The Effect of Roll Waves on the Hydrodynamics of Falling Films Observed in Vertical Column Absorbers PDF Download

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Languages : en
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A thin falling film is well suited to simultaneous heat and mass transfer because of the small thermal resistance through the film and because of the large contact surface achievable at low flow rates. The film enters as a smooth laminar flow and quickly transitions into small-amplitude wavy flow. The waves grown in length and amplitude and are identified as roll waves. This flow regime is termed wavy-laminar flow, and modern heat and mass transfer equipment operate in this complicated transition regime. Research published in open literature has shown the mass flow rate in the rollwaves to be about 10 to 20 times greater than that in the laminar substrate. As the film fully develops, the waves grow in mass and the film substrate thins because fluid is swept from the substrate by the secondary flows of the roll wave. Many studies have been conducted to measure and correlate the film thickness of wavy-laminar flows. Literature data show that Nusselt's theory for smooth laminar flow can over predict the film thickness by as much as 20% for certain wavy-laminar flow conditions. The hydrodynamics of falling films were therefore studied to measure the film thickness of a free-surface falling film and to better understand the parameters that affect the variations of the film thickness. A flow loop was set up for measuring the thickness, wave amplitude, and frequency of a film during hydrodynamic flow. Decreasing the pipe diameter caused the amplitude of the wavy flow to diminish. Measurements monitored from stations along the falling film showed a thinning of film thickness. Fully developed flow required large starting lengths of about 0.5 m. The film thickness increases as the Reynolds number (Re) increases. Increasing the Kapitza number (Ka) causes a decrease in the film thickness. Regression analysis showed that the Re and Ka numbers described the data trends in wavy-laminar flow. Rather than correlating the Re number in discrete ranges of the Ka number as earlier researchers have done, this research made the Ka number an independent regression variable along with the Re number. The correlation explains 96% of the total variation in the data and predicts the experimental data within an absolute average deviation of[+-] 4.0%. The correlation supports the calculation of a fully developed film thickness for wavy-laminar falling films.

Author: S. Mostafa Ghiaasiaan
Publisher: Cambridge University Press
ISBN: 1316785300
Category : Technology & Engineering
Languages : en
Pages : 1322

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Providing a comprehensive introduction to the fundamentals and applications of flow and heat transfer in conventional and miniature systems, this fully enhanced and updated edition covers all the topics essential for graduate courses on two-phase flow, boiling, and condensation. Beginning with a concise review of single-phase flow fundamentals and interfacial phenomena, detailed and clear discussion is provided on a range of topics, including two-phase hydrodynamics and flow regimes, mathematical modeling of gas-liquid two-phase flows, pool and flow boiling, flow and boiling in mini and microchannels, external and internal-flow condensation with and without noncondensables, condensation in small flow passages, and two-phase choked flow. Numerous solved examples and end-of-chapter problems that include many common design problems likely to be encountered by students, make this an essential text for graduate students. With up-to-date detail on the most recent research trends and practical applications, it is also an ideal reference for professionals and researchers in mechanical, nuclear, and chemical engineering.

Author: American Society of Mechanical Engineers. Advanced Energy Systems Division
Publisher:
ISBN:
Category : Heat pumps
Languages : en
Pages : 644

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Author: Hen-hong Chang
Publisher: Elsevier
ISBN: 0080529534
Category : Technology & Engineering
Languages : en
Pages : 413

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Wave evolution on a falling film is a classical hydrodynamic instability whose rich wave dynamics have been carefully recorded in the last fifty years. Such waves are known to profoundly affect the mass and heat transfer of multi-phase industrial units. This book describes the collective effort of both authors and their students in constructing a comprehensive theory to describe the complex wave evolution from nearly harmonic waves at the inlet to complex spatio-temporal patterns involving solitary waves downstream. The mathematical theory represents a significant breakthrough from classical linear stability theories, which can only describe the inlet harmonic waves and also extends classical soliton theory for integrable systems to real solitrary wave dynamics with dissipation. One unique feature of falling-film solitary wave dynamics, which drives much of the spatio-temporal wave evolution, is the irreversible coalescence of such localized wave structures. It represents the first full description of a hydrodynamic instability from inception to developed chaos. This approach should prove useful for other complex hydrodynamic instabilities and would allow industrial engineers to better design their multi-phase apparati by exploiting the deciphered wave dynamics. This publication gives a comprehensive review of all experimental records and existing theories and significantly advances state of the art on the subject and are complimented by complex and attractive graphics from computational fluid mechanics.

Author:
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ISBN:
Category :
Languages : en
Pages : 7

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Wave evolution on a falling film is important in many industrial processes such as fiber coating, paper coating, pollutant scrubbing, etc. If the film flows down a cylindrical fiber, the wave transitions are different from those on a vertical plane. Saturated waves are not observed unless film thickness is below a critical value; if above, the waves grow rapidly to form drops with characteristic dimension close to capillary length scale. Some important open problems in these two wave formation processes were studied.

Author: Claude Massot
Publisher:
ISBN:
Category : Chemical engineering
Languages : en
Pages : 604

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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 7

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Book Description
Wave evolution on a falling film is important in many industrial processes such as fiber coating, paper coating, pollutant scrubbing, etc. If the film flows down a cylindrical fiber, the wave transitions are different from those on a vertical plane. Saturated waves are not observed unless film thickness is below a critical value; if above, the waves grow rapidly to form drops with characteristic dimension close to capillary length scale. Some important open problems in these two wave formation processes were studied.

Author: Winston Walker Potts
Publisher:
ISBN:
Category :
Languages : en
Pages : 298

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Author: Nichigai Asoshiētsu
Publisher:
ISBN:
Category : AIChE symposium series
Languages : en
Pages : 640

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Author: Geoffrey Frederick Hewitt
Publisher:
ISBN:
Category : Liquid films
Languages : en
Pages : 32

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