Study of the synergistic effect of boron nitride and carbon nanotubes in the improvement of thermal conductivity of epoxy composites

The enhancement of the thermal conductivity, keeping the electrical insulation, of epoxy thermosets through the addition of pristine and oxidized carbon nanotubes (CNTs) and microplatelets of boron nitride (BN) was studied. Two different epoxy resins were selected: a cycloaliphatic (ECC) epoxy resin and a glycidylic (DGEBA) epoxy resin. The characteristics of the composites prepared were evaluated and compared in terms of thermal, thermomechanical, rheological and electrical properties. Two different dispersion methods were used in the addition of pristine and oxidized CNTs depending on the type of epoxy resin used. Slight changes in the kinetics of the curing reaction were observed in the presence of the fillers. The addition of pristine CNTs led to a greater enhancement of the mechanical properties of the ECC composite whereas the oxidized CNTs presented a greater effect in the DGEBA matrix. The addition of CNTs alone led to a marked decrease of the electrical resistivity of the composites. Nevertheless, in the presence of BN, which is an electrically insulating material, it was possible to increase the proportion of pristine CNTs to 0.25 wt% in the formulation without deterioration of the electrical resistivity. A small but significant synergic effect was determined when both fillers were added together. Improvements of about 750% and 400% in thermal conductivity were obtained in comparison to the neat epoxy matrix for the ECC and DGEBA composites, respectively. © 2019 Society of Chemical Industry     

Multi-input,multi-output flight control system design for the YF-16 using nonlinear QFT and pilot compensation

Nonlinear quantitative feedback theory (QFT) and pilot compensation techniques are used to design a 2 × 2 flight control system for the YF-16 aircraft over a large range of plant uncertainty. The design is based on numerical input-output time histories generated with a FORTRAN implemented nonlinear simulation of the YF-16. The first step of the design process is the generation of a set of equivalent linear time-invariant (LTI) plant models to represent the actual nonlinear plant. It has been proven that the solution to the equivalent plant problem is guaranteed to solve the original nonlinear problem. Standard QFT techniques are then used in the design synthesis based on the equivalent plant models. A detailed mathematical development of the method used to develop these equivalent LTI plant models is provided. After this inner-loop design, pilot compensation is developed to reduce the pilot's workload. This outer-loop design is also based on a set of equivalent LTI plant models. This is accomplished by modelling the pilot with parameters that result in good handling qualities ratings, and developing the necessary compensation to force the desired system responses.     

The group lock and its applications

Parallel algorithms that use shared resources are notoriously difficult to write and verify, especially when properties such as fairness and efficiency are desired. This paper introduces a new synchronization method called the group lock. This method has some of the advantages of strict synchronization methods such as monitors; algorithms written using group lock are quite clear and easy to verify. At the same time, these algorithms generally make better use of parallelism than those written using stricter synchronization methods. In many cases we can obtain worst case time bounds that are constant or logarithmic in the number of processes, thus also insuring bounded fairness. The paper begins with a discussion of related synchronization methods and an introduction to the group lock. This is followed by some examples of applications in algorithms for a readers/writers problem, parallel stacks and queues, implementation of fetch- and-phi routines, and others. Finally, two implementations of group lock are described. A detailed implementation is given for the paracomputer, an idealized MIMD multiprocessor that supports the fetch-and-add synchronization instruction. An implementation is also outlined for general CREW multiprocessors using only reads and writes to shared memory.     

Solar assisted cooling with sorption systems: status of the research in Mexico and Latin America

Solar refrigeration projects both national and international with sorption and other refrigeration systems have been developed in Mexico and other Latin American countries in the last 15 years. A review of the main projects, both for solar cooling and refrigeration and the results obtained are presented in this paper. A methodology where 19 solar technologies for cooling were identified is also presented. Although solar cooling is still not an economically viable technology, the advances made and the experience gained in the projects described and the improved systems envisaged, will make solar refrigeration systems play an important role in the future.     

Optimal control of accelerator concentration for resin transfer molding process

Resin cure following mold filling is an essential element in resin transfer molding. To fabricate a composite part with high dimensional stability and minimize residual stress, uniform resin cure should be achieved. This study considers a three-part resin system composed of epoxy, hardener and accelerator. The cure kinetics can be controlled by the accelerator concentration at the injection gate. A numerical method that can predict degree of cure distribution based on accelerator concentration at the gate was proposed. The degree of cure distribution is obtained by solving the resin flow, heat transfer, accelerator concentration and cure problems sequentially. Utilizing this numerical method, an optimal variation of accelerator concentration during mold filling was sought by solving a constrained optimization problem. The effect of accelerator control on degree of cure distribution was investigated and its validity was examined for two different geometries.     

A thermochemical concept-based equation to estimate waste combustion enthalpy from elemental composition

Waste combustion is an interesting alternative for waste management and energy recovery. Knowledge of the waste higher heating value (HHV) is important for judging it’s worth as fuel. This work introduces a new equation, based on thermochemical concepts, to calculate HHV from elemental composition. This equation is expressed in terms of mass percentages on a dry basis of carbon (%C), hydrogen (%H), oxygen (%O), nitrogen (%N), and sulfur (%S); the HHV is computed in MJ/kg. The equation is as follows: HHV=(1−(%H₂O/100))(−0.3708(%C)−1.1124(%H)+0.1391(%O)−0.3178(%N)−0.1391(%S)). The thermochemical concept on which this expression is based involves a wide applicability. This equation neglects the inorganic carbon, hence it is not very adequate when there is a significant concentration of it. The predictions from this approach were contrasted against those proceeding from equations currently used in combustion technology, and also against bomb calorimeter data. The new equation is clearly competitive with respect to other formulations, and it can be very helpful for presenting a waste HHV value based on different derivation suppositions.     

Dynamic analysis and drilling degrees of freedom

Finite element methods for dynamic analysis employing elements with drilling degrees of freedom are presented. The formulation is based on a variational principle in which displacements and rotations are interpolated independently. The issue of zero masses corresponding to rotational degrees of freedom is addressed and techniques for defining consistent and lumped rotational mass matrices are presented.     

Analysis of the cooling capability of steam generators during the loss of residual heat removal system at midloop operation with closed primary system

Loss of residual heat removal system (RHRS) at midloop operation is one of the most significant core damage risk contributor at low power and shutdown conditions. During this kind of transients the reflux-condensation is one of the cooling mechanisms anticipated in the abnormal procedure of loss of RHRS at midloop level. In this sense, several simulations of loss of the RHRS with closed primary system with the TRACE V4.160 code have been performed considering different availability of steam generators. The present study aims to analyze the thermal-hydraulic behavior after the loss of RHRS at midloop conditions with the reflux-condensation as the only cooling mechanism available and to investigate the capability of this cooling mechanism. The simulation results show that one steam generator is sufficient to remove core decay heat of 11 MW obtaining an equilibrium pressure, but the core uncovery depends on the number of steam generators operating. Finally, an analysis of the abnormal procedure and the event trees of the loss of RHRS sequences at midloop operation has been performed taking into account the results obtained in the simulation with TRACE.