Thermal and fluidic characterizations of piezoelectrically driven cantilevers
Abstract
Piezoelectric fans have been shown to provide substantial enhancements in heat transfer over natural convection while consuming very little power, and consist of a piezoelectric material attached to a flexible cantilever beam. These devices have received attention as a thermal management solution, but much remains to be understood about their operation, design and optimization. This thesis includes numerous experimental studies aimed at understanding the underlying physics of vibrating cantilevers. For quantification of thermal performance, local heat transfer coefficients induced by piezoelectric fans are determined through infrared thermography for a fan vibrating close to a constant heat flux surface. The two-dimensional contours of the local heat transfer coefficient transition from a lobed shape at small gaps to near-circular at intermediate gaps. At larger gaps, the distribution becomes elliptical in shape. Additional fans of varying geometries and operational frequencies are also considered and appropriately defined dimensionless parameters are found to describe the stagnation and area-averaged thermal performance over a wide range of operating conditions. For fans configured in an array, additional complexities are introduced, and conditions where synergistic interference exists are identified. Additional experiments are performed in order to directly compare these fans with their traditional counterparts (e.g., small axial fans). This is accomplished through pressure and flow rate measurements. The performance is highly dependent on both the vibration amplitude and frequency, and predictive relationships are established which express the pressure and flow rate in terms of the design parameters. Piezoelectric fans are found to compare quite favorably to conventional fans and are shown to provide a nearly order-of-magnitude increase in fan efficiency. Fans configured in an array are also found to interact through the fluid, ultimately affecting their dynamic behavior. The aerodynamic damping of the array is highly dependent on the vibration amplitude and array pitch. The damping is found to decrease significantly for in-phase vibration, while the opposite occurs for out-of-phase vibration. The experimental studies conducted in this thesis offer valuable insight into complex fluid thermal phenomena, and provide numerous tools and guidelines which assist the thermal engineer in designing and implementing piezoelectric fans.
Degree
Ph.D.
Advisors
Garimella, Purdue University.
Subject Area
Mechanical engineering
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