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The data in the spreadsheet ends at 15 seconds. It is critical that you match the end time in the spreadsheet to the step end time in ANSYS Mechanical. Click on under the tree in the outline window and enter 15 for the Step End Time.
Click on and you should see a constant temperature at 22 degrees from 0 to 15 seconds. Copy the time data in spread sheet and single click on the first cell in the first column (Time) and copy. Do the same for the temperature data in the first cell in the second column. The tabular data should look like this: