A Thorough Study over Spray Cooling Systems

Spray cooling systems have become essential in many industries due to their effectiveness in dissipating heat and keeping operating temperatures at the optimal level. The performance of these systems is challenging to accurately predict using traditional analytical methods due to the complex interactions between fluid mechanics, heat transfer, and mass transfer. The intricate nature of droplet formation, atomization, and impingement phenomena add an additional argue to the design process. However, thanks to Computational Fluid Dynamics (CFD) software like Ansys, engineers are able to overcome these challenges.

In this paper, we will explore the principles of spray cooling, examine the challenges related to their design, and showcase how Ansys can effectively tackle these issues.

Figure 1: A real-world example of spray cooling system

What are spray cooling systems?

A spray cooling system employs small droplets of liquid to capture heat from nearby surfaces, efficiently controlling thermal temperatures across different applications, such as cooling electronics. This approach is especially helpful for maintaining optimal temperatures in high-performance components across various industries such as aerospace, electronics, food processing, pharmaceuticals, automotive, and power generation, enabling them to function at higher power levels without the risk of overheating. These systems stand out as the top choice for cooling solutions, offering benefits like superior heat dissipation, boosted efficiency, decreased energy usage, extended equipment longevity, improved performance, and reduced operating expenses.

Major Application Areas of Spray Cooling System

Figure 2: Applications of spray cooling systems

Spray cooling finds extensive applications across various industries, including

           1.Fire Protection: Spray cooling is widely used in fire sprinkler systems, where water is atomized into tiny droplets to suppress or control fires. The small droplets have a large surface area, which promotes evaporation. Evaporation absorbs a significant amount of heat, helping to reduce the heat from the fire and prevent its spread.

           2. Cooling of Hot Gases: This is one of the major applications of sprays. For instance, in air-cooled chillers, Water mist spray systems can be used on the condenser coils of air-cooled chillers to make them work more effectively. In the following product which is accessible from here, a water mist spray system is simulated, validating a valuable paper considering Eulerian-Lagrangian approach.

Water Mist Spray Nozzle Considering Evaporation Using 2 CFD-way DPM –Numerical Paper Validation

         3. Dermatological Operations: Spray cooling is used in some medical and dermatological treatments, likely for cooling during certain procedures.

         4. Cooling Hot Surfaces in Metal Processing: In industries like steel production, spray cooling is used in processes such as steel strip casting and after hot rolling to control surface temperatures and optimize microstructure. A jet of gas with water droplets is directed at the hot metal surface to cool it effectively.

        5. Electronics: Cooling high-power electronic components like CPUs and GPUs to prevent thermal throttling and extend component lifespan.

        6. Automotive: Cooling engine components, brake systems, and electric vehicle batteries to maintain optimal operating temperatures. As an example, in one of spray applications, the Pintle Ejector is modeled that compress air to break up the liquid even more, making a fine mist or spray that can help with coating, mixing, or burning. Check this link out for more information.

Spray In Pintle Ejector CFD Simulation, ANSYS Fluent Training

        6. Energy: Cooling power electronics in renewable energy systems and nuclear reactors.

        7. Other Industries: Applications extend to aerospace, materials processing, and manufacturing processes where precise temperature control is crucial.

Spray cooling is valuable in both industrial and safety applications due to its ability to enhance heat transfer through phase change and evaporation.

What are the three types of cooling systems?

Innovative cooling methods have become extremely important in maintaining the performance and lifespan of state-of-the-art electronic devices. These methods, such as liquid cooling, phase change (evaporative) cooling, and air cooling, provide exceptional heat dissipation performance when compared to conventional techniques. By efficiently dissipating heat from essential parts, these cooling solutions enhance performance, improve energy efficiency, and extend the lifespan of devices.

Choosing the right cooling technique depends on various factors, including the maximum heat flux, total heat load, temperature tolerances, reliability, power consumption, operating environment, system complexity, technology maturity, and cost.

We will delve into the numerical modeling techniques that can be employed to simulate spray cooling systems. By gaining a comprehensive understanding of spray cooling, we aim to contribute to the development of more efficient and reliable thermal management solutions for future electronic devices.

