At supersonic speed, why does Mach number decrease?

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At supersonic speed, air become compressible. However why does Mach number decrease when flow area decrease? I understand that pressure will increase because the density will increase when flow area decrease. However shouldn't the Mach number increase in order to conserve mass flow? Unless the Increase in density is much higher than the loss in flow area.

As for expansion fan, the Mach number will increase due to increase in flow area. Again is it because just like the above, the loss in density is much greater than the increase in flow area, hence Mach number will increase to conserve the mass flow?
 
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Your question is not clear. What is the context? A supersonic airplane? A nozzle? Please provide a link to where you read about mach number decrease.
 
254
5
Your question is not clear. What is the context? A supersonic airplane? A nozzle? Please provide a link to where you read about mach number decrease.
Nozzle.
"When an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated. Across a shock wave, the Mach number decreases, the static pressure increases, and there is a loss of total pressure because the process is irreversible."

https://www.grc.nasa.gov/www/k-12/airplane/expans.html
 

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