Scholarly article on topic 'Research on U-tube Heat Exchanger with Shape-stabilized Phase Change Backfill Material'

Research on U-tube Heat Exchanger with Shape-stabilized Phase Change Backfill Material Academic research paper on "Materials engineering"

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{"tube heat exchanger" / "backfill material" / "shape-stabilized PCM"}

Abstract of research paper on Materials engineering, author of scientific article — Xiangli Li, Cang Tong, Lin Duanmu, Liangkan Liu

Abstract Fluent software was used to simulate the heat transfer performance of the U-tube heat exchangers with backfill materials of shape-stabilized phase change materials (PCMs) and crushed stone concrete in this paper. The shape-stabilized PCMs refer to a mixture of decanoic acid and lauric acid that the mass concentration of decanoic acid is 60% with10% silica and 6% expanded graphite. It makes the shape-stabilized PCM has the coefficient of thermal conductivity of 1.528W/(m•K) and the latent heat of 109.2kJ/kg. After the simulation of the time for 12hours, the heat exchange for unit borehole depth of backfilling with shape-stabilized PCM is 1.23 times of the heat exchange for unit borehole depth of backfilling with crushed stone concrete. And the influence radius of backfill materials of shape-stabilized PCM is 0.9 times of the influence radius of backfill materials of crushed stone concrete. So under same area of buried pipes region the shape-stabilized PCM backfill can get heat exchange is 1.37 times of crushed stone concrete backfill. In addition, the heat conductivity coefficient of PCMs has great influence on heat pump coefficient.

Academic research paper on topic "Research on U-tube Heat Exchanger with Shape-stabilized Phase Change Backfill Material"

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Procedía Engineering 146 (2016) 640 - 647

www.elsevier.com/locate/procedia

8th International Cold Climate HVAC 2015 Conference, CCHVAC 2015

Research on U-tube heat exchanger with shape-stabilized phase

change backfill material

Xiangli Lia, Cang Tonga, Lin Duanmu*a, Liangkan Liua

School of Civil Engineering, Dalian University of Technology, Dalian 116024, China)

Abstract

Fluent software was used to simulate the heat transfer performance of the U-tube heat exchangers with backfill materials of shape-stabilized phase change materials (PCMs) and crushed stone concrete in this paper. The shape-stabilized PCMs refer to a mixture of decanoic acid and lauric acid that the mass concentration of decanoic acid is 60% with10% silica and 6% expanded graphite. It makes the shape-stabilized PCM has the coefficient of thermal conductivity of 1.528 W/(m*K) and the latent heat of 109.2 kJ/kg. After the simulation of the time for 12 hours, the heat exchange for unit borehole depth of backfilling with shape-stabilized PCM is 1.23 times of the heat exchange for unit borehole depth of backfilling with crushed stone concrete. And the influence radius of backfill materials of shape-stabilized PCM is 0.9 times of the influence radius of backfill materials of crushed stone concrete. So under same area of buried pipes region the shape-stabilized PCM backfill can get heat exchange is 1.37 times of crushed stone concrete backfill. In addition, the heat conductivity coefficient of PCMs has great influence on heat pump coefficient.

© 2016 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of CCHVAC 2015 Keywords: tube heat exchanger; backfill material; shape-stabilized PCM

1. Introduction

Phase change materials (PCM) can be used as energy storage materials. And the thermal energy is stored in the material in the form of latent heat. In the process of phase changing the material absorbs or releases latent heat to achieve energy conversion. Compared to the sensible heat thermal storage, PCM has a higher energy density per unit volume and some improved phase change materials have wider range of phase change temperature selection and easier to control.

The research of application PCM to ground source heat pump system is very limited. Most of the application are independent heat/cold storage system coupled with the ground source heat pump systems, such as Huseyin Benli[1,2], who set up a ground-source heat pump systems with PCM storage tank. The system was installed in a

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of CCHVAC 2015

doi:10.1016/j.proeng.2016.06.420

greenhouse in order to avoid freezing in the spring. This experiment was used to study the heat storage properties of PCM and the influence of the PCM on ground source heat pump systems.

