Why Ethylene glycol does has produce greater enhancement on heat transfer rate than water its flow in a tube, considering that water has a higher thermal conductivity. What could be the possible reasons?
Water has higher thermal conductivity and lower viscosity than ethylene glycol, hence the heat transfer coefficient for water will be higher for both laminar and turbulent flow in a tube. I am not sure what you mean by 'enhancement on heat transfer', are you suggesting some augmentation technique? If so, it may be that the percentage increase in heat transfer for glycol may (under some circun\mstances) be higher than for water, but the absolute value for the heat transfer coefficient I believe will be higher for water.
Heat-transfer will be higher with pure water than with water-glycol mixtures. Water-glycol is used for mainly for freezing protection. If your system will never be used in freezing conditions, water will give better performance. Ethylene glycol does also protect to some extent against boiling, but it is not as efficient as against freezing. Propylene glycol is much more efficient there.
Thanks, but what I mean is that Ethylene glycol has a higher Nusselt number than water for flow through a tube
Nusselt number and heat transfer coefficient are two different things. Glycol has a higher Prandtl number than water (mainly due to higher viscosity and lower conductivity). So, for the same velocity the Reynolds number for glycol will be less, but the prandtl number higher than for water. These are just dimensionless numbers, don't over-interpret their meaning. (Basically Re is inertia forces divided by viscous forces, Pr is momentum diffusivity relative to thermal diffusivity and Nu is convection heat transfer to conductive heat transfer.) It is generally not wise to compare these for different fluids (e.g. liquid metal has very low Nu, but very high heat transfer coefficients.....)
According to me Ethylene glycol I think added in coolant only to not let it freeze.
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