# ASTM E457-08(R2020) pdf free download

ASTM E457-08(R2020) pdf free download.Standard Test Method for Measuring Heat-Transfer Rate Using a Thermal Capacitance (Slug) Calorimeter

3. Summary of Test Method

3.1 The measurement of heat transfer rate to a slug or thermal capacitance type calorimeter may be determined from the following data: 3.1.1 Density and specific heat of the slug material, 3.1.2 Length or axial distance from the front face of the cylindrical slug to the back-face thermocouple, 3.1.3 Slope of the temperature—time curve generated by the back-face thermocouple, and 3.1.4 Calorimeter temperature history.

3.2 The heat transfer rate is thus determined numerically by multiplying the density, specific heat, and length of the slug by the slope of the temperature–time curve obtained by the data acquisition system (see Eq 1).

3.3 The technique for measuring heat transfer rate by the thermal capacitance method is illustrated schematically in Fig. 1. The apparatus shown is a typical slug calorimeter which, for example, can be used to determine both stagnation region heat transfer rate and side-wall or afterbody heat transfer rate values. The annular insulator serves the purpose of minimizing heat transfer to or from the body of the calorimeter, thus approximating one-dimensional heat flow. The body of the calorimeter is configured to establish flow and should have the same size and shape as that used for ablation models or test specimens.3.3.3 Although the goal of good slug calorimeter design is to minimize heat losses, there can be heating environments, such as very high heat fluxes, where even a good slug calorimeter design cannot meet the recommended 5 % maxi- mum heat loss criterion of 6.1. Also, this criterion only deals with heat losses measured during the cooling phase, not losses during the heating phase, which can be greater than the cooling losses. Under these circumstances, significant heat losses from slug to holder during the heating phase, as well as other possible decaying processes such as a drop in surface catalycity, can cause the Temperature-Time slope to decrease significantly more than can be accounted for by the increasing heat capacity with temperature of the Copper slug alone, making it important that the slope be taken early in the process before the losses lower the slope too much, introducing more error to the downside on the heat flux calculated (see Fig. 3). The degree of losses affect the exact position where the best slope begins to occur, but typically it should be expected at about time τ = τ R calculated by Eq 2 for q indicated /q input = 0.99, which value of τ R is abbreviated as τ R0.99 . Fig. 2 and Fig. 3 assume that “heat source on” is a step function. This is an idealization, but the reality can be significantly different. For example, in some cases a calorimeter may experience a higher heat flux prior to reaching its final position in the heat source, which can cause the initial maximum slope to be higher than what is wanted for the calculation of the heat flux at the final position. Therefore, it is important to note that “zero” time, to which τ R0.99 is added to determine where to start looking for the desired slope, is when the calorimeter has reached its final slug should be modeled and accounted for by a correction term in the energy balance equation.

3.4 To minimize side heating or side heat losses, the body is separated physically from the calorimeter slug by means of an insulating gap or a low thermal diffusivity material, or both. The insulating gap that is employed should be small, and recommended to be no more than 0.05 mm on the radius. Thus, if severe pressure variations exist across the face of the calorimeter, side heating caused by flow into or out of the insulation gap would be minimized. Depending on the size of the calorimeter surface, variations in heat transfer rate may exist across the face ofthe calorimeter; therefore, the measured heat transfer rate represents an average heat transfer rate over the surface of the slug.