• KSTLE International Journal
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• v.4 no.2
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• pp.60-65
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• 2003

Tribological properties of novolac resin composites containing particulate barite (BaSO$_4$) or potassium titanate (K$_2$Oㆍ6(TiO$_2$))whiskers (two typical space fillers for commercial automotive brake linings) were investigated. The emphasis of the current investigation was given to the effect of the two fillers on the propensity of the stick-slip phenomena and formation of stable rubbing surface. A block-on-disk type tribometer was used for friction assessment. Results showed that the BaSO$_4$-filled composite produced large friction force oscillations at slow sliding speeds and created severe damage on the gray iron counter surface. On the other hand, the composite with $K_2$Oㆍ6(TiO$_2$) whiskers formed a stable rubbing surface and showed smooth sliding behavior without large friction force fluctuation. The microscopic observation of the rubbing surface revealed that the $K_2$Oㆍ6(TiO$_2$)whiskers played a key role in the formation of stable rubbing surface and smooth sliding behavior by effectively reinforcing the resin.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.11
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• pp.1527-1537
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• 1997

Extinction characteristics and acoustic response of hydrogen-air diffusion flames at various pressures are numerically studied by employing counterflow diffusion flame as a model flamelet in turbulent flames in combustion chambers. The numerical results show that extinction strain rate increases linearly with pressure and then decreases, and increases again at high pressures. Thus, flames are classified into three pressure regimes. Such nonmonotonic behavior is caused by the change in chemical kinetic behavior as pressure rises. The investigation of acoustic-pressure response in each regime, for better understanding of combustion instability, shows different characteristics depending on pressure. At low pressures, pressure-rise causes the increase in flame temperature and chain branching/recombination reaction rates, resulting in increased heat release. Therefore, amplification in pressure oscillation is predicted. Similar phenomena are predicted at high pressures. At moderate pressures, weak amplification is predicted since flame temperature and chain branching reaction rate decreases as pressure rises. This acoustic response can be predicted properly only with detailed chemistry or proper reduced chemistry.

• Nuclear Engineering and Technology
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• v.33 no.6
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• pp.638-651
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• 2001

The present study is to assess the applicability of the best-estimate thermal-hydraulic code, MARS 1.4, for the analysis of thermal-hydraulic behavior in PWRs during natural circulation conditions. The code simulates a natural circulation test, BETHSY test 4. la, which was conducted on the integral test facility of BETHSY. The test represented the cooling states of the primary cooling system under single-phase natural circulation, two-phase natural circulation and the reflux condensation mode with conditions corresponding to the residual power, 2% of the rated core power value and 6.8 MPa at the secondary system. Based on MARS 1.4 calculations, the major thermal-hydraulic behaviors during natural circulation are evaluated and the differences between the experimental data and calculated results are identified. The calculated results show generally good behavior with regard to the experimental results; the region of single-phase natural circulation is 100-92% of the initial mass inventory, two-phase natural circulation is 84-63 %, and the reflux condensation mode occurred below 58 %. U-tubes empty and the core uncovery are obtained at 39 % and 34 % of the initial mass inventory, respectively.

• Nuclear Engineering and Technology
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• v.16 no.2
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• pp.47-57
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• 1984

Mathematically-rigorous time-volume averaged conservation equations were simplified to established the differential equations of THERMIT-6S, which is a two-fluid 3-D code. The difference equations of THERMIT-6S were obtained by discretizing the proceeding set of differential equations. The spatial discretization is characterized by a first-order spatial scheme, donor cell method, and staggered mesh layout. For time discretization, a first order semi-implicit scheme treats implictly sonic terms and terms relating to local transport phenomena and explicitly convective terms. The results were linearized by the Newton-Raphson method. In order to construct the reduced pressure equation, the linearized equations were manipulated so that all variables are coupled between mesh cells through only the pressure variable. By simulating numerically the OPERA-15 experiment, it was found that THERMIT-6S is a very powerful code in predicting reactor behavior after sodium boiling including flow coastdown, reversal flow and flow oscillation.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.612-616
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• 2017

An experimental study investigates the flame response characteristics of non-premixed flame and premixed flame. Air was used as the oxidant. Hydrogen($H_2$)/methane($CH_4$) was used as the fuel, and the mixing ratio of the fuel was 50/50%. Flame response characteristics for various velocity perturbations were experimented. The flame images was acquired using the OH fluorescence measurement and the images were digitized using MatLab code. The results of the premixed flame show that flame perturbation increases as the oscillation amplitude increases. As the amplitude increases, the gain value of the flame transfer function is observed to be a linear behavior. The flame length of a nonpremixed flame decreases as the oscillation amplitude increases. Also, it was confirmed that the gain value according to the amplitude behaves nonlinearly.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.10
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• pp.720-727
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• 2019

