Flow and Heat Transfer in Liquid Films Flowing Over Highly Curved Surfaces

Large-scale evaporative cooling is one of the leading sources of fresh water consumption. Dry cooling based on existing heat exchangers, however, has found limited usage due to the high cost and large foot prints/weights. Development of alternative low-cost light-weight heat exchangers for dry cooling is therefore urgently needed. One promising design for such alternative heat exchangers is what we call Direct-contact Liquid-on-String Heat Exchangers (DILSHE). DILSHE consists of a vertically aligned array of inexpensive polymer strings. A nonvolatile liquid flows over the strings, forming thin liquid films. Large surface areas provided by these films enable efficient heat transfer to counter-flowing cooling air. Physics-based design and optimization of DILSHE requires rigorous understanding of flow and heat transfer phenomena of falling liquid films on highly curved surfaces. Formation of travelling beads through the Rayleigh-Plateau or Kapitza instability can enhance heat transfer across liquid-gas interfaces. We have developed a numerical model for liquid-gas flows and heat transfer in the drop-like regime, where the Rayleigh-Plateau instability dominates and the shape of travelling beads is governed mainly by the influence of surface tension. We solve the Young-Laplace equation to obtain the liquid bead shape, which was then used to construct a finite element model. The time-dependent Navier-Stokes equation and the energy equation were then solved to obtain velocity and temperature distributions in the liquid and the surrounding counter-flowing air. The temporal and spatial variations in the temperature of travelling beads are analyzed to evaluate the effective heat transfer coefficients, which are key input parameters for an overall heat exchange model to quantify the heat transfer characteristic of DILSHE. The present work helps build foundation for systematic design of new generations of heat exchangers for dry cooling.