324 Maefinal Searle Aremu 7651401 Temperature PDF

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Coventry University Faculty of Engineering, Environment & Computing Department of Mechanical, Aerospace & Automotive Engineering

324MAE Project Report

Computational Fluid Dynamics Investigation of 3D truss-based lattice structures Submitted in partial fulfilment of the requirements for the degree of Bachelor of Engineering

O. Searle SID: 7651401 Meng Mechanical Engineering Supervisor: Dr D. Aremu April 2021

Declaration: The work described in this report is the result of my own investigations. All sections of the text and results that have been obtained from other work are fully referenced. I understand that cheating and plagiarism constitute a breach of University Regulations and will be dealt with accordingly. Signed: O.Searle

Date: 16/04/21

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1 Acknowledgements I would like to thank Dr Deji Aremu for his guidance and support throughout the duration of this project. I would also like to acknowledge the software support team, with special thanks to Kyle Panton, who have helped me overcome many difficulties during the early stages of this project.

2 Table of Contents 1

Acknowledgements ............................................................................................................ 2

2

Table of Contents................................................................................................................ 2

3

Table of Figures .................................................................................................................. 3

4

Table of Tables ................................................................................................................... 4

5

Nomenclature...................................................................................................................... 4

6

Abstract............................................................................................................................... 5

7

Introduction ........................................................................................................................ 5 7.1 Background ................................................................................................................. 5 7.2

Problem Description and Scope .................................................................................. 6

7.3

Aims and Objectives ................................................................................................... 6

7.4 Hypothesis ................................................................................................................... 6 8 Literature Review ............................................................................................................... 6 8.1

Manufacturing Methods and Materials ....................................................................... 6

8.2

Computational Fluid Dynamics .................................................................................. 8

8.3

Existing Heat Exchangers ........................................................................................... 9

8.4 8.5

Industrial Applications ................................................................................................ 9 Existing Truss Lattice Literature ............................................................................... 10

8.6

Literature Review Closing Statements ...................................................................... 10

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Methodology ..................................................................................................................... 10 9.1

Unit Cell Creation ..................................................................................................... 11

9.2

CFD Model Set-Up ................................................................................................... 12

9.2.1

Geometry............................................................................................................ 13

9.2.2

Regions and Initial Conditions........................................................................... 14

9.2.3

Meshing.............................................................................................................. 15

9.2.4

Physics Models and Governing Equations ........................................................ 18

9.2.5

Exploratory Simulations .................................................................................... 19

9.3

Mesh Independence Study ........................................................................................ 20

9.4

Model Validation....................................................................................................... 20

10

Results and Discussion ................................................................................................. 21

10.1

Pressure Drop ............................................................................................................ 21

10.2

Temperature Change ................................................................................................. 23 2

10.3

Heat Transfer ............................................................................................................. 25

10.4 10.5

Reynolds Number ...................................................................................................... 27 Extended Fin HE ....................................................................................................... 27

10.6

2x2x4 Lattice ............................................................................................................. 27

10.7

Assumptions and Errors ............................................................................................ 28

11 11.1 11.2

Conclusions ................................................................................................................... 29 Considerations to Hypothesis and Aims ................................................................... 29 Future Work .............................................................................................................. 30

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References ..................................................................................................................... 30

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Appendix ....................................................................................................................... 32

