Validation of High Temperature Emissivity Results

In previous work we proposed a model for predicting the emissivity of stainless steel type 304 based on surface conditions and temperature. Such a model would be useful in the modeling of certain welding processes. We now repeat the previous experiments in an attempt to validate the model. Our results are promising, but point to the need to repeat the original experiment while eliminating several sources of experimental error.

Introduction

The purpose of this experiment is to validate a model for the emissivity of stainless steel type 304 which we proposed in previous work. Such a model would be useful in modeling the temperature profile of a metal during the welding process, which has important consequences for the strength of a weld, and of the regions surrounding a weld.

Our model predicts changes in emissivity due to varying surface conditions and temperature. During the welding process the surface of the metal undergoes varying degrees of carbonization, which appear as different colored bands surrounding a weld; we therefore modeled the changes in emissivity due to surface condition as a function of color. Emissivity also varies with temperature. Our model therefore contains three parts: the initial emissivity of the stainless steel, a function of surface color, and a function of temperature:

e_total = e_initial * C(color) * T(temp)

Figure 1: proposed emissivity model

We were able to attain graphs of both the C(color) and T(temp) functions:

Figure 2: C (color) function Figure 3: T(temp) function

The goal of this experiment is to validate our model. We will experimentally measure the emissivity of welded samples at various temperatures, and compare results with our model’s predictions.

Setup

For this experiment we will use a method similar to our previous work. A ceramic, open-ended oven is used to heat our samples to a maximum of 200 ^oC. A thermocouple lead is ultrasonically welded to the sample to obtain one direct temperature measurement. The thermocouple measurements are recorded by a Fluke Hydra DAQ unit, which is controlled with a LabView program. We use an infrared camera to capture the sample’s heating and emission; the IR image is recorded on VHS tape for later analysis.

Figure 4: Experimental Setup

Procedure / Experimental Technique

Each sample was placed on the oven’s tray and slowly heated to approximately 250°C. Starting at 150°C infrared images and measurements were captured every 40°C to 60°C. When taking the infrared readings, first an appropriate range was selected on the Infrared camera in order to obtain the best resolution for the emission levels being measured. This way the greatest contrast in the image could be obtained without losing any information due to the measurements being too high or too low and therefore out of range.

Once an appropriate range was obtained VCR recording of the images was started. Within the IR camera the emittance of the sample was set equal to 1. With the camera a point temperature measurement was read near the welded thermocouple. Simultaneously the actual temperature of the sample was read with the thermocouple. This information, along with the emission level at this point, can be used to calculate a reference emittance value for each image. Color and gray scale infrared images were recorded as well as the range of levels for those images.

A flat aluminum sheet was then placed in front of sample and infrared images were recorded at its appropriate range. If the aluminum sheet is placed directly in front of the sample, then it is assumed that all surround emissions are reflected off the sheet and read by the camera. This is a calibration procedure that takes into account all the emissions that are surrounding the sample but not from the sample itself.

The video recorded images were transported into computer image processing application (NIH Image 1.61) for analysis. The analysis procedure for the data was the same as that used in the previous high temperature emissivity experiment.

In order to measure the emittance of a particular region on a sample, we must be able to measure its infrared emission level relative to the emission level of a region of known emittance. This measurement can be performed by an IR camera alone. However, we must have a direct temperature measurement from the sample in order to establish a region of known emittance; due to existing surface imperfections, and the heating of the sample, we cannot simply use the published emittance of stainless steel 304 as a reference.

For a more complete description of the mathematical techniques used to produce emittance values, please refer to our previous report.

Analysis

Below are tables containing our measured emissivity values alongside the values predicted by our model. Also included are two graphs of the measured emissivities plotted next to the predicted values, one of the best matching data, and one of the worst. As a rule, the model’s predictions are higher than the measured values.

Figure 5: Sample 1 data

Figure 6: Sample 2 data

Figure 7: Sample 3 data

Figure 8: Our best data match Figure 9: Our worst data match

Clearly our model for high temperature emissivity is not perfected. The results are promising, however, considering the significant sources of error present in our initial experiments. This leads us to believe that it would be worthwhile to repeat our first set of experiments, while eliminating several sources of error, with the goal of refining our existing emissivity model. The following sources of error could be realistically eliminated:

· Place the IR camera closer to sample to increase image resolution

or

Use a zoom lens on the IR camera, to the same end

· Weld the thermocouples to the sample

· Take all data points precisely at 50 ^oC intervals

· Eliminate condensation on camera lens, perhaps with a heat shield