Inmaculada Arauzo

As a part of modeling the production process of activated biocarbon from barley straw, a model of the initial pyrolysis or carbonization stage was developed. Material samples were divided into five particle size ranges and pyrolyzed in a thermobalance at 900 °C under a nitrogen atmosphere with five different constant heating rates. For the modeling, after a brief review of the possible strategies, the Distributed Activation Energy Model (DAEM) was chosen as the most adequate. In particular, the distribution-free variant proposed by Miura and Maki was applied, allowing the activation energy distribution and the pre-exponential factor to be directly derived from experimental data without assuming a predefined analytical form for the energy distribution. The model implementation incorporates a new approximation for the temperature integral and uses five heating rates instead of the customary three. All this results in greater accuracy and detailed characterization, previously unavailable, of a widespread biomass material. A full description of the model hypotheses and procedures is given, and the results are discussed in detail, showing good agreement between experimental data and model predictions. Perspectives for the generalized use of the model, e.g., in CFD contexts, are also discussed, highlighting its robustness and computational efficiency for engineering applications.

The impact of the mushy zone parameter (Amushy) and the numerical model during the solidification of a commercial paraffin-type PCM in a vertical cylinder under T-history conditions was examined numerically. The cooling process was simulated using three methods implemented in the CFD code ANSYS Fluent 2020 R2: the enthalpy–porosity method, the AHC method, and a new model proposed by the authors, in which heat capacity is directly introduced into ANSYS Fluent. To determine the precise boundary conditions, radiative heat transfer between surfaces is considered. Moreover, the effect of the mushy zone parameter on simulation accuracy and solidification rate has been investigated, with a variable Amushyintroduced as a function of the liquid fraction. The results indicate that the proposed model demonstrates excellent agreement with experimental data for cooling temperature, providing a more accurate prediction than the other models. Additionally, it shows that temperature varies with time but not with position, suggesting that this model more effectively meets the lumped system condition—a key characteristic of the T-history experiment—compared to the other models. The effect of Amushyy indicates that simulations with a high mushy zone parameter are more accurate and predict a shorter solidification time. Additionally, using a variable mushy zone parameter based on liquid fraction yields similar results, leading to an increased solidification rate.

The growing interest in developing applications for the storage of thermal energy (TES) is highly linked to the knowledge of the properties of the materials that will be used for that purpose. Likewise, the validity of representing processes through numerical simulations will depend on the accuracy of the thermal properties of the materials. The most relevant properties in the characterization of phase change materials (PCM) are the phase change enthalpy, thermal conductivity, heat capacity and density. Differential scanning calorimetry (DSC) is the most widely used technique for determining thermophysical properties. However, several unconventional methods have been proposed in the literature, mainly due to overcome the limitations of DSC, namely, the small sample required which is unsuitable for studying inhomogeneous materials. This paper presents the characterization of two commercial paraffins commonly used in TES applications, using methods such as T-history and T-melting, which were selected due to their simplicity, high reproducibility, and low cost of implementation. In order to evaluate the reliability of the methods, values calculated with the proposed alternative methods are compared with the results obtained by DSC measurements and with the manufacturer’s technical datasheet. Results obtained show that these non-conventional techniques can be used for the accurate estimation of selected thermal properties. A detailed discussion of the advantage and disadvantage of each method is given.