Why does Ethylene glycol produce greater enhancement on heat transfer rate than water as its flow through a tube, considering that water has a higher thermal conductivity. What could be the possible reasons?
I assume that you are referring to the heat transfer coefficient (usually denoted with "h") that enhances, or? If I got it right, then you can do the comparison by yourself for a simple system of, let's say a cylinder in which the two fluids flow: the Water and the Ethylene Glycol. The heat transfer coefficient can be calculated using this formula:
h = k*Nu/L
Here k is the thermal conductivity of the fluid, Nu is the Nusselt number, and L is the characteristic length of the cylinder.
But the Nusselt number can be calculated depending on the Prandtl number and Reynolds number, which are related to the density, velocity, and viscosity [Nu = f(Re,Pr)]. The Reynolds number is given as:
Re = rho * v * L/mu
where rho is the density of the fluid, v is the velocity, L is the characteristic length and mu is the dynamic viscosity.
So, as you can see from the above formulas, the heat transfer coefficient doesn't depend only on the thermal conductivity of the fluid, but also on the density, velocity and viscosity. So it's not fair to compare them only by comparing the conductivities. If you need numbers for this, you can just replace the values in the above formula and get a more qualitative comparison. I hope this helps.
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