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## LS Sangweni

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**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**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.**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)**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.**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.**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.**Experimental Investigations (cont.)**P P P Single phase flow Two-phase flow Two-phase flow Closed-loop natural circulation NCL operating modes TCOLD TCOLD TCOLD THOT THOT THOT Shut-off valve Single and Two-phase Mode Constant-pressure closed-Loop single and two-phase thermosyphon Heat Pipe Mode Constant-volume closed-Loop two-phase thermosyphon Single Phase Mode Constant-pressure closed-Loop single-phase thermosyphon (NCL)**Mathematical model**Assumptions • One-dimensional two-fluid steady-state conditions apply. • Fluid is incompressible. • Boussinesq approximation is valid i.e. a hydrostatic pressure , and density is essentially constant density except in the buoyancy term where density [or ] and the fluid be it liquid or/and vapour is essentially incompressible. • Heat losses in the pipes are negligible. • Both two-phases are at equilibrium, i.e. particle velocity<<speed of sound.**Mathematical model (cont.)**Expansion Tank • Simulate the system by a series discrete-sized 1-D control flow volumes. Water cooled condenser**Mathematical modeling (cont.)**Conservation equation control volumes (a) Mass, (b) Momentum, (c) Energy**Theoretical Modeling (concluded)**Equations of change Conservation of mass Conservation of energy (Ignoring KE and PE ) and noting that h = u + PdV Conservation of momentum**Expected Outcomes**• Correlation between experimental and simulation results. Figure 2: Expected Results (White, 2010)**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)**Thank you**LS Sangweni Department of Mechanical Engineering,University of Stellenbosch,Stellenbosch, Western Cape Province SangweLS@Eskom.co.za