|M.Sc Student||Feldman Matan|
|Subject||Rate Limiting Constraints in Phase-Change-Materials (PCM)|
for Miniature Aircraft Propulsion Systems
|Department||Department of Aerospace Engineering||Supervisors||Dr. Dan Michaels|
|Professor Eran Sher|
|Full Thesis text|
With the immeasurably growth of use in Unmanned Aerial Vehicles (UAV), so does the need for Micro Aerial Vehicles (MAV), which require extremely lightweight propulsion systems. Whereas common MAV currently use electric batteries as their power source, this work examines the possibility of using Phase-Change Materials (PCM) in such propulsion system; where PCM are substances characterized by high latent heat, e.g. cryogenic fluids.
The proposed system involves a rapid depressurization of such substances from a stable liquid state to create vapor nuclei (bubbles), followed by nuclei growth till they coalesce, forming a continuous vapor phase which is heated and later drive a micro-turbine. Therefore, the transformation rate from liquid to vapor plays a prominent role in the system, and the limiting transition rates of several materials are investigated in terms of their initial temperature and pressure (with respect to the critical values), as well as of the dimensions of the orifice through which they exit.
While the applicable initial temperature range is quite narrow and restricted to the vicinity of the critical temperature due to nucleation considerations (both homogeneous and heterogeneous), the initial pressure is rather wide and was only limited to avoid heavy pressure vessels. However, changes in both parameters significantly affect the nucleation rates obtained and the products of propulsion systems, thus a parametric analysis of these impacts is reviewed using the Classical Nucleation Theory, Carey's estimation (2007) and two common heterogeneous nucleation theories - by Alamgir and Lienhard (1981) and Elias and Chambr`e (1993).
Out of 11 inspected substances, Hydrogen was found to be the most promising substance in terms of both specific energy and power, thanks to its remarkably low molar mass. With specific power values surpassing 1000 [W/kg] and specific energies of over 500 [W-h/kg] (under the dimensions constraints defined), it can overpower electric batteries and enables long-term missions of few hours, offering a real alternative to the current technologies powering MAV. Though being relatively inferior to Hydrogen, Methane also seems to be capable of propelling MAV; with specific power possibly surpassing 200 [W/kg] and specific energies of up to 80 [W-h/kg]. Comparison of the attained results was performed using a Ragone chart (1968).
If the volume of the pressure vessel becomes rather significant than the fluid mass, restricting the system dimensions; Neon offers rather elevated volumetric outputs, with power density of up to 105 [W/m3] and maximal energy density of 4.3∙104 [W-h/m3].
According to the abovementioned results, decompressing PCM can be clearly used as an alternative to the current MAV (and larger vehicles) propulsion systems.