toc1955's video: Thermoacoustic Cooling: a Demonstration

(Adolf Cortel, August 2020). This video shows how an acoustic standing wave in a tube produces a temperature gradient along a segment of a porous material (stack). I don't discuss the theory of thermoacoustics; those interested can find it in the book: Gregory W. Swift. Thermoacoustics. A Unifying Perspective for Some Engines and Refrigerators. In the book there is a link to download simulations which are very useful to understand how thermoacoustic gadgets work. This demonstration is based in the article by Daniel A. Russell and Pontus Weibulla “Tabletop thermoacoustic refrigerator for demonstrations”, Am. J. Phys. 70 , December 2002, pp1231-1233. The stack they propose is a roll of photographic film with segments of fishing line acting as separators. They get a temperature difference of 16ºC and, most important, the decrease of temperature at the cool end is about the double than the increase at the hot end. Since the prospect of building the stack with film by gluing fishing line looks to be very tedious I tried an alternative with an adhesive tape protected with corrugated plastic I came across in a hardware store. Once rolled this material has holes of around 0.3 mm, which are of a magnitude appropriate for thermoacoustic cooling. With this material I get a gradient of around 11-12 ºC, but the decrease of temperature at the cool end is only the half of the increase at the hot end. Anyway, it can be used for demonstration purposes: the gradient produced is a meaningful one and can be visualized very clearly by the insertion of thermistors, thermocouples or even with a thermochromic sheet attached to the tube. The thermistors (or a Vernier thermocouple) are connected to a Vernier SensorDAQ module and the data are captured, treated and visualized using simple Labview software. Another modification of the setup: I have checked that the position of the stack relative to the closed end of the tube is an important parameter to improve the gradient of temperature. In my apparatus the position of the stack (and the holes for the thermistors) is fixed and I use a movable plug to search for the position giving the maximum gradient. About the music: “Don't think twice it's alright” (Bob Dylan); the nice version by Leftover Cuties. I've no rights on this music and I use it only for the educational purposes which this video is aimed.