Technical Background: Flow Condensation

Flow Condensation in TubeModern technologies are demanding more effective schemes to tackle heat removal from very high power density devices.  The vast majority of published research addressing thermal management has been dedicated to heat extraction from the device itself rather than the ultimate heat rejection.  This trend is driven by the assumption that a commercial condenser could always be found to reject the heat from virtually any thermal management system.  But, while compact boiler designs, such as those employing micro-channels or jet-impingement, have been highly effective for these applications, packaging requirements impose equally stringent constraints on the condenser’s size, and commercial condensers are often far too large for these applications.

Flow Condensation in Micro-channelsPresently, there is limited understanding of interfacial behavior, and shortage of reliable predictive tools for pressure drop and heat transfer coefficient for flow condensation in small passages.  Several flow regimes are encountered in condensation, which include, in succession, pure vapor, annular, slug, bubbly, and pure liquid regimes.  The annular regime is of particular significance, given that it prevails over the largest fraction of condensation length, and is responsible for achieving the highest heat transfer coefficients among the different flow regimes.  The annular regime consists of a thin annular film that sheathes the flow passage’s inner wall, shear-driven by a central, fast-moving vapor core.  Two phenomena that are important to modeling annular flow are interfacial waves and turbulence in the annular liquid film.  The interfacial waves influence all aspects of mass, momentum, and heat transfer in the film.  Turbulence undergoes appreciable dampening near the liquid film-vapor core interface due to surface tension forces, and this behavior can have a significant influence on heat transfer across the annular film.  During the last three decades, Mudawar’s team has made significant advances in the development of predictive tools for flow condensation in both large and small flow passages, including experimental correlations and theoretical models.