhang ◽  
Issam Mudawar ◽  
Mohammad M. Hassan
Keyword(s):  
High Speed ◽  
Body Force ◽  
Flow Boiling ◽  
Imaging Techniques ◽  
Heated Wall ◽  
Vapor Flow ◽  
Vapor Layer ◽  
High Speed Video ◽  
Lift Off ◽  
Flow Velocities

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities under 0.2 m/s, where extremely low CHF values where measured, especially with a downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations for different orientations and velocities. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and downflow orientations.

A Method for Assessing the Importance of Body Force on Flow Boiling CHF

2004 ◽  
Vol 126 (2) ◽  
pp. 161-168 ◽  
Author(s):  
Hui Zhang ◽  
Issam Mudawar ◽  
Mohammad M. Hasan
Keyword(s):  
High Speed ◽  
Body Force ◽  
Flow Boiling ◽  
Imaging Techniques ◽  
Heated Wall ◽  
Vapor Flow ◽  
Vapor Layer ◽  
High Speed Video ◽  
Lift Off ◽  
Flow Velocities

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms corresponding to different flow orientations and liquid velocities. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities below 0.2 m/s, where extremely low CHF values where measured, especially with downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and for downflow orientations. A new method consisting of three dimensionless criteria is developed to determine the minimum flow velocity required to overcome body force effects on near-saturated flow boiling CHF.

Incipient Flow Boiling in a Vertical Channel With a Wavy Wall

2010 ◽  
Author(s):  
T. Netz ◽  
R. Shalem ◽  
J. Aharon ◽  
G. Ziskind ◽  
R. Letan
Keyword(s):  
Heat Transfer ◽  
Temperature Field ◽  
High Speed ◽  
Flow Boiling ◽  
Heated Surface ◽  
Vertical Channel ◽  
Infrared Camera ◽  
Heat Fluxes ◽  
Heated Wall ◽  
Boiling Incipience

In the present study, incipient flow boiling of water is studied experimentally in a square-cross-section vertical channel. Water, preheated to 60–80 degrees Celsius, flows upwards. The channel has an electrically heated wall, where the heat fluxes can be as high as above one megawatt per square meter. The experiment is repeated for different water flow rates, and the maximum Reynolds number reached in the present study is 27,300. Boiling is observed and recorded using a high-speed digital video camera. The temperature field on the heated surface is measured with an infrared camera and a software is used to obtain quantitative temperature data. Thus, the recorded boiling images are analyzed in conjunction with the detailed temperature field. The dependence of incipient boiling on the flow and heat transfer parameters is established. For a flat wall, the results for various velocities and subcooling conditions agree well with the existing literature. Furthermore, three different wavy heated surfaces are explored, having the same pitch of 4mm but different amplitudes of 0.25mm, 0.5mm and 0.75mm. The effect of surface waviness on single-phase heat transfer and boiling incipience is shown. The differences in boiling incipience on various surfaces are elucidated, and the effect of wave amplitude on the results is discussed.

scholarly journals Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

2020 ◽  
Vol 13 (3) ◽  
Author(s):  
Arash Sayyah ◽  
Mohammad Mirzadeh ◽  
Yi Jiang ◽  
Warren V. Gleason ◽  
William C. Rice ◽  
...  
Keyword(s):  
High Speed ◽  
Video Imaging ◽  
High Speed Video

Flow Boiling of Sub-Cooled Pentane on a Micro-Porous Coating

2012 ◽  
Author(s):  
Paul J. Laca ◽  
Richard A. Wirtz
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
High Performance ◽  
Flow Boiling ◽  
High Heat ◽  
Surface Coatings ◽  
Porous Coating ◽  
High Heat Flux ◽  
Characteristic Curves ◽  
High Speed Video

Flow boiling experiments with sub-cooled Isopentane and n-Pentane at 3.0bar pressure assess the utility of compressed copper- and steel-filament screen laminate surface coatings as high performance boiling surfaces. High-speed video show that at high heat flux ebullition is unsteady. Isopentane and n-Pentane are found to produce nearly identical boiling characteristic curves. At the same applied heat flux, the superheat of copper filament coatings are much smaller than the steel filament coating superheats.

