Journal of Aerospace Mechanics
Year:2020 | Volume:16 | Issue:3 (61)|Papers Count:8

This paper deals with designing the navigation scheme of a gimballed inertial system. This design is introduced and proved in the form of two theorems. Most of the gimballed navigation schemes proposed in the literature have the drawback of estimating position rates for alignment commands. Not only the estimating position rates are the basic source of the position errors, but also, they make the alignment commands and their implementation more complicated. The major advantage of the proposed design is that it eliminates the errors resulting from the estimation of the longitude and latitude rates because the angular velocity commands of gyroscopes are proportional to accelerations’ integrals and independent of the system position. In this paper, the stabilized platform is modelled, the platform alignment procedure is determined, and the initial conditions of the navigation phase are calculated. The results of the navigation scheme are compared with the wander-azimuth scheme in four scenarios and the performance of the position-independent navigation scheme is evaluated in practical tests and its results are presented.

In this article, a standard GAX absorption refrigeration cycle is investigated from the viewpoints of thermodynamics and thermos economics and its optimization is performed with the aim of obtaining maximum COP (or maximum exergy efficiency) or minimum unit product cost. The thermodynamic analysis of this system is done by applying the mass, energy and exergy balance equations on different components of the system, and its economic analysis is performed by solving simultaneously, the exergy-economics equations for each of the system components. The system is analyzed and optimized by defining the first and second laws of thermodynamics and also defining the unit exergy cost of the product. The key parameters selected as optimization parameters for the proposed system are the generator temperature, the condenser temperature and the degassing range (the difference between densities of the strong solution and that of the weak solution). Results indicate that in comparison with the thermodynamically optimized condition (maximum COP or exergy efficiency), minimum unit product cost is obtained at a lower generator temperature. Optimization results show that at minimum unit product cost, COP and 2 nd law efficiency are calculated to be 0. 602 and 0. 508, which are 57. 2% and 57. 35% less in comparison with their maximum values, respectively.

Oscillating disturbances of rotors in the turbo-machines, compressors, combustion engines and other rotary equipments are affected by various factors, such as the mass unbalance of parts, which cause the wear and collision between the rotor and bearing surfaces. This phenomenon always leads to the failure of elements and interruptions at work of the equipments. Today, hydrodynamic journal bearings are the most common type of rotor supports in rotary machines. Therefore, improving the ability of these supports in control of the rotor fluctuations, as an effective factor in enhancing the overall efficiency of the rotating systems, is important. In this study, the effect of external load, as an eccentricity ratio indicator of mass unbalanced rotor in the bearing clearance space, on the dynamic behavior of noncircular three lobe bearings with micropolar lubricant is investigated. For this purpose, the governing Reynolds equation and the equation of rotor motion have been rewritten and evaluated using FEM and 4 th order Runge-Kutta methods in static conditions and successive time steps after the occurrence of rotor disturbances. The results indicate that the amplitude and intensity of rotor disturbances are developed in the forms of periodic, KT periodic and quasi-periodic motions in the moderate values of eccentricity ratio and they are changed to limit cycle oscillations with small amplitude in low or especially high eccentricity ratios.

In this paper, the problem of intercepting very high speed non-maneuvering ballistic targets is addressed and a novel guidance law to enhance the probability of 'hit to kill' is designed. The most important challenges arise in interception of high speed ballistic targets include high closing velocity and consequent lack of time for the final reaction, the presence of noise with high standard deviation in the sensors and as a result of inability to accurately identify targets position and considerable time constant in the time order of final interception phase. The available guidance methods in literature review are usually designed by ignoring or reducing above items at low speed and quick response of the interceptor to guidance commands, which in some cases are not very effective in the practical cases mentioned in this article. In this paper, two ideas of using missile team in continuously to participate target detection information and a constant step guidance law to increase the quality of interception have been used. The amplitude of static guidance is deduced through a regression model using simulator software. The simulation of proposed guidance law on a high precision model shows the efficiency of the method.

