Question

7.17 The top surface of a compartment consists of very smooth (A) and highly roughened (B) portions, and the surface is placed in an atmospheric airstream. In the interest of minimizing total convection heat transfer to the surface, which orientation, (1) or (2), is preferred? If T, = 10°C, T = 35°C, and u = 25 m/s, what is the rate of convection heat transfer to the entire surface for this orientation? HT 0.5 m 0.5 m (2)

Answer 1

Answer:

Uosito 0.5m 0. 5m • Ups, Too > From configuration (2) results in a trobilant over the entire surface the lowest heat trauster is associated with configuration a. Tmean = 35tlo -2 Tmean = 22.50 At '32.5C K=0.02532 w/mk v = 1.539 x 106 m² /s 1. Por = 0.73 Reynald's number, Re = VL 25*1 1.539x106

31068.107 1.-) Transition will occur in configuration (J. Ree = sx Los vy= 2.5x8 1-539 x16 6 Ree - vw 5x105= 2.5xx 1-5398156 x = 5x108 x 1.539x106 ... 2.5 x= 0.03 m - Mul, = (0.037 x (1-6*167) 15 817] (0.7380. 33 ..: 2063.6.4178(0.73)®-33 = 18600 - For configuration (2), NuLz; = [0.037 (1.6x 107) 4187 (0.73 0.33 i. Mua 19337:> 19337 Nulz > Nuri

3 Nuri = n. 4 h = NUL, ok .: = 18600 x 0.02532 hei= 470.952 w/m²k a, the, A CTS-To) = 470-952 X1X0.525 a = 5886.9W