Woodruff School of Mechanical Engineering
Theoretical Modeling of Laminar Annular Film Condensation in Microchannels
Georgia Institute of Technology
Thursday, October 26, 2017 at 2:00:00 PM
Love Building, Room 109
Condensers are essential components of many energy systems. In particular, microchannel condensers have attracted a lot of attention because of the superior heat transfer coefficients offered by small hydraulic diameters. Many studies have focused on understanding the condensation process in micro channels. Several experimental (Garimella et al., 2016) or purely mechanistic approaches, where the authors derive a model from first principles with numerical solutions (Wang and Rose, 2005), have been used. In the present work, a mechanistic model is developed to determine the film thickness distribution in microchannels for an arbitrary shaped symmetrical microchannel with constant wall temperature. Initially a model for the condensation of thin films in circular channels is derived in polar coordinates. The continuity and energy equations, in addition to simplifying assumptions over the velocity field, are used to develop a governing equation for film growth. The main assumption needed to generalize this equation to an arbitrary shaped channel is that the flow in the liquid film is always tangential to the channel wall. In addition, a mathematical transformation is made by replacing the polar angle with the tangential angle. After performing the mathematical transformation and using the local radius of curvature for the wall and the interface, an equation describing the film growth in an arbitrary shaped channel is obtained. Effects of gravity, surface tension, axial shear stress and pressure gradient are included in the equation. The model has been applied for two geometries, namely square and circular channels. Simulations are conducted for R134a condensing at a Tsat of 50oC, with a wall-to-saturation temperature difference of 6oC and a mass flux of 500 kg m-2s-1 in circular and square channels of 1 mm hydraulic diameter. For the same conditions, the square channel offers higher heat transfer coefficients at high vapor qualities. This is because of the thinning of the liquid film due to the retention of liquid at the corners of the square channel. A new model for laminar annular film condensation in micro channels is developed using these formulations. This model can address a wide range of arbitrary channel cross sections. The results obtained for R134a, Tsat= 50 oC and hydraulic diameter of 1mm show that the square channel has higher heat transfer coefficients than circular channels.
Khoudor Keniar is a PhD student at the Sustainable Thermal Systems Laboratory working on different topics ranging from state-of-the-art energy systems to fundamental heat and mass transfer in two-phase flows. He graduated with ME and BE in Mechanical Engineering from the American University of Beirut. He is a recipient of the ASHRAE Grant-In-Aid award. He previously worked on research projects related to robotic snakes at Carnegie Mellon University and to novel thermo-electric storage systems at Google X.
Refreshments will be served.