The purpose of the PCM used as backfill material of the U-tube heat exchangers is as following: (1) Due to the high value of latent heat of the PCM and the suitable phase changing temperature, the ground source heat pump can run in suitable temperature and make the COP of equipment reach an optimal state for a long time. (2) Improve the heat storage of unit volume nearby the U-tube to reduce the influence radius of the borehole and reduce the horizontal spacing between the boreholes. More cold/heat can be got through setting more boreholes under certain area of buried region. (3) Use shape-stabilized PCM to make the phase change process stable just like the solid-solid phase transition, which can solve the storage problems of PCM in the borehole. It provides support on PCM when it is used in engineering project.

To prove that the purpose above can be achieved and to provide a quantitative reference for backfilling with shape-stabilized PCM, this paper uses Fluent software to simulate the U-tube heat exchangers with backfill of PCM and analyzes the results of simulation. This paper comes to a conclusion that under certain area of buried region, the heat exchange of the borehole group when backfilling with shape-stabilized PCM will be 1.37 times that backfilling with crushed stone concrete. In addition, the heat conductivity coefficient of shape-stabilized PCM has great influence on heat pump coefficient.

Nomenclature

t time(s)

T temperature (k)

href reference enthalpy (kJ/kg)

Tref reference temperature(k)

C specific heat capacity at constant pressure of PCM

Ts Solidus temperature

Tl liquidus temperature

Se source terms for energy equation

Si correction term of momentum equation

Amush constant

vp traction speed

vi velocity component of i direction

vw flow velocity of the water in U tube

Ef The total energy of fluid in porous medium

Es The total energy of the solid regionin porous medium

keff Effective thermal conductivity of porous medium

kf liquid thermal conductivity of porousmedium

ks solidthermal conductivity of porousmedium

k pcm thermal conductivity of PCM

^csc thermal diffusivity of the crushed stone concrete

2. The model of the U-tube heat exchangers with backfill of PCM

2.1. Mathematical model

The Solidification/melting model can be used to solve fluid flow problems involving solidification and/or melting taking place at one temperature (e.g., in pure metals) or over a range of temperatures (e.g., in binary alloys). Instead

of tracking the liquid-solid front explicitly, it uses an enthalpy-porosity formulation. The liquid-solid mushy zone is treated as a porous zone with porosity equal to the liquid fraction, and appropriate momentum sink terms are added to the momentum equations to account for the pressure drop caused by the presence of solid material. Sinks are also added to the turbulence equations to account for reduced porosity in the solid regions.

The mathematical description of the Solidification/melting model includes energy equation, momentum equation and continuity equation, etc.

The energy equation is written as

^ (pH) + V-(pvH) = V-(kVT) + Se (1)

The momentum equation is written as

f^Zp- + V(pv1v) = V(MVVl) + pgr + St (2)

The equation of continuity is written as

^ + = 0 (3)

This paper established a three-dimensional unsteady model of the U-tube vertical heat exchangers; same as the real profile, and the Gambit software was used to model and meshes the U-tube heat exchangers with backfill of PCM. This model sets the depth of the borehole to 50m, the borehole diameter to 150mm. The diameter of the U-tube is 40mm. The tube spacing is 70mm. And the minimum distance between the tube wall and the borehole wall is 20mm. The soil is simulated as a cylinder and according to the literature; the borehole spacing is generally 6m. Therefore this model adopts the soil cylinder radius of 3m.

