Experimental Study And Numerical Simulation Of Falling Film Flow Outside Of Horizontal Tubes

The open cycle absorption heat pump (OAHP) is one of the effective ways to recycle vapor and latent heat from flue gas of the gas combustion, and it is of great significance for the efficient use of natural gas and energy saving of gas appliances. Falling film absorption outside of horizontal tubes is a key process of the open cycle absorption heat pump, including complicated gas-liquid two phases flow and heat and mass transfer. Flow outside of horizontal tubes and the inter-tube flow have significant influence on heat and mass transfer between the gas phase including the non-absorbable gas and liquid film.This paper adopt the methods of experiment and numerical simulation, conducted research of falling film outside of multiple rows of horizontal tubes and characteristics and influence factors of flow mode transitions between tubes. The main research contents are as follows:Set up the test facility and conduct the visualization research of falling film flow outside of horizontal tubes with water and the concentration of20%,29%,40%,50%calcium chloride solutions. The least square method was used to fit the experimental data. Correlations for mode transitions were developed for considering and neglecting the direction of flow rate change (hysteresis phenomenon). The largest root mean square deviation for the correlation is9.478%. Meanwhile, the influence of countercurrent gas flow, tube diameter and pitch on the mode transitions of falling film flow between horizontal tubes was analyzed. The results show that, the transition Re of all four mode transitions neglecting the direction of flow rate change decrease, while the solution concentration increase. With the increase of We, the transition Re of droplet mode to droplet-jet mode, droplet-jet mode to jet mode, and jet-sheet mode to sheet mode increase, while, the Re of the jet to jet-sheet mode decrease. With the increase of tube spacing, the transition Re of all four mode transitions neglecting the direction of flow rate change increase. With the tube diameter increase from16mm to22mm, the transition Re of all four mode transitions neglecting the direction of flow rate change increase.Computational fluid dynamics software Fluent was used to simulate the flow. Half the length of two horizontal tubes was selected as the computational domain. And zoning and local encryption was used to discrete the domain. Assuming laminar flow, using the volume of fluid (VOF) model, setting gas-liquid two phases, specify the inter-phase interaction with surface tension and the contact angle between the liquid and wall. The results show that, droplet mode, droplet-jet mode and jet mode simulated which consistent with the flow regime observed in the experiment. And the deviation of the transition Re of droplet mode to droplet-jet mode, droplet-jet mode to jet mode, with the experiment results were13.67%and19.28%respectively, show that the model of falling film flow outside of the horizontal tubes set in this paper has a certain prediction accuracy while Re<400. Assumed the liquid mass flow rate of inlet is constant, and the flow mode between horizontal tubes are droplet, droplet-column and column, with the increase of the circular angle of measured point on tube surface, the film thickness decrease first and then increase. And the minimum value is near the105°～120°.Meanwhile, the fiｌｍ velocity at gas-liquid interface increases first, when increase to a certain value (for column of inter-tube mode, gets the maximum value near90°) and then decreases. For inter-tube mode are droplet, droplet-column and column, with the increase of Re, the film thickness increase. And for inter-tube mode is column, with the increase of Re, the film velocity at gas-liquid interface increase. For inter-tube mode is droplet and column, the film thickness of the first row is larger than the second for the upper tube, while the film thickness of the second row is larger than the first for the lower tube. For inter-tube mode is droplet, the film velocity of the first row is larger than the second. For inter-tube mode is typical column, with the circular angle between15°and45°, the film velocity of the first row is lesser than the second, while with the circular angle between45°and150°, the film velocity of the first row is larger than the second.