Flat Plate Heat Pipes: From Observations to the Modeling of the Capillary Structure

Abstract

This paper presents a synthesis of observations made inside six different flat plate heat pipes (FPHPs). Silicon and copper systems with different capillary structures and sizes are studied. They are hermetically sealed on their upper face with a transparent plate, which enables liquid-vapor interface observations. A confocal microscope is used to measure the liquid film shapes inside the systems and describe the meniscus curvature radius from the evaporator to the condenser. Physical mechanisms, including the boiling limit and the effect of the interfacial shear stress, involved in different capillary structures such as grooves or meshes are discussed. Experimental results are compared to an analytical model describing both the liquid and vapour flows inside the FPHP and coupled to an analytical solution for the wall temperature. The capillary structure is modeled by considering the permeability and the equivalent thermal conductivity of a porous medium. The comparison between the model and the experimental data makes possible the estimation of the physical properties of the capillary structure. These experimental parameters are introduced in the model to estimate the performance of the heat pipe in a real application with several electronic components.

Assessment of Vapor Formation Rate and Phase Shift between Pressure Gradient and Liquid Velocity in Flat Mini Heat Pipes as a Function of Internal Structure

“…Under the same conditions, there is a reduction in the radius of curvature of the liquid meniscus in the condenser. This has been experimentally found by F. Lefèvre et al [ 18 ], who showed that if the space through which the vapor flow decreases by 1 mm, the heat transfer capacity of the FMHP decreases by up to 35%. An important condition in the design of an FMHP is the capillary boundary.…”

A general analytical model for the design of conventional heat pipes

“…Estimations of the capillary structure properties have already been presented in previous works using the model of Lefèvre and Lallemand [20] in the case of configuration B. This approach was efficient to estimate the properties of longitudinal grooves [27], crossed grooves [26] and metallic meshes [28]. The present analytical model enables the generalization of this approach for other heat pipe configurations.…”

Designer fluid performance and inclination angle effects in a flat grooved heat pipe

“…Nucleation is typically considered a cause of dry-out in a variety of wicks and referred to as the boiling limit [18]. However, several studies have shown that nucleation in grooved wicks does not constitute an operation limit and may even increase the performance of the heat pipe [19,20]. Chen et al [21] studied capillary and boiling limits in 200-500 µm square cooper grooves.…”