Project 1

Analysis and Performance Evaluation of Vapor Chambers as an Advanced Thermal System

KQK7003

THERMAL SYSTEMS ENGINEERING

SESSION 2024/2025 SEMESTER 1

1.0 Introduction

1.1 Overview of Thermal Management Systems

Efficient thermal management is crucial for maintaining the performance and

reliability of modern electronic devices, high-performance computing systems, and aerospace technologies. Vapor chambers, as advanced thermal systems, have gained significant attention for their exceptional heat spreading and dissipation capabilities. Recent studies have

highlighted the increasing demand for efficient thermal management in data centres,

aerospace applications, and mobile devices (Weibel & Garimella, 2021; Hanlon & Ma, 2021; Davis & Garimella, 2022; Hwang et al., 2021). For instance, Hanlon and Ma (2021)

demonstrated the effectiveness of capillary-driven evaporation in enhancing the thermal

efficiency of vapor chambers in compact electronic devices. Similarly, Zhao and Chen (2023) investigated the role of micro grooved wick structures in improving heat dissipation

performance in high-power density applications. Furthermore, Ju et al. (2023) analysed the integration of vapor chambers in lateral artery structures for enhanced thermal control in

aerospace systems. These findings underscore the growing importance of vapor chambers in addressing thermal management challenges across diverse fields.

1.2 Introduction to Vapor Chambers

Vapor chambers are flat, two-phase heat transfer devices designed for efficient heat dissipation. They leverage the principles of phase change, utilizing a working fluid that

evaporates and condenses within a sealed enclosure to transfer heat across large surfaces uniformly. Recent advancements in vapor chamber technology have shown improved

performance under extreme thermal conditions (Li et al., 2022).

1.3 Objectives of the Study

1.Evaluate the thermal performance of vapor chambers under varying operational conditions.

2.Identify key design parameters influencing vapor chamber efficiency. 3.Explore emerging applications and technological advancements.

1.4 Scope and Limitations

This study focuses on analysing vapor chamber performance metrics, including

thermal resistance, heat spreading efficiency, and temperature gradients, while highlighting design considerations such as material selection, wick structures, and fluid properties.

2.0 Fundamentals of Vapor Chamber Technology

2.1 Working Principle of Vapor Chambers

The working fluid evaporates at the heat source, travels as vapor to the cooler region, condenses, and returns via capillary action through a wick structure. This cyclical phase

change ensures efficient heat transport. Recent research emphasizes the importance of

selecting appropriate working fluids to optimize thermal conductivity and phase change

efficiency (Hwang et al., 2021; Ju et al., 2023). For example, Hwang et al. (2021)

demonstrated that advanced liquid feeding structures improve vapor transport efficiency,

reducing dry-out conditions and enhancing performance stability. Ju et al. (2023) highlighted the role of lateral artery designs in improving liquid return through sintered powder wicks,

optimizing heat dissipation in high-power density applications.

Furthermore, wick structures play a critical role in maintaining capillary pressure and

ensuring effective fluid transport across the vapor chamber (Cai & Chen, 2023). Cai and Chen (2023) showed that thin-film evaporation from nanostructured wicks significantly  enhances heat transfer efficiency. Additionally, Zhao and Chen (2023) observed that

microgroove wick structures increase capillary pumping power, preventing dry-out and improving reliability under variable heat loads. These findings underscore the interplay between working fluid properties and wick design in achieving optimal vapor chamber performance.

2.2 Phase Change Heat Transfer

Phase change is central to vapor chamber functionality, enabling high heat transfer efficiency with minimal temperature gradient. Recent research emphasizes the role of

optimized wick designs and nanostructured surfaces in improving phase change efficiency (Weibel & Garimella, 2022; Hwang et al., 2021; Cai & Bhunia, 2022).

2.3 Comparison with Other Thermal Management Systems

1.Heat Pipes: Vapor chambers offer superior heat spreading due to their flat geometry.

2. Heat Sinks: Vapor chambers outperform. traditional heat sinks in terms of localized

heat dissipation. Recent studies have provided direct performance comparisons, showing vapor chambers' advantages in managing hotspots and uniform. temperature distribution   (Zhao & Chen, 2023; Li et al., 2022).

