Nusselt number? Please help?

Question

in an oil cooler the oil enters 10mm diameter tubes at 160 degrees celsius and is cooled to 40 degrees celsius, the mean velocity of the oil in the tubes is 1.5 m/s. calculate heat transfer coefficient h.

for turbulent flow of a liquid being cooled take Nu = 0.023

for laminar flow Nu= 3.65

Temp= 100

kg/m^3 = 839

m^2/s = 20.4 *10^-6

(W/mK) = 0.137

Answer 1

I'll let you do the actual math but here is what you should do:

The trick about this problem is understanding the definitions of the following dimensionless groups:

Nu = Nusselt Number = hl/k, where

h=heat transfer coefficient; l=a characteristic length; k=thermal conductivity

Re = Reynolds Number = rho*d*v/mu, where

rho=density of the fluid; d=characteristic length; v=bulk velocity of fluid; mu=viscosity of the fluid

Pr= Prandtl Number = Cp * mu/ k, where

Cp= heat capacity of the fluid; mu=viscosity of the fluid, and k=thermal conductivity of the fluid.

You'll need to look up these properties for your fluid.  The characteristic length for pipes is usually the diameter.  For properties that change with temperature (like viscosity, thermal conductivity) you should average the values for 160C and 40C.

Be sure all of your units are correct and consistent.  All of these numbers (Re, Pr, Nu) should have no dimensions left when calculated.

Once you calculate Re and Pr, you can use your correlation to calculate Nu.

Then it should be simple to calculate h from the definition of Nu.

Hope this helps.