what is the disadvantage of a karman vortex sensor?

Introduction

Karman vortex sensors are commonly used for flow measurement and fluid dynamic analysis in various industries and applications. These sensors make use of the Karman vortex street phenomenon, which occurs when fluid flows past a bluff body, resulting in alternating vortices shedding from its wake. While Karman vortex sensors offer numerous advantages, they also come with a few disadvantages that should be considered when deciding whether to use them for a particular application. In this article, we will delve into the disadvantages of Karman vortex sensors and explore their implications.

Understanding Karman Vortex Sensors

Before diving into the disadvantages, it is essential to understand the basic working principle of Karman vortex sensors. These sensors operate by placing a bluff body, such as a cylinder or a triangular prism, in the path of the fluid flow. As the fluid flows past the bluff body, it creates vortices that detach from the outer edges of the bluff body and form an alternating vortex street. The frequency of these vortices shedding is directly proportional to the fluid velocity, allowing for flow measurement.

The Disadvantages of Karman Vortex Sensors

While Karman vortex sensors offer many advantages, such as their compatibility with a wide range of fluids, accuracy, and relatively low cost, they also come with some drawbacks that should be taken into account. Let's explore these disadvantages in detail:

1. Sensitivity to Vortex Shedding Frequency

Karman vortex sensors' accuracy is highly dependent on the proper estimation of the vortex shedding frequency. The sensors require a stable and consistent vortex shedding pattern to measure fluid velocity accurately. However, the shedding frequency can be affected by various factors, such as fluid properties, flow disturbances, and changes in the geometry of the bluff body. This sensitivity to vortex shedding frequency can pose challenges in certain applications where the flow conditions are not constant or when there are disturbances in the flow.

In situations where the vortex shedding frequency deviates from the anticipated range, the accuracy of the measurements can be compromised. Prior calibration and careful selection of the correct bluff body geometry are crucial to minimize this disadvantage. Additionally, factors such as temperature variations and changes in fluid viscosity can further impact the accuracy of the measurements, making it necessary to consider compensation methods or additional calibration steps.

2. Limited Turndown Ratio

Turndown ratio refers to the range of fluid velocities that a sensor can accurately measure. While Karman vortex sensors offer a reasonably wide turndown ratio, it is still limited compared to certain other flow measurement technologies. The turndown ratio depends on various factors, such as the bluff body's size and shape, the sensor's design, and the fluid properties. In applications requiring a wide range of flow rates, Karman vortex sensors might fall short and may not be suitable without additional considerations.

When the fluid velocity approaches too low or too high values, the vortex shedding might become irregular or even cease, leading to inaccurate measurements. To overcome this limitation, it may be necessary to install multiple sensors with different bluff body sizes or consider alternative flow measurement techniques suitable for the specific range of flow rates.

3. Influence of Installation Conditions

The accuracy and performance of Karman vortex sensors can be influenced by their installation conditions. Improper installation can lead to disturbances in the fluid flow, causing the vortex shedding pattern to deviate from the expected behavior. Factors such as upstream and downstream piping, flow disturbances, and the presence of other nearby objects can affect the sensor's measurements.

To ensure reliable measurements, it is critical to carefully consider the installation location and conditions. Appropriate straight lengths of upstream and downstream pipe should be provided to minimize flow disturbances. Additionally, the sensor's alignment with the flow direction and proper positioning within the piping system are crucial for accurate and consistent measurements.

4. Limited Applicability for Non-Newtonian Fluids

Karman vortex sensors are primarily designed for Newtonian fluids, which have a constant viscosity regardless of the shear forces acting on them. Non-Newtonian fluids, on the other hand, exhibit variable viscosity depending on the shear rates. In such cases, the alternating vortex shedding pattern may be disrupted due to the fluid's complex rheological behavior.

When dealing with non-Newtonian fluids, the accuracy and reliability of Karman vortex sensors may be compromised. It is important to carefully evaluate the fluid's rheological properties before considering the use of these sensors. Alternative flow measurement techniques, specifically designed for non-Newtonian fluids, may need to be explored to ensure accurate measurements.

5. Sensitivity to Fluid Properties

Karman vortex sensors can be sensitive to changes in fluid properties, such as temperature, density, and viscosity. Variations in these properties can affect the vortex shedding behavior, leading to discrepancies in the velocity measurements. Temperature changes, for example, can impact the fluid's viscosity, altering the vortex shedding frequency and potentially affecting the accuracy of the measurements.

To mitigate the sensitivity to fluid properties, it is crucial to consider appropriate compensation methods, such as temperature and density corrections. Calibration with different reference fluids or conditions can also help understand and account for variations in fluid properties. However, it should be noted that additional calibration steps can add complexity to the measurement system.

Conclusion

Karman vortex sensors are popular tools for flow measurement and fluid dynamic analysis, thanks to their versatility, accuracy, and cost-effectiveness. However, it is important to be aware of their limitations and potential disadvantages. The sensitivity to vortex shedding frequency, limited turndown ratio, influence of installation conditions, limited applicability for non-Newtonian fluids, and sensitivity to fluid properties are important considerations when evaluating the suitability of Karman vortex sensors for specific applications.

Understanding these disadvantages allows engineers and technicians to make informed decisions and adopt appropriate corrective measures. By carefully addressing these limitations and considering alternative techniques when necessary, the reliability and accuracy of flow measurements can be improved, ensuring optimal performance in various industries and applications.