Abstract:The entropy generation analysis of fully turbulent convective heat transfer to nanofluids in a circular tube is investigated numerically using the Reynolds Averaged Navier-Stokes (RANS) model. The nanofluids with particle concentration of 0%, 1%, 2%, 4% and 6% are treated as single phases of effective properties. The uniform heat flux is enforced at the tube wall. To confirm the validity of the numerical approach, the results have been compared with empirical correlations and analytical formula. The self-similarity profiles of local entropy generation are also studied, in which the peak values of entropy generation by direct dissipation, turbulent dissipation, mean temperature gradients and fluctuating temperature gradients for different Reynolds number as well as different particle concentration are observed. In addition, the effects of Reynolds number, volume fraction of nanoparticles and heat flux on total entropy generation and Bejan number are discussed. In the results, the intersection points of total entropy generation for water and four nanofluids are observed, when the entropy generation decrease before the intersection and increase after the intersection as the particle concentration increases. Finally, by definition of E p , which combines the first law and second law of thermodynamics and attributed to evaluate the real performance of heat transfer processes, the optimal Reynolds number Re op corresponding to the best performance and the advisable Reynolds number Re ad providing the appropriate Reynolds number range for nanofluids in convective heat transfer can be determined.
Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4. Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (α-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and α-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D Sscheme α-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of α-Fe2O3/g-C3N4 (α-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2, removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using α-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe 3+ /Fe 2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the α-Fe2O3/g-C3N4 photocatalyst with a large surface area, adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe 3+ ) was reduced to Fe 2+ by the photogenerated electrons from the conduction band of α-Fe2O3. The resulting Fe 2+ could participate in the photo-Fenton reaction. The morphological structures of α-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. α-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the α-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of α-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single α-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater.
The gradual increase in the maturity of sensor electronics has resulted in the increasing demand for wireless sensor networks for many industrial applications. One of the industrial platforms for efficient usage and deployment of sensor networks is smart grids. The critical network traffic in smart grids includes both delay-sensitive and delay-tolerant data for real-time and non-real-time usage. To facilitate these traffic requirements, the asynchronous working–sleeping cycle of sensor nodes can be used as an opportunity to create a node connection. Efficient use of wireless sensor network in smart grids depends on various parameters like working–sleeping cycle, energy consumption, network lifetime, routing protocol, and delay constraints. In this paper, we propose an energy-efficient multi-disjoint path opportunistic node connection routing protocol (abbreviated as EMOR) for sensor nodes deployed in neighborhood area network. EMOR utilizes residual energy, availability of sensor node’s buffer size, working–sleeping cycle of the sensor node and link quality factor to calculate optimum path connectivity after opportunistic connection random graph and spanning tree formation. The multi-disjoint path selection in EMOR based on service differentiation of real-time and non-real-time traffic leads to an improvement in packet delivery rate, network lifetime, end-end delay and total energy consumption.
Primary frequency regulation capability of the wind turbine generators is an appealing topic in order to consider safe increasing of the wind power integration into power grids. This study introduces improvements in the primary frequency contribution of the grid-connected variable speed wind turbine generators, which operate under de-loaded conditions. A new dynamic droop control approach based on the fuzzy logic system is proposed. In this control approach, the droop setting is continuously adjusted in response to the de-loaded rotor speed, the frequency deviation, and the rate of change of frequency. In addition, a new dynamic de-loading technique based on the Sugeno-Fuzzy inference system is proposed where the de-loading percentage of the wind turbine generator is decided continuously according to the frequency deviation. The effectiveness of the proposed dynamic de-loading and dynamic droop control approaches is verified through extensive comparative studies. The simulation results confirmed that the proposed control approaches have the ability to provide transient and steady-state powersharing and improve effectively the frequency stabilisation of the power system within the stable and secure limits of the wind turbine.
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