Condensers are essential parts of any two-phase systems, such as air conditioners and thermoelectric power plants. A highly efficient flow of condensation is required for condensers to convert vapors into liquids for evaporators or boilers.
But the condensation process in industry adds an extra resistance for heat transfer, causing a lower conversion rate than boiling or evaporation and leads to oversized and overweighted condensers. Mechanical Engineering Professor Chen Li is currently examining transient flow condensation behaviors and their impacts on condensation rates.
“In two-phase systems, heat is taken by evaporation and rejected outside by condensation. For example, the working evaporator is basically a flow boiling [heat transfer mechanism] process inside an air conditioner,” Li says. “The condenser is a radiator placed outside a home or used for buildings, cars and other chiller systems.”
Li’s research is funded by a $240,000 grant from the National Science Foundation and a $284,000 grant from the International Space Station (ISS) National Laboratory. Computer Science and Engineering Professor Yan Tong is the co-principal investigator on the research, which began last August. Li has worked on similar research projects involving boiling, evaporation, condensation and associated two-phase systems for the last 10 years.
“If our technique can be proven, it can be used to improve the energy efficiency of refrigerators, air conditioners, power plants and other two-phase systems. We want our work to show that flow condensation can be used to increase the efficiency of the condenser since that’s part of the overall efficiency. It’s important for both terrestrial and space applications,” Li says.
Transient behavior refers to an unstable flow rate, and instabilities of the two-phase flow are a nature of physics in flow boiling and condensation. Even though these instabilities are potentially damaging, only a few studies have been conducted in better understanding the issue. Li’s research is linked to his other current project on understanding the gravity effect on flow boiling, which began in July 2021.
“We are fortunate to have both projects together. No matter whether it is for cooling or power generation, you need flow boiling to generate steam and cool a device. The steam eventually needs to be condensed back and pumped,” Li says.
Filmwise condensation wets the surface and forms a liquid film on the surface that continuously slides down under the influence of gravity. It is the current practice used in industry. But Li intends to determine whether the flow condensation process is more effective. Flow condensation can work for several days and be as much as 10 times more efficient, but it is hard to sustain and eventually transitions back to film condensation.
“When you impose pressure, the water will flow inside the pipe. However, this may not be the case for flow condensation,” Li says. “If you supply a vapor inside the pipe, you may see a very strong reverse flow, which means it’s unstable. It seems bad, but if we better understand this phenomenon, we can use it to help manage the flow instability. But more importantly, we can improve the heat transfer rate and efficiency.”
According to Li, it is hard to improve flow condensation. While there are approaches to improve flow boiling, it is harder to improve flow condensation, especially inside circular pipes. “It might be possible for us to understand transient behaviors much better. Then we can design operating conditions to make it a self-sustained two-phase oscillation with a high heat transfer rate,” Li says.
Part of Li’s research will be understanding why the flow becomes more unstable due to changes in working conditions. It is similar to feeling a shake when gears are shifted in a car. Li’s research team will start the system in two different ways to determine how it deals with each scenario. But modeling will be challenging since regular tools cannot correlate governing parameters. Tong will use her expertise to to build machine-learning based models to accurately predict transient behaviors of flow condensation, flow condensation rate and pressure drop.
“The ground experimental data can supply abundant data for training the models. But more importantly, the data collected from ISS will be utilized to fine-tune the models to better understand flow condensation behaviors in microgravity,” Tong says.
Li is excited about the potential of his research and the wide-ranging applications from everyday household items to more advanced equipment used in space missions.
“Since I have been working on flow condensation for a long time, we have great ideas and hopefully they [NASA] can let us try. The first step is to understand and predict the behaviors,” Li says. “This is the first time humans are doing a long term, two-phase flow boiling condensation experiment, and the results should be very significant.”