Browsing by Author "Mardolcar, Umesh V."
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- O Produto Viscosidade Densidade em Misturas Nafta+Dióxido de Carbono em Condições Supercríticas.Publication . Mardolcar, Umesh V.; Fialho, Paulo; Santos, F. J.; Nieto de Castro, Carlos A.Efectuou-se um estudo de viabilidade da medida do produto viscosidade (n) densidade (p) de misturas NAFTA+CO2 em condições supercríticas do dióxido de carbono com um viscosimetro de cristal de quartzo em vibração torsional, para uma gama de temperaturas entre (40º-60ºC) e pressões até 12MPa. Foram efectuadas medidas para o CO2 puro e a mistura NAFTA+CO2 a 40ºC e a pressões que atingiram os 9MPa. Foi assim possível tirar algumas conclusões acerca do sistema de trabalho, NAFTA+CO2, que sugerem modificações da célula de medida, tendo-se provado no entanto que o método de medida aplicado, sob a forma relativa, é um bom método para medir a viscosidade da mistura, desde que a viscosidade seja conhecida.
- Thermophysical properties of alkali metal vapours. Part II - assessment of experimental data on thermal conductivity and viscosityPublication . Fialho, Paulo; Ramires, Maria de Lurdes V.; Nieto de Castro, Carlos A.; Fareleira, João M.N.A.; Mardolcar, Umesh V.An analysis of the available data on the viscosity and thermal conductivity coefficients of the alkali metal vapours is presented. The analysis is based upon theoretical calculations of the properties of the monatomic systems, described in the preceding parts I and I.A of the present paper, and making use of the kinetic theory of a binary gas reacting mixture. A summary of the measurement techniques used, including, whenever possible, the magnitude of the corrections performed on the raw data and the nominal precision of the results, complements the analysis. It is concluded that it is possible to select precise and mutually consistent sets of experimental results for both viscosity and thermal conductivity coefficients of lithium, sodium and potassium vapours among the available literature data and to classify all the data in uncertainty classes. Regarding rubidium and cesium no systematic discussion is possible due to a lack of experimental data with sufficient precision.
- Thermophysical Properties of Alternative Refrigerants.Publication . Gurova, A. N.; Barão, M. T.; Sousa, Ana T.; Nieto de Castro, Carlos A.; Mardolcar, Umesh V.; Fialho, PauloThe need for new environrnentally acceptable refrigerants, with low or zero ozone depletion potential and low global warming potential, led the industrial and the scientific community to launch a significant research effort on the thermophysical properties of these compounds. It is the purpose of this paper to give an overall view of the current research in this field, started in 1990 by our group, in collaboration with the LIMHPCNRS, Villetaneuse, France, and the NIST, Boulder, USA. The effort has been concentrated in developing equipments to measure density dielectric constant and thermal conductivity in the liquid phase, covering the temperature and pressure ranges necessary for the normal duty of these fluids. The dielectric constant and the thermal conductivity of three provisional replacements, HCFC 123, HCFC 141b, HCFC 142b and two c1ass A fluids, HFC 134a and HFC 152a, have been measured in the temperature range 200 to 300 K and pressures up to 20 MPa, using a direct capacitance method, by measuring the capacitance of a cylinder filled with the sample and in vacuum, using an impedance analyzer, and the polarized hot wire technique. The density of HCFC 142b, HCFC 22 + HCFC 142b and HCFC 14lb was measured with a vibrating fork densimeter, calibrated with toluene, 2,2,4-trimethylpentane and vacuum. The instruments are capable of obtaining the dielectric constant with an absolute uncertainty of 0.1 %, the thermal conductivity with 0.5 % and the density with 0.1 %. An Hard Sphere De Santis equation of state has been used to develop an universal estimation scheme for the density of pure refrigerants and of their mixtures, based only upon the properties of pure fluids, capable of calculating the density of the liquid phase with an uncertainty better than 1.5 % for T/Tc < 0.9. The density dependence of the dielectric constant has been studied using the concept of Eulerian strain and the Onsager-Kirkwood theories. The dipole moments of the several refrigerants in the liquid phase were obtained.