Performance evaluation of a model thermocompressor using computational fluid dynamics

Thermocompressors are widely used in a large number of industries that use steam as their heating medium or as a power generating utility. They are devices that use the energy of a high pressure fluid to move a low pressure fluid and enable it to be compressed to a higher pressure according to the principle of energy conversion. They work like a vacuum pump but without usage of any moving part and so they can save energy. The performance of a thermocompressor highly depends on its geometry and operating conditions. This paper first describes the flow behavior within a designed model of a thermocompressor using the computational fluid dynamics code, FLUENT. Since the flow is turbulent and supersonic, CFD is an efficient tool to reveal the phenomena and mixing process at different part of the thermocompressor which are not simply obtained through an experimental work. Then its performance is analyzed by choosing different operating conditions at the boundaries and also different area ratios which is one of the significant geometrical factors to describe the thermocompressor performance. Finally, the effect of various nozzle exit plane diameters which cause different Mach numbers at the nozzle exit is investigated on the thermocompressor performance. The results indicate that these variables can affect both the entrainment ratio and critical back pressure. This device uses water vapor as the working fluid and operates at 7.5 bar motive pressure, 63°C and 80°C for suction and discharge temperatures, respectively

Simulation and measurement of condensation and mixing effects in steam ejectors

Vapour compression via ejectors has become a topic of interest for researchers in the field of air conditioning and refrigeration. Ejectors have the benefit of being extremely reliable with stable operation and no moving parts leading to essentially maintenance free operation. However, these devices typically have very low efficiencies due to the low entrained mass flow rate of the low pressure secondary stream relative to the high pressure primary stream mass flow rate. The entrainment of the secondary stream and mixing between the primary and secondary streams are therefore dominant features which require investigation. Entrainment and mixing typically occurs under conditions of compressible, turbulent flow with strong pressure gradients. Steam ejectors, which are the focus of the present work, have the added complexity of condensation effects which must be accommodated in modelling and simulation work. Condensation in the primary nozzle of steam ejectors alters the steam flow properties relative to properties derived from ideal gas modelling, which is sometimes used for steam ejector analysis work. By performing computational simulations for non-equilibrium wet steam flow in a representative primary nozzle, the altered steam jet properties that arise during the nozzle expansion process are demonstrated, via empirical correlations, to be of sufficient magnitude to affect the mixing rate, and thus the entrainment ratio, of steam ejectors. For the particular primary nozzle and flow conditions considered, it was estimated that these changes in steam properties would cause around 29% increase in the mixing layer growth rate for the wet steam case relative to the ideal gas case. To further explore the influence of wet steam mixing effects, the non-equilibrium wet steam computational simulation approach was then expanded to the case of a complete ejector. Under particular conditions for the choked flow ejector operation, results indicated that the non-equilibrium wet steam model simulates an entrainment ratio that is 10% higher than that for the ideal gas model. The non-equilibrium wet steam model also gives a higher critical back pressure by about 7% relative to the ideal gas model. Enhanced mixing layer growth, which arises due to steam condensation in the primary nozzle, was identified as the main reason for higher entrainment ratio of the ejector simulations using the wet steam model. Higher pitot pressure of the mixture at the diffuser entrance for the wet steam simulation was also identified as the reason for higher critical back pressure for the ejector relative to the case of ideal gas simulation. To estimate the relative significance of pressure-driven effects and mixing-driven effects on the secondary stream entrainment, ideal gas computational simulations were also performed. Under a fixed operating condition for the primary and discharge streams, the ejector entrainment ratio was more strongly influenced by the mixing effects at lower secondary pressure. For a particular ejector and associated operating conditions, about 35% of the ejector entrainment ratio was attributable to mixing effects when the secondary stream pressure lift ratio was 4.5, while this portion was reduced to about 22% when the secondary stream pressure lift ratio was 1.6. Given the significance of ejector mixing effects and the lack of consensus on the most appropriate model for turbulent mixing in steam ejectors, an experimental investigation was performed to provide direct data on the mixing of wet steam jets in steam ejectors for model development and validation of computational simulations. Pitot and cone-static pressures within a high pressure supersonic steam jet that mixed with low pressure co-flowing steam were obtained. Results from the non-equilibrium wet steam simulations were analysed to give values of pitot pressure and cone-static pressure values using both equilibrium and frozen-composition gas dynamic models. The equilibrium analysis appeared reasonable for the pitot pressure, whereas the frozen-composition analysis was a better approximation for the cone-static pressure. Differences between the experimental data and the wet steam computational simulations were in the vicinity of 25% at certain locations. The static pressures downstream of the nozzle exit were lower than the triple point, but energy exchanges associated with the transitions to and from the solid phase were not incorporated in the wet steam model. The development of such a model is required before definitive conclusions can be made regarding the accuracy of the turbulence modelling

Modeling and predicting the biofilm formation of Salmonella Virchow with respect to temperature and pH

