Heat Transfer Analysis of Melting Process Over A Horizontal Three Fins Tube with Different Orientations
Noor Jamal Younis1, Omar Mohammad Hamdoon1, Ziad Mohammed Majeed1*
1 Mechanical Engineering Department, University of Mosul, Mosul, Iraq
*Corresponding Author: [email protected] Accepted: 15 December 2022 | Published: 31 December 2022
DOI:https://doi.org/10.55057/ijarei.2022.4.4.8
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Abstract: In this paper, an experimental study was achieved for the melting process of a phase change material around a copper tube with three longitudinal copper fins that placed inside a rectangular plastic container as a horizontally latent thermal energy storage unit (LTESU). A paraffin wax was used as a phase change material (PCM). Some parameters were considerable in this work like the directions of fins at angles of (0, 30, 60 and 90), temperature of the water entering to the finned tube at (55, 65 and 75) C and the effect of water flow rate inside the finned tube at (7.5, 15 and 22.5) liters / min. The law of energy conservation was used to find the amount of potential energy stored inside paraffin wax by calculating the energy liberated from the hot water entering the system. The results showed that the effect of the temperature of water entering to the system has given increases in the melting with rate of (64.3)% at a temperature of (75) °C compared to the water temperature at (55) ° C. Also the increases in melting rate has reached to (17.5)% at the flow rate (22.5) liters / min compared to the flow rate (7.5) liters / min . The melting has reached to (25.6)% at the angle of (30) degrees compared to the angle of (90) degrees. The results also showed that when using temperature of (55) ° C, water flow rate (7.5) l/min and rotation angle of (30°) that the maximum enhancement ratio in the melting fraction reached to (5.74)% and when using the water temperature (65) degrees and flow rate (15) l/min and the angle of rotation (30°) the maximum enhancement ratio was (11.9)%. When using the incoming water temperature (75)
°C and flow rate (22.5) l/min and the angle of rotation at (30°) the maximum enhancement ratio was (12.03)%. The highest percentage of time savings was also reached (12.03%) at the angle of (30°) compared with (90°). Finally, the highest percentage of energy storage was reached at a water temperature (75) ° C and fin rotation angle (30°) and the flow rate (22.5) l / min was reached (502.4) watts.
Keywords: Melting process, Finned tube, Paraffin wax
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1. Introduction
In recent times thermal energy storage systems, particularly latent thermal energy storage (LHTES) in phase-changing materials (PCMS) have gained greater attention from the point of view of solving global environmental problems and improving energy efficiency. Variable- phase materials are increasingly used as thermally active materials in various engineering fields such as thermal storage, thermal insulation of buildings, outdoor air conditioning, heat recovery, cooling, solar thermal complex, and thermal regulation of systems. However, the challenge is to store thermal energy for later use, in a simple and effective way. There are three basic types of thermal energy storage systems, chemical thermal energy storage units (Nesrine,
& Abdelghani, 2022). The thermal energy storage units and latent thermal energy storage units, as shown in figure (1).
Figure 1: Classification of different types of energy storage
The increase in the discharge of ozone-depleting materials and the high cost of fuel are the main driving force behind efforts to increase the effectiveness of the use of various sustainable energy sources, as one alternative is the development of energy storage devices that are as important as the development of new sources of energy. Energy is stored in the right tanks that can be utilized by changing it to the desired shape which is a face for technologists at present.
Energy storage urges to spare premium fuels making the framework more realistic by reducing energy waste and the cost of capital (Mohammed, et. al. 2018).
The purpose of this paper is to store the thermal energy during peak pregnancy by obtaining continuous energy. This research will use paraffin wax as a phase change material. This work was to find the best position for fins orientation to get the optimum direction to store much energy as possible inside paraffin wax and that represent more efficient system in the practical and engineering scientific applications.
