One of the objectives of the In-Situ Resource Utilization (ISRU) RESOLVE project was to extract water from lunar regolith by heating and then capturing and quantifying the released water. A potentially large error in measurements would occur if some of the water condensed in the piping between the oven and the water capture bed. Questions arise if condensation were to occur (either because of a low-temperature or high-pressure area): 1) how long would it take for convection and diffusion to evaporate the droplet back into the process stream, 2) how will it affect the ISRU RESOLVE process, 3) can accurate data be obtained, 4) how much longer will the process have to run to capture all the water, and 5) what conditions are the most favorable to quicken the evaporation process?
From a conceptual viewpoint, the problem is easy to visualize — a half-spherical droplet evaporating into a flowing gas stream. The computation to calculate the evaporation rate, however, is very complex. If one attempted to solve the problem from first principles, the mass transfer, fluid flow, and heat transfer equations become much too difficult to solve analytically, and even extremely challenging to solve numerically. An alternative approach was taken.
Both diffusion and convection influence the evaporation rate. In addition, the streamlines are asymmetric, yielding a larger influence of convection on the upstream side. Other important parameters include the system geometry, temperature, flow rate, and initial droplet size. The solution method examines the influence of flow, temperature, and time needed to totally evaporate a specified initial size water droplet attached to the inside of a pipe in a flowing air stream.
The numerical technique described herein allows the computation of the time required to evaporate a water droplet in a process pipeline. In the ISRU processes, a critical concern is condensation at cool spots within the process. If water does condense as a droplet, the time needed for the fluid flow to re-evaporate the droplet is critical to determining both the overall process time and quantification of the water content in the system.
The software works by inputting an initial droplet volume, the range of temperatures to be considered, and a range of gas flow rates. Various parameters are then computed, such as the surface water concentration, diffusion coefficient, and flow velocity. Next, several dimensionless numbers are calculated, such as the Reynolds Number, Schmidt Number, and Nusselt Number. This leads to computing the mass transfer coefficient, molar transfer rate, evaporation rate, and finally, the time required to evaporate the entire water droplet. The software is separated into two worksheets: one for the spherical droplet and the other for the recessed cavity. The evaporation time results are displayed graphically as a function of temperature and flow rate.