ASTM E2683-17

5.1 The purpose of this test method is to measure the net heat flux to or from a surface location. For measurement of the radiant energy component the emissivity or absorptivity of the surface coating of the gage is required. When measuring the convective energy component the potential physical and thermal disruptions of the surface must be minimized and characterized. Requisite is to consider how the presence of the gage alters the surface heat flux. The desired quantity is usually the heat flux at the surface location without the presence of the gage.

5.2 The measured heat flux is based on one-dimensional analysis with a uniform heat flux over the surface of the gage. Measurements of convective heat flux are particularly sensitive to disturbances of the temperature of the surface. Because the heat-transfer coefficient is also affected by any non-uniformities in the surface temperature, the effect of a small temperature change with location is further amplified as explained by Moffat et al. (FIG. 2 Diagram of an Installed Insert Heat-Flux Gage

5.2.1 If the gage is water cooled, the thermal pathway to the plate is less important. The heat transfer to the gage enters the water as the heat sink instead of the surrounding plate. Consequently, the thermal resistance between the gage and plate may even be increased to discourage heat transfer from the plate to the cooling water. Unfortunately, this may also increase the thermal mismatch between the gage and surrounding surface.

5.2.2 Fig. 2 shows a heat flux gage mounted into a plate with the surface temperature of the gage of T_s and the surface temperature of the surrounding plate of T_p. As previously discussed, a difference in temperature between the gage and plate may also increase the local heat transfer coefficient over the gage. This amplifies the measurement error. Consequently, a well designed heat flux gage will keep the temperature difference between the gage surface and the plate to a minimum, particularly if any convection is being measured.

5.2.3 Under transient or unsteady heat transfer conditions a different thermal capacitance of the gage than the surrounding material may also cause a temperature difference that affects the measured heat flux. Independent measures of the substrate and the gage surface temperatures are advantageous for defining the heat transfer coefficient and ensuring that the gage thermal disruption is acceptably small.

5.3 The heat flux gages described here may also be water cooled to increase their survivability when introduced into high temperature environments. By limiting the rise in gage temperature, however, a large disruption of the measured heat flux may result, particularly if convection is present. For convection measurements to match the heat flux experienced by the surrounding surface, the gage temperature must match the temperature of that surface. This will usually require the surrounding surface to also be water cooled.

5.4 The time response of the heat flux sensor can be estimated analytically if the thermal properties of the thermal resistance layer are well known. The time required for 98 % response to a step input (where α is the thermal diffusivity of the TRL. Covering or encapsulation layers must also be included in the analysis. The calibrated gage sensitivity in Eq 3 applies only under steady-state conditions.