The air-cooled engine is common in motorcycles, lawn mowers, generators, etc. It dissipates heat from the engine through air circulation. The design of metal fins increases the surface area of the engine and thus improves the cooling rate through convection. The design of the air-cooled engine involves many aspects of engineering considerations, such as cooling rate, total mass, geometry restrictions, and so on.
This example isolates variables and greatly simplifies an engine design so that one only needs to focus on the heat transfer aspect. A simplified geometry is used that contains an engine cylinder and multiple heat-dissipating fins. Follow the instructions below and answer the questions:
1. Conduct a transient heat analysis on Design (a) and Design (b), and compare the time it takes to cool the maximum temperature of the engine cylinder down to 35℃. The modeling details are listed below:
Structural steel is used on both the engine cylinder and the metal fins. The boundary conditions are defined as follows: the cylinder’s initial temperature is set to 120℃ while the bulk temperature outside of the fins is set to 22℃; a convection boundary condition is applied to the outside surface of the cylinder and the fins with the ambient temperature as 22℃, and the convection coefficient as 1000W/(m2∙℃).
2. Conduct a transient heat analysis on Design (c) with the same modeling details in Step(1), and find out the time it takes design (c) to cool the maximum temperature of the engine cylinder down to 35℃. Assuming there are two design criteria:
a. The maximum temperature of the engine cylinder should drop to 35℃ within 50 secs.
b. Minimize the total mass.
Which of the design is better? Design(a), (b), or (c)?