IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Experiments have been conducted on countercurrent flow of air and water in tubes of 25, 67 and 99 mm diameter. Measurements of the pressure gradient, film thickness and down flow rate were made for a range of air and water flow rates under pre- and post-flooding conditions. The data showed that the pressure gradient did not increase appreciably until just before the onset of flooding. The mean film thickness under pre-flooding conditions was also predicted well by the falling film correlations. However, both quantities showed significant deviations from this pattern under post-flooding conditions. Existing correlations for predicting the pressure drop under post-flooding conditions gave poor prediction of the present data as well of the data from the literature. New correlations are presented for the pressure gradient and the mean film thickness under post-flooding conditions. © 2001 Published by Elsevier Science Ltd.

GENERAL

Department Of Chemical Engineering

International Journal of Multiphase Flow

Experimental study of air-water countercurrent annular flow under post-flooding conditions

Air-water upward annular flow experiments were conducted in a 10.8 m long tube of 31.8 mm internal diameter. Local values of the pressure gradient, film thickness, wall shear stress, film flow rate, disturbance wave velocity and frequency were measured over a range of gas and liquid flow rates. The results show that rapid changes in the film flow parameters occur within the first 50 tube diameters or so from the inlet. Entrained fraction and the film thickness appear to be the slowest to respond and these may take 100 to 300 tube diameters to develop fully. © 2001 Elsevier Science Ltd. All rights reserved.

Chemical Engineering Science

Flow development in vertical annular flow

Mixing of reactants, catalysts, etc., in a chemical reactor may be achieved using jets which offer the advantage of having no moving parts inside the reactor. While there have been many experimental studies and thumb rules for the design of jets, the detailed hydrodynamics of the mixing process is not properly understood. In the present paper, computational fluid dynamics (CFD) techniques are used to simulate jet mixing in a cylindrical vessel. The flow circulation patterns within the reactor and their effect on mixing of a soluble salt are studied. By simulating various configurations of jets, it is shown that the key factor in reducing mixing time is minimizing or eliminating dead zones in the reactor. This can be achieved, for example, by having a conical bottom. © 2001 Elsevier Science Ltd. All rights reserved.

Hydrodynamics of jet mixing in vessels

This paper describes an experimental study on the effect of tube diameter on the mechanism of flooding in vertical gas-liquid countercurrent annular flow. Flooding experiments were conducted with three different tube inner diameters, namely, 25, 67 and 99 mm with smooth inlet and outlet conditions for air and water. The results indicate that the mechanism of flooding is qualitatively different in the small and the large diameter test sections. While flooding in the 25 mm diameter section occurred essentially by the upward movement of large waves created near the liquid outlet, no such waves could be seen in the 67 and the 99 mm diameter test sections. Here, flooding occurred by droplet carryover or by an unstable, churn-like motion in the liquid film. The results are compared with existing correlations. The effect of test section length on flooding was also investigated. © 2001 Elsevier Science Ltd. All rights reserved.

Effect of tube diameter on flooding

Computational fluid dynamics techniques have been used to calculate the flow field and heat transfer through a wavy liquid film driven by gas shear. The geometry and flow conditions specified are typical of conditions encountered in gas-liquid annular flow. Results show that the flow in the substrate layer is laminar while that in the disturbance wave region is turbulent leading to a local enhancement of the transport coefficients. © 1997 Elsevier Science Ltd. All rights reserved.

International Journal of Heat and Mass Transfer

Hydrodynamics and heat transfer in wavy annular gas-liquid flow: A computational fluid dynamics study

The response of turbulent flow to abrupt changes in pipe wall roughness is studied here. It is shown that a circumferential flow will not develop near the wall in a pipe with a short patch of circumferentially varying roughness. It is therefore concluded that the secondary flow mechanism would play no part in the liquid film distribution in horizontal annular flow.

