Muhammad Ehtisham Siddiqui

The development of sustainable polymer hybrid composites combining high mechanical strength, thermal stability, and improved fire response is essential for lightweight engineering applications. The primary objective of this study was to develop bio-based polyamide 11 (PA11) hybrid composites reinforced with graphene nanoplatelets (Gr) and titanium dioxide (TiO₂) particles to enhance their mechanical, thermal, and fire-response properties. Scanning electron microscopy (SEM) revealed graphene nanoplatelets and TiO₂-rich particulate domains distributed within the PA11 matrix, highlighting the hybrid composite morphology and its potential contribution to improved mechanical reinforcement and thermal stability. Hybrid reinforcement enhanced crystallization and thermal stability, increasing crystallization temperature from 158.5 to 163.2 °C and crystallinity from 20.4 to 25.6%. The thermal degradation temperatures (T₅%, T₁₀%, and T_max) increased from 384.5, 398.5, and 434 °C to 411.0, 425.0, and 459 °C, respectively. The highest hybrid loading improved Young’s modulus, yield strength, and tensile strength by approximately 40%, 48%, and 30%, respectively, while DMA demonstrated enhanced thermomechanical stability. Furthermore, peak heat release rate (pHRR) decreased from 610 to 365 W g⁻¹, corresponding to a 40.2% reduction, while the temperature at peak heat release (TpHRR) increased from 438 to 460 °C. These findings highlight the potential of Gr/TiO₂-modified bio-based PA11 hybrid composites for sustainable, lightweight engineering applications requiring enhanced mechanical integrity, thermal stability, and fire resistance.

Etude des instabilités et de la transition de la couche limite produite par un disque en rotation: Ce travail de thèse expérimental étudie les instabilités et la transition de la couche limite produite par un disque en rotation. Pour l’écoulement naturel (c.-à-d. sans forçage extérieur), les mesures des profils de vitesse moyenne, de spectres en fréquence et de moyennes de phase des séries temporelles de vitesse ont permis de distinguer différents régimes en fonction de la distance adimensionnelle R à l’axe du disque. Pour les faibles valeurs de R, les profils de vitesse moyenne suivent la solution de von Kármán. Pour des valeurs plus importantes de R, des écarts à cette solution analytique sont observés et augmentent avec R. Ces écarts sont dus à la croissance spatiale de modes instables de la couche limite (vortex .cross-flow.), et la mesure du taux de croissance spatiale de ces modes correspond bien aux prédictions théoriques de l’analyse de stabilité linéaire. Dans cet écoulement, la transition se produit vers R ≈ 530 et la turbulence pleinement développée s’installe vers R ≈ 600. Les profils dans la région pleinement turbulente suivent la loi logarithmique des couches limites turbulentes et les spectres de vitesse présentent une loi en puissance de type Kolmogorov. Pour étudier la réponse au forçage, un dispositif expérimental a été mis au point qui permet d’exciter des perturbations stationnaires (dans le référentiel du laboratoire) ou en rotation à une fréquence qui peut être réglée indépendamment de la fréquence de rotation du disque. La réponse de l’écoulement à ces deux types de forçage et avec deux formes différentes pour l’élément de forçage a été étudiée. Un forçage stationnaire produit un sillage qui décroît avec la distance à l’élément de forçage, en accord avec la théorie. Le forçage avec des éléments en rotation peut produire un paquet d’ondes amplifié qui, bien que non linéaire, suit des trajectoires proches de celles prédites par la théorie linéaire.

This work demonstrates the potential of CO₂ + SiCl₄ binary mixture as a working fluid for power generation cycle. Recompression Brayton cycle configuration is considered due to its proven record of high performance for medium- to high-temperature sources. The objective of this study is to assess the thermodynamic performance of a recompression Brayton cycle using a CO₂ + SiCl₄ binary mixture as a working fluid, particularly under warm climate conditions. The cycle is simulated using the Peng–Robinson equation of state in Aspen Hysys (v11) software, and the model is validated by comparing VLE data against experimental data from the literature. The analysis involves the assessment of cycle’s thermal efficiency and exergy efficiency under warm climatic conditions, with a minimum cycle temperature of 40 °C. The results demonstrate a notable improvement in the cycle’s thermodynamic performance with CO₂ + SiCl₄ binary mixture compared to pure CO₂. A small concentration (5%) of SiCl₄ in CO₂ increases the thermal efficiency of the cycle from 41.7% to 43.4%. Moreover, irreversibility losses in the cooler and the heat recovery unit are significantly lower with the CO₂ + SiCl₄ binary mixture than with pure CO₂. This improvement enhances the overall exergy efficiency of the cycle, increasing it from 62.1% to 70.2%. The primary reason for this enhancement is the substantial reduction in irreversibility losses in both the cooler and the HTR. This study reveals that when using a CO₂ + SiCl₄ mixture, the concentration must be optimized to avoid condensation in the compressor, which can cause physical damage to the compressor blades and other components, as well as increase power input. This issue arises from the higher glide temperature of the mixture at increased SiCl₄ concentrations and the limited heat recovery from the cycle.

