BACKGROUND AND FORMULATION OF RESEARCH
1.3 Hydrodynamics in helical coil
1.3.1 Flow Pattern
Background and Formulation of Research
It also has attractive features as a chemical reactor where large volumes of vapour react with a liquid accompanied by a high heat of reaction, such as chlorination reactions (Ali et al., 1968).
It provides efficient contact and excellent heat transfer in coil tube heat exchanger (Ali et al., 1968)
Chapter-1
8
bubbly regime, small bubbles are present in the liquid slugs, something not observed for Taylor flow. The transition from Taylor flow to slug-bubbly flow occurs by increasing the liquid flow rate.
Churn flow regime: Churn flow occurs at very high gas velocities. It consists of very long gas bubbles and relatively small liquid slugs. Due to the high gas velocity, a wave or ripple motion is often observed at the bubble tail. Further increase in gas flow rate results in annular flow.
Annular flow: At excessively high gas velocities and very low liquid velocities, annular flow results. In this case a continuous gas phase is present in the central core of the capillary with the liquid phase which is displaced to form an annulus between the capillary wall and the gas phase.
Dispersed flow: Dispersed flow is characterized by the flow where one phase is dispersed in the other continuous phase. This flow configuration is observed in all types (gas-liquid, gas- solid, liquid-liquid and liquid-solid)
There are various methods are available to identify the regime transition. However following two methods are widely used in case of helically coiled system.:
Visual observation: Traditionally, flow regimes have been defined according to visual observations performed by viewing the flow through transparent pipe/tube. The majority of all the reported data in literature have been obtained in this manner. Although visual observation provides some information on the flow patterns, it is often difficult to identify the flow regime transitions without quantitative measurements, even in transparent pipe/tube, due to the relatively opaque nature of multiphase flow. At very high phase velocity due to intense in interaction of the phases, it is difficult to pinpoint the exact transition velocity by visual observation. It is admitted, however, that ambiguities regarding the exact nature of flow TH-1484_10610718
Background and Formulation of Research patterns may exist in the interpretation of such visual observations. In this connection, still pictures by high speed photography can be employed as a useful aid. Even though such pictures may give a clear view of flow at a certain moment of time. The interpretation regarding the flow regime may be arbitrary or somewhat subjective, depending on the observer. The difficulty of obtaining a clear view of the central parts of the flow cross section by high speed photography is due to the light diffraction at all gas-liquid interfaces which, in some cases, would make the clear observations limited to only a layer of the mixture near the pipe/tube walls. However, in spite of the limitation, the direct visual observation approach has been used for its simplicity and inexpensiveness. It certainly has its everlasting merit as the best tool for simple experiments.
Evolution of global hydrodynamic parameter: The global hydrodynamic parameter (pressure drop and holdup) displays the prevailing flow patterns varying with the regimes.
This fact has generally been utilized to identify flow regime transition point. Typically, the global hydrodynamics have been quantified based on overall gas holdup (it is defined as the percentage by volume of the gas in the two or three phase mixture in the system). The overall gas holdup increases with an increase in superficial gas velocity (the velocity of fluid moving through a tube, defined as the volumetric flow rate of that fluid divided by the cross-sectional area of the tube) linearly (g usg0.81) at low gas velocity but due to an intense nonlinear interaction of phases at high gas velocities, the relationship between overall gas holdup and superficial gas velocity deviates from linearity and it obeysg usg0.40.6. Hence, the change in slope of the gas holdup curve can be identified as a regime transition point. Sometimes, gas holdup shows an S-shaped curve, depending upon operating and design conditions (Shaikh and Dahhan, 2005) in two-phase flow system. In such cases, the superficial gas velocity at which maximum gas holdup attained is identified as the transition velocity. However, when the change in slope is gradual or the gas holdup curve does not show a maximum in gas
Chapter-1
10
holdup, it is difficult to identify the transition point. In such cases, the Zuber and Findlay (1965) drift flux method can be used extensively. They argued that gas holdup in two-phase flow depends on two phenomena: the gas rises locally relative to liquid due to phase density differences, and the gas holdup and velocity distribution across the diameter causes gas to concentrate in a faster or slower region of flow, thereby affecting the average gas holdup and flow pattern. The following drift-flux model of Zuber and Findlay (1965) is applicable to describe the flow regime criteria:
d sl sg o g
sg C u u V
u /
( ) (1.1)C0 is a distribution parameter and is a measure of the interaction of the holdup and velocity distribution according to flow pattern. The detailed literature on flow patterns is given in chapter 2.