2D CFD investigation of diagonal flow and thermal stratification in a packed-bed PCM thermal storage tank

Objective/short description

The project aims to develop and validate a two-dimensional CFD model of a packed-bed phase change material (PCM) thermal energy storage tank with non-axial (diagonal) flow. Unlike conventional top–bottom flow systems, this tank operates with two inlets and two outlets that switch between top–bottom and bottom–top configurations during charging and discharging.

The student will simulate the thermal and fluid dynamic behavior to identify how flow direction, flow rate, and buoyancy affect stratification, energy efficiency, and outlet temperature evolution.

Background

Packed-bed thermal energy storage systems filled with encapsulated PCM spheres are widely used in large-scale heating and cooling networks. Their design typically assumes axial flow through the bed, which allows simplified 1D or ε–NTU modeling.

However, in configurations such as the thermal storage tank, the system employs diagonal flow switching, from top inlet to bottom outlet during discharge, and bottom inlet to top outlet during charge, to enhance stratification and avoid mixing.

The complex flow field resulting from this diagonal configuration creates non-uniform velocity and temperature distributions. Predicting these distributions requires two-dimensional CFD modeling that accounts for porous flow, buoyancy-driven natural convection, and phase-change heat transfer.
Understanding the resulting lines of constant mass flow and temperature stratification is critical for optimizing the control strategy and validating simplified grey-box models (e.g., ε–NTU models).

The work is part of the HECTAPUS project (Heating Cooling Transition and Acceleration with Phase Change Energy Utilization Storage) which aims to accelerate the heating and cooling transition through the integration of Phase Change Materials (PCMs) with Underground Thermal Energy Storage (UTES) and heat pump systems. The project brings together partners from Norway, Türkiye, and Sweden, including KTH Royal Institute of Technology, to develop energy-efficient and cost-effective thermal storage concepts that contribute to Europe’s goal of achieving 100% renewable heating and cooling by 2050.

Task description

Literature Review

Review CFD and analytical models of packed-bed thermal energy storage, focusing on non-axial and radial flows. Summarize previous studies on ε–NTU and p-factor formulations for PCM beds.

Model Development

Build a 2D axisymmetric or rectangular CFD model (e.g., in ANSYS Fluent or COMSOL Multiphysics) of the PCM-packed bed treated as a porous medium with effective thermophysical properties. Implement appropriate energy equations with phase-change (enthalpy-porosity method).

Define boundary conditions for two operation modes:

• Charging: bottom-inlet → top-outlet

• Discharging: top-inlet → bottom-outlet
  (flow diagonally across the tank cross-section)

Parametric Study

Vary the inlet mass flow rate, flow direction, and inlet temperature. Evaluate resulting velocity, pressure, and temperature fields. Identify streamlines of constant mass flux and their dependence on Reynolds number. Quantify thermal stratification and outlet temperature profiles over time.

Model Validation

Compare key CFD results with available experimental results from HECTAPUS project. Discuss the relationship between observed flow fields and benchmark against the p-factor or effectiveness–NTU framework.

Analysis and Reporting

Assess the influence of flow direction on energy recovery efficiency. Provide design recommendations for optimized inlet/outlet configuration.

Learning outcomes

After completing the thesis, the student will be able to:

• Formulate and solve coupled fluid–thermal problems involving phase change in porous media.

• Apply finite volume CFD methods to simulate packed-bed energy storage systems.

• Interpret thermal stratification and flow distribution using physical and numerical reasoning.

• Compare CFD and reduced-order (ε–NTU) models and understand their respective limitations.

• Produce high-quality scientific visualizations and technical reporting.

Prerequisites

• Background in heat transfer, thermodynamics, and fluid mechanics.
• Analytical skills for data post-processing.
• Interest in thermal energy systems and modeling of energy storage.

Duration

Five to six months are expected to finish the thesis. Start date to be discussed.

Supervisors/Examiner

Reference

Amin, N. A. M., Belusko, M., & Bruno, F. (2014). An effectiveness-NTU model of a packed bed PCM thermal storage system. Applied Energy, 134, 356–362. https://doi.org/10.1016/J.APENERGY.2014.08.020