# Consider a system where a wall of a tank containing a hot liquid at a temperature T0, with an air stream of temperature T∞ flows on the outside of the tank, maintaining the outside wall temperature of TL. What is the expression for boundary condition of the system?

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Consider a system where a wall of a tank containing a hot liquid at a temperature T0,  with an air stream of temperature T∞ flows on the outside of the tank, maintaining the outside wall temperature of TL. What is the expression for boundary condition of the system?

(a) q=h(TL-T∞)

(b) T=T∞

(c) T=TL

(d) TL-T∞

The question was posed to me in final exam.

The origin of the question is Single Variable Problems in division Single Variable Problems of Finite Element Method

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The correct answer is (a) q=h(TL-T∞)

The explanation: The boundary conditions are mainly of three kinds: constant temperature, constant heat flux (zero for insulated), and convection. In this problem, the temperature difference between wall and surroundings creates the constant heat flux of q=h(TL-T∞). This heat flux is taken as a boundary condition.

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