Key Engineering Materials Vols. 326-328

Abstract: The present study is investigated the causes of enhanced heat transfer during the melting process of solid-liquid PCM (Phase Change Material) using an ultrasonic vibration. Paraffin (noctadecane) was selected as a PCM and experimental studies were performed as following. Heat transfer coefficient and enhancement ratio of heat transfer was measured, acoustic streaming induced by ultrasonic waves observed using a PIV (Particle Image Velocimetry) and thermally oscillating flow phenomenon observed using an infrared thermal camera during the melting process. For the numerical study, a coupled FE-BEM (Finite Element-Boundary Element Method) was applied to investigate acoustic pressure occurred by acoustic streaming in a medium. And then, the profiles of pressure variation compared with the enhancement ratio of heat transfer. The results of this study revealed that ultrasonic vibrations accompanied the effects like acoustic streaming and thermally oscillating flow. Such effects are a prime mechanism in the overall melting process when ultrasonic vibrations are applied. Also, as the acoustic pressure occurred by acoustic streaming increases, the higher enhancement ratio of heat transfer is obtained.

Abstract: Most rocks are saturated or partly saturated with different fluids under different depth, temperature and pressure conditions. It is generally acknowledged that fluids have the most important effect on the attenuation and dispersion of seismic waves. There exists a relation between frequency- and temperature- dependence on rock’s seismic properties. It is not yet clear in literature whether there exist other equally important attenuation mechanisms as that in Biot’s model, since there are other sources of dissipation, also related to fluids, that are not considered in Biot theory but that may also contribute to the overall dissipation of seismic energy. Identifying the precise relaxation mechanisms is still the subject of experimental and theoretical research. In this article, a series of experiments are conducted on dry and saturated rocks (sandstone, marble, granite) at different temperatures and frequencies to find the attenuation mechanism of interaction between rock skeleton and pore-fluid. Fluid viscosity generally depends on temperature, so the effect of pore fluid on attenuation is confirmed in terms of apparent viscosity variation of rock caused by the change of pore-fluid conditions (such as frequency or temperature). Based on our experimental data, we develop a new model of macroscopic apparent viscosity in saturated rock which is consistent with the nonlinear relaxation law. It helps to derive the analytical expressions to compute velocity dispersion and attenuation as functions of frequency and temperature.

Abstract: The filtration of soot in the metal foam DPF has been studied. INCOFOAM®HighTemp is selected for DPF material for its large specific area for filtration. The structural properties of the foam such as pore diameter, strut diameter and porosity are determined from the pictures by 3D Xray scope and SEM. The permeability and Forchheimer coefficient obtained by clean filter experiments are correlated with the structural properties. During the filtration process, the soot particles deposited inside the filter affect the local strut diameter, porosity and permeability, which determine the filtration efficiency and pressure drop. By the analytic model developed, the actual pressure drop in the engine operation can be predicted.

Abstract: As depositing layers with different thermal expansion coefficients, the residual gradient stress will cause the structure deformed. The deformation of structure in the free ending, middle section, and clamped end are detail investigated. It is found the clamped end often has complex deformation shape. The warpage due to buckling is found. The results show if the thickness of structure is much larger than above deposing layer, warpage will hardly happen and the free ending will have more flat region. As the thickness of structure layer being not much larger than above deposing layer, the warpage happens and the free ending is parabolic shape. In the clamped end, the complex deformation even is concave shape in the center part but protruding shape in the side region. The larger temperature difference will be more easily warpage and be no more flat. The results also show that as the ratio of length to width decreasing, seriously warpage and complex deformation happens. The free ending may be a little protruding shape. However, if the ratio of length to width is larger, the free ending will have concave shape.

Abstract: In order to protect turbine blades from high temperature, film cooling can be applied to gas turbine engine system since it can prevent corrosion and facture of material. To enhance the film cooling performance in the vicinity of the turbine blade leading edge, flow characteristics of the film-cooled turbine blade have been investigated using a cylindrical body model. Mainstream Reynolds number based on the cylinder diameter was 1.01×105 and the mainstream turbulence intensities were about 0.2%. CO2 was used as coolant to simulate the effect of coolant-tomainstream density ratio. The effect of coolant flow rates was studied for various blowing ratios of 0.5, 0.8, 1.1 and 1.4, respectively. Results show that the blowing ratio has a strong effect on film cooling effectiveness and the coolant trajectory is sensitive to the blowing ratio.

