Pulse-heating small-scale metal films for microscale boiling studies
Contact: Prof. C.T. Avedisian cta2@cornell.edu
Phone: 5-5105
Office: 193 Grumman Hall
The thermodynamics and interfacial transport associated with heating and boiling of liquids is important in a wide range of applications, including bubble pump technology used for inkjet printing and concepts that use pulse-heating of microscale metal films to screen liquids for fire suppression. These applications have in common that a small scale thin metal heater (of micrometer dimensions) is immersed in a liquid (e.g., water) and then pulse-heated electrically to produce enormously high heating rates. The surface temperature of the metal film increases rapidly, exceeding the normal boiling point of the liquid (e.g., water "boils" at 100C) and bubbles then form. The appearance of the bubbles fundamentally alters the evolution of temperature of the metal film by providing an insulating effect on the surface which creates an inflection point in the surface heating profile (figure 1a).
a) b)
Figure 3
The inflection point determines the thermodynamic state – temperature (Tnuc) – at which the phase change process begins. For water at atmospheric pressure, it occurs at about 300C.
The average film temperatures is inferred from measurement of its electrical resistance through a separate calibration of resistance with temperature. The metal film heaters are incorporated into one leg of a simple Whetstone bridge as shown schematically in Figure 3b. The circuit in figure 3b is typically incorporated on a breadboard with wire connections. The goal of this project will be to replace the breadboard by a printed circuit board that would ultimately be manufactured by a vendor.
The student would design a new bridge circuit based on a printed circuit board that has provision for replacement of one leg by the metal film. The design will then be tested by heating various fluids (e.g., water) to measure the evolution of electrical resistance during power pulses of several microseconds duration. The instrumentation includes a digital oscilloscope, pulse generator, and a programmable furnace for calibrating the electrical resistance with temperature of the thin film structures. Familiarity with LABView and a basic understanding of circuit design are important.