Table 1: Nomenclature
Melting Fraction (%)
𝑡𝑀𝑠 Time Saving
𝛼1(𝑡) Melting fraction at =90
𝛼𝑖(𝑡) Initial Melting fraction at each case Cps and Cpl Specific heat (solid and liquid) (kJ/kg) hfg Latent heat of fusion
LTESU latent thermal energy storage unit m˙ water flow rate (Liters/ min) mp Mass of paraffin wax (kg) PCM phase change material PCMS phase change materials
2. Literature Review
Several experimental and numerical studies were concerning with the effect of adding fins on the main tube of the heat exchanger to improve the performance of heat transmission. This added leads to increase the special area of heat transmission, as it turns out that most studies in this area were more theoretical than in practice, Therefore, the orientation of the thermal energy storage unit is of great importance in increasing the heat transfer rate and achieving low melting time of PCM. This studies have been performed in the recent past to increase the performance of the fins by finding the optimal design in terms of adding fins (annular or longitudinal) and also in terms of number was one or two fins around the main tube of the heat exchanger and using phase change materials, which are represented by (fatty acids, wax, salts), where they have varying capacity to store potential energy.
A theoretically studied improved heat transfer of a triple heat exchanger with rectangular fins mounted for the process of hardening the variable phase material. The main objective of this study was to arrange and select rectangular fins and this may maintain balance in heat transfer in the longitudinal and angular directions of the fin and also the phase variable material (PCM) converts from liquid state to solid simultaneously throughout the cross section of the fin and the hardening process is minimal when using the arrangement and other lengths of the fins with the area of the fin section remaining constant, (Junting, W.et. al. 2021).
A theoretically studied to develop a model for the hardening of a variable phase material (PCM) around a tube with an annular fin. The results of the study were verified through experimental work. A numerical and experimental study revealed that increasing the diameter of the fin will increase the speed of formation of the material on the façade and reduce the total time of the process of hardening the variable phase material. The cost of manufacturing pipes is relatively high due to the welding process, so it has been suggested that the alternative to this is the use of plastic pipes mixed with a metal powder to improve effective thermal conductivity (Daniel, B., et. al. 2021).
A numerically simulation study of the effect of the location and length of a single fin in the LHTES thermal energy storage unit was investigated using o melting and solidification of the phase variable material (PCM) through temperature distribution. The results showed that the total melting time decreased significantly when the length of the fin increased and decreased from changing the location of the fin simultaneously. On the other hand, lowering the location of the fin causes the temperature not to be uniformly distributed during the hardening process, (Jingjing, S. 2021) and (Birlie, F., & Mebratu, A. 2021).
A numerically studied using variable phase materials around a fined copper tube was investigated to obtain the best design of the fins to achieve less time for the hardening process
Qsen.liquid Latent heat storage (W) Qsen.solid Sensible heat storage (W) QT Average total heat srorage (W)
𝐸𝑟 Enhancement ratio
𝑡𝑀 Melting time for each case (min) 𝑡𝑀, 𝑚𝑎𝑥 Melting time of the case =90 (min)
and the best improvement of the performance of the thermal energy storage system specified in this numerical study. The results showed that the best performance of the fins is at a binding angle of 90 degrees and a length and width of 28 mm and 1 mm respectively. Using this condition in this mode can reduce the time of total sclerosis by about 42% compared to a finless system. In addition, it has been concluded that increasing the length of the fin can positively affect the improvement of system performance (Mohammad, J. 2020).
A studied the enhancement of the performance of a finned tube for heat transfer experimentally within a heat storage unit system using lauric acids and capric acid as phase-change material was achieved using set of longitudinal and annular fins were installed in the system. The performance of the fins was analyzed in terms of spacing between them in length and number in the system by the process of melting and solidification of phase-changing materials. Based on the experimental results, the researchers concluded that when combining annular and longitudinal fins, the researchers improved the melting performance of the system by 26.3%
and the solidification performance by 70% (Zhai, X. et. al. 2015).
A conducted an experimental study of the thermal energy storage performance of a casing- tube-type thermal unit using a phase variable material (paraffin) was studied with An open cylindrical heat storage tank filled with copper foam was proposed to study the characteristics of the melting process. A real-time temperature recording system for paraffin material was established and the results when compared with conventional smooth-tube heat exchangers showed that paraffin dissolves rapidly at the underside over time. Also, temperatures have become more homogeneous with time, and therefore, due to the expansion of the heat transfer area, the heat transfer coefficient has improved (Zhaoyang, N. et. al. 2019).
A numerical study using a copper tube with circular copper fins was presented in the vertical direction based on computational fluid dynamics (CFD) technique. A phase variable material was used with a melting point of 35°C and heat transfer depends on convection heat transfer.
