摘要:

Entrapment, E, in vertical gas-liquid annular flows may be pictured as a balance between the rate of atomization, RA, of the liquid layer and the rate of deposition of drops, R D. Laboratory measurements of RA, RD and E are reviewed. Theoretical analyses are discussed which picture RA as related to the growth of wavelets through a Kelvin-Helmholtz instability and RB as being directly proportional to the root-mean-square of the turbulent velocity fluctuations of the drops. An equation for E can be developed, which assumes that the deposition constant is independent of drop concentration and that the rate of atomization varies linearly with the flow rale of the liquid in the film. Limitations of this approach, suggested by measurements of RA and RD at large liquid flows, are discussed.

ISSN:0098-6445    

DOI:10.1080/00986449608936417

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The response of turbulent pipe flow to sudden changes in wall roughness and flow cross-sectional area has been studied experimentally and numerically. Changes typical of those encountered by the gas phase in annular gas-liquid flow have been considered. The results show that the flow field and the pressure field can be significantly distorted at these transitions. Good agreement has been obtained between the measured results and those calculated using the Harwell-FLOW3D computational fluid dynamics (CFD) code.

ISSN:0098-6445    

DOI:10.1080/00986449608936418

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Condensation from air-steam mixtures on falling water layers is investigated experimentally and theoretically. The thin film flows on the inner surface of a 5cmi.d. vertical pipe. This film is wavy turbulent while the gas phase is kept saturated with steam. Experiments are conducted with the gas mixture effectively stagnant, compared with the fast moving liquid film. Measurements are also made under a mild vacuum applied on the gas phase. Heat transfer coefficients averaged over the entire length of the condensing surface, tend to increase by decreasing the liquid flowrate, by increasing the steam fraction, and by applying a mild vacuum on the gas phase. However, for the cases examined, there is a liquid flowrate above which the heat transfer coefficient becomes almost constant.

ISSN:0098-6445    

DOI:10.1080/00986449608936419

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Many different forms of the phasic conservation equations of two-phase flow have been presented in the literature. Unfortunately, there is still a lot of controversy as to "best" form for two-fluid modeling. In addition, while there have been many attempts to constitute the intraphase and interfacial closure laws to achieve closure, ill-posed models are still the rule rather than the exception. The purpose of the paper is to show how various popular forms of the two-fluid model are relaled, and to summarize the current state-of-the art in the modeling of interfacial and intraphase closure laws. Moreover, new closure laws are proposed which attempt to account for the effect of dispersed phase (i.e., inter-partical) interactions. It is hoped that this paper will help stimulate multiphase flow researchers to redouble this effort to achieve accurate, properly closed, two-fluid models which can used for the mechanistic predictions of multidimensional two-phase flows.

ISSN:0098-6445    

DOI:10.1080/00986449608936420

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In this paper are analysed the interactions between a surface wave field and the kinematic structures above and below the waves, in gas-liquid stratified flow in a rectangular cross sectional channel. The analysis is based on experimental data both on the local structure of the flows and on the deformation of the gas-liquid surface. The basic phenomena that have been observed are: on the one hand, the waves that propagates over the liquid surface can exhibit a crosswise distribution of amplitude; on the other hand, secondary flows can be generated both in the gas and in the liquid. A theoretical attempt is developed to explain the distribution to wave amplitude: in fact, the waves propagate over a non uniform liquid current. On the one hand, the wave field interact with the liquid current to generate secondary flows below the waves. On the other hand, the wave amplitude distribution interacts with the gas flow; the wave distribution can be considered as a non uniform interfacial roughness which generates Prandtl second type secondary flows above the waves. These physical mechanisms which are based on the analysis of experimental results are also validated with numerical simulations.

ISSN:0098-6445    

DOI:10.1080/00986449608936421

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This paper presents an experimental study of dispersed flow heat transfer in 90-degree circular bends. From extensive measurements, two different heat transfer patterns are identified, i.e. heat transfer without and with rewetting. Their intrinsic mechanisms are analysed, based on the present experimental evidence and our previous theoretical studies. Effects of mass flow rate, wall heat flux, system pressure and curvature ratio on heat transfer are also investigated. An empirical criterion is developed to identify the heat transfer pattern in the bend.

ISSN:0098-6445    

DOI:10.1080/00986449608936422

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The processes lhat lead to formatioh of large secondary disturbances are different for sheared channel flows and falling films largely as the result of different linear celerity and growth behavior. Close to neutral stability, sheared films usually have a band of linearly unstable wave modes with wavenumbers bounded away from 0 with a maximum growth rate at frequency ωmThese conditions have been observed to produce steady moderate wavelength waves with frequency close to ωmthat remain at small amplitude. For more severe conditions, long waves can become unstable although their growth rate is significantly lower than that at ωmand there still may be an intermediate band of stable waves. However, under some conditions, the linear dispersion relation stipulates that a low frequency unstable mode, ωmis resonant or nearly resonant with the fastest growing mode, ωm( ωt≪ ωm). Experiments suggest that this mechanism may trigger significant growth of the to, mode by feeding additional energy from the faster growing mode. This mechanism selects the frequency of the low mode and enhances its growth rate. This causes the large disturbances that eventually form to be of a particular frequency. For unstable inclined films, all the wavenumber modes less than the peak are always unstable and the possibility of resonance does not exist because the speeds do not match. Thus no specific frequency is selected. Evolution towards solitary waves, which involves a nonlinear phase-locking mechanism of all unstable modes, occurs under all conditions due to the absence of resonant energy transfer from the most unstable modes. Furthermore, there is no specific frequency selected so lhat there is no preferred separation for the solitary waves that are formed.

ISSN:0098-6445    

DOI:10.1080/00986449608936423

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The superimposed oscillations method for generating waves of many different types was used in the laboratory to study the wave effect on the desorption of carbon dioxide from falling liquid films. The mechanisms for heat mass transfer augmentation by waves were analyzed, taking into consideration the processes of absorption for a dilute gas, non-isothermal absorption, evaporation, and condensation.

ISSN:0098-6445    

DOI:10.1080/00986449608936424

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The usual two-fluid models for disperse two-phase flows feature an average disperse-phase stress in the disperse-phase momentum equation. It is shown that it is possible to derive an expression for this quantity without considering the detailed nature of the constitutive relation of the disperse phase. The result, which is more general than other relations of the same nature, is illustrated with a number of examples. It is also shown that a new formulation of the averaged equations recently proposed by the authors is equivalent to the more usual one.

ISSN:0098-6445    

DOI:10.1080/00986449608936425

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We propose a practical method for the treatment of turbulent mixing rate in a two-phase subchannel flow in a hydrodynamic non-equilibrium state. Based on the assumption that the fundamental modes of the inter-subchannel fluid transfer in such a state are turbulent mixing, void drift, and diversion cross flow, the turbulent mixing rate is considered to be equal to that in the hydrodynamic equilibrium state that the flow will attain. The applicability of the method is examined by experiments concerning the axial variation in tracer concentration in a non-equilibrium flow without diversion cross flow. A good agreement is seen between the calculations and the measurements.

ISSN:0098-6445    

DOI:10.1080/00986449608936426

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor