Motivation
Different concepts are available for the cooling of temperature-sensitive components. In comparison to air or liquid cooling, direct-evaporation of a refrigerant has the following advantages:
- Highly efficient heat transfer during boiling of the refrigerant,
- thus more compact designs of heat exchangers,
- large cooling capacities,
- uniform temperature distribution by constant evaporation temperature.
In the case of the parallel connection of evaporator tubes, however, a maldistribution of the refrigerant can occur. The target of the project is the examination of this mechanism for different refrigerants.
Flow instabilities during phase change
If a refrigerant flows through an evaporator pipe with a constant heat flow, it may happen that a decrease in the pressure loss occurs when the mass flow density is increased. A corresponding state point is potentially unstable, so that the smallest disturbances cause a sudden response of the system (Ledinegg instability). This phenomenon particularly affects the behavior of parallel evaporator tubes, e.g. in a cooling plate. If there is still a uniform distribution at higher mass flows, a reduction of the flow rate can lead to a direct superheating in individual pipes, whereas in other pipes there's almost no evaporation happening. A uniform cooling of the plate is thus prevented.
This problem will be investigated experimentally and numerically with field simulations for different refrigerants at variable heat and mass flows, as well as at different pressure levels.