2.4 Factors Influencing Vapor Chamber Performance

Several key factors influence the performance of vapor chambers, and recent research has provided in-depth insights into each aspect:

1. Wick Structure Morphology and Porosity: The design and porosity of the wick structure significantly  affect  the  capillary  action  and  fluid  return  efficiency.  Zhao  et  al.  (2023) demonstrated that microgroove wick structures enhance capillary pumping power, reducing dry-out risks under high heat flux conditions. Similarly, Cai and Chen (2023) analysed thin- film evaporation behaviour in nanostructured wicks, revealing improved thermal efficiency under steady-state conditions.

2. Material Thermal Conductivity: The thermal conductivity of the chamber material

directly affects heat spreading performance. Hanlon et al. (2023) studied copper and graphene composite materials, showing a 20% improvement in heat dissipation compared to traditional  aluminium designs.

3. Working Fluid Properties: Optimal selection of the working fluid is essential for  maximizing phase change efficiency and maintaining operational stability. Hwang et al.

(2021) explored the impact of nanofluids as working fluids, demonstrating enhanced thermal conductivity and improved boiling efficiency in vapor chambers.

4. Geometric Configuration: The shape and size of the vapor chamber influence heat

input distribution and overall performance. Ju et al. (2023) studied lateral artery wick designs, revealing their effectiveness in reducing thermal gradients across high-power density zones.

These findings highlight the interplay between structural, material, and fluid

properties in optimizing vapor chamber performance, emphasizing the need for integrated design approaches supported by empirical data from recent studies.

3.0 Design Considerations

3.1 Geometric Design and Material Selection

Geometric design and material selection are fundamental factors affecting vapor

chamber efficiency. Studies have shown that the material's thermal conductivity, such as

copper and graphene composites, significantly impacts heat dissipation performance (Hanlon et al., 2023). Geometrically optimized designs, including thinner profiles and uniform. wick    structures, contribute to minimizing thermal resistance (Ju et al., 2023).

3.1.1 Selection of Shell Material

The shell of the vapor chamber is designed using copper alloy C5191, known for its excellent thermal conductivity and mechanical strength. The key properties of this material include a density of 8.84 ⋅ 103 kg⁄m3  , a thermal conductivity of 67 W⁄(m ⋅ K) , and a yield strength of 450–550 MPa. This material ensures efficient heat transfer under extreme conditions while maintaining mechanical stability under high-pressure differentials.

3.1.2 Optimization of Shell Thickness

The shell thickness was optimized to maintain stiffness and minimize deformation  under high-pressure conditions. The maximum deformation of the shell is calculated using the formula:

Where:

•     q is the applied pressure (Pa),

•     a is the boundary length (m),

•    E is the elastic modulus (Pa),

•    h is the shell thickness (m).

For an internal pressure of 1 MPa and a boundary length of 0.01 m, the deformation is

controlled within 10 μm, ensuring that the vapor chamber's overall performance remains unaffected.

3.2 Working Fluid and Capillary Wick Structure

The properties of the working fluid and the design of capillary wick structures play a critical role in determining vapor chamber performance. Research indicates that nanofluids   and hybrid fluids exhibit higher thermal conductivity and phase change efficiency compared to conventional fluids (Hwang et al., 2021; Wong et al., 2022).

3.3 Heat Input Distribution and Heat Spreading

In this case, the heat dissipation area almost entirely covers the heat source, resulting in an HSR of approximately 1. This reflects the heat dissipation characteristics of vapor

chambers commonly utilized in electronic devices. Additionally, fins are incorporated in this case to assist in heat dissipation. Due to the complexity of calculations and the difficulty of   conducting experiments, only the simplified model's key parameters are calculated.

3.4 Effect of Orientation and Gravity

Direction and gravity play an important role in the heat transfer process of the Vapor

Chamber because they affect the flow pattern of vapor and liquid refrigerants in the chamber, the phase change efficiency, and the overall thermal resistance.

The orientation of vapor chambers also affects fluid return and overall efficiency.

In a vapor chamber, vapor generated in the evaporation region diffuses toward the

condensation region, forming a high-temperature vapor zone. Gravity can impact the vapor   flow path: In a gravity-assisted mode (evaporation region at the bottom, condensation region on top), vapor flow only needs to overcome the internal pressure gradient, resulting in lower flow resistance. While in a reverse gravity mode (condensation region at the bottom,

evaporation region on top), vapor must work against gravity, increasing flow resistance and potentially elevating the equivalent thermal resistance.