Applied Mathematical Modelling

Response of turbulent flow to abrupt changes in surface roughness and its relevance in horizontal annular flow

The hydrodynamics and heat transfer of thin film flow with a wavy interface has been studied using computational fluid dynamics techniques. The velocity and temperature fields are obtained for periodic laminar flow with an assumed interface shape. The effect of sinusoidal and solitary waves on the heat transfer across the film is investigated. It is shown that the overall heat transfer coefficient is determined mainly by conduction through the film, rather than by the recirculation, if any, under the waves. However, the presence of interfacial waves still enhances the heat transfer coefficient due, mainly, to the effective thinning of the film. Copyright © 1996 Elsevier Science Ltd.

Hydrodynamics and heat transfer of wavy thin film flow

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.

Chemical Engineering Communications

Turbulent flow in a pipe with intermittent rough patches: an analogue of annular two-phase flow

Flow visualization experiments and film thickness measurements were conducted in air-water upward annular flow in a tube of 31.8 mm i.d. As the liquid flow rate was increased at a constant gas flow rate, first disturbance waves and then ephemeral waves were found to occur on the liquid film. A model to determine the film Reynolds number has been developed, and is used to show that the onset of disturbance waves and of ephemeral waves happens at a substrate film Reynolds number of about 500. It is proposed that the ephemeral waves are none but developing disturbance waves.

On the nature of ephemeral waves in vertical annular flow

Calculations have been performed using the CFDS-FLOW3D computational fluid dynamics code to determine the force exerted on a standing wave by gas flowing over it. It is found that the drag on the wave is mainly due to the pressure variation around it and that it is a strong function of the channel dimensions. Extension of these results to typical flooding conditions in countercurrent flow in vertical tubes shows that the gas velocity required to transport waves upwards increases significantly as the tube diameter increases. It is suggested therefore that the mechanism of flooding depends on the diameter of the tube and that flooding is induced by upward transport of waves from near the bottom of the tube in small diameter tubes whereas in large diameter tubes, it may occur due to entrainment and carryover of droplets near the liquid entry.

Theoretical investigation of the diameter effect on flooding in countercurrent flow

Theoretical and experimental studies were carried out on a possible compression ignition problem arising in an air supply line of 50.8 mm (2 inch) ID. A valve was used to close one end of the line and another valve located at a distance of 2.2m and connected to a high pressure air source was opened suddenly. The temperature of the air near the closed end was measured using a fast response thermocouple for a range of pressures. It was found that the measured temperature rise was much lower than that calculated thermodymically assuming isentropic compression of the low pressure air, but higher than that assuming complete mixing of the high and the low pressure air streams. Calculations using a transient flow simulation code (PLAC) also predicted values which were much higher than the measured ones. It is concluded that partial mixing of the two streams follows a sudden compression and that this effect must be properly accounted for in calculations to get an accurate estimate of temperature rises.

World University root node

Process Safety and Environmental Protection: Transactions of the Institution of Chemical Engineers, Part B

Measurement and prediction of temperature rise following sudden compression in a high-pressure pipeline

An assessment is made of the various viewpoints on the slug-to-churn flow transition in vertical upward flow in the light of recent experimental results obtained at Harwell Laboratory. It is found that the flooding model of McQuillan & Whalley and the bubble entrainment model of Barnea & Brauner give satisfactory results at low and high liquid flow rates, respectively. An improved model for flooding, which takes account of the effect of the falling film, has been proposed. It is shown that this new model is in good agreement with experimental results at both low and high liquid flow rates. © 1992.

Prediction of the slug-to-churn flow transition in vertical two-phase flow

Depending on the flow conditions, the liquid film in annular two-phase flow in coiled tubes may be pushed towards the outer or the inner side by the centrifugal force. It is important to understand the mechanism of this ‘film inversion’ in order to develop a predictive model for the film thickness distribution. In this paper, this phenomenon is studied analytically, and a new criterion, based on the secondary flow in the thin liquid film, is proposed to predict its occurrence. The criterion shows good agreement with available experimental data. It is suggested that the analytical model can readily be extended to predict the distribution of the film thickness and film flow rate in coiled tubes. © 1992, Cambridge University Press. All rights reserved.

