Chapter 8 Internal Forced ConvectionPropertiesWe assume the water temperature not to drop significantly since the pipe is not very long. We will check this assumption later. The properties of water at 90°C are (Table A-9) 96.1PrCJ/kg.4206/s;m10326.0/CW/m.675.0;kg/m3.96526-3=°=×=ρμ=υ°==ρpCkAnalysis(a) The mass flow rate of water is kg/s0.9704m/s)8.0(4m)(0.04)kg/m3.965(23=π=ρ=VAmc&Water 90°C 0.8 m/s 10°C L= 15 mDi= 4 cm Do= 4.6 cm The Reynolds number is 062,98/sm10326.0m)m/s)(0.04(0.8Re26=×=υ=−hmDVwhich is greater than 10,000. Therefore, the flow is turbulent and the entry lengths in this case are roughly m4.0m)04.0(1010==≈≈DLLthwhich are much shorter than the total length of the pipe. Therefore, we can assume fully developed turbulent flow in the entire pipe. The friction factor corresponding to Re = 98,062 and ε/D = (0.026 cm)/(4 cm) = 0.0065 is determined from the Moody chart to be f= 0.034. Then the Nusselt number becomes 6.52196.1062,98034.0125.0PrRe125.0
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