And there is importance of gravity for liquid return. After condensation, the liquid must return to the evaporation region. This process depends on capillary forces, gravity, or a

combination of both:

In a gravity-assisted mode, liquid flows back to the evaporation region along the

direction of gravity, reducing dependence on capillary structures (e.g., wicks) and lowering

overall thermal resistance. In a reverse gravity mode, liquid must rely entirely on capillary

forces to overcome gravity, placing higher demands on wick design and potentially leading to insufficient liquid supply, reducing the vapor chamber's heat transfer performance

In conclusion, gravity has a significant impact on liquid return, with gravity-assisted modes offering higher heat transfer efficiency, while reverse gravity or horizontal modes  increase reliance on capillary structures.

Directional effects influence vapor flow and liquid distribution, indirectly altering evaporation and condensation efficiency.

Optimized designs such as enhanced wick structures can mitigate the impact of gravity and orientation on vapor chamber performance, enabling stable operation across various orientations.

Research shows that optimized wick structures can mitigate gravitational effects,

ensuring consistent performance in varying orientations (Ju et al., 2022).

4.0 Heat Transfer Mechanisms in Vapor Chambers

4.1 Evaporation and Condensation Process

Evaporation and condensation are the primary mechanisms governing heat transfer in vapor chambers. During evaporation, heat applied to the vapor chamber surface causes the working fluid to absorb energy and transition into vapor. This vapor then travels to the condenser region, where it releases heat and transitions back into liquid form. Recent studies by Cai and Chen (2023) demonstrate that optimizing the evaporation interface through nanostructured wick surfaces significantly enhances heat transfer efficiency.

4.2 Capillary Action and Wick Functionality

Capillary action within the wick structure ensures the return of condensed fluid to the evaporation zone. The effectiveness of this process is highly dependent on wick morphology, porosity, and permeability (Zhao & Chen, 2023). Advanced designs, including microgrooves and nanoporous wicks, have been shown to reduce fluid resistance and improve thermal performance (Hwang et al., 2021).

4.3 Temperature Distribution and Steady-State Conditions

Uniform. temperature distribution is essential for minimizing thermal stresses and

ensuring the reliability of vapor chambers in high-heat flux environments. Recent findings indicate that optimized wick designs and advanced material selections contribute to achieving steady-state temperature profiles across the vapor chamber surface (Weibel & Garimella, 2022).

5.0 Performance Analysis

5.1 Thermal Resistance and Conductivity

Thermal resistance is a critical performance parameter in vapor chambers, directly

affecting their ability to dissipate heat efficiently. Advanced materials, such as graphene-

enhanced composites, have demonstrated reduced thermal resistance and increased thermal conductivity (Hanlon et al., 2023).

5.1.1 Selection of Working Fluid

Distilled water was chosen as the working fluid due to its high latent heat of

vaporization (2257 kJ/kg) and low viscosity, which enhance heat transfer efficiency. The mass flow rate of the working fluid is calculated as:

Where:

Q is the heat input (W),

Hfg   is the latent heat of vaporization (J/kg).

5.1.2 Design and Optimization of Wick

The wick structure is composed of multiple layers of sintered mesh with a total

optimized thickness of 0.24 mm (each layer being 0.08 mm). After oxidation treatment, the   wick surface roughness increased significantly, and the contact angle decreased to below 5° . The capillary pressure is calculated as:

Where:

σ=0.072 N/m is the surface tension of distilled water,

θ=5ois the contact angle,

reff   =0.1 mm is the effective pore radius.

The calculated capillary pressure is 1.44 kP, providing sufficient driving force for liquid flow. The permeability of the wick is given by:

Where:

•    ϵ=0.6 is the porosity,

•    τ=2.5 is the tortuosity factor.

The calculated permeability is 2.88X10−12 m2 , ensuring smooth liquid return flow. 5.1.3 Heat Input Distribution and Heat Spreading

Uniform. heat input distribution is essential for effective vapor chamber operation.   Optimized wick structures and improved fluid distribution mechanisms have demonstrated significant improvements in heat spreading efficiency (Li et al., 2022).

