نشریه مهندسی مکانیک تبدیل انرژی
سال هشتم شماره 4 (پیاپی 29، زمستان 1400)

In thermodynamic analysis of internal combustion engine, the specific heats of working fluid is considered to be constant. Due to specific heats are functions of operation temperature of cycle, this consideration cause to several errors in the thermal analysis. In this paper, standard Otto cycle with three various types of specific heats ratios has been analyzed numerically and effect of these ratios on the thermal performance of cycle such as specific output work and thermal efficiency has been studied and results has been investigated. For three types of specific heats ratio, these ratios are considered constant, linear function of operation temperature of cycle and polynomial function of cycle temperature. Results showed that both linear and polynomial specific heats ratios gave approximately same results such as specific output work and thermal efficiency and there are difference between thermal result of both linear and polynomial type with constant ratio. Also CPU time consumption and required memory usage for analyzing these here type had been evaluated . all results mentioned that linear specific heats ratio should be considered for the design of practical Otto cycle.

A compact fin-tube heat exchanger is used to transfer current fluid heat inside the tubes into the air outside. In this study, entropy production and optimized Reynolds number for finned-tube heat exchangers based on the minimum entropy production have been investigated. As a result, the total entropy of compact heat exchangers, which is the summation of the production rate of fluid entropy inside the tube, and the entropy production rate of fluid in the air, is minimum due to the changes in the Reynolds number of the fluid inside the tubes. Based on thermodynamic analysis and study of parameters affecting the entropy production, one expression is proposed for the optimal Reynolds number. The expression is a function of the thermodynamic properties of the fluid in the pipe, fluid thermodynamic properties of air, the effects of ambient temperature, input temperature of the cooling fluid, and geometrical dimensions of the heat exchanger. According to this optimization, effective information for the design of a compact finned-tube heat exchanger would be obtained. Therefore, in practical conditions, using optimal Reynolds number, the system has the lowest irreversibility, and as a result, the best exergy will be accessible. Finally, an empirical correlation is proposed for the optimum Re number that can predict Reopt with less than 0.6 % error.

Energy systems, despite their key roles and diversity, have serious implications for human health, ecosystems, and natural resources; Thus, in the last two decades, the focus of more than 1,000 LCA studies on energy systems has been to determine and mitigate these effects. This paper examines the applications of LCA in energy and heat generation energy systems to determine environmental hotspots, as well as brief and important information about 1) methodology, including defining the objectives and scope of studies, covering the system life cycle, and Environmental Impacts; 2) The main findings of these studies, especially with the aim of determining environmental hotspots and patterns of effects in different energy sources, are presented. In this article, an attempt has been made to review the recommendations and guidelines in the field of LCA on the main aspects of the methodology in order to properly guide LCA studies on energy systems and the reliability of LCA results.

Due to the application of heat exchange equipment such as converters in various industries, improving heat transfer in this equipment is very important. Various methods are used to improve heat transfer. In this paper, the effect of nanofluid and quadruple barrier type barrier on heat transfer coefficient has been investigated numerically with Ansis Fluent software. The nanofluidused is considered as a homogeneous fluid and the effect of increasing the nanoparticle is applied as an effect on the properties of the nanofluid. The heat transfer coefficient for pure and nanofluid water and barriers in different states was obtained and the amount of heat transfer improvement was obtained. The flow inside the pipe was turbulent and The Reynolds number was considered in the range of 5600 to 12000. Simulation for simple tube mode with pure water, simple tube with nanofluid with volume fraction of 0.5%, tube with barrier with 3, 4 and 5 mm offerings with nanofluid with volume fraction of 0.5% The results showed that with the introduction of nanoparticles, the heat transfer coefficient increased by 52.87%. It also showed that the highest value of Nusseltnumber was obtained in 4 mm width of torsional strips. In this case, the increase in Nusselt number compared to the unobstructed mode in Reynolds 12000 increased by 64.58%. The value of the coefficient of friction was evaluated to determine the pressure drop. At the 3 mm width of the barrier, the maximum coefficient of friction was obtained. In this case, the coefficient of friction increased by 14.26% compared to the unobstructed mode.

The aim of this study was to provide a new model for identifying the sources and sources of waste gas in Mahdishahr city gas department and to define corrective measures and prioritize measures to help managers to make appropriate decisions to reduce waste gas. The research method is descriptive-analytical in terms of nature and is applied in terms of purpose. The statistical sample of the research is 10 managers, experts of Semnan Gas Company. Interview tools were used to collect field data. To analyze the collected data, thematic analysis method, FMEA analysis of triple fuzzy number and fuzzy hierarchical analysis were used, respectively. The results showed that 9 of the existing risks of urban gas loss in the study area are the most effective factors in creating these risks. To fix them, a set of corrective measures should be taken, including reducing flagship operations that wear out the ball valve, installing a second valve on the outlet collector valve after the filters, and sending a second discharge valve on the filter collector outlet at stations with this second valve and installing a sample location. Leakage should be considered in the way of filter drain pipe and leakage control. Therefore, paying attention to these results of provincial and city gas companies as an important and defined strategy can be used to identify waste points and define corrective measures for all cases and take care to eliminate them and ensure sufficient effectiveness and adequate measures.

In this study, experimental energy and exergy efficiency analysis of a solar desalination have been presented in two cases; with concentrator and without concentrator. Solar concentrator was used to increase production of clean water. Because production rate of clean water is slow in solar desalination devices. Hence, a solar device with ability of adding concentrator was designed and performed.   In experimental investigations, amount of produced water has been measured in periods of one hour and by using ambient parameters, these two cases have been compared each other. Results show that the amount of produced water increases about 40% in case of with concentrator rather than without concentrator. Also, energy and exergy analysis of solar desalination display rising of efficiencies in case of with concentrator, as energy and exergy efficiencies grows up to 30% and 15.19%, respectively.