Heuristics for a Two-Stage Assembly-Type Flow Shop with Limited Waiting Time Constraints

This study investigates a two-stage assembly-type flow shop with limited waiting time constraints for minimizing the makespan. The first stage consists of m machines fabricating m types of components, whereas the second stage has a single machine to assemble the components into the final product. In the flow shop, the assembly operations in the second stage should start within the limited waiting times after those components complete in the first stage. For this problem, a mixed-integer programming formulation is provided, and this formulation is used to find an optimal solution using a commercial optimization solver CPLEX. As this problem is proved to be NP-hard, various heuristic algorithms (priority rule-based list scheduling, constructive heuristic, and metaheuristic) are proposed to solve a large-scale problem within a short computation time. To evaluate the proposed algorithms, a series of computational experiments, including the calibration of the metaheuristics, were performed on randomly generated problem instances, and the results showed outperformance of the proposed iterated greedy algorithm and simulated annealing algorithm in small- and large-sized problems, respectively.

Dispersion in doublet-type flows through highly anisotropic porous formations

Steady doublet-type flow takes place in a porous formation, where the log-transform $Y = \ln K$ of the spatially variable hydraulic conductivity $K$ is regarded as a stationary random field of two-point autocorrelation $\rho _Y$ . A passive solute is injected at the source in the porous formation and we aim to quantify the resulting dispersion process between the two lines by means of spatial moments. The latter depend on the distance $\ell$ between the lines, the variance $\sigma ^2_Y$ of $Y$ and the (anisotropy) ratio $\lambda$ between the vertical and the horizontal integral scales of $Y$ . A simple (analytical) solution to this difficult problem is obtained by adopting a few simplifying assumptions: (i) a perturbative solution, which regards $\sigma ^2_Y$ as a small parameter, of the velocity field is sought; (ii) pore-scale dispersion is neglected; and (iii) we deal with a highly anisotropic formation ( $\lambda \lesssim 0.1$ ). We focus on the longitudinal spatial moment, as it is of most importance for the dispersion mechanism. A general expression is derived in terms of a single quadrature, which can be straightforwardly carried out once the shape of $\rho _Y$ is specified. Results permit one to grasp the main features of the dispersion processes as well as to assess the difference with similar mechanisms observed in other non-uniform flows. In particular, the dispersion in a doublet-type flow is observed to be larger than that generated by a single line. This effect is explained by noting that the advective velocity in a doublet, unlike that in source/line flows, is rapidly increasing in the far field owing to the presence there of the singularity. From the standpoint of the applications, it is shown that the solution pertaining to $\lambda \to 0$ (stratified formation) provides an upper bound for the dispersion mechanism. Such a bound can be used as a conservative limit when, in a remediation procedure, one has to select the strength as well as the distance $\ell$ of the doublet. Finally, the present study lends itself as a valuable tool for aquifer tests and to validate more involved numerical codes accounting for complex boundary conditions.

Performance Optimization of μDMFC with Foamed Stainless Steel Cathode Current Collector

The micro direct methanol fuel cell (μDMFC) has attracted more and more attention in the field of new energy due to its simple structure, easy operation, and eco-friendly byproducts. In a μDMFC’s structure, the current collector plays an essential role in collecting the conduction current, and the rational distribution of gas and water. The choice of its material and flow fields would significantly impact the μDMFC’s performance. To this end, four different types of cathode current collector were prepared in this study. The materials selected were stainless steel (SS) and foam stainless steel (FSS), with the flow fields of hole-type and grid-type. The performance of the μDMFC with different types of cathode current collector was investigated by using polarization curves, electrochemical impedance spectroscopy (EIS), and discharging. The experimental results show that the maximum power density of μDMFC of the hole-type FSS cathode current collector is 49.53 mW/cm2 at 70 °C in the methanol solution of 1 mol/L, which is 115.72% higher than that of the SS collector. The maximum power density of the μDMFC with the grid-type FSS collector is 22.60 mW/cm2, which is 27.39% higher than that of the SS collector. The total impedance of the μDMFC of the FSS collector is significantly lower than that of the μDMFC of the SS collector, and the total impedance of the μDMFC with the hole-type flow field collector is lower than that of the grid-type flow field. The discharging of μDMFC with the hole-type FSS collector reaches its optimal value at 70 °C in the methanol solution of 1 mol/L.