To more intuitively demonstrate the efficient heat transfer performance of VC, the following case is assumed:

An electronic device operates at a stable temperature of 75°C. To optimize its heat

dissipation, a VC is attached to itstop surface, with fins placed on top of the VC. A fan blows air at 25°C parallel to the passages between the fins. The VC dimensions are 100 mm × 15 mm with a thickness of 0.27 mm. Each fin measures 15 mm × 30 mm × 2.5 mm, with a spacing of 5 mm between fins.

Figure1. 2D model

Figure2. 3D model

5.1.4 Heat Resistance Calculation

The total thermal resistance Rtotal   during the operation of the vapor chamber consists of the following components:

1. Thermal Resistance of the Evaporation Section  Rce :

where δc   is the thickness of the chamber wall, kc is the thermal conductivity of the chamber material, and Ae is the area of the evaporation section.

2. Thermal Resistance of the Condensation Section  Rce : This follows the same formula as Rce , reflecting the thermal performance of the condensation area.

3. Thermal Resistance in the Lengthwise Direction  Racross :

where Leff   is the effective heat transfer length and Acorss   is the cross-sectional area of the chamber wall.

4. Total Thermal Resistance of the Wick Rwtotal :

Where nw   and kw represent the number of wick channels and the thermal

conductivity of the wick, respectively, and Aw is the total cross-sectional area of the wick.

5. Total Thermal Resistance of the Vapor Chamber  Rvtotal :

where P is the vapor pressure, Rgas   is the gas constant,  nv   and  wc   represent the number and width of the vapor channels and  ℎc   is the height of the vapor channel.

The total thermal resistance Rtotal   is calculated as:

5.1.5Thermal Conductivity Calculation

The effective thermal conductivity  keff   of the vapor chamber, a critical measure of its heat transfer capability, is calculated as:

where AUTV is the total cross-sectional area for heat transfer. Substituting the design and experimental parameters of the gas-liquid coplanar ultrathin vapor chamber into the

equations,the following results are obtained at an operating temperature of 55°C:

total thermal resistance R: 0.930 ℃/W

Effective thermal conductivity keff   = 16805 W/(m·K) For the fins

The temperature T0  = 75 C  Ts   = 25 C

According to the table-A15, we get  pr   =0.7228, k=0.2735W/(m*k), θ = 1.798 × 10−5m2 /s

5.2 Heat Spreading Ratio

The Heat Spreading Ratio (HSR) quantifies the ability of a heat dissipation device, such as a vapor chamber, to spread heat from a small heat source area to a larger heat dissipation  area. It is defined as the ratio of the dissipation area to the source area.

For the physical significance of HSR: Low HSR indicates the heat source and dissipation areas are similar in size, making heat spreading relatively easy. While high HSR indicates the heat source area is much smaller than the dissipation area, requiring efficient thermal spreading to minimize temperature differences.

The Heat Spreading Ratio is crucial in VC design because it directly affects temperature Gradient: as a higher HSR demands better thermal spreading to avoid excessive temperature differences. And Uniformity: Higher HSR makes achieving a uniform. temperature

distribution more challenging. Also affects heat transfer efficiency: Optimizing HSR improves the overall heat dissipation performance.

This case is a Low HSR (HSR < 2), we need suitable for small-scale devices where heat spreading is less challenging to ensure high heat transfer efficiency. (Cai & Chen, 2023).

5.3 Temperature Gradient Analysis

Minimizing temperature gradients across the vapor chamber surface is essential for

ensuring stable thermal performance. Research indicates that nanostructured wick designs reduce localized hotspots and achieve consistent temperature profiles (Weibel & Garimella, 2022).

5.4 Comparison with Other Heat Dissipation Methods

5.4.1 Heat Pipes

Vapor chambers outperform. heat pipes in applications requiring flat geometries and uniform. heat distribution (Hwang et al., 2021).

5.4.2 Traditional Heat Sinks

Traditional heat sinks are less efficient in high heat flux scenarios compared to vapor chambers, which exhibit superior performance in temperature uniformity and dissipation

(Zhao & Chen, 2023).

