TRANSCRITICAL CO2 REFRIGERATION SYSTEMS COMPARISON WITH CONVENTIONAL SOLUTIONS AND APPLICATIONS

JAHAR SARKAR 대한설비공학회 2012 International Journal Of Air-Conditioning and Refr Vol.20 No.4

Carbon dioxide ($CO2) is one of the natural refrigerants which can be used as working fluid in various refrigeration applications along with the ammonia and hydrocarbons due to its eco-friendliness, higher volumetric capacity, good heat transfer properties, etc. The present article consists of two parts: A detailed comparative study of $CO2-based transcritical refrigeration systems with conventional refrigerants-based systems in terms of both thermodynamic and heat transfer performances, and review of both theoretical and experimental researches on transcritical $CO2 vapor compression cycle for various refrigeration applications including commercial product status. Suitability of the $CO2 system in specific refrigeration application is also discussed.

Extended Exergy Analysis Based Comparison of Subcritical and Transcritical Refrigeration Systems

JAHAR SARKAR,Dnyanesh Joshi 대한설비공학회 2016 International Journal of Air-Conditioning and Refr Vol.24 No.2

The main purpose of this study is to apply advanced exergy analysis to the transcritical CO2 vapor compression refrigeration system, and compare with the analysis of subcritical cycle using ammonia and R404a. Endogenous, exogenous, avoidable and unavoidable exergy destructions are determined for each component of these systems. For CO2 system, compressor contributes highest avoidable endogenous exergy destruction and gas cooler contributes highest avoidable exogenous exergy destruction. It is concluded that compressor is the ¯rst component for CO2 and R404a, and evaporator is the ¯rst component for NH3 to be improved. System improvement options to reduce the exergy destruction are discussed as well.

Energy-Economic Analysis of Plate Evaporator using Brine-based Hybrid Nanofluids as Secondary Refrigerant

Atul Bhattad,JAHAR SARKAR,Pradyumna Ghosh 대한설비공학회 2018 International Journal Of Air-Conditioning and Refr Vol.26 No.1

Energy and economic analyses of corrugated plate evaporator have been performed in this study by using brine-based hybrid nanofluids as secondary refrigerant for low temperature applications (milk chilling, ice making and fish freezing). Various hybrid nanofluids containing alumina with different nanoparticles (copper, silver and multi walled carbon nanotube) dispersed in ethylene glycol/water, propylene glycol/water, calcium chloride/water and potassium acetate/water brines have been considered. A comparison has been made based on overall heat transfer coefficient, evaporator size (heat transfer area), pump work, COP, annual cost and payback period. The maximum reduction in heat transfer area has been observed for PG brine-based alumina–silver hybrid nanofluid. Whereas, the maximum reduction in pump work and augmentation in COP have been obtained for PG brine-based alumina–MWCNT hybrid nanofluid. EG brine-based Alumina–Cu hybrid nanofluid yields minimum annual cost. Most of the nanofluids (except PG brine-based Al2O3–Ag nanofluid) seem to be not beneficial for practical use due to comparatively long payback period; however, that can be significantly reduced in future by decreasing nanoparticle cost and increasing nanofluid stability. The present study reveals that the brine-based hybrid nanofluids may be the potential option as next generation secondary refrigerants for low temperature applications.

Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability

Lee, Min-Jae,Kang, Shin-Hyun,Dey, Jahar,Choi, Sung-Min American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.26

<P>A method to fabricate porous silica-coated Au sponges that show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG), which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges, which contain hierarchically connected micro- and mesopores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH<SUB>4</SUB> to 4-aminophenol as high as 93% even after 10 catalytic cycles. The method may be applicable for other porous metals, which are of great interests for catalyst, fuel cell, and sensor applications.</P> [FIG OMISSION]</BR>

Ongoing angiogenesis in blood vessels of the abdominal aortic aneurysm

David C. Paik,Chenzhong Fu,Jahar Bhattacharya,M. David Tilson 생화학분자생물학회 2004 Experimental and molecular medicine Vol.36 No.6

Pathogenesis of the abdominal aortic aneurysm has been atributed to neovascularization of the aortic wall. However, it is not clear whether angiogenesis persists in the aneurysm. In sec-tions of aneurysms, we determined the imuno-histochemical distributions of the αvβ3 integrin, tenascin and endothelial nitric oxide synthase (eNOS), which are markers respectively, of an-giogenesis, matrix remodeling and vasoregula-tory function. In addition, we used reverse trans-cription folowed by in situ PCR, to determine the distribution of αv mRNA. Al aneurysm spec-imens exhibited extensive increases of wal vas-cularization as compared with the control aortic wall, and showed the presence of perivascular inflamatory exudates containing macrophages and lymphocytes. The neovascularization con-sisted of thick-waled vessels in the media and majority of vessels stained positively for the αvβ3 antigen and eNOS. Tenascin was deposited as bands that circumscribed thick-waled ves-sels. The distribution of av mRNA was extensive and was positive even in those vessels that failed to stain for the αvβ3 protein. No staining was evident in control aortas for the αvβ3 anti-gen, tenascin or αv mRNA. The upregulation of αv αvβ3 integrin in blod vessels surounded by a matrix expresing tenascin, indicates that angiogenesis is an ongoing pro-cess in the mature aortic aneurysm.

Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions

Ha, Jae-Min,Lim, Sung-Hwan,Dey, Jahar,Lee, Sang-Jo,Lee, Min-Jae,Kang, Shin-Hyun,Jin, Kyeong Sik,Choi, Sung-Min American Chemical Society 2019 Nano letters Vol.19 No.4

<P>Nanoparticle superlattices (NPSLs) are of great interest as materials with designed emerging properties depending on the lattice symmetry as well as composition. The symmetry transition of NPSLs depending on environmental conditions can be an excellent ground for making new stimuli-responsive functional materials. Here, we report a spherical micelle-assisted method to form exceptionally ordered NPSLs which are inherently sensitive to environmental conditions. Upon mixing functionalized gold nanoparticles (AuNPs) with a nonionic surfactant spherical micellar solution, NPSLs of different symmetries such as NaZn<SUB>13</SUB>, MgZn<SUB>2</SUB>, and AlB<SUB>2</SUB>-type are formed depending on the size ratio between micelles and functionalized AuNPs and composition. The NPSLs formed by the spherical micelle-assisted method show thermally reversible order-order (NaZn<SUB>13</SUB>-AlB<SUB>2</SUB>) and order-disorder (MgZn<SUB>2</SUB>-isotropic) symmetry transitions, which are consistent with the Gibbs free energy calculations for binary hard-sphere model. This approach may open up new possibilities for NPSLs as stimuli-responsive functional materials.</P> [FIG OMISSION]</BR>

Seedless Synthesis of Monodisperse Cuboctahedral Gold Nanoparticles with Tunable Sizes

Umar, Aminah,Lee, Jeeun,Dey, Jahar,Choi, Sung-Min American Chemical Society 2016 Chemistry of materials Vol.28 No.14

<P>Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA(+), the surface self diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.</P>