wton’s law of cooling:−=W/m 2dxdtq λ−=Contents第一节对流换热概述Analysis on Convection第二节对流换热微分方程组The Convection Heat Transfer Equations第三节边界层换热微分方程组Convection Differential Equations of Boundary Layer 第四节边界层换热积分方程(自学)第五节动量传递和热量传递的类比(自学)第六节相似理论基础Basis of similarity theoryConvection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.(流体与固体壁直接接触时所发生的热量传递过程，称为对流换热)The faster the fluid motion, the greater the convection heat transfer.We will study how to calculate the convection heat-transfer coefficient h in Chapter 5 and Chapter 6.5-1 Analysis on Convection(对流换热概述) Convection transfer problemHeat exchangers Tubes in steamboiler (蒸汽锅炉的管束)Tube-shell heat exchanger (管壳式换热器)Condenser ofrefrigerator Tubes withfins (翅片管束)图5-1几种常见的换热设备示意图Factors influencing convection heat transfer ：Flow causes of fluid (流体流动的起因)、flow states ( 流动状态)、properties of fluid (流体物性)、change of phase of fluid (流体物相变化)、geometry parameters (壁面的几何参数),and so on 。
1.Causes and States of Flow (流动的起因和状态)(1) Causes of flow(流动起因)Forced convection(受迫对流):if the fluid is forced to flow over the surface by external means such as a fan ,pump,or the wind .naturalforced h h >Natural (or free) convection(自然对流):if the fluid motion is caused by buoyancy (浮力) forces that is induced(引起) by density difference due to the variation of the temperature in the fluid.Examples ：air free convection ：h=5~25W/（m 2.K ）air forced convection ：h=10~100W/（m 2.K ）(2) flow states ( 流动状态)Whatever the causes of flow are ,the flow states are consist of laminar flow (层流)and turbulent flow (紊流)Laminar flow ——characterized by smooth streamlines (流线) and highly ordered motion due to the fluid flowing in laminae(层) or layers(整个流场呈一簇互相平行的流线)Turbulent flow ——characterized by velocity fluctuation (波动) and highly disordered motion （流体质点做复杂无规则的运动）lt h h >2.The thermal properties of fluid (流体热物性)Properties:density(密度) ρ[kg/m 3], specific heat(比热) c [J/（kg. ℃）]thermal conductivity(导热系数) λ[ W/(m. ℃)],dynamic viscosity(动力粘度) μ[kg/（s.m)]kinematic viscosity(运动粘度) ν（m 2/s ），volume expansion coefficient (体积膨胀系数) α(1/K),ρμ=v p p T T ⎟⎠⎞⎜⎝⎛∂∂−=⎟⎠⎞⎜⎝⎛∂∂=ρρυυα11自然对流换热增强↑⇒α)( 多能量单位体积流体能携带更、↑↑⇒h c ρ)( 热对流有碍流体流动、不利于↓↑⇒h μ↑↑⇒h λ)(间导热热阻小流体内部和流体与壁面where Ideal gasα=1/T***Characteristic temperature (定性温度)Average temperature of fluid （流体平均温度）t fthe surface temperature of wall （壁表面温度）t wthe arithmetic average temperature of the fluid and the wall surface (流体与壁的算术平均温度)(t f +t w )/2.3.Change of phase (流体的相变）Change of phase heat transfer(相变换热)：Condensation (凝结)、Boiling(沸腾)、sublimation(升华)、freeze(凝固)、thaw(融化) and so onSingle phase heat transfer (单相换热)glechange h h sin >4. Geometry parameters of convection surface（换热表面几何因素）shape, sizes, position and roughnessExterior flow (外掠): flow across the flat plate,flow across cylinders and tubes (外掠平板、外掠圆管及管束)Interior Flow (内流): flow in a pipe ,flow in slot (管内、槽内流动)。
Forced convectionNatural convectionLower surface of a hot plateUpper surface of a hot plate Flow in a pipe Flow across a pipe***Characteristic Length (定型尺寸) l由于壁面几何形状的影响，在分析计算中可采用对换热有决定意义的特征尺寸作为依据，这个尺寸称定型尺寸。
Example :for a flat plate, l is the length of the plate in the flow direction .If the fluid flow in a pipe , l is the diameter d of the pipe, l=d),,,,,,,,(l c t t u f h p f w μαρλ=5. Sorts of ConvectionGenerally ,the convection heat transfer coefficient h can be expressed the function of those factors mentioned above5-2 The Convection Heat Transfer Equations对流换热微分方程组To simplify the analysis we assume:（1）The fluid is incompressible and Newton-style ；（2）The properties of fluid are constant and no heat resources. ；（3）The energy generated by viscosity is negligible (粘性耗散产生的耗散热可以忽略不计)。
**Newton-style fluid (牛顿型流体)服从y u∂∂=μτ定律的流体称牛顿型流体。
The viscous forces are described in terms of a shear stress between the fluid layers ,that this stress is assumed to be proportional to the normal (法线)velocity gradient .yu∂∂=μτ2-1 Differential Equation for the convection heat transfer process(对流换热过程微分方程式)xy q u Where y=0，u=0，no-slip conditionHeat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conductionThe fluidlayer ismotionlessHeat transfer can be expressed as xw x y t q ,⎟⎟⎠⎞⎜⎜⎝⎛∂∂−=λ:0=y W/㎡λis thermal conductivityof fluidx w y t ,⎟⎟⎠⎞⎜⎜⎝⎛∂∂——is the temperature gradient at the surface x from leading edge of the flat plate(x 点帖壁处流体的温度梯度，K/m)According to Newton’s law of coolingxx x f w x x t h t t h q Δ⋅=−=)(h x ——the local convection heat transfer coefficient at surface x (壁面x 处的局部表面传热系数)，W/（m 2.K ）。