The results of meshing by Gambit are shown in figure 1. The volume of the U-tube bending is very small to the whole U-tube and the heat transfer is also small that could be ignored. Meshing can be greatly simplified by reasonably ignoring. Xiaochun Chen made a three-dimensional unsteady numerical simulation study on the heat transfer of the U-tube heat exchangers [3]. In his research, the heat transfer of the bending was also ignored. The error of the simulation results was 6.4% and it can meet the requirements in the engineering project area. So the ignoring of the heat transfer of the bending is reasonable and practicable. So in this paper, the wall of the U-tube bending is set as an adiabatic wall. The flow of the medium is considered through mesh encryption in the tube. But the heat transfer is ignored.

Fig. 1 Grid partition sketch 3. The verification of the model with the backfill materials of PCM

In verifying the accuracy of the model with the backfill materials of PCM, an experiment platform of the U-tube heat exchanger with the backfill materials of PCM which is used to study the heat transfer performance is built. The schematic diagram of the experiment platform is shown in figure 2 the system is mainly composed of the following parts: the constant temperature bath, U-tube heat exchanger system, backfill materials, soil and the data acquisition system. A mixture of decanoic acid and lauric acid that the mass concentration of decanoic acid is 60% is used. the parameters are shown in table2.

Fig.2 The schematic diagram of the experiment platform

Table 2 The parameters of the PCM

Parameter Value Unit

Density 880 kg/m3

Specific heat capacity 1960 J/(kg-K)

thermal conductivity 0.235 W/(m-K)

Melting heat 128.6 kJ/kg

Solidus temperature 293.3 K

Liquidus temperature 293.3 K

The comparison of the simulation results and the experimental results of the model with the backfill materials of PCM is shown in figure 3, with the inlet velocity of the U-tube 0.16 m/s, inlet water temperature 45°C.

Time/h

Fig. 3 The experiment and simulation results of the temperature at different distance

As is shown in figure6, the simulation results and the experimental results fitted well, and their relative error is within 5%. Thus it can also be proved that this model is effective.

4. The preparation of the shape-stabilized PCM

As can be seen from the foregoing, the current widely used organic PCM applied in engineering project faces the following problems: (1) When the PCM has a phase transition, the liquid PCM will be lost in the borehole, which affecting the heat transfer and making damage to the soil environment. (2) Organic PCM has low thermal conductivity generally. The thermal conductivity of the organic PCM this article used in part.2 is only 0.235 W/(m • K). However, the thermal conductivity of the backfill materials should be slightly higher than the soil surrounding the borehole [5].

In response to the first problem, methods at present are mainly making the PCM react with the materials such as polyethylene, silica to reach shape-stabilized composite PCM [6,7] or using the microcapsule method, making the PCM encapsulated[8]. In response to the second problem, in order to improve the heat transfer performance of the PCM, methods at present is mainly adding additives of a high thermal conductivity, such as some metal compounds, expanded graphite and carbon fiber [9]. In this paper, the way is a mixture of capric acid / lauric acid with10% of silica and 6% of expanded graphite and among them, organic compounds including 40% lauric acid and 60% decanoic acid. It makes the PCM into a shape-stabilized PCM with the coefficient of thermal conductivity of 1.528 W/(m • K) and the latent heat of 109.2kJ/kg.

5. The simulation and the result analysis

After the verification of the model, the model which is exactly the same as the real one is set. This model sets the depth of the borehole to 50m, the borehole diameter to 150mm. The diameter of the U-tube is 40mm. The tube spacing is 70mm. The minimum distance between the tube wall and the borehole wall is 20mm and the soil cylinder radius is 3m.

Before the analysis of simulation results, the concept of heat exchange for unit borehole depth should be introduced, the calculating formula is written as

q = pcPG {T,n - Tout)/ h

Through the simulation of the time for 12 hours, figure 4 is the simulation results of the heat exchange for unit borehole depth with different backfill materials in the condition of heat emission. The simulated condition is the velocity of the fluid flow is 0.27m/s and the inlet fluid temperature is 35 °C. As shown in figure 4, when the backfill material is crushed stone concrete, the heat exchange for unit borehole depth is down to 83.46W/m from the initial value of 152.40W/m. As a result, total heat storage of a single borehole is 2.15x105kJ. When the backfill material is shape-stabilized PCM, the heat exchange for unit borehole depth is down to 85.74W/m from the initial value of 227.89W/m. And its heat storage is 2.59x105 kJ Therefore, the shape-stabled phase change materials used, a single borehole can store 20.47% calories than ordinary backfill materials.

