Sharif Digital Repository

An algorithm for guiding a launch vehicle carrying a small satellite to a sun synchronous LEO is presented. Before the launch, a nominal path and the corresponding nominal control law for the entire journey are computed. For each sampling instant during the guided flight, a linear equation approximately relating the differences between the actual and nominal values is considered, and a LeastSquares formula using data from on-line state measurements is applied to compute the actual control. The coefficient matrices of the Least-Squares formula can be determined by off-line computations. The method enjoys simplicity of implementation by onboard computers, as well as robust accuracy against... 

Droplet microfluidics enables the generation of monodisperse droplets of desired size using immiscible multiphase flows. These droplets serve as individual reactors in bio (chemical) analysis. In addition to monodispersity, high throughput is also necessary for many applications, especially in synthesizing nano- and microscale materials. Therefore, high-frequency droplet formation in the stable regimes (dripping and squeezing) is of great importance. In this work, the flow-focusing geometry is numerically investigated to determine the geometrical dimensions and flow rate ratio by which high-frequency, high mass flow rate, and monodispersed droplet formation are achievable. 3D numerical... 

The focus of present numerical study is on assessment of control of laminar separation bubble phenomenon using Micro-scale combustion actuators in an airfoil with low Reynolds number under surface effect and free flows. In this way, the characteristics of laminar separation bubble such as its formation, geometry, and transition from laminar to turbulent around airfoil SD8020 in attack angles of 5 and 8° are investigated. Following that, the new combustion actuators in Micro-scale, cold, and hot air-jet injection are introduced to control boundary layer flow in terms of eliminating the separation bubble. Some mechanisms are identified for improvement of methane-air premixed flame... 

This study is on combustion modeling of gaseous hydrogen and cryogenic liquid oxygen at the subcritical condition for the well-known Mascotte laboratory combustor. The proposed strategy relies on the hybrid Eulerian-Lagrangian framework in which the continuous phase is evaluated by Reynolds Average Navier-Stokes (RANS) equations and the quick discretization method. The dispersed phase of the combustion field is evaluated by the Discrete Phase Method (DPM). The Eddy Dissipation Concept (EDC) has been performed for combustion-turbulence interaction modeling. Effects of the turbulence model, chemical kinetic mechanism, equation of state, and inlet momentum jet flux are investigated in terms of... 

Moderate or Intense Low-oxygen Dilution (MILD) combustion which known as a combustion under highly preheated and diluted condition is a relatively new combustion regime including many differences in comparison with the traditional one. Among many distinguished disparities between combustion modeling of conventional and MILD ones, turbulent-chemistry interaction modeling is an open field of study. Current knowledge on modeling this combustion regime shows that the eddy dissipation concept (EDC) model could be successfully used for modeling of this combustion regime. In this paper, it has been tried to propose some guidelines to modify the semi-empirical constants of the original EDC model for... 

Objectives. Musculoskeletal disorders (MSDs) are prevalent among airline baggage handlers due to manual materials handling. In this study, the Nordic musculoskeletal questionnaire (NMQ), the revised National Institute for Occupational Safety and Health (NIOSH) lifting equation, and the University of Michigan 3D Static Strength Prediction Program™ (3DSSPP) were used to analyze MSDs among baggage handlers. Methods. The NMQ was filled out by 209 baggage handlers and 46 arbitrarily selected baggage handlers were evaluated using the NIOSH method and 3DSSPP. Results. The obtained results showed that the most common MSDs occurred in the lower back region. The next risky regions included knees,... 

Due to the wide usage of rotary equipment and the necessity of their testing for maintenance and repair, test rigs have become necessary. The mechanical closed-loop test rig developed in Sharif University of Technology branch of ACECR (Academic Centre of Education, Culture and Research) is a test rig with low energy losses that is suitable for testing high power gearboxes such as aerospace or wind turbine gearboxes. It can be loaded up to 489Hp at a maximum speed of 3000 rpm, and the test components can be tested in different testing conditions including a variety of torques and speeds. This paper describes the preliminary, conceptual, and detailed design, steps including frame work design,... 

