how to do recirculation problems in fluent

arvind

i hve to analyse a cross flow HEX of air and cylinder encapsulated with phase change material ( two rows) for an air conditioning application.

can anybody tell me how to recirculate the air through the HEX till the required temperature is achieved. i modelled it for one pass and i dont know how to recirculate it.

if i give recirculation boundary type in gambit what models i have to enable in fluent to run the simulation.