SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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.

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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)

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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.

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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.

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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.

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC Experimental Setup

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

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

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

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

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

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

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC 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)

SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, Aug 2013 iThemba LABS EPC LS Sangweni Department of Mechanical Engineering, University of Stellenbosch, Stellenbosch, Western Cape Province Thank you