Buoyancy driven heat transfer behavior of [C4mim][NTf2] ionic liquid: An experimental study

• [C4mim][NTf2] ionic liquid has lower thermal conductivity and higher viscous force compared to De-Ionized (DI) water.
• Natural convection heat transfer coefficient of ionic liquid is lower than De-Ionized (DI) water.
• New correlation of natural convection heat transfer for ionic liquid is proposed.

This paper presents the experimental results of natural convection heat transfer for 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide ([C4mim][NTf2]) ionic liquid (IL), in rectangular cavity with different aspect ratios. In addition to evaluating the natural convection heat transfer, thermophysical properties of [C4mim][NTf2] such as density, viscosity, heat capacity, and thermal conductivity were also experimentally measured. The results show that the density of [C4mim][NTf2] decreases with increased temperature within the investigated temperature range of 10–70 °C, the shear viscosity of IL reduces with increasing temperature in an exponential manner within temperature 10–70 °C. Its heat capacity increases linearly with temperature from 20 to 300 °C, and the thermal conductivity varies from 0.131 to 0.122 W/m K within the measured temperature range. A lower natural convection heat transfer coefficient is observed compared to that of the De-Ionized (DI) water for all aspect ratios (AR). Meanwhile, higher measured dimensionless Nusselt number is observed for the IL than that of DI water due to its lower thermal conductivity (approximately 21% of DI water) and higher viscous force. For the two aspect ratios, larger aspect ratio has higher natural convection heat transfer compared to the lower aspect ratio. Finally, the experimental results of IL for aspect ratios studied are expressed as the conventional natural convection correlation of the form of Nu = cRan with different c and n values.