Frequency response limitation of heat flux meters

TL;DR

This paper investigates the theoretical frequency response limitations of heat flux meters, revealing that their transfer functions can behave as high pass filters contrary to physical expectations, which impacts their accuracy under dynamic conditions.

Contribution

It introduces a theoretical analysis transforming the heat equation into models showing potential high pass filter behavior of heat flux meters.

Findings

Heat flux meters can exhibit high pass filter characteristics.

Theoretical models challenge the assumption of low pass filter behavior.

Implications for dynamic heat flux measurement accuracy.

Abstract

Heat flux meters are used for measuring the heat flux densities going through walls, usually at quasi-steady state. The limitations of heat flux meters under dynamic conditions are well documented in the literature; nonetheless there is a theoretical limitation which is mostly not considered and should be also taken into account. Since heat transfer is a dissipative process, it would be expected to obtain transfer functions which act as low pass filters. Nonetheless, this paper shows that the transfer functions modeling heat flow rate may become high pass filters, which is against the physical evidence. In order to show this theoretical limitation of the heat flux meters, the heat equation is transformed in different classes of models, from partial differential equations to transfer functions related to surface temperatures and heat flux density going through a wall.