Simultaneous Measurements of Unsteady Thermal and Flow Fluctuations Using High-Speed Infrared Thermography and PIV Over a Backward Facing Step

2017 ◽  
Author(s):  
Shunsuke Yamada ◽  
Hajime Nakamura
Keyword(s):  
Heat Transfer ◽  
Infrared Thermography ◽  
Vortex Structure ◽  
High Speed ◽  
Stereoscopic Particle Image Velocimetry ◽  
Heated Wall ◽  
Wall Region ◽  
Backward Facing Step ◽  
Combined System ◽  
Near Wall

In order to investigate the flow and heat transfer fluctuations in the near-wall region downstream a backward facing step, a Time-resolved Stereoscopic Particle Image Velocimetry (TS-PIV) and a high-speed infrared thermography (IRT) combined system was constructed. Using this measurement system, the time series of the velocity in the vicinity of the heated wall and the heat transfer on the heated wall were measured at Reynolds number, which is based on the step height and inlet mainstream velocity, of 2.5 × 103. It confirmed the validity of the velocity fluctuation obtained by using TS-PIV. The results showed that the forward and downwash flows correspond to the enhancement of the heat transfer in the near-wall region. Also, the vortex structure in the yz plane was detected by Qyz-criterion, and the locational relationship between the vortex structure and the heat transfer enhancement was investigated.

Using High Speed Video Imaging in the Study of Cracking Processes in Rock

2009 ◽  
Vol 32 (2) ◽  
pp. 101631 ◽  
Author(s):  
L. D. Suits ◽  
T. C. Sheahan ◽  
L. N. Y. Wong ◽  
H. H. Einstein
Keyword(s):  
High Speed ◽  
Video Imaging ◽  
High Speed Video

Large-eddy simulation of the turbulent flow through a heated square duct

2002 ◽  
Vol 453 ◽  
pp. 201-238 ◽  
Author(s):  
M. SALINAS VÁZQUEZ ◽  
O. MÉTAIS
Keyword(s):  
High Speed ◽  
Secondary Flows ◽  
Heated Wall ◽  
Wall Region ◽  
Central Plane ◽  
Square Duct ◽  
Low Speed ◽  
Size Limitation ◽  
Large Eddy ◽  
Near Wall

Large-eddy simulations of a compressible turbulent square duct flow at low Mach number are described. First, we consider the isothermal case with all the walls at the same temperature: good agreement with previous incompressible DNS and LES results is obtained both for the statistical quantities and for the turbulent structures. A heated duct with a higher temperature prescribed at one wall is then considered and the intensity of the heating is varied widely. The increase of the viscosity with temperature in the vicinity of the heated wall turns out to play a major rôle. We observe an amplification of the near-wall secondary flows, a decrease of the turbulent fluctuations in the near-wall region and, conversely, their enhancement in the outer wall region. The increase of the viscous thickness with heating implies a significant augmentation of the size of the characteristic flow structures such as the low- and high-speed streaks, the ejections and the quasi-longitudinal vorticity structures. For strong enough heating, the size limitation imposed by the lateral walls leads to a single low-speed streak located near the duct central plane surrounded by two high-speed streaks on both sides. Violent ejections of slow and hot fluid from the heated wall are observed, linked with the central low-speed streak. A selective statistical sampling of the most violent ejection events reveals that the entrainment of cold fluid, originated from the duct core, at the base of the ejection and its subsequent expansion amplifies the ejection intensity.

scholarly journals Simulation of Boiling Flow Experiments Close to CHF with the Neptune_CFD Code

2008 ◽  
Vol 2008 ◽  
pp. 1-8 ◽  
Author(s):  
Boštjan Končar ◽  
Borut Mavko
Keyword(s):  
Volume Fraction ◽  
Flow Boiling ◽  
Three Dimensional ◽  
Heated Wall ◽  
Wall Region ◽  
Two Phase ◽  
Subcooled Flow Boiling ◽  
Boiling Flow ◽  
Flow Experiments ◽  
Near Wall

A three-dimensional two-fluid code Neptune_CFD has been validated against the Arizona State University (ASU) and DEBORA boiling flow experiments. Two-phase flow processes in the subcooled flow boiling regime have been studied on ASU experiments. Within this scope a new wall function has been implemented in the Neptune_CFD code aiming to improve the prediction of flow parameters in the near-wall region. The capability of the code to predict the boiling flow regime close to critical heat flux (CHF) conditions has been verified on selected DEBORA experiments. To predict the onset of CHF regime, a simplified model based on the near-wall values of gas volume fraction was used. The results have shown that the code is able to predict the wall temperature increase and the sharp void fraction peak near the heated wall, which are characteristic phenomena for CHF conditions.

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