Aerodynamic Criterion for Anti-Entrainment Spray Design in Handheld Fan-Mist Devices from Shenzhen Huizhizhen Smart Technology Co., Ltd ; Aerodynamics expert

Abstract：
Handheld fan-mist devices must deliver water mist effectively while avoiding mist entrainment into the fan inlet. This paper proposes an aerodynamic design criterion: the spray jet dynamic pressure should be greater than or equal to the fan suction static pressure at the nozzle location, with an appropriate safety margin. Based on this principle, the required spray jet velocity and corresponding water supply pressure are estimated for typical handheld fan configurations. The analysis shows that mist entrainment risk depends not only on jet velocity, but also on nozzle position, droplet size, fan type, and local flow structure. For small high-speed axial fans, a jet velocity of approximately 16–32 m/s may be required under different inlet proximity conditions. For general engineering applications, avoiding direct alignment with the fan inlet, increasing nozzle-to-inlet distance, selecting moderate droplet sizes, and maintaining sufficient spray momentum are recommended to improve spray delivery performance.

1. Introduction

Handheld spray fans are widely used for personal cooling, humidification, and portable mist delivery. In typical designs, a small fan generates airflow while a mist nozzle releases water droplets into the surrounding air. A common failure mode is that fine mist is drawn back into the fan inlet due to local negative pressure, causing reduced mist output, internal wetting, noise increase, and potential reliability issues.

Many existing designs rely mainly on empirical adjustment of pump pressure or nozzle position. However, mist entrainment is essentially an aerodynamic interaction problem between the spray jet and the fan-induced flow field. Therefore, a simple quantitative criterion can help guide early-stage product development.

This study focuses on handheld fan-mist devices equipped with small high-speed axial fans. The main objective is to establish a practical design rule that relates fan suction pressure, spray jet dynamic pressure, required jet velocity, and water supply pressure.

2. Problem Definition and Assumptions

The analysis is based on the following assumptions:

• The fan suction effect is represented by the local static pressure drop, or negative pressure, near the fan inlet.
• The spray is generated by a water mist nozzle operating under ambient air conditions.
• The nozzle is arranged near the fan inlet, with the spray direction facing toward, parallel to, or obliquely toward the fan.
• The primary design goal is to prevent mist droplets from being entrained into the fan inlet.
• Air density is taken as
  [ \rho \approx 1.2 \ \mathrm{kg/m^3}. ]

The key variables are defined as follows:

3. Aerodynamic Design Criterion

To prevent mist from being drawn into the fan, the spray jet must possess sufficient momentum to resist the local suction flow. A simplified criterion is expressed as:

[ P_d \geq k \cdot P_{fan} ]

where the spray jet dynamic pressure is given by:

[ P_d = \frac{1}{2}\rho v^2 ]

The safety factor (k) accounts for edge turbulence, non-uniform inlet flow, droplet size distribution, and transient fluctuations in fan operation. For handheld mist fans, a typical range is:

[ k = 1.5 \sim 2.0 ]

A higher safety factor is recommended when:

• the nozzle is very close to the fan inlet;
• the droplet size is very small;
• the fan produces strong local suction;
• the inlet flow field is highly turbulent.

From the dynamic pressure requirement, the minimum required jet velocity can be estimated as:

[ v = \sqrt{\frac{2 k P_{fan}}{\rho}} ]

This velocity refers to the air-mist jet velocity at the nozzle exit, not the water pump pressure. Confusing jet velocity with water supply pressure is a common design error.

4. Estimation of Fan Suction Pressure

For a small high-speed axial fan operating at approximately 16,000 rpm, the local negative pressure near the inlet varies significantly with distance. Typical reference values are:

• Very close to the inlet:
  (P_{fan} \approx 250 \sim 350 \ \mathrm{Pa})
• At a distance of 1–2 cm from the inlet:
  (P_{fan} \approx 80 \sim 150 \ \mathrm{Pa})

These values are approximate and depend on fan geometry, rotational speed, housing structure, and operating conditions. They should be verified by experimental measurement in actual product development.

