Hydrocarbon Micro-heat Exchangers for Process Applications a report by Shripad T Revankar1 and Nicholas R Brown2
1. Professor, School of Nuclear Engineering, Purdue University, Visiting Professor, Division of Advanced Nuclear Engineering, Pohang University of Science and Technology; 2. Temporary Postdoctoral Employee, School of Nuclear Engineering, Purdue University, School of Nuclear Engineering, Purdue University
Considerable advances have been made in recent years on the development of micro- or meso-scale heat exchangers that fully utilise both the high-energy density and heat transfer properties of hydrocarbons. In particular, long-chain hydrocarbon fuels have the highest energy density of known chemical energy carriers. In some applications, small-scale mechanical and electrical heat exchangers referred to as micro-electro-mechanical systems (MEMS) heat exchangers operate at high temperatures, enhancing the thermodynamic efficiency of the heat transfer. Higher temperatures can also be used to significantly reduce heat exchanger size with area densities in the range of 15,000 m2/m3.1
tool for macro-scale processes.3
These highly parallel systems allow for
increased energy transfer, reduction in moving parts and decreased probability of a catastrophic failure. Additionally, MEMS heat exchangers for power generation applications could serve important roles in vital systems, such as back-up power for telecommunications systems or emergency power for a nuclear generating station. There are also many military and medical applications for the use of hydrocarbons in MEMS heat exchangers. MEMS heat exchangers are the crux for the realisation of micron-to-millimetre scale energy systems and full utilisation of the energy density of hydrocarbons.
This high area density
results in swift and efficient heat transfer within the MEMS heat exchanger. Potential applications of hydrocarbon MEMS heat exchangers include energy converters, combustors, fuel processors, reformers, reactors and refrigerators.
Hydrocarbons do not cause depletion of the ozone layer and when used as refrigerants, have no direct impact on global warming.1
micro-scale heat exchangers may be used as a process intensification
Shripad T Revankar is a Professor of Nuclear Engineering and Director of the Multiphase and Fuel Cell Research Laboratory in the School of Nuclear Engineering at Purdue University, West Lafayette. He is also World Class University (WCU) Visiting Professor in the Division of Advanced Nuclear Engineering at Pohang University of Science and Technology (POSTECH), Pohang. He has received the American Society of Mechanical Engineers (ASME) heat transfer best paper and best teacher and
research awards from Purdue University. He has published extensively in major journals and conference proceedings with more than 250 peer-reviewed technical papers. He is an active member of the following societies: the American Nuclear Society (ANS), ASME, the Indian Society of Heat and Mass Transfer (ISHMT), the American Institute of Chemical Engineers (AIChE), the American Association for the Advancement of Science (AAAS), the American Society for Engineering Education (ASEE) and the Electrochemical Society (ECS). He has was Chair of Professional Divisions and Committees in ANS, ASME and ASEE, and is a Fellow of ASME.
E:
shripad@purdue.edu
Nicholas R Brown has experience as a student intern at Sandia National Laboratories and as a US Department of Homeland Security Fellow at Lawrence Livermore National Laboratory. He is also a member of the American Nuclear Society (ANS). Nicholas earned his Bachelor of Science in Nuclear Engineering (BSNE), summa cum laude, from the University of New Mexico (UNM), Albuquerque, in 2005 and his Master of Science in Nuclear Engineering (MSNE) from Purdue
University in 2007. He successfully defended his PhD dissertation, entitled: ‘Study of Hydrogen Generation Plant Coupled to High Temperature Gas Cooled Reactor’ in February 2011.
Additionally,
Flow Considerations in Micro-electro-mechanical Systems Heat Exchangers
There are many differences between flow through mini-channels and flow through macro-channels. Factors that are important include the surface area-to-volume ratio of micro-scale channels, the increased relevance of the surface tension and associated capillary forces and the relative magnitude of the surface roughness compared with the size of the flow channel. For very small channels (micron-scale), the length scale of the channel may approach the physical length scales involved in turbulent mixing; hence the vorticity of the flow itself may be suppressed.4
Thus, micro-channels may impact the inertial energy transfer through the turbulent flow.
Molecule polarity has also been shown to impact friction in micro-channels. For polar molecules, the friction coefficient has been shown to depend mainly on channel size, but for non-polar molecules the friction coefficient has been shown to depend mainly on Reynolds number.6
Micro-channel flow may encounter increased friction and the friction coefficient has been shown to increase with increasing channel aspect ratio.5
Polar molecules are also subject to electrical effects due to the charge of the channel walls. Transition from laminar-to-turbulent flow is also impacted by micro-channel geometry. As the characteristic length of the channel decreases, the transition Reynolds number decreases.5
Compressibility affects impact gas- phase flow in micro-channels significantly.
On the micron-scale, surface tension becomes more important as the relative magnitude of the capillary forces increases. Surface tension dominates when the characteristic length satisfies the Moalem-Maron criteria:4
2π2 σ ρι − ρ gD2 g > 1 (1)
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© TOUCH BRIEFINGS 2011
Heat Exchangers
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