IRANIAN JOURNAL OF MECHANICAL ENGINEERING (ENGLISH)
Year:2018 | Volume:20 | Issue:2 |Papers Count:12

Increasing energy demand is the main reason of designing the most efficient energy systems. Using fossil fuels to produce power in world with finite energy sources brings some problems in greenhouse gas control domain in addition to energy crisis. Since 1/3 emitted CO2 associated to human activities releases from power producing sections, oxy-fuel cycles are promising technology. Matiant cycle is one of the most well-known and developed oxy-fuel cycles where it is able to separate and capture the produced CO2 in combustion chamber. In order to propose efficient energy systems, combination of Matiant cycle with an Organic Rankine Cycle (ORC) studied from view of energy and exergy and also a parametric study carried out to show the most effective components in exergy destruction. Proposed combined cycle keeps the CO2 capturing quality of Matiant cycle besides increasing of its efficiency due to waste heat recovery. Energy and exergy efficiency of proposed cycle is 51. 45 and 45. 63% respectively. In exergy analysis domain, in addition to parametric study results of exergy efficiency and exergy destruction of each component has been outlined.

Ignition delay time is an important factor in internal combustion engines design, performance and emissions. In this work, ignition delay time of dual fueled (Diesel – Natural Gas) diesel engine with constant speed in 1500 rpm and 44 kW was investigated. Experimental tests done with diesel in different loads and fuel injection timings primarily and then engine converted to dual mode and experimental tests continued in various loads (10%, 25%, 50%, 75% and 100 of full load) and different pilot advance timings (20, 17, 14 crank angle degree before TDC) and different pilot fuel to gaseous fuel ratios (50%, 40%, 30%). Results shown that by increasing engine load, ignition delay decreases in both diesel and dual mode. Ignition delay time in diesel mode was lower than dual mode in all of condition of the tests. In dual mode increasing pilot injection timing decreases ignition delay time, although, in diesel mode it was non – significant. Numerical simulations done for estimating ignition delay time in different conditions of the tests. Simulations results shown that suggested model can estimate ignition delay time in dual fueled engine and have a good agreement with experimental results.

In the present research the effect of adding multi wall Carbon nano tubes (with 10-15 nano meter diameter) on the heat transfer coefficient of a heat exchanger is investigated. This work is performed by a co current flexible double pipe heat exchanger made of PVDF. To this end, the viscosity and thermal conductivity of nano fluids is measured experimentally for various temperatures and concentrations. The nano fluid temperature is set at 35° C, 40° C, 45° C and 50° C. The flow rate of nano fluid inside the inner tube was adjusted at 100 (L/h), 200(L/h), 300(L/h) while, the cold water flow rate of the outer tube is 100 (L/h)The results shows that heat transfer coefficient of the heat exchanger improves with increasing the fluid temperature, concentration and flow rate. The most effective factor on the heat transfer coefficient is flowrate. Obtained results indicate that using this nano fluid enhances heat transfer coefficient 75%.

In the present research, performance of harmonic components identification and elimination techniques in practical operational modal data is investigated. Firstly, classical modal testing has been carried out on a cantilever steel beam and modal data are extracted. Four bending modes were available in the frequency band of interest. Afterwards, the same structure underwent operational modal tests with simultaneous random and harmonic excitation forces. In OMA, measurement data obtained from the operational responses are used to estimate the parameters of models that describe the system behavior. Extracted modal parameters from these data compared with baseline parameters. Extraction of modal parameters from operational data was fulfilled through different common OMA techniques. Each set of obtained results was evaluated in comparison with reference results. Accuracy and capability of each method in eliminating spurious modes caused by harmonic input components have been studied. For data analysis, in addition to developed MATLAB codes, commercial software of PULSE has been utilized in parallel as a tool for verification. The results of this survey demonstrate that the accuracy of the Stochastic Subspace Identification method is higher compared to Frequency Domain Decomposition and Enhanced Frequency Domain Decomposition methods. However, when the system has low modal damping, Frequency Domain Decomposition methods provide better estimates.

In this paper, improving the performance of the boilers 4th refinery of south pars gas complex, using energy and exergy analysis is discussed. After modeling the factors affecting the performance boilers of the refinery such as variations in ambient temperature, relative humidity, percent excess air and preheat boiler feed water are investigated. According to the results of thermal efficiency and exergy efficiency of 4the boilers 4th refinery operating conditions (ambient temperature 25 C, relative humidity of 70 % air and excess air of 35 %), respectively 75. 78 % and 39. 64 % has been obtained, Also by setting the percentage of excess air in the combustion chamber of the boiler, exergy efficiency of the boiler can be increased to 41. 46%.

