An experimental and numerical study of two-phase flow in horizontal tube bundles

An effective design for a kettle reboiler is dependent on fitness for purpose while reducing costs. Thus, accurate information concerning two-phase flow behaviour within it is important. Experimental and numerical studies have been carried out in this research to gain a more detailed understanding of the phenomena associated with two-phase flow in a thin-slice kettle reboiler. The kettle reboiler contained 241 electrically heated tubes arranged as 17 rows of 17 columns in an in-line layout with an outside diameter of 19 mm and a pitch-to-diameter ratio of 1.34. The working fluids used in this investigation were pentane and the refrigerant R113. They were boiled at atmospheric pressure at uniform heat fluxes in the range of 10 to 40 kW/m2. The patterns of flow inside the kettle reboiler were investigated experimentally using ordinary and high speed cameras. Visual observation of the flow patterns showed that the flow in the tube bundle was two-dimensional at heat fluxes of 20 kW/m2 and above. The quantity of foam and recirculation above the tube bundle were found to depend on both the heat flux and the working fluid used. Observations of the two-phase flow pattern in the shell indicated that the movement of fluid from the centre column of the bundle was affected by the down flow into the top of the tube bundle. Two flow patterns in the tube bundle were identified: bubbly and intermittent. At low heat fluxes, bubbly flow dominated, then, with increasing heat flux, bubble coalescence led to the development of vapour slugs and intermittent flow was observed. Pressure drop measurements were made in three columns within the tube bundle. The results showed that at heat fluxes below 20 kW/m2, the pressure drop remained nearly constant and equal to the all-liquid value. At a heat flux of 20 kW/m2 and above, the pressure drop was found to increasingly fall below the all-liquid value as the bundle row number increased. This effect was especially evident in the centre of the bundle. A change in the flow pattern caused the pressure distribution up the tube bundle to change from roughly constant to decaying with height. Based on a number of assumptions, the two-fluid model has been applied. The two-fluid model’s drag coefficient and tube resistance were deduced from a one-dimensional model. The two-fluid model predictions show good agreement with the experimental results for the pressure distribution and flow distribution. Grid sizes of 10, 8 and 4 mm for the bundle and the pool were considered. It was found that the predicted bundle results were not affected by changing the grid size. However, in the pool region, a small grid size was needed. A grid size of 10 mm was used in the bundle while 4 mm was used in the pool. The pool velocity predictions compared well with measured values available in the open literature. The results indicated that the bundle flow is not significantly affected by the pool flow. This allows the two-fluid model to be further refined: simplifying it and reducing the computational time. A bundle-only two fluid model has been developed to accurately predict two-phase flow behaviour in the kettle reboiler tube bundle. Information available from earlier studies has been used to develop this model because of the difficulties associated with measuring the void fraction and velocities within the tube bundle. The model uses two different boundary conditions: (1) static liquid pressure in the pool and (2) variation of pressure in the pool based on the flow pattern transition. The results predicted by the model have been compared with experimental data and with one and two-fluid models at different heat fluxes. Boundary condition (1) was found to be in good agreement with experimental data and the one and two-fluid models at a heat flux of 10 kW/m2. This was because the transition flow pattern was not achieved and the bundle was surrounded by a static pool. Boundary condition (2) is based on the Kutateladze number (Ku), which sets the transition point from bubbly to intermittent flow at a certain height in the bundle. For Ku ≤ 1.09, the bundle flow would be surrounded by liquid, and if Ku > 1.09, the bundle flow would be surrounded by two-phase flow. At heat fluxes of 20 kW/m2 and above, boundary condition (2) has been found to be in good agreement with experimental data and the values predicted from the one and two-fluid models for liquid velocities, vertical mass flux and void fraction. The bundle-only model accurately predicts the trend line of constant and decaying pressure drop measured at low and high heat fluxes, respectively, and the observed flow phenomena in the kettle reboiler. The key feature of the model presented is that it allows two-phase flow in the kettle reboiler to be simulated by only modelling the tube bundle. Thus the model is simplified and less computational time is required. A central column model was developed using the minimum pressure gradient approach. The predicted results from this model were compared with experimental data and the values predicted by the two-fluid model and the bundle-only model. Reasonable agreement was obtained indicating that the flow distribution may be linked to the minimum pressure gradient

MHD heat transfer in W-shaped inclined cavity containing a porous medium saturated with Ag/Al2O3 hybrid nanofluid in the presence of uniform heat generation/absorption

