Convection Calibration of Heat Flux Gages in Stagnation and Shear Air Flow

Introduction

Measurement of heat flux in addition to temperature gives a more complete understanding of thermodynamic systems because it provides direct information about energy transfer and heat paths. In order to make accurate heat flux measurements, however, it is important to calibrate heat flux gages in environments close to their use. The presence of the gage can cause a disruption of the thermal conditions that is particularly important when convection is present at the surface.

Most methods for measuring heat flux are based on insertion of a sensor either onto the surface or into the material, which can cause physical and thermal disruption of the surface. The goal is to minimize these disruptions, which requires an understanding and analysis of the gage system. These effects are generally not present during the usual radiation calibration done by the manufacturers or NIST. The figure shows a general situation for an insert heat flux gage with three modes of energy transfer acting on it.

Heat flux sensor with the three modes of energy transfer

Consultant Results

Convection calibration of Schmidt-Boelter heat flux gages was performed in both shear and stagnation air flow. The gages were standard design using a copper heat sink with water cooling channels around the active sensing element. The gages were provided included two one-inch diameter and two one-and-one-half-inch diameter Schmidt-Boelter heat flux gages. The effect of the thermal resistance between the surface of the sensor and the heat sink was estimated from the calibration results. The model predicts a non-linear dependency of the gage sensitivity over a range of heat transfer coefficients. All of the measured sensitivities for shear as well as for stagnation flow fit with the theory and show the non-linear decay of apparent sensitivity for increasing heat transfer coefficient values. This was true for both water-cooled and non-water-cooled tests.

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