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Augmentation of Boiling and Evaporation
DOI 10.1615/hedhme.a.000199
2.7.9 Augmentation of boiling and evaporation
J. R. Thome and G. Ribatski
The augmentation of boiling heat transfer is one of the most exciting and dynamic areas of thermal engineering. Although utilization of enhanced boiling surfaces is now a standard practice of heat exchangers designers and manufacturers, especially for refrigeration and climatization industries, extensive research on this topic continues. Potentially, heat transfer augmentation techniques are capable of being applied to any heat exchanger having the following limiting parameters: reasonable manufacture processes and a favorable reduction in the initial and operational costs. Here, an overview is presented on the principal, commercially available heat transfer augmentation techniques used in evaporation, covering pool boiling, flow boiling within a tube and bundle boiling. The main techniques are identified, the literature on these topics described and, when available, pressure drop, heat transfer coefficients and CHF predictive methods are presented. The emphasis will be on more recent work while previous literature reviews will be cited for those interested in older work.
Other boiling enhancement techniques exist, such as the use of an aqueous surfactant or a polymeric additive, electric fields (EHD), etc. to enhance heat transfer, but these are still either not widely used or are not yet appropriate for practical application. Literature surveys by Cheng et al. (2007), Webb (1994) and Wasekar and Manglik (1999) on the use of surfactants and additives and by Eames and Sabir (1997) and Webb (1994) on electro-hydrodynamic (EHD) enhancement of boiling heat transfer are suggested here as reference studies.
A. Pool boiling
Over the past 70 years, the mechanisms of pool boiling heat transfer have been intensively investigated to better understand the boiling phenomenon of nucleate pool boiling, viz. nucleation site characteristics, pool boiling regimes, critical heat flux, bubble growth, bubble departure dynamics and the development of physical models and correlations to predict heat transfer. In addition to the studies for plain surfaces and tubes, there have been extensive efforts made to augment nucleate boiling heat transfer by means of special structures and plain surfaces covered with novel porous coatings. The joint effort by academic research, providing a better understanding of the boiling phenomenon, and by industry, providing both new geometries and technology for their fabrication, has led to the development and continuous improvement of commercially viable enhanced boiling surfaces.
Enhanced boiling surfaces are widely used in flooded evaporators, falling-film evaporators, direct-expansion evaporators, compact heat exchangers and cooling coils in refrigeration and air-conditioning systems, and to a lesser extent in reboilers in chemical processing plants. In these applications, the improvement of the heat transfer performance minimizes the evaporator size, resulting in reduced initial costs and space requirements, and can also be used to increase the evaporation temperature, improving the efficiency of the system. Moreover, the need for smaller and more effective heat exchangers has also motivated the development of enhanced surfaces for the electronics industry for cooling of high-power density components. Figure 1 shows schematically the structure of some earlier pool boiling enhanced surfaces. In this figure, it can be noted that the main point in the development of such surfaces is obtain a high density of reentrant grooves and tunnels interconnected below the surface to mimic that of metallic porous coatings.
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