Majid Sharifi

Room Temperature Self-Healing Thermoset Based on the Diels–Alder Reaction

A self-healing epoxy-amine thermoset based on the compatible functionalization of the thermoset and encapsulated healing agent has been successfully developed. Healing of the thermoset resulted from the reaction of furans in the thermoset and multimaleimides (MMIs) in the healing agent solution. The healing agent, MMI dissolved in phenyl acetate, was encapsulated using a urea-formaldehyde encapsulation method. Autonomic healing of the thermoset was achieved by incorporating microcapsules filled with the healing agent solution within a furan-functionalized epoxy-amine thermoset. The resulting self-healing thermoset recovered 71% of its initial load after fracture.

Toughened epoxy polymers via rearrangement of network topology

The highly crosslinked molecular architecture of thermosets makes this class of material strong but brittle. It is advantageous to enhance the ductility without sacrificing strength, glass transition temperature (Tg), and modulus. The hypothesis was tested that without altering the chemical structure, the network topology of a dense thermoset can be engineered to dissipate more energy before rupturing covalent bonds, producing a tougher material without sacrificing ultimate tensile strength, density, or glass transition temperature. A processing technique termed “Reactive Encapsulation of Solvent/Drying” (RESD) was used in which epoxy curing was conducted in the presence of varying amounts of inert small-molecule solvent, followed by a drying/annealing process in which the solvent was removed. Density measurements and freeze-fracture surface analysis revealed that the resulting RESD materials are not porous in their relaxed state after annealing. Comparing the dynamic mechanical response of the modified (RESD) to the unmodified (conventional) structures revealed no significant differences in glassy modulus and Tg. However, quasi-static mechanical testing showed that upon stretching, the modified structures have a volumetric energy capacity of up to 2.5 times that of the unmodified ones. SEM micrographs of fracture surfaces of RESD specimens indicated nano-sized cavities on the surface of the modified thermosets, which were not present before breaking. Therefore, the presence of distinct topological features in the modified network is likely the origin of the large improvement in ductility. Topology-based toughening is potentially an important step toward developing better high performance network polymers and composites.

NUMERICAL INVESTIGATION OF THE EFFECTS OF STEAM NUCLEATION ON THE STEAM EJECTORS PERFORMANCE

Steam ejectors are widely used in different applications such as propulsion, refrigeration, evacuation and aerospace. The fundamental numerical approach in evaluating the characteristic parameters of steam ejectors was through considering steam as a single-phase gas. But, at some regions in steam ejectors, nucleation of steam is occurred. It is very important to evaluate the amount of wet steam at the first step. At the second step, it is vital to estimate the effect of steam condensation on the aerodynamics and thermodynamic performance of steam ejectors. In the present study, numerical simulation of a steam ejector at normal operation is undertaken. In the mathematical modeling of compressible flow within such equipments, wetsteam nucleation theory is employed to investigate the effects of wetness condition inside the ejectors. In order to verify the numerical simulation, wet steam results have been compared with a set of experimental data reported in previous literatures. By comparison of the numerical results with experimental data, it was concluded that steam condensation in the nozzle declines the maximum Mach number of supersonic flow which has a closer agreement with experimental reports. The numerical calculations were performed with a commercial code with a supplementary user-defined code and applied to a multi-block computational domain with structured elements. Some important items such as shock location, shock strength and pressure distribution along the centerline of the ejector were compared in the cases of wet and ideal steam simulations. Furthermore, the average compression ratio and entraining capability of the ejector were compared for both simulation methods. Moreover, the results show a strengthened shock wave under the assumption of wet steam which leads to an intensified pressure recovery inside the ejector.

CFD EVALUATION OF STEAM PROPERTIES WITHIN THERMO-COMPRESSORS WITH TWO DIFFERENT NUMERICAL APPROCHES

Thermo-compressors are among the most important facilities in thermal desalination systems. Such devices are compressing and recycling the useless vapors and hence effectively enhancing the efficiency of desalination units. Since the connection between the evaporator box and suction surface is not perfectly symmetric, it is necessary to consider the effect of this curved path on pressure loss at the inlet boundary. In this study, a numerical procedure is developed to achieve reliable results in thermo-compressors through using CFD simulations. Two approaches are examined: axisymmetric and full three-dimensional method. The results are compared and the deviations of velocity, pressure and temperature are evaluated in both methods. The flow pattern in the steam collector is investigated afterwards. The distributions of velocity and pressure through this canal are illustrated and the critical point where pressure loss originates is revealed. In the meantime, the effect of this non-symmetric path on the flow is compared with the axisymmetric results. Finally, it is shown that the influence of the curved-shape inlet on flow properties is insignificant such that it can be neglected, because the flow swirl at the inlet is very negligible. Therefore, the axisymmetric model is capable of producing reliable results for thermo-compressors in a more advantageous way with a simpler mesh generation and reduced computational cost.Copyright

Numerical Investigation of Heat Transfer in Wet Steam Flow in Convergent-Divergent Nozzles

Nucleation and condensation phenomena are of fundamental importance in many fields such as steam-turbine design and power generation technologies. Wet steam flows are typically considered as multiphase gas droplet mixtures in which both vapor and liquid droplets coexist. In such flows, spontaneous nucleation leads to the formation of liquid droplets from vapor. Our key goal is to determine the rate of nucleation and droplets growth correctly. This will enable us to predict the variations of thermodynamic properties along the nozzle axis. In this study, a CFD code is generated based on the assumption of non-isothermal homogenous nucleation rate. A “Used-Defined Function” (UDF) was written in a compatible format with FLUENT solver such that it implements all the required wet steam calculations through a finite-volume method. The predicted numerical results were well supported by experimental data from literature for a specific nozzle. The predicted distribution of pressure ratio along the main axis of the nozzle shows a reasonable agreement with experimental data. Moreover, the droplets sizes predicted were in good agreement with experimental data too. Besides, the variations of some important thermodynamic properties along the nozzles were determined as well. The predicted results were compared to available data from relevant literature. The outcome of current numerical procedure confirms the superiority of this module for wet steam considerations in supersonic flow. It can further be applied to wet flow analysis in so many applications such as steam turbine cascades.Copyright

EXPERIMENTAL IMPROVEMENT OF EJECTOR PERFORMANCE THROUGH NUMERICAL OPTIMIZATION OF NOZZLE GEOMETRY

Ejectors are widely used in different applications such as refrigeration, propulsion, evacuation and aerospace. They use a pressurized flow as a motive stream to entrain a secondary flow or suction flow. In the current study, a malfunctioning steam ejector is studied experimentally to identify the sources of low compression ratio. This ejector was designed to operate under a motive pressure of 6 bar. However, the required vacuum in the system was not attained unless the pressure of motive steam was increased to 8 bar. The steam ejector was coupled with other unit operating facilities and hence, the ejector replacement was very costly. Therefore, the fastest and the most inexpensive way of improving the device performance was considered as replacing just the primary nozzle and without any further change in ejector’s geometry. To achieve the required vacuum under the available motive pressure (i.e. 6 bar), a CFD−based optimization procedure was performed and different nozzle shapes were numerically investigated. The CFD Models were constrained to a fixed constant throat since the optimized nozzle shall not consume more flow rate than the former one. Ten different nozzle geometries were scrutinized in this numerical simulation and the one, which yields the highest entraining performance under the given boundary condition (i.e. motive flow pressure of 6 bar), was selected as the most optimized nozzle and manufactured. After installing the designed nozzle, an improved entrainment capability was observed and a desired vacuum level was attained under the nominal pressure of 6 bar. NOMENCLATURE English letters