I am guessing that you want some kind of separation between dining room and kitchen? You want to keep steam and any oil in the air, from cooking, out of the dining room, stopping it at the ceiling with a wall but keeping the dining room open so the light from the large kitchen window can filter into the dining space? If I am right, the shape of an arch would best match the architecture of your house or of these rooms. An arch is an architectural element as well as a design element. It is possible though that an arch will be totally out of place in your space when a cleaned lined separation would look very nice. Hard to tell. Can you post a picture or give some verbal detail about the style of your spaces?

Size of kitchen is 12 ft by 16 ft

The distance between chimney and hob needs to be 26"-30" : 65cm-80cm. placing it above the window, suction power is reduced, placing it in middle of the window, will make the window ineffective. Island chimneys are best suited to be positioned in middle of the room, the duct running through false ceiling. also they fall in the expensive category. it is best that you move the hob position to side of the window, or totally to the other side and do chimney core cutting there.

Hi ma'am Please contact me as under. I am an architect and interior designer from Gurugram deals in architecture n interiors. Hope we get in touch.  Thanks Alokendu  A+m21 architects studio Gurgaon 98181908.61