Failure Mode and Effect Analysis (FMEA) is an effective risk analysis and failure avoidance approach in the design, process, services, and system. With all its benefits, FMEA has three limitations: failure mode risk assessment and prioritization, complex FMEA worksheets, and difficult application of FMEA tables. This paper seeks to overcome the shortcomings of FMEA using an integrated approach based on a developed Pythagorean fuzzy (PF) k-means clustering algorithm and a popular MCDM method called PF-VIKOR. In the first step, Pythagorean fuzzy numbers (PFNs) were used to collect Severity (S), Occurrence (O), and Detection (D) factors for failure modes to incorporate uncertainty and fuzziness... 

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO2 dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier-Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence-chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of... 

One way of improving the performance of diesel engines is to produce modifications in the fuel supply systems. In this article, the effects of creating rotationary flow inside the nozzle and changing fuel injection angle on the performance and emission of caterpillar diesel engine have been examined in two separate stages using AVL FIRE software. First, the injector and its spray have been simulated with various geometries. The numerical results of this step indicate that creating rotationary flow inside the nozzle decreases the penetration length, while increases fuel spray cone angle and improves atomization quality. In the subsequent step, the diesel engine has been simulated with its... 

In this article, NO production mechanisms for CH 4-H 2 combustion under MILD (moderate or intense low-oxygen dilution) conditions are studied using CDF and also zero-dimensional well stirred reactor (WSR) analysis. A H 2/CH 4 jet into a heated and diluted coflow is modeled in CFD analysis. The RANS equations with modified k equations are solved in an axisymmetric 2D computational domain. The GRI2.11 full mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration, fuel hydrogen content, and fuel jet Reynolds number are studied on NO formation reactions. Results show that the measurements are predicted with an acceptable accuracy. The NNH and N 2O... 

In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local... 

This study investigates GH2/LOX coaxial jet flame at trans- and supercritical conditions using the Reynolds averaged Navier-Stokes approach. Four two-equation-turbulence models, three real equation of states, two chemical mechanisms, and three different chamber pressures are examined. Predictions show good agreement with measurements qualitatively and quantitatively. Based on the results, the predictions of the Soave-Redlich-Kwong equation of state (EOS) are closer to the experiment, while the Aungier-Redlich-Kwong EOS has more deviation than the others. Moreover, the k-ω shear stress transport model has better performance than the other turbulence models. It is also found that the flow... 

One promising solution to achieve moderate or intense low-oxygen dilution (MILD) combustion under lower temperatures may be using non-equilibrium plasma discharge. Plasma assisted combustion can extend the flammability limits by reducing the auto-ignition temperature of the reactants and can be a great approach to achieve steady MILD combustion under extreme conditions through decreasing ignition delay time. In particular, the influence of the outlet flows from a co-flowing cylindrical dielectric barrier discharge (DBD) on the CH4/N2 combustion under MILD combustion has been investigated numerically through some simplifications on plasma discharge. A three streams coaxial burner are modeled... 

A major challenge for designers of competitive electricity markets is to devise market rules that limit the capability of electricity producers to exercise market power. Market power is the ability of a firm owning generation assets to raise the market price by its bidding behavior and to profit from this price increase. In addition, an important plan of market monitoring units in long term and competition policy framework is to persuade the participants to bid prices near to their marginal costs. Therefore, estimation of marginal cost function is important in way that Regulatory use it to increase competition in electricity markets. Bid price of the power plant to the power market contains... 

The Eddy Dissipation Concept (EDC) combustion model, in comparison with some other combustion models, has drawn attention, especially for the Moderate or Intense low oxygen Dilution (MILD) combustion. The original formulation of the EDC combustion model is not developed for the MILD combustion regime, and a revision of the model could be considered. In this study, the effect of the characteristic frequency on the EDC combustion model has been investigated, and some parametric studies on the ratios of length and time scales of the fine structures to the Kolmogorov scales have been performed. Results revealed that finding optimum model constants for all combustion field with a wide range of...