Inside-pipe heat transfer coefficient

Description

Inside-pipe heat transfer coefficient characterisation of a one third height scale model of a natural circulation loop suitable for a Reactor Cavity Cooling System of the Pebble Bed Modular ReactorThesis presented in partial fulfilment of the requirements for the degree Master of Science in Engineering at the University of Stellenbosch, 2011, 141 p.The feasibility of a closed loop thermosyphon for the Reactor Cavity CoolingSystem of the Pebble Bed Modular Reactor has been the subject of many researchprojects. Difficulties identified by previous studies include the hypotheticalinaccuracies of heat transfer coefficient correlations available in literature. Theaim of the research presented here is to develop inside-pipe heat transfercorrelations that are specific to the current design of the RCCS.In order to achieve this, a literature review is performed which identifies reactorswhich employ closed loop thermosyphons and natural circulation. The literaturereview also explains the general one-dimensional two-fluid conservationequations that form the basis for numerical modelling of natural circulation loops.The literature review lastly discusses available heat transfer coefficientcorrelations with the aim of identifying over which ranges and under whichcircumstances these correlations are considered accurate. The review includescorrelations commonly used in natural circulation modelling in the nuclearindustry in aims of identifying correlations applicable to the modelling of theproposed RCCS.One of the objectives of this project is to design and build a one-third-height-scalemodel of the RCCS. Shortcomings of previous experimental models wereassessed and, as far as possible, compensated for in the design of the model.Copper piping is used, eliminating material and surface property uncertainties.Several sight glasses are incorporated in the model, allowing for the visualidentification of two-phase flow regimes. An orifice plate is used allowing for bidirectionalflow measurement. The orifice plate, thermocouples and pipe-in-pipeheat exchangers are calibrated in-situ to minimize experimental error and aidrepeatability.Twelve experiments are performed with data logging occurring every ten seconds.The results presented here are limited to selected single and two-phase flowoperating mode results. Error analyses and repeatability of experimentalmeasurements for single and two-phase operating modes as well as cooling watermass flow rates are performed, to show repeatability of experimental results.These results are used to mathematically determine the experimental inside-pipeheat transfer coefficients for both the evaporator and condenser sections. Trendsin the heat transfer coefficient profiles are identified and the general behaviour ofthe profiles is thoroughly explained.The RCCS is modelled as a one-dimensional system. Correlations for the frictionfactor, heat transfer coefficient, void fraction and two-phase frictional multiplierare identified. The theoretical heat transfer coefficients are calculated using themathematical model and correlations identified in the literature review. Fluidparameters are evaluated using experimentally determined temperatures and massflow rates. The resulting heat transfer coefficient profiles are compared toivexperimentally determined profiles, to confirm the hypothesis that existingcorrelations do not accurately predict the inside-pipe heat transfer coefficients.The experimentally determined coefficients are correlated to 99% confidenceintervals. These generated correlations, along with identified and established twophaseheat transfer coefficient correlations, are used in a mathematical model togenerate theoretical coefficient profiles. These are compared to the experimentallydetermined coefficients to show prediction accuracy.