Thermal stratification in a hot water tank established by heat loss from the tank

Abstract This paper presents numerical investigations of thermal stratification in a vertical cylindrical hot water tank established by standby heat loss from the tank. The transient fluid flow and heat transfer in the tank during cooling caused by standby heat loss are calculated by means of validated computational fluid dynamics (CFD) models. The measured heat loss coefficient for the different parts of the tank is used as input to the CFD model. Parametric studies are carried out using the validated models to investigate the influence on thermal stratification of the tank by the downward flow and the corresponding upward flow in the central parts of the tank. Tank design parameters such as tank volume, height to diameter ratio and insulation and different initial conditions of the tank are investigated. It is elucidated how thermal stratification in the tank is influenced by the natural convection and how the heat loss from the tank sides will be distributed at different levels of the tank at different thermal conditions. The results show that 20–55% of the side heat loss drops to layers below in the part of the tank without the presence of thermal stratification. A heat loss removal factor is introduced to characterize the effect of the buoyancy driven flow on exchange of heat loss between tank layers by natural convection. Based on results of the parametric studies, a generalized equation for the heat loss removal factor is obtained by regression which takes into account the influences of tank volume, height to diameter ratio, tank insulation and initial conditions of the tank. The equation is validated for a 150–500 l tank insulated with 0–7 cm mineral wool and a tank height to diameter ratio of 1–5. The equation will be implemented in an existing tank optimization and design program for calculation of thermal performance of a hot water tank.