Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.     

The Effect of Operating Conditions on Permeation of Water-methanol Mixtures by a Pervaporation Membrane System

In this work, the effects of various operating parameters on the membrane performance in separation of water from methanol by pervaporation (PV) using a commercial poly (vinyl alcohol) (PVA) membrane have been studied.

Experimental results were obtained at different feed temperatures (30–60°C) and permeate pressures (1–10 mm Hg) as well as concentrations of 1–10 wt% water in feed. Investigations indicate that increasing feed temperature and vacuum (lowering permeate pressure) increase flux and selectivity resulting in improvement of membrane performance. Also, increasing water concentration in feed from 1 to 10 wt% increases permeate flux from 0.4 to 0.6 kg/hm² while selectivity first decreases from 10.6 to 4.8 and then increases to 6.8.     

Novel direct designs for 3-input XOR function for low-power and high-speed applications

Novel direct designs for 3-input exclusive-OR (XOR) function at transistor level are proposed in this article. These designs are appropriate for low-power and high-speed applications. The critical path of the presented designs consists of only two pass-transistors, which causes low propagation delay. Neither complementary inputs, nor V _DD and ground exist in the basic structure of these designs. The proposed designs have low dynamic and short-circuit power consumptions and their internal nodes dissipate negligible leakage power, which leads to low average power consumption. Some effective approaches are presented for improving the performance, voltage levels, and the driving capability and lowering the number of transistors of the basic structure of the designs. All of the proposed designs and several classical and state-of-the-art 3-input XOR circuits are simulated in a realistic condition using HSPICE with 90 nm CMOS technology at six supply voltages, ranging from 1.3 V down to 0.8 V. The simulation results demonstrate that the proposed circuits are superior in terms of speed, power consumption and power-delay product (PDP) with respect to other designs.     

Differential Cascode Voltage Switch (DCVS) Strategies by CNTFET Technology for Standard Ternary Logic

Differential Cascode Voltage Switch (DCVS) is a well-known logic style, which constructs robust and reliable circuits. Two main strategies are studied in this paper to form static DCVS-based standard ternary fundamental logic components in digital electronics. While one of the strategies leads to fewer transistors, the other one has higher noise margin. New designs are simulated with HSPICE and 32 nm CNTFET technology at various realistic conditions such as different power supplies, load capacitors, frequencies, and temperatures. Simulations results demonstrate their robustness and efficiency even in the presence of PVT variations. In addition, new noise injection circuits for ternary logic are also presented to perform noise immunity analysis.     

Green solid-state fabrication of new nanocomposites based on La–Fe–O nanostructures for electrochemical hydrogen storage application

Nano-sized La–Fe–O (LFO) structures were fabricated via novel free-solvent and green solid-state route using La (acac)₃. H₂O and Fe (acac)₃ complex precursors. Acetylacetonate (acac) in organometallic complex precursors control nucleation and growth of formed crystals with creation spatial barrier around the cations, and prevent nano-product agglomeration. The mechanism of role of acac has been explained in nanostructure formation. Changing of parameters in synthesis reaction consisting La:Fe molar ratio, calcination time and temperature in turn offer a virtuous control over the nanocomposites size and shape which various compositions of La₂O₃/LaFeO₃, LaFeO₃/La₂O₃ and LaFeO₃/Fe₂O₃ obtained. The as-prepared La–Fe–O nano-products were characterized thorough Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), UV–Vis, BET and energy dispersive X-ray (EDX) analysis in terms of crystallinity structure, composition, porosity and morphology. Different formed La–Fe–O nanostructures were evaluated for electrochemical hydrogen storage capacity through chronopotentiometry technique in stable current (1 mA). The achieved La–Fe–O nanoparticles could be applied as a favorable candidate active material for electrochemical hydrogen storage. Optical, magnetic and reducible characteristics of La–Fe–O nanostructures have positive effect on electrochemical hydrogen storage capacity. It was found out that the LaFeO₃/Fe₂O₃ nanocomposites have the best electrochemical hydrogen storage performance due to oxidation-reduction process of Fe²⁺/Fe³⁺ components which can help to charge-discharge process of hydrogen to increase the storage capability to 790 mAhg^?1 after 20 cycles. Also, the mixed metal oxides illustrate advanced discharge capacity than other binary oxides.     

