Heat Pipe Performance Improvements
Our lab is currently investigating the performance of heat pipes. We are interested to see if changing the surface properties or geometries of the heat pipe will have an effect on the overall performance.
The shape of the liquid-vapor interface along the length of the adiabatic section of a heat pipe - a passive heat transfer device that relies on capillary pumping - is investigated to identify the mechanisms behind both performance enhancement techniques as well as operational failure. In formulating the associated mathematical model, the liquid flow within the (screen type) wick is described using Darcy’s law while Poiseuille’s equation is applied for the vapor phase. The liquid and vapor phases are coupled together using the Young-Laplace equation with additional closure provided by select thermodynamic relations e.g. to relate the temperature and pressure changes experienced by the vapor. Our model thereby predicts the shape of the liquid-vapor interface and its deflection owing to the variations in the heat load, operating temperature and the wick-surface characteristics. Together, these details can prescribe the parameter space over which the liquid-vapor interface remains mechanically-stable meaning that the heat pipe can function effectively. The possibility of employing a wick with non-uniform wetting characteristics is also explored with comparisons drawn against the uniform wettability case. The comparison in question reveals some of the benefits of the former approach. For instance, a wick exhibiting a contact angle that varies as a function of the axial coordinate allows for better control of (i) the depth to which the interface is recessed within the wires comprising the wick, and, therefore (ii) the entrainment of liquid droplets into the counter-flowing vapor stream. As such, the space of acceptable operating parameters can be suitably expanded.
In this study, we present experimental and theoretical analyses of evaporating a double-emulsion drop resting on a substrate. Multistage evaporation of the outer and inner droplet is witnessed. The complete evaporation of the outer drop and the initialization of the inner drop evaporation demonstrate an interesting transition dynamics. After the apparent completion of evaporation of the inner phase of a double-emulsion drop, surprisingly, formation of a daughter droplet is observed. We further investigated to hypothesize this phenomenon and achieved the formation of the daughter droplet for a single-phase drop as well. While engineering the “daughter drop formation” phenomena, we also proposed a way to obtain prolonged fixed contact line evaporation for a single-phase drop.