HEAT TRANSFER ENHANCEMENT IN THE RADIATOR OF THE FUEL
CELL HYDROGEN CAR (Dhir)
One important engineering issue concerning fuel-cell vehicles is the radiator system. Because fuel-cell engines are different than their IC counterparts, their radiators are also different. In particular, they are much larger. "We're going to have to find innovative ways to get the heat rejection you need in fuel-cell vehicles," said McCormick [1].
"PEM fuel cells run at 80°C (175°F) because the membrane needs that temperature to keep the process going," explained DaimlerChrysler's Mohrdieck, "but we would really rather have a 120°C (250°F) operating temperature like IC engines because it's difficult to get rid of low-temperature heat, particularly when exterior temperatures are high. It's actually one of the biggest technical challenges; fuel cells need a very large radiator, which makes packaging and styling difficult by adding size and large air openings that hurt the drag coefficient. It's going to be easier for American cars to accommodate these concerns because of their larger size," he noted. "In addition, fuel-cell vehicles require two radiator systems - one for cooling the electronics and the electric motor and another for cooling the stack, which makes it awfully complicated."
The heat transfer rate on the gas side can be enhanced in a number of ways: increase in flow velocity; increase in surface area by surface modifications; and altering the flow field which in turn leads to enhanced heat transfer coefficient. In the proposed concept we will use swirl flow to enhance heat transfer on the gas side. Steam condensation will take place on the outside of the tube. Swirl flow can be created either by placing injector heads at the tube inlet or by placing twisted tape in the tube. Both methods will be examined. There is enough experience in the Boiling and Phase Change Heat Transfer Laboratory at UCLA in the use of swirl flows and twisted tapes to enhance heat transfer.
In fact UCLA has a patent on swirl flow enhancement technique. Experiments will be conducted in conditions that simulate the implementation of fuel cells in vehicles. An operational map will be developed under the constraint of pumping power and space available for dissipation of given amounts of thermal energy.
[1] http://www.sae.org/automag/features/fuelcells/fuelcell7.htm