Flow Regime Observations in a Vertical Annulus With an Inner Roughened Tube

2008 ◽  
Author(s):  
G. D. Harvel ◽  
B. Torica
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Multiphase Flow ◽  
Flow Regime ◽  
Nuclear Power ◽  
Single Phase ◽  
Flow Regimes ◽  
Smooth Tube ◽  
Surface Conditions ◽  
Vertical Annulus

The efficiency and safe performance of a nuclear power plant is often reliant upon a solid understanding of the multiphase flow phenomena that occurs in various components and piping networks throughout the heat transport loops. Depending upon the conditions, various flow regimes can be observed and each of these flow regimes would have a different impact upon the heat transfer and pressure drop of the component or piping that the two-phase mixture is traveling in. Thus, many researchers have performed significant studies to predict flow regime. Over the past several years, the maintenance and inspection efforts at various nuclear power plants have shown that ageing phenomena are occurring including corrosion, cracking, erosion phenomena, and deposition of materials. One of the effects of such phenomena is the roughening of the surface which would impact the nature of fluid behaviour, heat transfer behaviour, and mass transfer at the surface. Relationships for surface conditions and pressure drop/heat transfer are already established for single phase flow conditions albeit with significant uncertainty. Yet, the relationships for multiphase flow have not been established and it is assumed that the surface effects are covered through the single phase component. In this work, experimental studies are performed for a vertical annulus geometry. The apparatus consists of glass tubing to allow visualization of the flow regime inside the tubes. The glass tube material is also a reference case for the smooth tube. Experiments are performed in a bubble column mode for superficial gas velocities up to 0.5 m/s. Observations are taken at various developing lengths and varied water inventories. The results are compared to previous work which shows that the smooth tube results match previous measurements. The inner surface of the tube is modified by forming a thin layer of material over the glass surface. The material is initially moldable and allows for the imprinting of different surface shapes and roughnesses. The material cures in a few hours and is resistant to erosion or dissolution in water. Thus, the surface conditions are changed on the inner tube. The experiments indicate that the flow regime transition from bubbly to slug flow and from slug to churn flow occur at lower superficial gas velocities.

Correlations Between Pressure Drop and Two-Phase Flow Regimes in Microchannel Networks

2009 ◽  
Author(s):  
Christian Weinmu¨ller ◽  
Dimos Poulikakos
Keyword(s):  
Experimental Data ◽  
Pressure Drop ◽  
Flow Regime ◽  
Single Phase ◽  
Two Phase Flow ◽  
Flow Regimes ◽  
Phase Flow ◽  
Two Phase ◽  
Research Activities ◽  
Wide Range

Microfluidics has experienced a significant increase in research activities in recent years with a wide range of applications emerging, such as micro heat exchangers, energy conversion devices, microreactors, lab-on-chip devices and micro total chemical analysis systems (μTAS). Efforts to enhance or extend the performance of single phase microfluidic devices are met by two-phase flow systems [1, 2]. Essential for the design and control of microfluidic systems is the understanding of the fluid/hydrodynamic behavior, especially pressure drop correlations. These are well established for single phase flow, however, analytical correlations for two-phase flow only reflect experimentally obtained values within an accuracy of ± 50% [3, 4]. The present study illustrates the effect of two-phase flow regimes on the pressure drop. Experimental measurement data is put into relation of calculated values based on established correlations of Lockhart-Martinelli with Chisholm modifications for macroscopic flows [5, 6] and Mishima-Hibiki modifications for microscale flows [7]. Further, the experimental pressure drop data is superimposed onto two-phase flow maps to identify apparent correlations of pressure drop abnormalities and flow regimes. The experiments were conducted in a square microchannel with a width of 200 μm. Optical access is guaranteed by an anodically bonded glass plate on a MEMS fabricated silicon chip. Superficial velocities range from 0.01 m/s to 1 m/s for the gas flow and from 0.0001 m/s to 1 m/s for the liquid flow with water as liquid feed and CO2 as gas. The analysis of the flow regimes was performed by imaging the distinct flow regimes by laser induced fluorescence microscopy, employing Rhodamine B as the photosensitive dye. The pressure drop was synchronically recorded with a 200 mbar, 2.5 bar and 25 bar differential pressure transmitter and the data was exported via a LabView based software environment, see Figure 1. Figure 2 illustrates the experimentally obtained pressure drop in comparison to the calculated values based on the Lockhard-Martinelli correlation with the Chisholm modification and the Mishima-Hibiki modification. For both cases the predications underestimate the two-phase pressure drop by more than 50%. Nevertheless, the regression of the experimental data has an offset of linear nature. Two-phase flow is assigned to flow regime maps of bubbly, wedging, slug or annular flow defined by superficial gas and liquid velocities. In Figure 3 the pressure drop is plotted as a surface over the corresponding flow regime map. Transition lines indicate a change of flow regimes enclosing an area of an anticline in the pressure data. In the direct comparison between the calculated and the measured values, the two surfaces show a distinct deviation. Especially, the anticline of the experimental data is not explained by the analytical correlations. Figure 4 depicts the findings of Figure 3 at a constant superficial velocity of 0.0232 m/s. The dominant influence of the flow regimes on the pressure drop becomes apparent, especially in the wedging flow regime. The evident deviation of two-phase flow correlations for the pressure drop is based on omitting the influence of the flow regimes. In conclusion, the study reveals a strong divergence of pressure drop measurements in microscale two-phase flow from established correlations of Lockhart-Martinelli and recognized modifications. In reference to [8, 9], an analytical model incorporating the flow regimes and, hence, predicting the precise pressure drop would be of great benefit for hydrodynamic considerations in microfluidics.