© 2020 by the Authors. In this paper, a 2D numerical study of natural convection heat transfer in a W-shaped inclined enclosure with a variable aspect ratio was performed. The enclosure contained a porous medium saturated with Ag/Al2O3 hybrid nanofluid in the presence of uniform heat generation or absorption under the effect of a uniform magnetic field. The vertical walls of the enclosure were heated differentially; however, the top and bottom walls were kept insulated. The governing equations were solved with numerical simulation software COMSOL Multiphysics which is based on the finite element method. The results showed that the convection heat transfer was improved with the increase of the aspect ratio; the average Nusselt number reached a maximum for an aspect ratio (AR) = 0.7 and the effect of the inclination was practically negligible for an aspect ratio of AR = 0.7. The maximum heat transfer performance was obtained for an inclination of ω = 15 and the minimum is obtained for ω = 30. The addition of composite nanoparticles ameliorated the convection heat transfer performance. This effect was proportional to the increase of Rayleigh and Darcy numbers, the aspect ratio and the fraction of Ag in the volumetric fraction of nanoparticles

3D Magneto-buoyancy-thermocapillary convection of CNT-water nanofluid in the presence of a magnetic field

Anumerical study is performed to investigate the effects of adding Carbon Nano Tube (CNT) and applying a magnetic field in two directions (vertical and horizontal) on the 3D-thermo-capillary natural convection. The cavity is differentially heated with a free upper surface. Governing equations are solved using the finite volume method. Results are presented in term of flow structure, temperature field and rate of heat transfer. In fact, results revealed that the flow structure and heat transfer rate are considerably affected by the magnitude and the direction of the magnetic field, the presence of thermocapillary forces and by increasing nanoparticles volume fraction. In opposition, the increase of the magnetic field magnitude leads to the control the flow causing flow stabilization by merging vortexes and reducing heat transfer rate. © 2020 by the authors

Numerical study of periodic magnetic field effect on 3D natural convection of MWCNT-water/nanofluid with consideration of aggregation

In this paper, a numerical study is performed to investigate the effect of a periodic magnetic field on three-dimensional free convection of MWCNT (Mutli-Walled Carbone Nanotubes)-water/nanofluid. Time-dependent governing equations are solved using the finite volume method under unsteady magnetic field oriented in the x-direction for various Hartmann numbers, oscillation periods, and nanoparticle volume fractions. The aggregation effect is considered in the evaluation of the MWCNT-water/nanofluid thermophysical properties. It is found that oscillation period, the magnitude of the magnetic field, and adding nanoparticles have an important effect on heat transfer, temperature field, and flow structure. © 2019 by the authors

Natural Convection and Irreversibility Evaluation in a Cubic Cavity with Partial Opening in Both Top and Bottom Sides

A numerical study on natural convection in a cubical cavity with partial top and bottom openings is performed in this paper. One of the vertical walls of the cavity has higher temperature than that of the opposite one; the remaining walls are insulated perfectly. Three-dimensional simulations of governing equations have been performed using a finite volume technique. The results are presented for different parameters such as opening length and Rayleigh number. It is observed that heat transfer rate and fluid flow can be controlled via opening ratio size and Rayleigh number

Physico-chemical characterization of Grewia Monticola Sond (GMS) fibers for prospective application in biocomposites

New fibers extracted from plant barks are a recent subject of investigation as possible fillers for polymer composites. In this work, Grewia Monticola Sond (GMS) fibers have been characterized from a morphological, chemical, and thermal point of view. This involved using a number of techniques, including Fourier infrared spectroscopy (FTIR), X-ray diffraction (×RD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), atomic force microscopy (AFM), and 13C nuclear magnetic resonance (NMR). Results indicated that GMS fibers have relatively high cellulose (55%), with 33.5% crystallinity index and a crystallite size of 4.89 nm, and lower hemicellulose (14%) and lignin (15%) contents. The fibers showed cellulose degradation onset at around 324°C, while kinetic activation energy (74.18 kJ/mol) is quite low. The roughness of the fibers was in the range expected for similar natural fibers, and they may be considered silky according to their kurtosis values. As for possible application in composites, they would definitely withstand process temperatures with thermoplastics. However, their not very high crystallinity and low crystallite size might suggest a limited strength, to be verified in future after optimizing GMS fiber extraction from bark

Numerical Study of Periodic Magnetic Field Effect on 3D Natural Convection of MWCNT-Water/Nanofluid with Consideration of Aggregation

In this paper, a numerical study is performed to investigate the effect of a periodic magnetic field on three-dimensional free convection of MWCNT (Mutli-Walled Carbone Nanotubes)-water/nanofluid. Time-dependent governing equations are solved using the finite volume method under unsteady magnetic field oriented in the x-direction for various Hartmann numbers, oscillation periods, and nanoparticle volume fractions. The aggregation effect is considered in the evaluation of the MWCNT-water/nanofluid thermophysical properties. It is found that oscillation period, the magnitude of the magnetic field, and adding nanoparticles have an important effect on heat transfer, temperature field, and flow structure