Computational Fluid Dynamics and Particle Image Velocimetry Supported Examination of Bidirectional Velocity Probes for Measurements in Flames

Abstract

The bidirectional velocity probe has been used in various flames to measure local velocity. The device is based on the pressure difference between a closed forward facing cavity and a closed rearward facing cavity. The probes have been noted to indicate a pressure difference greater than that which would be predicted based on Bernoulli's equation. Each device must be experimentally calibrated in a wind tunnel at similar Reynolds number to determine its “amplification factor.” This study uses PIV, flow visualization and CFD to examine the flow field around the probe, as well as an experimental study which compares various probe configurations for measurement of velocity by pressure differential. The conclusion is that the amplification factor is indeed greater than unity but use of the wind tunnel for calibration is questionable.

Evaluation of a new device for simultaneous measurement of heat flux and gas velocity in a diffusion flame

“…However, review of the method for characterizing the probe and analysis of the flow field around the probe has led to the conclusion that this approach is fraught with uncertainty due to the irreversible influence of a detached boundary layer on the reading obtained in the rear port and the unknown temperature distribution along the boundary layer, which appears to be detached, based on careful imaging and CFD studies performed on the BDVP. 2 A spherical shape, however, has the ability to keep the boundary layer attached over much of its surface, which augments a better analytical understanding and interpretation of the yielded data. Hence, the method based on flow around a sphere and analysis of the influence of a non-isothermal boundary layer is felt to offer significant advantage over previously utilized tools.…”

“…Since the thermal properties of the Inconel and the insulation are known, the measured temperatures in conjunction with inverse heat transfer calculations can be used to determine the net heat flux at the exposed surface of the plates. Previously used devices for velocity measurement, such as the bidirectional velocity probe (BDVP), 2,3 require wind tunnel calibration to correlate pressure differential to gas velocity. Such wind tunnel calibration is at isothermal flow conditions so this approach requires additional, unknown corrections to lead to results which are useful in the flame environment.…”