Condensation Flow Mechanisms in Microchannels: Basis for Pressure Drop and Heat Transfer Models

2003 ◽  
Author(s):  
Srinivas Garimella
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Visualization ◽  
Flow Regime ◽  
Annular Flow ◽  
Flow Regimes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Flow Mechanisms ◽  
Transfer Models

This paper presents an overview of the use of flow visualization in micro- and mini-channel geometries for the development of pressure drop and heat transfer models during condensation of refrigerants. Condensation flow mechanisms for round, square and rectangular tubes with hydraulic diameters in the range 1–5 mm for 0 < x < 1 and 150 kg/m2-s and 750 kg/m2-s were recorded using unique experimental techniques that permit flow visualization during the condensation process. The effect of channel shape and miniaturization on the flow regime transitions was documented. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. These flow regimes were further subdivided into several flow patterns within each regime. It was observed that the intermittent and annular flow regimes become larger as the tube hydraulic diameter is decreased, at the expense of the wavy flow regime. These maps and transition lines can be used to predict the flow regime or pattern that will be established for a given mass flux, quality and tube geometry. These observed flow mechanisms, together with pressure drop measurements, are being used to develop experimentally validated models for pressure drop during condensation in each of these flow regimes for a variety of circular and noncircular channels with 0.4 < Dh < 5 mm. These flow regime-based models yield substantially better pressure drop predictions than the traditionally used correlations that are primarily based on air-water flows for large diameter tubes. Condensation heat transfer coefficients were also measured using a unique thermal amplification technique that simultaneously allows for accurate measurement of the low heat transfer rates over small increments of refrigerant quality and high heat transfer coefficients characteristic of microchannels. Models for these measured heat transfer coefficients are being developed using the documented flow mechanisms and the corresponding pressure drop models as the basis.

Development of a Flow Regime Map for a Horizontal Pipe With the Multi-Classification Support Vector Machines

2008 ◽  
Author(s):  
L. M. Tam ◽  
A. J. Ghajar ◽  
H. K. Tam ◽  
S. C. Tam
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Mixed Convection ◽  
Flow Regime ◽  
Single Phase ◽  
Flow Regimes ◽  
Support Vector ◽  
Flow Maps ◽  
Vector Machines ◽  
Flow Regime Maps

For horizontal circular pipes under uniform wall heat flux boundary condition and three different inlet configurations (re-entrant, square-edged, bell-mouth), Ghajar and Tam (1995) developed flow regime maps for the determination of the boundary between single-phase forced and mixed convection using experimental data of Ghajar and Tam (1994). Based on the ratio of the local peripheral heat transfer coefficient at the top and the bottom, the heat transfer data was classified as either forced or mixed convection among the different flow regimes. The forced-mixed convection boundary was then obtained by empirical correlations. From the flow maps, heat transfer correlations for different flow regimes were recommended. Recently Trafalis et al. (2005) used the Multiclass Support Vector Machines (SVM) method to classify vertical and horizontal two-phase flow regimes in 4 pipes with good accuracy. In this study, the SVM method was applied to the single-phase experimental data of Ghajar and Tam (1994) and new flow regime maps were developed. Five flow regimes (forced turbulent, forced transition, mixed transition, forced laminar, mixed laminar) were identified in the flow maps using Reynolds and Rayleigh numbers as the identifying parameters. The flow regimes on the boundaries of the new maps were represented by the SVM decision functions. The results show that the new flow regime maps for the three types of inlets can classify the forced and mixed convection experimental data in different flow regimes with good accuracy.

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