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PSYCHO15rus [73]
2 years ago
9

An electronic device dissipating 25 W has a mass of 20 g and a specific heat of 850 J/kg·0K. The device is lightly used, and it

is on for 5 min and then off for several hours, during which it cools to the ambient temperature of 250C. Let the mass of heat sink vary from 0 to 1 kg. Plot the maximum temperature against the mass of heat sink, and discuss the results. U

Engineering
1 answer:
7nadin3 [17]2 years ago
7 0

Answer:

Explanation:

FIrst let c be the specific heat, now we need the heat that the device is able to provide:

so it is turned on for 5 minutes (300 seconds) with 25 Watts

Q = Power*time = (25)*(300) = 7500J

Now heat is equal to the change in internal energy, where we will use the remaining terms.

Now we continue

Q = \Delta U = M*C(\Delta T)

where \Delta T is the change of temperature ( \Delta T = T_{final} - T_{initial} )

now Q = \Delta U = M*C*(T_{final} - T_{initial})

We solve T_{final} = \frac{Q}{M*C} +T_{initial}

now we just plot it for different values of M using any software..

please find attached the plot of the temperature.

The result indicates that as the mass increases it is harder for the system to reach higher final temperatures.

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From the question we are told that:

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A cyclic tensile load ranging from 0 kN to 55 kN force is applied along the length of a 100 mm long bar with a 15 mm x 15 mm squ
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square cross section. The bar is made of a 7075-T6 aluminum alloy which has a yield strength of 500 MPa, a tensile strength of 575 MPa, and a fracture toughness of 27.5 MPaâm.

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Water at atmospheric pressure boils on the surface of a large horizontal copper tube. The heat flux is 90% of the critical value
masya89 [10]

Answer:

The tube surface temperature immediately after installation is 120.4°C and after prolonged service is 110.8°C

Explanation:

The properties of water at 100°C and 1 atm are:

pL = 957.9 kg/m³

pV = 0.596 kg/m³

ΔHL = 2257 kJ/kg

CpL = 4.217 kJ/kg K

uL = 279x10⁻⁶Ns/m²

KL = 0.68 W/m K

σ = 58.9x10³N/m

When the water boils on the surface its heat flux is:

q=0.149h_{fg} \rho _{v} (\frac{\sigma (\rho _{L}-\rho _{v})}{\rho _{v}^{2} }  )^{1/4} =0.149*2257*0.596*(\frac{58.9x10^{-3}*(957.9-0.596) }{0.596^{2} } )^{1/4} =18703.42W/m^{2}

For copper-water, the properties are:

Cfg = 0.0128

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q_{n} =uK(\frac{g(\rho_{L}-\rho _{v})     }{\sigma })^{1/2} (\frac{c_{pL}*deltaT }{c_{fg}h_{fg}Pr  } \\16833.078=279x10^{-6} *2257x10^{3} (\frac{9.8*(957.9-0.596)}{0.596} )^{1/2} *(\frac{4.127x10^{3}*delta-T }{0.0128*2257x10^{3}*1.76 } )^{3} \\delta-T=20.4

The tube surface temperature immediately after installation is:

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For rough surfaces, Cfg = 0.0068. Using the same equation:

ΔT = 10.8°C

The tube surface temperature after prolonged service is:

Tprolo = 100 + 10.8 = 110.8°C

8 0
3 years ago
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