Investigation of Air Humidification Using a Pressure-Based Measurement Technique

The need of fresh water is a global concern that needs no introduction as it is on the rise on daily bases. A huge body of scientific research is active in producing alternative resources of potable water. In water desalination research field, a reputable body of studies was carried in the literature aiming at maximizing the throughput of the process by investigating the evaporation rate in sprays as in Humidification-Dehumidification (HDH) water desalination. In this technique, air stream with precisely controlled thermodynamic conditions is used to atomize the saline water, transport the droplets, and evaporate them to extract the salt particles. The water vapor is, thereafter, condensed and collected as fresh water.

Currently, most of the active atomization researches are focused on the spray quality. They investigate the best flow conditions and geometrical considerations to achieve the finest spray quality and best size/velocity distributions. However, a solid understanding of the spray evaporation rate along the spray path, based on air/water flow rates, temperature of the streams, and spray characteristics, is needed to maximize fresh water production.

The studies we have found in the literature can be summarized to either measure the evaporation rate in a small scale (single droplet) through high temperature mediums such as in fuel evaporation and combustion, or in large scales through moderate temperature mediums such as in HDH. Air humidification measurements were always taken at two extremes of the evaporation medium without giving any insights of the evaporation rate at the early stages of contact between the air and water streams or the change of air humidification along the flow path.

As a part of $2M award from US Department of Energy, Solar Energy Technologies Office, a new water desalination technology based on HDH technique patented under the application number US62882953 is being developed. To achieve the designated energy consumption and cost targets, accurate quantification of evaporation/humidification is critical. This is primarily driven by spray rate of evaporation. Understanding the state of the flow close to the tip of the atomizer is a challenge in both the analytical and experimental realms. In the analytical realm, using the conservation equations will return a wide array of different solutions that will satisfy the mass and energy continuities at the tip of the atomizer. We can have high temperatures and low humidity content or vice versa. As for the experimental realm, taking temperature measurements is challenging due to the fact that we are dealing with wet streams. Temperature measurements will show the wet bulb temperature which in return has no value in fixing the thermodynamic state as it will be very close for all the possible states.

In this work we aim to capture a comprehensive representation of the evaporation/humidification rate at the initial stages of contact of the two streams through an air-blast atomizer. We developed a humidity measurement technique based on pressure measurement to eliminate the temperature measurement error arising from the liquid phase interference and the complexity associated with optical techniques. Pressure is a more dependable metric to evaluate the state as it can be isolated from fluid interference. By measuring the partial pressure of dry air and the total pressure of the mixture stream we can solve for the absolute humidity and quantify the trends manifesting from changing the inlet conditions such as the air/water flow rate ratio, air and water temperatures, and the water dissolved salt concentration.