During a cold winter day, wind at 42 km/h is blowing parallel to a 6-m-high and 10-m-long of a house. If the air is outside at 5oC and the surface temperature of the wall is 12oC, determine the rate of heat loss from that wall by convection. What would your answer would be if the wind velocity was doubled

a) , b)

Explanation:

a) The problem depicts a case of heat transfer due to natural convection. The wall can be modelled as vertical plate. As first step, the Raleigh number is found:

Where:

- Coefficient of volume expansion of air, in .

- Kinematic viscosity of air, in .

- Gravitational acceleration, in .

Properties of air at 5 °C are presented below:

Let assume that air behaves ideally. Then, the coefficient of volume expansion is:

The Rayleigh number is:

The following formula is adecuate to estimate the Nusselt number:

Convection coefficient can be determined in terms of Nusselt number:

The rate of heat loss by natural convection is:

b) The rate of heat loss is directly proportional to mass flow, and mass flow is directly proportional with speed. Then, the heat rate is found by simple rule of three:

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