Measurements of an elliptic jet

• Main Author: Zhang, Kenny Zhenkuan
• Other Authors: Chua Leok Poh
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/54729
• Institution: Nanyang Technological University

This report presents the measurements of the velocity and temperature fields of the free turbulent jet issuing from a smooth contoured contraction elliptic nozzle with an aspect ratio (AR) of 2:1 (50mm x 25mm). The motivation for this investigation is understanding the possible enhanced mixing performance afforded by low AR elliptic jets over other noncircular jet shapes. Given the widespread use of noncircular jet nozzles in passive flow control and the numerous engineering applications involving heated jet flows, the current work aims to address the lack of published literature relating the momentum transfer with the heat transfer characteristics ofthe elliptic jet. Due to their compactness, sensitivity and good frequency response making them ideal for use in the highly turbulent jet flow, measurements were carried out using constant temperature and constant current anemometry systems with single hot-wire, single cold-wire, X-wire and three-wire probes. The characteristic lengthscale used is the equivalent diameter ofthe elliptic nozzle De, at 35.4 mm. The jet exit velocity, "Uj was maintained at 25±O.5 mls throughout the experiments, giving an exit Reynolds number (Re == VjDe I v) of 56441. The boundar~ layers measured at the nozzle exit plane agreed with the Blasius profile, indicating laminar initial conditions for the present jet. For the heated jet, the jet exit temperature, 1j was held constant at 20±O.5°C above the reference ambient temperature to avoid buoyancy effects. In the first part ofthe study, streamwise mean and root-mean-square (RMS) velocities were measured along both axes. The potential core was found to have ended by x == 5De . In the near-field region, the shear layers spread at different rates in both axes, showing the effects of the non-uniform distribution of boundary layer momentum thickness along the nozzle exit and the consequent enhanced mixing properties of elliptic jets. The mean streamwise velocity profiles became self-similar by x == 12De and x == 8De in the major and minor axes respectively, indicating a short interaction region. The jet decay and spreading rates in both axes as measures of entrainment and mixing performances were also determined. The jet spreading rate in the minor axis was found to be greater than that in the major axis, leading to the single axis-switchover observed at x == 8.8De . In the second stage of the work, the Reynolds normal and shear stresses along both axes were measured. The normalized RMS velocities in both lateral and span-wise directions were consistently smaller than those in the streamwise direction, indicating t~e anisotropy of the turbulence in the flow. Within the entire measurement range, all RMS velocity profiles did not become self-similar although asymptotic states were observed in the far-field. The Reynolds shear stress distributions became self-similar at x == 30De • In all velocity measurements, flow development was observed to be more rapid in the minor axis plane. In the heated jet experiments, the mean temperature profiles became self-similar by x == 7De in both axes. Both RMS temperature and turbulent heat flux distributions exhibited peaks in the shear layer, reflecting the rapid transport of heat due to turbulent mixing in this region. The turbulent heat flux distributions achieved selfpreservation at x == 30De • Jet temperature decay and spreading rates were found to be greater than those of velocity. The turbulent Prandtl number evaluated based on the self-preserved data of Reynolds shear stress and heat flux at x == 30De showed unique trends along the major and minor axes, which should be taken into account when simulating a heated elliptic jet flow. All the results presented suggest that the use of heated elliptic jets could offer enhanced mixing performance in relevant applications.