• Energy Research

Abstract The utilization of pumps in reverse function is one of the economically beneficial methods for off-grid power generation in micro-hydropower capacities. The traditional method of hydraulic loss calculation in turbomachinery based on pressure drop calculations is unable to determine the exact location of losses. In this paper, the irreversible energy losses within the PAT has been studied for the first time using entropy generation theory and the second law of thermodynamics point of view. In order to conduct numerical simulation, the 3-dimensional incompressible steady-state flow within the PAT is simulated by solving the Reynolds averaged Navier-Stokes (RANS) equations. The shear stress transport (SST) turbulence model is considered for turbulence modeling. The quantity of direct (viscous) and turbulent entropy generation rate is calculated in different PAT components in 9 different flow rates in the range of 0.7QBEP to 1.3QBEP. The numerical results show that the turbulent term is the main factor of entropy production within the PAT (86.89%–90.98%), and thus, turbulent entropy generation is the dominant mechanism for hydraulic losses. More than 50% of the energy dissipation occurs within the PAT runner. Most of the losses within the runner take place at the blade leading edge, blade trailing edge and flow separation regions of the blade suction and pressure sides. The volumetric entropy generation rate analysis demonstrates that the draft tube has the most potential to generate irreversible losses among all the components (47.37%). Flow field analysis reveals that the blade inlet shock, flow deviation at the blade outlet, flow separation, backflow and vortices in flow passages are categorized as the main reasons for entropy production and irreversible hydraulic losses within the PAT components. The advantages of the entropy generation method including the determination of the exact location and quantity of energy dissipation within the PAT are indicated in this investigation.

Abstract Utilization of pump as turbine (PAT) systems is known as an economically beneficial method to harvest hydropower energy, especially in a remote area. Despite the extensive investigations on the operation and parametric study of the PAT components, the turbine mode of multistage PATs in case of a viscous working fluid is found as a literature gap. In this paper, a numerical simulation method is employed as a predicting tool to investigate the consequences of changing the fluid viscosity affecting the flow patterns within the multistage PATs. The commercial solver ANSYS CFX 16 was used for simulation. Validation of numerical solution was done for the single-stage pump considering water and 48 cSt viscous fluid. The numerical results in the single-stage PAT showed that increasing the viscosity causes an efficiency drop mainly at the part-load condition and translates the maximum efficiency point to higher flow rates, where the vortices at impeller passage and draft tube are weaker. Increasing the viscosity up to 48 cSt, results in 12.5% reduction of efficiency at the best efficiency point (BEP) in two stage PAT. Based on the two-stage PAT results, the total hydraulic efficiency at the BEP has been increased by 1% and 1.5% for water and the 48 cSt viscous fluid, respectively, in comparison with the single-stage PAT. According to the post-processed streamlines at different stages of the two-stage PAT, the undesired vortical structures were caused by the diffuser and the return channel, which were mainly designed for the pump mode. The hydraulic analysis of two-stage PAT shows that using multistage pumps in reverse mode is reasonable for power generation in high head and flow rate sites.

Optimization of vacuum cleaner fan components is a low-cost and time-saving solution to satisfy the increasing requirement for compact energy-efficient cleaners. In this study, surrogate-based optimization technique is used and for the first time it is focused on maximization of Airwatt parameter, which describes the fan suction power, as an objective function (Case II). Besides, the shaft power is minimized (Case I) as another optimization target in order to reduce the power consumption of the vacuum cleaner. 11 geometrical variables of 3 fan components including impeller, diffuser and return channel are selected as the optimization design variables. 80 training points are distributed in the sample space using Advanced Latin Hypercube Sampling (ALHS) technique and the outputs of sample points are calculated by means of CFD simulations. Kriging and RSA surrogate models have been fitted to the outputs of the sample space. Through coupling of constructed Kriging models and Multi-Island Genetic Algorithm (MIGA), the optimal design for each of the optimization cases is presented and evaluated using numerical simulations. A 20.22% reduction in shaft power in Case I and an improvement of 27.73% in Airwatt in Case II have been achieved as the overall results of this study. Despite achieving goals in both optimization cases, a slight decrease in Airwatt in Case I (−6.20%) and a slight increase in shaft power in Case II (+4.82%) are observed relative to primary fan. Furthermore, the Analysis of Variance (ANOVA) determines the importance level of design variables and their 2-way interactions on the objective functions. It was concluded that geometrical parameters related to all of the fan components must be considered simultaneously to conduct a comprehensive optimization. The reasons of enhancement in optimal cases compared with the reference design have been further investigated by analysis of the fan internal flow field. Post-processing of the CFD results demonstrates that the applied geometrical modifications cause a more uniform flow through the flow passages of the optimal fan components.

