(A) study on the numerical optimization of the thermal performance of pulsating heat pipes

Abstract

In this thesis, the thermal performance of pulsating heat pipes is numerically optimized in terms of the number of turns and the channel diameter under the constraint of fixed space. Thermal optimization is performed for tubular pulsating heat pipes and flat-plate pulsating heat pipes. For this, numerical models for predicting the thermal performance of tubular pulsating heat pipes and that of flat-plate pulsating heat pipes are developed, respectively. To predict the thermal performance of tubular pulsating heat pipes, a one-dimensional slug/plug flow model is developed. Conjugate heat transfer between the solid wall and oscillating liquid slugs/vapor plugs is simulated. The solid wall and liquid films are described in the Eulerian reference frame which is fixed in space while liquid slugs and vapor plugs are described in the Lagrangian reference frame which follows the corresponding liquid slugs and vapor plugs. Heat transfer between the solid wall and liquid slugs/vapor plugs is classified into sensible heat transfer via the oscillation of liquid slugs and latent heat transfer via liquid films enclosing vapor plugs. The liquid film thickness can vary as a result of evaporation or condensation, therefore fitting parameters with regard to the liquid film thickness are not required. Merging of liquid slugs and the formation of new vapor plugs by nucleation are also accounted for. Based on the developed slug/plug flow model, a new numerical model for predicting the thermal performance of flat-plate pulsating heat pipes is proposed. To consider heat transfer between two adjacent channels through heat conduction, a three-dimensional heat conduction model is combined with the developed slug/plug flow model. The flow inside the channels is treated in one dimension while heat conduction within the substrate material and conjugate heat transfer between the solid wall and liquid slugs/vapor plugs are treated in three dimensions. The developed numerical models are validated by comparing the numerical results with the experimental data available in literature. Using the developed models, flow and heat transfer characteristics of pulsating heat pipes are numerically investigated. Finally, the thermal performance of pulsating heat pipes is numerically optimized in terms of the channel diameter and the number of turns, under the constraints of fixed space and fixed section lengths. The optimum number of turns appears because either an increase of the number of turns or an increase of the channel diameter enhances the thermal performance but these two variables are limited by each other from the geometric point of view. A merit number is proposed to find an optimum number of turns of pulsating heat pipes under the constraints of fixed space and section lengths. It is numerically found that the thermal performance is optimized when the merit number is maximized. Therefore, the proposed merit number provides a design guideline for pulsating heat pipes under the constraints of fixed space and section lengths.