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^{1}Xi'an Jiaotong University, P.R. China

**Keywords:** dynamic light scattering, photon-correlation spectroscopy, spectrum analyzing technology**property:** thermal conductivity coefficient**material:** benzene and toluene

Thermal conductivity coefficient of fluid is an indispensable thermophysical property in the design and operating of the thermodynamic cycle which is related to heat transfer process, especially, in the design of low- and medium-temperature thermodynamic cycle (organic Rankine cycle, heat pumps and refrigeration processes). The literatures show that the experimental data of thermal conductivity coefficients of many new substitute working fluids and the working fluid additive are very lacking. So, it became more and more necessary to obtain the accurate fluid thermal conductivity coefficient by the experimental measurement or theoretical prediction. The traditional methods to measure the thermal conductivity coefficient mostly need to establish temperature gradients inside of the sample by the method of local heating [1]. The most commonly used methods are the steady-state plate method and the transient hot-wire method, and The measured deviations of this two methods are approximate 3% and 1.5%, respectively [2]. Since the convection and radiation effect increase with the temperature increasing, the traditional measuring methods of fluid thermal conductivity are difficult to obtain the accurate experimental data in the high temperature region [3]. As an effective experimental method for measuring the fluid thermal coefficient, dynamic light scattering method can significantly reduce the measurement error derived from the convection and radiation effect in the high temperature region [4]. In this paper, the dynamic light scattering system for measuring fluid thermal conductivity coefficient was established and presented in detail. Two data processing methods of light-scattering information (time-dependent photon correlation spectroscopy and frequency-dependent spectrum analyzing technology) were presented and compared. Then the uncertainty of the key component of the experimental system were analyzed, the uncertainty for the scattering angle is estimated to be not greater than ±0.22%, the uncertainty caused by the dark current and the afterpulse of the photomultiplier is estimated to be not greater than ±0.1%, the uncertainty in the determination of the decay time is estimated to be not greater than ±0.4% and the uncertainty in the measurement of the linewidth of the spectrum is ±0.2%. Acetone, benzene and toluene were selected as the standard substance and their thermal conductivity coefficient were measured at temperature ranging from (233.15 to 473.15) K by investigation of the line width of the central scattering spectrum component and the decay time of entropy fluctuation process. It is verified that the accuracy of thermal conductivity coefficient from this system can be kept within 0.5-1.5%.

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- B. Kruppa, J. Straub, Exp. Therm. Fluid Sci. 6, 1 (1993)

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