There is nothing wrong with a fasting person smelling good smells, from food, perfume etc, but he should not inhale bukhoor (incense) or the steam coming out of the cooking pot because these have particles which may reach his stomach.
It says in Haashiyat al-Dasooqi (1/525):
If the smoke of bukhoor or steam from the cooking pot reaches the throat, the day must be made up… if it reaches it through inhaling deliberately, whether the one who does that is the one who is making it or someone else. But if one of them reaches the throat without him wanting it to, then he does not have to make it up, whether he is the one who is making it or someone else according to the correct view. End quote.
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked:
Is it permissible to use perfume, such as putting on ‘ood, cologne and bukhoor during the day in Ramadaan?
Yes, it is permissible to use it, so long as one does not inhale bukhoor. End quote.
Fataawa Ibn Baaz, 15/267
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: What is the ruling on a fasting person using perfumes during the day in Ramadaan?
There is nothing wrong with using them during the day in Ramadaan and inhaling them, except for bukhoor, which should not be inhaled, because it has particles which may reach the stomach, i.e., the smoke. End quote.
Fataawa Ramadaan, p. 499
It says in Fataawa al-Lajnah al-Daa’imah (10/271):
If a person puts on any kind of perfume during the day in Ramadaan when he is fasting, that does not invalidate his fast, but he should not inhale bukhoor and powdered perfume such as powder of musk. End quote.
The point is that simply smelling food does not affect the fast, but one should not inhale the steam from the cooking pot.
And Allaah knows best.