Title:

An experimental and numerical study of twophase 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 twophase 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 twophase flow in a thinslice kettle reboiler. The kettle reboiler contained 241 electrically heated tubes arranged as 17 rows of 17 columns in an inline layout with an outside diameter of 19 mm and a pitchtodiameter 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 twodimensional 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 twophase 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 allliquid value. At a heat flux of 20 kW/m2 and above, the pressure drop was found to increasingly fall below the allliquid 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 twofluid model has been applied. The twofluid model’s drag coefficient and tube resistance were deduced from a onedimensional model. The twofluid 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 twofluid model to be further refined: simplifying it and reducing the computational time. A bundleonly two fluid model has been developed to accurately predict twophase 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 twofluid models at different heat fluxes. Boundary condition (1) was found to be in good agreement with experimental data and the one and twofluid 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 twophase 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 twofluid models for liquid velocities, vertical mass flux and void fraction. The bundleonly 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 twophase 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 twofluid model and the bundleonly model. Reasonable agreement was obtained indicating that the flow distribution may be linked to the minimum pressure gradient.