Comparison of Cooling Techniques

Different cooling techniques are available to manage heat dissipation in high-performance devices with varying heat transfer efficiencies and capacities. The effectiveness of each method depends on factors such as heat flux, cooling mechanism, and the specific application. By comparison of several cooling methods based on heat transfer coefficient (HTC) and maximum heat flux, two-phase cooling methods were more effective at higher heat fluxes, and among related techniques, Spray Cooling Offers very high heat flux removal, with an HTC of 20-40 W/cm²·K and the ability to manage up to 1200 W/cm², making it particularly preferred under certain conditions for high heat fluxes. Jet Impingement was the most powerful of methods, with an HTC of 28 W/cm²·K and the ability to handle up to 1820 W/cm² in heat flux.

Spray cooling is often preferred over other high-heat flux techniques, particularly in situations requiring extremely high heat dissipation. However, the choice of cooling method depends on the specific needs of the system, which we talk about continuously.

Heat-Transfer Mechanisms of Spray Cooling

Heat-Transfer Mechanisms of Spray cooling can be analyzed using boiling curves and temperature-time cooling curves. The process typically involves three stages. Single-Phase Regime with Low heat flux and minimal phase change, Two-Phase Regime with Significant heat transfer enhancement due to nucleate boiling, and Critical Heat Flux (CHF) with Maximum heat flux before film boiling, beyond which heat transfer deteriorates.

Understanding these regimes is crucial for optimizing spray cooling systems and preventing overheating in electronic devices.

1. Single-Phase Regime:
   • Low surface temperature, leading to minimal phase change (liquid to vapor).
   • Heat transfer primarily occurs through conduction and convection.
   • Heat flux is relatively low and increases slowly with increasing surface temperature.
2. Two-Phase Regime (Nucleate Boiling):
   • Increased surface temperature triggers the formation of vapor bubbles at nucleation sites.
   • Rapid vaporization and bubble departure enhance heat transfer significantly.
   • The steep slope of the boiling curve in this regime indicates a high rate of heat transfer.
3. Critical Heat Flux (CHF) Regime:
   • As the surface temperature reaches a critical point, vapor blankets start to form around the heated surface.
   • This reduces effective heat transfer, leading to a peak in the boiling curve.
   • Beyond CHF, the heat transfer deteriorates rapidly, potentially causing overheating and damage to the surface.

Key Performance Metrics of spray cooling include:

1. Critical Heat Flux (CHF):
   • Represents the maximum heat flux that can be removed from the surface before the onset of film boiling.
   • A higher CHF indicates a better cooling performance and a higher thermal load capacity.
2. Heat Transfer Coefficient (HTC):
   • Defines the rate of heat transfer per unit temperature difference between the surface and the coolant.
   • A higher HTC implies more efficient heat dissipation.
3. Cooling Efficiency:
   • Measures the effectiveness of the spray cooling system in utilizing the coolant.
   • A higher efficiency indicates better utilization of the coolant and reduced energy consumption.

Factors to Consider in ANSYS Simulations for Spray Cooling Systems

The heat-transfer mechanism of spray cooling is influenced by various factors, including spray parameters, working fluid properties, surface modifications, and system/environmental conditions.

Spray Parameters of spray cooling

Spray parameters that influence calculations include Nose type, flow rate, spray distance, spurt duration, and spray orientation. We cannot consider them separate from each other as the following experiments show the relationship between nose and surface distance. It presents that for different nozzle types, the optimal distance for maximizing heat transfer is noticeably less than the distance where the spray cone fully covers the heated area. To achieve the highest critical heat flux (CHF), it is essential to select the distance that minimizes the local temperature maximum. The temperature field non-uniformity in spray cooling depends on the nozzle-to-surface distance, spray flow parameters, and heat transfer mechanism. It’s revealed that the development of intensive boiling in the liquid film changes the heat transfer curve slope and significantly reduces cooling non-uniformity. (2)

Regarding the experiment of Kewie Dong and others about Transient heat transfer characteristics in spray cooling in 2022 (3), The effect of spray height on liquid film is more significant when compared with the effect of pressure, which demonstrates that the heat transfer capacity at a spray height of 5 mm is significantly higher than at 10 mm and 15 mm, leading to the conclusion that the appropriate spray height is an important factor in maximizing the efficiency of spray cooling.

