For most heat exchangers, in the bottom and out the top is the safe bet in absence of a solid reason contrary. No matter how hard you try to burp the air out, there will always be more leaking in via pump seals and the like, and it will accumulate in HEs that flow downwards, gradually reducing capacity and causing corrosion.
Automotive radiators, for example, generally flow cross/down, BUT there is an air release at the top. Heater cores and engines flow up. Where there is down flow, it is with enough velocity to sweep the air through.
Automotive radiators, for example, generally flow cross/down, BUT there is an air release at the top. Heater cores and engines flow up. Where there is down flow, it is with enough velocity to sweep the air through.