5. Required Spray Jet Velocity

Using (k = 2.0) and (\rho = 1.2 \ \mathrm{kg/m^3}), the required jet velocities under representative conditions are summarized below.

These results indicate that mist entrainment risk increases sharply when the nozzle is placed directly adjacent to the fan inlet. Even a small increase in distance can significantly reduce the required jet velocity.

6. Relationship Between Jet Velocity and Water Supply Pressure

For conventional fine-mist nozzles, the relationship between jet velocity and water supply pressure can be roughly estimated as follows:

• Jet velocity 15–25 m/s:
  water pressure approximately 0.4–0.8 MPa (4–8 bar)
• Jet velocity above 30 m/s:
  water pressure approximately 0.9–1.2 MPa (9–12 bar)

These estimates are approximate and depend on nozzle type, orifice diameter, flow coefficient, liquid properties, and atomization mechanism. Therefore, they should be treated as preliminary design references rather than exact values.

7. Practical Design Considerations

7.1 Droplet Size Effect

Very fine droplets, especially those smaller than 10 μm, have low inertia and are easily influenced by surrounding airflow. Even when the main spray jet has sufficient momentum, fine mist at the spray edge may still be entrained by local vortices near the fan inlet. Therefore, simply increasing water pressure may not fully solve the entrainment problem.

For handheld fan-mist devices, a droplet size range of approximately 20–40 μm may provide a better balance between mist visibility, cooling effect, and anti-entrainment performance.

7.2 Nozzle Position and Spray Angle

Nozzle placement has a strong influence on entrainment behavior. Directly facing the fan inlet creates the highest risk. A more robust arrangement is to:

• avoid aligning the nozzle directly with the inlet center;
• adopt an oblique spray direction;
• maintain a separation distance of at least 15 mm between the nozzle and the fan inlet when structural constraints allow.

7.3 Fan Type Influence

Centrifugal or turbo fans usually generate higher static pressure than small axial fans at similar rotational speeds. If a high-static-pressure fan is used, the local suction near the inlet may be stronger, and the required spray jet velocity will increase accordingly.

8. Engineering Recommendations

For handheld high-speed fan-mist devices, the following design strategies are recommended:

Preferred configuration
Place the nozzle obliquely relative to the fan inlet, maintain a distance of at least 15 mm, and operate the mist system at approximately 5–7 bar. This configuration generally provides stable mist delivery with low entrainment risk.

Compact configuration with limited space
If the nozzle must be placed very close to the inlet, the water supply pressure should be increased to at least 10 bar, and the droplet size should be moderately increased to around 20–40 μm. Extremely fine dry mist is not recommended in this case.

Low-performance or cost-sensitive configuration
If slight mist entrainment is acceptable, a water supply pressure of 3–4 bar may be sufficient for basic operation, although mist output stability may be reduced.

9. Conclusion

This study proposes a simple aerodynamic criterion for spray fan design: the spray jet dynamic pressure should exceed the local fan suction static pressure by a suitable safety margin. For small high-speed axial fans, the required jet velocity may range from approximately 16 m/s to 32 m/s, depending on nozzle position and inlet suction intensity. Corresponding water supply pressures typically fall within 3–12 bar, subject to nozzle characteristics and system design.

The proposed method is suitable for early-stage conceptual design and engineering estimation. For final product validation, experimental measurement of fan inlet pressure distribution, droplet size, spray trajectory, and entrainment rate is still necessary. Future work may further incorporate computational fluid dynamics simulation, multi-phase flow modeling, and human-perception-based evaluation of mist comfort and cooling performance.

EU new battery regulation: new EU battery rules, so the user can be able to remove and replace the battery.

Shenzhen HuiZhiZhen Smart Technology Co., Ltd. ( JMK Smart ) is a manufacturing export enterprise that continuously pursues: innovation, technology, and quality. Handheld fans, desktop fans, outdoor camping fans, and neck bladeless fans lead the industry.