BOS-correction of refraction for water based PIV measurements within a natural convection boundary layer

Abstract

Particle image velocimetry (PIV) is a ubiquitous optical based experimental technique for providing high resolution measurements of the velocity and derived turbulent quantities. Such data is relied upon for the validation of computer codes which are used to design and safety assure many engineering systems in use today. A desire for greater performance and safety within nuclear reactor designs increasingly demands the use of computational fluid dynamics (CFD). To fulfil this role CFD codes and models must be validated to the exceedingly high standards of the nuclear industry which in large part is expected to be performed using PIV. For nuclear reactors, water is the most commonly used coolant and thus the performance of CFD to simulate the related flow physics in water is of great interest. One particular area of interest is natural convection, which is relied upon in reactor designs as a passive heat removal capability in the event of emergency. Performing PIV measurements in water based natural convection experiments has traditionally been limited by the distorting effect of refraction which may result in substantial measurement error. In this thesis high resolution PIV measurements of natural convection from a uniformly heated vertical surface submerged in thermally stratified water are performed. The relatively high heat flux results in a transition to turbulence and substantial refractive distortion and blurring. A correction method is developed to correct for the refractive distortion and some amount of blurring. The method involves performing simultaneous background orientated schlieren (BOS) measurements to measure the distortion and provide a correction for the instantaneous PIV images as a pre-processing operation. The technical challenges of performing measurements in such circumstances and the methods employed to overcome them are discussed through this thesis. Novel methods for scaling the measured distortion and accounting for differing thermal conditions between measurements are presented. Finally, the uncorrected PIV result is compared against an instantaneous and time averaged corrected results. It is found that whilst the flow is laminar the effect of refraction is negligible, and as the flow transitions to turbulence the applicability of a time averaged correction method is limited.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Hector Iacovides (Supervisor) & Timothy Craft (Supervisor)