Numerical Modeling of Condensation from Vapor-gas Mixtures for Forced Down Flow Inside a Tube PDF Download
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Languages : en
Pages : 25
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Laminar film condensation is the dominant heat transfer mode inside tubes. In the present paper direct numerical simulation of the detailed transport process within the steam-gas core flow and in the condensate film is carried out. The problem was posed as an axisymmetric two dimensional (r, z) gas phase inside an annular condensate film flow with an assumed smooth interface. The fundamental conservation equations were written for mass, momentum, species concentration and energy in the gaseous phase with effective diffusion parameters characterizing the turbulent region. The low Reynolds number two equation [kappa]-[epsilon] model was employed to determine the eddy diffusion coefficients. The liquid film was described by similar formulation without the gas species equation. An empirical correlation was employed to correct for the effect of film waviness on the interfacial shear. A computer code named COAPIT (Condensation Analysis Program Inside Tube) was developed to implement numerical solution of the fundamental equations. The equations were solved by a marching technique working downstream from the entrance of the condensing section. COAPIT was benchmarked against experimental data and overall reasonable agreement was found for the key parameters such as heat transfer coefficient and tube inner wall temperature. The predicted axial development of radial profiles of velocity, composition and temperature and occurrence of metastable vapor add insight to the physical phenomena.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 25
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Book Description
Laminar film condensation is the dominant heat transfer mode inside tubes. In the present paper direct numerical simulation of the detailed transport process within the steam-gas core flow and in the condensate film is carried out. The problem was posed as an axisymmetric two dimensional (r, z) gas phase inside an annular condensate film flow with an assumed smooth interface. The fundamental conservation equations were written for mass, momentum, species concentration and energy in the gaseous phase with effective diffusion parameters characterizing the turbulent region. The low Reynolds number two equation [kappa]-[epsilon] model was employed to determine the eddy diffusion coefficients. The liquid film was described by similar formulation without the gas species equation. An empirical correlation was employed to correct for the effect of film waviness on the interfacial shear. A computer code named COAPIT (Condensation Analysis Program Inside Tube) was developed to implement numerical solution of the fundamental equations. The equations were solved by a marching technique working downstream from the entrance of the condensing section. COAPIT was benchmarked against experimental data and overall reasonable agreement was found for the key parameters such as heat transfer coefficient and tube inner wall temperature. The predicted axial development of radial profiles of velocity, composition and temperature and occurrence of metastable vapor add insight to the physical phenomena.
Author: Ruey Yng Yuann
Publisher:
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Category :
Languages : en
Pages : 432
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Author:
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Languages : en
Pages :
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The present analysis deals with steady-state, internal-flow, laminar film condensation of vapour-gas mixtures in horizontal and downward-inclined parallel-plate channels. The cooled lower plate is maintained at a uniform temperature, while the upper plate is insulated. Saturated vapour mixed with a non-condensable gas enters the channel at prescribed pressure, velocity, and gas mass fraction. As a result of condensation on the lower plate, distinct regions of liquid condensate and vapour-gas mixture are formed inside the channel. Each phase is described with a set of complete boundary layer equations governing the conservation of mass, momentum, and energy. These two phases are related at the interface with the continuity of velocity, temperature, mass flux, shear stress, and heat flux. The focus of this thesis is on developing a robust numerical solution for the above model and assessing the effect of each independent parameter on the condensation process. The numerical approach utilized a variable-properties finite-volume method with a marching technique. (Abstract shortened by UMI.).
Author: Daniel Grant Ogg
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ISBN:
Category :
Languages : en
Pages : 308
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Author: Esam Saleh
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Category :
Languages : en
Pages : 0
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A complete two-phase numerical model of film condensation from a mixture of a vapour and a non-condensing gas that is based on the two-dimensional elliptic governing equations with variable physical properties is presented. The model predicts the full viscous flow and heat and mass transfer for the mixture around the tube and in the entire liquid film from the top of the tube to the falling film below the tube. A finite volume method is used with a segregated solution approach and a dynamically moving computational grid that tracks the phase interface sharply. Fundamental balances of mass, energy, and force are enforced accurately at the phase interface. The model was developed in steps and validated against various experimental and theoretical works in the literature for different two-phase flows. The validation tests included stratified flow of liquid and gas in a horizontal channel, falling liquid film over a vertical wall, and condensation of steam from a steam-air mixture in a vertical channel. The model was used to simulate laminar film condensation from a downward flowing steam-air mixture over an isothermal horizontal tube. The validity of this new model is demonstrated by comparisons with previous theoretical and experimental studies. New results are presented on the effects of free-stream-to-tube temperature difference, upstream Reynolds number, free-stream gas mass fraction, and free-stream pressure on the condensate film development, the local and average heat transfer coefficients, and the total condensate mass flow rate. It was found that the temperature difference had the greatest effect on the condensation rate and film thickness. The presence of non-condensing gas in the vapour has a strong negative impact on the condensation process. For the pure steam case, moderate changes in the upstream Reynolds number showed slight increases in condensate mass flow rate with increased Reynolds number. For the mixture case, however, moderate increase in upstream Reynolds number increases significantly the condensate mass flow rate and film thickness. This trend becomes more noticeable as the free-steam gas mass fraction increases. Changing the free-stream pressure demonstrated that property variation had a relatively smaller effect than temperature difference and gas mass fraction changes.
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Category :
Languages : en
Pages :
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Author: Tongran Qin
Publisher: Springer
ISBN: 3319613316
Category : Science
Languages : en
Pages : 217
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This thesis represents the first systematic description of the two-phase flow problem. Two-phase flows of volatile fluids in confined geometries driven by an applied temperature gradient play an important role in a range of applications, including thermal management, such as heat pipes, thermosyphons, capillary pumped loops and other evaporative cooling devices. Previously, this problem has been addressed using a piecemeal approach that relied heavily on correlations and unproven assumptions, and the science and technology behind heat pipes have barely evolved in recent decades. The model introduced in this thesis, however, presents a comprehensive physically based description of both the liquid and the gas phase. The model has been implemented numerically and successfully validated against the available experimental data, and the numerical results are used to determine the key physical processes that control the heat and mass flow and describe the flow stability. One of the key contributions of this thesis work is the description of the role of noncondensables, such as air, on transport. In particular, it is shown that many of the assumptions used by current engineering models of evaporative cooling devices are based on experiments conducted at atmospheric pressures, and these assumptions break down partially or completely when most of the noncondensables are removed, requiring a new modeling approach presented in the thesis. Moreover, Numerical solutions are used to motivate and justify a simplified analytical description of transport in both the liquid and the gas layer, which can be used to describe flow stability and determine the critical Marangoni number and wavelength describing the onset of the convective pattern. As a result, the results presented in the thesis should be of interest both to engineers working in heat transfer and researchers interested in fluid dynamics and pattern formation.
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Category :
Languages : en
Pages : 24
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Previous experiments have been carried out by Vierow, Ogg, Kageyama and Siddique for condensation from steam/gas mixtures in vertical tubes. In each case the data scatter relative to the correlation was large and there was not close agreement among the three investigations. A new apparatus has been designed and built using the lessons learned from the earlier studies. Using the new apparatus, an extensive new data base has been obtained for pure steam, steam-air mixtures and steam-helium mixtures. Three different correlations, one implementing the degradation method initially proposed by Vierow and Schrock, a second diffusion layer theory initially proposed by Peterson, and third mass transfer conductance model are presented in this paper. The correlation using the simple degradation factor method has been shown, with some modification, to give satisfactory engineering accuracy when applied to the new data. However, this method is based on very simplified arguments that do not fully represent the complex physical phenomena involved. Better representation of the data has been found possible using modifications of the more complex and phenomenologically based method which treats the heat transfer conductance of the liquid film in series with the conductance on the vapor-gas side with the latter comprised of mass transfer and sensible heat transfer conductance acting in parallel. The mechanistic models, based on the modified diffusion layer theory or classical mass transfer theory for mass transfer conductance with transpiration successfully correlate the data for the heat transfer of vapor-gas side. Combined with the heat transfer of liquid film model proposed by Blangetti, the overall heat transfer coefficients predicted by the correlations from mechanistic models are in close agreement with experimental values.
Author: Vlajko Srzić
Publisher:
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Category :
Languages : en
Pages : 432
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Author:
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Category :
Languages : en
Pages :
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The effects of a lighter noncondensable gas on laminar film condensation from moving vapor-gas mixtures was investigated. Condensation occurred on the top of an isothermal flat plate with an arbitrary inclination. The liquid film and the mixture boundary layers were described with the conservation equations for mass, momentum, energy, and gas species (for the mixture boundary layer only). A finite volume method was applied on a staggered grid in the numerical solution domain. The properties for both liquid and mixture were evaluated at the local temperature. The solution procedure was terminated either when the separation criteria were met or when the flow reached the transition to turbulence. The main objectives of the study were to investigate the mixture boundary layer separation distance and the reduction in heat transfer to the wall due to the presence of a lighter noncondensable gas. Three vapor-gas combinations were studied: steam-hydrogen, Freon12-air, and mercury-air. Applying two simple collapsing procedures, a set of graphs is presented for each vapor-gas combination which can be used to estimate the separation length for a given set of input parameters. For each mixture and for a given free stream temperature, the variation of Nusselt number (normalized by the square root of the local Reynolds number) along the plate is also presented for different values of gas concentration and wall temperature. (Abstract shortened by UMI.).
