Bench Talk

My family is mildly crazy and the stunt for this past Spring was driving from Los Angeles to Indianapolis in 72 hours. One of my brothers was graduating in Los Angeles and another was getting married near Indianapolis.

About 90 minutes out of LA, our car overheated. Naturally my mom and I decided to open the hood and see what we could do. As I poured water over the radiator, I noticed that in some spots the water was boiling on contact and other places it was rolling off and dripping onto the ground.

I thought about my thermal class. In particular I was thinking of thermal contact resistance. I recalled a particularly illustrative picture from Chapter 10 of my thermal book and the heat conduction of road dust versus metal (Hint: it is not helping with the heat transfer).

I thought about the mechanical stress that must occur if part of the radiator is being hit with cool water while the rest is being protected from this shock by dust. Yet, how much is this mechanically stressing the radiator? It was around 110°C (230°F) and the water around 25°C (77°F). I do not know enough about the mechanical properties of aluminum to make an educated guess. I realize 85°C (153°F) is a fairly wide range, but I think perhaps the water took most of the stress by boiling away.

These are just the musings of an interested novice. If you have anything knowledgeable to say on this, please leave a comment.

Later in the journey I took on a much easier problem. How much would my brother’s uncooled reception hall heat up in two hours given 200 not active people and a volume of 13,068m³? Also, because we all like a frictionless vacuum, we assumed even air circulation, and the people were the only source of heat transfer.

Answer: 8.9°C (16°F) since we started around a cool 20°C (68°F) which would leave the room around a balmy 28.9°C (84°F) and that is, falsely, assuming no dancing which would lead to 3X the power output per person dancing.

Fortunately for the happy couple, industrial grade fans were quickly brought in that blew air conditioned and cool May Indiana air into the space. I still think it reached a mildly uncomfortable 27°C (80.6°F).

The Indianans asked “hot enough for you yet?” to which the collection of continent driving Texans just gave them a look of “you don’t know hot.”