Carbon dioxide (CO₂) has been recognized as one of the potential working fluids to operate power generation cycles, either in supercritical or transcritical configuration. However, a small concentration of some of the additives to CO₂ have shown promising improvements in the overall performance of the cycle. The current study is motivated by the newly proposed additive silicon tetrachloride (SiCl₄), and so we perform a detailed investigation of SiCl₄ along with a few well-known additives to CO₂-based binary mixtures as a working fluid in transcritical organic Rankine cycle setup with internal heat regeneration. The additives selected for the study are pentane, cyclopentane, cyclohexane, and silicon tetrachloride (SiCl₄). A comprehensive study on the energy and exergy performance of the cycle for warm regions is conducted at a turbine inlet temperature of 250 °C. The performance of the heat recovery unit is also assessed to highlight its importance in comparison to a simple configuration of the cycle. This study shows that the cycle operating with binary mixtures performs significantly better than with pure CO₂, which is mainly due to its better heat recovery in the heat recovery unit. The results show that the optimal molar concentration of the additives is in between 20% and 25%. Besides having better thermal stability, SiCl₄ shows an improvement in the cycle thermal efficiency by 6% points which is comparable to cyclopentane (7.3% points) and cyclohexane (7.8% points). The optimal cycle pressure ratio for SiCl₄ is also relatively lower than for other additives. The energy efficiency of the cycle with pure CO₂ is around 45% which is also increased to 58%, 63%, 64%, 60% with pentane, cyclopentane, cyclohexane, and SiCl₄, respectively. These results suggest that additives like SiCl₄ could make CO₂-based cycles more viable for power generation in warm regions.

There are various organic compounds that can be utilized in the organic Rankine cycle as working fluids. The selection of a suitable working fluid is complicated due to the large number of options and factors affecting the choice, such as thermodynamic properties, environmental impact, cost, etc. This study evaluates seven different pure organic compounds and twenty-one of their binary zeotropic mixtures as potential working fluids for the organic Rankine cycle powered by a heat source at 200 °C. The pure organic fluids show higher exergy efficiency, higher specific net power output, and lower heat exchange area requirements compared to the binary mixtures. Among the pure fluids, RE347mcc performs the best in terms of exergy efficiency, followed by neopentane, isopentane, and pentane. Cyclopentane exhibits the highest power production capacity per unit mass flow rate of the working fluid. Two mixtures, pentane/Novec 649 and cyclopentane/Novec 649, showed significantly higher exergy efficiency than their individual components, but at significantly lower specific power production capacity. The study presents an interesting trade-off between exergy efficiency and heat exchange area, indicating that a small increase in exergy efficiency can lead to a large decrease in the required heat exchange area. The outcomes of this study can help in selecting suitable working fluids for ORC operation with a heat source at 200 °C.

The organic Rankine cycle (ORC) exhibits considerable promise in efficiently utilizing low-to-medium-grade heat. Currently, there is a range of organic fluids available in the market, and selecting the appropriate one for a specific application involves considering factors such as the cycle’s thermodynamic performance, plant size, and compatibility with turbomachinery. The objective of our study is to examine the exergetic performance of the ORC with internal heat regeneration. We analyze 12 different organic fluids to evaluate their suitability based on parameters like exergy efficiency and heat exchange area requirements. Additionally, we investigate the need for internal heat regeneration by comparing the overall exergy performance with a simpler ORC configuration. To ensure broad applicability, we consider source temperatures ranging from 150 to 300 °C, which are relevant to industrial waste heat, geothermal sources, and solar energy. For each case, we calculate specific net power output and the UA value (heat exchanger conductance) to gain insights into selecting the appropriate organic fluid for specific source temperatures. Cyclohexane, benzene, isopropyl alcohol, and hexafluorobenzene show poor exergy efficiency due to their high boiling points. Pentane and cyclopentane provides the highest exergy efficiency of 62.2% at source temperature of 300 °C, whereas pentane is found to be the most suitable at source temperatures of 200 and 150 °C with exergy efficiency of 67.7% and 61.7%, respectively. At 200 °C source temperature, RE347mcc achieves 65.9% exergy efficiency. The choice of organic fluid for a given heat source is highly influenced by its critical properties. Moreover, the normal boiling temperature of the organic fluid significantly impacts exergy destruction during the condensation process within the cycle.

These days, the low efficiency of solar-based thermal power plants results in uneconomical performance and high-cost uncompetitive industries compared with conventional fossil fuels. In order to overcome such issues, a novel combined cooling–power–heating (trigeneration) system is proposed and analyzed in this paper. This system uses an ammonia–water binary mixture as a working fluid and a solar heat source to produce diverse types of energy for a multi-unit building in a sustainable fashion. In addition to the basic cooling–power cogeneration cycle, a flashing chamber that will boost the flow rate of refrigerant without any additional heat supply is employed. By developing a mathematical model, the system performance is analyzed using varying parameters of solar irradiation, hot oil temperature, process heat pressure, and ambient temperature to investigate the influence on electrical power, cooling capacity, refrigeration exergy, energy utilization factor (EUF), and system exergy efficiency. Increasing direct normal irradiation (DNI) from 500 W/m² to 1000 W/m² reduces the system EUF and exergy efficiency from 53.62% to 43.12% and from 49.02% to 25.65%, respectively. Both power and refrigeration exergy increase with increasing DNI and ambient temperature, while heating exergy remains constant. It is demonstrated that of 100% solar energy supplied, 46.03% is converted into energetic output and 53.97% is recorded as energy loss. The solar exergy supplied is distributed into 8.34% produced exergy, 29.78% exergy loss, and the remaining 61.88% is the destructed exergy. The highest destruction of solar exergy (56.89%) occurs in the central receiver.