Integrated Structural–Architectural Design for Interactive Planning

Traditionally, building floor plans are designed by architects with their usability, functionality and architectural aesthetics in mind; however, the structural properties of the distribution of load‐bearing walls and columns are usually not taken into account at this stage. In this paper, we propose a novel approach for the design of architectural floor plans by integrating structural layout analysis directly into the planning process. In order to achieve this, we introduce a planning tool which interactively enforces checks for structural stability of the current design, and which on demand proposes how to stabilize it if necessary. Technically, our solution contains an interactive architectural modelling framework as well as a constrained optimization module where both are based on respective architectural rules. Using our tool, an architect can predict already in a very early planning stage whose designs are structurally sound such that later changes due to stability reasons can be prevented. We compare manually computed solutions with optimal results of our proposed automated design process in order to show how much our proposed system can help architects to improve the process of laying out structural models optimally.     

Multicast-Unicast Data Delivery Method in Wireless IPv6 Networks

Multicast IPv6 is an efficient way of transmitting data simultaneously to a group of IPv6 users. It has the advantage of reducing the required bandwidth of IPv6 data delivery compared to unicast transmission. The data rate of multicast transmission over WLAN is confined by the user with the lowest rate in the multicast group, which is called the fixed base rate problem. This paper proposes a delivery method that incorporates both multicast and unicast transmissions to solve the fixed base rate problem. The proposed method divides the IPv6 network into two levels: multicast mode for the upper level of the network [IPv6 server to Access Point (AP)], and unicast mode for the lower level (AP to mobile nodes). To maintain the end-to-end multicast transmission, the AP is responsible for converting multicast packets to unicast packets. Such a combination enables the proposed method to inherit the advantages of both multicast and unicast transmissions. The performance of our proposed method is evaluated in a test-bed environment that considers the transmission of real-time video application. The proposed multicast-unicast is able to improve the throughput and video quality experienced by the end user, with low packet loss and transmission delay.     

Adhesion of tin droplets impinging on a stainless steel plate: effect of substrate temperature and roughness

We photographed impact of small tin droplets on stainless steel surfaces of varying temperature and roughness. To achieve high impact velocities the test surfaces were mounted on the rim of a rotating fly wheel. Substrate temperature (T_s) was varied from 120 to 220 °C and surface roughness (R_a) kept at either 0.05 or 2 µm. We kept constant the impact velocity (30 m/s) and droplet diameter (0.6 mm). To form a coating 60 droplets were deposited randomly on each stainless steel test coupon. Deposition efficiency was evaluated by dividing the mass adhering to the coupon by the mass of sixty droplets prior to impact. The maximum deposition efficiency was achieved at a substrate temperature of 160 °C. For T_s < 160 °C the deposition efficiency was higher on a rough surface (R_a = 2 µm) than on a smooth surface (R_a = 0.05 µm), since splats did not adhere well to the smooth surface. For T_s≥ 160 °C the deposition efficiency was higher on a smooth surface (R_a = 0.05 µm) than on a rough surface (R_a = 2 µm), since splats splashed less on the smooth surface.

© 2003 Elsevier Science Ltd. All rights reserved.     

Exergoeconomic optimization of a 1000 MW light water reactor power generation system

A typical 1000 MW pressurized water reactor nuclear power plant is considered for optimization. The thermodynamic modeling is performed based on the energy and exergy analysis, while an economic model is developed according to the total revenue requirement method. The objective function based on the exergoeconomic analysis is obtained. The exergoeconomic optimization process with 10 decision variables is performed using a hybrid stochastic/deterministic search algorithm namely as genetic algorithm. The results that are obtained using optimization process are compared with the base case system and the discussion is presented. Copyright © 2009 John Wiley & Sons, Ltd.