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Foster Kwame Kholi,Alberto Mucci,Hariharan Kallath,하만영,Jason Chetwynd-Chatwin,민준기 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.6
Previous correlations that have been used to predict the heat transfer performance of pulsating heat pipes (PHPs) offer limited thermal predictions within a narrow range of fluid-filling ratios and PHP inclinations. In this paper, a novel semi-empirical correlation with improved scope is proposed, with an increased range of fluid-filling ratios and PHP inclinations. The proposed correlation employs the dimensionless numbers governing the thermohydrodynamic operation of PHPs, and achieved ±30 % accuracy when predicting selected experimental data, showing reasonably good agreement. Unlike previous correlations, the new correlation can be used for different working fluids, geometrical aspect ratios, and heat loads. A comprehensive assessment of the relative significance of the correlation parameters on the total heat transfer performance is discussed. The new correlation with its flexible application range is expected to assist in faster and more enhanced thermal predictions as interest in PHPs grows.
Experimental study of effects of wicks and boundary conditions on thermal performance of heat pipes
Foster Kwame Kholi,Hariharan Kallath,Alberto Mucci,Man Yeong Ha,Jason Chetwynd-Chatwin,민준기 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.1
The inherent two-phase heat transport of heat pipes (HPs) is progressively being examined for potential uses. These thermal devices are affected by many operating factors, prompting this study to investigate the effects of different types of wicks and working conditions on the time-dependent thermal behavior. Primarily, the effects of different wick performances were investigated under various operating conditions. The resulting surface temperatures depicted in the time to steady performance and the dry-out behavior revealed the conditions to improve the HPs design. The thermal resistance decreased from 0.6 K/W (at 25 W) to 0.05 K/W (at 200 W) by increasing the HP diameter from 6 to 10 mm; these values are relative to those of copper rods, which decrease from 2.70 K/W (at 25 W) to 0.40 K/W (at 200 W). Nonlinear and linear temperature responses were recorded when the HPs diameter and length were varied. Compared to conventional mesh and groove wicks, the composite groovesintered, mesh-sintered, and groove-mesh wicks recorded lower thermal resistance with distinctively faster startup times, lower startup temperatures, better temperature uniformity and less dynamic instability. Tilting the HPs relative to the horizontal position lessens failure tendencies. Usually, dynamic responses are typically first-order under the conditions studied. Hence, proper sizing of HPs and correct wick selection can improve their performance.
Foster Kwame Kholi,Albert Mucci,Mimbo Shim(심민보),J. K. Min(민준기),M. Y Ha(하만영) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11
Heat pipes, HP, are phase change devices with high conductivities compared to existing technologies of the same dimensions. However, for HP with multi-layer wicks, the layers and location of the dense wicks significantly affect the thermo-fluidics of the device. The influence of these factors on the overall performance of HP is presented for a standard mesh-screen wick HP with different layers - uniform and non-uniform wick layers. An in-house code assisted in predicting the limit of HP, which show increased heat transfer limit as the number of layers increase, especially for a mixture of layers of uniform and non-uniform wicks. The heat transfer improves in different inclinations when the dense wicks located in the core of the HP, in contact with the vapour flow. This arrangement restricts the entrainment of liquid from the wick surface. The results have shown that the performance of HP can easily be improved through proper wick arrangement.
