For a trade show, you are going to have a lot of people looking at it and asking questions. Hacking together an Arduino for the display is a good idea but the temperature sensors have to be the actual ones that your customers will use. You're trying to demonstrate the performance of the heat exchanger, aren't you? Then you need to do it in a way that your customers will understand.
If this was just a home project then I would say just tape or glue a 5-cent sensor to the outside of the pipe.
For industrial/automotive/aerospace use, you are going to have specialised sensors that everyone uses. They may even have worse performance than the homemade sensor, but they will have a known performance that your customers are used to. With a sensor that they are comfortable with, they will be comfortable with your performance claims.