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