combo questions iuui ECU stions 12 -1 e following wil allow you to demonstrate c

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combo questions

iuui ECU stions 12 -1 e following wil allow you to demonstrate competency in #T applying the first w of thermodynamics to a steady flow device: i) calculating power and fii) using tract thermophysical data. The solutions provided should also be done n.a tables to ext way that allows you to practice systematic problem solv al management in electronic d herm rough a power supply evices and computers is an important design consideration. The motherboard on a small laptop operates by drawing 20V and 4.5A This motherboard is assembled into a plastic case , with negligible h eat transfer assumed to occur through solid walls. To avoid the electronics in the case from overheating a single motherboard powered) 10W electric fan is mounted inside the casing Th e fan sits flush against the base of the laptop and functions by drawing ambient air (coofing ain) into the case via suction. Ambient air is drawn into the fan through a series of openings in the base of,the protective case. The air passes over a heat exchanger (mounted on motherboard components) and then exits through a side port. The side port has 10 openings, each measuring about 3mm (wide) by 10mm (high) Assume that for steady operating conditions, the air temperature inside the case should not exceed 6OC (before it reaches the side port) On a typical day, the temperature of ai rawn into the computer case reaches 27C. Assume that the absolute pressure (Pabs) inside the case is 101.325kPa and air behaves as an ideal gas with constant specific heats CV 0.721 koikg.K and Cp- 1.007kJ/kg.K Outlet of cooling ai Intake fi Note from tables in solution, clearly identify these data and f property data are ext he tables they are extracted from

Explanation / Answer

Given data:

Air inlet temperature(T1)= 27 °C

Air outlet temperature(T2)= 60 °C

CP= 1.007 kJ/kg.K

Cv=0.0721 kJ/kg.K

Pabs= 101.325 kPa

Power consumed by fan=Work done on system= 10 W or 10 J/s

Power generated by motherboard= Internal energy generated by the system= 20 V * 4.5 A= 90 W

Solution to Q12

According to first law of thermodynamics,

Q =E + W ,                           where Q= heat transferred,E=internal energy of the system,W=work doneby the system

Now, E= 90 J/s and W= -10 J/s

So, Q=90 +(-10) =80 J/s

Since pressure is constant,

Q= ? Cp ?T = ? Cp (T2-T1)

=> 80= ? *1.007*(60-27)

=> ? =2.4074 g/s

Mass flow rate of air is 2.4074 g/s.                                                                                                           (Answer)

Solution to Q13

Volumetric flow rate of air inlet=Volumetric flow of air outlet

?/?air = A*v ,where A=total area of cross section of 10 openings, v= average flow speed of air

Also ?air= Pabs/(T*Rspecific)              ,where T=absolute temperature,

                                                                Rspecific= specific gas constant of dry air (287.058 J/kg.K)

Hence ?air=101.325 kPa/(300 K * 287.058 J/kg.K)

         => ?air= 1.1766 kg/m3 = 0.0011766 g/cm3

A = 10*3*10 mm2= 300 mm2=3 cm2

Now using this relation,

?/?air = A*v

=>2.4074/0.0011766 = 3*v

=>v=682.02 cm/s

=>v=6.82 m/s approx.

Average flow speed of air is 6.82 m/s.                                                                                                     (Answer)

Solution to Q14

New inlet temperature of air (T1*)

Since pressure is constant,

Q= ? Cp ?T = ? Cp (T2- T1*)

=> 80= ? *1.007*(60-35)

=> ? =3.177 g/s

Mass flow rate of air is 3.177 g/s.                                                                                                             (Answer)

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