# Determining Reynolds number

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,

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.

• 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) – agentp Sep 29 '17 at 3:17
• Looks fine as per agentp, turbulent but that is to be expected.. – Solar Mike Sep 29 '17 at 6:42
• Use this site and check : engineeringtoolbox.com/reynolds-number-d_237.html – Solar Mike Sep 29 '17 at 15:49

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