Journal of Fluid Mechanics

Prediction of film inversion in two-phase flow in coiled tubes

The bifurcation phenomenon whereby multiple-vortex secondary flow occurs in place of the normal two-vortex flow in laminar flow in curved ducts has previously been studied numerically by several researchers. However, the various results have been conflicting on many points. The present paper describes a set of numerical experiments conducted to study the effect of numerical accuracy on the solution. The results show that the transition from two- to four-vortex structure depends strongly on the differencing scheme and to a lesser extent on the grid size. The study also shows that as the Reynolds number of the flow increases, a two-vortex structure is re-established via a path which involves strongly asymmetric secondary flow patterns. These results are in agreement, at least qualitatively, with recent experimental theoretical and numerical results.

A numerical study of bifurcation in laminar flow in curved ducts

The heat transfer characteristics of a coiled steam generating tube are very different from those of a straight tube. For example, the critical heat flux (CHF) in coiled tubes is less than that in straight tubes for subcooled boiling and significantly higher at higher qualities. In this paper, predictive methods are developed for the determination of the point of onset of nucleate boiling, the point of net vapour generation and the critical heat flux condition in curved ducts. The methods are based on the assumption that these phenomena are governed by the local hydro- and thermodynamic conditions which can be predicted using computational fluid dynamics techniques. Good agreement has been obtained with available experimental data. © 1991 Springer-Verlag.

Wärme- und Stoffübertragung

Prediction of onset of nucleate boiling, net vapour generation and subcooled CHF in coiled tubes

The prediction of the occurrence of dryout in helical coils is important for the proper design of the steam generators of some LMFBRs. Past work in this area is not satisfactory either in the prediction of the first dryout quality or in the treatment of the partial dryout region. Qualitative arguments are presented to show that dryout data can be classified into stratification-, redeposition- and entrainment-dominated dryout. Results obtained by various laboratories including experiments carried out at CENG, Grenoble, are used to develop transition criteria for these zones in a 'dryout map'. Correlations are given for dryout quality in each group. An analysis of the partial dryout region is also presented. © 1990.

Characterization of dryout in helical coils

The structure of the liquid film in horizontal annular flow is studied visually using the refractive index matching technique. The liquid film is found to contain significant amount of air bubbles, which are continuously entrained, broken up and released by the rolling motion within the film. A new conceptual picture of the gas-liquid interface is presented. © 1990.

Structure of thin liquid films in gas-liquid horizontal flow

A crucial point still to be established in the prediction of the film thickness distribution in horizontal annular two-phase flow is the mechanism(s) for transporting liquid from the bottom to the top part of the tube. To resolve this issue, the time-dependent behaviour of the liquid film is studied. Wave characteristics such as velocity and frequency are measured around the circumference. It is inferred from the autospectral density functions of film thickness variation that disturbance waves play an important, but as yet unclear, role in the formation of a liquid film in the top part of the tube. A new mechanism, based on the shape of disturbance waves is proposed. © 1990.

Time-dependent behaviour of the liquid film in horizontal annular flow

With the use of helical coils in steam generators of LMFBR, the problems associated with dryout are receiving wide attention. This paper describes the results obtained at the CEN-Grenoble with a freon-12 loop with electrical heating. The influence of pressure, coil diameter, mass flux and heat flux is analysed qualitatively, and a first attempt at correlating the results is presented. © 1990.

Nuclear Engineering and Design

High-quality dryout in helical coils

The advent of standard algorithms for the numerical solution of partial differential equations has given researchers a new tool for fluid flow calculations. In this paper, single-phase flow in curved ducts is numerically simulated by imposing a spatially varying centrifugal force on a fluid flowing in a straight tube. The resulting set of partial differential equations is solved using the HARWELL-FLOW3D computer program. Comparison with other numerical and experimental results shows that this simplified formulation gives accurate results. The model neglects certain geometric terms of the order d/D, the duct-to-coil diameter ratio. The effect of these terms is investigated by considering the flow in a 90° bend for large d/D. It is shown that while there may be significant error in the prediction of the local variables for large d/D, the circumference-averaged quantities are well predicted.