טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRaizner Mordehai
SubjectHeat Transfer and Flow Field Measurements of a Pulsating
Round Jet Impinging on a Flat Heated Surface
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Rene Van Hout
Professor Emeritus Gershon Grossman
Full Thesis textFull thesis text - English Version


Abstract

Impinging jet heat transfer has been widely investigated due to its strong ability to achieve high heat fluxes from surfaces. Applications include e.g. cooling/heating of electronic components, laser diodes, turbine blades, engine parts, paper drying. High heat transfer coefficients are obtained in the stagnation region, decaying sharply with increasing distance from it. Higher average heat transfer coefficients can be obtained by using an array of impinging jets. However, this increases the fluid flow rate and leads to larger pumps. In order to maintain a low flow rate, pulsating jets were also investigated. These effectively enhance the heat transfer coefficient both over the stagnation region and the wall jet region. However, for a specific jet configuration the optimal pulsation frequency and setup needs to be determined.

Here, heat transfer and flow field characteristics of a pulsating, round jet impinging

on a flat plate were measured and compared to those of a steady jet. The surface to jet

nozzle stand-off distance equaled two nozzle diameters (D) and Reynolds numbers based on the average steady jet exit velocity were: Re = 4,606, 8,024 and 13,513. Strouhal numbers ranged between 2 ?10−3 < Sr < 15.6 ? 10−3. Jet exit velocities varied nearly sinusoidal, having a radially uniform exit velocity at each phase. The effective pulsation amplitude decreased with increasing pulsation frequency. The heated target surface was located at a stand-off distance of 2D.

Radial distributions of the Nusselt number, Nu, showed a secondary peak at r/D

2 (r denotes the radial distance away from the jet centerline) due to the generation of

secondary vortices. An additional Nu peak appeared at r/D ≈ 4 as a result of partial jet

confinement. Overall heat transfer enhancement, Λ, of maximum 15.7% and 4% at Re = 4,606 and 8,024, respectively, were obtained within the studied Sr range. At Re = 13,513, Λ was slightly negative. In addition, at Re=8,024 a local maximum Λ was obtained for Sr = 4.1. Existing empirical correlations were unable to predict decreasing enhancement with increasing Sr for a given Reynolds number. We showed that overall heat transfer enhancement as a function of Sr was well correlated to the primary vortex generation Strouhal number, Srv. Our results indicate that heat transfer enhancement is associated with increased Srv and heat transfer attenuation with decreased Srv.

Jet pulsation did not affect the normalized, phase-averaged radial and axial velocity distributions in the impingement region and these could be predicted based on a quasi-steady assumption. However, at the jet’s center, rms values of radial and axial velocity fluctuations were enhanced at pulsation phases for which the jet exit velocity was lowest. In contrast, profiles of radial and axial velocity fluctuations and the Reynolds shear stress in the wall jet were greatly affected by pulsation. For the here investigated range of parameters, our results indicate that outer layer self-similarity in the wall jet is not attained for a pulsating jet, and the wall jet cannot be treated as quasi-steady.