The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor
(PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of
the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the
evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed
pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal
conductivity due to turbulent mixing that occurs as the flow criss–crosses between the
pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the
thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to
an effective Peclet number eff Pe based on the inverse of the effective thermal
conductivity eff k .
This thesis describes the experiments carried out on three different BETS test sections
with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also
provides the values derived for the enhanced fluid thermal conductivity for the range of
Reynolds numbers between 1,000 and 40,000.
The study includes the following:
* Compilation of a literature study and theoretical background.
* An uncertainty analysis to estimate the impact of instrument uncertainties on the
accuracy of the empirical data.
* The use of a Computational Fluid Dynamics (CFD) model to simulate the heat
transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to
extract the effective fluid thermal conductivity values from the measured results.
* The assessment of the results of the experiments by comparing it with the results
of other investigations found in the open literature.
The primary outputs of the study are the effective fluid thermal conductivity values
derived from the measured data on the HPTU plant.
The primary variables that were measured are the temperatures at radial positions at
different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80%
and 0.06% respectively.
The overall effective thermal conductivities that were calculated at the minimum and
maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK
respectively. A sensitivity study was conducted on the experimental data and the CFD
data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal
conductivities.
The results show that relatively high values of thermal dispersion quantities or effective
Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to
randomly packed beds. Therefore, the effective thermal conductivity is low and it can be
concluded that the radial mixing in the structured packing is low relative to the mixing
obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/4864 |
| Date | January 2010 |
| Creators | Kgame, Tumelo Lazarus |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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