Arvind Pattamatta

The thermal performance and the internal flow regimes of a closed-loop flat plate pulsating heat pipe (FPPHP) are experimentally investigated. Reports on the FPPHP using the ethanol-water mixtures as working fluids are scarce in the literature. The binary mixtures with different boiling point components are suitable for a wide range of heat fluxes. Therefore, the results are reported for the ethanol-water mixtures of ratios 3:1, 1:1, and 1:3, and the corresponding pure liquids filled in the range of [40–80] % with power inputs given from 40 to 200 W. The effect of different condenser cooling modes, such as forced convective water cooling, forced convective air cooling, and natural convective air cooling on the thermal performance of the FPPHP, is also reported. With the increase in the power input, the observed flow characteristics in the FPPHP channels are: no oscillations, slug-plug oscillations, droplet oscillations, and the evaporator dry out. The binary mixtures with increased ethanol content give better slug-plug flow oscillations with smaller thermal resistances and fewer evaporator drying out instances than the pure working fluids. For power inputs of less than 120 W, the ethanol:water mixture ratio of 3:1 at all filling ratios gives a larger slug departure frequency in the evaporator. The smallest thermal resistance measured is 0.1 K/W, a decrease of 27% over pure ethanol. For power inputs greater than 120 W, the mixture ratio of 1:1 at all filling ratios performs better with continuous droplet oscillations. The smallest thermal resistance measured at the 80% filling ratio is 0.12 K/W, a decrease of 22% over pure ethanol. When the condenser cooling mode is changed to air cooling, the evaporator temperatures reach around 100 °C for power inputs greater than 40 W and 80 W for natural and forced air convection. Thus, the FPPHP filled with ethanol-water mixtures with the water-cooled condenser gives a stable flow regime and better thermal performance for a long-range of power inputs.

In the present work, surface wettability modifications were utilized to enhance the phase change heat transfer in a water-charged two-phase flat thermosyphon. A flat thermosyphon's thermal performance with various surface wettability modifications on evaporator and condenser plates was investigated for various heat inputs and filling ratios in the horizontal orientation. The evaporator and condenser's surface wettabilities were varied to superhydrophilic (contact angle of 0-1°) and superhydrophobic (contact angle of 155.4 ± 3°). Changing the evaporator's surface wettability to superhydrophilic nature increased the thermal resistance of thermosyphon due to the high superheat requirement and delay during bubble nucleation. A 43.74% decrease in the thermal resistance was observed for a thermosyphon with a superhydrophobic condenser due to the dropwise condensation and faster condensate return to the evaporator compared to the bare one. A lumped parameter model was used to predict the thermal resistance of flat thermosyphon with a superhydrophobic condenser and hydrophilic evaporator, which is in good agreement with the experimental results. The experimental results encourage research on a two-phase flat thermosyphon with a superbiphilic evaporator and superhydrophobic condenser as it can further improve thermal performance.

Thermal management is critical in improving the efficiency, lifespan, and reliability of sustainable energy devices such as LEDs, batteries, and solar cells. The compact flat thermosyphon (CFT) is more reliable for the effective thermal management of multiple heat sources in energy-intensive devices. A comprehensive study on the thermal performance of a two-phase CFT is conducted in the present study. The volume of the CFT is 72 × 24 × 15 mm3 and is made up of copper. The thermal performance of the CFT is analyzed by varying the filling ratio (20%, 50%, 80%), working fluid (acetone, ethanol, water), number of heat sources, and condenser surface wettability. The best-performing filling ratio is found to be 50%. Water is the best performing working fluid over a wide range of heat fluxes (10 – 70 W/cm2) due to high heat capacity, latent heat, and low vapor pressure. The CFT can safely cool up to four heat sources of size 12 × 24 mm2 at a heat flux of 30 W/cm2 each. The lifespan of the devices can be increased by 2.49 – 6.62 times when the CFT is used to cool multiple heat sources instead of a cold plate. The superhydrophobic condenser surface further improved the thermal performance of the CFT by 28 – 47% because of the quicker return of condensate and dropwise condensation.