In this paper, nonlinear vibration analysis of a composite cylindrical shell with internal pressure, subjected to a low velocity impact is investigated using analytical and FE methods. The governing coupled partial differential equations of motion are derived using Donnel’ s nonlinear shell theory. The impact force is modeled by employing the modified Hertzian contact theory. To model the low velocity impact, the model proposed by Shivakumar is exploited. The governing nonlinear coupled partial differential equations of motion for the shell are solved using the Galerkin method. Then, the dynamic response and generated stresses of the cylindrical shell subjected to a low velocity impact are analyzed using the ABAQUS FE software and a mathematical code developed in the environment of Mathematica software. Finally, the effect of some parameters on the impact response is studied. These parameters include the number of layers, ply orientation, shell thickness and shell radius and characteristics of the striker. It is seen that ply orientation of the composite layer has a significant effect on the dynamic response of the shell under impact and when the ply angle increases the amplitude of dynamic response of the shell decreases. Moreover, for a shell with smaller radius the response frequency is higher and when the radius of the shell increases, its effect on the frequency variation, decreases.

Improving the performance of solar heat pumps has always been of interest to researchers. The purpose of this study has been to find the optimal state of carbon nanotube (CNT) concentration in the refrigerants and condenser copper tubes as well as the total tube length and the diameter of the condenser helical coil, in order to maximize the exergy and heat transfer. The investigation has been carried out by the numerical method using finite volume, unsteady and laminar flow. The genetic algorithm has been used to obtain the optimum choice for the condenser in two states of constant and variable diameter. Three concentrations of 0, 0. 5 and 1 percent for the carbon nanotubes in the refrigerant, three concentrations of 0, 1 and 3 percent for the carbon nanotubes in the copper tube, three different values for the condenser diameter in each of the two states of constant and variable diameter, and three different condenser tube lengths have been investigated. The results show that the use of CNT in the refrigerant, always increases the Nusselt number and the exergy while, the use of CNT in the wall structure of the condenser copper tube reduces them. The results also show that the use of carbon nanotubes in the refrigerant increases the Nusselt number by 16% to 21%, and the exergy by 9% to 13%. However, using carbon nanotubes in the condenser copper tube wall reduces the Nusselt number and the exergy up to 9. 5% and 18% respectively.

Exergy analysis is a method for recognizing value and type of system availability variations respect to the surrounding environment. It also helps to find how macroscopic properties of the system change during a process. By exergy analysis, proportion of each process in the internal availability transmission and the places of useful energy losses in the system can be determined. In this research, the energy and exergy analysis and the parametric study of a gas to liquid compact heat exchanger (11. 32-0. 737-SR type) are accomplished for various working conditions. So, the appropriate model for the heat exchanger is achieved based on theoretical and experimental thermodynamic relations. The heat exchanger performance in the presented model is estimated based on the ε-Ntu method. In this study, various working conditions are determined by changing some parameters like temperature, pressure and volumetric flow rate of air and mass flow rate of water. The variation range of these parameters is chosen to cover a real intercooler working conditions. For assessment of system performance, energy analysis is performed by definition and evaluation of parameters such as effectiveness and pressure drop coefficient. Also, for exergy analysis parameters such as exergic efficiency and irreversibility coefficient are considered. The presented results imply how variation of working conditions affects the characteristic parameters of compact heat exchanger.

Among the available cooling methods in the combustion chamber of liquid propellant engine, regenerative cooling is widely used due to its good performance used. In this paper, a combustion chamber of liquid propellant engine is simulated numerically and effect of increasing cooling channel surface roughness on cooling performance is studied. The results showed that maximum temperature of combustion chamber wall is in the throat and by increasing surface roughness in cooling channel, the temperature in throat decrease and the heat transfer from the combustion gases to the cooling fluid increases. The temperature drop in the throat is 9. 9% and 32% when water and liquid hydrogen is used as cooling fluid respectively. Also in the roughness height between 0 and 24 µ m by increasing the surface roughness, temperature drop in throat increase.