Performance Of Flat Heat Pipes Research Articles

Experimental Investigation on Affecting Air Flow against the Maximum Temperature Difference of a Lithium-Ion Battery with Heat Pipe Cooling

Research on battery thermal management systems (BTMSs) is particularly significant since the electric vehicle sector is growing in importance and because the batteries that power them have high operating temperature requirements. Among them, heat pipe (HP)-based battery thermal management systems have very high heat transfer performance but fall short in maintaining uniform temperature distribution. This study presented forced air cooling by an axial fan as a method of improving the cooling performance of flat heat pipes coupled with aluminum fins (FHPAFs) and investigated the impact of air velocity on the battery pack’s maximum temperature differential (ΔTmax). All experiments were conducted on lithium nickel manganese cobalt oxide (NMC) pouch battery cells with a 20 Ah capacity in seven series connections at room temperature, under forced and natural convection, at various air velocity values (12.7 m/s, 9.5 m/s, and 6.3 m/s), and with 1C, 2C, 3C, and 4C discharge rates. The results indicated that at the same air velocity, increasing the discharge rate increases the ΔTmax significantly. Forced convection has a higher ΔTmax than natural convection. The ΔTmax was reduced when the air velocity was increased during forced convection.

The visualized investigation on flow and heat transfer characteristics of flat-plate heat pipe with micro rectangular grooves

With the development of information technology, the heating power of data transmission equipment (mobile phones, notebook computers, data center processors, etc) continues to rise, and the high temperature failure has become a major cause of equipment damage. At present, the heat flux of high-power chips has reached 2 ´ 106 W/m2. Therefore, the traditional single-phase heat transfer is unable to meet the heat dissipation requirements of new data equipment, and more efficient heat exchange methods are urgently needed to ensure the normal operation of the equipment. As a highly efficient phase change heat transfer element, the thin plate grooved heat pipe is researched in the order of millimeters or even micrometers. Therefore, flat grooved heat pipes simultaneously satisfy the heat dissipation and packaging requirements of high power data transmission equipment, and thus have been extensively studied. In the present study, the phase change characteristics and heat transfer performance of flat-plate heat pipes with rectangular grooves were experimentally studied. Each micro heat pipe test section consisted of twenty parallel grooves with the same total length of 90 mm. The cross-sectional area of G-400 heat pipe and G-800 heat pipe were 400 μm ´ 400 μm and 800 μm ´ 800 μm, respectively. The evaporation section, adiabatic section and condensation section had the same size of 30 mm, and vapor space height was equal to 3.6 mm. The flat-plate was sealed on its upper face with transparent plate, and the phase change phenomenon in the grooves could be observed. A high speed camera was used to visualize the phase change phenomenon in evaporation and condensation regimes. The experiments were carried out with the same cooling water temperature of 30°C under the conditions comprising the heat power range of 18–90 W and filling ratio of 150%–400%, respectively. In order to systematically investigate the performance of flat heat pipes with different dimensions of micro grooves, the analysis was focused on the heat resistance and phase change phenomenon in evaporation section and condensation section. The results showed that the heat pipe with wide grooves was able to maintain stable heat transfer performance at higher heat power, while heat pipes with narrow grooves would dry out at lower heat power. The optimal liquid filling rate of G-400 heat pipe was 400%, and the optimal liquid filling ratio of G-800 heat pipe was 150%. Furthermore, the heat transfer in the evaporation section of flat heat pipe with narrow grooves was mainly dominated by liquid film evaporation. Comparatively, the heat transfer phenomenon in the evaporation section of a heat pipe with wide grooves was affected by the liquid filling ratio. The liquid film evaporation was the main solution at low liquid filling ratio, and continuous bubble behavior was visualized in high filling ratio. In addition, the phase change behaviors in the condensation section of rectangular-grooved flat heat pipe not only occured at the gas-liquid interface, but also had droplet condensation phenomenon on the top surface of the groove rib. The cycle of condensation consisted of three stages: Droplet growth, droplet coalescence, and droplet department.

Topology optimization of the wick geometry in a flat plate heat pipe

In the present study, a topology optimization approach is proposed to determine an optimal geometry of a wick sintered inside a flat plate heat pipe. Total thickness of the heat pipe is assumed as fixed, and thus the task involves redistributing the wick material and selecting the internal shape of the vapor core to provide a minimum power dissipation in the liquid flow and minimum total pressure drop in the entire heat pipe. Constraints are used for the maximum volume of the wick and maximum allowable temperature of the heat pipe wall. With respect to the preliminary assessments, the simulations are realized based on simplified 2D steady state thermal and hydrodynamic models assuming constant temperature and laminar flow in the vapor core and Darcy’s law for the liquid flow through the porous wick. Optimization results are presented for rectangular flat heat pipes with different lengths and thicknesses. Optimal placement of the wick columns and grooves are obtained by using the proposed topology optimization scheme. The results indicate that the use of the wicks of optimal shape increases the operating performance of flat heat pipes and especially increases their heat transfer capability up to twice that of heat pipes with flat wicks of constant thickness.

Transport in Flat Heat Pipes at High Heat Fluxes From Multiple Discrete Sources

A three-dimensional model has been developed to analyze the transient and steady-state performance of flat heat pipes subjected to heating with multiple discrete heat sources. Three-dimensional flow and energy equations are solved in the vapor and liquid regions, along with conduction in the wall. Saturated flow models are used for heat transfer and fluid flow through the wick. In the wick region, the analysis uses an equilibrium model for heat transfer and a Brinkman-Forchheimer extended Darcy model for fluid flow. Averaged properties weighted with the porosity are used for the wick analysis. The state equation is used in the vapor core to relate density change to the operating pressure. The density change due to pressurization of the vapor core is accounted for in the continuity equation. Vapor flow, temperature and hydrodynamic pressure fields are computed at each time step from coupled continuity/momentum and energy equations in the wick and vapor regions. The mass flow rate at the interface is obtained from the application of kinetic theory. Predictions are made for the magnitude of heat flux at which dryout would occur in a flat heat pipe. The input heat flux and the spacing between the discrete heat sources are studied as parameters. The location in the heat pipe at which dryout is initiated is found to be different from that of the maximum temperature. The location where the maximum capillary pressure head is realized also changes during the transient. Axial conduction through the wall and wick are seen to play a significant role in determining the axial temperature variation.