Chapter7 - Chapter 7 External Flow Objectives Determine...

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Copyright 2003 John Baldwin 1 ChEn 323 – Chapter 7 – Slide 1 Chapter 7 – External Flow – Objectives Determine external flow convection coefficients for various flow geometries Know when to use various expressions Understand general methodology of convection calculations The geometries to be considered are: § Flat plate § Cylinder § Sphere § Tube bank ChEn 323 – Chapter 7 – Slide 2 From Last Chapter Covered low speed, forced convection with no phase change Heat transfer: § Nu x = f 4 (x * , Re x , Pr) § Nu x = f 5 (Re x , Pr) Mass transfer: § Sh x = f 7 (x * , Re x , Sc) § Sh x = f 7 (Re x , Sc) We will use this basis for the solution approaches: § Experimental (empirical) § Theoretical Boundary layer method Exact solution (similarity method) Approximation solution (integral method)
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Copyright 2003 John Baldwin 2 ChEn 323 – Chapter 7 – Slide 3 Empirical Correlations Objective: determine an equation by which h can be calculated. Empirical (Webster): Relying on experience or observation without due regard for system or theory. Basic concept: An equation representing one situation can be used to predict other similar situations. Steps for empirical approach: § Choose parameters you think are important for describing the situation of interest. § Arrange the parameters into a set of dimensionless groups. This will reduce the number of variables. § Perform experiments. § Determine a suitable equation. § Modify parameters as required. ChEn 323 – Chapter 7 – Slide 4 Flat Plate in Parallel Flow – Similarity Approach Restrictions: § Steady flow § Incompressible § Laminar flow § Constant fluid properties § Negligible viscous dissipation § dp/dx = 0 Equations: § Continuity: § Momentum: § Energy: § Species: 0 = + y v x u 2 2 y u y u v x u u = + n 2 2 y T y T v x T u = + a 2 2 y D y v x u A AB A A = + r r r
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Copyright 2003 John Baldwin 3 ChEn 323 – Chapter 7 – Slide 5 Solution Approach Since there are constant properties, conditions in the velocity boundary layer are independent of temperature and species concentration. Begin solution by solving the hydrodynamic problem (e.g., the
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This note was uploaded on 03/26/2008 for the course CHEN 323 taught by Professor Johnbaldwin during the Fall '03 term at Texas A&M.

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Chapter7 - Chapter 7 External Flow Objectives Determine...

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