The basis of planing vessels motion is the use of low drag force to lift force. One of the main goals in the design of planing vessels is to reduce the hydrodynamic resistance and achieve higher speeds. For this purpose, using a hydrodynamic correct analysis is an important step. In this study, a three-dimensional model of computational fluid dynamics is presented using a finite volume discretization on a structured network to study the flow pattern under the body of COUGAR model in different modes. Therefore, for the numerical simulation of COUGAR model, the commercial software of the ANSYS-CFX has been used. The VOF method was used to study the distribution of two fluid phases and free surface modeling. Thus, the fundamental equations governing the flow of fluid around the cougar model are solved and the distribution of velocity and pressure in the computational domain will be obtained. In these simulations, the model is considered constant and the fluid moves. Simulations at trim angles of 1, 2 and 3 degree and drafts of 10. 7, 11, 77 and 12/84 cm (equivalent to weights 9. 5, 10. 5 and 11. 5 tons in the main vessel) and three speeds of 8. 52, 11. 5 and 14. 95 meters per second (35, 50 and 65 knots for the main vessel) is done. The Studied geometry is two-steps cougar model. The length of the second step is three different distances from the aft and values 34, 38. 5, and 43 cm. Thus, the exact results of the vessel hydrodynamic resistance, the pressure distribution on the body, the waveform and the lifting force will be obtained for all three bodies. For verification, the independence of the network as well as the validation of the results with the laboratory model is carried out. Comparison of the simulation results with the experimental results of the 38. 5 cm model, which was tested in the Persian Gulf National Laboratory, is in good agreement. The maximum error rate for drag of two-step vessel at a speed of 11. 5 m / s is 12. 4% and satisfies the experimental results. Studies show that in a loading and constant velocity, the vessel drag increases with the increase of the second step length from of aft vessel.

Solar radiation data play an important role in designing solar systems. These data measured are not easy for some locations; Therefore, solar radiation data have to be predicted by using solar radiation estimation models. In this paper was to investigate the effect of meteorological parameters sunshine, temperature and relative humidity on solar radiation based on Stepwise Multidimensional regression analysis according to the research literature and empirical studies (eight models including models suggested by Angströ m and Prescott, Ogelman, El-Sebaii, Karakoti and Abdalla) and we used daily solar radiation data of Bandar Abbas station during 19-year statistical period (1987-2005). The accuracy of the models was evaluated using the statistical indicators of Coefficient of Determination (R2), Root Mean Square Error (RMSE), Mean Percentage Error (MPE) and Mean Bias Error (MBE). The results of this analysis show that the variables of “ sunshine duration (square)” and “ temperature” are significantly associated with solar radiation with a coefficient of determination of 0. 7566 (%99 confidence level). The results of this research necessarily cannot be generalized to other stations and in each Region, according to the specific characteristics of that Regions، we can find other variables effects on the solar radiation and its changes.

In current study, three dimensional numerical analysis has been employed to investigate the flow and thermal fields, for different geometry of the hole injection of the film cooling over a flat plate. The computational methodology includes the use of a structured, non-uniform hexahedral grid consisting of the main flow channel, the coolant delivery tube and the feeding plenum, applying the SIMPLE algorithm for pressure-velocity coupling. The two sister holes injection located in the vicinity of to the main injection the cylindrical and the fan-shaped hole by angles of 15, 0 and-15 degree to decrease the strength of counter rotating vortex pairs of the main injection hole and the film cooling performance investigated for two blowing ratios of 0. 5 and 1 and density ratio of 1. 6. The results show that for blowing ratio of 1, the using of sister holes with the angle of 15 degree with fan-shaped hole injection has the best film cooling performance and increases the performance of film cooling about 37. 44% more than the base case which is used from sister holes with the angle of 0 degree with cylindrical hole injection.

In this study, the variation of aspect ratio (height to width) and nanoparticles volume fractions of on natural convection heat transfer of Alumina-water nanofluid in enclosures was investigated. It is supposed that the nanofluid are applied as incompressible fluid and single phase base on water (as basefluid) include Alumina (Al2O3), with volume fraction (concentration) 0-4%, under laminar flow. Governing equations were solved numerically by finite volume method (Fluent). Rayleigh number is in the range of 4×105-108 in 2D enclosures with specific aspect ratios (0. 6-3. 2) and temperature difference (Δ 𝑇 ). In the present work with due attention to the used models for the viscosity and the nanofluid coefficientof thermal conduction was showed that the Nusselt number of the nanofluid with more concentration, is increasing due to more viscosity and coefficient of thermal conduction rather than pure water. In other words the Nusselt number in every specific aspect ratio, in minimum of concentration equals 0. 3% and in maximum of concentration equals 14% would be decreased. In a specific concentration, The Nusselt number was more related with aspect ratio rather than the coefficientof natural convection heat transfer and thermal conduction.

Geometric details are very influential on transfer phenomena such as fluid flow and heat transfer in porous media. But in practice, the simulation of the whole problem is not possible by taking into account these details. To overcome this limitation, one can simulate a part of the media taking into account the geometric details to study the functional parameters. In the present study, structural parameters are determined using an electron microscopy (SEM), and the suitable algorithm for determining the fibers orientation in computational domain is developed. Navier-Stokes equations with continuity assumption in a small scale is solved numerically. Here permeability coefficient considered as functional parameter. For validation, the permeability of the media is experimentally determined. Then the effect of geometric parameters was investigated. By increasing (solid volume fraction (SVF) and angles between the flow direction and the fiber axis (θ ), the permeability decreased. Also, by increasing fiber diameter, the permeability increased.