They concluded that increasing the fin length leads to a decrease in the dissolution time of the phase variable. Also, note that there is a difference in the numerical method used compared to the finite volume method in terms of the speed of program implementation. (Bacellar, T. 2021) A numerical study of the process of melting a phase change material (PCM) was achieved based on eccentric horizontal cylinders. The simulation of the symmetrical melting of phase change materials between the two cylinders was performed using the finite volume method. In this study, the researchers relied on an inner cylinder, which is a finned tube to improve heat transfer between the inner cylinder and the phase change material. The inner cylindrical wall was considered a hot wall while the outer cylinder is insulated and therefore this simulation shows the melting process from start to finish. The researchers concluded that the use of fins on the inner tube increases the melting process by reducing the melting time by 72.72%.
(Khaldi, A. and Abboudi, S. 2018).
A numerical study of a group of finned heat tubes with the aim of improving the performance of the thermal storage system was presented by different numbers of fins were used and a simulation was conducted to study the effect of the length, number and spacing of the fins as parameters for the study. The simulations were carried out using different numbers of heat pipes and found that the system is ideal when it consists of 3 heating tubes and 20 fin with a length of 35 mm. The researchers also concluded that increasing the fin length and thickness improved system performance by 10%.(Saeed, I. et. al. 2015).
A numerical study in the process of melting in a three-tube heat exchanger was conducted with a phase change material. The study included a two-dimensional model using Fluent 6.3.26.
Three heating methods were used to melt the phase variable material from the inner tube, the outer tube and both tubes. The heat transfer of the phase change material is improved by placing internal and external fins. A comparison was made when the tube was heated from the inside and heated from the outside and the effect of the fin length on the improvement process was studied using a triple heat exchanger with internal - external fins, the results indicated a reduction in the melting time to 43.3% in the triangular tube without fins, and experiments were conducted to verify the validity of the proposed model and it was found that the simulation results are consistent with the experimental results. (Mat A. etl al. 2013).
A practical study of the formation of ice around fined pipes was carried out by freezing water around a tube with ring fins to illustrate the effect of the fin and the working conditions of the cold liquid on the cohesion of the ice. Experimental results revealed that increasing the diameter of the fin increases the speed of hardening of the snow. Lowering the coolant's temperature and increasing its flow rate also promotes increased solid mass formation. . (Kamal et.al. 1999).
3. Problem Formulation
The computational domain consists of a two-dimensional cross-section of a concentric shell and tube heat exchanger as presented in Figure 2 (a) and (b). The inner diameter of copper tube of 𝑫𝒕 = 1𝟐.7 𝒎𝒎 and outer diameter = 19.5 𝒎𝒎 is placed at the center of a rectangular shell of length, width and height (130,130,230) respectively and thickness ts = 8 𝒎𝒎. Three equally spaced (120°) copper fins of radial length 𝒍𝒇 = 45 𝒎𝒎 and thickness 𝒕𝒇 = 1 𝒎𝒎 are attached to the tube as shown in Figure 2 (b). PCM is placed between the spacing of shell and tube. In this study, Paraffin wax is used as phase change material which is an appropriate choice of PCM because of its non-toxicity and non-corrosiveness. The melting temperature changes in three phases, the first at the melting point of 55°C, the second at 65°C and the third at 75°C.
During the process of changing the stage remains chemically and thermally stable. Table 1 lists all thermos-physical properties of Paraffin wax.
Figure 2: (A) Two dimensional finned tube inside enclosure (B) three dimensional for finned tube inside enclosure
(A) (B)
Table 2: Thermo-physical properties of Paraphine wax
[°C]
(38-43) main peak 41°C Melting point
43 [°C]
- 37 Congealing area
[KJ/Kg]
165 Heat storage capacity± 7.5%
[KJ/KgK]
2 Specific heat capacity
3] [Kg/m 880
Density solid at 15°C
3] [Kg/m 760
Density Liquid at 80°C
3K)]
[W/(m 0.2
Heat conductivity Both phase) )
12.5 [%]
Volume expansion
186 [°C]
Flash point
72 [°C]
Max. operation temperature
The Figure 3 shows four different cases of configurations that are investigated in this work.
