بسم الله الرحمن الرحیم

1. بسم الله الرحمن الرحیم

2. Seminar Dr.Sarreshtehdari Farhad Abbassi Amiri Shahrood university of technology

3. Numerical Investigation of Nanofluids’ Heat Transfer

4. Introduction • Nanofluids are a relatively new class of fluids which consist of a base fluid with nano-sized particles (1–100 nm) suspended within them. It is introduced by choi on Argonne National Laboratory at1995.

5. Concept of Nanofluids • Conventional heat transfer fluids have inherently poor thermal conductivity compared to solids. • Conventional fluids that contain mm- or m-sized particles do not work with the emerging “miniaturized” technologies because they can clog the tiny channels of these devices. • Modern nanotechnology provides opportunities to produce nanoparticles. • Argonne National Lab (Dr. Choi’s team) developed the novel concept of nanofluids. • Nanofluids are a new class of advanced heat-transfer fluids engineered by dispersing nanoparticles smaller than 100 nm (nanometer) in diameter in conventional heat transfer fluids. Solids have thermal conductivities that are orders of magnitude larger than those of conventional heat transfer fluids.

6. Why Use Nanoparticles? • The basic concept of dispersing solid particles in fluids to enhance thermal conductivity can be traced back to Maxwell in the 19th Century. • Studies of thermal conductivity of suspensions have been confined to mm- or mm-sized particles. • The major challenge is the rapid settling of these particles in fluids. • Nanoparticles stay suspended much longer than micro-particles and, if below a threshold level and/or enhanced with surfactants/stabilizers, remain in suspension almost indefinitely. • Furthermore, the surface area per unit volume of nanoparticles is much larger (million times) than that of microparticles (the number of surface atoms per unit of interior atoms of nanoparticles, is very large). • These properties can be utilized to develop stable suspensions with enhanced flow, heat-transfer, and other characteristics

7. Advantages of nanofluids • Compared to conventional solid-liquid suspensions for heat transfer intensifications, properly engineered thermal nanofluids possess the following advantages: • 1. High specific surface area and therefore more heat transfersurface between particles and fluids. • 2. High dispersion stability with predominant Brownian motion ofparticles. • 3. Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification. • 4. Reduced particle clogging as compared to conventionalslurries, thus promoting system miniaturization. • 5. Adjustable properties, including thermal conductivity andsurface wettability, by varying particle concentrations to suitdifferent applications.

8. Materials for Nanoparticles and Base Fluids • 1.Nano-particle materials include: • Oxide ceramics – Al2O3, CuO • Metal carbides – SiC • Nitrides – AlN, SiN • Metals – Al, Cu • Nonmetals – Graphite, carbon nanotubes • Layered – Al + Al2O3, Cu + C • PCM – S/S • Functionalized nanoparticles • 2.Base fluids include: • Water • Ethylene- or tri-ethylene-glycols and other coolants • Oil and other lubricants • Bio-fluids • Polymer solutions • Other common fluids

9. Methods for Producing Nanoparticles/Nanofluids • Two nanofluid production methods has been developed: • In two-step process for oxide nanoparticles (“Kool-Aid” method), nanoparticles are produced by evaporation and inert-gas condensation processing, and then dispersed (mixed, including mechanical agitation and sonification) in base fluid. • A patented one-step process (see schematic) simultaneously makes and disperses nanoparticles directly into base fluid; best for metallic nanofluids.

10. Applications of nanofluids • Transportation (Engine cooling/vehicle thermal management) • Electronics cooling • Defense • Space • Nuclear systems cooling • Heat exchanger • Biomedicine • Other applications (heat pipes, fuel cell, Solar water heating,chillers, domestic refrigerator, Diesel combustion, Drilling,Lubrications, Thermal storage,…).

11. Applications in heat transfer Properties of consideration • Single phase convective heat transfer • Properties of interest: k, cp, viscosity • Particle size, shape and concentration • Multiphase flow and heat transfer • Properties of interest: k, cp, viscosity, surface tension, wetting. • Particle size, shape and concentration • Interaction of particles with surface

12. Two-phase Methods • Eulerian-Eulerian • Eulerian-Lagrangian • Mixture • VOF (volume of fluid)

13. Results and discussion: • Numerical and experimental studies in this field show that there are some parameters that can enhance the heat transfer coefficient, includig: • Nano-particle concentration • Nano-particle size • Re number • Pe number • Interaction between particles • Sphericity of nano-particles • Axial distance from the channel inlet

14. Nano-particle concentration & size

15. Re & Pe numbers

16. Axial distance from the channel inlet

17. Challenges of nanofluids • lack of agreement of results obtained by different researchers • lack of theoretical understanding of the mechanismsresponsible for changes in properties • poor characterization of suspensions • stability of nanoparticles dispersion • Increased pressure drop and pumping power • Nanofluids thermal performance in turbulent flow and fullydeveloped region • Higher viscosity, Lower specific heat • High cost of nanofluids • Difficulties in production process

18. References: • thermalscienceapplication.asmedigitalcollection.asme.org • Argonne National Laboratory (ANL), Dr. S.Choi & Dr. J.Hull • www.kostic.niu.edu • www.researchgate.net • www.nanoscalereslett.com

19. Thank you