Efforts to phase down refrigerants with significant global warming impacts are accelerating research and development to implement natural refrigerants. Some of these refrigerants are flammable and thus require compliance with various safety codes and regulations. One approach to mitigating risk is to reduce the refrigerant charge in a given system. Reducing the refrigerant charge also reduces the direct emissions associated with its use. In many air-conditioning and heat pump systems, the majority of the refrigerant charge resides in the heat exchangers, which is why there is a push towards more compact heat exchangers. In the context of air-to-refrigerant heat exchangers, this implies the use of smaller hydraulic diameter flow channels. This presentation summarizes advances over the last ...