Abstract
Stratified tank models are used to simulate thermal storage in applications such as residential or commercial hot-water storage tanks, chilled-water storage tanks, and solar thermal systems. The energy efficiency of these applications relates to the system components and the level of stratification maintained during various flow events in the tank. One-dimensional (1D) models are used in building energy simulations because of the short computation time but often do not include flow-rate dependent mixing. The accuracy of 1D models for plug flow, plug flow with axial conduction, and two convection eddy-diffusivity models were compared with experimental data sets for discharging a 50-gal residential tank and recharging the tank with hot water from an external hot-water source. A minimum and maximum relationship for the eddy diffusivity factor were found at Re <2100 and >10,000 for recirculation of hot water to the top of the tank and vertical tubes inletting cold water at the bottom. The root mean square error decreased from >4 °C to near 2 °C when considering flow-based mixing models during heating, while the exponential decay of the eddy diffusion results in a root mean square error reduction of 1 °C for cone-shaped diffusers that begin to relaminarize flow at the inlet.
Highlights
The study aimed to identify a flow-rate dependent 1D thermally stratified tank model for building energy simulations that compute faster than 3D models
The additional diffusion in the thermocline region of a hot-water tank appears to be related to the inlet flow rate. We suggest that these 1D seasonal models for hot-water tank modeling incorporate a correction factor for flow rates throughout the tank because the diffusion is greater than conduction alone under flow conditions
The cases of hot-water flow from the top of a tank and different vertical tube inlets (VTIs) that deliver cold water to the bottom of the tank have been modeled and compared with experimental data
Summary
The study aimed to identify a flow-rate dependent 1D thermally stratified tank model for building energy simulations that compute faster than 3D models. These 1D models can still include the mixing flow dynamics related to the inlet turbulence. For the most robust 1D models, nondimensional base models were reviewed in detail as they contain the many physical parameters mentioned above. There are many valid choices of nondimensional numbers or combinations of numbers to model thermally stratified flows with a 1D equation. The complexity is in choosing valid nondimensional numbers to develop correlations for a robust range of flow-rates experienced in hot-water tanks (e.g., 0.5 GPM to 3 GPM)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
