Keynote Talk 9

Tuesday, January 5, 2010 / 17:30 – 18:15 hrs

Dropwise Condensation on Textured Surfaces: Issues and Prospects

Sameer Khandekar
Associate Professor, P. K. Kelkar Research Fellow, Department of Mechanical Engineering,
Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, 208016, India.
samkhan@iitk.ac.in

ABSTRACT

Heterogeneous condensation phase-change process is one of the most efficient ways of heat transfer in engineering systems, the heat transfer coefficients being orders of magnitude larger than single-phase convective paradigms. This phase-change process may result, under quasi-steady-state conditions, in either, (i) the formation of distinct droplet ensemble mode of condensing fluid, (ii) the formation of a continuous film on the cold substrate or, (iii) there can be a mixed mode too, having fuzzy overlapping characteristics of dropwise and film mode simultaneously. The heat transfer coefficient of dropwise mode is usually one order of magnitude higher than the other modes of condensation process. The preferred mode of condensation depends not only on the thermophysical properties of the fluid getting condensed, but also on the chemo-physical properties of the cold substrate.

Deploying dropwise condensation process in advanced engineering systems involves not only an understanding of the fundamental thermo-fluidic transport behavior but also the microscale issues associated with the substrate material. In this background, the following major research and development issues and problems are identified, amongst others:

Sustaining dropwise mode of condensation for prolonged periods of time on engineering surfaces by controlling the surface energy profile, so as to take the obvious advantage of high transfer coefficients, has been a classical technological problem which the industry has faced.
As the ensuing driving temperature difference is very small in dropwise mode of condensation, experimental determination of heat transfer coefficient is a challenge. The statistical nature of droplet distribution in the ensemble population, further contributes to the intricacy of analysis and interpretation.
Mathematical modeling of the entire dropwise condensation process, from atomistic level to macroscale individual droplet formation followed up by the creation of quasi-steady droplet population continues to be a challenge.
Since individual droplet formation, shape and resulting dynamics depends on microscale surface energy distribution, surface roughness characteristics and thin film characteristics etc. near the surface, macroscale effects are intrinsically linked to microscale phenomena which necessitate hierarchical modeling.
Understanding of the contact line motion, droplet merging and instabilities, contact angle hysteresis and metastable thermodynamics continue to pose a challenge in macroscale models.

In this paper we present a holistic view of the complete hierarchy of the processes involved in dropwise mode of condensation on plain and textured surfaces. The present state-of-the art on the subject has been critically scrutinized. The subject matter is presented in the following sequence:

Atomistic viewpoint of the droplet formation, involving cluster of adatoms, which eventually leads to thermodynamically stable nuclei is presented (refer Figure 1-a, b). A population balance model which captures the major engineering process parameters is highlighted.
Experimental and numerical analysis of thermo-fluid transport behavior of an individual droplet is discussed. (refer Figure 2 and 3). Quantities reported are 3Dvelocity, pressure, temperature and transport fluxes inside a single droplet.
A macroscopic model, incorporating the statics of an isolated individual droplet and dynamics of an ensemble of condensing droplets, formed under dropwise condensation is presented. Numerical computations are conducted for various contact angles and substrate inclination angle as a parameter. Drop size distributions and spatial patterns of condensation are presented for various liquid. The model predictions are compared to experimental data. Experiments have also been done to study the contact angle hysteresis and droplet stability under physically and chemically textured surfaces (refer Figure 4 and 5).
Experimental heat transfer measurements by Liquid Crystal Thermography, under condensing droplets are presented. Local heat transfer below a droplet and average heat transfer coefficients under controlled conditions are reported (refer Figure 6, 7 and 8).

It is concluded that the classical art of dropwise condensation process continues to pose challenges in contemporary times too. Advent of nano-technology, breakthroughs in thin film coating, physical and chemical texturing technologies, advanced manufacturing and superior experimental techniques has allowed deeper understanding of the process control parameters.