Biofilm formation of Salmonella Virchow was monitored with respect to time at three different temperature (20, 25 and 27.5 °C) and pH (5.2, 5.9 and 6.6) values. As the temperature increased at a constant pH level, biofilm formation decreased while as the pH level increased at a constant temperature, biofilm formation increased. Modified Gompertz equation with high adjusted determination coefficient (Radj2) and low mean square error (MSE) values produced reasonable fits for the biofilm formation under all conditions. Parameters of the modified Gompertz equation could be described in terms of temperature and pH by use of a second order polynomial function. In general, as temperature increased maximum biofilm quantity, maximum biofilm formation rate and time of acceleration of biofilm formation decreased; whereas, as pH increased; maximum biofilm quantity, maximum biofilm formation rate and time of acceleration of biofilm formation increased. Two temperature (23 and 26 °C) and pH (5.3 and 6.3) values were used up to 24 h to predict the biofilm formation of S. Virchow. Although the predictions did not perfectly match with the data, reasonable estimates were obtained. In principle, modeling and predicting the biofilm formation of different microorganisms on different surfaces under various conditions could be possible

Facility layout design for hybrid cellular manufacturing systems

Facility layout which is the arrangement of facilities in the shop-floor has a great impact on the performance of manufacturing systems. An effective layout decreases material handling cost, throughput time, lead-time and results in increasing productivity and efficiency of manufacturing systems. Although layout problems have significant roles on the efficacy of manufacturing systems, scant attention has been paid to the layout design in hybrid cellular manufacturing systems. In this paper, a mathematical model for layout problems in a hybrid cellular manufacturing system is proposed that minimizes the total material handling cost (both inter-cell and intra-cell material handling cost). To solve the model, a variant of a simulated annealing algorithm is developed. The results show that the developed algorithm outperforms the algorithm that was benchmarked from the literature in terms of solution quality and computation time

Penile Girth Enhancement using Amniotic Membrane in a Rabbit Model: A stereological study

Objectives: This study aimed to evaluate the efficacy of Penile Girth Enhancement (PGE) using Amniotic Membrane (AM) as a graft in a rabbit model. Additionally, stereological studies were used to obtain quantitative histological data regarding the structure of the penis. Methods: In this study, 20 adult male rabbits of similar age and weight were allocated to two sham and surgery+AM groups. Both groups underwent surgery by longitudinal Ishape midline incision of the tunica albuginea on the dorsal surface of the penis. The surgery +AM group underwent PGE by AM graft. The penile length and mid circumference were measured using a Vernier caliper before and two months after the surgery. Stereological studies were used to obtain quantitative histological data regarding the structure of the penis. Results: The mean total volume and diameter of the penis increased in the surgery +AM group (p<0.03 and p<0.04, respectively). The stereological evaluation showed a significant increase in the mean volumes of the tunica albuginea and corpora cavernosa in the surgery +AM group compared to the sham group (p<0.01, p< 0.03). Additionally, the mean volume density of the collagen bundles, muscle fibers, and cavernous sinuses and the total number of fibroblasts and smooth muscle cells increased in the surgery +AM group compared to the sham group (p<0.01, p<0.01, p<0.03, p<0.01, and p<0.05, respectively). No infections, bleedings, or other complications were seen. Conclusions: AM is a method that has appeared promising for material use in penile enhancement. Thus, it may be used for PGE in the future. Keywords: Amniotic Membrane; Histopathology; Animal; Penile Girth Enhancement

A genetic algorithm for solving supply chain network design model

Network design is by nature costly and optimization models play significant role in reducing the unnecessary cost components of a distribution network. This study proposes a genetic algorithm to solve a distribution network design model. The structure of the chromosome in the proposed algorithm is defined in a novel way that in addition to producing feasible solutions, it also reduces the computational complexity of the algorithm. Computational results are presented to show the algorithm performance

An optimization approach for a joint location inventory model considering quantity discount policy

Joint location inventory problems involve determination of two key decisions in a supply chain network: inventory control decisions and facility location decisions. This paper addresses a supply chain network consisting of a supplier and a set of retailers, in which a number of distribution centers (DCs) are to be located. The objectives of a joint location inventory problem are to identify the optimal number and location of the DCs, the assignment of the retailers to the DCs and determining the inventory control decision of the DCs so that the total cost is minimized. This paper extends the traditional joint location inventory model that considers the basic EOQ policy to replenish the inventory of retailers, by considering quantity discount as the inventory policy of the supply chain network. The new model can be applied to both EOQ and quantity discount policies. An efficient two-stage heuristic algorithm is developed to solve the model. Numerical experiments show the efficiency of the proposed algorithm

Design of a facility layout model in hybrid cellular manufacturing systems under variable demand

Changes in demand, as one of the issues of volatile manufacturing systems, decline the performance of manufacturing systems over the time; especially, it degrades the effectiveness of layout in manufacturing systems. Although the layout of the arrangement of facilities on the shop floor play a significant role in the effectiveness of manufacturing systems, it has not absorbed the attention of researchers in hybrid manufacturing systems. In this paper, a new mathematical model for facility layout in a hybrid cellular manufacturing system has been proposed, which considers demand varying over the planning horizon. The model minimises the material handling cost. To solve the model, a simulated annealing algorithm from literature has been improved. The comparison of results between two algorithms shows the superiority of the improved algorithm in both the quality of solutions and computational time