Number Nusseit

Nud Nusseit number based on diameter D, hD/k newtonian shear rates ... 

Overall reaction order Nusseit Number (dimensionless)... 

TABLE 5-4 Values of Limiting Nusseit Number in Laminar Fiow in Ciosed Ducts... 

Convective Heat Transfer in Microchanneis, Tabie 3 Nusseit numbers Nuj3 for iaminar fuiiy deveioped fiow as a function of the dimensioniess waii thermai resistance and of the Peciet number for the T3 boundary condition... 

Convective Heat Transfer in Microchanneis, Table 7 Nusseit numbers Nuj for laminar fully developed flow for a rectangular cross-section with three (3bc) and four (4bc) heated sides as a function of the aspect ratio S for the T boundary condition... 

The value of the Nusseit numbers obtained for the same conditions mentioned above but for slug flow can be calculated analyrticaUy as a function of the aspect ratio by means of the foUowing relations for three and four heated walls ... 

For slug flow under H2 boundary conditions it is possible to demonstrate that the Nusseit number in the 4bc case (four heated sides) is independent of the aspect ratio and its value is 6. The Nusseit number for slug flow with three heated sides can be calculated by means of the following analytical expression ... 

In Tab. 8 the values of the Nusseit number for HI and H2 boundary conditions (with three (3bc) and four (4bc) heated walls) in the case of pressure-driven flow with negligible external volume forces (fext,z = 0). axial heat conduction (Pe —> oo), viscous dissipation (Br = 0), flow work (fi = 0) and thermal energy sources (Sg = 0) within the fluid is quoted as a function of the aspect ratio Su. 

In Tab. 9 the fully developed Nusseit numbers for rectangular microchanneis in which rarefied gases flow under the HI boundary condition with four heated sides (4bc) are quoted. These values are obtained for a gas having Pr = 0.7 by neglecting viscous dissipation, axial conduction and flow work and using in the slip boundary conditions (Eqs. (8) and (19)) a = ov = o / = <7/ = 1. It is evident that the Nusseit number decreases when the Knudsen number increases and it tends to increase when the aspect ratio decreases. 

Tunc and Bayazitoglu [4] have calculated in the case H2 (4bc) the fully developed Nusseit numbers for rectangular microchanneis through which a rarefied gas flows by neglecting the viscous dissipation, axial conduction in the fluid and the flow work. They used in the slip boundary conditions (Eqs. (8) and (19)) a =ay = at = at = I The values of the Nusseit number for fully developed laminar flow determined by Tunc and Bayazitoglu [4] are quoted in Tab. 10 as a function of the aspect ratio 3, of the Pran-dlt number and of the Knudsen number for four sides heated (4bc). By observing these data it is evident that... 

Convective Heat Transfer In Microchanneis, Table 10 Nusseit numbers Nuh2 for rarefied gases in iaminar tuiiy deveioped fiow in a rectanguiar microchannei w/ith four sides heated as a function of the Knudsen number, of the Prandti number and of the aspect ratio S for the H2 boundary condition... 

Nonino et al. [5] computed numerically the value of the laminar fully developed average Nusseit number for trapezoidal and double trapezoidal KOH-etched microchanneis under the T boundary condition with four and six heated... 

The thermal behavior of turbulent flows in a microchannel, under conditions of constant properties and moderate velocity can be described by means of the Nusseit number. Under certain thermal boundary conditions the Nusseit number in the turbulent regime becomes independent on the channel length. As in the case of laminar flow the... 

For turbulent flow the thermal entrance region is shorter than for laminar flow (with the exception of liquid metals which have a very low Prandtl number) and thus the fully developed values of the Nusseit number are frequently used directly in heat transfer design without reference to the thermal entrance effects. The turbulent fully developed Nusseit number in a smooth channel can be expressed as a function of the Reynolds number and of the Prandtl number. 

Since the local value of the bulk temperature is not easy to measure along a microchannel, the local value of the Nusseit number cannot be calculated in general. Only the... 

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