This invention in general relates to the void fraction measurement in gas/liquid two-phase flow system. Particularly, the invention relates to a method and a pressure-differential device to measure the void fraction of a gas/liquid two-phase stratified flow within a horizontal pipeline system. The said method and said device are applicable to flow at steady and transient states, and at high-temperature/pressure conditions.
Void fraction is a crucial parameter in the determination of energy and momentum transfer in a two-phase flow system. In usual practice, the void fraction is defined two-dimensionally, in reference to a chosen cross section of the pipe under measurement, as the area of gas phase passing through that cross section divided by the whole cross-sectional area. At present, the instruments most often used for void fraction measurement are the expensive and complicated xcex3-ray and "khgr"-ray densitometers, in which void fraction is derived from radiation attenuation of one or several xcex3-ray or "khgr"-ray beams traversing through the gas/liquid two-phase flow. FIG. 6 illustrates such a densitometer commonly used in industry, which includes radiation source 60, collimators 61, two-phase flow measurement section 62, radiation shields 63, and a detector and counting system 64. This layout was first reported by Edelman et. al. in International Journal of Heat and Mass Transfer, vol. 28, no. 7, 1985, where xcex3-ray is applied to measure the average cross-sectional void fraction of a steady-state boiling flow within a stainless steel pipe. FIG. 7 is another example, presented by Kagawa et. al. in International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NUREG/CP-0014, vol. 2, 1990, where "khgr"-ray scanning method is applied to a high-temperature/pressure flow system to measure the distribution of cross-sectional void fractions in a horizontal pipe and the system is at transient state due to a severe leakage somewhere in the pipeline system. This is an even more complicated instrument than the xcex3-ray densitometer just mentioned, in that, more than those basic component parts shown in FIG. 6 the instrument has, referring to FIG. 7, a radiation scanning location-control mechanism 65 and a radiation scanning beam-control electronic system 66. The measurement accuracy of xcex3-ray or "khgr"-ray densitometer depends on the degree of homogeneity of the gas/liquid two-phase flow: better homogeneity of the fluid obviously promises more reliable measurement result. However, it is well known that, the void fraction of a gas/liquid two-phase flow seldom is, if ever, homogeneously distributed. The measurement accuracy of a radiation densitometer also depends on the incident angle and location of each radiation beam, because each beam, different from the rest, is specifically attenuated by a certain wall thickness of pipe and by a certain portion of gas/liquid two-phase flow. As a rule, the farther away is a radiation beam from pipe axis, the less reliable is the measurement data. Moreover, depending on the material and thickness of pipe wall, the radiation attenuation may be so great as not to leave any useful information for the detector to resolve. For a specific example, one may consider the case of a horizontal natural circulation of a gas/liquid two-phase thermal-hydraulics system, in which the density difference between low flow rate liquid phase (e.g. water) and high flow rate gas phase (e.g. vapor) is usually so large that, under the influence of gravity, the two phases virtually separate and become two quite distinct flows, namely the upper layer gas flow and the lower layer liquid flow, thus the term flow stratification. Neither xcex3-ray nor "khgr"-ray densitometer is at all suitable for such flow condition for the want of fluid homogeneity.
To overcome the aforesaid deficiencies of radiation method for void fraction measurement, the present invention, taking advantage of the fact that flow stratification phenomenon often occurs in natural circulation of gas/liquid two-phase flow within horizontal pipeline system, discloses a method to determine the void fraction of gas/liquid stratified flow; wherein a couple of pressure-signal tubes are vertically flush-mounted to the top and bottom interior surface of the horizontal pipeline to measure the pressure differential across the pipe. A thermocouple-rake temperature probe assembly is used to obtain the average temperature of the liquid phase of a gas/liquid two-phase stratified flow, in which the thermal stratification also occurs. Therefore, the average mass density of the liquid phase of the stratified flow is obtained. Then the liquid-phase level of the stratified flow is calculated from the aforesaid pressure-differential data and liquid-phase mass density data. Finally the void fraction is derived from the known geometric relation between said liquid-phase level and the cross section of the pipe.
