Monthly Archives: January 2016

Visualizing thermal equilibration: IR imaging vs. Energy2D simulation

Figure 1
A classic experiment to show thermal equilibration is to put a small Petri dish filled with some hot or cold water into a larger one filled with tap water around room temperature, as illustrated in Figure 1. Then stick one thermometer in the inner dish and another in the outer dish and take their readings over time.

With a low-cost IR camera like the FLIR C2 camera or FLIR ONE camera, this experiment becomes much more visual (Figure 2). As an IR camera provides a full-field view of the experiment in real time, you get much richer information about the process than a graph of two converging curves from the temperature data read from the two thermometers.
Figure 2

The complete equilibration process typically takes 10-30 minutes, depending on the initial temperature difference between the water in the two dishes and the amount of water in the inner dish. A larger temperature difference or a larger amount of water in the inner dish will require more time to reach the thermal equilibrium.

Another way to quickly show this process is to use our Energy2D software to create a computer simulation (Figure 3). Such a simulation provides a visualization that resembles the IR imaging result. The advantage is that it runs very fast -- only 10 seconds or so are needed to reach the thermal equilibrium. This allows you to test various conditions rapidly, e.g., changing the initial temperature of the water in the inner dish or the outer dish or changing the diameters of the dishes.

Figure 3
Both real-world experiments and computer simulations have their own pros and cons. Exactly which one to use depends on your situation. As a scientist, I believe nothing beats real-world experiments in supporting authentic science learning and we should always favor them whenever possible. However, conducting real-world experiments requires a lot of time and resources, which makes it impractical to implement throughout a course. Computer simulations provide an alternative solution that allows students to get a sense of real-world experiments without entailing the time and cost. But the downside is that a computer simulation, most of the time, is an overly simplified scientific model that does not have the many layers of complexity and the many types of interactions that we experience in reality. In a real-world experiment, there are always unexpected factors and details that need to be attended to. It is these unexpected factors and details that create genuinely profound and exciting teachable moments. This important nature of science is severely missing in computer simulations, even with a sophisticated computational fluid dynamics tool such as Energy2D.

Here is my balancing of this trade-off equation: It is essential for students to learn simplified scientific models before they can explore complex real-world situations. The models will give students the frameworks needed to make sense of real-world observation. A fair strategy is to use simulations to teach simplified models and then make some time for students to conduct experiments in the real world and learn how to integrate and apply their knowledge about the models to solve real problems.

A side note: You may be wondering how well the Energy2D result agrees with the IR result on a quantitative basis. This is kind of an important question -- If the simulation is not a good approximation of the real-world process, it is not a good simulation and one may challenge its usefulness, even for learning purposes. Figure 4 shows a comparison of a test run. As you can see, the while the result predicted by Energy2D agrees in trend with the results observed through IR imaging, there are some details in the real data that may be caused by either human errors in taking the data or thermal fluctuations in the room. What is more, after the thermal equilibrium was reached, the water in both dishes continued to cool down to room temperature and then below due to evaporative cooling. The cooling to room temperature was modeled in the Energy2D simulation through a thermal coupling to the environment but evaporative cooling was not.

Figure 4

An infrared investigation on a Stirling engine

Figure 1
The year 2016 marks the 200th anniversary of an important invention of Robert Stirling -- the Stirling engine. So I thought I should start this year's blogging with a commemoration article about this truly ingenious invention.

A Stirling engine is a closed-cycle heat engine that operates by cyclic compression and expansion of air or other gas by a temperature difference across the engine. A Stirling engine is able to convert thermal energy into mechanical work.

You can buy an awesome toy Stirling engine from Amazon (perhaps next Christmas's gift for some inquisitive minds). If you put it on top of a cup of hot water, this amazing machine will just run until the hot water cools down to the room temperature.

Figure 2
Curious about whether the Stirling circle would actually accelerate the cooling process, I filled hot water into two identical mugs and covered one of them with the Stirling engine. Then I started the engine and observed what happened to the temperature through an IR camera. It turned out that the mug covered by the engine maintained a temperature about 10 °C higher than the open mug in about 30 minutes of observation time. If you have a chance to do this experiment, you probably would be surprised. The flying wheel of the Stirling engine seems to be announcing that it is working very hard by displaying fast spinning and making a lot of noise. But all that energy, visual and audible as it is, is no match to the thermal energy lost through evaporation of water from the open hot mug (Figure 1).

How about comparing the Stirling engine with heat transfer? I found a metal box that has approximately the same size and same thickness with our Stirling engine. I refilled the hot water to the two mugs and covered one with the metal box and the other with the Stirling engine. Then I started the engine and tracked their temperatures through the IR camera. It turned out that the rates of heat loss from the two mugs were about the same in about 30 minutes of observation. What this really means is that the energy that drove the engine was actually very small compared with the thermal energy that is lost to the environment through heat transfer (Figure 2).

This is understandable because the speed of the flying wheel is only a small fraction of the average speed of molecules (which is about the speed of sound or higher). This investigation also suggests that the Stirling engine is very efficient. Had we insulated the mug, it would have run for hours.