Thermodynamics heat sink questions

[tamumechengr]

1.Task is to maintain the temperature a device, lower than 40 C. The unit is operating for 5 minutes and then turned off for a time period that the device temperature reaches ambient temp 25 C. Suppose you have 10 heat sinks of different weights...0.1,0.2,0.3,...1.0 kg. Determine the temperature of the unit when the heat sinks are attached to the unit. Plot the temperature without the heat sink and together, (i.e. total 11 points) and find the weight that you achieve this objective. Discuss outcomes what will happen if you use materials of lower or higher specific heats than that of aluminum for the heat sink.

specs of device
30 W power consuming unit
mass= 20 g
specific heat= 850 J/kg-K

Anyone have any ideas to solve this out theoretically... I have EES to solve equations. Not sure exactly where to start.

Homework Equations

Second Law of thermodynamics?

The Attempt at a Solution

(Qin + Win +(m(h+v^2/2+gz))in)-(Qout + Wout +(m(h+v^2/2+gz))out)=Esys
if you put this into rate form i believe the DEsys/dt=0...I don't think you need the m(h+v^2/2+gz part on either...
Win would be 30 W...
Qin+Win-Qout-Wout=0? not sure where to go from here

 

[RTW69]

I'm not sure what exactly is happening with this problem but it looks like you have an electrical device that you need to run for 5 minutes (300 seconds) at a maximum temperature of 40 C. Q*Time=M*C*(delta T) shows that the device reaches 40 C in about 8.5 seconds so you need to add aluminum heat sinks to absorb some of the heat generated. We are assuming no convection or radiation heat transfer.

Q_gen*time= Q_device+ Q_alum heat sink where Cp for alum is 0.9 J/g*K. You can solve directly for the mass of the heat sink needed to keep temp < 40 C