Ten percent doesn't seem like a lot of anything, but when it represents the reduction of warm air infiltrating refrigerated grocery store display cases, it can add up to big energy and monetary savings.
Dr. Homayun Navaz, associate professor of Mechanical Engineering at Kettering University, has been researching ways to improve the efficiency of refrigerated display cases for four years. He and other researchers at Kettering have been exploring new designs to minimize the mixing of warm air into the cold air curtain.
The Kettering researchers are part of a national energy improvement effort that includes research teams at the University of Illinois-Champagne Urbana and Oakridge National Laboratory with funding from Southern California Edison through the California Department of Energy and the California Energy Commission. Support also came from Dr. Joel Berry, department head, and the Kettering Mechanical Engineering Department.
"Energy savings is a big thing in California," Navaz said, "they are ahead of other states in relation to environmental issues. Initially, the project was to perform Computational Fluid Dynamics (CFD) simulations, then later on they wanted some measurement and flow visualization to see the flow of the cold air curtain," he said.
Former Kettering faculty member Brenda Henderson was involved in the initial studies utilizing Digital Particle Image Velocimetry (DPIV). DPIV technology uses digital photography and lasers to compile data on the velocity field of air flow. The researchers used DPIV to track the infiltration of warm air into open-front refrigerated display cases commonly used for meat and diary products in grocery stores.
Navaz explained the refrigerated display cases function using a "cold air curtain" with a duct in the top that discharges cold air, and another duct at the bottom to suck the cold air down, creating a curtain of cold air. The air is then re-circulated through the system to be recycled after it is re-cooled.
The job of the air curtain is to create a barrier between the outside warm air and the cold air inside the case to maintain food temperature at Food and Drug Administration (FDA)-prescribed specifications.
"As the air moves downward toward the air return grill it gets mixed with warm air, what we call entrained," said Navaz, "and some of the warm air becomes infiltrated. It is because some of the air that is recycled is warmer than the discharge air that it has to be cooled down again."
How much of the warm air has infiltrated through the return air grill into the display case defines the cooling load. The more warm that has infiltrated into the display case the harder the compressors have to work to make the warm air cool again. "Ideally you don't want any mixing," Navaz said. "You want whatever temperature you have at the top, around 27 to 28 degrees farenheit, to be what comes in at the bottom. However, the air going in at the bottom is closer to 45 degrees." Approximately 70 percent of the cooling load comes from the cold air curtain.
"Our job was to work on minimizing this mixing of warm air into to the cold air curtain. At first we tried to understand the problem. Kettering bought DPIV equipment and Dr. Henderson initially worked on the project. DPIV at that time was very new," said Navaz, "and we are still probably one of a handful of schools in the country that have the equipment."
In using DPIV technology the air is seeded with smoke containing very small particles and then lasers are shot through the air and a digital camera records the data by tracking the particles of smoke in the airflow. That data is put into a computer, which compiles statistical analyses and identifies the velocity field. The velocity field indicates which direction the airflow is moving.
The images taken of the laser are the "streamlines" showing where the outside air is being entrained into the air curtain. "By looking at this data we can measure how much air is being entrained - based on actual measurements - so we don't have to rely on computational modeling or guesswork," said Navaz. "this is actually what is happening. So, we can calculate the amount of warm air - or entrained air - and find out how much of this has infiltrated the cold air curtain.
"It took us about three years to understand the problem and one year to get all the terminology right and get the message out to the research community," Navaz said. "Now, the terminology we established are widely used and people in the industry and researchers distinguish between entrained and infiltrated air," he said.
Prior to the Kettering research, there was no methodology for calculating the infiltration rate. "There was a misconception throughout the whole industry," said Navaz, "everybody thought that the amount of entrained air meant also infiltration, but that is not true. You could have 10 pounds of air coming in and 10 pounds of air going out and if you run it into a solid wall it bounces back you haven't infiltrated anything."
"Entrainment means how much air is being pulled in, but how much of it goes in the return grill is infiltration," Navaz said. Entrainment means the air can be pulled in but some of it bounces out. Infiltration is how much actually enters into the cold air curtain. Reducing entrainment does not necessarily mean a reduction in infiltration.
"We needed to get our jargon right so everybody would be talking about the same thing," he said. "Now that we have identified the difference and published our findings we have established that terminology."
After calculating and analyzing the rate of entrainment and infiltration, the Kettering research team projected that a 10 percent energy savings from each display case, multiplied by the number of display cases in the world, would translate into a lot of energy saved.
"Ten percent is do-able because these display cases are not built 100 percent efficient," said Navaz. "So now we need to come up with a design and play with different parameters to examine the way we can minimize the infiltration of warm air."
Putting doors on refrigerated cases is not a viable option. "Doors cause sales to go down," explained Navaz, "one of the big chains in California tried using cases with doors and sales went down. The manufacturers of the cases and the stores that use them have been very cooperative. They have provided us with invaluable field data related to cooling loads, electric bills and information about sales relating to doors and display cases."
Navaz is currently in the process of setting up a permanent DPIV lab on campus, complete with lasers, that will enable further research efforts and expand undergraduate lab opportunities. "We are including these experiments as part of our energy systems lab for undergraduates," he said. "We are gradually bringing this into our curriculum. We had a display case donated by Hill-Phoenix to use for experiments in the energy systems lab."
Mazyar Amin, a Ph.D. student from the University of Washington-Seattle, is at Kettering as part of his degree program, to help set up the lab. The lasers will be set up in the old energy systems lab in the Academic Building and will include a closed circuit television and safety shields. The targeted time frame for completion is September 2004.
The Energy Systems Lab at Kettering encompasses a wind tunnel, super sonic nozzle experiments, a mini jet engine and fuel cells. The addition of DPIV will enhance the lab offerings and will set Kettering apart among its peers. "We'll probably be one of very few undergraduate schools to offer DPIV to students," said Navaz.
It's a great opportunity for our students," Navaz said, "they can get exposure to latest technology, and when they go to work and someone says DPIV they know something about it."
In addition to incorporating the DPIV technology and cold air curtain research into the curriculum, Navaz is currently putting together a 15-member world wide advisory committee of fluid dynamics experts from England, Sweden and the U.S. to critique the research and make suggestions. "We want our work to go through a lot of analysis and critique, our data has to be flawless," he said.
Written by Dawn Hibbard
(810) 762-9865
dhibbard@kettering.edu