Resumen de A numerical study of turbulent heat transfer in pipes
The present work is devoted to the detailed study of the turbulent heat transfer problem in a pipe flow with variable fluid properties subjected to highly non-uniform heat flux distribution on the pipe surface. This problem aims to be representive of the conditions found in the tubes of heat receivers in Solar Power Tower (SPT) plants where sun radiation is converted into electric energy. The operational and economical performance of SPT plants rely on an accurate prediction of the thermal field, sometimes beyond the current semi-empirical approach to the heat transfer problem. Moreover, this study might be of interest for many other heat transfer applications in engineering.
The primary intention of this thesis is to shed some light on the influence of the circumferentiallyvarying heat flux conditions and of the temperature-dependent fluid properties in the statistics of a fully-developed turbulent flow. To that end, we create and analyze a numerical database modifying the main flow parameters of interest: the friction Reynolds number (Retau = 180 − 360), the Prandtl number (Pr = 0.7 − 4), the heat flux distribution and the sensitivity to temperature of the fluid variables.
From the analysis, we observe that, while the friction Reynolds number has little effect on the temperature distribution on the pipe wall, the impact of the Prandtl number is significant. This is of importance for the heat receiver application, as the maximum flow temperature at the inner wall, the so-called film temperature, might be always below certain limit.
Another important outcome from the analysis of a case with a heated half of the pipe but cooled in the other half is that, because of the influence of the variable fluid properties, the temperature turbulent fluctuations are enhanced near the cold wall but damped near the hot wall. This behavior is seen to reduce the maximum temperature peaks in the flow while maintaining a similar wall temperature distribution. The variation of the fluid properties are also shown to induce small but discernible secondary velocities in the pipe cross-plane. These velocities are responsible of a non negligible contribution to the heat flux from the heated wall to the cooled wall.
To gain a deeper insight in the turbulent heat transfer phenomena, we perform a modal decomposition of the instantaneous velocity and temperature fields to extract the information on the most energetic coherent structures in the flow. The use of a relatively new technique, Extended Proper Orthogonal Decomposition, allows us to discern how the velocity fluctuations are correlated to the temperature fluctuations, hence increasing our knowledge in the heat transfer process. From this analysis, we characterize the modes that are responsible of the heat transport in the pipe crossplane, obtaining that the scales of the structures bringing hot fluid from the heated wall to the pipe core are strongly affected by the Prandtl number.