Since commercialized instruments for void fraction measurement of gas/liquid two-phase flow, such as xcex3-ray or "khgr"-ray densitometer, are far from satisfactory when flow stratification occurs, owing to the reasons briefly discussed in the foregoing, it is the principal objective of this disclosure to resolve this problem by providing a novel method for void fraction measurement, applicable to vapor/water, or any other kind of gas/liquid two-phase stratified flow within a horizontal pipeline system. According to this disclosure, a couple of pressure-signal tubes are vertically flush-mounted to the top and bottom interior surface of the pipe at a selected location, so as to measure the pressure differential across the cross section at that location; and, through liquid-phase mass density correction, this pressure-drop data is transformed into the liquid-phase level within the pipe; and the said liquid-phase level, by its geometric relation with the cross section of the pipe, is then transformed into void fraction. The device herein disclosed can accurately determine the void fraction of a horizontal gas/liquid two-phase stratified flow, be the flow at steady or transient state, and whether or not the flow is at high-temperature/pressure condition.
When the flow being measured is at high-temperature/pressure condition or is at transient state, physical effects like gravity and heat that may render pressure-differential measurement difficult must be put into consideration. Not only does gravity produce flow stratification, but it may also pull down some pressure-signal transmitting medium from the upper pressure hole, resulting in voids or even void sections present within the upper pressure-signal tube. To obviate the gravitational effect four measures are taken: (1) keep as small as possible the diameter of pressure-signal tube that is near the pressure hole, (2) keep as short as possible the vertical section of the upper pressure-signal tube that is near the pressure hole; (3) keep as small as possible the two locations at top and bottom pressure holes where pressure-signal transmitting medium is present; and (4) have pressure-signal tubes flush-mounted to the top and bottom interior surface of the pipe. As to heat, obviously it can increase the temperature of pressure-signal tubes, especially at the sections near the pressure holes, because of thermal conduction. A density gradient will therefore occur within pressure-signal transmitting medium. This problem will be especially serious at the upper vertical section of the pressure-signal tube, and will greatly affect measurement accuracy of the liquid-phase level of a gas/liquid two-phase stratified flow within a horizontal pipeline system. If accident such as leakage or cleavage occurs in a high-temperature/pressure closed system, resulting in pressure decrease faster than temperature decrease, vaporization of the liquid phase within the system may begin rapidly (this is called flashing phenomenon), and this may sometimes lead to vaporization of pressure-signal transmitting medium near the pressure holes. Flashing of pressure-signal transmitting medium may also result in voids or even void sections within pressure-signal tube. Once voids or void sections are present within the vertical section of pressure-signal tubes, any further measurement is not merely futile but, in some cases, possibly dangerous may result. This is because the measurement device is no longer reliable: it may produce false signals which may accordingly trigger the safety systems of, say, a nuclear reactor to work, or, contrariwise, it may ignore accident evolution and delay the safety systems, and neither case is exactly desirable. To mitigate the effects of thermal conduction and flashing phenomenon, this invention has each one of the pressure holes and its nearby space encased within a cylindrical coolant cladding to keep the temperature of the pressure-signal transmitting medium at safe condition. Experiments done in this laboratory prove that this local cooling strategy can effectively prevent loss of pressure-signal transmitting medium due to density gradient or vaporization. Thus, more than providing a method and a device to measure the void fraction of a gas/liquid two-phase stratified flow within a horizontal pipeline system, this invention provides a preventive method and a device to effectively overcome the problem of loss of measurement accuracy due to loss of pressure-signal transmitting medium encountered in transient flow system by the commonly used pressure-based instruments such as fluid-level meter, flowmeter, pressure-drop meter, pressure meter, etc.