Each case is represented orientation angle (𝜃) which is measured from horizontal centerline of fin 1 to the +x-axis. The angle 𝜃 is systematically reduced in counter clockwise direction to change the orientation of the fins while keeping angle fixed between each fin. The Case 1 refers to fin angle 𝜃 = 90° and it is considered as a base case. The Case 2 has 𝜃 = 60° producing circumferentially asymmetric orientation of the fins. The Case 3 and Case 4 represented 𝜃=
30° & = 0°, respectively.
4. Experimental Setup
The experimental setup consists of a shell and tube latent heat storage unit along with a hot and cold reservoir as shown in Figure 4. shows the actual image of the experimental setup. The hot reservoir has a capacity of (125) liters containing three thermal heaters of 3000 watts. A digital thermometer has been linked to measure the temperature of the water when it comes out of the heater in addition linked the tow of the Flow meter to measure the amount of water flow inside the pipes, which is read by an Orduino card attached to a digital screen which is read the amount of water flowing through the system. A tank has been placed to store water, which is connected to a DC motor (3phase) connected with an Inverter to convert the electrical phase from 3Phase to single phase and to control the speed of the pump inside the system, which determines the amount of water running through the pipes. The shell and tube latent heat storage unit containing a three-fin longitudinal heat exchanger on which a number of Thermocouples K type are installed distributed in different places on the main pipe and fins.
A thermocouple calibration was performed to ensure the accuracy of their reading. After that, the thermocouples were distributed three thermocouples on the main pipe of the heat exchanger
Figure 3: Illustrates the four cases studied
in several angles and three other thermocouples were connected in the center of each of the three fins and towards a transverse direction, and also two thermocouples were connected when the water entered and exited the thermal storage unit to make the total eleven thermocouples attached to the thermal storage unit, which in turn is connected to a control device to measure the temperatures of each of the aforementioned doubles. Paraffin wax has been used as a phase variable material and is used for the purpose of thermal storage around the fin tube.
5. Results and discussions
5.1. Time variation of molten fraction when previewing image during experiments Figures (5-1), (5-2) and (5-3) are showing a set of photographs taken using a camera from type (SONY) to showing the melting stages of the wax material during the experiments at temperatures of water inlet and amount of water flow rate at (55°C and 22.5 l/min), (65°C and 15 l/min) and (75° C and a flow rate of 7.5 l/min) respectively at different times and different angles. By conducting the experiment and examining the images, it turns out that the angle orientation of the finned tube at (30°) has the largest melting amount compared with the other angles at (0°, 60° and 90°) and during the time period from (5 to 100) minutes. The reason is due to the availability of sufficient upper space to form and grow the load currents between the upper fins more than the lower side, followed by the orientation of the heat exchanger at the angle (0°) by obtaining the best melting amount, and then the angle (60°) and observed The minimum melting amount when directing the angle at (90°), Where vortex currents are formed only around the upper fin, and due to the buoyancy force and the difference in density between molten wax and hard wax, the load currents rise upwards and begin to widen until the upper part is completely melted, which explains the melting of the upper part before the lower part of the wax inside the container.
