16 th Australasian Fluid Mechanics Conference Crown Plaza, Gold Coast, Australia 2-7 December 2007 Liquid Film Falling on Horizontal Circular Cylinders F. Jafar, G. Thorpe and O.F. Turan School of Architectural, Civil and Mechanical Engineering Victoria University (F113), PO Box 14428, Melbourne Australia 8001 Abstract The objective of this study is to investigate experimentally and numerically the behaviour of liquid film flow over horizontal cylinders. Numerical simulations are performed using a CFD code (FLUENT) for 2D configurations with one, two and three cylinders. The numerical results have been compared with the present experimental results as well as those from the literature. The flow modes and film thickness are reported for the Reynolds numbers range of 400 and 3200. The effect of cylinder separation on the film flow is investigated. List of Symbols Ar Archimedes number = g ! 2 D 3 / µ 2 D Tube diameter, m g acceleration due to gravity, m/s -2 Ga Galileo number = ! 3 ! / µ 4 g (dimensionless) Re Film Reynolds number = 4 / ! µ S Distance between tubes, m Greek symbols ! Density, = 998.2 kg/m 3 for water µ Newtonian dynamic viscosity, kg/m.s = 0.001 kg/m.s for water ! Surface tension, kg/s 2 = 0.073 for water ! Mass flow rate of the liquid film per unit length of tube (for each side), kg/ms ! Liquid film thickness, m ! Capillary constant = [ ! / ( ! g)] 5 . 0 , m ! Instability wavelength, spacing between neighbouring jet or droplets, m 1. Introduction Liquid films flows over horizontal tubes are encountered in several industrial processes such as in chemical petroleum refining, and heat exchangers. These flows are significant in desalination, refrigeration and food and dairy industries. This paper is a progress report of the present study towards effective cooling of fresh horticultural produce. It contains experimental and 2D numerical predictions compared with those obtained from the literature. In 1936, Adams and Conn [1] studied falling liquid film in heat exchangers. They reported advantages where the pressure drop of the liquid flowing over tubes is negligible, the quantity of cooling liquid required was small and the heat transfer coefficients were high. Maron – Moalem et al [2] experimentally observed several working media of falling film liquid over a tube. These researchers focused on dripping characteristic and not on mode transitions. In 1980, Yung et al. [3] studied the flow mode transitions, they associated the mass flow rate for a transition from the droplet mode to the jet mode with the capillary effect of through the capillary constant, ! . The flow rate per unit length of the tube at the transition was given by ! = 0.81 ! " ( 6 3 dp ! ) 3 2 !" #$ 2 1 (1) where ! = ! [4 2 ! n] 5 . 0 (2) n = 2 and d p is the diameter of primary drops, experimentally determined for water and alcohol to be d p = 3 ! (3) Rogers [4] solved the motion and energy equations for laminar film flow falling on horizontal tubes. He calculated the laminar film thickness at any position as function of Reynolds number (Re), Archimedes number (Ar) and the angular position on the horizontal tube. Mitrovic [5] investigated the falling film - flow mode-transitions of adiabatic and non–phase change film for plain tubes and found that transition from the droplet mode to the jet mode occurred at Reynolds numbers between about 150 and 200. The transition from jet mode to the sheet mode occurred at Reynolds numbers between 315 and 600. He also found that the heat transfer coefficient increased with feeder height, along the whole surface perimeter. The falling-film mode-transitions from Mitrovic are shown in Figure 1. Honda et al. [6] presented several transition expressions for fluids condensing on low finned tubes, their study are similar to that of an adiabatic falling film of Mitrovic [5]. Rogers and Goindi [7] experimentally calculated the film thickness of water falling on a circular horizontal tube as follows: 3 1 3 1 min Re 186 . 1 ) ( ! = Ar d " (4) Armbruster and Mitrovic [8] modelled the mode transitions for droplet, jet and sheet modes according to Re = A Ga 4 1 (5) where A is an empirical constant whose value defines the type of transition. They observed the mode transition depended on the liquid mass flow rate, only, regardless of increasing or decreasing mass flow rates. Fujita and Tsutsui [9] defined the two flow modes as follows: 1. Separate droplet mode, characterized by the liquid falling as jet and then, separated into droplets as the falling velocity increases 1193