טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentJulius Shimon Eliahu
SubjectConduction and Forced Convection under Periodic Forcing
Conditions
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Beni Cukurel


Abstract

This work is comprised of two paths of investigation; the study of acoustically enhanced forced convection heat transfer in roughened channels, and the study of heat-flux driven sound production. These two topics represent the two components for a novel methodology for deploying acoustic emitters towards heat transfer enhancement in complex environments. The topics are further linked by a common theme: the study of heat transfer under periodic forcing conditions, whether acoustic, as in the first part, or as a heat-flux oscillation, as in the second.

The transport of heat and momentum in non-equilibrium separating and reattaching flows is studied experimentally. The wall-bound heat transfer characteristics around various simple rib topologies were studied under acoustic forcing conditions through the use of the Thermochromic Liquid Crystal (TLC) measurement technique. The flow field was examined via Particle Image Velocimetry (PIV) measurements, complemented by static and unsteady wall-bound pressure measurements. A relationship was established between heat transfer enhancement and the rate of pressure recovery in the recirculation region. It was observed that there is a direct link between the pressure gradient and the turbulent transport & production in the near-wall region of the reattachment zone. It was further shown that there is strong evidence that the trigger for the acoustic enhancement mechanism can be found in the perimeter of the recirculation bubble downstream of the obstacle, and that the instability dynamic is receptive to fluctuations in the velocity field (and not the pressure field). In conjunction to this, a novel integral equation and methodology was developed for the determination of boundary layer thickness and skin friction from wall-bound heat transfer and static pressure measurements independent of knowledge of flow history. In order to accomplish this feat, novel composite velocity and temperature profile formulations were proposed.

In the case of heat-driven sound production (thermophone transducers), in-roads were made in the theoretical understanding of joule heating transduction into sound. Redressing inaccuracies in the literature, a simple point-source acoustic modelling approach for thermal and mechanical sound sources is proposed and validated via the novel demonstration of sound cancellation of a mechanical source by a conterminous heat flux source. The factors contributing to thermophone conversion efficiency are identified and investigated. A study into nanoscale thermophone sound production demonstrates evidence of non-Fourier heat conduction effects. A complete generalized analytical solution to the non-homogeneous Dual Phase Lag (DPL) heat equation is presented, and the ramifications of non-Fourier heat conduction behaviour are extensively explored and discussed.