High-pressure water jet cleaning is one of the internationally recognized scientific, economical, and environmentally friendly cleaning methods. It works by using a power unit to drive a high-pressure plunger pump to generate high-pressure water for flushing the surface of an object. When the impact force of the water exceeds the adhesion between the dirt and the surface, the high-pressure water will remove and carry away the dirt, achieving the purpose of cleaning the object's surface.
At the same time, high-pressure water jet cleaning machines are also very professional equipment. Although they are simple and convenient to use, improper operation can lead to significant loss of equipment efficiency, or even personal and equipment safety accidents. When using a high-pressure gun for cleaning, many people are accustomed to gradually bringing the nozzle closer to the object being cleaned, believing that as the nozzle approaches the object, the impact force of the water jet will gradually increase. However, this conclusion is not accurate.
First, let's look at the basic structure of a high-pressure water jet. The internationally recognized jet structure diagram in the water jet academic community is as follows:
It divides the water jet into three segments, respectively: initial segment, basic segment, and dissipation segment.
1、Jet Initial Segment
The jet flow from the nozzle exit to the turning surface area. The jet immediately interacts with the surrounding medium upon exiting the nozzle, experiencing intense momentum exchange and turbulent diffusion. However, a portion of the jet medium near the central axis still maintains the initial velocity at the nozzle exit, forming an equal-velocity core, which is the essence of the jet. The initial segment has concentrated energy, mainly used for water cutting and not suitable for cleaning.
2. Jet Basic Segment
The area from the turning surface to the dissipation segment. The jet basic segment follows the initial segment and is a longer section of the jet. Within this segment, the axial velocity and dynamic pressure of the jet gradually decrease, following a hyperbolic relationship. Turbulent characteristics are fully manifested in this segment, with the jet medium mixing with surrounding media such as air to form a turbulent mixing zone. The jet remains intact with a compact internal structure in this segment. The greater the target distance, the lower the axial velocity, meaning the striking force decreases, but the striking area increases, resulting in higher cleaning efficiency. Additionally, due to the jet's entrainment effect, surrounding air is also entrained, forming a jet with air bubbles that enhances the jet's destructive power. This segment is mainly used for cleaning.
3. Jet Dissipation Segment
The jet dissipation segment follows the basic segment. At this stage, the jet has fully mixed with the surrounding medium, with relatively low axial velocity and dynamic pressure. In the atmosphere, the jet becomes a mixture of water droplets and air or atomized mist. Clearly, in the dissipation segment, the jet's ability to entrain surrounding media is nearly exhausted, and the boundary layer is blurred by the atomization zone. The line from the boundary to the axis indicates the start of the atomization zone. The striking force at the dissipation end is very small or even nonexistent, and this segment is mainly used for spraying, cooling, dust removal, and landscape creation.
According to the characteristics of water jet, from the above analysis, the jet basic segment is mainly used in the cleaning industry. Mathematical analysis and experimental data confirm that there exists an optimal target distance to achieve maximum striking force. According to experimental results, the optimal target distance for achieving maximum striking force is approximately 25 to 150 times the nozzle diameter.
Taking a high-pressure washer with a pressure of 15MPA, flow rate of 10L/MIN, and a nozzle diameter of 1.25 mm as an example, its optimal standoff distance is approximately 120 mm. When the standoff distance is less than 120 mm, the impact force slightly decreases but the change is not significant, but the impact area is greatly reduced, significantly lowering the cleaning efficiency. When the standoff distance exceeds 120 mm, especially beyond 140 mm, the impact force drops rapidly.
For professional use models, the optimal standoff distance is usually used to achieve maximum efficiency. The same model applied to other scenarios may not necessarily use the optimal standoff distance to achieve the highest efficiency. For instance, for light dirt and floating dust, an impact force of 5~10N is sufficient to clean effectively. For the above high-pressure washer used for car washing, it is best to use a 30° fan nozzle and a standoff distance exceeding 200 mm, which ensures high efficiency and most importantly, does not damage the paint surface. The main reason is that for car washing, this model's pressure, flow rate, and total power configuration are much higher than needed.
Of course, the optimal jet standoff distance is a rather complex issue, not only related to jet pressure and nozzle diameter, but also highly related to factors such as the hardness and adhesion strength of the cleaning target material, the matching of nozzle type with jet pressure and flow rate, nozzle type and design, manufacturing quality, jet impact angle, and cleaning movement speed. In short, professional equipment requires professional technical guidance to achieve maximum efficiency.
Main references: Xue Shengxiong's 'High-Pressure Water Jet Technology Engineering', Fushen Environmental Protection's 'Discussion on Jet Standoff Distance' etc.
