Author's Accepted Manuscript

Effect of inclination angle on the melting process of phase change material

CASE STUDIES IN THERMAL ENGINEERING

H. Zennouhi, W. Benomar, T. Kousksou, A. Ait Msaad, A. Allouhi, T. El Rhafiki

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PII: S2214- 157X(16)30116-2

DOI: http://dx.doi.Org/10.1016/j.csite.2016.11.004

Reference: CSITE160

To appear in: Case Studies in Thermal Engineering

Received date: 5 September 2016 Revised date: 27 October 2016 Accepted date: 24 November 2016

Cite this article as: H. Zennouhi, W. Benomar, T. Kousksou, A. Ait Msaad, A. Allouhi and T. El Rhafiki, Effect of inclination angle on the melting process o phase change material, Case Studies in Thermal Engineering http ://dx.doi.org/ 10.1016/j. csite.2016.11.004

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Effect of inclination angle on the melting process of phase change

material

H. Zennouhi(a), W. Benomar(a), T. Kousksou(b), A. Ait Msaad(c), A. Allouhi(c), T. El Rhafiki(a)

(a) Ecole Nationale Supérieure d'Arts et Métiers, ENSAM Marjane II, BP - 4024 Meknès Ismailia,

(b)Laboratoire des Sciences de l'Ingénieur Appliquées à la Mécanique et au Génie Electrique (SIAME), Université de Pau et des Pays de l'Adour - IFR - A. Jules Ferry, 64000 Pau France

(c)École Supérieure de Technologie de Fès, U.S.M.B.A, Route d'Imouzzer BP 242, Morocco

Abstract:

A two-dimensional numerical simulation of the melting process in a rectangular enclosure for different inclination angles, has been carried out. Galium as a phase change material (PCM) with low Prandtl number is used. A numerical code is developed using an unstructured mesh, finite-volume method and an enthalpy porosity technique to solve for natural convection coupled to solid-liquid phase change. The validity of the numerical code used is ascertained by comparing our results with previously published results. The effect of the inclination angle on the flow structure and heat transfer characteristics is investigated in detail. It is found that the melting rate inside the rectangular cavity increases by decreasing the inclination angle from 90° to 0°.

Key words: PCM; Natural convection; Inclination angle; Melting; Energy storage.

Nomenclature

Ap : the linearized coefficient c : specific heat capacity (J.kg-1K-1) f : liquid fraction

g : the acceleration of gravity vector ( m.s- ) H : the height of the rectangular cavity (m) ZF : melting heat (J kg-1) Nu: Nusselt number p : pressure (Pa)

pc p H3 T - Tm ) Ra : Rayleigh number ( Ra =-)

S : surface (m )

T : temperature (°C) t : time (s)

u : vitesse vector ( m.s-1)

V: control volume (m )

x : coordinate (m)

Greek symbols

f : the viscous stress tensor

A : thermal conductivity (W.m-1.K-1)

^ : scalar

r : diffusion coefficient U : dynamic viscosity (kg.m-1.s-1)

p : density (kg.m- )

P : the coefficient of volumetric thermal expansion (K-1)

Subscripts

i: initial m: melting nb: neighboring

1. Introduction

Latent energy storage (LES) is required to ensure the continuity of a thermal process in energy systems where a temporal difference exists between the supply of energy and its utilization [1-3]. Certainly, LES is of particular interest and significance in using this essential technique for solar thermal applications such as heating, hot water, cooling, air- conditioning, etc., because of its intermittent nature. In these applications, a LES system must be able to retain the energy absorbed for at least a few days in order to supply the energy needed on cloudy days when the energy input is low. Good understanding of heat transfer during melting process is essential for predicting the storage system performance with accuracy and avoiding costly system overdesign [4-5]. Natural convection heat transfer around and within cylindrical capsules finds various practical applications in space heating, heat exchangers, solar energy collectors, energy storage systems, and electronic devices. During the solidification process, conduction is the sole transport mechanism but in the case of melting natural convection occurs in the melt region and this generally enhances the heat transfer rate compared to the solidification process.

Various investigations have been performed to analyze the effect of the natural convection on the melting process of the PCM [6-10]. These investigations can be classified into categories based on the Prandtl number of the PCM: low Prandtl number (Pr < 1) and high Prandtl number (Pr > 1).

The influence of the inclination angle on melting process of PCM in an enclosure has been studied by few investigators [11-13]. Webb and Viskanta [11] investigated the melting heat transfer of n-octadecane in an inclined rectangular enclosure. During the experiments the only recorded parameter was the interface shapes which were then used to infer the flow structure. It was found that decreasing the inclination angle increases the three-dimensionality of the flow field and results in non-uniform melting of the solid PCM.

Akgun et al. [12] experimentally investigated the melting and solidification process of paraffin in a vertical annular enclosure. It was found that the melting time can be decreased by 30% when the enclosure is tilted 5° from its vertical position. Sharifi et al. [13] investigated the effect of tilting during the outward melting from a vertical warm cylinder. Experiments were performed for small inclination angles of 5° and 10°. It was observed that modest tilting of the enclosure significantly affects the temperature distribution within the PCM, as well as the

temporal evolution of the solid-liquid interface with a three-dimensional shape. This is a result of the interaction between 3D convection currents in the liquid PCM with the solid interface.