 

• Nozzle Type: Different nozzle types result in varying droplet sizes, velocities, and spatial distributions, impacting heat transfer.
• Flow Rate: Higher flow rates generally enhance heat transfer but can reduce cooling efficiency due to thicker liquid films.
• Spray Distance: Optimal spray distances exist, balancing droplet coverage and impinging energy.
• Spurt Duration: Moderate spurt durations can improve cooling efficiency and temperature reduction.
• Spray Angle: Inclined sprays can enhance heat transfer by reducing stagnation zones but may also increase temperature non-uniformity.

Working Fluid Properties and Heat Transfer Performance

The choice of working fluid significantly impacts the efficiency and effectiveness of spray cooling systems. Key properties of the working fluid that influence heat transfer include Thermophysical Properties such as viscosity, thermal conductivity, latent heat of vaporization, and Saturation Temperature, in which lower saturation temperatures allow for higher heat fluxes before CHF.

Heat transfer enhancement due to fins depends on the material used for the construction of fins, particle deposition, surface treatment, and thermal contact, which have been proven. (4)

• Thermophysical Properties:
  • Thermal Conductivity: Higher thermal conductivity enables faster heat transfer.
  • Specific Heat Capacity: A higher specific heat capacity allows the fluid to absorb more heat energy per unit mass.
  • Latent Heat of Vaporization: A higher latent heat of vaporization means more energy is required to vaporize the fluid, leading to increased cooling capacity.
  • Viscosity: Lower viscosity facilitates better fluid flow and heat transfer.
  • Surface Tension: Lower surface tension promotes better wetting of the surface and droplet formation.

Types of Working Fluids:

1. Non-dielectric Liquids:
   • Water: While water offers excellent thermal properties, its high boiling point and electrical conductivity limit its applications in high-temperature and electronic cooling scenarios.
   • Aqueous Solutions: By adding salts, alcohols, or other substances to water, it’s possible to enhance its thermal properties, such as thermal conductivity and boiling point elevation.
2. Dielectric Liquids:
   • Fluorocarbons: These fluids offer excellent dielectric properties, low boiling points, and high thermal conductivity, making them suitable for high-heat-flux applications.
   • Hydrofluorocarbons: These fluids are often used as refrigerants and have good thermal properties, but their environmental impact is a concern.

Working Fluid Modifications:

To further enhance the performance of spray cooling systems, researchers have explored various modifications to the working fluid:

1. Surfactants:
   • Surfactants reduce the surface tension of the fluid, promoting better wetting of the surface and droplet formation.
   • They can also enhance bubble nucleation, leading to improved heat transfer.
2. Salt Additives:
   • Salt additives can influence the boiling curve and heat transfer mechanisms.
   • They can affect bubble formation, film evaporation, and critical heat flux.
3. Nanofluids:
   • Nanofluids are engineered fluids containing nanoparticles dispersed in a base fluid.
   • They can significantly enhance the thermal conductivity and specific heat capacity of the fluid.
   • However, challenges such as nanoparticle stability, dispersion, and potential clogging issues need to be addressed.

Regarding an article by Saee Thakur, “A Review of Parameters and Mechanisms in Spray Cooling,” 2023, p.19, Spray cooling heat transfer is a challenging but essential technology for miniaturized devices. The heat transfer performance of spray cooling is affected by a variety of parameters, including water inlet pressure, temperature, spray angle, volumetric flow rate,  surface geometry, micro-level surface features, and fluid types. Increasing the contact angle, surfactants, and other additives can reduce fluid surface film development. To explore these factors, spray cooling must advance.   Focus on film thickness, critical heat flux point, and nozzle characteristics to improve heat transfer. Spray cooling regimes are crucial to many high-heat-flux removal applications.

In miniature electronics, spray cooling seems promising for quick heat dissipation. Water inlet pressure, temperature, spray angle, volumetric flow rate, surface geometry, micro-level surface properties, and fluid types affect spray cooling heat transfer microstructure. Modifying the heat transfer surface or utilizing different coolants improves spray cooling. Surface geometry or micro-characteristics can be altered.  Coolant changes include adding surfactants or other fluids, alone or with water. Spray cooling works well in many applications, notably at high heat fluxes and low coolant flow rates. Improved wick ability, triple contact line length, active nucleation sites, and flow confinement for nanostructured surfaces and increased surface area, flow confinement, and triple contact line length for macrostructure surfaces are the main spray cooling enhancement processes.

