Applied Mechanics and Materials Vol. 598

Abstract: In this paper, the feature vector of the roller bearing signals are extracted on the basis of wavelet analysis and a fault diagnosis experiment is carried through wavelet neural network in detail. The method and the theory of fault diagnosis based on BP neural network and the radial basis function neural network are studied and the results of diagnosis based on relax-type Neural-Networks and close-type Neural-Networks are compared.

Abstract: The objective is to study on inverse analysis for the mechanical properties of circulating tumor cells and estimate the mechanical properties. For the adsorbed cells in the micro flow path, experimental image of cell deformation under flow was obtained. The strain is defined based on the image and it is shown for the relationship with flow rate. The fluid analysis was performed to compute the stress acting on the spheres in the micro flow path. The amount of deformation of the cells was examined with changes in the Young's modulus. It consists combining experimental results with a numerical finite element model in order to find out the mechanical properties of circulating tumor cells. The procedure proposed may be also applicable in estimating other physical and mechanical properties of cells.

Abstract: Solar thermal propulsion is a kind of space propulsion technology with great potential applications. Due to the difficulty of hydrogen storage in orbit, ammonia becomes an ideal candidate propellant as its stability and easier storage. In solar thermal propulsion system, the working temperature is usually above 2300K, and in this condition the dissociation of ammonia will occur. Thus, using ammonia as a single component propellant to compute and analysis the performance of thruster is not precise, and the mixture components produced from ammonia dissociation must be taken into account. In this paper a novel heat exchanger configuration with platelet technology is designed, and based on finite-rate chemical reaction method and computational fluid dynamics, the dissociation process of ammonia in the heat exchanger and nozzle is simulated. Then the influence to the specific impulse of the solar thermal propulsion system is comparatively analyzed. The simulation result indicates that the main productions of the dissociation are N₂ and H₂, and the mole fraction of other components is small value that can be neglected. The specific impulse considering dissociation reaction is higher than not considering, so that this research can estimate the performance of solar thermal propulsion with ammonia propellant more precisely.

Abstract: In this paper, the effect of crosswind on triple natural draft dry cooling towers is studied and analyzed. Concerning this area, many researches have concentrated on one cooling tower. This research focuses on the mutual effects of the adjacent towers' performance, and also makes a comparison between the efficiency of the three cooling towers in windy and no-wind conditions, using Computational Fluid Dynamics (CFD). In modeling the crosswind condition, at first solid windbreakers, and then radiator-type windbreakers are used for each cooling tower. Finally, the water outlet temperatures of the radiators' cooling towers are analyzed, and the total heat rejections at different conditions are compared. Numerical results show that radiator-type windbreakers can substantially improve cooling efficiency more than usual solid-types. It should be mentioned that a complete grid study is done to achieve a grid-independent solution.

Abstract: The enhancement of heat transfer by corona wind reported in the literature has ranged from 16% to 2500%. It is suspected that such a huge variation has to do with the measurement of temperatures involved. The objective of the present study is to reevaluate the heat transfer enhancement by such technique. To accurately assess the heat transfer enhancement, a thermal imaging system has been used for temperature measurement. A corona wind generator is used to produce air jet directly impinging the heat transfer surface (which is a grounded copper plate of 10 cm by 10 cm uniformly heated by a thermofoil heater). The applied voltage varies from 8 kV to 12 kV with two gap distances of 3 cm and 4 cm between the electrodes.

Abstract: Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.

Abstract: Solar thermal propulsion is a potential technology in aerospace applications, and it is a significant issue to improve the heat transfer efficiency of the solar thermal thruster. This paper proposes a novel platelet configuration to be used in the heat exchanger core, which is the most important component of solar thermal system. The platelet passage can enhance the heat transfer between the propellant and the hot core heated by the concentrated sunlight. Based on fluid-solid coupled heat transfer method, the paper utilized the platelet heat transfer characteristic to simulate the heat transfer and flow field of the platelet passage.The simulation result shows that the propellant can be heated to the design temperature of 2300K in the platelet passage of the solar thermal propulsion system, and the fluid-solid coupled method can solve the heat transfer in the platelet structure more precisely.

Abstract: In this research, numerical simulation is adopted to study the cooling characteristic of a bracket-type heat sink assembly. Analysis on thermal/fluid fields are used to justify the physical mechanisms inducing the excessive heat dissipation and temperature rise. In order to enhance the cooling capacity of the heat sink assembly, two types of vortex generator, flow-up and flow-down were used to create the turbulent flow for increasing the contact opportunitybetween fluid and plate fin.Also, thermal resistance is used to evaluate the cooling capacity of the heat sink assembly in this work. As a result, the numerical simulations show thatapparent reductions on source temperature (from 348.0K to 346.2K) and thermal resistance (from 0.266 K/W to 0.253 K/W) are observed for a 150W power input. Besides, significant improvement on the heat concentrated phenomenon anduniform temperature distribution were acquired after installing the vortex generators.Furthermore,a larger longitudinal flow was generated when the flow-up type vortex generator was mounted; therefore the temperature distribution is more uniform than that of the flow-downtype. Clearly, the cooling capacity of heat sink assemblyis enhanced since the fluid is guided to the heat concentrated area when theflow-down vortex generators are adopted.

Abstract: In the present work, the surface temperature history of a metal shell of the blunt nose of supersonic launch vehicle which is covered by a thermal protection coating is numerically predicted and compared with experimental data. The full Navier-Stokes equations are used to estimate the aerodynamic heat flux during flight, coupled with the governing equations for the thermal protection system to study the erosion rate and temperature variations. The results show the importance of the properties of the coating on accuracy of the numerical predictions.

Abstract: At this paper, the thermal behavior of ablative materials as heat shields for reentry vehicles is investigated numerically. A one-dimensional finite difference solver is developed to simulate governing mass and energy equations. Four ablative materials; AVCO 5026-HCG, Carbon-Phenolic, Nylon-Phenolic, and Silica-Phenolic; are considered as a heat shield material for a reentry capsule with the diameter 2.8 meter. A heat flux profile from a simulated trajectory of a reentry capsule is used for investigation the performance and essential thickness of these four ablative materials. The only restriction for this simulation is the ultimate temperature of the backup structure which is beneath the ablative heat shield. At all simulations, the final thickness is defined by reaching the interface temperature, the temperature of the border between the ablative heat shield and solid shell, to 80±0.5 degrees Celsius. In addition, the sensitivity analysis is carried out to investigate the effect of properties variations on Carbon-Phenolic’s thickness for this specific heat flux profile.