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CHME 333 Workbook Handouts - Chapter 7

CHME 333 Workbook Handouts - Chapter 7 - CHME 333 Workbook...

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1 CHME 333 Workbook Chapter 7 – External Flows Objective - Calculate local and average convection coefficients for external flow heat and mass transfer applications In Chapter 6 we learned the basics about how heat transfer and mass transfer coefficients are calculated. By understanding the geometry, the flow conditions, and the fluid properties, we can calculate Nu and Sh , and then get values of h and h m . In Chapter 7, we consider External Flow – flow of a fluid that is bound by only one surface; the boundary layer is unconstrained. We will start by considering the simplest of cases – laminar flow over an isothermal flat plate – and develop equations for Nu (and Sh by using the heat/mass transfer analogy). We will then progress to more complicated geometries and flow conditions. The keys to solving problems involving external convection are the following: o Determine flow geometry o Specify appropriate reference temperature, and evaluate the fluid properties at that temperature o In mass transfer, the fluid properties are that of species B (because the mole fraction of species A is often << 1.0. o Calculate Re o Determine if local or average convection coefficients are needed o Select the appropriate correlation
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2 External flow – BL development is not constrained by another surface Forced Convection – a pressure gradient is driving the flow (pump, fan, etc.) Remember from Chapter 6 material o Nu x = f (x*, Re x , Pr) and Pr) , (Re __ x x f Nu = o Sh x = f (x*, Re x , Sc) and ) , (Re __ Sc f Sh x x = o Our objective from Chapter 6 was to set the stage for calculating the convection coefficients Pure analytical derivation and calculation of convection coefficients is not always feasible – many are determined empirically for certain geometries, and then correlations are developed o Under controlled conditions, one can measure parameters and determine how h avg depends on flow conditions and fluid properties.
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