Question

What is transient heat transfer? why it is important?

Answer 1

Transient heat transfer, is the temperature within an object itself keeps changing with time. The heat entering a section thus might not be the same as the heat exiting the section, as the temperature difference across the section keeps changing with time.

"Transient" implies the heat transfer is changing with time. If a block of material is held at room temperature and heated at one side, the temperature distribution in the material will change: the heated side will heat first and heat will travel through the material (following Fourier's law of heat transfer). As heat travels from the hot side to the room temperature side, a steady state temperature distribution will form, but since the entire block starts at room temperature initially, the temperature distribution will not form immediately.

Transient heat transfer is the non-steady state transfer of energy through a medium. The transient state refers to a non-constant flow of energy. This changing rate of heat transfer could be due to fluctuating temperature difference over the medium or changing properties throughout the medium.

For example heating up of gas turbine compressors as they are brought up to speed during take-off. The disks that hold the blades are large and take a long time to come to temperature, while the casing is thin and in the path of high velocity compressor flow and thus comes to temperature rapidly. The result is that the case expands away from the blade tips, sometimes enough to cause serious difficulties with aerodynamic performance.