3 Table of Figures Figure 1 - Lattice Structure Types: BCC (A), BCCZ (B), FCC (C), FCCZ (D) (Maconachie. T et al. 2019) .............................................................................................................................. 5 Figure 2 - Nickel Plated Lattice from Maloney et al. (2012) page 2488. .................................. 7 Figure 3 - Scan of SLM Truss Structures Demonstrating Surface Finish from Leary. M et al. (2016) page ................................................................................................................................ 8 Figure 4 - Extended Surface Heat exchanger (Kwon et al., 2020) page 5. ................................ 9 Figure 5 - Lines of Code from Matlab Script .......................................................................... 11 Figure 6 - BCCZ Truss Lattice Unit Cell Dimensions ............................................................ 12 Figure 7 - Imported Unit Cell .................................................................................................. 13 Figure 8 - United Lattice with Extended Faces ....................................................................... 13 Figure 9 - VOI Dimensions...................................................................................................... 14 Figure 10 - Side View of Volume Mesh with Volumetric Control Applied ........................... 16 Figure 11 - Detailed View of Refined Prism Layers ............................................................... 17 Figure 12 - Wall y+ Monitor Plot ............................................................................................ 17 Figure 13 - Mesh Independence Study .................................................................................... 20 Figure 14 - Typical Extended Surface HE Validation Model.................................................. 20 Figure 15 - Pressure Gradient Contour Plot............................................................................. 21 Figure 16 – Porosity (%) vs Pressure Drop (Pa) ...................................................................... 22 Figure 17 - Velocity Streamline Around 47.5% Porous Lattice .............................................. 22 Figure 18 - Top-Down View of Velocity Streamlines Through A) 94.5% Porous Lattice and B) 2x2x4 Lattice ...................................................................................................................... 23 Figure 19 - Temperature Contour Plot Across Lattice and Fluid ............................................ 23 Figure 20 – Porosity (%) vs Temperature Change (K) ............................................................ 24 Figure 21 - Velocity Vector Contour Plot at Plane in A) 78.2% Porous, B) 47.5% Porous, C) 94.5% Porous and D) 2x2x4 Lattice ........................................................................................ 25 Figure 22 - Porosity (%) vs Heat Exchange (W) ..................................................................... 26 Figure 23 - Porosity (%) vs Heat Exchange (W) Including 2x2x4 Lattice.............................. 26 Figure 24 – Porosity (%) vs Reynolds Number ....................................................................... 27 Figure 25 - Normalised Heat transfer and Temperature Change ............................................. 28

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4 Table of Tables Table 1 - Unit Cell Parameters................................................................................................. 11 Table 2 - Truss Radius and Porosity ........................................................................................ 11 Table 3 – Fluid Inlet Initial Conditions.................................................................................... 14 Table 4 - Lattice Initial Conditions .......................................................................................... 15 Table 5 - Applied Meshing Tools ............................................................................................ 15 Table 6 - Basic Mesh Controls................................................................................................. 16 Table 7 - Applied Fluid Physics Models.................................................................................. 18 Table 8 - Fluid Properties of Air Table 9 - Solid Properties of Aluminium.......................................................................................................... 19

5 Nomenclature CFD – Computational Fluid Dynamics BCC – Body centred cubic BCCZ – Body centred cubic with Z axis struts FCC – Face centred cubic FCCZ – Face centred Cubic with Z axis struts

NSE –Navier Stokes Equations HE – Heat exchanger SLM – Selective laser melting CAD – Computer aided design VOI – Volume of Interest

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6 Abstract Truss lattices are promising structures for a multitude of functions such as energy absorption, lightweight structural components, and compact heat exchangers. They possess excellent mechanical strength for their weight and can be used as effective load-bearing structures. In addition to this, they have large surface areas for their size and, as a result, can be used as highly efficient compact heat exchangers. The combination of these properties and advances in modern additive manufacturing techniques leads to the potential for some highly effective multifunctional structures. This study will detail an investigation into the heat transfer performance of varying porosity BCCZ truss lattice unit cells to determine the optimum geometry for heat transfer, as this has not been investigated in the existing literature. Considerations such as pressure drop, specific heat transfer, flow turbulence and potential applications are also discussed. It is found that the optimum unit cell porosity is 78.2%, which performs 4.7% better than a typical extended fin HE, in terms of heat transfer, of the same external volume, whilst using 15.2% less material and maintaining significantly better mechanical strength properties.

7 Introduction 7.1 Background A lattice is defined as a regular repeated three-dimensional arrangement of unit cells; they can take many forms and are often based on crystalline structures (Zok et al., 2016). Figure 1 shows a selection of typical truss lattice unit cells. This set is based on metallic crystalline shapes in BCC, FCC and their Z strut variants. A truss lattice is made from unit cells that consist of truss struts arranged regularly.

Figure 1 - Lattice Structure Types: BCC (A), BCCZ (B), FCC (C), FCCZ (D) (Maconachie. T et al. 2019)

Trusses are a series of connected beams that create a rigid structure. They are widely used across all industries in structural applications such as bridges and buildings due to their excellent structural properties (Lin & Yoda, 2017). Combining truss structures and metallic crystalline structures produces truss lattices. These structures are known for their high specific compressive strength, meaning they have various uses within industry where strength and weight are considerations, particularly within the automotive and aeronautical industries (Frulloni et al., 2007). Truss lattices have high specific surface areas due to their complex cylindrical truss structures. High specific surface area can lead to excellent heat transfer performance. The combination of these factors justifies the interest in these structures as multifunctional structural heat exchangers. Efficient heat transfer is becoming more of a concern as the global energy consumption is increasing significantly every year. Around 50% of this energy is heat energy meaning efficient heat transfer is vital. Truss lattice HEs can provide very efficient heat transfer in small volumes and therefore contribute to improving heat transfer efficiency.