Resonating thermal lags of solid fuel with heat transfer oscillations generated by boundary layer oscillation is the primary mechanism of the occurrence of the LFI (Low Frequency Combustion Instability) in hybrid rocket combustion. This study was experimentally attempted to confirm that how the boundary layer was perturbed and led to the LFI. Special attention was also made on oxidizer swirl injection to investigate the contribution to combustion stabilization. Also the overall behavior of fluctuating boundary layer flow and the occurrence of the LFI was monitored as swirl intensity increased. Fluctuating boundary layer was successfully monitored by the captured image and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p', q' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decreased. Thus, results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p', q' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel.

• Smart Structures and Systems
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• v.24 no.3
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• pp.303-317
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• 2019

Magneto-rheological fluid (MRF) rotatory dampers are normally used for controlling the constant rotation of machines and engines. In this research, such a device is proposed to act as variable stiffness device to alleviate the rotational oscillation existing in the many engineering applications, such as motor. Under such thought, the main purpose of this work is to characterize the nonlinear torque-angular displacement/angular velocity responses of an MRF based variable stiffness device in oscillatory motion. A rotational hysteresis model, consisting of a rotatory spring, a rotatory viscous damping element and an error function-based hysteresis element, is proposed, which is capable of describing the unique dynamical characteristics of this smart device. To estimate the optimal model parameters, a modified whale optimization algorithm (MWOA) is employed on the captured experimental data of torque, angular displacement and angular velocity under various excitation conditions. In MWOA, a nonlinear algorithm parameter updating mechanism is adopted to replace the traditional linear one, enhancing the global search ability initially and the local search ability at the later stage of the algorithm evolution. Additionally, the immune operation is introduced in the whale individual selection, improving the identification accuracy of solution. Finally, the dynamic testing results are used to validate the performance of the proposed model and the effectiveness of the proposed optimization algorithm.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.50 no.5
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• pp.339-347
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• 2022

The post chamber in hybrid rocket is installed to induce additional increase in combustion enthalpy by allowing continuous burning of the liquefied fuels. When paraffin wax fuel is used, unsteady pressure oscillations are observed only at the beginning of combustion. This study intends to investigate the effect of additional combustion of liquefied fuel droplets on the occurrence of unsteady pressure fluctuations. For this, the combustion in post-chamber was visualized and image analysis using POD(Proper Orthogonal Decomposition) technique was performed. In addition, the hypothesis was proposed on the occurrence of unsteady pressure oscillations by identifying the modes including the behavior of droplets through mode reconstruction. Conducting a series of combustion tests, the amount of liquefied fuel flowing into the post chamber and the generation of fuel droplets were controlled. Also, the changes in frequency characteristic of unsteady pressure oscillation were monitored. As a result, the unsteady pressure oscillations observed in paraffin wax combustion were the result of additional combustion of fuel droplets generated in the post chamber.

• Journal of Mechanical Science and Technology
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• v.15 no.4
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• pp.510-521
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• 2001

A nonlinear acoustic instability of subcritical liquid-oxygen droplet flames burning in gaseous hydrogen environment are investigated numerically. Emphases are focused on the effects of finite-rate kinetics by employing a detailed hydrogen-oxygen chemistry and of the phase change of liquid oxygen. Results show that if nonlinear harmonic pressure oscillations are imposed, larger flame responses occur during the period that the pressure passes its temporal minimum, at which point flames are closer to extinction condition. Consequently, the flame response function, normalized during one cycle of pressure oscillation, increases nonlinearly with the amplitude of pressure perturbation. This nonlinear response behavior can be explained as a possible mechanism to produce the threshold phenomena for acoustic instability, often observed during rocket-engine tests.

• Journal of Institute of Control, Robotics and Systems
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• v.15 no.3
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• pp.258-264
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• 2009

This paper presents two control algorithms for the frequency and amplitude of the resonator of a micro sensor. One algorithm excites the resonator at its a priori unknown resonant frequency, and the other algorithm alters the resonator dynamics to place the resonant frequency at a fixed frequency, chosen by the designer. Both algorithms maintain a specified amplitude of oscillations. The control system behavior is analyzed using an averaging method, and a quantitative criterion is provided for the selecting the control gain to achieve stability. Tracking and estimation accuracy of the natural frequency under the presence of measurement noise is also analyzed. The proposed control algorithms are applied to the MEMS dual-mass gyroscope without mechanical connecting beam between two proof-masses. Simulation results show the effectiveness of the proposed control algorithms which guarantee the proof-masses of the gyroscope to move in opposite directions with the same resonant frequency and oscillation amplitude.