Leslie A. Momoda

Heat transfer and pressure drop in narrow rectangular channels

Recently, with the advent of more powerful electronic chips and the miniaturization of electronic circuits and other compact systems, a great demand exists for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, both single-phase forced convection and subcooled and saturated nucleate boiling experiments have been performed in small rectangular channels using FC-84 as test fluid. The test section used in these experiments consisted of five parallel channels with each channel having the following dimensions: hydraulic diameter (Dh)=0.75 mm and length to diameter ratio (L/Dh)=409.8. The experiments were performed with the channels oriented horizontally and uniform heat fluxes applied at the top and bottom surfaces. The parameters that were varied during the experiments included the mass flow rate, inlet liquid subcooling, and heat flux. In each of the experiments conducted, the temperature of both the liquid and the wall were measured at various locations along the flow direction. Based on the measured temperatures, pressure drops, and the overall energy balance across the test section, the heat transfer coefficients for both single-phase forced convection and flow boiling has been calculated. Additionally, in these experiments, the single- and two-phase pressure drop across the channels was also measured. A correlation has been developed for two-phase flow pressure drop under subcooled and saturated nucleate boiling conditions. Furthermore, two new correlations are proposed – one for subcooled flow boiling heat transfer and the other for saturated flow boiling heat transfer.

Manufacturing issues of thin film NiTi microwrapper

Manufacturing issues related to a thin film NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous film produces a cleaner pattern than the crystallized film. Transformation temperatures are not affected by the pre-exposure of the NiTi film to air before crystallization. However, this process produces breakage in the NiTi film at sacrificial layer steps. This is believed to be due to residual stresses developed between the film and substrate during sputtering. The film breakage is overcome by dry etching the film with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally gradated NiTi film. Results show that when heated the microwrapper arms flatten due to shape memory effect and curl up to form a cage-like structure when cooled.

Miniature thin-film NiTi hydraulic actuator with MEMS microvalves

The implementation of smaller, lighter, and more agile military systems requires new actuation technologies that offer high power density in compact form factors. The Compact Hybrid Actuator Program (CHAP) is pursuing active material based, rectifying actuators to create new actuation solutions for these demanding applications. Our actuator approach is based on thin film NiTi membranes operating in parallel (high intrinsic power density, >125 kW/kg) combined with liquid rectification, MEMS passive check valves, and commercially available power electronics. Previous results demonstrated 8 micron thick membrane actuation with 150 Hz forced convection response and force output of 100N. This paper focuses on two developments critical in scaling up previous single membrane results to power levels sufficient for military applications. This first is the development of SOI MEMS fabricated microvalve arrays which exhibit high flow rate at high frequencies. The second focus area is the design, fabrication, and assembly of a form factor compact actuator. The initial prototype demonstration of this concept shows great promise for thin film NiTi based actuation both in military technologies and in other areas which demand extremely compact actuation such as embedded fluid delivery for biomedical applications.

Compact Hybrid Actuator Device (CHAD) for ultracompact, navigation, guidance and control

The objective of our Compact Hybrid Actuator Device (CHAD) program is to produce a novel, ultra-compact, high force actuator to meet the aggressive requirements for navigation, guidance and control of a compact missile as well as other military and commercial applications confronted with tight volume constraints. Our approach to this challenge uses the high power density of thin film shape memory alloys coupled with fluid rectification and commercial power electronics. Phase One of our program demonstrated the performance of critical technical elements in a non-compact form factor. NiTi films were reproducibly deposited and then fabricated into bubble actuators that demonstrated ≥ 100 Hz performance when forced convection heat transfer to a liquid was optimized. Increased efficiency in thermal activation was achieved through high Joule heating rates for short duty cycles; this allowed simplification of the power electronics. These technical elements were combined to produce a thin film SMA pump which ultimately demonstrated force outputs on the order of 250 N and average power densities on the order of 50 W/kg when operated at 100 Hz. The demonstrated performance shows great promise for applications requiring ultracompact form factors with high output force.

Processing Effects on the Microstructure of Sol-Gel Derived SBN Thin Films

Microstructural changes in sol-gel derived Sr{sub x}Ba{sub 1{minus}x}Nb{sub 2}O{sub 6} (SBN) thin films were monitored as a function of chemical variations in the precursor sol and of processing variations of the thin films. 6000{angstrom} Sr{sub 0.5}Ba{sub 0.5}Nb{sub 2}O{sub 6} thin films were deposited from different alkoxide based precursors under various hydrolysis conditions. The tetragonal tungsten bronze structure was achieved in films at temperatures on the order of 700 C. Preferential (001) orientation of the tetragonal lattice was found to be very dependent upon the substrate orientation as well as thermal treatment schedule. Transmission electron microscopy (TEM) was used to examine the film microstructure. Orientational effects were verified by TEM as well as a tendency for a porous structure in the alkoxide derived films. Substrate and processing dependent grain size and size distributions were also noted.

Technical Issues Governing the Selection and Development of Dielectric Coolants for Fuel Cell Systems

The selection of a heat transfer fluid that meets all of the requirements for fuel cell operation is daunting. There are multiple competing fluid properties needed for optimal performance: high dielectric strength, low ionic and electronic conductivity, high heat capacity, high thermal conductivity, low viscosity at low temperatures, high flash points, low toxicity and no reactivity with contacting materials. In this paper we report the results of our evaluation of numerous basic and commercial heat transfer fluids with respect to their prospective fuel cell performance. We have also examined some of the fundamental physical and chemical properties of fluids to provide guidance on engineering a fluid based temperature regulation system for a fuel cell stack.Copyright