5.5 Factors Affecting Thermal Performance

Several factors, including material selection, wick structure, and working fluid

properties, collectively influence vapor chamber performance. Recent studies emphasize the importance of integrated design optimization for maximum efficiency (Ju et al., 2023).

6.0 Applications of Vapor Chambers

6.1 Electronic Cooling Systems

Vapor chambers are extensively used in electronic cooling systems, particularly in

CPUs, GPUs, and high-power amplifiers. Their ability to uniformly spread heat and prevent localized hotspots significantly improves the reliability and longevity of electronic devices.  Studies have shown that vapor chambers can reduce core temperatures by up to 20%

compared to traditional heat sinks (Weibel & Garimella, 2021).

6.2 High-Performance Computing

In high-performance computing (HPC) systems, thermal management is critical to    maintaining consistent computational performance. Vapor chambers have been successfully integrated into servers and data centers to ensure uniform temperature distribution across

processors. Research indicates a 15% improvement in energy efficiency when vapor chambers replace conventional cooling systems (Hwang et al., 2021).

6.3 Spacecraft and Aerospace Applications

Vapor chambers play a vital role in spacecraft and aerospace applications, where weight, space, and reliability are critical concerns. They are used to dissipate heat from   electronic control systems, power modules, and communication devices. Zhao and Chen (2023) highlighted the effectiveness of vapor chambers in maintaining stable thermal

conditions in microgravity environments.

6.4 Power Electronics and Renewable Energy

In power electronics, vapor chambers help manage the significant heat generated by power conversion devices, ensuring stability and prolonged service life. Renewable energy  systems, such as solar inverters, have also benefited from vapor chamber technology by

enhancing thermal efficiency and preventing overheating during peak operations (Ju et al.,

2023).

6.5 Emerging Applications and Future Trends

Emerging applications of vapor chambers include wearable electronics, IoT devices, and biomedical equipment. Recent studies have explored the integration of vapor chambers  into flexible electronics, enabling lightweight and compact designs without compromising   thermal performance (Hanlon et al., 2023). Furthermore, advancements in nanotechnology   and hybrid wick structures are expected to drive innovation in vapor chamber designs,

expanding their applications across various industries.

7.0 Conclusion

7.1 Summary of Key Findings

This study highlights the critical role of vapor chambers in modern thermal management systems. Key findings include:

1.Vapor chambers significantly reduce thermal resistance and enhance heat transfer efficiency (Weibel & Garimella, 2021; Zhao & Chen, 2023).

2.Advanced wick structures and optimized working fluids play a crucial role in enhancing capillary action and preventing dry-out conditions (Cai & Chen, 2023).

3.Vapor chambers demonstrate superior performance compared to traditional heat sinks and heat pipes in high heat flux applications (Hanlon et al., 2023).

4.Emerging applications, including wearable technology and renewable energy systems, present promising opportunities for vapor chamber integration (Ju et al., 2023).

7.2 Limitations of Vapor Chambers

Despite their numerous advantages, vapor chambers face several limitations:

1.High manufacturing costs, particularly for advanced wick structures and nanomaterials. 2.Limited performance in certain orientations due to gravitational influences.

3.Challenges in maintaining long-term reliability under cyclic thermal loads (Zhao & Chen, 2023).

4.Complex design and material integration processes.

7.3 Future Research Directions

Future research on vapor chambers should focus on:

1.Developing cost-effective manufacturing processes for advanced wick structures. 2.Exploring novel working fluids with superior thermal properties.

3.Investigating the integration of vapor chambers in flexible and compact electronic devices.

4.Enhancing the performance of vapor chambers in microgravity and extreme environmental conditions (Hwang et al., 2021; Ju et al., 2023).

5.Utilizing artificial intelligence and machine learning for optimizing vapor chamber designs.

7.4 Conclusion on the Role of Vapor Chambers in Thermal Management

Vapor chambers represent a cornerstone technology in thermal management, offering unparalleled performance in heat spreading, thermal resistance reduction, and reliability in

extreme conditions. Their integration into diverse applications, including electronics cooling, aerospace systems, and renewable energy technologies, highlights their versatility and

importance. Future advancements in materials, design optimization, and cost-effective manufacturing will further enhance their applicability and effectiveness in addressing  emerging thermal management challenges.

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