QUESTION: Air at 35°C enters a 10 m long section of a rectangular duct of cross section 25 cm by 15 cm made of commercial steel at an average velocity of 10 m/s.

*Take density of air and dynamic viscosity as 1.145 kg/m^3 and 1.895 x 10^-5 kg/m.s respectively.

Determine: (a) Reynolds number

see my working here below, enter image description here

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i take the cross section 25cm * 15 cm as my area. I need help finding the diameter of pipe and please do confirm my workings cause i feel like its somewhat off since from my calculation above my Reynolds number is quite big.

  • $\begingroup$ looks ok, what makes you think something is wrong. (I'm cringing at the line where you appear to multiply (a+b)(a+b), but you correct that on the next line) $\endgroup$
    – agentp
    Sep 29, 2017 at 3:17
  • $\begingroup$ Looks fine as per agentp, turbulent but that is to be expected.. $\endgroup$
    – Solar Mike
    Sep 29, 2017 at 6:42
  • 1
    $\begingroup$ Use this site and check : engineeringtoolbox.com/reynolds-number-d_237.html $\endgroup$
    – Solar Mike
    Sep 29, 2017 at 15:49

1 Answer 1


It's correct. Even if you take the area to be half of the current area and reduce the velocity of air by a factor of 10, even then you will get turbulent flow.I took these numbers just to put things in perspective.

1.145*1*0.09375/(1.89*10^-5) ~ 5679

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