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Title: Transient surface cooling by non-contacting droplet impingement
Author: Chatzikyriakou, Despoina
ISNI:       0000 0004 2688 6698
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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Following a large loss of coolant accident in a PWR, cooling is performed by superheated vapour with entrained droplets, which bounce from the hot metal without wetting it. This thesis describes experimental and modelling studies aimed at the evaluation of the direct cooling by these droplets. Droplet diameters are less than 2 mm, they spend ~15 ms near the surface, extract ~1/5 J, cooling the metal by ~50 oC with heat fluxes of the order of MW/m2. An interface-tracking CFD code was used to model the droplet approach, the generation of vapour from its underside and its rebound or break-up, and to compute the transient cooling of the hot metal below the droplet. Validation of this model requires measurements of the heat transfer. A novel method to measure the transient surface temperature beneath the droplet is reported, using transient high resolution infra-red spectroscopy. Spatial and temporal resolutions of ~100.μm and ~4ms respectively are achieved, observing an opaque metallic layer from beneath through an infrared-transparent substrate. Post-processing via transient finite elements permits all thermal quantities (heat flux, energy, etc) to be determined. Associated simultaneous high speed optical recording of the droplet motion and deformation provided data for validation of the hydrodynamic aspect of the prediction. It is estimated that these methods allow the heat extracted by (for example) a 1.5 mm droplet during the 10 ms it spends in the vicinity of the hot surface to be obtained with an uncertainty of 15%. This heat extracted is approximately 0.19 J, associated with a transient temperature reduction of ~47 oC, and is removed by a heat flux peaking at 3.5 MW/m2. Encouraging agreement was obtained between these measurements and the computational simulations. For this same case, the CFD analyses predict 0.12 J and a peak heat flux of 5 MW/m2.
Supervisor: Walker, Simon ; Richardson, Stephen Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral