Numerical Studies on the Effect of Diffuser Rotational Speeds on Low Pressure Ratio Centrifugal Compressor Performance

Numerical investigations are conducted to analyze the effect of rotational speeds of rotating vaneless diffuser on performance of a low-pressure ratio centrifugal compressor stage at off-design and design flow coefficients. Four different rotational speeds are selected for the rotating vaneless diffuser. Free type rotating vaneless diffusers are rotated at speed ratios SR0.25, SR0.50 and SR0.75. Total temperature of the fluid at stage exit increases with increase in rotational speeds of the free rotating vaneless diffuser. This affects the efficiency of free rotating vaneless diffusers. Its flow angles are smaller resulting in a shorter flow path length thereby reducing the frictional losses substantially. Gain in stagnation pressure is observed for all free rotating vaneless diffuser configurations. The rotational speeds determine the extent of net gain in energy level of the fluid and drop in total pressure losses. Based on static and stagnation pressure distributions at stage exit, the flow in FreeRVDSR0.75 undergoes a comparatively better diffusion process. In general, efficacy of diffusion process in a compressor stage with free rotating vaneless diffuser is better at speed ratio above 0.50. Subsequently, the speed ratio is increased to SR1.0 thereby the rotating vaneless diffuser behave like a forced type. Various free rotating vaneless diffuser configurations are compared with the forced type of rotating vaneless diffuser. Based on this study, it is understood that an optimum rotational speed of the rotating vaneless diffuser plays an important role in facilitating the effective diffusion process within the diffuser passage

Performance Enhancement of a Low-Pressure Ratio Centrifugal Compressor Stage with a Rotating Vaneless Diffuser by Impeller Disk Extended Shrouds

Numerical simulations have been carried out to examine the performance and flow parameters of forced rotating vaneless diffuser obtained by the extension of impeller disks of a low-pressure ratio shrouded type centrifugal compressor stage with diffuser diameter ratio 1.40. Four different levels of shroud extensions (i.e., impeller disks alone) forming the rotating vaneless diffusers are analyzed at four different flow coefficients. The extension of impeller disks alone by 40% of impeller exit diameter leads to a fully forced rotating vaneless diffuser thereby replacing the existing stationary vaneless diffuser. The comparative studies are performed using the same impeller with a stationary vaneless diffuser also having a diffuser diameter ratio of 1.40. Static pressure rise in ES40 is found to be higher than SVD by around 9.84% at design flow coefficient and also at above off-design flow rates. Energy coefficient is highest for ES40, followed by ES30 compared to SVD. For ES40, the static pressure recovery coefficient also is higher compared to SVD. The efficiency of ES40 is lesser by around 5.40% to 3.43% compared to SVD, at design as well as at above off-design flow coefficients. The stagnation pressure losses for ES40 drastically reduced compared to SVD. The comparison of stagnation pressure contours and absolute velocity contours near the hub and shroud walls of ES40 and SVD configurations shows that the rotating diffuser walls as in ES40 causes further addition of energy to the fluid. This adds up the kinetic energy level of the fluid which due to better diffusion, results in gain of static pressure rise. Moreover, there is a net increase in stagnation pressure distribution at the exit of diffuser due to rotating vaneless diffuser. Also, the presence of a fully rotating vaneless diffuser (ES40) smooth out the distorted entry flow profiles, thereby improving the performance of the centrifugal compressor stage

Optimization of Job Shop Scheduling Problem using Tabu Search Optimization Technique

ABSTRACT-The Job shop scheduling (JSS) problem consists of "n" jobs and "m" operations on each of the jobs and it is hardest combinatorial optimization problems for which it is extremely difficult to find optimal solutions. Past two decades, much attention has been made to general heuristics such as Genetic algorithm, Ant Colony Optimization, Tabu Search and Simulated Annealing to solve this type of combinatorial optimization problems. In this paper we present how the adaptive search algorithms namely Tabu search is applied to solve Job shop scheduling (JSS) problem. The method uses dispatching rules to obtain an initial solution and searches for new solutions in a neighborhood based on the critical paths of the jobs. Several benchmark problems are tested using this algorithm for the best makespan and the obtained results are encouraging when compared with benchmark values

