Abstract

The aim of this chapter is to report the study of a novel liquid-vapour separatorincorporated gravity-assisted loop heat pipe (GALHP). This involves a dedicated conceptual formation, thermo-fluid analyses, and computer modelling and experimental validation. The innovative feature of the new GALHP is the integration of a dedicated liquid vapour separator on top of the evaporator section, eliminating the potential entrainment between the heat pipe liquid and the steam stream, while addressing the inherent 'dry-out' problem exhibited in the traditional GALHP. Based on this recognised novelty, a dedicated steady-state thermal model covering the mass continuity, energy conservation and Darcy equations were established. Under the specifically defined operational condition, the proposed GALHP has more evenly distributed axial temperature profile. The effective thermal conductivity in the proposed GALHP was 29,968 W/C m. It is therefore concluded that the novel heat pipe could achieve a significantly enhanced heat transport effect. The results derived from this research enabled characterisation of the thermal performance of the proposed GALHP and validation of the developed computer simulation model. The research will enable design, optimisation and analysis of such a new GALHP, thus promoting its wide application and achieving efficient thermal management.

Keywords: LHP, composite wick, start up, thermal conductivity

## 1. Background

A heat pipe [1, 2] is an effective heat transfer device functioning through the evaporation and condensation cycles without external driving forces, which is normally used under three circumstances: (1) transferring heat from the heat source to heat sink at a distance; (2) transporting a thermal shunt in an effective way and (3) dissipating heat effectively across a plane. To date, various types of heat pipes have been investigated for various applications, for example, electronics cooling, solar devices, heat exchangers, aerospace, medical applications and transportation system [3]. Of these, the most commonly used one is a gravitational straight type, which possesses the problems of increased flow resistances within the vapour and liquid flows and reduced overall heat transport capacity of the heat pipe.

Loop heat pipe (LHP) [3–5] is a two-phase (liquid/vapour) heat transfer device allowing a high thermal flux to be transported over a distance of up to several tens of metres in a horizontal or vertical position owing to its capillary or gravitational structure. LHP has a separate evaporator and condenser, thus eliminating an entrainment effect occurring in between. LHP can operate under different gravitational regimes, regardless of whether the evaporator is above or below the condenser.

In order to overcome the problems exhibited by the conventional GALHP, a novel liquid-vapour separator-incorporated GALHP was proposed, which is dedicated to simplify the wick structure, eliminate the 'dry-out' potential and, thus, create a high-efficient and cost-effective heat transport solution. Through the theoretical and experimental analysis, the analysis results will be compared with the conventional GALHP and conventional straight heat pipe. The research results could be directly used for design, optimisation and analyses of the new GALHP configuration, thus promoting its wide applications in various situations to enable

Study of a Novel Liquid-Vapour Separator-Incorporated Gravitational Loop Heat Pipe

Schematic of the proposed GALHP is shown in Figure 1, and the novel liquidvapour separator-incorporated GALHP is shown in Figure 2. This separator is configured as a three-way structure, internally containing a tubular pipe with a downward expanding opening, which is fitted into the top of the evaporator while the edge of the expanding opening is tightly attached to the wicked inner surface of the heat pipe. In this way, the return liquid will be reserved in the liquid reservoir above the evaporator, thus formulating a certain liquid head. Under the action of the liquid head, the liquid will penetrate through the peripheral gap between the pipe and the expanding opening, and flow evenly downward along the wicked surface of the heat pipe. Meanwhile, the evaporated fluid, in the form of vapour,

the enhanced performance of the GALHP heat transport to be achieved.

2. Description of the proposed GALHP

DOI: http://dx.doi.org/10.5772/intechopen.86048

Figure 1.

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Schematic of the proposed GALHP.

A conventional LHP is usually composed of the complex capillary pumps (evaporators), compensation chambers (storage), condensers and vapour and liquid transfer lines [6–8]. The working principle of the LHP device could be described as follows [1]: the heat transfer fluid in the wick absorbs the heat added to the evaporator and vaporises via the vapour line to the condenser. Within the condenser, the vapour will be condensed to the liquid of the same temperature and return to the compensation chamber through the liquid line. The liquid will then be accumulated and stored in the compensation chamber and further saturate the wick.

Numerous works in relation to LHP have been developed, for example, loop component designs, mathematical models, working fluid and wick structures. The first LHP was developed and tested in 1972 by Russian scientists Gerasimov and Maydanik [9]. A book written by Peterson [10] illustrated the performance limit approach for the heat pipe in the steady-state condition. Peterson [11] also analysed the heat pipe's heat transfer processes in the steady-state condition by using thermal resistances calculating method. In order to simplify the existing engineering models and reduce the required computing resources, Zuo and Faghri [12] developed a thermal network model to analyse the circulation of the working fluid in the heat pipe by using the thermodynamic cycle approach. Kaya and Hoang [13] modelled the performance of a LHP based on steady-state energy balance equations at each component of the loop. The loop operating temperature was found to be a function of the applied power at the given loop condition. Bai et al. [14] established a mathematical model for the start-up process of a LHP based on the node network method. Pauken and Rodriguez [15] modelled and tested a LHP with two different working fluids, that is, ammonia and propylene. Hoang et al. [16] mentioned that the heat transfer characteristic of a LHP was difficult to predict, owing to the complicated nature of the thermal interaction between the LHP and environment. Riehl [17] tested a LHP system operating with acetone as the working fluid. Zan et al. [18] established an experimental formula for a sintered nickel powder wick. Riehl and Dutra [19] presented the development of an experimental LHP. Vlassov and Riehl [20] explored LHP modelling by developing a relatively precise condenser sub-model from the solutions of the conjugate equations of energy, momentum and mass balances, and only describing a few transient nodes within the evaporator and compensation chamber. A more comprehensive dynamic model was published by Launay et al. [21], who proposed a transient model to predict the thermal and hydrodynamic behaviour of a standard LHP.

In recent years, the application of the LHP in solar thermal field has become more and more attractive owing to the significant technical advance in renewable energy [22–24]. LHP is suitable for use in building solar hot water system, owing to its unique features, that is, highly effective thermal conductance and flexible design embodiment and installation [1]. For such an application, the LHP was mostly operated under gravity-assisted conditions, and termed gravitation-assisted loop heat pipe 'GALHP'. The GALHPs have been identified with two shortfalls that need to be tackled with, that is, complicated wick structure and liquid film 'dry-out' problem [3, 4].

Study of a Novel Liquid-Vapour Separator-Incorporated Gravitational Loop Heat Pipe DOI: http://dx.doi.org/10.5772/intechopen.86048

In order to overcome the problems exhibited by the conventional GALHP, a novel liquid-vapour separator-incorporated GALHP was proposed, which is dedicated to simplify the wick structure, eliminate the 'dry-out' potential and, thus, create a high-efficient and cost-effective heat transport solution. Through the theoretical and experimental analysis, the analysis results will be compared with the conventional GALHP and conventional straight heat pipe. The research results could be directly used for design, optimisation and analyses of the new GALHP configuration, thus promoting its wide applications in various situations to enable the enhanced performance of the GALHP heat transport to be achieved.
