A silicon computer chip (k = 150 W/m · K) is attached to the top of a motherboard tha generating heat at a rate of 10 W/cm². A fan blows air (T. = 300 K) over the topside of chip, resulting in a convection coefficient of h = 1 kW/m2 · K. The silicon chip has a thickness of 1 mm
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- 1.63 Liquid oxygen (LOX) for the space shuttle is stored at 90 K prior to launch in a spherical container 4 m in diameter. To reduce the loss of oxygen, the sphere is insulated with superinsulation developed at the U.S. National Institute of Standards and Technology's Cryogenic Division; the superinsulation has an effective thermal conductivity of 0.00012 W/m K. If the outside temperature is on the average and the LOX has a heat of vaporization of 213 J/g, calculate the thickness of insulation required to keep the LOX evaporation rate below 200 g/h.1.4 To measure thermal conductivity, two similar 1-cm-thick specimens are placed in the apparatus shown in the accompanying sketch. Electric current is supplied to the guard heater, and a wattmeter shows that the power dissipation is 10 W. Thermocouples attached to the warmer and to the cooler surfaces show temperatures of 322 and 300 K, respectively. Calculate the thermal conductivity of the material at the mean temperature in W/m K. Problem 1.41.3 A furnace wall is to be constructed of brick having standard dimensions of Two kinds of material are available. One has a maximum usable temperature of 1040°C and a thermal conductivity of 1.7 W/(m K), and the other has a maximum temperature limit of 870°C and a thermal conductivity of 0.85 W/(m K). The bricks have the same cost and are laid in any manner, but we wish to design the most economical wall for a furnace with a temperature of 1040°C on the hot side and 200°C on the cold side. If the maximum amount of heat transfer permissible is 950 , determine the most economical arrangement using the available bricks.
- 2. In determining the thermal conductivity of a plateflat insulator, the temperature was measured on both sides of the 25 mm thick plate and gave the results of 318.4 K and 303.2 K. FluxThe measured heat is 35.1 W.m-2. Illustrate the problem in a simple schematic/drawing, then calculate the thermal conductivity inbtu/h.ft.oF and in W/m.K! (0.033 ; 0.058) 3. A coil-shaped cooling pipe is made of SS-304 material. This pipe is 1 ft long, 0.4 inch outside diameter, and inch inside diameter. This coil cooling pipe is used to cool the water in the bath. The temperature of the inner coil pipe is 40oF while the outer coil in contact with water is 80oF. The thermal conductivity of SS-304 is a function of temperature where k(T) = 7.75 + (7.78 x 10-3).T where k is in Btu/h.ft.oF and T is in oF. Calculate the rate of heat dissipation in watts! (1287.7)A composite wall of a refrigerator consists of three different materials; A, B, and C. Two of the used materials have known thermal conductivity, KA = 30 W/m. K and kc-45 W/m. K, and known thickness, LA=0.20 m and Lc = 0.15 m. The third material, B, which is sandwiched between materials A and C, is of known thickness, LB = 0.15 m, but unknown thermal conductivity kB. Under steady-state operating conditions, measurements reveal an outer surface temperature of Ts,o= 20°C, an inner surface temperature of Ts,i = 600°C, and a refrigerator air temperature of Too = 800°C. The inside convection coefficient h is known to be 25 W/m².K. (a) First, draw the thermal circuit for this wall. (b) What is the value of thermal conductivity for material B? Tsi Air To, h KA K₂ kc Lg Lc Figure 2: Composite wall configuration described in problem 3.Q2- Water flows inside a steel pipe with an ID of 2.5 cm. The wall thickness is 2 mm, and the convection coefficient on the inside is 100 Wm? C. The convection coefficient on the outside is 10 W/n C. The pipe is covered with a layer of asbestos. 1-determine the thickness of the asbestos layer at the critical insulation radius of the pipe. 2-calculate the change percent of heat transfer with and without the insulation. 3-commet on your results,
- A finned surface has been added to cool an electronic part. The surface temperature of the electronic part in contact with the fins is 60 ° C, the convection coefficient between the environment and the finned surface is 40 W / m? .K and the thermal conductivity coefficient for the fin material (aluminum) is 180 W / m.K. a. The amount of Heat (W) per unit time thrown into the environment in case the electronic part has wings or no blades, b. Find out wing effectiveness and efficiency?Problem 4 A cork board (k = 0.039W/m K) 3 cm thick is exposed to air with an average temperature T.. = 30°C with a convection heat transfer coefficient, h = 25 W/m².K. The other surface of the board is held at a constant temperature of 15°C. A volumetric heat generation of 5 W/m³ is occurring inside the wall. Assuming one dimensional conduction, write the governing equation and express the boundary conditions for the wall. (solve the equation is not required) ProblomA hollow cylindrical copper conductor 1.27cm. i.d. and 5.1cm. o.d. carries a current density 5000 amp/cm². For copper K = .38 kW/m°K and electrical resistivity = 2 x 10-6 ohm cm. Find the position and magnitude of the maximum temperature and the internal and external heat removal when (a) the outside temperature is 37.8°c and no heat removal occurs on the inside and (b) the outside is at 37.6°C and the inside at 27.2°C.
- For your company you need to mass produce 20-cmx10-cm heat sinks to dissipate heat quickly from an equipment. After considering several other parameters, you have short-listed two possible designs: A. 72 straight pin fins made of an alloy (thermal conductivity 240 W/m-°C) that costs $9,000 per m³. Each pin is 4 cm long and 1-cm diameter. E.=3'18; $204 B. 64 straight rectangular fins made of another alloy (thermal conductivity 180 W/m.°C) that costs $12,000 per m³. Each pin is 4 cm long and has 2-cmx1-cm cross-section. Eo =7:94%$ 6·14 The convection coefficient of the operating condition is approximated as 15 W/m2.°c. Consider that the manufacturing cost is same for both designs but the material cost differs. Calculate and compare the cost and performance (in terms of heat transfer gains by the heat sinks) of the two heat sinks so that a decision can be made to select a design.The composite wall of an oven consists of three materials, two of which has known thermal conductivity, kA = 40 W/m.K and kc = 30 W/m.K. The third material, B, sandwiched between these two has an unknown thermal conductivity kB. The thickness of the materials LA = 0.2 m, LB = Lc = 0.1 m. Under steady-state operating conditions, measurements reveal an outer surface temperature Ts,o = 40°C, an inner surface temperature Tsi = 550°C, and oven air temperature To = 700°C. The inside convection coefficient h = 25 W/m2.K. (a) Draw the thermal circuit and temperature profile across the wall. (b) Determine the heat flux through the wall. %3D %3D %3D (c) Find the value of kB. Draw sketch and show all calculations.(50 marks) Q: A hot surface at 125°C is to be cooled by attaching 4 cm long, 0.2 cm diameter aluminum pin fins (k =237 Wim. °C) to it, with a center-to- center distance of 0.5 cm. The temperature of the surrounding medium is 25 °C, and the heat transfer coefficient on the surfaces is fos cm 25 W/m?. C. Determine the rate of heat transfer from the surface 0.2 for a Ix0.5 m section of the plate. Also, determine the overall effectiveness of the fins.