Ph.D Thesis

Ph.D StudentKlein Daniel
SubjectThe Influence of Actuation in the Flow of a Jet on the Wall
Jet Heat Transfer Coefficient
DepartmentDepartment of Mechanical Engineering
Supervisor PROFESSOR EMERITUS Gad Hetsroni (Deceased)


Recent technological developments have lead to significant increase in the generated heat by electronic components. Removal of high heat fluxes can be successfully treated by several methods, among which a leading candidate is the impinging jet. Further improvement is offered by incorporating arrays of jets or causing jet pulsation. The research reported here introduces a new method which is based on the actuation of a slab against a two dimensional steady micro-impinging laminar liquid jet, leading to the enhancement of heat transfer coefficients down the wall jet region.

In order to investigate the potential improvement in heat removal capability, a two-dimensional laminar jet impinging on an actuating slab was numerically simulated, using commercial CFD software. The slab motion in the flow field was simulated using a dynamic mesh, which was integrated to the rest of the static meshed regions via a designated interface. In order to evaluate the heat transfer enhancements, the thermal response of the chip to triangular actuation profiles at up to 140 µm and 500 Hz, was studied. The simulations revealed the existence of a minimal actuation height, below which reduced heat transfer coefficients are obtained. In addition, it was found that for given flow rate and frequency, the heat transfer coefficient raises monotonously with the increase in actuation height. Further examination of the frequency effects at constant flow rate and actuation amplitude also revealed a monotonous increase in average heat transfer coefficient with frequency. A maximal enhancement up to 100% was achieved for the highest values of amplitude and frequency.

Following the preliminary numerical analysis an experimental setup which included a PZT actuator, a designated silicon chip and a steady slot impinging jet, was assembled. Using a high speed IR radiometer, the cooling process of the chip was recorded and the heat transfer enhancement values were determined for normalized actuation amplitudes, Reynolds and Strouhal numbers in the ranges of 0.45<d<0.75, 756<Re<1260 and 0<St<0.052, respectively. It was experimentally found that heat transfer coefficients were enhanced up to 35%. The increase is attributed to the combination of two phenomena: (a) repetitive buildup and breakdown of the boundary layers, effectively thinning the time averaged layers depth along the surface; (b) significant enhancement of fluid mixing process between the thin boundary layer at elevated temperatures (close to the hot surface) and the low temperature bulk fluid flowing above.