Jourabian et al. [14] performed a numerical analysis of the melting process with natural convection in an inclined cavity using the enthalpy-based lattice Boltzmann method. The study was carried out for Stefan number of 10, Rayleigh number ranging from 104 to 106, and inclination angle ranging from -30 ° to +30 The predicted results indicated that an increase in Rayleigh number leads to intensifying the melting rate at each inclination angle.

Recently Kamkari et al. [15] investigated experimentally the heat transfer process and melting behavior during the solid-liquid phase change of lauric acid (as a high Prandtl number PCM) in a rectangular enclosure at different inclination angles. They founded that the heat transfer enhancement ratio for the horizontal enclosure is more than two times higher than that of the vertical enclosure.

This paper investigates the heat transfer process and melting behavior during the solid-liquid phase change of galium (as a low Prandtl number PCM) in a rectangular enclosure for different inclination angles. The problem of the melting process is formulated using the enthalpy-porosity based method. A numerical code is developed using an unstructured mesh and a finite-volume method.

2. Physical model and basic equations

The general assumptions considered in this work include transient formulation and two dimensional Newtonian incompressible fluid where the natural convection effects are considred. The thermophysical properties of the PCM are assumed to be constant but may be different for the liquid and solid phases. The Boussinesq approximation is valid, i.e., liquid density variations arise only in the buoyancy source term, but are otherwise neglected. Since the present formulation deals with solutions on unstructured grids, it is essential to represent the conservation laws in their respective integral forms.

With the foregoing assumptions, the conservation equations for mass, momentum and energy may be stated as

J u.n dS = 0 (1)

— Jpu dV + Jpuu.ndS = -JVpdV + Jz.ndS + J AvdV (2)

— J pcpT dV + JpcpTu.n dS = JAV Tn dS + JpLF — (3)

dt* p J' p J J %

vu V S S V

where u is the velocity vector, p the pressure and T the temperature. z is the viscous stress tensor for a Newtonian fluid :

f = u + (? u (4)

The integration occurs over a control volume V surrounded by a surface S , which is oriented by an outward unit normal vector n. The source term in Eq.(2) contains two parts:

Au =pp(T - Tm )g + Au (5)

where [3 is the coefficient of volumetric thermal expansion and g the acceleration of gravity vector. The first part of the term source represents the buoyancy forces due to the thermal dilatation. Tm is the melting temperature of the PCM. The last term is added to account for the

velocity switch-off required in the solid region. The present study adopts a Darcy-like momentum source term to simulate the velocity switch-off [16]

A-Cpz- <6)

The constant C has a large value to suppress the velocity at the cell becomes solid and e is small number used to prevent the division by zero when a cell is fully located in the solid region, namely f = 0 . In this work, C = 1x1015 kg / m3 s and e = 0.001 are used.

The conservation Eqs.1-3 are solved by implementing them in an in house code. The present code has a two dimensional unstructured finite-volume framework that is applied to hybrid meshes. To evaluate the source term in the energy equation, the new source algorithm proposed by Voller [17] is used. The numerical procedure to study phase change processes were validated elsewhere and will not presented here [18-19]. It has seen that the validation of

the results obtained with this formulation and the code resulted in excellent agreement with those of the literature [20].

4. Results and discussions

The characteristics of the flow and temperature fields in the rectangular cavity are examined by exploring the effect of inclination angle on the melting process inside a rectangular cavity. In the current numerical investigation, the dimension of the rectangular cavity (see Fig.1) used are 120 mm in height and 50mm in width and it was filled by solid PCM initially at

temperature T = 15°C. Galium is selected as PCM that its thermophysical properties are

taken in Table 1. The Prandtl number for liquid gallium at this temperature is 0.0216. The

right wall of the enclosure was maintained at constant temperature ^ = 35 °C. The others

walls are assumed to be adiabatic. Numerical investigations were conducted using 92000 cells and the time step of 10-4s was found to be sufficient to give accurate results.

Figs.2, 3 and 4 present the melting process in the vertical cavity for three inclinations angle 90°, 60° and 30°. We note that for the three inclinations angle 90°, 60° and 30°, at early times, heat transfer in the melt zone predominantly by conduction and the liquid-solid interface mimics the wall's profile. This mode of heat transfer prevails as long as the viscous force opposes the fluid motion during which the solid-liquid interface remains almost uniform and parallel to the hot wall. However, as melting progresses, the cavity expands and convection takes over conduction. High melting occurs near the top of the solid-liquid interface where warm fluid impinges after being heated by the hot wall. As the fluid descends along the interface, heat is transferred to the melting front, and the liquid cools. As a result, the melting rate at the bottom is significantly lower than that at the top.

