Heat flux sensors are transducers that are used to convert thermal energy into electrical signals.
The electrical signal generated in a heat flux sensor is usually a voltage signal proportional to the heat rate.
Gardon gauge or foil-type heat flux sensors are used in heat flux measurements under high-intensity radiation environments.
Heat flux is the amount of heat energy that passes through the surface
Heat flux measurements are of primary importance in chemical industries, heat transfer systems, furnaces, and almost all manufacturing industries. Heat flux is the amount of heat energy that passes through the surface.
Heat flux sensors are usually employed for heat flux measurements. In the smart textile industry, heat flux sensors collect information on the thermal exchange between the environment and the body. The heat flux sensor output values significantly improve the garment's thermal comfort. This is just one of many examples of heat flux sensor applications.
In this article, we will discuss the types of heat flux sensors as well as how to distinguish between heat flux and temperature sensors.
Heat Flux Sensors
The heat flux cannot be measured directly. Generally, the effect of heat energy in transit is monitored and the heat flux is inferred. Heat flux sensors are transducers that are used to convert thermal energy into electrical signals. Heat flux sensors generate electrical signals equivalent to the rate of thermal energy transfer per unit area. The electrical signal generated in a heat flux sensor is usually a voltage signal proportional to the heat rate. From the electrical signals, the heat flux is determined. Heat flux sensors can measure the heat flux in all three heat transfer mechanisms: conduction, convection, and radiation.
Types of Heat Flux Sensors
Heat flux sensors are available in different types. A few of them are listed in the table below.
Heat Flux Sensors vs. Temperature Sensors
Heat flux and temperature are two entirely different quantities. Generally, there is confusion regarding the purpose of heat flux sensors and temperature sensors.
Whenever there is a temperature gradient, heat energy flow takes place and the heat flux sensor measures the thermal energy flow. The measured quantity is energy. In a temperature sensor, the measured quantity is the temperature in degrees Celsius, Fahrenheit, or Kelvin.
As thermal energy flows through an element, its temperature builds up. It is possible to determine which of the two components is hotter when using a heat flux sensor. With the help of a heat flux sensor, the final temperature of an element or system can be predicted. However, to exactly measure the temperature, a temperature sensor should be used.
Determining Heat Flux Using Heat Flux Sensors
When using heat flux sensors, the output voltage is recorded. Mathematically, the heat flux is calculated from the voltage. Most heat flux sensors are available with precalibrated data to convert the voltage measurements into heat flux. When employed in applications outside of normal room temperature, the heat flux sensor behaves differently. The multiplication graph and heat flux sensors are useful for finding the multiplication factor corresponding to a certain temperature. The multiplication graph is created by testing the heat flux sensor at different temperature ranges. This graph is utilized whenever the heat flux sensor is used outside the normal temperature range.
Measurement Techniques in Heat Flux Sensors
Heat flux measurement techniques can be classified into four categories.
- Calorimetric method - Heat balance is applied to a particular region of the heat flux sensor and the temporal measurement of the temperature gives the heat flux quantity.
- Based on the differential temperature - The determination of heat flux across a spatial distance is based on the measurement of discrete location temperatures, the related temperature gradient, and material properties.
- Mass-transfer analogy - Mass transfer is measured instead of heat transfer and heat flux is inferred using the analogy.
- Energy supply or removal - A heater or cooler is used for heat supply and removal. A heat balance is applied and the temperature measured helps determine the heat flux.
Gardon Gauge or Foil Heat Flux Sensors
Gardon gauge or foil heat flux sensors are used in heat flux measurements under high-intensity radiation environments. In these sensors, a thin metallic circular foil is used as a constant temperature reservoir that absorbs heat. The absorbed heat is transferred radially to the copper heat sink welded around the circumference of the foil. Due to heat dissipation to the heat sink, the maximum temperature distribution is experienced at the center of the foil, whereas the minimum is at the circumference of the foil. The thermocouple attached between the center and edge of the foil generates electrical voltage output.
Use of Heat Flux Sensors
In extreme environments such as modern turbomachinery, aero-engine systems, and thermal protection systems, heat flux measurements are challenging. There are variants of heat flux sensors that are characterized to overcome the difficulty in heat flux measurements under harsh and extreme atmospheric conditions with great accuracy.
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