Online Measuring Of Grain Flowrates and Its Measurement Sensitivities with Pitch and Roll Angles

A complete calibration test stand was constructed and instrumented to examine the effect of varying pitch and roll positions on the measurement errors of a microwave solid type flow sensor. Results indicated that measurement errors ranging from 2.50% to 6.82% and 1.80% to 8.86% were obtained by the changing of chute pitch (descending and ascending) and roll angle positions from 1.5° to 4.5°, respectively. Greater measurement errors were found at the low screw auger conveyor speed range. However, the magnitude of errors is within the acceptable margin for any typical wet paddy land topography.

CFD Simulation and Performance Investigation on a Novel Bionic Spider-Web-Type Flow Field for PEM Fuel Cells

The products generated by the electrochemical reaction in the PEM fuel cell (PEMFC) are mainly concentrated in the flow field on the cathode side of the bipolar plate, and the oxygen introduced on the cathode has higher requirements to improve its diffusion performance by using the flow field structure. For this reason, the optimization of the cathode flow field of the PEMFC is essential. Inspired by the structure of a spider web, this paper proposes a novel spider-web-type flow field. In this kind of flow field, the shape of a polygonal structure and the number of layers of spiral flow channels are the two most crucial variables. In order to explore the impact of these two variables on the cathode flow field, complete three-dimensional PEMFC models with different values of the two variables were established, and the models were simulated by the method of CFD. By observing the results of oxygen distribution, the water removal performance and fuel cell output performance of different schemes, the optimal scheme of the polygonal structure and layer number are determined. Compared with the traditional flow field, it is proved that the optimization scheme is desirable in improving the performance of the cathode flow field in PEMFC.

Highly Transient High Speed Jet Flow From Different Orifice Geometries

The majority of the many types of different industrial flows are not in the laminar flow regime, but rather these flows are well beyond laminar and continue to exceed the turbulent flow transition criterion for internal and free shear flows to be fully turbulent and highly unsteady involving the transfer of fluid through circular conduits or round pipes as well as other conduit geometries including the issuing of jet flows into some type of ambient environment. High speed jet flows have a wide range of applications in many areas of engineering. The understanding of jet flow theory has progressed substantially. However, there is a very little investigation into the transient nature of the high-speed jet flows and how the structure of these jet flows differs according to the geometry of the orifice out of which the jet flow emanates. The transient nature of these jet flows allows the applications into which they are installed to be optimized according to the characteristics of the jet flow and the configuration of the system. The focus of the present work is to characterize transient high speed jet flows from the differing orifice nozzle geometries and the introduction of a swirling motion into the jet flow and how this affects the characteristics of the jet flow from the reference jet flow that is free from swirling motion momentum. Another important but niche or specialised jet flow application is in reverse pulse-jet (RPJ) cleaning systems, of which is the focus application of the present work. A typical RPJ cleaning system consists of three main components: compressed air supply, valve and blowtube. The blowtube is the name given to the pipe connected to the valve and configured with a number of outlets or orifices where the flow exits into a plenum thereby entraining and inducting flow into a filter that is aligned with the orifice from which the jet flow issues. In the present work, improvements are sought from the blowtube or more specifically the exit pipe orifices for a more efficient operation of a well-designed cleaning system. The present paper will discuss and compare the flow through a number of different orifice geometries for the type flow that is typically experienced in this type of application. The operation of a single event or an actuation of the pulse-jet valve, is extremely rapid; typically approximately 300 ms. The valve is actuated and the diaphragm moves and allows the compressed air to travel from the pressure vessel or header tank through the valve past the valve seat into the blowtube and exits through plain orifices or nozzles. The extremely rapid event generates highly transient, highly turbulent free shear jet type flow from either the plain orifice or nozzle with a circular orifice geometry through which the flow exits. Advancements made by the author in subsonic flows and high-speed gas dynamic flows could provide not only improvements to the flow but further insight to the physics of high speed flows in particular around pipe exit orifices. This investigative study of the jet flow was based upon a computational analysis. It was shown that the base reference case of the jet flow that was solely a non-swirling flow although the jet flow was highly transient in nature that the centreline velocity of the jet flow had variability from the inner core to the outer extremities of the jet flow. The comparison of the base reference case with swirling jet flow will produce a longer coherent jet flow using the different orifice geometries. The stability of the jet flow was improved with the introduction of the swirling motion to the jet flow. Future developments of the transient nature of the jet flow will include experimental studies to verify the flow control methods that were used in the swirling jet flow cases.