传热英文讲义-热对流 heat transfer-convection.docx

Convection As mentioned earlier, there are three mechanisms of heat transfer: conduction, convection, and radiation. Conduction and convection are similar in that both mechanisms require material medium, but the difference is that convection requires the presence of fluid motion. And heat transfer through a liquid or gas can be by convection or conduction, depending on if there is presence of bulk fluid motion. In other words, if there exists bulk fluid motion, it is convection; if there is none, then it is conduction.Convection is complicated because it involves fluid motion and heat conduction. Thus, rate of heat transfer by convection is higher than conduction. And the higher the fluid velocity, the higher the heat transfer rate.Further reading about convection is available at Although, convection is complex, the rate of convection heat transfer is observed to be proportional to the temperature difference and can be conveniently expressed by Newtons law of cooling as qconv=h(Ts-T) (W/m2) (x.1)or Qconv=hAs(Ts-T) (W) (x.2)Where h = convection heat transfer coefficient, W/m2oCAs = heat transfer surface area, m2Ts = temperature of the surface, CT= temperature of the fluid sufficiently far from the surface, CConvection heat transfer coefficient h can be defined as the rate of heat transfer between a solid surface and a fluid per surface area per unit temperature difference. The convection coefficient is decided by variables influencing convection such as surface geometry, the nature of fluid motion, the properties of the fluid, and the bulk fluid velocity. Typical values of h are given in Table x.1. Table x.1Nusselt NumberIn convection studies, to nondimensionalise the governing equations, dimensionless numbers are introduced to reduce the number of total variables. Nusselt number, viewed as the dimensionless convection heat transfer coefficient is defined as:Nu=hLckWhere k is the thermal conductivity of the fluid and Lc is the characteristic length. The physical significance means the heat transfer ratio of convection to conduction. For Nu=1, the heat transfer is pure conduction.Fluid flowsHeat transfer between moving fluid and solid surface or between moving fluid and interface (to/from air to falling drop). It is commonly assumed that all resistance to heat/momentum transfer occurs in boundary layer defined as part of fluid adjacent to the surface where velocity/temperature changes. Outside boundary layer velocity/temperature is constant. (a) Velocity boundary layerThe fluid flow is characterised by two regions: - Thin fluid layer (boundary layer) in which velocity gradients and shear stresses are large - Free stream (region outside boundary layer) where velocity gradients and stresses are negligible (b) Thermal (temperature) boundary layer- Thin fluid layer of fluid in which temperature gradients are large-exists only when there is a difference between surface temperature and bulk temperatureThickness of boundary layer t is the value of y for which: Ts-TTs-T=0.99If the temperature distribution in boundary layer is known local heat flux from/to the surface can be calculated from Fouriers law in the fluid (there is no fluid motion on the surface)qs=-kfTyy=0 and q=h(Ts-T)In such case convective heat transfer coefficient can also be calculated (no need for experimental data or Buckingham theorem).Local heat transfer coefficientIntegrating local heat transfer coefficient over the entire surface the average value can be calculated: This method of calculation of heat flux to or from the surface is used in CFD packages (numerical solution of momentum and energy balance). In engineering calculations fully developed flows are usually considered therefore average heat transfer coefficients are commonly used (but not always).Structures of boundary layers, local/average heat transfer coefficients1. External flow (a). Flow parallel to flat plateLaminar part:- Local convective heat transfer coefficient:- average heat transfer coefficient ( integrate above from 0 to x):Turbulent part:- Local heat transfer coefficient:- Mixed boundary layer conditions (part of the plate laminar, part turbulent): (b). Flow around cylinderLocal heat transfer coefficient:Average:m and n are constants that can be found from literature.(c). Flow around sphere (similar to flow around cylinder)Internal flow:The extent of boundary layer can be estimated from Re numberD Tube diameter m, - dynamic viscosity Pa s, m& - mass flow rate kg/s, um- mean fluid velocity m/sThermal entrance regions:(a). Laminar flow: (b). Turbulent flow: Nu number for different types of flow:Fully developed laminar flow:(a). Constant temperature at the wall NuD=3.66(b). constant heat flux at the wall NuD=4.36Laminar flow including entry region:Fully developed turbulent flow (properties at Tm)(a). Chilton-Colburn equation:(b). Dittus-Boelter equation: (c). Sieder-Tate equation:For noncircular tubes-hydraulic diameter Dh=4Ac/P, Ac flow cross sectional area, P wetted perimeter (both in Re and Nu numbers)Summary You should:A) know/understand that convective heat transfer coefficients depends on:1. Type of flow: internal or