However when CHF is exceeded, film boiling occurs leading to the sudden drop of the heat transfer rate and metal temperature rise.
The drawback is that if film boiling appears, wall temperature increase drastically and the part fails very quickly.
When film boiling occurs, stresses in the part increase drastically causing an early failure.
Due to thermal stresses in the components, which depend mainly on temperature field, durability drops drastically when metal temperature increases due to film boiling.
Figure 3 shows the effects on the inner tubes of an EGR cooler when CHF is exceeded, metal shows deep temper color marks due to high temperature and coolant silicate depositions appear when film boiling occurs.
In an EGR cooler normally the areas more susceptible to have film boiling are located near the hot header, so although the thermocouples are located in several zones inside the cooler, the instrumentation is focused around the hot header.
The objective is to test points with no boiling or low boiling level, and also points with film boiling.
Absolute values of temperature for every single sweep give important information about when film boiling occurs, but the full view cannot be obtained without a global analysis.
In one case the inlet coolant port has been oriented in order to maximize the coolant velocity in the hot header (Good cooling) and in the other case the inlet port has been oriented deliberately for obtaining a bad coolant distribution to force film boiling (Bad cooling).
With Good cooling it was impossible to reach film boiling.