Experimental and numerical study of microchannel heater/evaporators for thermal phase-change actuators

The performance of thermal phase-change actuators is strongly affected by the design of their heater/evaporators. The use of microchannels to control the liquid working fluid via capillary forces promises one approach to optimize the operation of heater/evaporators and the performance of the actuators. An experimental and numerical study of heater/evaporators for phase-change actuators, based on open rectangular microchannels, is presented. The microchannel heater/evaporators consist of SU8 walls fabricated in a radial pattern on either silicon or silicon nitride membranes. The microchannels, of rectangular cross section, taper from a width of 80 μm at their outer radius to a width of five microns at their inner radius, and have a constant depth of 40 μm. The energy balance for the heater/evaporators is determined experimentally through measurements of electrical power dissipated as heat in a thin-film resistance heater, sensible heat transfer by conduction radially out of the evaporator, and latent heat transfer by evaporation from the microchannels themselves. A finite difference code is then used to calculate liquid flow pumped by capillary forces in the microchannels as well as sensible and latent heat transfer rates. The numerical code is shown to predict the experimental measurements well. The performance of this type of heater/evaporator for phase-change actuators is shown to be limited by two mechanisms: dry-out of the working fluid in the microchannels due to the capillary limit determines maximum evaporative mass transfer rates, while conduction heat losses out of the evaporators determines maximum evaporator efficiencies.