2. Description of the proposed GALHP

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

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

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

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

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

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'

the condenser.

Recent Advances in Heat Pipes

saturate the wick.

behaviour of a standard LHP.

problem [3, 4].

50

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,

Figure 1. Schematic of the proposed GALHP.

• The axial pressure drop is negligible due to less magnitude against the

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

• The working fluid is incompressible and has a constant property value in each

• The wick is liquid saturated and wick material is assumed homogenous and

• A local thermal equilibrium exists between the porous structure and the

• Heat loss to the surroundings is ignored due to the well-insulated pipes.

The mass flow rate within the wick structure is considered to be constant owing

hfg

� � (2)

<sup>4</sup> (4)

<sup>∂</sup><sup>r</sup> <sup>¼</sup> <sup>0</sup> (6)

(1)

(3)

(5)

(7)

<sup>m</sup>\_ <sup>¼</sup> <sup>ρ</sup>lμl2πγLevaε<sup>w</sup> <sup>¼</sup> <sup>Q</sup>\_

The energy conservation equations of the single wick structure are given by

The effective thermal conductivity of liquid-saturated wick in a cylindrical

keff <sup>¼</sup> kl½ � ð Þ� kl <sup>þ</sup> kw ð Þ <sup>1</sup> � <sup>ε</sup><sup>w</sup> ð Þ kl � kw ½ � ð Þ� kl þ kw ð Þ 1 � ε<sup>w</sup> ð Þ kl � kw

<sup>ε</sup><sup>w</sup> <sup>¼</sup> <sup>1</sup> � <sup>1</sup>:05πnwDw

<sup>α</sup> <sup>¼</sup> mC\_ pl 2πrLevakeff

ð Þ <sup>1</sup> � <sup>α</sup> <sup>∂</sup>Tw

T rð Þj<sup>r</sup>¼rw,<sup>0</sup> <sup>¼</sup> Tw,<sup>0</sup>

T rð Þj<sup>r</sup>¼rw,i <sup>¼</sup> Tw,i

1 r 1 r ∂ ∂r r ∂Tw ∂r

3.1 Energy conservation and temperature profile in the evaporator

∂Tw <sup>∂</sup><sup>r</sup> <sup>¼</sup> keff

mC\_ pl 2πrLeva

in which, the porosity of screen wick is expressed as [3, 4]

∂2 Tw ∂r<sup>2</sup> þ

> 8 < :

gravitational head.

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

phase.

isotropic.

[4, 25]

53

geometry is [3, 4]

Define the variable α as

Then, rewrite Eq. (2) as

The boundary conditions are

working fluid.

to the mass conservation law, given by [4, 16]

Figure 2. Top-positioned vapour-liquid separator of new GALHP.

will flow upward through the central tubular pipe and enter into the vapour transport line. If the liquid level is further controlled by a valve mounted on the liquid transfer line, the downward liquid flow rate can be controlled to match the rate of evaporation on the inner surface of the heat pipe. So the wicked inner heat pipe surface will be constantly in 'wet'state, thus preventing the potential 'dry-out' problem with the conventional GALHP. Meanwhile, the vapour and liquid flows will be regulated in the same direction and separated clearly during the operation, thus preventing the potential entrainment problem with the conventional straight heat pipe.

Based on the above innovative concept, a dedicated mathematical model and the associated computer program will be developed to analyse the characteristics of the new GALHP.
