Interaction of flow pattern and heat transfer in oscillating heat pipes for hot spot applications

Abstract

Oscillating heat pipes (OHP) are increasingly used for the thermal management of hot spots. Thereby, a local evaporation zone occurs that is still not fully understood due to complex interactions of flow patterns and heat transfer. This study quantifies the thermal performance and the flow behavior of oscillating heat pipes with a centrally located hot spot heater. The copper OHPs were filled with acetone and tested with different orientations. The filling ratios varied from 0% to 90%, the heat inputs increased from 25 W to 200 W. Our results illustrate that the flow velocity values and the resulting thermal resistance are highly interrelated. Within one channel, there is a wide range of velocity values from 30 mm/s to 1200 mm/s; the thermal resistance decreases with higher average flow velocity. By characterizing the flow regime, we show that a reliable start-up depends on the initial vapor liquid pattern. Depending on the initial pattern, an individual temperature difference is required for the start-up, even for similar operating conditions. The analysis of the stopping behavior shows that a uniform temperature difference of 14 K is required for a stable fluid oscillation. Our study substantiates a strong interrelation between flow pattern and heat transfer in OHPs, which also shows a strong potential for future optimizations of OHP applications.