Principles of Chemical Engineering: A fluid with a certain mass flow rate flows turbulently inside a straight pipe with dimensions of φ25mm×2.5mm. . . Help needed
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When a fluid with a certain mass flow rate flows turbulently in a straight pipe with dimensions of φ25mm×2.5mm, if the flow rate and physical properties of the fluid remain unchanged and it flows instead in a straight pipe with dimensions of φ19mm×2mm, its convective heat transfer coefficient will be approximately a multiple of the original convective heat transfer coefficient multiplied by ( ). A0.79 B1.06 C1.26 D1.68 Answer D1.68
The formula for the turbulent heat transfer coefficient in circular tubes is α=0.023λ/di*(di*u*ρ/μ)^0.8*(μ*Cp/λ)^n, where di*u*ρ/μ=Re and μ*Cp/λ=Pr. Given that the flow rate remains constant, when the tube diameter is reduced from DN20 to DN15, the flow velocity becomes 20*20/15/15=1.78; thus, the flow velocity increases by a factor of 1.78. Then the change amount is as follows: when di=20 and di=15 are used, with flow rates of 1 and 1.78 respectively, for di=20 and a flow rate of 1, α=1/20*(20*1)^0.8*other constant parameters=0.549*other constant parameters; for di=15 and a flow rate of 1.78, α=1/15*(15*1.78)^0.8*other constant parameters=0.923*other constant parameters. 0.923/0.549=1.68, so the answer is D