Example: Calculation of average bulk temperature for property determination

Problem Statement:

A pipe has flowing water at an inlet temperature of 80°C. Heat is removed using cooling water outside the pipe. The final outlet temperature of the water is 40°C. Calculate the average bulk temperature of the water. Determine the viscosity of the bulk fluid and compare this value to the viscosity of water at 80°C and 40°C.

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Solution:

To calculate the bulk average temperature of a material, find the average temperature of the material:

$T_{bulk}=\frac{T_{out}+T_{in}}{2}=\frac{80+40}{2}=60^{\circ }C$

Now find the viscosity of water at 40°C, 60°C and 80°C and compare.

at 60°C: $\mu =0.4656mPa\ast s$

at 80°C: $\mu =0.3537mPa\ast s$

at 40°C: $\mu =0.6539mPa\ast s$

Clearly, there is a large difference between the viscosities at the three different temperatures. This shows the importance of using bulk material temperature when determining properties instead of inlet or outlet temperature only.