Davoud Jafari

Theoretical analysis of screened heat pipes for medium and high temperature solar applications

A mathematical model is applied to study the cylindrical heat pipes (HPs) behaviour when it is exposed to higher heat input at the evaporator for solar collector applications. The steady state analytical model includes two-dimensional heat conduction in the wall, the liquid flow in the wick and vapour hydrodynamics, and can be used to evaluate the working limits and to optimize the HP. The results of the analytical model are compared with numerical and experimental results available in literature, with good agreement. The effects of heat transfer coefficient, power input, evaporator length, pipe diameter, wick thickness and effective pore radius on the vapour temperature, maximum pressure drop and maximum heat transfer capability (HTC) of the HP are studied. The analysis shows that wick thickness plays an important role in the enhancement of HTC. Results show that it is possible to improve HTC of a HP by selecting the appropriate wick thickness, effective pore radius, and evaporator length. The parametric investigations are aimed to determine working limits and thermal performance of HP for medium temperature solar collector application.

An investigation of porous structure characteristics of heat pipes made by additive manufacturing

Specific properties of porous media such as thermal conductivity and wicking of liquid into the porous structure are of great importance to many applications. Typically, such porous structures are found in two-phase devices, such as heat pipes (HPs). In this study, we have experimentally analysed the effective thermal conductivity and the wicking of different liquids into a stainless steel 316L porous structure fabricated through selective laser melting (SLM) technology. The sample was rectangular shaped with a porosity of 46.5% and outer dimensions of 20×40×1 mm3. An experimental apparatus and related procedures for the determination of the effective thermal conductivity of the porous structure saturated with distilled water and Ethylene glycol are discussed. The experimentally measured values of effective thermal conductivity are compared with correlations available in the literature. The standard Washburn wicking model is taken into account for the analysis. We describe the application of the Washburn equation to measure the contact angle of a printed porous sample with three test liquids; n-Hexane, water and Ethylene glycol are used to measure contact angles. The experimental results verify that SLM technology can be used to fabricate porous structures for HP technology. The results show an effective thermal conductivity in the range of 1.8–6.0 W/mK for different working fluids.

Design and experimental analysis of a screened heat pipe for solar applications

This paper summarizes the design, the construction and the preliminary results of a transient and steady state investigation of the heat transfer mechanisms of a horizontal heat pipe (HP). The experiments are performed using a custom-made HP constituted by copper tube with outer diameter and length as 35 mm and 510 mm, respectively, with the inner surface covered by three layers stainless steel mesh wick (100 mesh/inch). Water is used as a working fluid. The evaporator section is heated by electrical resistances wrapped around the tube and the cooling system consists of an insulated water manifold with inner diameter of 39 mm, connected to chilled water bath to maintain the inlet temperature of the circulating cooling water at 25 °C for various heat loads (30-100 W). The aims of this activity is to obtain data to verify the steady state HP analytical model already presented by authors at a fixed filling volume and to determine the effect of the heat transfer load on the heat transfer performance of screen mesh HPs. The heat transfer coefficients are determined using thermocouples on the outer wall and within the core of the HP. The agreement between the analytical results and the preliminary experimental data appears to be very good.