Development Of Surface Tension Measurement System With Surface Laser Light Scattering Method

Surface tension is a thermophysical property of fluids, and is used in many engineering applications such as thermal and chemical engineering etc. There are many ways to investigate it, but because of the limits of theoretical and half-empirical ways, the reliable data of surface tension come from experiments.Recently, as the progress of laser technology, it is intruduced in measurement field more and more. It is same that more and more scholars apply the laser technology to the measurement of thermophysical properties. In the surface tension measurement, surface laser light scattering method (SLLS) was noticed more and more. The surface tension measurement system with surface laser light scattering method was developed and the surface tensions of some new alternative clean fuels were measured using the system. Based on the experiment, a prediction method of surface tension was provided in this paper. The following is the main works of this paper.1. Firstly, the SLLS model was introduced in details, and the affection elements were analyzed too.The experiment system was developed, in which a wedge prism was choosen as beamsplitter through the theoretical calculation. Based on the optical heterodyne detection technique, a new optical system was designed, in which the interference intensity was increased. Compared with other researches, the laser power used in this paper was lower (20mW). To increase the S/N ratio, the average method was used to deal signal. Through experiment, the best sweep time was determined to be 12ms, the best number of the average times was determined to be 600, and the sigle measurement time is about 7s. The affection of the vessel dimension was analyzed too, and the diameter of the vessel was determined to be 106mm. The surface tension of water and ethanol was measured using this system, and the deviations was less than±2%.2. The surface tensions of six mixtures of diesel oil/dimethoxymethane (C₃H₈O₂), diesel oil/dimethyl carbonate (C₃H₆O₃), diesel oil/ethanol (C₂H₆O₂), gasoline/dimet- hoxymethane, gasoline/dimethyl carbonate and gasoline/ethanol were measured with surface laser light scattering method (SLLS), and 126 surface tension data were obtained. The excess surface tensions were calculated and the surface tension data were correlated as a function of compositions, and the standard diviations were less than 0.097mN·m^-1.3. The surface tension of saturated dimethyl carbonate has been firstly measured with a differential capillary rise method (DCRM). Thirty-one surface tension data have been obtained in the temperature range from 282 to 371K. The uncertainties of temperature were less than±20mK (ITS-90). The uncertainties of surface tension measurements were estimated to be less than±2%. The results were correlated as the van der walls type and polynominal functions of temperature, and the average absolute deviation were 0.0776 and 0.0270mN·m^-1, respectively.4. A generalized corresponding-states model based on two reference fluids was developed for the prediction of surface tensions for non-polar, weakly polar pure fluids and their binary mixtures. Four parameters, Pc, Tc, Vc andω, were used in this model, and the acentric factorωwas used as scaling parameter; the other three parameters were used to calculate the corresponding surface tensions. Methane and n-octane were chosen as reference fluids in the calculation of pure fluids, and in the calculation of binary mixtures, the pure components were chosen as reference fluids. This model has been tested on 985 surface tension data points of 69 pure substances and 20 binary mixtures; the average absolute deviations are 0.28mN·m^-1 and 0.20 mN·m^-1, respectively. The results indicated that, the predictions of this model were in good agreement with experimental data. Especially for binery mixtures, the results of this model are better than the other two corresponding models. In addition, the calculated deviation would increase with the excess surface tension rising, and if excess surface tensions were less than 3mN·m^-1, the deviations were less than 0.5mN·m^-1.