Chapter 2, Problem 2.4DP

You are designing an enclosure which houses a PCB. Mounted outside the enclosure on its top is a set of aluminum fins, as seen in the sketch below. The enclosure (and fins) are 150mm deep long. There is no forced airflow. Neglect the thickness of the enclosure walls.  Neglect heat transfer out of the sides and bottom of the enclosure.  If the PCB produces 4000 W/m2: a) Most industrial-grade ICs have a maximum temperature of 85oC. If you must keep your PCB below this what is the maximum thermal resistance from the PCB to the air surrounding the fins?  Assume the air around the fins stays at room temperature. b) If the top of the enclosure (and bottom of the fin base) runs at 50°?, what is the effective thermal conductivity above the PCB? Hint: Because the air inside the enclosure is trapped and heats up, any air properties NOT given below are calculated at the average air temperature (T1+T2)/2. c) What is the temperature at the base of the fins? d) What is the efficiency of the fins? e)…

When the sun is high in the sky, it delivers approximately 1000 watts of power to each square meter of earth's surface. The temperature of the surface of the sun is about 6000 K, while that of the earth is about 300 K. Suppose you plant grass on this square meter of earth. Some people might argue that the growth of the grass (or of any other living thing) violates the second law of thermodynamics, because disorderly nutrients are converted into an orderly life form. How would you respond?

A wall consists of a layer of wood and a layer of cork insulation of the same thickness. The temperature inside is 34.0°C, and the temperature outside is 0.0°C. The thermal conductivity of wood is 0.130 W/(m·K) and the thermal conductivity of cork is 0.0460 W/(m·K). A) What is the temperature at the interface between the wood and the cork if the cork is on the inside and the wood on the outside? in C  B) What is the temperature at the interface if the wood is on the inside and the cork is on the outside? in C C) It doesn’t matter whether the cork is placed on the inside or the outside of the wooden wall because the temperature at the interface differs for the two cases.the temperature at the interface differs for the two cases. the temperature at the interface is the same for the two cases.the temperature at the interface is the same for the two cases. the total thermal resistance is the same for the two cases.the total thermal resistance is the same for the two cases.…

Thermal energy of a substance is the sum of the potential energy and kinetic energy of the particles that make it up. Let’s say you had a hot cup of water and a cold cup of water, and then mixed them. Illustrate the behavior of the particles before and after thermal equilibrium is reached.

Q3. What is the analogical reason between heat transfer by conduction and flow of electricity through ohmic resistance? Use a composite wall of a building to illustrate the concept. A composite slab with three layers of thermal conductivities k1, k2, k3 and thickness t1, t2, t3 respectively, are placed in a close contact. Derive an expression from the first principle for the heat flow through the composite slab per unit surface area in terms of the overall temperature difference across the slab.

1. The two faces of a thin metal sheet that is 3 units wide and 5 units long are insulated, and the sheet is so thin that the heat flow inside of it is essentially two-dimensional. The bottom edge is kept at zero degrees, and the top edge is kept at fifty degrees. The vertical edges are insulated. Determine the temperature distribution throughout the sheet in a steady state.

Nanotechnology, the field of building ultrasmall structures one atom at a time, has progressed in recent years. One potential application of nanotechnology is the construction of artificial cells. The simplest cells would probably mimic red blood cells, the body’s oxygen transporters. Nanocontainers, perhaps constructed of carbon, could be pumped full of oxygen and injected into a person’s bloodstream. If the person needed additional oxygen—due to a heart attack or for the purpose of space travel, for example—these containers could slowly release oxygen into the blood, allowing tissues that would otherwise die to remain alive. Suppose that the nanocontainers were cubic and had an edge length of 25 nanometers. What is the volume of one nanocontainer? (Ignore the thickness of the nanocontainer’s wall.)

Nanotechnology, the field of building ultrasmall structures one atom at a time, has progressed in recent years. One potential application of nanotechnology is the construction of artificial cells. The simplest cells would probably mimic red blood cells, the body’s oxygen transporters. Nanocontainers, perhaps constructed of carbon, could be pumped full of oxygen and injected into a person’s bloodstream. If the person needed additional oxygen—due to a heart attack or for the purpose of space travel, for example—these containers could slowly release oxygen into the blood, allowing tissues that would otherwise die to remain alive. Suppose that the nanocontainers were cubic and had an edge length of 25 nanometers. Suppose that each nanocontainer could contain pure oxygen pressurized to a density of 85 g/L. How many grams of oxygen could be contained by each nanocontainer?

Q2: A: The exterior wall of a single-story office building near Chicago is 3 m high and 15 m long. The wall consists of 100-mm face brick, 40-mm polystyrene insulating board. 150-mm lightweight concrete block, and an interior 16-mm gypsum. The wall contains three single-glass of 3 mm thickness windows 1.5 m high by 2 m long. Calculate the heat loss through the wall at design conditions if the inside temperature is (20 °C). The outside temperature is (-18 "C).