Figure :4 The experimental device image
Angle 0°
Angle 30°
Angle 60°
Angle 90°
Time (min)
10
35
70
100
Figure 5 -1: The actual images of wax fusion at a temperature of 55 ° C and a flow amount of 22.5 l/min
Angle 0°
Angle 30°
Angle 60°
Angle 90°
Time (min)
10
35
70
100
Figure 5-2: The actual images of wax fusion at a temperature of 65 ° C and a flow amount of 15 l/min
Angle 0°
Angle 30°
Angle 60 ° Angle 90°
Time (min)
10
35
70
100
Figure 5-3: The actual images of wax fusion at a temperature of 75°C and a flow amount of 7.5 l/min
5.2. Variation of melting faction when the temperature of the water entering the system The rate of the molten part of paraffin wax over time by changing the temperature (55, 65 and 75) °C and the amount of flow at (7.5, 15 and 22.5) l/min respectively when directing the rotation of the fins at an angle of (30°), illustrated in the figure 6. In general, we find that increasing time gradually leads to an increase in the melting ratio of phase variable matter (paraffin wax) and it is also clear that the state of stability is gradually reached when time increases. of the general behavior of the scheme (6-A) the ratio of the molten part of the wax during the first ten minutes is very close at different temperature values and the reason is that melting depends on heat transfer through conduction only where the load currents are very small during this period of time. After the passage of time for more than twenty minutes, the currents of pregnancy begin to arise and increase, which increases the amount of molten part, as the highest percentage of molten wax in time (100) minutes is about )%72.54( at a temperature of 75 ° C, the highest increase was (63.7%) compared to the temperature of 55 ° C. As for the two schemes (6-B-C), the highest melting rate in them was (77.66%) and (83.29%) respectively at a temperature of 75 ° C, which shows an increase of (63.9%) and (64.3%) compared to the temperature of 55 ° C for each case. The ratio of the molten part was calculated during each period of time and using the following equation:
Melting fraction ()% = 𝑀𝑒𝑙𝑡𝑖𝑛𝑔 𝑣𝑜𝑙𝑢𝑚𝑒
𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 × 100 % ………….. (1)
(A)
(B)
(C)
Figure 6: The effect of temperature change (T = 55°C, 65°C, 75°C) at the angle of 30° and the amount of flow (A) 7.5 l/min (B) 15 l/min (C) 22.5 l/min
5.3. Variation of melting faction when Effect of changing the amount of flow entering the system
Figures (7-A, B and C) illustrate the relationship between the ratio of the molten part over time by the effect of changing the amount of flow at (7.5, 15 and 22.5) liters/min when directing the rotation of the fins of the heat exchanger at an angle of (90°). Figure (7-A) shows the gradual increase in the proportion of the molten part with the increase of time until it reaches the stability ratio at the end of the time period of the experiment at a temperature of (55) degrees Celsius. It is noticeable that in the first minutes, which are less than ten minutes, heat transfer by conduction, which plays the largest role in heat transfer at this particular time, then the load currents are very few. After increasing the melting time and reaching more than twenty minutes, the load currents increase gradually and work to increase the amount of heat transfer in addition to heat transfer by conduction, as it was observed that the highest melting rate of wax was obtained at the highest amount of flow used (22.5 liters / min), where the melting rate reached (23.61%) and the percentage of increase was (12%) compared to the amount of flow 7.5 liters / min, We note from the general shape of the diagram the convergence of the behavior of each of the three curves at different flow amounts due to the use of medium temperatures, which leads to slow heat transfer, whether by conduction or by load currents. We also note the increase in the amount of molten part shown in the form (7-B) as it reached the highest percentage of fusion when the temperature increased to 65 ° C (55.05%) at the highest amount of flow (22.5 liters /
0 50 100
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION(%)
TIME (MIN)
T=55°C T=65°C T=75°C
0 50 100
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
T=55°C T=65°C T=75°C
0 50 100
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
T=55°C T=65°C T=75°C
min) and that the percentage of increase amounted to (11%) compared to the amount of flow 7.5 liters / min, The melting rate also increases when the temperature increases to 75
° C, where it reached the highest percentage (72%) at the highest amount of flow (22.5 liters / min) as shown in Figure (7-C) and the percentage of increase amounted to (25%) compared to the amount of flow 7.5 liters / min.