Experimental Analysis of Anaerobic Co-digestion: Potential of Fruit Wastes

This study focuses on converting fruit waste into usable clean energy by an innovative, cost-effective anaerobic biodigester. The biodigester is designed to anaerobically digest various fruit wastes and starter inoculums of cow dung that are locally obtained. A batch vertical digester of 1000 liters capacity built of fiber with a phonematic agitator positioned in the center is used to improve mixing. The retention time is 30 days with a substrate of banana peels co-digested with mango and papaya peels individually in the ratio of 50:50. The combined wastes generated the biogas and the total quantity of biogas generated for all combined wastes over 21 days varies between 530L/day and 480L/day respectively. In this work, banana and mango peel (waste/water) split 50:50 gives a peak yield of 530L/day. The average ambient temperatures are kept in the range of 25°C to 35°C (i.e., mesophilic range). The pH range of 6.4 to 7.8 is consistently maintained and seems to be stable. Therefore, this proposed anaerobic digester would reduce the disposal of solid waste, and it is cost-effective. After cleaning, it is observed that the combined peels of bananas and papaya contained 91.95% of the estimated biogas and methane, which can be used to solve energy issues such as electricity production and cooking purposes

Scheme of squares: Part I. Systems formalism for potentiostatic studies

The so-called "Scheme of Squares", displaying an interconnectivity of heterogeneous electron transfer and homogeneous (e.g., proton transfer) reactions, is analysed. Explicit expressions for the various partial currents under potentiostatic conditions are given. The formalism is applicable to several electrode geometries and models (e.g., semi-infinite linear diffusion, rotating disk electrodes, spherical or cylindrical systems) and the analysis is exact. The steady-state (t→∞) expressions for the current are directly given in terms of constant matrices whereas the transients are obtained as Laplace transforms that need to be inverted by approximation of numerical methods. The methodology employs a systems approach which replaces a system of partial differential equations (governing the concentrations of the several electroactive species) by an equivalent set of difference equations obeyed by the various partial currents

Scheme of squares: Part III. The single and two square schemes

The special cases of one and two square (horizontal, diagonal and vertical) schemes are analysed in detail. Several limits are considered

Free Rotating Vaneless Diffuser of Diffuser Diameter Ratio 1.30 with Different Speed Ratios and its Effect on Centrifugal Compressor Performance Improvement

AbstractFree rotating vaneless diffuser is one of the novel methods in the design of radial diffuser for reducing the energy losses associated with diffusion. In the present study, the impeller with a stationary vaneless diffuser of diffuser diameter ratio 1.40 and impeller with a free rotating vaneless diffuser of diffuser diameter ratio 1.30 along with stationary vaneless diffuser at downstream for the remaining radius ratio, running at speed ratios 0.25 and 0.75 times the impeller rotational speed are analyzed. A higher static pressure rise with reduced losses is achieved by free rotating vaneless diffuser configuration, Free RVD30 SR0.75. The static pressure recovery coefficient increased by 23 to 80%, by rotating the diffuser independently over the entire flow range. Also, losses are lesser due to reduced shear between the through flow and independently rotating walls of the diffuser. The efficiency of both Free RVD30 SR0.25 and Free RVD30 SR0.75 are marginally lesser by 3.5 to 5.3% with SVD. This clearly reveals that the diffusion rate is higher in the free rotating vaneless diffuser configuration

Fluctuation phenomena in electrochemistry: Part I. The formalism

A unified approach to the problem of electrochemical fluctuations is presented. On the basis of the Langevin procedure, primary noise sources are introduced in the basic phenomenological equations and a discussion of the secondary noise sources arising in the expressions for the power spectra of currents is given

Scheme of squares: Part II. Systems formalism for sinusoidal perturbations

We extend here the formalism developed in Part I (for the potentiostatic response) to the admittance analysis of the scheme of squares. The results are applicable, as before, to several configurations of the electrode such as the rotating disk or the planar. All that one has to do is "to plug in" the appropriate matrices relating the interfacial concentrations to the fluxes