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7.2 Problem Description and Scope As established, truss lattice structures have excellent heat transfer and structural properties, leading to the next step of optimising these structures. Optimising these structures will provide a more appropriate comparison to typical extended surface heat exchangers as these HEs have had many years of development and optimisation. Existing literature covers many different unit cell structures, but are no studies optimising any particular unit cell type. This gap in literature provides an opportunity to further the research in this area and aid the development of truss lattices. Due to limited time and access to computational resources, this study will focus on only one type of truss lattice unit cell, BCCZ. BCCZ has been selected as it is a commonly used structure with excellent compressive strength and specific surface area, making it a good candidate for optimisation.

7.3 Aims and Objectives This project aims to determine the optimum unit cell porosity for heat transfer and pressure drop performance by studying the flow through various truss lattice unit cells in a Star CCM+ CFD simulation. A secondary aim is to compare the optimised truss lattice unit cell to a current typical CPU HE, providing context for the real-world application and if these truss lattices are viable to replace current designs. Objectives: - Create a series of truss lattice unit cells with varying porosities using a Matlab script - Construct a lattice array using unit cells within Star CCM+ - Create a CFD model in Star CCM+ - Conduct a mesh independence study - Collect Data from the CFD simulations - Analyse fluid flow and HE performance - Validate CFD simulations by comparing results to the existing literature - Select the most effective truss lattice unit cell for heat transfer and pressure drop - Compare most effective truss lattice unit cell to typical existing HE

7.4 Hypothesis It is hypothesised that as the porosity of the unit cell decreases, there will be an increase in the pressure drop across the lattice. It is thought that as the porosity of the unit cell decreases, there will be an increase in heat transferred and therefore the temperature change of the fluid.

8 Literature Review 8.1 Manufacturing Methods and Materials Truss lattice structures are complex, and therefore difficult to manufacture. Traditional techniques such as brazing can be used to create truss lattices. This process is time-consuming and labour-intensive as each unit cell is made individually and then assembled. This assembly process makes creating accurate lattices extremely difficult (Helou & Kara, 2017). Techniques such as wire-woven metals are quicker to produce but may not yield strong structures as the bonds between the weaves are often flawed. An investigation by Khoda et al. (2021) uses a dipped continuous rod technique that claims to improve nodal bonding. It is in the early stages of research but has produced promising results.

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In a study by Maloney et al. (2012), nickel plating via electrolysis was used to create hollow lattice structures. This process uses a polymer lattice scaffold coated in a conductive seed layer. It is then electroplated in nickel with a thickness of 50µm, and the scaffold is then etched away. Figure 2 shows the result of this process. An advantage of this manufacturing method is the hollow truss struts which can be used in a crossflow heat exchanger with fluid passing through the inner tubes and across the outside of the lattice.

Figure 2 - Nickel Plated Lattice from Maloney et al. (2012) page 2488.

Investment casting is a conventional method that can yield complex and accurate lattice structures. A sacrificial scaffold is created in a volatile wax or polymer using an injection moulding or additive manufacturing method. This scaffold is then coated in a ceramic slurry. Once the ceramic has dried, the volatile scaffold is removed by melting it away, and liquid metal is then poured into the ceramic mould. This process is costly and time-consuming due to the number of steps involved (Rashed et al., 2016). The current preferred method of manufacture is additive manufacture, which covers a wide range of techniques. One example is SLM, which builds up thin layers of material using a laser to melt material on top of each layer (Lei et al., 2019). This method means lattices can be made in one process, and the internal geometry can be highly complex. Due to the novelty of this method and the expensive machinery used, SLM is an expensive and time-consuming option. A potential issue with SLM is the surface finish of the product, as it can have a high roughness value due to the nature of the layer-by-layer build process. This roughness is seen clearly in Figure 3. However, this may be advantageous for heat transfer applications due to the increased surface area and boundary layer disruption (Leary et al., 2016).

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Figure 3 - Scan of SLM Truss Structures Demonstrating Surface Finish from Leary. M et al. (2016) page

A technique called wire arc additive manufacturing has been investigated by Zhang et al. (2020), which uses an au...