Question Bank • QB_EXP_DFE_CNV_003 MSQ • 2 Marks

Section 7: Dairy and Food Engineering

Convective Heat Transfer & Dimensionless Numbers

Subtopic: Dittus-Boelter correlation exponents

Question Statement:

Which of the following conditions and parameters are valid for the Dittus-Boelter turbulent pipe flow correlation $Nu = 0.023 Re^{0.8} Pr^n$?

Options:

  • (A) Fully developed turbulent flow with $Re > 10000$ and $0.6 \le Pr \le 160$
  • (B) The exponent is $n = 0.4$ when the fluid is being heated
  • (C) The exponent is $n = 0.3$ when the fluid is being cooled
  • (D) The correlation is applicable to creeping laminar flow ($Re < 100$)

Answer Key:

A, B, C

Step-by-Step Solution & Explanation

1. Valid for turbulent flow $Re > 10^4$ and moderate Prandtl range (A is true). 2. Heating fluid: $n = 0.4$ (wall hotter than bulk fluid) (B is true). 3. Cooling fluid: $n = 0.3$ (wall colder than bulk fluid) (C is true). 4. For laminar flow ($Re < 2100$), $Nu = 3.66$ (constant wall temp) or $4.36$ (constant heat flux); Dittus-Boelter is completely invalid for laminar flow (D is false).
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