Abstract Fluids slip on superhydrophobic surfaces. The slip velocity is modeled by Navier's slip-length. A user defined function (UDF) in ANSYS Fluent 15.0 was developed to implement the slip boundary condition. The UDF was validated for different values of slip length by two benchmark solutions in laminar and turbulent flows. We utilized a periodic approach to model the VLH turbine in steady-state condition. For modeling, single-phase is assumed and the shear stress transport model was used for turbulence modeling. The results of employing partial slip at different positions of the runner blade and other turbine components with a constant slip-length of 50 μm showed an approximately 4% improvement in the turbine hydraulic efficiency, at the design point and when only the runner blade is superhydrophobic. In addition to the design point analysis, the turbine efficiency values were studied at part and high load ranges. The entropy generation method was applied to the output results of the simulated cases to determine the energy dissipating zones and to detect the effect of superhydrophobic walls on the dissipation mechanisms. Since this work is the first numerical evaluation of superhydrophobic surfaces for VLH, the conclusions can be meaningful for future numerical and experimental studies.

Abstract Pumps in reverse function are one of the most cost-effective industrial devices to generate power from small hydropower capacities. Due to the fact that the pump components are not inherently designed for the turbine mode, the hydraulic performance of the pump as turbine (PAT) is usually not ideal. One low-cost solution to improve the PAT performance is to redesign the pump Impeller without changing of any other components. In this study a multi-objective optimization process is applied based on the surrogate models with focus on minimizing the entropy generation rate (EGR) at 2 flow rates of the QBEP (best efficiency point) and Q1.3BEP in order to improve the hydraulic performance of the PAT over a wide range of operating points. In the optimization process, the sample space points are determined by the optimal Latin hypercube sampling (OLHS) technique. The values of the objective functions and the constraints at the sample points are specified by numerical solution of the 3D incompressible steady-state RANS equations. Kriging (KRG) surrogate models have been constructed to estimate the objective functions and constraints at the whole sample space. The blade inlet and outlet angles, the blade wrap angle, the runner inlet width and the number of blades are defined as the design variables. Finally, by using non-dominated sorting genetic algorithm II (NSGA-II), in addition to obtaining the Pareto optimal fronts (POFs), the optimal combinations of the design variables are determined. Analysis of the importance level of the design variables has been performed using the RSA surrogate model. Optimization results reveals that the maximum reduction in the EGR occurs in the optimal 8-bladed runner by approximately 50%. The decrease in the entropy production leads to a decrease in irreversible hydraulic losses and an increase in the hydraulic efficiency of the PAT, e.g. 3.8% and 5.72% increase in efficiency at the QBEP and Q1.3BEP, respectively. Flow field analysis demonstrates that the entropy generation minimization causes a reduction in flow disorders within the optimal PATs. As a result, inlet shock, flow deviation at the blade outlet, flow separation at the blade passage, backflow and swirling flow at the draft tube are dramatically reduced or completely eliminated. The optimization of the PAT with the help of surrogate models and evolutionary algorithms (EAs), from the point of view of the second law of thermodynamics, has been successfully investigated in this work and the improvement of hydraulic performance was achieved.