Enhancing the cooling capacity of heat pipes : wicking in micropillar arrays and electrowetting-based droplet pumping
Thermal management is an important consideration for many technologies, ranging from electronic devices to solar thermal energy. Heat pipes transfer heat by cyclically evaporating and condensing a working fluid, and their main benefits are high thermal conductance and little to no power consumption. In a standard heat pipe, an internal wicking structure utilizes capillary action to drive fluid circulation. Heat transfer capacity is limited by the maximum fluid velocity the wick can sustain, among other factors. Because viscous pressure losses increase with distance, the maximum heat transfer capacity of a heat pipe decreases over extended distances. This work explores two ways to increase the maximum cooling capacity of heat pipes: first, by designing an alternative wicking geometry, and second, by removing the wick entirely and replacing it with electrowetting-based droplet pumping technology. The capillary limit is largely determined by the permeability and driving capillary pressure of the wick. These two factors typically depend on the same geometric parameter, such that the two are inseparable and inversely related. Micropillar arrays are wicking materials where the distances between pillars are independent and variable, which could lead to a decoupled capillary pressure and permeability. The first half of this dissertation work designs, optimizes, manufactures, and tests micropillar wicks for use in a heat pipe. To accomplish this, an analytical model for fluid flow through a micropillar array with independent x- and y- dimensions is developed, enabling the exploration of non-symmetrical pillar arrangements. For pillar dimensions outside of the range of the analytical model, numerical simulations are employed. Using these models, the dimensions of pillar arrays are optimized for maximum fluid flow rate. The findings indicate that arrays where the pillars are arranged in a rectangular pattern exhibit the ability to maintain high capillary pressures even at high porosities, which increases the overall cooling capacity above square arrays by 1.5x in the absence of gravity and 5x – 7x in the presence of gravity. To verify the maximum fluid velocities predicted by modeling, a range of pillar configurations are manufactured and tested. However, conclusions about the fluid velocity are obscured because atmospheric conditions of the experimental apparatus allow the boiling limit to occur before the capillary limit. Electrowetting is a microfluidic pumping technique which operates by applying a voltage across a liquid droplet to change the surface energy balance of the liquid/solid interface. If several electrodes are placed in succession, a discrete droplet can be pulled along a surface. Replacing the internal wick of a heat pipe with an electrowetting system reduces the viscous pressure losses of a long pipe, potentially revolutionizing heat transfer over long distances. The second half of this dissertation work explores the feasibility of replacing the internal wick in a heat pipe with electrowetting-based droplet pumping to create an electrowetting heat pipe (EHP) with water as the working fluid. First-order models show that an EHP with a cross-sectional area of 10 cm x 4 mm can transport 1.6 kW over extended distances (1 m) using only 1.2 mW of power. Compared to three heat pipes and one thermosyphon of similar dimensions, the EHP can transport more than twice the maximum heat capacity of other devices while offering a low thermal resistance of 0.01 K/W, comparable to wicked heat pipes. Compared to pumped microchannel systems of equivalent cooling capacity, the EHP achieves lower thermal resistances than both single- and two-phase pumped microchannels. The EHP offers lower power requirements than a single-phase pumped system and similar power requirements compared to a two-phase system. To experimentally prove the feasibility of an EHP condenser, this work first develops a novel, rapid manufacturing method for an electrowetting droplet pumping platform, then demonstrates droplet generation from an open liquid source. This work therefore constitutes a starting point for the development of a new class of electrically controlled thermal management devices for long-distance heat transport.