A 15 cm x 15 cm circuit board dissipating 15 W of power uniformly is cooled by air, which approaches the circuit board at 20°C with a velocity of 5 m/s. Disregard any heat transfer from the back surface of the board and evaluate properties of air at 300 K. Assume the flow to be turbulent across the entire length of the board since the electric components will have tripped the boundary layer. (a) Determine the surface temperature of the electronic components at the leading edge of the board, and (b) at the end of the board. (c) Ideally, at what temperature would the properties of air be evaluated at?

Metal plates of size 60 cm × 50 cm, and thickness 1 cm are initially at 100◦C. They are to be cooled by blowing chilled air at 2 m/sec over both surfaces, the flow being directed along the side whose length is 60 cm. Incoming air temperature is 10◦C. Data for the metal: ρ = 7854 kg/m3, cp = 434 J/kg/K, k = 60.5 W/m/K Data for air: Use values at 50◦C. (a Determine the convective heat transfer coefficient at the surfaces of the plates.(b Determine whether lumped system analysis is applicable(c Assume the convection coefficient is constant, determine how much time is necessary for the temperature of the plates to drop to 25◦C.(d Estimate the total heat lost by a plate when it has reached 25◦C.

solve, show assumptions and full solutions

Fruit is impeded from ripening when it reaches high temperatures (>32°C). Farmer Joe isworried that Deleware's summer heat wave will affect the ripening of his ground cherries.The spherical fruit (1 cm in diameter) is surrounded by a spherical papery husk (λhusk = 0.3W/m K) with an inner diameter of 2 cm and an outer diameter of 2.1 cm. The air in betweenthe fruit and the husk is stagnant (λair = 0.03 W/m K). On a calm day in July, the delewareair (outside of the husk) is very warm, with a temperature of ~38°C and a heat transfercoefficient of h deleware,calm = 10 W/m 2K. (a) Sketch the system, labeling all temperatures, dimensions, and proportionality constants. (b) sketch the electrical analogy tothis system, labeling all temperatures, resistances, and heat flows (c) what is the overall resistance to heat transfer from the air in Deleware to the fruit?