Designing grocery display cases has a lot in common with aeronautical engineering. Refrigerated display cases shoot jets of air across their front openings, creating an invisible shield that aims to keep cold air in and warm air out. Current technology does this with limited success. Engineers at the University of Washington and Kettering University have found that tweaking the physics can reduce energy by as much as 15 percent.
“Most of the energy these cases use goes into cooling infiltrated air,” explains Dana Dabiri, a UW associate professor of aeronautics and astronautics. “Some energy goes to extract the heat from lighting and fan motors, some goes to remove the heat gain from radiation and conduction, but 75 percent of the cooling load is attributed to infiltration of warm and moist air from the surrounding environment.”
Open-air coolers are increasingly popular compared to other options. Refrigerated cases with doors are good at keeping cold air in, but they fog up when opened and can frustrate shoppers who want to look at more than one product while making a choice. Another design is to hang sheets of clear plastic in front of the opening, but some see this as tacky. Refrigerated bins that are open on top waste less energy because the cold air is heavier, and tends to stay inside the case. The big energy hog, and the focus of the UW research, are open-air vertical shelves.
The team includes principal investigator Homayun Navaz, a professor of mechanical engineering at Kettering who specializes in computational fluid dynamics and fluid flow simulations, and Mazyar Amin, a former UW doctoral student now doing postdoctoral research at Missouri’s Saint Louis University.
The researchers built a modular mock display case and an air curtain simulator to test various designs. They measured how much air was infiltrated for various air curtain speeds, angles, and other factors to minimize the amount of warm, moist air entering the chilled compartment of the case.
Key variables strongly affect the amount of warm air penetrating the air curtain. Results show that the most important factors are the angle between the case’s discharge and return air grilles, and the jet’s exit Reynolds number, a figure that depends on the air speed and density, and the jet's turbulence intensity.
Combining experimental results and mathematical models, the team developed a tool that lets manufacturers optimize their particular design. Researchers collaborated with a leading display-case manufacturer to retrofit a proof-of-concept case. Tests showed the retrofit was a cost-effective way to get a 10 percent reduction in infiltration of warm air. Calculations for other display designs show potential savings of up to 15 percent.
An industry-wide implementation of the findings across the U.S. would save roughly $100 million in electricity costs each year.