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This study investigated the boiling heat transfer and pressure drop of hydrocarbon refrigerant (R-600a) flowing in six minichannels with fillisters made by electrical-discharge machining. The tests were run at a inlet pressure of 259-293 kPa and under saturated conditions, with the Reynolds number of 4,400-11,000 (i.e. mass flux of 195-487 kg/m 2 s), heat flux of 1,790-8,950 W/m 2 and outlet vapor quality of 0.041-0.25. Effects of the geometries of the fillisters, Reynolds number, heat flux and refrigerant quality on the heat transfer coefficient, friction factor and enhancement performance ratio were examined. The results of the minichannels with fillisters (Tests 1-4) compared to the minichannels without any fillister (Test 5) showed that the heat transfer coefficients increase about 1.05-1.34, 1.11-1.25, 1.23-1.59 and 1.07-1.21-fold, respectively. In addition, the friction factors for Tests 1, 2, 3 and 4 were about 3.3-9.4, 11.2-15.6, 14.7-21.9 and 5.0-6.3 % larger compared to that of the minichannels without any fillisters for Test 5. It was also found that Test 3 had the best enhancement of the performance. In summary, this study strongly suggests the use of fillisters constructed in minichannels. List of symbolsA Heat transfer surface area of a single mini-channel (m 2 ) A c Cross-sectional area of a single mini-channel (m 2 ) B Width of the minichannels (m) D h Hydraulic diameter (m) h Heat transfer coefficient (W/m 2 K) H c Depth of the minichannel (m) H f Depth of the fillister (m) I fg Latent heat (J/kg) k Thermal conductivity (W/m K) L Length of the mini-channels (m) _ m Liquid mass flow rate (kg/s) n Number of fillisters N Number of mini-channels P Pressure (kPa) Pr Prandtl number, Pr ¼ C p' l ' =k ' DP Pressure drop (kPa) Q Actual heat transfer rate to the working fluid (W) q Heat flux (W/m 2 ) T Temperature (K) X Length of the fillister (m) Y Gap between fillister (m) DT Wall superheat, DT ¼ T w À T sat (K) h Enhancement performance ratio, defined in Eq. (7) u Velocity (m/s) l Liquid viscosity (Pa s) q Fluid density (kg/m 3 ) Subscripts f Minichannels with fillisters '

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