"F 160 -

§ 150 -

o 140-

crushed stone concrete shape-stabilized PCM

I ' I—i—i—1—T" 1 2 3 4

(—'—I—<—\—i—I—'—I 9 10 11 12 13

Time(h)

Fig.4 The simulation results of the heat exchange for unit borehole depth with different backfill materials

The figure 5 shows the comparison of the heat exchange for unit borehole depth with the backfill materials of shape-stabilized PCM and organic PCM. The organic PCM means the mixture of decanoic acid and lauric acid that the mass concentration of decanoic acid is 60%. The physical properties of the two materials are the same, except for the thermal conductivity. Due to the low thermal conductivity of the mixture, the heat exchange for unit borehole depth is only 37.98W/m after 12h. And it is far less than the backfill material of shape-stabilized PCM which has much better thermal performance. It is fully proved the importance of improving the performance of the thermal conductivity of the PCM.

240 220 200 180 160 140

E? g 120

organic PCM shape-stabilized PCM

Time/h

Fig.5 the comparison of the heat exchange for unit borehole depth with the backfill materials of shape-stabilized PCM and organic PCM

The influence radius is defined as the distance from the center of the borehole where the excess temperature has reached 0.05°C after running a certain time. Under the conditions of this definition, the influence radius of backfill materials of crushed stone concrete and shape-stabilized PCM has been got after 12h simulation. The results are shown in figure 6.

crushed stone concrete shape-stabilized PCM

Fig.6. The temperature distribution at 25m depth of different backfill materials

As shown in figure 6, the influence radius of backfill materials of crushed stone concrete is 0.60m after 12h simulation. However, the influence radius of backfill materials of shape-stabilized PCM is only 0.54m, 90% of the former. The reason is that the PCM has a large latent heat value that it can absorb a lot of energy, so that the heat storage of unit volume nearby the U-tube has been increased, thereby reducing the influence radius. Thus the application of the shape-stabilized PCM can greatly reduce the horizontal spacing between the boreholes in the project. And more cold/heat can be got through setting more boreholes under certain area of buried region.

As can be seen from the foregoing, the heat exchange for unit borehole depth of backfilling with shape-stabilized PCM is 1.23 times of the heat exchange for unit borehole depth of backfilling with crushed stone concrete. And the influence radius of backfill materials of shape-stabilized PCM is 0.9 times of the influence

radius of backfill materials of crushed stone concrete. So under certain area of buried region, the heat exchange of the borehole group when backfilling with shape-stabilized PCM will be 1.37 times of the he heat exchange when backfilling with crushed stone concrete. The utilization factor of the ground is improved.

6. conclusion

In this study, Fluent software was used to simulate the U-tube heat exchangers with backfill materials of shape-stabilized PCM and crushed stone concrete. The results indicate that When the backfill material is shape-stabilized PCM, the total heat storage capacity of a borehole is 1.23 times that of the backfill material of crushed stone concrete. The influence radius of backfill materials of shape-stabilized PCM is 90% of the influence radius of backfill materials of crushed stone concrete. Thus the application of the shape-stabilized PCM can greatly reduce the horizontal spacing between the boreholes in the project. And more cold/heat can be got through setting more boreholes under certain area of buried region. The heat exchange of the borehole group when it is backfilled with shape-stabilized PCM will be 1.37 times of the he heat exchange when it is backfilled with crushed stone concrete under certain area of buried region.

Acknowledgements

We sincerely appreciate the Project supported by the National Natural Science Foundation of China (Grant No. 51278076).

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