microchannel, capillary flow, liquid metal
Aluminum micro-channel coils are extensively used in automotive air conditioning systems, owing to the usage of multi-port micro-channel tubes and effective fin design for the air-side heat transfer enhancement. The superior thermal performance and compact structure are attracting new applications of aluminum micro-channel heat exchangers in HVAC&R industries. The increased market demands on micro-channel heat exchangers require better understanding and subsequent good control of the manufacturing process. For example, fabrication of the microchannel coils involves brazing of multi-port tubes with header manifolds and other components. Since capillary flow of molten filler metal is involved in a typical brazing process, there is a possibility of the liquid metal flowing into the micro-channels driven by surface tension force and subsequently blocking the refrigerant passages. This phenomenon can be overlooked during the manufacturing process due to the difficulty in identifying channel blockage using non-destructive testing methods. Undesired consequences, such as a depletion of the filler metal needed for brazing and non-uniform distribution of refrigerant flow during heat exchanger operation may occur. Current trends of reducing multi-port tube diameters and refrigerant charge lead to even smaller micro-channel port dimensions. Therefore, a good control of the manufacturing process becomes more critical. In this paper, the phenomena closely related to liquid filler metal flowing through micro-channels driven by surface tension force are studied. A hot stage microscopy system and a transparent lab furnace are used to experimentally visualize the capillary flow of molten filler metal through the micro-channels. Influences of manufacturing conditions, such as heat exchanger geometry, brazing parameters, filler metal and substrate materials on flow behavior are examined. Theoretical models for liquid metal flowing through capillary channels are also discussed. The overall goals are to better understand the capillary flow phenomena occurring during the fabrication of micro-channel heat exchangers, and to explore the potentials of control and/or utilization of the liquid metal flow characteristics in various manufacturing applications.