Heat loss
 

Hello

I'm trying to find the effect of different initial swirl ratio on NG/diesel dual-fuel combustion. According to literature, in diesel engine heat losses increase when increasing swirl ratio. However, I increased the swirl ratio from 1 to 3 and the heat losses decreased. I used the ''Han and Reitz'' model for the wall heat transfer. I used 1 layer with the cell size of 0.0005 for the walls. Does anyone suggest smaller cell size around walls? Do I need to increase the number of layers?

It is weird that the heat loss of piston increased, however, the heat loss of cylinder wall and cylinder head decreased when the swirl ratio increased from 1 to 3.

I even compared the Heat Transfer Coefficient (CHT) and found attached results. I expected that the HTC increases with increasing swirl ratio for the whole crank angles, but it did not.


I appreciate if anyone helps me.

Regards,
Amin.

Hello amin,

All three of your plots show that for most of the crank angle range that HTC increases with swirl ratio. Since HTC does not tell the whole story, have you looked at the time-integrated heat transfer to the wall boundaries? Referring to CONVERGE 2.4 manual, Ch 23.6, if you set wall_output_flag = 1 in inputs.in, you will get a file bound<ID>-wall.out for each wall boundary. Column 17 of that file is the total heat transfer to the boundary. You can compare this cumulative heat flow [J] for the three cases.

As to wall spacing, please check y+ at the walls. Using 30 < y+ < 300 is recommended for standard law of the wall models such as Han and Reitz or O'Rourke.

Hello Dan,

I found what is the issue.I carefully look at the Han&Reitz model and there is (y) in the formula which shows the volume of the cell near the wall. First time the boundary layer cell size was 0.5 mm and the smallest cell of AMR reached 0.25. When the flame reached the piston wall the cell size of boundary layer was changed according to the flame shape which is different based on different swirl ratios. So, to keep constant the boundary layer cell size I used the cell size of 0.25mm for near wall the same as smallest cell size of AMR. Actually, the boundary layer cell size affects the heat transfer. Now, when the flames come to the boundary layer, the cell size near the wall is still 0.25 and does not change. I got results under different swirl ratios which show that increasing swirl increases the heat losses which make sense.