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• A thinner inverted flag starts flapping at a lower wind speed. • The heat dissipation effect is strongly affected by the inverted flag motion. • The flapping mode induces the highest average vorticity compared with other modes. • The flapping mode induces the largest heat transfer enhancement compared with other modes. It has been demonstrated that flexible vortex generators, e.g., flapping flag, can significantly enhance heat transfer inside a heat sink. However, their heat transfer enhancement is only effective when they exhibit flapping behaviors, which require a flow velocity higher than the heat sink working velocity, and thus restraint their application. Minimizing the critical flapping velocity of the flags without sacrificing the heat transfer performance is needed. In this work, we study the cases of inverted flags with different thicknesses in a channel flow. Three flag motion modes are identified by a high-speed camera with increasing flow velocity. In the first mode transition, i.e., the flag starts flapping, the heat dissipation has the highest enhancement. Numerical simulation reveals that compared to the other motion modes, the flapping mode has the strongest average vorticity along the channel wall, leading to the highest heat dissipation among all flag motion modes. Experimental results show that the critical velocity can be as low as 1.5 m/s, at which the heat dissipation enhancement can be as high as 100%. The findings in this work significantly benefit the application of flexible vortex generators in heat sinks, by enabling a decrease in critical velocity and a good enhancement in heat dissipation. [ABSTRACT FROM AUTHOR]

• Inserting a fluttering flag into a channel enhances its heat dissipation. • Heat dissipation effect by a split flag is superior to that by a full flag. • Hydraulic loss of a split flag is lower than that of a full flag. • A split flag starts flutter at a lower wind speed than a full flag. • The fluid-structure interaction of a split flag is stronger than that of a full flag. The use of flags as vortex generators inside heat sinks has been successfully demonstrated as a heat transfer enhancement technique. However, their thermal-hydraulic performance is usually diminished by the blocking effect induced by the fluttering phenomenon. To tackle this problem, with the expectation to maximize the fluid mixing while minimizing the pressure drop, we report a simple and direct design by splitting a flag into multiple strips. Flags with different strip widths were compared with a full flag on the performance of pressure drop and heat transfer. A high-speed camera is used to investigate their fluttering motion. A piezoelectric plate is attached to the wall to measure the flag's flutter frequency. The results show that the performance of a flag with multiple strips outperforms that of the full flag for its lower pressure drop and higher heat dissipation effect. The performance of split flag is not linear with the strip number and the optimal way in our study is to split the full flag into 4 strips. The maximum thermal-hydraulic performance factor of the split flag is 1.91, which is 26% higher than that of the full flag. Besides, the split flag starts fluttering at a lower wind velocity. All these results demonstrate that the split strategy of a flag acting as a vortex generator is of great potential for improving the heat sink performance. [ABSTRACT FROM AUTHOR]