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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Lesson 2,Fluid,Flow,Li Qiong,College of Architecture Engineering,North China Institute of Science and technology,Version:2021-2021,Content,2.1Fluid properties,2.2 Basic relations of fluid,2.3 Basic flow process,Lesson 2,Fluid,Flow,Flowing fluids in HVAC&R systems can,transfer,heat,mass and,momentum,.This chapter introduces the basics of,fluid,mechanics,that are,related to,HVAC process,review,pertinent,flow process.,Lesson 2,Fluid,Flow,2.1 Fluid properties,Fluid,s,differ from,solids in their reaction to shearing.When placed under,shear stress,a solid deforms only a finite amount,whereas a fluid deforms continuously for as long as the shear is applied.,Lesson 2,Fluid,Flow,Both liquids and gases are fluids.Although liquids and gases differ strongly in the nature of molecular actions,their primary mechanical differences are in the degree of compressibility and liquid formation of a free surface(interface),Fluid motion can usually be described by one of several simplified modes of action or models.The simplest is the ideal-fluid model,which assumes no resistance to shearing.,许多简单运行形式或模型通常可以描述流体移动.,Lesson 2,Fluid,Flow,Most fluids in HVAC applications can,be treated as,Newtonian,where,the rate of deformation,i,s directly proportional to the shearing stress.,Turbulence complicates fluids behavior,and,viscosity,influences the nature of the,turbulent flow,.,Turbulence,which complicates fluid behavior,and,viscosity,does tend to influence turbulence.,Lesson 2,Fluid,Flow,Fluid properties,Density 密度,kg/m3,The densities of air and water at standard indoor conditions of 20oC and 101.325 Pa(sea level atmospheric pressure)are,air=1.20 kg/m3 water=998 kg/m3,Viscosity(粘性,Ns/m2,Absolute viscosity or dynamic viscosity 绝对粘度，动力粘度,Differential equation:,Lesson 2,Fluid,Flow,The velocity gradient associated with viscous shear for a simple case involving flow velocity in the,x,direction,but,of,varying magnitude,in the,y,direction,is illustrated in Figure 1B.,Lesson 2,Fluid,Flow,Absolute viscosity depends primarily on temperature.For gases,viscosity increases with the square root of the absolute temperature.Liquid viscosity decreases with increasing temperature.,Shearing stress(切应力,N,Kinematic viscosity(运动粘度，m2/s,The ratio of absolute viscosity to density.,Velocity gradient(速度梯度,Lesson 2,Fluid,Flow,2.2 Basic relations of fluid dynamics,This section considers homogeneous,constant-property,incompressible fluids and introduces fluid dynamic considerations used in most analyses.,Continuity(连续性,Conservation of matter applied to fluid flow in a conduit requires that,Lesson 2,Fluid,Flow,Both,and v may vary,over the cross section A,of the conduit.If both,and v are constant over,the cross-sectional area normal to,the flow,then,For the ideal-fluid model,flow patterns around bodies(or in conduit section changes),result from,placement effects,.,An obstruction in a fluid stream,such as a strut in a flow or a bump on the conduit wall,pushes the flow smoothly out of the way,so that behind the obstruction,the flow becomes uniform again.,The effect of fluid inertia(density)appears only in pressure changes.,Lesson 2,Fluid,Flow,Pressure Variation Across Flow,Pressure variation in fluid flow is important and can be easily measured.Variation across streamlines involves fluid rotation(vorticity).,This relation explains the pressure difference found between the inside and outside walls of a,bend,and near other regions of conduit section change.It also states that pressure variation is,hydrostatic,across any conduit where streamlines are parallel.,Lesson 2,Fluid,Flow,Bernoulli equation and,pressure variation,along flow,A basic tool of fluid flow analysis is the Bernoulli relation,which involves the principle of,energy conservation,along a,streamline,.,The first law of thermodynamics,can be applied to mechanical flow energies(,kinetic and potential,)and thermal energies,:,heat is a form of energy,and,energy is conserved,.,The change in energy content,E per unit mass of flowing material is,a,result from,the,work,W done on the system plus the heat Q absorbed:,Lesson 2,Fluid,Flow,Fluid energy is,composed of,kinetic,potential,(due to elevation z),and,internal,(u)energies,.,Per,unit mass of fluid,the above energy change relation between two sections of the system is,the external work from a fluid machine,the pressure or flow work,Lesson 2,Fluid,Flow,Rearranging,the energy equation can be written as,the generalized Bernoulli equation,:,Lesson 2,Fluid,Flow,Laminar flow,For steady,fully developed laminar flow in a para
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