Ene-39.4055 Irreversible Thermodynamics

1. Ene-39.4055 IrreversibleThermodynamics Energy Conversion and Management Volume 52, Issue 2, February 2011, Pages 1397-1405 M. Moghaddami, A. Mohammadzade, S.A.V. Esfehani Second law analysis of nanofluid flow Arttu Meriläinen

2. A nanofluid is a suspension of ultrafine particles (<100 nm) in a conventional base fluid SiO2-H2O nanofluid: Picture taken with a transmission electronmicroscope (cryogenicsample) SiO2-H2O nanofluid: Particlesizemeasurementsbased on dynamiclightscatteringusingMalvernZetasizer

3. Introduction • A nanofluid is a suspension of ultrafine particles (<100 nm) in a conventional base fluid • Nanofluids have unique transport properties that differ from conventional suspensions. Nanoparticles do not settle under gravity • Experimentally studied water based metal oxide nanofluids have enhanced heat transfer characteristics even with a low volume fraction of nanoparticles • Numerous parameters affect the thermal conductivity and convective heat transfer of nanofluids, which leads to significant discrepancies between different publications • 1. Particle volume fraction 5. Temperature • 2. Particle material 6. Suspension stability • 3. Particle size and size distribution 7. Additives • 4. Particle shape 8. Acidity (pH) • Convection: Heat transfer is enhanced but also the pressure drop is increased • This paper (Second law analysis of nanofluid flow) analytically studies convective heat transfer inside a circular pipe with constant heat flux. The objective is to find an optimum trade-off between enhanced heat transfer performance and pressure drop by minimizing entropy generation

4. Al2O3-H2O nanofluidproperties (d = 28 nm) densityρ [kg/m3] specificheatcapacity cp [kJ/kg] dynamicviscosity µ [kg/(s·m)] thermalconductivity k [W/(m·K)] nanoparticlevolumefraction nfnanofluid bfbasefluid p particle

5. Entropygeneration in pipeflow with constantheatflux Entropygenerationrate per unitlengthS’gen [J/(K·s·m)] 1st law 2nd law Frictionfactor f q’: heatflux per unitlength T: bulkfluidtemperature ΔT: temperaturedifferencebetweenfluid and wall Tw: walltemperature Entropygenerationnumber Augmentationentropygenerationnumber

6. Laminarflow Entropy generation number of water–Al2O3nanofluid in laminar flow. Augmentation entropy generation number of water–Al2O3nanofluid in laminar flow. • Heattransferirreversibility is dominant; pressuredropalmostnegligible • Entropygeneration is decreased, whennanoparticlesareadded • OptimumRe-numberexists

7. Turbulentflow Entropy generation number of water–Al2O3nanofluid in turbulent flow. Augmentation entropy generation number of water–Al2O3nanofluid in turbulent flow. • OptimumRe-numbersexist • For lowRe-numbers the heattransferirreversibility is dominant • For highRe-numbers the fluidflowirreversibility is dominant • For Re < 50000, the thermodynamicperformance is increased • For Re > 50000, the thermodynamicperformance is decreased

8. Summary • Optimumdesign of a thermalsystembyminimizing the entropygeneration • Nanofluidscanbeused to increasethermodynamicperformance • However, addingnanoparticles to the basefluid is disadvantageouswhenRe > 50000 • Questionableassumptions: • thermalconductivity and viscosity of nanofluidsbased on verylimited and oldexperimental data (1999) • the enhancedconvectiveheattransfer is basedsolely on the increasedthermalconductivitywhichhasbeenprovenwrong in the literature