Investigations on the Thermal Performance of Miniature Flat Heat Pipe

Abstract

Modern electronic devices face the problem of effective cooling due to their miniaturization and quickly increasing heat loads. The most recent additions to the spectrum of cooling methods used for electronic device cooling are two-phase heat dissipation devices, like heat pipes. Heat pipes are passive cooling systems that have recently been used in several electronic applications due to their extraordinarily high thermal conductivity and reliability. To provide thermal management for applications involving the cooling of next generation electronics, passive cooling methods using miniature flat heat pipes have garnered a lot of attention in the last decade. Due to their small size, miniature flat heat pipes (mFHP) create additional hurdles in manufacturing and in determining the influence of different variables on their thermal functioning. The limited exploration of miniature heat pipes and their recent applications in the thermal control of modern portable electronic devices motivates us to carry out these studies. The portable electronics devices may operate at different inclinations depending upon applications. Similarly, such devices may employ natural convection or forced convection cooling depending upon available spaces. So, it is important to study the influence of tilt angle, working fluid, and condenser cooling mode on the heat transfer potential of flat heat pipes (FHP). Five different tilt angles (0˚, 30˚, 45˚, 60˚, and 90˚), four various working fluids (water, acetone, ethanol, and methanol), and two condenser cooling modes (natural convection and fan assisted forced convection) were considered. The horizontal orientation of FHP had a better thermal functioning for low heat load applications with DI water, acetone, and ethanol as the working fluids. The vertical orientation of FHP produced a stronger thermal effect than other orientations using methanol as the working fluid. Due to their lower boiling temperatures, methanol and acetone are suitable working fluids for low heat loads and natural convection. The heat transfer performance of ethanol as a working fluid was better under larger heat loads. Under forced convection cooling mode and various inclined orientations, methanol outperformed other working fluids. With a 24 W heat load and a vertical orientation under forced convection condenser cooling, an FHP filled with methanol achieved the maximum effective thermal conductivity of 7846 W/mK. A working fluid for a flat heat pipe-based system that satisfied the specifications for the specific heat input and condenser cooling applications may be selected using the findings of this experimental study. The maximum heat carrying capacity of the miniature flat heat pipe utilizing conventional working fluids is limited by the reduced vapor core volume of mFHP. Using nanofluids as a working fluid is an appropriate approach to improve the mFHP's thermal characteristics. In the present study, graphene and Al2O3 nanoparticles were utilized in mass fractions of 0.1 weight percent and 0.5 weight percent, respectively. The thermal conductivity and viscosity of nanofluids, among other thermophysical characteristics, are crucial to the thermal behavior of FHP. From the perspective of heat transmission, thermal conductivity is crucial, while dynamic viscosity is essential from the perspective of working fluid circulation. Compared to other nanofluids, graphene has a better heat conductivity while Al2O3 has a lower viscosity. Because of this, both of these nanofluids were selected for the present experimental study. By substituting graphene and Al2O3 nanofluids for DI water as working fluids, the thermal behavior of FHP was enhanced. The FHP filled with Al2O3-DI water nanofluid achieved the maximum effective thermal conductivity of 6271 W/mK under a heat load of 14 W and 60˚orientation with natural convection condenser cooling. Due to larger mass fractions and lower wettability of Al2O3-based nanofluid, the effect of tilt angle on FHP thermal behavior was reduced in the case of Al2O3 nanofluid compared to graphene nanofluid. The electronics devices may have varying spaces to incorporate heat pipes for thermal management. Therefore, sometimes heat pipes are bent and used in different shapes for thermal management. The shape of the FHP impacts the working fluid's gravitational movement and, consequently, the thermal operation of the FHP. Therefore, it is important to investigate the effect of shape on the heat transfer functioning of FHP. In the present study, the four different geometries: horizontal, L-shaped (3 different orientations), Stair step-shaped, and U-shaped of FHPs were considered. The shape and orientation of FHP significantly affected its heat transfer behavior in addition to its physical and operational properties. In comparison to all other designs, the L-shaped FHP with case 1 arrangement offered the best heat transfer capability for forced and free convection condenser cooling. The L-shaped FHP with case 1 configuration at 12 W heat load and free convection condenser cooling achieved the maximum effective thermal conductivity of 9756 W/mK for this set of experimental trials. The present parametric study on FHP revealed that the heat input, the working fluid utilized in the heat pipe, and the type of condenser cooling can all affect the gravitational effect and the quantity of condensate movement; consequently, the ideal inclination angle can also change depending on the operating circumstances. The use of graphene and Al2O3 nanofluid improved the heat transfer behavior of FHP. The greater wettability, higher capillary pressure of the nanofluid, and the nucleation sites produced by nanoparticles are the responsible factors for such improvement. In addition to its physical and operational characteristics, FHP's shape and orientation have a considerable impact on its thermal performance.