1. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 1 Investigating Instabilities of a Single and Two Phase Flow in a Multi-Channel Natural Circulation Loop LS Sangweni Department of Mechanical Engineering, University of Stellenbosch, Stellenbosch, Western Cape Province

2. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 2 Contents Two-phase flow in a multi-channel natural circulation loop. Why are we interested in two phase flow Natural Circulation Loops? How is the investigation going to be conducted? Expected Outcomes. Conclusions and Recommendations.

3. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 3 Two-phase flow in a multi-channel natural circulation loop. Fluid filled loop formed by pipe connections between a heat source and sink. Sink is located at a higher elevation than the source to enhance circulation. Figure 1: Operating principle of NCL (Reys & Cleveland, 2005:14)

4. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 4 Motivation Natural Circulation Loop are safety passive systems in nature. Two phase flow in Multi-channel systems knowledge is limited ( Reys & Cleveland, 2005:37). Nuclear Reactor core cooling utilizes NCL systems. SA is looking at nuclear power stations for future base load power generation.

5. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 5 Scope of work and Objectives Conduct extensive literature study on NCL operation. Construct a triple-channel NCL. Perform a single and two phase flow experiments on the NCL. Write a simulation programme to predict experimental behavior of the NCL. Distinguish the instabilities associated with a two phase flow multi- channel system from a single phase flow. Simulation outcomes to correlate with the experimental and literature outcomes. Document all work done.

6. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 6 Literature Study Extensive studies on Natural Circulation Systems have been conducted World wide. Not much has been done on two-phase multichannel systems. Research shows that NC system is the preferred option for heat removal in nuclear reactors. NC system is likely to suffer instabilities. Methods of improving the stability of the NC systems have been investigated.

7. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 7 Experimental Setup

8. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 8 Experimental Investigations (cont.) Closed-loop natural circulation NCL operating modes

9. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 9 Mathematical model Assumptions

10. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 10  Simulate the system by a series discrete-sized 1-D control flow volumes. Mathematical model (cont.) Water cooled condenser Expansion Tank

11. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 11 Mathematical modeling (cont.) Conservation equation control volumes (a) Mass, (b) Momentum, (c) Energy

12. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 12 Theoretical Modeling (concluded) Equations of change

13. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 13 Expected Outcomes  Correlation between experimental and simulation results. Figure 2: Expected Results (White, 2010)

14. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 14 Conclusion and Recommendations  Thermosyphon is an effective way of removing heat.  Effects of the multiple loop systems on natural circulation instability is the main objective of this study.  Natural cooling Systems can be used in the design of chemical reactors, nuclear reactors, steam generators and solar heaters to eliminate problems associated with pump operation and maintenance. Figure 3: Pebble bed reactor (White, 2010)

15. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS EPC 2013 15 LS Sangweni Department of Mechanical Engineering, University of Stellenbosch, Stellenbosch, Western Cape Province SangweLS@Eskom.co.za Thank you