Abstract: The objective of this study is to develop a local failure criterion at wall-thinning defect of piping components. For this purpose, a series of tensile tests was performed using several types of specimens with different stress state under tension, including smooth bars, notched round bars and grooved plates. In addition, finite element (FE) simulations were performed for all tests, and its results were compared to the test results. From the comparisons, the equivalent stress and strain corresponding to maximum load and final failure of the notched round bar specimens were proposed as the local failure criterion which is a function of stress triaxiality at notched area. The criteria were verified by employing them to the estimation of maximum load and final failure of grooved plate specimen tests.

Abstract: Drill pipe fatigue damage occurs under cyclic loading conditions due to, for instance, rotation in a dogleg region. This paper presents two approaches to evaluate damage in drillpipes; First, Finite Element Method is used to evaluate cumulative effects of fatigue damage with respect to the actual drilling conditions and the fatigue damage curves for smooth and die-marked drill pipes are obtained. Second, as a case study, the Cox Regression Model, a broadly applicable method of survival analysis is used to analyze the failure data of the southern oilfields of Iran. The resultant cumulative survival and hazard functions can reliably predict the time of failure.

Abstract: The influence of surface treatment such as nitriding, TNHT(Ti Nano Heat-treatment, PHILOS TECHNOLOGIES, INC.) and PVD coating on the thermal crack propagation behavior of hot work die steel was investigated. To examine the thermal fatigue resistance, the cyclic thermal shock system consisted of induction heating and water spray quenching unit was constructed and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Thermal stress depending on test temperature was also simulated by FEM. The TNHT specimen showed lower Lm value than as-heat treated specimen but, in the case of maximum and average crack length, the TNHT specimen exhibited higher value than those of as-heat treated specimen. This means that the small number of large cracks were initiated and propagated selectively in the TNHT specimen. This result can be caused by two contrary effects of diffusion layer, those are introduction of the residual compressive stress good for mitigation of thermal tensile stress and very high surface hardness harmful for crack initiation. However, Lm value of salt-bath nitriding specimen was very high due to the white layer in spite of the existence of diffusion layer.

Abstract: The tensile damage evolution of 2D plain woven C/SiC composites strengthened with 1K and 3K carbon fiber bundles and microstructure’s influence on material’s damage evolution were investigated using the Acoustic Emission technology (AE) and failure observation. Experimental results reveal that damage evolution of these two kinds of composites is a gradual procedure and this procedure consists of three phases. There is no damage during the first phase. During the second phase, the damage, mainly consisting of matrix microcrack cracking, interface debonding of fiber and joining of microcrack, random takes place in the whole area of specimen. During the third damage phase, the damage, mainly consisting of macrocrack cracking, fibers breaking and fibers pulling out, mainly takes place in the local failure area of specimen. Because the microstructures of composites with 1K and 3K carbon fiber bundles are different, their damage mechanisms are different. Composite strengthened with 1K carbon fiber bundles get in second phase at 90% failure stress, and their main energy dissipation occurred during the second damage phase. While Composite strengthened with 3K carbon fiber bundles get in second phase at 80% failure stress, and their main energy dissipation occurred during the third damage phase.

Abstract: The Effect of die surface modification on the physiochemical melt-out phenomenon was investigated. To measure the melt-out resistance more accurately, the mean depth measurement method after immersion in molten Al-alloy was proposed instead of the conventional weight change method. The validity of the mean depth method was verified by the comparison with the field service test of core-pin. The several kind of nitriding, such as gas nitriding, ion nitriding, salt-bath nitriding and TNHT
process of PHILOS TECHNOLOGIES, INC were employed to change surface condition and their effects on melt out resistance were examined by the immersion test and the field service test of core pins. The melt-out depth was decreased with the nitriding treatments and in case of ion-nitrided specimen showed 2.8 times lower dissolution depth than as heat treated specimen. The white compound layer play a important role in the prevention of physiochemical reaction, therefore, the TNHT process, which does not form the white layer, was less effective in the view point of melt-out phenomenon only. The field test result using core-pins for producing automobile engine parts showed similar tendency with the mean depth method and this implies that the mean depth method reflects the melt-out resistance of the modified surfaces effectively.