Fig.5 shows the melting process in the enclosure with and inclination angle of 0°. Note that, in the present study three-dimensional convection is neglected since a two dimensional model is used. However, the duration of the three dimensional convection is very short compared to the whole melting process. After the conduction dominating stage, a complex structure of the fluid dynamic is characterized by a multi-cellular flow patterns. The number and the size of the Benard cells are dependent on the height of the molten phase. As the liquid layer

increases, the rolls at the centre disappear to let those at the bottom corners to increase. We can also note that the warm fluid rises through the vertical walls and falls from the centre of the cavity. For this reason, the interface shows a trough in the centre of the cavity.

Fig. 6 presents the effect of the inclination angle on the temperature history at different positions inside the cavity. For all inclination angles, initially the temperature of the PCM at different locations increases due to the heat conduction in the solid PCM until the melting temperature is reached. We note that the rate of temperature increase at positions 3, 6, 9 and 12 is much higher than those at the other ones. This can be explained by the fact that the heat transfer to the solid PCM at positions 3, 6, 9 and 12 is by strong heat conduction through the thin layer of the liquid PCM. The decreasing trend of temperature from the upper to the lower positions in the cavity implies the thickening of the thermal boundary layer along the interface liquid-solid and confirms the presence of the counter-clockwise rotating flow in the liquid. We can also note some minor fluctuations in temperature at positions 9 and 12 for inclination angles 30° and 60° (see Figs. 6-b and 6-c). These fluctuations can be attributed to three dimensional and unstable flow structures in the liquid PCM. The presence of three-dimensional flow structures in inclined rectangular cavity were also observed and reported by Webb and Viskanta [11] and by Kamkari et al. [15].

During the melting process of PCM from below (see Fig.6-d), temperatures at positions 3, 6, 9 and 12 increase uniformly, with no significant fluctuations. We note that the temperatures at positions 2, 5, 8 and 11 show minor fluctuations immediately after the melt front touch them. These fluctuations can be attributed to the regular convection cells in the liquid region and development of turbulent convection currents in the melt layer.

From an engineering point of view, the rate of melting is one of the most important parameters of the problem. The time evolution of the total liquid fraction in the cavity (ratio of volume of melt to volume cavity) is a factor that has been widely used as a monitoring parameter in earlier publications. From the liquid fraction versus time plot, one can get both the rate melting (slope of the tangent line at a given time) and the average melting rate (ratio of current liquid fraction and time). Fig.7 displays the time evolution of the total fraction of the liquid in the cavity for different inclination angles. It appears from this figure that when the convection takes over conduction, the evolution of liquid fraction versus time does not seem to vary by varying the inclination angle. This is attributed to the overwhelming effect of the natural convection regime which suppresses the effect of the inclination angle during the

melting process. We can also note that for the horizontal cavity, the liquid fraction vary linearly with time until the end of the melting process, but for the inclination angles 30°, 60° and 90°, the liquid fraction variation deviates from a linear trend and the melting rate decreases by increasing the inclination angle. This is attributed to the suppression of the convection current inside the cavity. This result confirm that in the horizontal position (inclination angle = 0°), the heat transfer rate of a low Prandtl number PCM is not affected by the melt layer thickness during the melting process. Similar results are obtained by Kamkari et al. [15] for high Prandtl number PCM.

Conclusion

Melting process inside a rectangular cavity for different inclination angles has been studied numerically. Decreasing the inclination angle from 90° to 0° produces irregular liquid-solid interface and increases the strength of the vortical flow structures in the liquid region. The shapes of the liquid-solid interfaces during the melting process in the horizontal cavity show the generation of Benard convection cells in the liquid region. It is also found that the rate of the melting increases by decreasing the inclination angle from 90° to 0°.

References

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[4] J.M. Khodadadi, Y. Zhang, Effets of buoyancy-driven convection on melting within spherical containers, International Journal of Heat and Mass Transfer 44 (-001) 1605-1618.

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Table 1: Physical Properties of Pure Gallium

Density (liquid) 6093 kg.m-3

Reference density 6095 kg.m-3

Reference temperature 29.78 °C

Volumetric thermal expansion coefficient of liquid 1.2 10-4 K-1

Thermal conductivity 32 W.m-1.K-1

Melting temperature 29.78 °C

Latent heat of fusion 80160 J.kg-1

Specific heat capacity 381.5 J.kg-1

Dynamic viscosity 1.81 10-3 kg.m-1.s-1

Prandtl number 2.16 10-2

Fig.1: The rectangular enclosure

T10 T11 T12

• • •

T7 T8 T9

• • •

T4 T5 T6

• • •

T1 T2 T3

• • •

t= 1300 s

t = 300 s

t = 800 s

t= 1300 s

t = 300 s

t = 800 s

t= 1300 s

iSïniûl OiSi^^

t = 300 s

aflBMi -----

t = 800 s

t= 1300 s

0_10_20_30_40 _

40 —i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—140

Inclination angle = 90°

Time (min)

Time (min)

0 5 10 15 20 25 30 35 40 in 5 10 15 20 25 30 35,,.

401 i i i i | i i i i | i i i i | i i i i | i i i i | i i i i| i i i i| i i i i [ 40 4U I i i i | i i i i | i i i i | i i i i | i i i i | i i i i | i i i i 140

Time (min)