Thermal conductivity of water increases when carbon nanotubes are dispersed in it

Carbon nanotubes, by virtue of their high surface area and aspect ratio, are known for their excellent thermal conductivity — 3,000 W/mK. Even as scientists across the world are looking at ways of adding carbon nanotubes to fluids for use in heat exchangers to enhance heat transfer efficiency in steel plants and automobile coolants, IIT Madras has already tested the nanofluid’s superiority.

In 2010, a joint patent was filed by Tata Steel and IIT Madras for the use of nanofluids to increase the thermal conductivity in industries such as steel and automobiles.

IIT Madras has developed the process of manufacturing 10 grams of nanotubes per batch and successfully used the material in increasing the thermal conductivity of cooling fluids. The challenge is to increase the amount of nanotubes produced per batch and test the heat transfer efficiency in heat exchangers.

“There will be up to a 30 per cent increase in thermal conductivity in the case of nanofluid when compared with distilled water,” said Prof. S. Ramaprabhu of the Alternative Energy and Nanotechnology Laboratory, Department of Physics, IIT Madras. Prof. Ramaprabhu and his team synthesised better quality carbon nanotubes and increased the yield by successfully using a novel catalyst; the process of synthesising the carbon nanotubes was patented in 2004.

Increase in efficiency

“The thermal conductivity increased from 0.635 W/mK in the case of distilled water to 0.7W/mK-0.78 W/mK when carbon nanotubes are dispersed in water,” he said. “Thermal conductivity rises with temperature.”

The thermal conductivity of nanofluid was tested and compared with distilled water at different temperatures within the 30 degree C to 70 degree C range.

The nanofluid’s thermal conductivity was next compared with distilled water using a more direct test. The time taken to cool a hot steel plate at 800 degree C to room temperature using normal water and nanofluid was compared. “Normal water took 15 to 18 seconds to cool the hot steel plate to room temperature, while the nanofluid took just five to six seconds,” he recalled.

An increase of up to 30 per cent in thermal conductivity directly translates into commercial gains. In the case of steel industry, huge quantities of water used in heat exchangers can be greatly reduced by using water containing carbon nanotubes.

Steel industry is water intensive. For instance, hot rolling operations and coke oven gas treatment independently use 7,000-9,000 gallons of water per tonne of product produced.

Added to these operations are about 2,500-4,000 gallons of water per tonne of product produced in each of the following cases — blast furnace, basic oxygen furnace and cold rolling operation.

Automobile industry

The automobile industry is another important field where nanofluids would find great application. When nanofluid is used as a coolant, the size of the radiator and other components for heat dissipation can be reduced. This reduction would directly translate to a decrease in the size or weight of the car. The net effect would be a decline in fuel consumption.

Tests demonstrated that nanofluid performed well even when the viscosity of water was raised by adding ethylene glycol. Ethylene glycol is used in coolants for its antifreeze properties.

“The increase in thermal conductivity in the case of ethylene glycol mixed with water [50:50] was up to 10 per cent,” said Prof. Ramaprabhu. “The [relatively] reduced heat transfer efficiency is because of the higher viscosity of ethylene glycol.”