TrtComb	  Airflow
(1)   	0.46
a     	0.42 
b     	0.57 
ab    	0.45 
c     	0.73 
ac    	0.71 
bc    	0.70 
abc   	0.70 
d     	0.42
ad    	0.28
bd    	0.60
abd   	0.29
cd    	0.70
acd   	0.71
bcd    	0.72
abcd   	0.72