Author: Tanjebul Alam
Date: Wednesday, January 11, 2023, at 9:30 am
Location: Room EGR-2164
Committee Members:
Research Professor Vikrant Aute, Mechanical Engineering, Chair/Advisor
Professor Bao Yang, Mechanical Engineering
Associate Professor Damena Agonafer, Mechanical Engineering
Title of Thesis: DEVELOPMENT, VALIDATION AND APPLICATION OF RESISTANCE -CAPACITANCE BASED MODELS (RCM) FOR PHASE CHANGE MATERIAL HEAT EXCHANGERS
Abstract:
Latent thermal energy storage use Phase Change Material (PCM) because of its ability to absorb or release a large amount of latent heat within a narrow temperature range. The low conductive heat transfer in PCMs can be improved with thermal enhancement techniques such as the addition of highly conductive metal foams and extended surfaces like fins or periodic metal structures within the PCM domain. High-order modeling tools like Computational Fluid Dynamics (CFD) are widely used for the simulation of different types of PCM heat exchangers (HXs). High computing costs are typically associated with CFD, particularly for the complex transient phase-change processes. This becomes restrictive in some applications such as PCMHX optimization where the conventional process is limited by the computational cost of the high-order physics models. A simulation tool with a faster turnaround is necessary for such cases, even if it comes with a small accuracy penalty. Resistance-capacitance based model (RCM) can be a suitable solution for this type of problem as the model is computationally inexpensive. RCM does not solve for the mass and momentum governing equations as in CFD, but can still predict the PCMHX characteristics with reasonable accuracy, especially for configurations where conduction is the dominant heat transfer mechanism.
This work presents the development of RCMs for four types of PCMHXs which are a rectangular PCM enclosure enhanced with copper foam subject to constant heat flux, a geometry enhanced with 3D lattice structures subject to constant heat flux, a cylindrical PCMHX with annular fins and tube in conjugate heat transfer with single-phase heat transfer fluid and a cylindrical PCMHX with annular fin and tube enhanced with copper foam. In all these geometries the effect due to the flow of molten PCM can be considered negligible and the geometries are regarded as structured. The models were validated against experimental data and compared against CFD models for computational cost and prediction accuracy. Both of the models predicted the energy storage within 0.8% of the experimental data for the rectangular PCM enclosure enhanced with copper foam. For the cylindrical PCMHX with annular fins, the maximum RMSE for average PCM temperature prediction was found to be 0.62K for CFD and 0.7K for RCM. These results show that RCM can predict the average temperature profile and energy storage up to 5 orders of magnitude faster than CFD while having negligible prediction deviation. The validated model for annular finned PCMHX is used with a multi-objective genetic algorithm to optimize a PCMHX integrated with a domestic water heater. Additionally, thermal Ragone plots were generated to compare different designs at various operating conditions which can be used for optimal design selection.