Section02_ Chapter 3 Convection.pdf

Section02_ Chapter 3 Convection.pdf - BKF2423 HEAT TRANSFER...

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1 BKF2423 HEAT TRANSFER CHAPTER 3 Principles of steady-state heat transfer in convection CO1 - Solve heat transfer problems that involve conduction, convection and radiation in steady state heat transfer PO1 Engineering Knowledge - Apply knowledge of mathematics, science, engineering fundamentals and an engineering specialisation to the solution of complex engineering problems. PO2 Problem Analysis - Identify, formulate, research literature and analyse complex engineering problems reaching substantiated conclusions using first principles of mathematics, natural sciences and engineering sciences. CONTENT Define and differentiate between forced convection , natural convection, boiling and condensation. Calculate the heat transfer coefficient (h) & heat transfer rate (q) for Forced convection inside pipe Fluid flow inside pipe Forced convection outside various geometries Fluid flow across flat plate Fluid flow across tube/cylinder Fluid flow across sphere Fluid flow across bank of tubes/cylinders Natural convection Fluid flow across flat plate Fluid flow across cylinder Fluid flow in enclosed spaces Boiling Nucleate boiling Film boiling Condensation Film condensation The rate of heat transfer : T w = 80 o C T f = 30 o C q ) ( f w T T hA q The heat transfer coefficient is a measure of how effective a fluid is at carrying heat to or away from the surface. h = heat transfer coefficient (W/m 2 .K) A= surface area (m 2 ) Fluid flow Convection: Heat transfer using movement of fluids
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2 Determination of heat transfer coefficient The determination of heat transfer coefficient is affected by the: physical properties of fluid type and velocity of flow temperature difference geometry of the specific physical system Dimensionless numbers (Reynolds, Prandalt, Nusselt numbers) are used to correlate the data for heat transfer coefficient. 1. Prandtl Number k c N p Pr 2. Reynolds Number  D N Re 3. Nusselt Number k hD N Nu Where, = viscosity of fluid c p = heat capacity of fluid ν = velocity of fluid k = thermal conductivity of fluid D = pipe diameter (for flow inside pipe) ρ = density of fluid Determination of heat transfer coefficient All the physical properties are evaluated at bulk fluid temperature Determination of heat transfer coefficient inside a pipe m, Cp, Fluid flow, v T w L D T bo q T bi For laminar flow of fluids inside horizontal pipe, N Re <2100 : 100 2100 Pr Re Re L D N N N Limitations All the physical properties are evaluates at T b , except μ w 2 bo bi b T T T For turbulent flow of fluids inside horizontal pipe, N Re >6000 : 60 16000 7 . 0 6000 Pr Re D L N N Limitations 14 . 0 3 / 1 Pr Re 86 . 1 w b a a Nu L D N N k D h N 14 . 0
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