11 Convection Part 2

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L.D.D Advanced Building Physics - Heat transfers by air movement This chapter focuses on heat transfers between air and the solid surfaces bonding the room. Convection is much more complicated than trensfer by conduction and radiation. Convection is described by the Navier-Stokes equations for a fluid moving under the action of a body force Fi and a heat input Q. They contain: - Conservation of mass - Conservation of momentum - Conservation of energy To these must be added some model of turbolence (the k-ε model). --> this defines the turbulent exchange coefficient from the turbolent kinetic energy k and its dissipation rate ε. The convective solutions requires for determining the thermal behaviour of the room are relatively simple, consisting of expressions for heat flow through boundary layers. 1 - LAMINAR AND TURBULENT FLOW When a fluid moves relative to a surface, the flow may be laminar or turbolent. The air in contact with the surface is stationary and the velocity of air particles near the surface follows the direction of the bulk flow --> laminar flow. Away from the surface, instability may set in and the velocity vector may have x,y,z components. --> turbulent flow The degree of turbolence is usually about 10%. Reynolds --> introduce adimensional numbers ρVD/µ (fluid density - mean fluid velocity - pipe diameter - fluid viscosity) that form together the reynolds number Re. If Re < 2300 the flow is laminar. Up to Re = 50000 the flow may be laminar if sufficient care is taken, but the regime is unstable. A study of fluid motion near a solid surface is conducted using the boundary layer concept introduced by Prandtl in 1904. ---> the flow is divided into two regions: 1 - Thin boundary layer very close to the solid wall: important frictional forces 2 - External…