Error reduction, evaluation and correction for the intrusive optical four-sensor probe measurement in multi-dimensional two-phase flow

The objective of the present study is to increase the reliability of multi-dimensional two-phase flow measurement using an intrusive optical four-sensor probe. We investigated the error reducing ways in fabricating an optical conical four-sensor probe from its basic principles and sought for a control technique to sharpen the optical fiber tip and a sensor assembling method for a four-sensor probe. According to the measuring process by a multi-sensor probe, measurement errors were classified into signal processing errors and hydrodynamic errors. The signal processing errors in the void fraction due to the threshold setting and those in the interfacial area concentration (IAC) due to the interface-pairing scheme and the threshold setting were analyzed and concluded to be tiny and negligible in the measurement by an optical four-sensor probe. The hydrodynamic errors were classified into oncoming bubble errors, receding bubble errors and transversal or missing bubble errors according to the bubble motion relative to the probe. The maximum errors in both IAC and void fraction due to oncoming bubbles in a four-sensor probe measurement were estimated to be 10%. The maximum underestimation for IAC in the traditional transversal bubble recovering way of a four-sensor probe was reported up to 30% when the intensity of bubble velocity fluctuation equaled to 1 and the bubble size was close to the probe separations between sensor tips. The maximum measurement errors in IAC and void fraction for the receding bubbles were valued at 31% and 38%, respectively, at low liquid and high gas flow rates conditions by performing evaluation experiments using downward-facing and upward-facing probes. To overcome the unsatisfactory measurement errors for the receding and transversal bubbles, we proposed expressions for the correction of IAC and void fraction in the four-sensor probe measurement in a multi-dimensional two-phase flow by adding the contribution of escaped bubbles due to the hindrance of the probe rear parts and that of transversal bubbles due to the existence of finite distance separation between the sensor tips.