Flow measurement method of complex flow in heat exchange casing

Abstract

Fluid plays an important role in engineering application and the accurate control of the carrier is the premise of energy efficient conversion and management, such as the primary air volume of the boiler, the reactor inlet flowrate, the amount of oil and air intake of internal combustion engine, etc. Therefore, flowmeter plays an important role in energy conversion and management. In this work, a high-low frequency combined with reflection-penetration synchronization flowmeter was designed to measure the flowrate in casing device based on the characteristics of different frequencies of sound. The flowrate calculation method of casing device is given and verified by numerical simulation. The results show that the flowrate of different flow states can be calculated accurately by using the high-low frequency combined with reflection-penetration synchronization method. At the same time, the method can also be used in electromagnetic wave, optical and other fields.

Introduction

With the development of industry, people are gradually aware of the energy crisis [1]. Efficient and clean use of energy is a hot topic in recent years. The conversion and transportation of energy are inseparable from the carrier [2], so the transmission and transportation of the carrier directly determines the conversion and management of energy. For example, the role of the tertiary air in the boiler is to promote the burnout of pulverized coal and improve the energy utilization efficiency. However, too much tertiary air will reduce the temperature and thus reduce the heat exchange efficiency, too little tertiary air will lead to the inadequate combustion of pulverized coal, which will also cause energy waste [3]. In nuclear power plants, in order to ensure the safety under the uncertainty error of main feedwater, high safety margin is usually used. The core safety is ensured by reducing the power level. Flowmeter not only affects the effective of energy, but also relates to the safety of equipment [4].

Casing device is a common heat exchange device, as show in Fig. 1, which is widely used in industrial production and experimental equipment [5]. The casing device is mainly used for heat exchange and transportation of two different working fluids. Therefore, the two working fluids usually have different temperature, different flow rate, and even different flow directions in most cases. And the head transfer coefficient has a closely relationship with the flow velocity of the fluid, so in the application of the casing device, the flow rate is usually adjusted to control the heat exchange and outlet temperature. For the casing once-through steam generator [6,7], the outlet temperature directly affects the safe operation of the turbine. Therefore, the flow rate measurement of casing device can not only provide a reference for improving the heat exchange efficiency, but also ensure the safe operation of subsequent equipment. So it is significant to measure the flow of the casing device without affecting the flow field.

At present, there are many types of flow meters used for flow measurement in different occasions, such as rotameter, electromagnetic flowmeter, mass flowmeter, ultrasonic flowmeter, etc. Chen et al. [8] designed a high precision magnetoresistive rotameter based on the detection mechanism of Quadrupole magnet. Yang et al. [9] used the electromagnetic flowmeter and the image processing to measure vertical gas-liquid two-phase flow. Yang et al. [10] improved a huge-scale capacitance mass flowmeter to measure the gas/solid two-phase flow rate in rectangular vertical tubeline. Although these flowmeters improve the application scope of flowmeter, there is a common disadvantage that the existing tubelines need to be modified, or even destroy the original flow field to increase a certain flow resistance. In addition, the particularity of the casing device as show in Fig. 1, the rotameter and mass flowmeter cannot be installed, and the electromagnetic flowmeter is limited by the fluid and tube material. Therefore, ultrasonic flowmeter becomes the best way to measure the flow of casing device.

Ultrasonic flowmeter is a kind of non-contact flowmeter, which uses velocity superposition principle to measure carrier velocity [11]. The advantage of ultrasonic flowmeter is that it does not damage the flow field, does not make special modifications to the tubeline, and almost no additional flow resistance [12]. A large number of scholars have studied the ultrasonic flowrate. Chen et al. [13] improved a multipath ultrasonic gas flowmeter based on transit-time technique. The result shows that the synchronization of transducers can eliminate the influence caused by the inherent switch time in simple chord flowmeter. Then, Chen and Wu [14] analyzed the inherent error of ultrasonic flowmeter in measuring hydrogen flow by numerical simulation. Du and Liu [15] analyzed the factors that affect the ultrasonic flowmeter based on the study of time difference ultrasonic flowmeter, and the relationship between the factors and flowmeter coefficient is given, which provides a reference for the field calibration of the flowmeter. Hao et al. [16] analyzed the influence of bubbles in water on ultrasonic flow measurement by experiment, and pointed out that the error can be reduced to about 2% by appropriate correction. Zheng et al. [17] optimized the integration method of ultrasonic flow in the measurement of non-ideal fluid by numerical analysis based on Gauss quadrature and carried out experimental verification.

Recent years, with the development of ultrasonic technology, ultrasonic flowmeter has more comprehensive functions. Ultrasonic heat meter is a device that combines ultrasonic flowmeter with temperature sensor to calculate the heat flow of fluid. Shi et al. [18] analyzed the influence of impurity type and concentration on ultrasonic heat meter by flow test of calcium carbonate and yellow mud. Shi's work provided some theoretical guide on improving the heat meter measurement accuracy when the water contains impurities. Setareh et al. [19] studied the heat transfer of casing device by experiment and simulation, and found that ultrasonic can enhance the heat transfer of casing device by comparing the results with and without ultrasonic.

In summary, although the ultrasonic flowmeter has been widely used, most of the applications are single channel, single flow direction. However there are two different flow states in the casing device, and the current flowmeter can not accurately measure the flow rate. The flow rate not only affects the heat exchange of the casing device, but also affects the safe operation of the subsequent equipment. Therefore, it is significance to design an ultrasonic flowmeter suitable for casing device.

In this paper, a high-low frequency combined with reflection-penetration synchronization (HLFC-RPS) ultrasonic flowmeter be designed to measure the flowrate in casing device based on the characteristics of different frequencies of sound waves. The flow rate measurement and calculation method of casing device are given and verified by numerical simulation.