Research Paper Aspect ratio dependence of turbulent natural convection in Al 2 O 3 /water nanofluids Rajesh Choudhary, Sudhakar Subudhi ⇑ Department of Mechanical & Industrial Engineering, IIT Roorkee, 247667, India highlights  Nu-Ra-Pr correlation for / = 0.01 vol.% and d p = 40 nm nanofluid Nu ¼ 0:1199Pr 1=12 Ra 1=4 þ 2:17  10 3 Pr 1=7 Ra 3=7 .  Nu-Ra-Pr correlation for / = 0.1 vol.% and d p = 40 nm nanofluid Nu ¼ 0:132Pr 1=12 Ra 1=4 þ 1:66  10 3 Pr 1=7 Ra 3=7 .  The thermal boundary layer for / = 0.01 vol.% and d p = 40 nm nanofluid is d th H  NF ¼ 122:55Ra 0:427 .  The thermal boundary layer for / = 0.1 vol.% and d p = 40 nm nanofluid is d th H  NF ¼¼ 44:873Ra 0:3728 .  The r.m.s. temperature distributions are ð h C D Þ NF ¼ 0:6693Ra 0:195 for / = 0.01 vol.% and d p = 40 nm nanofluids h C D  NF ¼ 1:5284Ra 0:2391 for / = 0.1 vol.% and d p = 40 nm nanofluids. article info Article history: Received 14 March 2016 Revised 5 July 2016 Accepted 2 August 2016 Available online 3 August 2016 Keywords: Nanofluid Turbulence Natural convection Rayleigh number Aspect ratio abstract A systematic study of the turbulent natural convection in an enclosure filled by water based Al 2 O 3 nanofluids for aspect ratio (AR = height/width) range of 0.3 6 AR 6 2.5 and Rayleigh number range of 10 7 < Ra < 10 12 is conducted. Results show that with the increase in Ra, the heat transfer is augmented in the presence of nanoparticles in low concentration, while a deterioration is observed for the higher concentration of nanoparticles. The variation of Nusselt number (Nu) with Rayleigh number (Ra) is for- mulated in the form of power laws. The variation of thermal boundary layer thickness and r.m.s. temper- ature distributions are also shown in the power laws for all the reported aspect ratios. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction In heat transfer processes, a detailed exploration of natural convection phenomena was prompted due to the vast area of applications such as energy conservation in buildings, solar engi- neering, geothermal power plants, cooling of electronic appliances, energy conservation in building and environmental processes etc. As a working fluid, water was used extensively due to its immense availability. Fiscaletti et al. [1] performed experiments to identify the beginning of natural convection with a water filled square- section enclosing a horizontal cylindrical heat source and found a flow transition due to transformation of system from steady state laminar flow to unsteady oscillatory flow. The employment of micron size solid particles as a dispersion in the conventional or base fluids was resulted in the increased heat transfer rate, but with the downsides of sedimentation, clogging and wear and tear of application areas. The advancement of tech- nology in manufacturing the nano size solid particles (nanoparti- cles) vanquishes the downsides due to dispersion of micron size particles. Choi and Eastman [2] dispersed the nanosized solid particles (nanoparticles) having a particle size in the range of 1–100 nm in the base fluid by different techniques to maintain a uniform suspension of nanoparticles in the base fluid and the mix- ture was called nanofluids. The thermal conductivity of nanofluid was found higher than that of base fluid without the rapid sedi- mentation and the aggregation of the solid particles, resulted in a stable dispersion. The applicability of nanofluids in the forced convection systems was widely probed theoretically and experimentally and enhance- ment in the heat transfer coefficient was observed. Heris et al. [3] examined the effect of Al 2 O 3 nanoparticles suspension in water flowing inside a circular tube with constant wall temperature condition. The enhancement in the heat transfer coefficient was http://dx.doi.org/10.1016/j.applthermaleng.2016.08.016 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: subudhi2@yahoo.com (S. Subudhi). Applied Thermal Engineering 108 (2016) 1095–1104 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng