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Ebook A heat transfer textbook (3rd edition): Part 2

(BQ) Part 2 book "A heat transfer textbook" has contents: Laminar and turbulent boundary layers, forced convection in a variety of configurations; natural convection in single-phase fluids and during film condensation; heat transfer in boiling and other phase-change configurations,. and other contents. | Part III Convective Heat Transfer 267 6. Laminar and turbulent boundary layers In cold weather, if the air is calm, we are not so much chilled as when there is wind along with the cold; for in calm weather, our clothes and the air entangled in them receive heat from our bodies; this heat. . .brings them nearer than the surrounding air to the temperature of our skin. But in windy weather, this heat is prevented. . .from accumulating; the cold air, by its impulse. . .both cools our clothes faster and carries away the warm air that was entangled in them. notes on “The General Effects of Heat”, Joseph Black, c. 1790s 6.1 Some introductory ideas Joseph Black’s perception about forced convection (above) represents a very correct understanding of the way forced convective cooling works. When cold air moves past a warm body, it constantly sweeps away warm air that has become, as Black put it, “entangled” with the body and replaces it with cold air. In this chapter we learn to form analytical descriptions of these convective heating (or cooling) processes. Our aim is to predict h and h, and it is clear that such predictions must begin in the motion of fluid around the bodies that they heat or cool. It is by predicting such motion that we will be able to find out how much heat is removed during the replacement of hot fluid with cold, and vice versa. Flow boundary layer Fluids flowing past solid bodies adhere to them, so a region of variable velocity must be built up between the body and the free fluid stream, as 269 Laminar and turbulent boundary layers 270 Figure 6.1 §6.1 A boundary layer of thickness δ. indicated in Fig. 6.1. This region is called a boundary layer, which we will often abbreviate as b.l. The b.l. has a thickness, δ. The boundary layer thickness is arbitrarily defined as the distance from the wall at which the flow velocity approaches to within 1% of u∞ . The boundary layer is normally very thin in comparison with the dimensions of the body immersed