| Along with the rapid development of global economy, energy shortage and environment pollution are getting more and more serious. The implementation of heat metering is an important measure for avoiding energy waste and reducing polluted emission. As the core of heat metering, heat meter must meet the measure heat accuracy according to the "Technical specifications for heat metering of district heating system" (Ministry of Construction). Due to the poor water quality in China, impurities are the main factor that influences the measurement precision of heat meter. The change of flow field caused by impurities, the sound attenuation caused by the particles on the propagation path and the deposition of impurities on the reflecting surface will reduce the measurement accuracy of the ultrasonic heat meter. In this paper, the propagation rules of ultrasonic wave in water containing impurities and deposition of impurities are studied, the flow characteristics of water flow containing impurities are obtained and effects of impurities on the measurement accuracy of ultrasonic heat meter are discussed. The researches in this paper have important scientific significance and practical value.Acoustic wave propagation in the water containing impurities involves the coupling of sound field, flow field, solid phase and other physical fields, which lead to a complicated acoustic propagation. In this paper, theoretical analysis,experimental study and numerical simulation are used to study the flow characteristics of the impurity flow in the ultrasonic heat meter with different structures. The theoretical analysis is used to get attenuation characteristics caused by impurities. The standard experiment setup is built to determine the measurement accuracy while water containing impurities is used to verify the validity of two-phase flow turbulent models. On this basis, the numerical simulation method is used to obtain impurity distribution properties in heat meter and the effect of impurities on ultrasonic propagation. The main works of this paper are listed as follows:Firstly, the propagation of acoustic wave in the water containing impurities is studied by using theoretical analysis. Based on the velocity distribution.temperature change, flow disturbance, noise and detection methods, the influence factors of measurement accuracy in water are given. The attenuation mechanism of acoustic propagating in water containing impurities is analyzed. The absorption,scattering and diffusion of ultrasonic wave are combined with the effects of the deposition of impurities on the reflection of ultrasonic waves. The effect of impurities on measurement accuracy of ultrasonic heat meter are analyzed systematically.Secondly, the measurement performances of ultrasonic heat meter are experimental studied with water containing impurities as the working fluid. The tap water, water in heat-supply system and suspensions of different mass concentration calcium carbonate and yellow mud are selected as water quality models. Experiments are carried out to test measurement errors when U-type heat meter measure different water model at different flow rate. The impact law of different kinds of impurities with different concentrations on measurement errors is studied and waveform display of oscilloscope is recorded.Thirdly, the numerical simulation of the flow field in the ultrasonic heat meter is carried out. Two-phase flow model is used to study internal flow field characteristics and impurities distribution in the ultrasonic reflection path and reflecting surface of ultrasonic heat meter. The standard k-s model is used to calculate the turbulent characteristics of the flow field and the discrete phase model is used to track solid particles. The experimental results are compared with the numerical simulation results to verify the correctness of the two-phase flow model used in this paper. The line-averaged velocity and area-averaged velocity are obtained directly by numerical simulation. The k coefficient of ultrasonic heat meter is also extracted based on the numerical results. The effects of particle of different size with different flow rates on k coefficient were compared. The particle mass concentration on the propagation path, the particle concentration changes with flow rate are obtained by numerical simulation and the distributions of impurities in different flow rate are gotten.Finally, the flow filed of V-type ultrasonic heat meter is also be studied. Due to the reflection surface is horizontal, impurities are easy to deposit on it. A vortex generator locates in front of reflection region which can generator stronger vortex to remove impurities and clean reflection surface. The comparison of flow field between before and after the installation showed that the vortex generator can effectively remove impurities near the reflection area.In the paper, the numerical computation method is decided for flow containing impurities in ultrasonic heat meter small body. The results of the theoretical analysis have been proved by experiments. Through the analysis of the numerical simulation results, the flow characteristics of U-type ultrasonic heat meter in the water with different impurities are observed; the influences ofthe different concentrations of particle and the size of the impurities on the measurement accuracy in different flow are analyzed; U-type ultrasonic heat table errors inherent mechanisms caused by the variation of flow coefficient k and the sound attenuation induced by impurity distribution are given; the vortex generators is installed in front of the reflection area of V-type ultrasonic heat meter.The results of the numerical simulation show that the vortex generators have obvious effect on the deposition of impurities under 1 ?m.This study can provide theoretical basis for improving the detection accuracy of ultrasonic heat meter and optimization of heat meter cavity body, which has certain theoretical significance and practical application value. |
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