Heat Transfer Performance for a Falling-Film on Horizontal Flat Tubes

Abstract

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach. Experiments are conducted using water, ethylene glycol. and their mixture (50% by volume) under different heat fluxes and tube spacing, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube. The distribution of the heat transfer coefficient along the axial direction depends on the flow mode: it is constant for the sheet mode, shows small variations for the jet mode, and has variations as large as 20% for the droplet mode. Heat flux has almost no effect on the average Nusselt number within the experimental range. The average Nusselt number for the flat tube is close to that for round tubes in the droplet flow mode, however, in the jet and sheet modes the flat-tube Nusselt number is much larger than the round-tube Nusselt number. Boundary-layer theory is used to explain the local heat transfer coefficient distribution and the experimental data show good agreement with the boundary-layer theory for most cases. New curve fits for the average heat transfer coefficient for three flow modes at different tube spacing are provided and the maximum deviation of the data from the fit is less than 14%.

Dominant flow mechanisms in falling-film and droplet-mode evaporation over horizontal rectangular tube banks

“…In addition, insight has been gained into flow mode transitions, wave characteristics, dryout, and other key mechanisms. To the authors' knowledge, only Wang et al (2010Wang et al ( , 2011 have studied falling-films over similar rectangular tube geometries. Their initial work (Wang et al, 2010) focused on the flow modes and transitions of water and ethylene glycol in adiabatic conditions with no vapor flow over tubes with a height of 25.4 mm and width of 3.18 mm, and recognized the potential for a microchannel evaporator with in-tube condensation and external evaporation.…”

“…In addition, they noted several potential differences between round and flat tubes, including the possibility that the Taylor instabilities known to determine the spacing between droplet departure sites will not be as significant on flat tubes, and that different gravitational and shear force distributions could result in new velocity profiles, leading to new flow patterns. A second study (Wang et al, 2011) explored the local and average sensible heat transfer coefficients for falling-films on horizontal flat tubes for a range of heat fluxes, tube spacings, and flow rates. They found that the average Nusselt numbers for flat tubes are similar to those for round tubes in the droplet mode, but are approximately double those of round tubes in the jet and sheet flow modes.…”

Film Flow of Nano-Micropolar Fluid with Dissipation Effect

“…• Charogiannis et al [34] conducted experiments on thin film flows over inclined glass planes driven by gravity, studying the relationship between the Nusselt number and Reynolds number. • Wang et al [35] investigated the thermal influence on thin films under a thermal source environment.…”