The flow field of air in the headspace induced by the natural convection inside the menu box was included in the numerical simulation. The air gaps in the head spaces of beefsteak box, French toast box and Danishes boxes were very small, so the air there was assumed to be stagnant. Natural convective flow was considered only for the headspace of the menu box and the flow field was assumed to be laminar. The velocity of air at the inner walls of the menu box was specified as zero. The conservation of momentum equation (Eq. (10)) was used for solving natural convective air flow.
equation(10)