Pseudocriticai temperature

Fig. 2.4 Heat transfer coefficient near the pseudocriticai temperature comparison with the calculated results, experimental results of Yamagata et ai. [8] and results from the Dittus-Boeiter correlation...

Prandtl numbers. Thus, the coefficient near the pseudocriticai temperature, where the Prandtl number becomes large, may be smaller. The ideal coefficient calculated by the Jones-Launder k-e model at the pseudocriticai temperature is plotted in Fig. 2.4. It is calculated by fixing the thermophysical properties at the pseudocriticai temperature. This value is higher than that shown by the curve of 2.33 x 10 W m . When the Jones-Launder k-e model is used, it is known that the wall shear stress is relatively large and the heat transfer coefficient is also large with a constant turbulent Prandtl number. As indicated by Jackson and Hall [4], the heat transfer coefficient is the maximum when the heat flux is zero and it monotonically decreases as the heat flux increases. The calculation supports their assertion. 

To obtain the deteriorated heat flux, calculations have been carried out with various combinations of flow rate G and heat flux q". Deterioration is assessed where the bulk temperature reaches the pseudocriticai temperature. The deterioration ratio a/ao is defined where ao is the ideal heat transfer coefficient. Some calculation results are shown in Fig. 2.5. The heat transfer coefficient monotonically decreases when the flow rate is large. On the other hand, it abraptly drops at a certain heat flux and maintains a constant value or increases with larger heat fluxes when the flow rate is small. The boundary is around 200 kg m s under the analyzed flow... 

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