The main reason to increase is the fan diameter is because it is more efficient to move large volume of air at lower speed compared to a smaller volume at higher speed, at the end of the day m.V gives you the thrust you need.  Hence larger diameter the better, however, there are engineering constraints such as ground clearance of the size, transportation (a huge issue) and of course the increase in weight, but the exchange is very favourable and pays to increase if it was not because of the previous aforementioned issues.  Of course there are other benefits such as shielding effect of the core (hot) jet, hence reduction of the noise.  There should not be any major effect on the combustion; however the LP turbine needs to be redesigned to be able to drive the larger diameter fan.

Hope this is helpful.   A question for you: what cycle analyse software do you use for the above analysis?

Hi

I am sure there are many other reasons, but one reason is to reduce noise as the mechnical power scales as flow velocity, U, to the power of three (U**3) while noise, according to Lighthill, scales as the power of eight (U**8).

http://qringtech.com/2014/04/04/flow-induced-noise-explained/

This fact has been a driving force for airplanes for many years. I am sure those working the subject are able to tell you much more than I can.

Sincerely

Claes

if you need to increase the by pass ratio of the turbofan engine you need to increase the fan diameter, which in turn increases the cold thrust of the turbofan engine.

If you want to increase the diameter of the turbofan .we want do lots of modification in the the engine diameter in the combustion chamber also otherwise it will affect the major parameters like velocity and pressure.these are very essential for the gas turbine engine .,

When your requirement is to increase the thrust of the engine for HIgh Bypass turbofan engine, High diameter is advisable as will increase overall propulsive efficiency of aircraft engine. But at the same time it increases the drag of aircraft and increases the fuel consumption. 

The main reason to increase is the fan diameter is because it is more efficient to move large volume of air at lower speed compared to a smaller volume at higher speed, at the end of the day m.V gives you the thrust you need.  Hence larger diameter the better, however, there are engineering constraints such as ground clearance of the size, transportation (a huge issue) and of course the increase in weight, but the exchange is very favourable and pays to increase if it was not because of the previous aforementioned issues.  Of course there are other benefits such as shielding effect of the core (hot) jet, hence reduction of the noise.  There should not be any major effect on the combustion; however the LP turbine needs to be redesigned to be able to drive the larger diameter fan.

Hope this is helpful.   A question for you: what cycle analyse software do you use for the above analysis?