(A)
(B)
(C)
Figure 7: The effect of changing the amount of flow (7.5 l/min, 15 l/min and 22.5 l/min) at the angle of 90° C and temperature (A) 55°C (B) 65°C (C) 75°C
5.4. Variation of melting faction when changing the angles of the heat exchanger
Figure 8 shows the special behavior of the heat exchanger when changing its rotation angles for both (θ = 0°, 30°, 60°,90°C) and at a temperature of 55°C where figure (8-A) represents the
0 10 20 30 40 50 60 70 80
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
m˚= 7.5 m˚= 15 m˚= 22.5
0 10 20 30 40 50 60 70 80
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
m˚= 7.5 L/min m˚= 15 L/min m˚= 22.5 L/min
0 10 20 30 40 50 60 70 80
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
m˚= 7.5 m˚= 15 m˚= 22.5
relationship between the amount of fusion over time under the above conditions and with the amount of flow of 7.5 l/min It is clear from the diagram that the behavior of the four curves is very similar at the beginning of melting, especially after the passage of less than twenty minutes where the currents of load are very small and that the melting gel depends on the transfer of heat by conduction, so the direction of rotation of the four corners does not at all clearly affect the behavior of the curve in this period of time, After the passage of time for more than twenty- five minutes, the convection currents begin to widen and take the largest role in the process of heat transfer from the heat exchanger to the wax where the vortex currents form from the bottom and begin to rise under the influence of the force of buoyancy and widening towards the top, in addition to the transfer of heat through the conduction and thus accelerate the process of melting wax in the upper part more than in the lower part of the container, Where the amount of melting per minute was obtained by one hundred (21%) at the angle of 90° and the percentage (23.6%), (26.3%), (25.6%) at each of the angles of 60°, 30° and 0° respectively from that the highest percentage of increase was obtained by the amount of melting at the angle of 30° (20%) compared to the angle of 90°, After increasing the amount of flow to 15 liters / min and at a temperature of 55 ° C, a percentage (22.5%) of the amount of melting of wax was obtained within 100 minutes of time for the angle of 90° and as shown in Figure (8-B) and (24.48%, 28%, 27%) for each of the angles (60°, 30°, 0°) respectively, so it was observed that the highest percentage of increase in the amount of fusion was obtained at the angle of 30°
(19.6%), In Figure (8-C), when the amount of flow increased to 22.5 l/min and the temperature remained at 55 °C, melting (23.61%) was obtained for the angle of 90° and (25.5%), (32.5%), (28.96) for each angle of 60°, 30° and 0° respectively, while the highest increase was obtained by (18.4%).
(A)
(B) 0
20 40 60
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
ɵ=0°
ɵ=30°
ɵ=60°
ɵ=90°
0 20 40 60
0 20 40 60 80 100
MELTING FRACTION (%)
TIME (MIN)
ɵ=0°
ɵ=30°
ɵ=60°
ɵ=90°
(C)
Figure 8: The effect of the heat exchanger rotation at angles of (0°, 30°, 60°,90°C) and a temperature of (55°C) at the amount of flow (A) 7.5 l/min (B) 15 l/min (C) 22.5 l/min
5.5. Melting Enhancement ratio and Time Saving
The percentage enhancing of LTESU can be quantitatively analyzed through enhancement ratio. Enhancement (𝐸𝑟) ratio in equation (2) can be calculated by the difference melting fraction of each case from the case (=90). Similarly, the saving of time (𝑡𝑀𝑠) in equation (3) is based on the difference of melting time between the case (=90) (𝑡𝑀, 𝑚𝑎𝑥) and melting time for each case (𝑡𝑀).
𝐸𝑟 = 𝛼𝑖(𝑡)−𝛼1(𝑡)
1.0 × 100% ………….. (2) 𝑡𝑀𝑠 = 𝑡𝑀,𝑚𝑎𝑥− 𝑡𝑀
𝑡𝑀,𝑚𝑎𝑥 × 100% …………. (3)
The increase in Er indicates an improvement in the melting performance of different fin arrangements compared to the arrangement of fins at the angle of 90°, as Figure 9 shows the temporal evolution of the improvement ratio Er at a temperature of 55 ° C and the amount of flow of 7.5 l / min, when the time is less than ten minutes the improvement is almost the same for all cases because thermal transfer is the prevailing mode and the effect of normal convection is negligible. However, after ten minutes the convection heat transfer becomes important and this is where the effect of fin arrangement begins to be significant. The first peak in the optimization ratio is observed at time = 50 minutes, which occurs at a time when the temperature zones are high due to the arrangement of the fins at the angle of 90°. The value was (4%) for the angle of 30°, (3.22%) for the angle 0°, (2.19%) for the angle of 60°. At 70 minutes there was a decrease in the improvement rate for all cases, with the ratio (3.77%) for the angle 30°, (3.37%) for the 0° angle, (2.35%) for the 60° angle. Then the value of Er begins to increase when a second peak is observed at time of 100 minutes, with the value of Er (5.74%) for the angle of 30°, (4.84%) for the angle 0°, and (3.84%) for the angle of 60°.
