Heat Pipes and Thermosyphons for Solid Sorption Machines and Fuel Cells Thermol Management

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Heat pipes are very flexible systems with regards to the effective thermal control of different heat loaded devices. Their heat transfer coefficient in the evaporator and condenser zones is 103 - 105 W/m2K, heat pipe thermal resistance is 0.01-0.03 K/W, therefore lead to smaller area and mass of heat exchangers. Heat pipes and thermosyphons are easily integrated into fuel cells, simple and multistage adsorption systems and ensure the heat and mass recovery from one stage to another. In this chapter different types of heat pipes are considered, including miniature and micro heat pipes for fuel cells, electronic components cooling and space two-phase thermal control systems, loop heat pipes, pulsating heat pipes and sorption heat pipes. Vapordynamic thermosyphons are suggested as the novelty for modern heat exchangers and air-conditioning systems. The new stream in the heat pipe technology is related also with nano fluid application and nano coating of the heat pipe evaporators, You M., Kim J.H, Kim K.H., 2003; Das S.K, Putra N., Roetzel, W., 2003. The generation of engineered nanomaterials represents a major breakthrough in material science and nanotechnology-based materials. Recent advances in nanotechnology have allowed development of a new nanofluids (NF), to describe liquid suspensions containing nanoparticles (NP) with thermal conductivities, orders of magnitudes higher than the base liquids, and with sizes significantly smaller than 100 nm, Vassallo, P., Kumar, R., Amico, S. 2004; Bang, I.C, Chang, S.H., 2005. Considering the rapid increase in energy demand worldwide, intensifying heat transfer processes and reducing energy losses due to ineffective use have become increasingly important task (Wen, D., Ding, Y. 2005). NFs and NPs have some interest to be applied as the means to increase the heat transfer intensity in heat pipes and thermosyphons, Mitrovic. J.,2006; Vasiliev L.L.,et al, 2004. NFs are very stable due to the small size and volume fraction of NPs needed for heat transfer enhancement. When the NPs are properly dispersed, NFs can offer numerous benefits besides the anomalously high effective thermal conductivity, such as improved heat transfer and stability, microchannel cooling without clogging, the possibility of miniaturizing systems scaling, or reduction in pumping power, among others, Vasiliev, L., Lapotko, D., et al.,2007. Thus, NFs have a wide range of industrial, engineering, and medical applications in fields ranging from transportation, micromechanics, heating, ventilating and air conditioning systems, biomolecules trapping, or enhanced drug delivery. Nanostructures and nanomaterials are getting more and more commonly used in various industrial sectors like cosmetics, aerospace, communication and computer electronics. Nanocoatings (NCs) have a grand potential to increase the wettability and heat transfer intensity in small size heat transfer devices as mini heat pipes, miniature heat exchangers. In addition to the associated technological problems, there are plenty of unresolved scientific issues that need to be properly addressed. Nucleate boiling of NFs is the subject of numerous studies, which cover questions related with formation of bubble nuclei in metastable liquids and on the micro/nano porous coatings. Some further detailed investigations are necessary to understand the phenomena of boiling of NFs. In particular experiments are lacking on the effects of NPs material and heat loaded surface on the boiling heat transfer from horizontal smooth tubes. The general conclusion is that micro and nano particles on the heat loaded surface initiate a bubble generation and enhance two phase heat transfer, You M., Kim J.H, Kim K.H. 2003; Das S.K, Putra N., Roetzel, W. 2003. Some experimental investigations have revealed that NFs have remarkably higher thermal conductivity and greater heat transfer characteristics than conventional pure fluids, Vassallo, P., Kumar, R., Amico, S. 2004; Bang, I.C, Chang, S.H. 2005. Till now all above mentioned experiments with nanofluids are performed in conventional heat pipes, Wen, D., Ding, Y. 2005.

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