Development of a convective boiling model

for the liquid coolant side in an internal combustion engine

The present project was carried out within the framework of a larger cooperative project entitled: "Virtual engine". The individual project partners were the Dept. for Fluid Mechanics and Heat Transfer (ISW), the Dept. for Internal Combustion Engine, both at the Technical University Graz, the AVL-List in Graz, and the BMW Motoren GmbH in Steyr.

The part of the ISW was the development of a convective boiling model to be applied in CFD of the coolant jackets in combustion engines. Putting the focus on the boiling heat transfer was motivated by the following considerations:

The rapid increase of the specific power output of modern internal combustion engines is accompanied by increasing thermal loads on the structure of the combustiion chambers. Highly efficient cooling systems are needed to keep the wall temperatures below acceptable limits. Concerning the heat flux from the cylinder wall to the liquid coolant jacket the desired high heat transfer rates can be achieved by providing for a transition from single-phase to subcooled boiling heat transfer. To be useful for the modern combustion engine design this concept necessitates realistic and accurate models for subcooled convective boiling, which give, when applied in CFD, reliable predictions for the cylinder wall temperatures.

In the first step an experimental test channel was specially designed and constructed at the ISW Dept. for boiling flow measurements. As schematically shown in Figure 1 the square duct test section was heated from beneath using an aluminium heater. The test fluid was a 50/50 Vol% mixture of water and glycantin, which is commonly used in car engines. In this experimental set-up a large number of flow boiling curves were measured varying the inlet bulk velocity and the pressure of the system, respectively. An experimentally obtained flow boiling curve is exemplarily shown in Figure 2. Using a high-speed camera the bubble dynamics could be investigated for different flow conditions as well (see Figure 3).