6 research outputs found
Development and Validation Of Resistance-Capacitance Model (RCM) For Phase Change Material (PCM) Embedded In 3D Periodic Structures
No full textDEVELOPMENT, VALIDATION AND APPLICATION OF RESISTANCE-CAPACITANCE BASED MODELS FOR PHASE CHANGE MATERIAL HEAT EXCHANGERS
No full textLatent 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
A Study on Computational Cost Reduction of Simulations of Phase-Change Material (PCM) Embedded Heat Exchangers
No full textThermal storage can be implemented using Phase-Change Materials (PCM), which absorb significant latent heat with a relatively small temperature change. PCM phase-change processes are transient and are driven by thermal diffusion and natural convection – the latter, especially for melting process. Modeling and simulation of PCM heat exchangers (HX’s) is typically computationally intensive due to the relatively complex time-dependent physics. Most of the PCM modeling work in the literature uses high-order modeling tools such as Computational Fluid Dynamics (CFD) and Lattice-Boltzmann Method (LBM). For design purposes, the existing PCM modeling approaches are not practical, limiting researchers in their ability to investigate new ideas and different PCM’s with faster turnarounds. This paper presents a study investigating the reduction of computational cost of PCM embedded HX’s CFD models by evaluating the feasibility of spatial reduction without losing accuracy. The analysis consists of comparing full and partial domain under full melting conditions. The subject of this study is a single straight tube with circular transverse fins in the vertical orientation, using PCM’s with 35oC nominal melting temperature. Different tube and fin dimensions are investigated. Results indicate that the reduced domain reproduces -in half the run time -the same behavior as the full domain since the buoyancy effects are localized and patterned. The outputs from the partial domain simulation were used to build a non-general correlation for the PCM heat transfer characteristics and demonstrated how it can be implemented in a Finite Control Volume Reduced Order Model (ROM). The ROM can accurately reproduce the CFD simulations at 4 to 5 orders of magnitude faster
Development and Validation Of Resistance-Capacitance Model (RCM) For Phase Change Material (PCM) Embedded In 3D Periodic Structures
No full textNumerical Study And Validation Of Melting And Solidification In PCM Embedded Heat Exchangers With Straight Tube
No full textLatent heat thermal energy storage (LHTES) systems have shown great potential to enable reliable use of renewable energy and load shifting. LHTES offer high storage density and release energy at near constant temperature because of its use of phase change materials (PCMs). The cylindrical PCM heat exchangers (PCMHX) are one of the most used technologies due to their simplicity. Numerical models for such PCMHX enable engineers to estimate their performance for different design parameters and operating conditions without having to test them all. However, modeling the phase change phenomena can be challenging. To better understand the difficulties involving accurate modeling of PCMHX, a cylindrical latent storage unit filled with PCM and water as in-tube heat transfer fluid (HTF) is numerically investigated. This paper presents a study based on a 2D-axisymmetric model of a straight tube embedded in PCM in a cylindrical container. CFD is used to study the charging (melting) and discharging (solidification) phenomena. The models are validated against experimental and numerical data from the literature. The predicted local PCM temperature profile over time agrees within 2K compared to the experimental values. The paper also presents a simple method to estimate the melting and solidification phase change temperature range from limited data provided by PCM manufacturers