Foster Kwame Kholi,Jaehyun Park(박재현),Hyo Je Son(손효제),Hariharan Kallath,Michael Klingsporn,Jason Chetwynd-Chatwin,June Kee Min(민준기) 한국전산유체공학회 2020 한국전산유체공학회지 Vol.25 No.4
This study examined the cooling performance of a fuel-cooled oil cooler (FCOC) under typical operating conditions. The oil cooler can fail due to the congealing of oil in the FCOC core at low-temperature conditions, which could be recovered by de-congealing the oil flow. The de-congealing time can be affected by changes in oil pressure and temperature conditions, flow re-routing, and malfunctioning of the oil bypass valve (OBV). In this study, we proposed a prediction model for the transient oil de-congealing phenomenon in the FCOC. The numerical procedure was based on the one-dimensional (1D) flow and thermal network analysis and the effectiveness (ε)-NTU method. The commercial code was customized to implement the de-congealing phenomena. Pre-defined empirical correlations and property corrections aided the modeling of the detailed de-congealing process. Moreover, the 3D CFD temperature contours provided a visual insight into the internal flow inside the FCOC core during the de-congealing process. An experimental study was conducted in parallel to validate the prediction result. Both experimental and numerical methods showed a loss of cooling when the OBV fails. The oil temperature and pressure, and flow directions affect the de-congealing process. The present model proved useful for various oil cooler configurations that are difficult to investigate experimentally and could quickly provide oil de-congealing results.
박재현,Foster Kwame Kholi,Michael Klingsporn,Jason Chetwynd-Chatwin,하만영,민준기 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.1
In the present study, an improved heat exchanger model is proposed to efficiently analyze the cold oil removal process inside a fuel-cooled oil cooler. When exposed to low temperature condition, oil within a heat exchanger begins to congeal, preventing oil flow and causing loss of cooling. The conventional heat exchanger models, however, shows limits in reflecting the highly varying viscosity effects due to large temperature difference. To overcome this, the conventional porous media model was rewritten with the friction and Colburn j-factors that include the temperature dependent property variation. The property correction method by Shah and Sekulic [31] was added to enhance the prediction accuracy. Also, a relief valve model based on the porous media approximation is developed. The developed models were validated against the experimental data. Transient three-dimensional numerical simulations are carried out to analyze the effects of operating conditions on de-congealing phenomenon inside the FCOC.
Alberto Mucci,Foster Kwame Kholi,Hariharan Kallath,Thierry Sibilli,June Kee Min(민준기) 한국전산유체공학회 2020 한국전산유체공학회지 Vol.25 No.1
Experimental studies on gas turbines face severe difficulties in obtaining an accurate and representative model of the internal environment. High temperature and pressure in the core flow require specific test rigs to guarantee the safety and representativeness of the experiment. Moreover, complexities arise when testing novel designs. Due to increasing concerns about flying gas turbines environmental footprint, the Advisory Council for Aeronautics Research in Europe (ACARE) proposed a reduction in aircraft emissions. From a concept proposed by the NEW Aero Engine Core (NEWAC) grant, we developed a methodology to test a novel Cooled Cooling Air Heat Exchanger (CCAHX) system at real steady-state operative conditions. The method involves the creation of one- and three-dimensional Computational Fluid Dynamics (CFD) models to support the experiment and verify the safety of the testing environment. The study presents the steps required to achieve this test, which involves the correct definition of an ε-NTU method for the one-dimensional model and a correct design of the three-dimensional exchange model. Additionally, the CFD model provides insight into the CCAHX operations, showing the temperature gradient inside the bulk structure and inducing further studies on the unit. The experiment reached the required temperature reduction and induced testing in case of a cooling failure to verify the structural safety of the CCAHX device.
Hariharan Kallath,Foster Kwame Kholi,June Kee Min(민준기) 한국전산유체공학회 2021 한국전산유체공학회지 Vol.26 No.4
The significance of low Reynolds flows increased due to the emergence of low speed flying vehicles. Therefore, the flow control methods deserve more attention. In the present study, the application of a differential temperature on the surfaces of a slotted double-element airfoil is numerically evaluated at a low Reynolds number turbulent flow condition. The main objective is to study the effects of differential temperature on the aerodynamics of the airfoil at various flap positions. Six flap positions were considered utilizing the overset meshing techniques after appropriate parameterization of the flap locations. The numerical results were validated against previous experimental results, and subsequently, the effects of the differential temperature on the aerodynamics of baseline and other configurations were clearly explained. The best configurations were identified with and without the presence of differential temperature. Moreover, it was found out that among all the considered configurations, a maximum of 4% increase in lift and 5% reduction in drag can be achieved through the application.