Surface Modification for Enhanced Spray Cooling

Surface modification is a crucial technique to improve the heat transfer performance of spray cooling systems. By altering the surface structure at macro and micro scales, various benefits can be achieved:

The choice of materials for surface modification must be compatible with both the working fluid and operating conditions. Surface modification includes:

Macro-scale Modification:

• Increased Heat Transfer Area: Larger surface area allows for more efficient heat dissipation.
• Improved Wettability: Enhanced wetting promotes better contact between the surface and the coolant.
• Optimized Flow Patterns: Proper flow distribution can improve heat transfer.

Micro-scale Modification:

• Increased Nucleation Sites: More nucleation sites lead to an earlier onset of nucleate boiling and improved heat transfer.
• Reduced Surface Temperature: Lower surface temperatures can prevent boiling crises and improve overall performance.

Combined Approaches:

• Composite Structures: Combining macro and micro-scale modifications can yield synergistic effects, leading to even better performance.

While surface modification techniques can significantly enhance cooling performance, they can also be complex and costly to implement.

System and Environmental Parameters of Spray Cooling

System and environmental parameters can significantly influence the performance of spray cooling systems. Here are some key factors:

Non-condensable Gases:

• Impact on Heat Transfer: The presence of non-condensable gases can affect the heat transfer mechanism by altering the pressure and temperature conditions within the system.
• Effect on CHF: Non-condensable gases can influence the critical heat flux, potentially leading to higher or lower values.
• System Design Considerations: Removing non-condensable gases is crucial to optimizing system performance and preventing potential damage to components.

Gravity:

• Influence on Bubble Dynamics: Gravity affects the buoyancy forces that drive bubble detachment from the surface.
• Impact on Film Formation: Gravity can influence the thickness and stability of the liquid film on the surface.
• Experimental Challenges: Short-duration microgravity experiments limit the ability to fully understand the long-term effects of reduced gravity on spray cooling performance.

By addressing these factors and conducting further research, we can continue to improve the performance and reliability of spray cooling systems for CFD simulation.

Conclusion

Understanding the complex interplay of those factors is crucial for optimizing spray cooling systems and achieving efficient thermal management.

Simulation software like Ansys plays a crucial role in improving the design and performance of spray cooling systems. By utilizing these software tools, engineers can create detailed 3D models of spray cooling systems, predict system performance under various operating conditions, conduct sensitivity analyses to understand the impact of design parameters, and optimize designs for enhanced heat transfer, pressure drop, and droplet distribution. This ultimately leads to reduced development costs and improved overall system performance.

References:

1. Xi Meng, Li Meng, Yi Goa, Haoran Li( June 2022), “ A comprehensive review on the spray cooling system employed to improve the summer thermal environment: Application efficiency, impact factors, and performance improvement”, Volume 217.
2. Jing Yin, Shangming Wang, Xuehao Sang, Zhifu Zhou, Bin Chen, Panidis Thrassos, Alexandros Romeos, Athanasios Giannadakis (2022, Energies), “Spray Cooling as a High-Efficient Thermal Management Solution”.
3. Vladyko, N.Miskiv (December 2024 ), “Heat transfer peculiarities and crisis phenomena development in spray cooling using various types of nozzles”, Volume 159, Part B
4.  Sadeghian Jahromi, A. and C.-C. Wang(2021), “Heat transfer enhancement in fin-and-tube heat exchangers–A review on different mechanisms. Renewable and Sustainable Energy Reviews”.
5. Saee Thakur et al “A Review of Parameters and Mechanisms in Spray Cooling”, 2023, p.19
6.  Hamid Motazeri, Bert Blocken, Jan LM Hensen ( March 2014), “ Evaporative cooling by water spray systems: CFD simulation, experimental validation and sensitivity analysis”
7. Mascarenhas, N.; Mudawar, I. Analytical and computational methodology for modeling spray quenching of solid alloy cylinders. Int. J. Heat Mass Transf. 2010, 53, 5871–5883