Topic: thermodynamics

1. An engine block with a surface area measured to be 0.90m2 generates a power output of 55 kW with a net engine efficiency of 35%. The engine block operates inside a compartment at 157oC and the average convection heat transfer coefficient of 60 W/m2K.
. a) If convection is the only main heat transfer mechanism occurring, determine the engine ?block surface temperature.
. b) Ifthereisalsoradiationtosurfaceofthecompartment(Tsurfacecompartment=87oC) ?in addition to the convection, what would be the surface temperature of the engine block (use emissivity=0.8 for engine surface).

2. A molten mixture of salt (NaNO3-KNO3) is used as the heat transfer fluid for application in a parabolic trough solar field. The molten salt at 300oC is flown through a steel pipe (k = 14 W/moC) whose inner and outer diameters are 9 cm and 10 cm, respectively, in an environment at 32oC. If the heat transfer coefficients on the inside and the outside of the pipe are 150 and 30 W/m2C, respectively:
a. Determine the rate of heat loss from the molten salt per meter length of the pipe.
b. If the pipe is insulated with 5-cm-thick SiO2 wool insulation (k = 0.017 W/moC), what would be the rate of heat loss per meter length of the pipe. What is the reduction in heat loss (in %) if we use the insulation.

3. Hot water at an average temperature of 90oC passes through a row of eight parallel pipes that are 4 m long and have an outer diameter of 3 cm, located vertically in the middle of a concrete wall (k = 0.75 W/mK) that is 4 m high, 8 m long, and 15 cm thick. If the surfaces of the concrete walls are exposed to a medium at 32oC, with a heat transfer coefficient of 12 W/m2K, determine the rate of heat loss from the hot water and the surface temperature of the wall.

4. A hot dog can be considered to be a cylinder 12 cm long and 2 cm in diameter whose properties are ??= 980 kg/m3, cp = 3.9 kJ/kgoC, k = 0.76 W/moC, and ??= 1.99×10-7 m2/s. A hot dog initially at 4oC is dropped into boiling water at 100oC. If the heat transfer coefficient at the surface of the hot dog is estimated to be 680 W/m2oC, determine the center temperature of the hot dog after 5, 10 and 15 min by treating the hot dog as a finite cylinder.

NOTE:
4. – Do the problems in a systematic way.
5. – Include the units in the calculation.
6. – State any assumptions.
7. – For Problem 1, you will need to solve an equation with power of 4 (as it involves radiation). You can use EES, Excel, ?Matlab, Mathematica to solve this.