NUMERICAL INVESTIGATION OF FINS EFFECT FOR MELTING PROCESS OF
PHASE CHANGE MATERIALS
Yasin VAROL
Firat University, Technology Faculty, Department of Automotive Engineering
Elazig, Turkey
Mutlu OKCU
Ardahan University, Engineering Faculty, Department of Mechanical Engineering
Ardahan, Turkey
ABSTRACT
The present study numerically explores the melting process of a phase change materials in a rectangular geometry. More specifically, it investigates flow field, thermal phenomena, detailed phase and melting process by means of numerical simulation which is using as parameter; melting temperature, latent and specific heat capability, thermal conductivity and density in solid and liquid states, are based on paraffin wax. The study is performed for melting in rectangular container of without fin and 5 fins, when the wall temperature is uniform. Transient numerical simulations are performed using the ANSYS-Fluent 12.0 commercial software. The simulation results show that the transient phase change process depends on PCM properties, thermal condition and geometrical parameters of system. It includes the paraffin wax properties, thermal difference between hot and cold wall, and number of fins in the rectangular container. Results also indicate that the presence of fins embedded in the container significantly accelerates the melting process.
NOMENCLATURE Amush porosity function C mush zone constant. Cp specific heat, J/kgK h specific enthalpy, (J/kg) href reference enthalpy, (J/kg) k the thermal conductivity (W/mK)
P pressure, (Pa)
S source term
t time (s)
Tliquid liquids temperature, Tm transformation temperature, K
Tref reference temperature, K Tsolid solidus temperature, K
density, kg/m3V
velocity vector
dynamic viscosity, Ns/m2
liquid volume fraction INTRODUCTIONPhase change materials (PCMs) are used to balance temporary temperature and to store energy in several practical application areas from electronics to the automobile industry and also buildings. In recent years, PCMs have been widely examined as alternative cooling and heating methods such as transient electronic cooling, mobile phones, digital video cameras, solar energy systems, thermal energy storage etc. [1-3]. In this context, the primary objective of the study is to investigate alternative solutions related to the efficient use of existing energy resources and developing new ways on different energy sources to supply the demand of energy. Fundamental understanding of the heat transfer process that occurs during melting is crucial for designing more efficient thermal energy storage systems. Many PCMs have unacceptably low heat conductivity. As a result, internal heat transfer enhancement techniques such as fins are required in thermal energy storage applications [4,5]. Soares et al. [6] aims to explore how and where phase change materials (PCMs) are used in passive latent heat thermal energy storage (LHTES) systems, and to present an overview of how these construction solutions are related to building’s energy performance. This review shows that passive construction solutions with PCMs provide the potential for reducing energy consumption for heating and
Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition IMECE2013 November 15-21, 2013, San Diego, California, USA
IMECE2013-64092
cooling due to the load reduction/shifting, and for increasing indoor thermal comfort due to the reduced indoor temperature fluctuations. Kandasamy et al. [7] investigated experimentally thermal management of portable electronic devices for effects of various parameters e.g. power input, orientation of package, and various melting/freezing times under cyclic steady conditions. Tay et al. [8] investigated heat transfer enhancement of phase change materials. They developed a new heat transfer enhancement technique using recirculation of melted PCM. Sharifi et al. [9] developed a numerical model for simulating the melting of a phase change material housed within an internally-finned metal enclosure. They reported the influence of the number of fins, the fin length and thickness, and the hot wall temperature on the melting process. Al-Abidi et al. [10] investigated the application of a triplex tube heat exchanger with a phase-change material in the middle tube to power a liquid-desiccant air-conditioning system. Results show temperature contour and profile during phase change process. Mat et al. [11], numerically investigates the melting process in a triplex-tube heat exchanger with PCM RT82. They were developed a two-dimensional numerical model using the Fluent 6.3.26 software program. At this paper, Internal, external, and internal–external fin enhancement techniques were studied to improve the heat transfer between the PCM and heat transfer fluid.
Based upon the literature review, this numerical study aims to compare the heat transfer during PCM melting in a rectangular container based on two geometrical cases (without fins and consist of five fins). In this study, researchers used an efficient approach to simulate heat transfer during PCM melting in an enclosure including the effects of transient conduction.
NUMERICAL PROCEDURE
In the present investigation, we explore details of melting in a rectangular container of 100 cm height and 20 cm length, at the right wall-temperatures of 320 K and other wall is insulated. The properties of the PCM are based on a commercially available material, paraffin. The thermo physical properties of the paraffin are shown in Table 1.
Table 1. Thermo‐physical properties of PCM
Melting range
28‐30
oC
Latent Heat
179 kJ/kg
Heat storage capacity of solid state
2400 J/kgK
Heat storage capacity of liquid state
1800 J/kg/K
Thermal conductivity in solid state
0.24 W/mK
Thermal conductivity in liquid state
0.15 W/mk
Solid density
870 kg/m
3Liquid density
760 kg/m
3Dynamic viscosity
3.42*10
‐3kg/ms
The numerical approach makes it possible to calculate the processes that occur inside the solid PCM (conduction), liquid PCM (convection) and to account for the phase-change, moving boundary due to the variation of the PCM volume, and solid phase motion in the melt. Computational domain of the physical model was defined, as shown in Fig. 1.
a) Without fins b) With five fins Fig. 1. Physical model
In this study, solidification process has been simulated using ANSYS-FLUENT 12.0 commercial code [12]. This code uses finite volume method in order to solve Navier–Stokes and energy equations. The finite volume method can accommodate any type of grid. Thus, it is suitable for complex geometries, like fins geometries. The CFD code is based on the pressure-correction and uses the SIMPLEC algorithm of Patankar [13]. The first order upwind difference scheme (UDS) is used to discretize the momentum and energy equations.
