External Forces Convection
4
External Forced Convection
4.1 Introduction to Laminar Boundary Layers 4.1.1 Introduction
Chapters 1 through 3 consider conduction heat transfer in a stationary medium. Energy transport within the material of interest occurs entirely by conduction and is governed by Fourier’s law. Convection is considered only as a boundary condition for the relatively simple ordinary or partial differential equations that govern conduction problems. Convection is the transfer of energy in a moving medium, most often a liquid or gas flowing through a duct or over an object. The transfer of energy in a flowing fluid is not only due to conduction (i.e., the interactions between micro-scale energy carriers) but also due to the enthalpy carried by the macro-scale flow. Enthalpy is the sum of the internal energy of the fluid and the product of its pressure and volume. The pressure-volume product is related to the work required to move the fluid across a boundary. You were likely introduced to this term in a thermodynamics course in the context of an energy balance on a system that includes flow across its boundary. The additional terms in the energy balance related to the fluid flow complicate convection problems substantially and link the heat transfer problem with an underlying fluid dynamics problem. The complete solution to many convection problems therefore requires sophisticated computational fluid dynamic (CFD) tools that are beyond the scope of this book. The presentation of convection heat transfer that is provided in this book looks at convection processes at a conceptual level in order to build insight. In addition, the capabilities and tools that are required to solve typical convection heat transfer problems are presented. As engineers, we are most often interested in the interaction between a fluid and a surface; specifically the transport of momentum and energy between the surface and the fluid. The transport of momentum is related to the force exerted on the surface and it is
External Forced Convection
4.1 Introduction to Laminar Boundary Layers 4.1.1 Introduction
Chapters 1 through 3 consider conduction heat transfer in a stationary medium. Energy transport within the material of interest occurs entirely by conduction and is governed by Fourier’s law. Convection is considered only as a boundary condition for the relatively simple ordinary or partial differential equations that govern conduction problems. Convection is the transfer of energy in a moving medium, most often a liquid or gas flowing through a duct or over an object. The transfer of energy in a flowing fluid is not only due to conduction (i.e., the interactions between micro-scale energy carriers) but also due to the enthalpy carried by the macro-scale flow. Enthalpy is the sum of the internal energy of the fluid and the product of its pressure and volume. The pressure-volume product is related to the work required to move the fluid across a boundary. You were likely introduced to this term in a thermodynamics course in the context of an energy balance on a system that includes flow across its boundary. The additional terms in the energy balance related to the fluid flow complicate convection problems substantially and link the heat transfer problem with an underlying fluid dynamics problem. The complete solution to many convection problems therefore requires sophisticated computational fluid dynamic (CFD) tools that are beyond the scope of this book. The presentation of convection heat transfer that is provided in this book looks at convection processes at a conceptual level in order to build insight. In addition, the capabilities and tools that are required to solve typical convection heat transfer problems are presented. As engineers, we are most often interested in the interaction between a fluid and a surface; specifically the transport of momentum and energy between the surface and the fluid. The transport of momentum is related to the force exerted on the surface and it is