Heat transfer coefficients entrance region effect

Prediction of the heat-transfer coefficient in the transition flow regime is uncertain due to the strong effects of entrance conditions and instability of the flow pattern. Gnielinski [18] modified the Petukhov-Popov equation to accommodate the transition region and extend it into the turbulent flow range ... 

It is interesting to note that the nonnewtonian effect has been taken into account by simply multiplying the corresponding newtonian result by [(3n + l)/4n]1/3. Equations 10.48 and 10.49 may be used to predict the local heat transfer coefficient of purely viscous and viscoelastic fluids in the thermal entrance region of a circular tube. Figure 10.6 shows a typical comparison of the measured local heat transfer coefficient of a viscoelastic fluid with the prediction for a power-law fluid. The good agreement provides evidence to support the applicability of Eq. 10.48 in the case of the constant heat flux boundary condition. 

Length Effect. The heat transfer coefficient can vary significantly in the entrance region of the laminar flow. For hydrodynamically developed and thermally developing flow, the local and mean heat transfer coefficients h, and h, for a circular tube or parallel plates are related as [19]... 

F Entrance-Region Effect on Heat-Transfer Coefficient... 

The development of the velocity and temperature fields has a marked effect on the friction factor and the heat transfer coefficient near the channel entrance region. Correlations for the calculation of the friction factor and of the convective heat transfer coefficients in the entrance region of a microchannel are given in entrance region. 

In the Hausen correlation the Nusselt number is calculated as a sum of two terms the first term (Nu) is the fully developed value of the Nusselt number its value can be found in convective heat transfer in microchannels for the most common microchannel cross-sections. The second term takes into account the effects of the thermal entrance region. The values of the coefficients K, K2 and b for circular microchannels are quoted in Table 3. 

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