An insulation layer of 20 mm thick refractory material is placed on top of a layer of mineral wool to insulate a hollow, spherical steel ball with an outside diameter of 60 mm and an internal diameter of 50 mm wool. The table below lists the thermal conductivities: Substance a. 6.0 mm above Steel Refractory Mineral wool b. 6.1 mm k (W/m-K) c. 6.2 mm 48 While the outside temperature is 30 °C, the temperature within the steel is 200 °C. Heat effects from convection and radiation are disregarded. Find the needed thickness of the mineral wool if the heat loss is 50 W. 0.346 0.047 d. 6.3 mm e. No to all of the

An insulation layer of 20 mm thick refractory material is placed on top of a layer of mineral wool to insulate a hollow, spherical steel ball with an outside diameter of 60 mm and an internal diameter of 50 mm wool. The table below lists the thermal conductivities: Substance a. 6.0 mm above Steel Refractory Mineral wool b. 6.1 mm k (W/m-K) c. 6.2 mm 48 While the outside temperature is 30 °C, the temperature within the steel is 200 °C. Heat effects from convection and radiation are disregarded. Find the needed thickness of the mineral wool if the heat loss is 50 W. 0.346 0.047 d. 6.3 mm e. No to all of the

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.37P: 1.37 Mild steel nails were driven through a solid wood wall consisting of two layers, each 2.5-cm...

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1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss through a wall of 10-cm thickness is . If a thermocouple at the inner surface of the wall indicates a temperature of 22°C while another at the outer surface shows 6°C, calculate the thermal conductivity of the concrete and compare your result with the value in Appendix 2, Table 11.
2.30 An electrical heater capable of generating 10,000 W is to be designed. The heating element is to be a stainless steel wire having an electrical resistivity of ohm-centimeter. The operating temperature of the stainless steel is to be no more than 1260°C. The heat transfer coefficient at the outer surface is expected to be no less than in a medium whose maximum temperature is 93°C. A transformer capable of delivering current at 9 and 12 V is available. Determine a suitable size for the wire, the current required, and discuss what effect a reduction in the heat transfer coefficient would have. (Hint: Demonstrate first that the temperature drop between the center and the surface of the wire is independent of the wire diameter, and determine its value.)
1.37 Mild steel nails were driven through a solid wood wall consisting of two layers, each 2.5-cm thick, for reinforcement. If the total cross-sectional area of the nails is 0.5% of the wall area, determine the unit thermal conductance of the composite wall and the percent of the total heat flow that passes through the nails when the temperature difference across the wall is 25°C. Neglect contact resistance between the wood layers.
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.
As a designer working for a major electric appliance manufacturer, you are required to estimate the amount of fiberglass insulation packing (k = 0.035 W/m K) that is needed for a kitchen oven shown in the figure below. The fiberglass layer is to be sandwiched between a 2-mm-thick aluminum cladding plate on the outside and a 5-mm-thick stainless steel plate on the inside that forms the core of the oven. The insulation thickness is such that the outside cladding temperature does not exceed 40C when the temperature at the inside surface of the oven is 300C. Also, the air temperature in the kitchen varies from 15Cto33C, and the average heat transfer coefficient between the outer surface of the oven and air is estimated to be 12.0W/m2K. Determine the thickness of the fiberglass insulation that is required for these conditions. What would be the outer surface temperature when the inside surface of the oven is at 475C?
Using Table 1.4 as a guide, prepare a similar table showing the orders of magnitude of the thermal resistances of a unit area for convection between a surface and various fluids.
1.2 The weight of the insulation in a spacecraft may be more important than the space required. Show analytically that the lightest insulation for a plane wall with a specified thermal resistance is the insulation that has the smallest product of density times thermal conductivity.
One end of a 0.3-m-long steel rod is connected to a wall at 204C. The other end is connected to a wall that is maintained at 93C. Air is blown across the rod so that a heat transfer coefficient of 17W/m2 K is maintained over the entire surface. If the diameter of the rod is 5 cm and the temperature of the air is 38C, what is the net rate of heat loss to the air?
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)
GChoose the right answer 1. The temperature profile for steady hest conduction through a plane wall of constant thermal conductivity is a. logarithmic b. parabolic 2. A brick wall (k-0.75 W/mK ) transmits $02 of heat loss through another wall (k-0.25 W/mK, x-100 mm). If the temperature difference across both walls is the same, the thickness of the brick wall would be a.150 mm c. linear 6T b. 240 mm c. 300 mm d. 375 mm 3. The maximum temperature at the center of the solid sphere of radius R having heat generation q (W/m') is given by g'R +9R b. T, + 4k T+9R 6k d. T+9R 8k a. 2k C. 4. The internal thermal resistance of a solid can be ignored if the Biot number is less than a. 1 b. 0.5 c. 0.1 d. Fourier number 5. The ratio of heat transfer of a fin to the heat transfer without fin is referred to as a. fin resistance b. fin efficieney efin effectiveness d. fin conductance.
A ball made of steel alloy with a diameter of 2 millimeters is first heated to 800 degrees and then suddenly immersed in a water bath to cool down to 100 degrees. The materials from which the ball is made have a thermal conductivity coefficient of 54.94 w / mk, a heat capacity of 7860 kg / m3, a heat capacity of 450 j / kg. Also, the heat transfer coefficient is 41.66 2 / m2. a-)Find the temperature of the ball immersed in water after 10 seconds. b-)Find the time required to cool the ball to 400 degrees.
Q1. Double glazed window is made of 2 glass panes of 6 mm thick each with an air gap of 6 mm between them. Assuming that the air layer is stagnant and only conduction is involved, determine the thermal resistance and overall heat transfer coefficient. The inside is exposed to convection with h - 15 W/m?. K and the outside to 9 W/m'. K. Compare the values with that of a single glass of 12 mm thickness. The conductivity of the glass - 1.4 W/m. K and that for air is 0.025 W/m. K. h, = 15 Wim'k Glass Air gap Giass 14 Wmk =0.025 WimK h Wim'k 6 mm Q2. A 2m long, 0.3cm diameter electrical wire extends across a room at 15°C. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 152°C in steady operation. Also, the voltage drop and electric current through the wire are measured to be 60 V and 1.5 A, respectively. Disregarding any heat transfer by radiation, determine the convection heat transfer coefficient for heat transfer…