Standard Test Method for Measuring Heat Flux Using Flush-Mounted Insert Temperature-Gradient Gages

Importancia y uso:

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.

5.1.1 Temperature limitations are determined by the gage material properties, the method of mounting the sensing element, and how the lead wires are attached. The range of heat flux that can be measured and the time response are limited by the gage design and construction details. Measurements of a fraction of 1 kW/m ² to above 10 MW/m² are easily obtained with current gages. With thin film sensors a time response of less than 10 μs is possible, while thicker sensors may have response times on the order of 1 s. It is important to choose the gage style and characteristics to match the range and time response of the required application.

5.1.2 When differential thermocouple sensors are operated as specified for one-dimensional heat flux and within the corresponding time response limitations, the voltage output is directly proportional to the heat flux. The sensitivity, however, may be a function of the gage temperature, which is often important for uncooled gages.

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. (2) and Diller (3). Moreover, the smaller the gage surface area, the larger is the effect on the heat transfer coefficient of any surface temperature non-uniformity. Therefore, surface temperature disruptions caused by the gage should be kept much smaller than the surface to environment temperature difference driving the heat flux. This necessitates a good thermal path between the sensor and the surface into which it is mounted. If the gage is not water cooled, a good thermal pathway to the system’s heat sink is important. The gage should have an effective thermal conductivity as great or greater than the surrounding material. It should also have good physical contact insured by a tight fit in the hole and a method to tighten the gage into the surface. An example method used to tighten the gage to the surface material is illustrated in Fig. 2. The gage housing has a flange and a separate tightening nut tapped into the surface material.

The value of the step change in imposed heat flux is represented by q_ss. The resulting time constant characterizes the first-order sensor response.

5.4.3 The time response of the gage can be improved by up to a factor of 28 by using a simple data processing routine (8). It uses a combination of the temporal and spatial temperature measurements of the sensor. This is another reason for measuring and recording temperature signals along with the heat flux.

Subcomité:

E21.08

Referida por:

F1930-23, E3057-19, E1529-22

Volúmen:

15.03

Número ICS:

17.200.10 (Heat. Calorimetry)

Palabras clave:

heat flux gage; temperature gradient; thermal transport;