Two-phase closed thermosyphons: A review of studies and solar applications

Abstract

The use of the two-phase closed thermosyphons (TPCTs) is increasing for many heat transfer applications. This paper reviews the most recent published experimental and theoretical studies on the TPCT. After a description of the TPCT operating principle and the performance characteristics, the heat transfer analysis in condenser and evaporator sections that depends on the complex two-phase process are described. The influence of the affecting parameters on the performance of TPCTs such as the geometry (diameter, shape and length), the inclination angle, the filling ratio (FR), the working fluid, the operating temperature and pressure analyzed by various researchers is discussed. The various operating limits occurring in a thermosyphon includes viscous, sonic, dryout, boiling and flooding are also analyzed. Considering the application of TPCTs, the paper presents a review of experimental tests and applications. This paper can be used as the starting point for the researcher interested in the TPCTs and their renewable energy applications.

Introduction

Heat pipes (HPs) have emerged as the most appropriate technology and cost effective thermal control solution due to their excellent heat transfer capabilities [1], [2]. Interesting connection can be evidenced in the literature between HPs and thermal engineering applications [3]. A special HP where the condensed liquid moves to the condenser by gravity is two-phase closed thermosyphon (TPCT). In this device no capillary structure is present and it works in a two-phase close cycle where latent heat of evaporation and condensation is used to transfer heat. The TPCT can work in gravity [4] or antigravity mode with the condenser above the evaporator [5]. In this last case the system operates in an unsteady, e.g. periodic regime. There are other kinds of two-phase thermosyphon systems such as two-phase closed loop thermosyphons (TPCLTs) [6] and two-phase microchannel thermosyphon [7].

A TPCT can be used in much wider thermal and temperature ranges than a wicked HP, since it does not have the large flow resistance or low boiling limit inside the wick, as the condensate liquid in the TPCT is returned to the heated side of the system under the effect of gravity, instead of capillary forces in wicked HPs [8]. However, TPCTs have major limits on the maximum amount of thermal energy that can be transferred due to viscous, sonic, dryout, flooding and entrainment limits [9].

The TPCT technology has found increasing interest of the researchers in a wide range of applications from small-scale to large-scale systems. TPCTs are used in chemical and petroleum industries applications [10], electronic cooling [11], [12], telecommunication devices [13], energy storage systems [14], the railway transportation systems [15], thermoelectric power generators [16], seasonal cooling load reduction of buildings [17], cooling of super conducting bearings [18] and various heating and cooling applications [19]. TPCTs can be also used for thermal control in particular in connection with solar energy systems like solar collectors [20] or photovoltaic systems [21]. A general description of TPCT systems is reported in Fig. 1.

The widely spread use of TPCTs has led to increase the demand for analytical and numerical models and predictive tools that would allow the thermal engineers to predict their performance. There are experimental studies which have tried to predict the complex mechanism inside the TPCT. Although considerable studies have been undertaken to investigate the behavior of the TPCT in steady operating conditions, there still exists considerable uncertainty in the description of the complex physical phenomena of TPCTs. The heat and mass transfer through a TPCT is, in fact, significantly affected by different parameters and design variables such as: geometry, inclination angle, filling ratio (FR), thermophysical properties of working fluid, and vapor temperature and pressure. The large number of physical parameters affecting the TPCT performances has, therefore, prevented a complete understanding of the mechanisms of the heat and mass transfer in the TPCT and the possibility of analyzing the system performance under a variety of operating conditions and design variables. This complex mechanisms include: wall and interfacial heat transfer; thin film flows, interfacial mass transfer, compressible vapor effects and phase-change phenomena (boiling, condensation and evaporation). Considering the above, the present review outlines the state-of-the-art of TPCT technology, design methodologies and applications. The paper is organized into two main sections: thermal analysis of TPCTs and applications. The first section is divided into five different parts: background and operating principle, heat transfer analysis, analytical and numerical approaches, operating limit, and experimental devices. The second section analyzes the applications with specific attention to recent published renewable energy application including: solar applications and air to air heat exchangers.