Studies of the flow pattern in short term hot water storages

Abstract: This thesis relates to the study of thermally stratified hot water storages used in district heating systems and is an extension of the doctoral theses of Jan Dahl and Roger Hermansson. The thermal performance of a stratified water storage is reduced by different phenomena such as heat transfer to the surroundings and mixing between hot and cold water during the charging of the tank. The study of the flow field in the storage tank gives a better understanding of these phenomena. The work described here is composed of two parts. The first concerns the development of a Particle Tracking Velocimetry (PTV) technique, described in Paper I, which can be used to measure the velocity field in a model water heat storage. Paper II presents a study of phenomena affecting the accuracy of the PTV technique. The inaccuracy of the velocity measurements due to the limited resolution of the video camera increases rapidly if the time-step between pictures is too small. The number of incorrect identifications is also an important factor for the determination of the time-step. A method of selecting the appropriate time-step has been proposed in Paper III. With the help of a procedure to remove incorrect identifications, a method for evaluation of the velocity field aimed at reducing errors caused by resolution inaccuracy and incorrect particle identifications has been developed. The second part of this thesis concerns studies of flow patterns in a model water heat storage. Using the PTV technique presented previously, the natural convection boundary layer has been observed and measured in Paper IV. Flow phenomena caused by heat losses to the surrounding are found to have an important effect on the nature of the gradient zone. The gravity current resulting from the inflow of hot water in the tank filled with cold water has been studied in Paper V. A numerical simulation has been performed. The mixing has been quantified through an efficiency based on exergy. Phenomena leading to a loss of efficiency during the formation of the thermocline have been identified. The conditions of vortex formation at the edge of the inlet plate during the formation of the thermocline are investigated in Paper VI. A range of critical Richardson numbers, based on a comparison of experimental results, numerical simulation and theorietical analysis has been proposed.