Experimental analysis of thermal mixing at reactor
conditions
MATTIA BERGAGIO
Licentiate Thesis
Stockholm, Sweden 2016
Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges till offentlig granskning för avläggande av teknologie licentiatexamen i reaktortek-nologi fredagen den 16 december 2016 kl 10.00 i FB55, AlbaNova, Stockholm.
Abstract
High-cycle thermal fatigue arising from turbulent mixing of non-isothermal flows is a key issue associated with the life management and extension of nu-clear power plants. The induced thermal loads and damage are not fully understood yet.
With the aim of acquiring extensive data sets for the validation of codes mod-eling thermal mixing at reactor conditions, thermocouples recorded tempera-ture time series at the inner surface of a vertical annular volume where turbu-lent mixing occurred. There, a stream at either 333 K or 423 K flowed upwards and mixed with two streams at 549 K. Pressure was set at 72 × 105Pa. The annular volume was formed between two coaxial stainless-steel tubes. Since the thermocouples could only cover limited areas of the mixing region, the in-ner tube to which they were soldered was lifted, lowered, and rotated around its axis, to extend the measurement region both axially and azimuthally. Trends, which stemmed from the variation of the experimental boundary conditions over time, were subtracted from the inner-surface temperature time series collected. An estimator assessing intensity and inhomogeneity of the mixing process in the annulus was also computed. In addition, a fre-quency analysis of the detrended inner-surface temperature time series was performed. In the cases examined, frequencies between 0.03 Hz and 0.10 Hz were detected in the subregion where mixing inhomogeneity peaked. The uncertainty affecting such measurements was then estimated.
Furthermore, a preliminary assessment of the radial heat flux at the inner surface was conducted.
Keywords: Mixing estimator, empirical mode decomposition,
Hilbert-Huang transform, uncertainty assessment, radial heat flux