Different grid elements, such as square and triangular shapes were tried for this study. As geometry is not very complicated, square-element grid structure has a more appropriate result. The results were obtained from different grid densities such as 3000, 12000, 50000 and 160000 These results compared with each other and 50000 grid distribution is chosen for this study.
0
)
.(
V
t
T
k
t
Dh
2
S
g
V
p
Dt
V
D
2
T T p ref refdT
c
h
h
katı sııv katıT
T
T
T
V
A
S
mush
*
Governing EquationsConsidering a pure conduction model for the melting process, one can write the governing equations with the respective initial condition as:
The mass conservation equation can be written as follows:
(1) Energy equation:
(2) Momentum equations can be written as:
(3)
Where
is the density, k is the thermal conductivity,
is the dynamic viscosity,S
is the momentum source term,
V
is velocity vector, T is temperature and h is specific enthalpy. (4) T < T solid
0
T < T liquid
1
T solid < T < T liquid (5))
(
)
1
(
3 2
C
A
mush(6)
(7)where
is liquid volume fraction,
= 0.001 is a small computational constant used to avoid division by zero,V
velocity vector, Amush porosity function and C is mush zone constant.RESULTS AND DISCUSSION
A computational study has been performed in this work for different fin number of rectangular container. In this part of the study, the results are presented with velocity vectors, temperature contours, liquid fraction contour, and profile. The liquid volume fraction can be monitored to follow the evolution of the melting in two containers, as shown in Fig. 2. We observe that the curve of without fins was linear during all melting process. On the other hand, the curve of 5 fins was faster during the first five hours and after this time, the rate of melting decreases because the temperature difference decreased. The five fins container had the fastest rate of melting, and the solid PCM was less than the without fins container. This figure clearly shows that the effect of fins on melting process. Time (h) 0 2 4 6 8 10 Li qui d v o lu m e frac ti on 0.0 0.2 0.4 0.6 0.8 Without fins 5 fins
Fig. 2. Comparison of temporal variation liquid volume fraction for without fins and 5 fins
Figure 3 shows the time evolution of the predicted temperatures, liquid fractions and velocity vectors in the melt for heated right vertical wall at 320 K without fins. As seen, the simulated phase distribution for paraffin melting was obtained numerically during at first, fifth and tenth hour after the start of the process. It is seen from the figure that the solid phase typically descended. This result is in very good agreement with the literature. It is seen that the mush zone consist of the stalactites at all the melting process. These stalactites are occurred due to very big container sizes. At early times, heat transfer in the melt zone was dominated by conduction. Then, convective heat transfer contributed to melting process. As the melt was heated with time, liquid PCM moved upward, and reached its maximum temperature in the upper portion of the container. Then the hot liquid deflected away from the heated wall toward the solid-liquid interface.
Temperature (K)
Liquid Fraction
Velocity Vector
1 Ho ur318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
5 Ho urs
318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
10 Ho urs
318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
Fig. 3. Melting process with time for without fins
Temperature (K)
Liquid Fraction
Velocity Vector
1 H our318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
5 Ho urs 318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
10 Ho urs 318 316 314 312 310 308 306 304 302 300 298 296 294 292
0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
Fig. 4. Melting process with time for 5 fins
Therefore, significantly more melting occurred in this region. The velocity vectors show that the flow rate was higher in this region. Figure 4 illustrated temperature contour, liquid fraction and velocity vector in the melting process for heated right wall 320 K with five equal distance and length fins. Comparison of the results for without fins and fins case shows that fins very accelerated the melting process. Heat transfer rates increased due to the increase in the heated surface area. Under the fins, liquid PCM moved upward, and this hot liquid deflected down from the fins to solid surface. Thus, small vortexes occurred in under each fin. These vortices got lost in the main melting area with time. Melting area gradually increased with time as shown velocity vectors. This result is in very good agreement with the literature.
CONCLUSIONS
In this study, we numerically modeled the melting of a PCM inside a container used to control the temperature. We investigated the impact of the fins of this container on the flow and heat transfer. We used a fixed grid enthalpy-porosity technique coupled with a two-dimensional flow solver based on finite-volume method involving second-order accurate spatial and temporal numerical schemes. Several efficiency indicators were simulated, including the temperatures, the total liquid fraction and the velocity vector. All of these indicators demonstrate the importance of the effect of the fins on melting process. Result show that the placed of fins embedded in the PCM significantly accelerates the melting process. it was show that embedding fins in the PCM more efficient for reducing the melting times. This type container are used especially of solar assisted heating systems in many industrial system. This systems are stored solar energy during the day, and provide a more effective use at night. In this respect, this paper is important for an industrial applications.
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