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An electronic device, such as a power transistor mounted on a finned heat sink, can be modeled as a spatially isothermal object with internal heat generation and an external convection resistance.
(a) Consider such a system of mass M, specific heat c, and surface area
(b) An electronic device, which generates 60 W of heat, is mounted on an aluminum heat sink weighing 0.31 kg and reaches a temperature of 100°C in ambient air at 20°C under steady-state conditions. If the device is initially at 20°C, what temperature will it reach 5 min after the power is switched on?
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Chapter 3 Solutions
Introduction to Heat Transfer
- 1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.arrow_forwardQ5Two large containers A and B of the same size are filled with different fluids. The fluids in containers A and B are maintainedat 0° C and 100° C, respectively. A small metal bar, whose initial temperature is 100° C, is lowered into container A. After1 minute the temperature of the bar is 90° C. After 2 minutes the bar is removed and instantly transferred to the othercontainer. After 1 minute in container B, the temperature of the bar rises 10°. How long, measured from the start of theentire process, will it take the bar to reach 99.9° C?arrow_forward3.4 Estimate the rate of heat loss due to radiation from a covered pot of water at 95 ° C. How does this compare with the 60 W that is lost due only to convection and conduction losses? What amount of energy input would be needed to maintain the water at its boiling point for 30 minutes? The polished stainless steel pot is cylindrical, 20 cm in diameter and 14 cm high, with a tight-fitting flat cover. The air temperature in the kitchen is about 25 ° C. State any assumptions you make in deriving your estimatesarrow_forward
- In thermodynamics, how do you differentiate a closed system with an adiabatic system? What is the difference in the equation used for calculating heat transfer? (If you know a good book or scientific article that contains info regarding this, please include. Thank you.)arrow_forwardI need the answer as soon as possiblearrow_forwardHeat transfer allows us to study non-equilibrium processes which is otherwise impossible in thermodynamic analysis O Not sufficient information O True O Falsearrow_forward
- Q2 FigureQ2showsaHeatingRoom,whichpartlyisolated.Thereisathermalsystemconsisting of an electric heater in a room. Heat input by the heater is Q1and heat loss from room is Q2. Air in the room is at uniform temperature, ?, and surrounding temperature is ?o. Thermal capacity of the room is C, and wall thermal resistance is R. (a) Determine a mathematical model of temperature of the room. (b) DeterminetheTransferFunctionoftheheatingroom. (c) Determine the output response of the system when a step input 25°? is given.arrow_forwardThermodynamics: Can you show me how to solve for the answer that is written below? Please show it in step by step solution Thank you!arrow_forwardHi, can you help me with the following problem? A thermally insulated tank of water 2 m x 2 m, depth 1 m is stirred with a paddle for threehours. If the paddle is driven by a motor whose shaft power is 8 kW determine the rate ofchange of temperature in degrees C per hr and the increase in temperature of the water.The specific heat of water is 4200 J/kgC. Identify a system and start your analysis from anappropriate form of the Fist Law of Thermodynamicsarrow_forward
- 6. a. The heat flux applied to the walls of the biomass combustion furnace is 20 W/m2. The furnace walls have a thickness of 10 mm and a thermal conductivity of 12 W/m.K. If the wall surface temperature is measured to be 50oC on the left and 30oC on the right, prove that conduction heat transfer occurs at a steady state!b. Heating the iron cylinder on the bottom side is done by placing the iron on the hotplate. This iron has a length of 20 cm. The surface temperature of the hotplate is set at 300oC while the top side of the iron is in contact with the still outside air. To reach the desired hotplate temperature, it takes 5 minutes. Then it takes 15 minutes to measure the temperature of the upper side of the iron cylinder at 300oC. Show 3 proofs that heat transfer occurs transientlyarrow_forward3. A well-mixed cement-lined storage tank contains water initially at 50°C. The water loses energy to its surroundings via conduction through the tank's concrete-lined walls. The tank sits in an unheated warehouse with an air temperature of 0°C. A diagram of the storage tank is shown below. The cement lining is 1.5-cm thick and has a thermal conductivity (htc) of 2.0 W/(m K). height = 1.5 m diameter = 2.0 m a. How long will it take (in minutes) for the water to cool to 40°C? b. Why might you be over- or underestimating the time it will take to cool the contents of the reactor?arrow_forward3. A lumped system with a volume of 0.003 m³ and a surface area of 0.08 m² is made of a material with density of 3800 kg/m³, thermal conductivity of 300 W/m K, and specific heat of 200 J/kg K. If the system is exposed to a convection environment with h = 60 W/m2 K, what is the approximate time it will take for this system to reach equilibrium with the environment? Express your answer in minutes.arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning
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