Encyclopedia of Two-Phase Heat Transfer and Flow II

The following sections are included:

• Contents
• About the Editors
• List of Contributors

An overview concerning commercially available heat transfer augmentation techniques used in evaporation covering pool boiling, falling film on horizontal tubes, flow boiling inside tubes, and external to tube bundles is presented. The main techniques are identified, the literature on these topics is described and, when available, predictive methods for pressure drop and heat transfer coefficient are presented and compared against independent data available in the literature.The emphasis is on studies published after 2005. Previous literature reviews are cited for those interested in earlier studies.

In this chapter the main aspects of the heat transfer phenomena of nucleate pool boiling with smooth and enhanced surfaces are presented and discussed. This chapter first covers pool boiling on smooth surfaces and afterwards pool boiling on enhanced surfaces. Also, a section on boiling of refrigerant mixtures presents a method to calculate the effects of mass transfer of refrigerant mixtures on the pool boiling heat transfer. For both smooth and enhanced surfaces, first are described the physical processes guiding the heat transfer phenomena. These points are then illustrated by a set of experimental data which allows one to observe the effect of each experimental parameter on the heat transfer process. A set of selected nucleate pool boiling heat transfer correlations is then presented, discussed and compared. Through this presentation, the most important aspects of nucleate pool boiling will be highlighted and explained, and thus provide a guide to the selection of the most appropriate prediction model for the application.

In this chapter flow patterns, heat transfer and pressure drops characterizing two-phase flows in flooded evaporators (bundle boiling) are presented and discussed. First, the particularities of bundle boiling with regards to pool boiling are introduced. Flow patterns and respective measurement techniques and experimental results are presented. Bundle boiling heat transfer and different bundle effects are then discussed before introducing plain and enhanced tubes bundle boiling heat transfer correlations. Dryout mechanisms are then discussed, followed by pressure drops in bundle boiling. The objective of this chapter is to capture the most important phenomena related with heat transfer and pressure drops in flooded evaporators. In particular, the models presented here will allow one to correctly size and optimize the design of this type of two-phase heat exchanger.

This chapter presents an overview of two-phase heat transfer and pressure drop within plate heat exchangers. First, a comprehensive literature survey of experimental evaporation and condensation studies and their associated prediction methods is detailed. Next, a sensitivity analysis on the prediction methods is performed to consider the effect of plate geometry and dimensions on thermal-hydraulic performance. Furthermore, a comprehensive experimental databank has been collected from numerous independent research studies. The databank is utilized here to evaluate the existing PHE's two-phase prediction methods available in the open literature. This evaluation can be practical for design engineers to select the most suitable/reliable correlations according to the relevant operating conditions to perform a finer design. Based on the present databank, the Würfel and Ostrowski [2004] prediction method was in the best agreement with the condensation heat transfer coefficients data; in the case of evaporation, the Danilova et al. [1981] correlation exhibits the best agreement with saturated flow boiling data. The frictional pressure drop data were well predicted using the Huang et al. [2012] method.

The effects of low gravity on pool boiling heat transfer are presented. A short overview of boiling and the facilities used are provided and recent results are reviewed. Topics covered include bubble dynamics, nucleate pool boiling, critical heat flux, transition boiling, boiling of mixtures, electric field effects, acoustic effects, and quenching.

Flow boiling in reduced gravity conditions provides important information needed for the development of high-performance space thermal management systems in addition to the means to obtain fundamental scientific knowledge about the interfacial phenomena in the presence of various dynamic forces. In this chapter, the objectives of the studies on flow boiling under reduced gravity are presented followed by descriptions of experimental facilities. A review of recent flow boiling research concerning heat transfer characteristics and critical heat flux conditions is presented including quenching experiments. In the last section, the proposed research activities for a planned experiment on the International Space Station are described.

Boiling is used in a vast number industries, playing a critical role in electrical power generation, chemical processing, water purification, and HVAC, to name a few. As such, even modest enhancements of boiling heat transfer will translate directly to substantial energy and cost savings on a large scale. The realization of next-generation technologies to enhance boiling heat transfer is of crucial importance due to their impact on energy, the environment, and water resources, as well as their potential in high-heat-flux thermal management schemes. While the enhancement of boiling heat transfer using “macroscale” structures has been in practice since the 1950's, scaling these structures down radically increases surface area and capillarity, leading to substantial increases in heat transfer. Recent studies have shown that high-surface-area coatings comprised of micro and nanoscale structures can be used to significantly increase performance during pool boiling. Using cutting-edge fabrication techniques, researchers have developed novel structures to enhance both critical heat flux as well as heat transfer coefficient by influencing the contact between liquid, vapor, and solid interfaces. This chapter provides an in-depth review of state-of-the-art micro and nano fabrication techniques, the novel surface modifications that can be created using these techniques, and their impact on various processes and phenomena during pool boiling heat transfer.

Heat pipes are thermal devices that can be used to transfer heat from one location to another. Their advantages include high thermal conductivity, no mechanical or electrical input, low maintenance, low operation cost, and structural flexibility. This chapter provides an overview of heat pipe technology and applications. The first part of this chapter discusses heat pipe fundamentals, heat transfer limitations, wick structures, working fluids, and non-conventional heat pipes. The second part provides a review of heat pipe applications in the areas of electronics cooling, dehumidification, recovery of waste heat, thermal management of satellites, cooling of PVcells, solar thermal water heating, and stabilization of permafrost.

This chapter covers flow pattern based models for evaporating flows inside plain horizontal smooth tubes. First, the various fundamental elements of two-phase flows are presented, which are the building blocks of flow pattern based models. Then, the flow pattern map of Wojtan et al. (2005b) is presented and described. Next, the flow pattern map based boiling heat transfer model of Wojtan et al. (2005c) is presented and then finally the flow pattern based frictional pressure drop model of Da Silva Lima and Thome (2012c) is described. For all presented models, a parametric study is then provided, including graphical outputs that will help the reader understanding and implementing these models.

This chapter presents an experimental study on saturated flow boiling heat transfer in a multiport extruded aluminum tube. Upward flow boiling experiments were carried out in a multiport extruded aluminum tube, which is composed of seven parallel rectangular channels (1.1mm × 2.1mm) with a hydraulic diameter of 1.4mm. Two refrigerants, R245fa and R1234ze, the latter a recent environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new data reduction approach that accounts for either uniform or nonuniform local heat flux distribution along the channel was developed to reduce the flow boiling heat transfer data. Effects of heat flux, mass flux, vapor quality and saturation temperature on flow boiling heat transfer in multiport tubes were covered. A new flow regime, termed “intermittent dryout”, was realized to prevail at low mass flux and relatively high heat fluxes and thus the corresponding experimental data have been used to propose a new updated microscale diabatic flow pattern map for identifying the flow regimes, including: isolate bubbly (IB), coalescing bubbly (CB), annular (A), intermittent dryout (ID) and post dryout regimes (PD). The experimental results were compared with several well-known correlations to evaluate their capabilities. The analysis showed that the three-zone model for transient elongated bubbles flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannelswith measured surface roughnesses. Finally, a flow pattern-based asymptotic prediction method for flow boiling in small channels was proposed, consisting of the two well-known methods (i.e. Thome et al. [2004] for the isolated and elongated bubbles regime together with that of Cioncolini and Thome [2011] for the annular regime) together with a method for the new ID regime.