Figure 9: The improvement ratio at inlet temperature of (55 °) C and a flow rate of (7.5 l/min) 0
20 40 60
0 2 0 4 0 6 0 8 0 1 0 0
MELTING FRACTION (%)
TIME (MIN)
ɵ=0°
ɵ=30°
ɵ=60°
ɵ=90°
0 1 2 3 4 5 6
0 5 0 1 0 0 1 5 0
ENHANCEMENT RATIO (%)
TIME (MIN)
0°
30°
60°
5.6. Comparison Energy Storage
The rate of storage of thermal energy is an important aspect when designing an underlying LTESU thermal energy storage unit. Energy storage is combination of stored energy during temperature increase in the solid state of PCM, potential energy during the phase change process of PCM and again the storage of perceived energy during the temperature increases in the liquid state of PCM and can be calculated as:
QT=Qsen.solid + Qlat. + Qsen. liquid ……….(4)
QT=mp[Cps(Tm-Tini)+hfg+ Cpl(Tend-Tm)] …….(5)
The thermal energy storage rate of LTESU is displayed with a length of 23 cm for all four cases, Figure 10 shows the total energy stock in the latent thermal energy storage unit for all angle states, as Figure (11-A) shows the energy stored inside the LTESU when passing water inside the system at a temperature of 55 ° C, as it is clear from the general behavior of the diagram that the rotation state at the angle of 30° and at time 100 minutes amounted to 246.96 W, which is the highest storage ratio in this case compared to With other cases, Figure (11-B) shows the amount of energy stored when passing water inside the system at a temperature of 65 ° C, where the amount of stored energy reached 414.62 W at the angle of 30°, which is higher than the amount of energy stored in the rest of the cases, while for Figure (11-C) shows the amount of energy stored inside the LTESU when passing water at a temperature of 75 ° C, where the amount of energy stored reached 502.4 W at the angle of 30° at a time of 100 minutes, Therefore, when comparing the above figures, it turns out that the largest amount of energy storage lies at the direction of rotation at an angle of 30°, the amount of flow of 22.5 l/min and the temperature of 75 ° C. For the rest of the cases, it requires extra time to get the same amount of energy stored at the 30° angle.
(A) (B)
(C)
Figure 10: The amount of energy stored in the storage unit (LTESU) at angles of (0°, 30°, 60°,90°), water flowrate (7.5 L/min, 15 L/ min, 22.5 L/ min) and temperatures at (A) 55°C, (B) 65°C and (C)
75°C
0 100 200 300 400 500
0° 30° 60° 90°
ENERGY STORAGE (W)
ORENTATION ANGLE (°)
m˙= 7.5 L/min m˙= 15 L/min m˙= 22.5 L/min 0
50 100 150 200 250 300
0° 30° 60° 90°
ENERGY STORAGE (W)
ORENTATION ANGLE (°)
m˙= 7.5 L/min m˙= 15 L/min m˙= 22.5 L/min
0 100 200 300 400 500 600
0° 30° 60° 90°
ENERGY STORAGE (W)
ORENTATION ANGLE (°)
m˙= 7.5 L/min m˙= 15 L/min m˙= 22.5 L/min
6. Conclusion
In this paper, a practical study was conducted on finding the best guidance for the fins in improving the performance of the horizontal latent thermal energy storage unit (LTESU).
Paraffin wax is used as a phase change material (PCM) where it is placed in a rectangular plastic container and with a three-fin copper heat exchanger inside. Four different angle positions of the fins are searched at θ = (90°, 60°, 30°, 0°). The law of energy conservation was used to find the amount of potential energy stored inside paraffin wax by calculating the energy released from the hot water entering the system. The effect of the water temperature entering the system at (55 ° C, 65 ° C, 75 ° C), and the effect of the amount of water flow at multiple speeds at (7.5 l/min, 15 l / min, 22.5 l / min), as well as the effect of changing the direction of rotation of the fins of the heat exchanger at θ = (90°, 60°, 30°, 0°) was studied.
The melting rate increases significantly in the case of fins towards 30° and decreases significantly in the case of fins towards 90°. The total melting time of the system is reduced while the accumulation of energy is increased by arranging the fins towards 30°. The high ratio of the length of the fin to its thickness and the high thermal diffusion of the casing significantly enhance the melting performance of the LTESU. The increase in HTF temperature also increases the heat transfer rate and reduces melting time.
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