Analysis and Modeling

 

A realistic model for convection and boiling heat transfer coefficient in engine cooling systems is under development at Western Michigan university. The data available from the various tests with the 1.6L DaimlerChrysler S.I. engine were superimposed on the theoretical boiling curve predicted by the new model developed. The model curve shows the wall temperature overestimated, which is believed to attribute to the dissolved air boiling as shown in the next figure (McAdams, 1949)

The presence of gas dissolved in the water increases the heat transfer in subcooled boiling. Gas bubbles can form at the surface and agitate the liquid in the same way that vapor bubbles do. McAdams et al. [1949] demonstrated the effect of dissolved gases in forced convection subcooled boiling. It can be seen that the presence of gas has a considerable effect on the position where the first evolution of bubbles is seen and also on the curve for the boiling process at the lower heat fluxes. However, the effect dies out at the higher heat fluxes near the critical heat flux.

In order to investigate the effect of 3-D on the linear prediction of coolant wall temperature with the measurement of a pair of metal temperatures in the cylinder head, numerical computations were performed using a 2-D rectangular channel with the boundary conditions shown in the following figure. The rectangular channel simulates a narrow flow at the exhaust-exhaust gas valve bridge in the cylinder head. The top surface is exposed to ambient air, while the sides are exposed to the exhaust gas valve exit flow and the bottom face is exposed to the combustion chamber near the spark plug in the cylinder head. The convection heat transfer coefficient in the coolant channel was incorporated with the new model, so that the coolant wall temperature leads to whether boiling at the surface takes place or not. The results using a Mathcad software with typical operating conditions were shown in the 2-D rectangular channel computation.

The results of the computations indicate that the linear assumption of temperature at hot spots might slightly overestimate both the coolant wall and gas face temperatures, but slightly underestimates the heat flux at coolant wall, which is associated with the heat flow into the spot as shown in the left figure.

 

 

 

 

 

 

 

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Mechanical and Aerospace Engineering

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