Refrigerant distribution effects on the performance of microchannel evaporators.
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant maldistribution as they typically have many parallel channels fed by a common header or manifold. This paper outlines general trends in refrigerant flow distribution in such evaporators taking into account various heat exchanger orientations. In addition, links between heat exchanger performance and refrigerant flow distribution characteristics in microchannel evaporators are made using both analytical and experimental methods. These results show that unification of the liquid refrigerant flow distribution in the evaporator can lead to quantifiable improvements in evaporator performance.
Flow boiling of ammonia and flow instabilities in mini-channels
Abstract A simultaneous visualization and measurement study has been carried out to investigate the flow boiling and two-phase flow instabilities of ammonia in a mini-channel evaporator at various heat and mass fluxes. The evaporator consisted of 4 parallel 1 × 1.1 mm channels with a uniformly heated length of 250 mm. The visualization study showed that periodic reverse flow occurred in the mini-channel evaporator with liquid rewetting stage and annular film evaporating stage repeated alternately. Both damped and stable oscillations of pressure drop, wall temperature and system pressure were observed. The liquid rewetting stage approximately coincided with the decreasing stage of system pressure. Three typical two-phase flow patterns: bubbly flow, elongated bubble/slug flow and annular flow were identified. Low-frequency high-amplitude oscillation and high-frequency low-amplitude oscillation were identified through wavelet analysis. The high-frequency oscillation mainly occurred at the wave crest of the low-frequency oscillation. In addition, high-order harmonics were found in almost all the low-frequency oscillation waves. The effects of mass flux and saturation temperature on flow boiling instabilities were also investigated.
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.