Heat flux measurement techniques

# Heat flux measurement techniques

This short review gives the reader an overview of the various heat flux measurement techniques with the direct and indirect methods with the use of heat flux and temperature sensors respectively.

## Direct and Indirect heat flux measurement techniques

Heat flux can be measured using two different techniques. It can be either measured directly using Heat Flux Sensors, or alternatively, indirectly using Temperature Sensors.

### Direct Heat Flux Measurement Technique: Heat flux sensor

This Heat Flux Measurement Technique relies on one Heat Flux Sensor.

The gSKIN® Heat Flux Sensors measure the heat passing through the sensor surface. The single heat flux sensor is mounted to the position of interest (see “HFS” on the image on the left). The main advantages of this method lies in it simple application and the very high heat flux resolution. Typically, the heat flux resolution is < 0.1 W/m2. An explanation of the working principle of gSKIN® Heat Flux Sensors is available here.

### Indirect Heat Flux Measurement Technique: Temperature sensors

The Indirect Heat Flux Measurement Technique uses the relationship between Heat Flux (HF) and the temperature difference (∆T) and absolute thermal resistance (Rth). The Heat Flux HF is calculated from the measurement of two temperature sensors, and the exact absolute thermal resistance between the two spots A and B such as:
HF = ∆T / Rth

with HF = Heat Flux, in W, ∆T = Temperature difference TA – TB, in K, Rth = Absolute thermal resistance between point A and B, in K/W.

While this Technique is tried and tested, it has major disadvantages with regards to Heat Flux accuracy. This technique relies on highly accurate and costly temperature sensors. Furthermore, it is very difficult to define the absolute thermal resistance between two points. The absolute thermal resistance is a component property and depends on the component shape and size, material properties and the distance between points A and B. Knowing the exact value of these properties is challenging.

## The combination of both techniques allows indirect temperature measurement

The same relationship as the one used in the Indirect Heat Flux Measurement Technique is applied here. Instead of using only two temperature sensors, here it uses one Heat Flux Sensor and one Temperature Sensor mounted at the same spot (see figure on the right). Now the temperature at Spot A can be calculated such as
TA = (HF x Rth) + TB where TA = Temperature at spot A, in K, HF = Heat Flux, in W ,Rth = Absolute thermal resistance, in K/W, TB = Temperature at spot B, in K. This is especially useful when Spot A is difficult to reach.