Masoud Ziabasharhagh

This study numerically investigates the capability of cylindrical phase change material (PCM) encapsulations to attenuate inlet-temperature fluctuations in water as the heat transfer fluid (HTF). A sinusoidal inlet profile with a 20 K amplitude is imposed, and melting and solidification are modeled using the enthalpy–porosity method. The analysis begins with a single encapsulation, which reduces the outlet temperature amplitude by 45.06%, and extends to three, nine, and 15 cylinders in aligned and staggered arrangements. Increasing the cylinder count enhances fluctuation reduction but with diminishing returns, as the HTF thermal energy reaching downstream cylinders decreases. Spatial arrangement is equally important: a staggered array of nine encapsulations achieves a 65.91% reduction, surpassing a 15-cylinder aligned configuration (65.80%), while the highest reduction, 73.01%, is obtained with a 15-cylinder staggered configuration. Across all configurations, the outlet fluctuation reduction follows a single near-linear relationship with the total melted PCM mass (R² = 0.96), across cylinder count and arrangement, identifying melted mass as a controlling parameter for fluctuation mitigation rather than the cylinder-averaged liquid fraction. These results indicate that the total melted PCM mass is a practical criterion for comparing PCM encapsulation configurations, while the arrangement remains a distinct factor.

Ejectors are devices designed to suck fluid, steam or gas (primary fluid) from a closed space using a powerful jet of steam (secondary fluid), usually operated under specified boundary conditions using specific working fluids. If ejectors are to be used under new boundary conditions, predicting their performance requires either numerical or experimental studies. This paper presents a simple theoretical model capable of accurately predicting the performance of an ejector, given its geometry and boundary conditions, under different operating conditions. The model can predict the entrainment ratio, critical back pressure, and break-up back pressure using a given simple performance curve. The accuracy of the model is validated using computational fluid dynamics (CFD) simulations. Two ejectors with different geometries, dimensions, and boundary conditions are studied using ANSYS Fluent 19.2, and the results are compared with those from two other studies. The model successfully predicts the performance of all four ejectors across a wide range of operating conditions. Finally, the model is extended to any working fluid and temperature and validated numerically using air as the working fluid instead of water vapor. The results show that the model has an entrainment ratio error of less than 2%. It’s worth noting that this model’s applicability is contingent upon simultaneous changes to both the primary and suction streams by the same factor. Under these conditions, the model aligns closely with CFD-simulations.

This paper introduces a novel 1D model approach to optimize a steam ejector entrainment ratio. New definitions of ejector efficiency are defined and the increase in ejector efficiency is studied using this novel 1D model approach. Based on this novel 1D model, an ejector is designed for specified boundary conditions for which CFD entrainment ratio results are available. The ejector's dimensions and geometry are calculated. Based on the main dimensions extracted from the theoretical 1D model, dozens of geometries and dimensions were used to calculate the entrainment ratio using computational fluid dynamics (CFD). As compared to previous literature, the optimal design had a significantly higher entrainment ratio. The entrainment ratio closely approximated the theoretical maximum entrainment ratio as determined by the innovative 1D model approach, given the identical boundary conditions.This paper introduces innovative definitions of ejector efficiency. These definitions can be used to evaluate how well an ejector is designed and how far it can be improved.

The effectiveness of using wetted cellulose pads on improving the performance of two conventional passive cooling systems has been evaluated. First, an experimental design was developed to determine the impact of using a wetted cellulose pad on the temperature and velocity of the airflow. A cellulose pad (7090 model) with a cross-sectional area of 0.5 × 0.5 m² and three different thicknesses of 10, 15, and 30 cm were selected and tested. The results indicated that using wetted cellulose pads with thicknesses ranging from 10–30 cm decreased the outlet airflow temperature from 11.3 to 13.7 °C on average. For free airflow at velocity 3.5 m/s, the outlet airflow velocity from the wetted cellulose pad decreased to 0.9, 0.7 and 0.6 m/s, respectively, for cellulose pads with thicknesses of 10, 15, and 30 cm. By applying experimental results on a psychrometric chart, the humidity ratio of outlet airflow was obtained between 40–70%. The study established airflow velocity as the critical parameter in passive cooling systems. With the novel concept of combining wetted cellulose pads for passive cooling systems (i.e., wind catchers and induced ventilation), there is good potential to reduce the energy requirements for thermal comfort in buildings in regions with a hot and arid climate.