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Park, Hyunwook,Lee, Sumi,Kim, Jong-Man ROYAL SOCIETY OF CHEMISTRY 2007 PHOTOCHEMICAL AND PHOTOBIOLOGICAL SCIENCES Vol.6 No.10
<P>Photopolymerization of a diacetylene monomer having terminal pyrene groups afforded formation of polydiacetylene nanoparticles in aqueous solvent along with fluorescence quenching of pyrene moieties.</P> <P>Graphic Abstract</P><P>Photopolymerization of a diacetylene monomer having terminal pyrene groups was investigated to probe the molecular stacking of the diacetylene monomer in aqueous solvent. UV-irradiation of a monomer suspension afforded formation of polydiacetylene nanoparticles, confirming certain amounts of intermolecular Type B packing. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b702466e'> </P>
Park, Hyunwook,Pan, Xiaomin,Lee, Changhoon,Choi, Jung-Il Elsevier 2016 Journal of computational physics Vol.314 No.-
<P><B>Abstract</B></P> <P>A novel immersed boundary (IB) method based on an implicit direct forcing (IDF) scheme is developed for incompressible viscous flows. The key idea for the present IDF method is to use a block LU decomposition technique in momentum equations with Taylor series expansion to construct the implicit IB forcing in a recurrence form, which imposes more accurate no-slip boundary conditions on the IB surface. To accelerate the IB forcing convergence during the iterative procedure, a pre-conditioner matrix is introduced in the recurrence formulation of the IB forcing. A Jacobi-type parameter is determined in the pre-conditioner matrix by minimizing the Frobenius norm of the matrix function representing the difference between the IB forcing solution matrix and the pre-conditioner matrix. In addition, the pre-conditioning parameter is restricted due to the numerical stability in the recurrence formulation. Consequently, the present pre-conditioned IDF (PIDF) enables accurate calculation of the IB forcing within a few iterations. We perform numerical simulations of two-dimensional flows around a circular cylinder and three-dimensional flows around a sphere for low and moderate Reynolds numbers. The result shows that PIDF yields a better imposition of no-slip boundary conditions on the IB surfaces for low Reynolds number with a fairly larger time step than IB methods with different direct forcing schemes due to the implicit treatment of the diffusion term for determining the IB forcing. Finally, we demonstrate the robustness of the present PIDF scheme by numerical simulations of flow around a circular array of cylinders, flows around a falling sphere, and two sedimenting spheres in gravity.</P>
Park, Hyunwook,Shim, Euijoon,Bae, Choongsik Elsevier 2019 Energy conversion and management Vol.194 No.-
<P><B>Abstract</B></P> <P>Dual-fuel premixed charge compression ignition (DF-PCCI) combustion can achieve low nitrogen oxides (NO<SUB>X</SUB>) and particulate matter (PM) emissions for wide ranges of engine operations. However, the deterioration in thermal efficiency, and hydrocarbon (HC) and carbon monoxide (CO) emissions at low loads were recognized as the barriers for expanding the low-load operating range. In this study, the causes of the barriers were investigated and a mixture preparation strategy was suggested for overcoming the barriers in a natural gas (NG)-diesel DF-PCCI engine. Combustion and energy balance analysis was conducted to evaluate the strategy. Baseline DF-PCCI was determined by combinations of diesel start of injection (SOI) and NG substitution ratio (SR) at low loads from 0.3 to 0.6 MPa indicated mean effective pressure (IMEP). An increase in the homogeneity of a fuel-air mixture in the baseline DF-PCCI effectively reduced the NO<SUB>X</SUB> and PM emissions but increased the HC and CO emissions in each low-load operation. As the engine load was decreased, the formation of an overly-lean mixture intensified the effects of the mixture homogeneity. Therefore, the thermal efficiency, and HC and CO emissions deteriorated at 0.3 MPa IMEP. A mixture stratification strategy was established to increase the local equivalence ratio and reactivity of the fuel-air mixture. The strategy was realized by a retarded diesel SOI, a lower NG SR, and a higher exhaust gas recirculation rate. The strategy increased the degree of constant volume combustion by enhancing the combustion performance. The enhanced combustion reduced the combustion loss, and thus, improved the thermal efficiency. The HC and CO emissions also decreased mainly due to the improved combustion and the reduced mass flow rates of the NG.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Dual-fuel PCCI reduced NO<SUB>X</SUB> and PM emissions by increasing mixture homogeneity. </LI> <LI> The increase of the mixture homogeneity increased HC and CO emissions at low loads. </LI> <LI> Mixture stratification improved the thermal efficiency in dual-fuel PCCI at low loads. </LI> <LI> Mixture stratification effectively reduced the HC and CO emissions at low loads. </LI> </UL> </P>
Park, Hyunwook,Shim, Euijoon,Bae, Choongsik Elsevier 2019 Fuel Vol.235 No.-
<P><B>Abstract</B></P> <P>Dual-fuel premixed charge compression ignition (DF-PCCI) combustion has been demonstrated as a promising solution for simultaneous reduction of nitrogen oxides (NO<SUB>X</SUB>) and particulate matter (PM) emissions in heavy-duty compression ignition engines. The use of natural gas (NG) as the low-reactivity fuel in DF-PCCI combustion can expand the limited range of high load operations owing to the lower reactivity of NG than that of gasoline. However, the lower reactivity of NG results in significant hydrocarbon (HC) and carbon monoxide (CO) emissions at the low load operations. In this study, the mixture formations with and without exhaust gas recirculation (EGR) in NG-diesel DF-PCCI combustion were assessed to reduce the HC and CO emissions as well as to improve the fuel economy at low load operations. Diesel injection timing and NG substitution ratio (SR), which is defined as the proportion of energy stored in NG with respect to the total energy amount, were changed to examine the effects of the mixture formation on the DF-PCCI combustion. The NG SR, which was required to maintain the combustion phasing at a constant crank angle degree (CAD), was increased as the diesel injection timing was retarded in the mixture formation without EGR. The introduction of EGR, in addition to the diesel injection timing and the NG SR, contributed to the favorable mixture formation for the low load operations. The NO<SUB>X</SUB> and PM emissions were lower than the EURO VI limitations in both the mixture formations with and without EGR. When the EGR rate of 50% was applied, the indicated thermal efficiency (ITE) increased compared to the case without EGR. The increased ITE was due to the improved combustion efficiency, the higher peak heat release rate (HRR), and the shorter combustion duration. The HC and CO emissions also decreased significantly with the EGR.</P>