A one-tube pass cross-flow heat exchanger is considered for recovering energy from the exhaust gases of a turbine-driven engine. The heat exchanger is constructed of flat plates, forming an egg-crate pattern as shown in the sketch below. The velocities of the entering air (10°C) and exhaust gases (425°C) are both equal to 61 m/s. Assuming that the properties of the exhaust gases are the same as those of the air, estimate for a path length of 1.2 m the overall heat transfer coefficient U, neglecting the thermal resistance of the intermediate metal wall. Then determine the outlet temperature of the air, comment on the suitability of the proposed design, and if possible, suggest improvements. State your assumptions.
GIVEN
The heat exchanger shown above
Air and exhaust velocities (Va = Ve) = 61m/s
Inlet temperatures
Air (Ta,in) = 10°C
Exhaust (Te,in) = 425°C
Path length (L) = 1.2 m
ASSUMPTIONS
Steady state
Exhaust gas properties are the same as air
Thermal resistance of the metal walls is negligible
Thermal properties can be evaluated at the average temperature
Heat losses from the exterior walls are negligible
Dividing walls within one side of the exchanger do not participate in the heat transfer process