Header design tradeoffs in microchannel evaporators

This paper uses a simulation model to quantify design tradeoffs associated with refrigerant maldistribution caused by header pressure gradients in microchannel evaporators. Mass flow maldistribution was investigated by using well-known pressure drop and heat transfer correlations for two-phase flow, selected after comparing with the experimental results obtained by using different correlations. Results show that mass flow maldistribution cannot be controlled by changing either port/header diameter or the refrigerant state at the inlet to the ports, only by minimizing pressure gradients along header. At the same time the need to avoid gravitationally induced phase stratification in the header places a lower bound on mass flux. The dependence of heat exchanger capacity on header diameter and the length was investigated in detail. Results demonstrated that a requirement for inertially dominated flow in the inlet header severely limits the set of feasible evaporator geometries. Therefore a fundamentally different concept for header design was identified and characterized, based on parametric analyses.