______________________________________ +Corresponding author. Tel.: +91-261-2201575, fax: +91-261-2228394 E-mail: jbaner@med.svnit.ac.in Estimation of Wall Shear Stress in Two-Phase Flow Using Hot Film Anemometry Mahesh J Vaze and Jyotirmay Banerjee + S V National Institute of Technology Surat-395007, India Abstract. Measurement of velocity gradient near the wall and wall shear stress for air- water two phase flows through pipe is reported. The measurements are carried out using hot film anemometer. Mean and time varying fluctuation of the local wall shear stress and velocity of the horizontal air-water two-phase flow is measured using TSI 1210-20W hot film. Measured wall shear stress distributions are found to be strongly influenced by the condition of gas-liquid interface. Results reveal that an increase in water flow rate increases the wall shear stress while the variation of wall shear stress with air flow rates is negligible. The data collected is utilized to obtain time averaged value of shear stress for the fluctuations. The axial velocity distribution is measured by traversing the probe in radial direction. Keywords: Two-phase flow; Hot Wire Anemometer; Shear Stress; Reynolds number 1. Introduction Air-water two-phase flow involves complex shear stress mechanisms which are very difficult to correlate. Shear stress models generally rely on a number of basic assumptions and empirical closure equations. Taitel and Dukler [1] first presented a shear stress model based on momentum balance of the gas and liquid phases. Andritsos and Hanratty [2] reported that liquid-wall shear stress is better predicted via a characteristic stress τC (taken as the weighted average of liquid-wall and characteristic stress). According to the authors, the characteristic stress can be calculated from a dimensionless liquid height, which is a known function of the liquid Reynolds number. Early works for the measurement of shear stress was limited to two-phase flow through the rectangular channels (Davis [3], Fabre et al. [4]). A variety of techniques have been used to measure wall shear stress. Direct measurements of average wall shear stress by measuring the restoring force are reported by Cravarolo et al. [5] and Kirillov et al. [6]. However, most researchers have used indirect methods based on the analogy between momentum transfer and mass transfer (e.g. Cognet et al. [7]) or heat transfer (e.g. Whalley and McQuillan [8]). These methods however cannot produce the fluctuations in wall shear stress. In order to obtain fluctuating component of shear stress, hot film probes are utilized. Kowalski [9] has presented measurements of the wall shear stress for circular pipe at various radial locations in the gas region, and has concluded that existing models for estimation of the gas wall shear stress seem to be adequate. The shear stress relation is essential as closure to the analytical model. An experimental investigation to establish the wall shear for air-water two-phase flow through pipe is presented in this paper. The wall shear stress and the velocity gradient in the radial direction in both the phases is measured using hot film anemometer. Experimental values of wall shear stress are plotted as functions of gas and liquid superficial Reynolds numbers and are is then utilized in obtaining two-phase skin friction coefficient. 2. Experimental Methodology The established experimental setup (Fig 1) is discussed in an earlier paper by the authors [10]. The experimental setup consists of air-circuit, water circuit, flow visualization section and measuring section. The measurement section consists of pressure and temperature measuring sections. Hot film anemometry is utilized in predicting wall shear stress and measuring the velocity profile in radial direction. An attachment is prepared having provision of traversing the probe. The attachment consists of SS pipe with flange 2012 International Conference on Fluid Dynamics and Thermodynamics Technologies (FDTT 2012) IPCSIT vol.33(2012)©(2012) IACSIT Press, Singapore 58