Viscosity of Fluids

When a fluid is flowing through a closed channel such as a pipe or between two flat plates, either of two types of flow may occur, depending on the velocity of this fluid. At 

A fluid can be distinguished from a solid in this discussion of viscosity by its behavior when subjected to a stress (force per unit area) or applied force. An elastic solid deforms by an amount proportional to the applied stress. However, a fluid when subjected to a similar applied stress will continue to deform, i.e., to flow at a velocity that increases with increasing stress. A fluid exhibits resistance to this stress. Viscosity is that property of a fluid which gives rise to forces that resist the relative movement of adjacent layers in the fluid. These viscous forces arise from forces existing between the molecules in the fluid and are of similar character as the shearforces in solids. 

It has been found experimentally for many fluids that the force F in newtons is directly proportional to the velocity Av. in m/s, to the area Ain m of the plate used, and inversely proportional to the distance Ay in m. Or, as given by Newton s law of viscosity when the flow is laminar. 

2 Principles of Momentum Transfer and Overall Balances 

Other conversion factors for viscosity are given in Appendix A.l. Sometimes the viscosity is given as pjp, kinematic viscosity, in m /s orcm /s, where p is the density of the fluid. 

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S = Specific gravity at flowing temperature versus water at 15°C fi= Viscosity of fluid at flowing temperature, centipoises or mPa-s A = Effective orifice area, mm from the manufacturer s literature). 

Pf = density of fluid, Ib/ff or kg/m p. = viscosity of fluid, cp bj = constant given above n = constant given in text. 

X, y, z are empirical exponents k = thermal conductivity d = heat transfer tube diameter p = density of fluid or specific gravity p = viscosity of fluid... 

L = total heated or cooled length of heat transfer path, ft w = weight rate of fluid flow per tube, Ib/hr p, = viscosity of fluid, lb/ (hr) (ft)... 

Reynolds Re Velocity of fluid Density of fluid Viscosity of fluid Dimension of surface... 

Prandtl Pr Specific heat capacity of fluid Viscosity of fluid Thermal conductivity of fluid... 

D = impeller diameter, ft N = impeller speed, rpm p = density, Ib/cu ft ft = bulk viscosity of fluid... 

O In CF, the cystic fibrosis transmembrane regulator (CFTR) chloride channel is dysfunctional and usually results in decreased chloride secretion and increased sodium absorption, leading to altered viscosity of fluid excreted by the exocrine glands and mucosal obstruction. 

The unit of viscosity of fluids in CGS units. It is measured in dynes/cm2 per unit velocity gradient. Poisson s Ratio... 

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