Q2/ The wall of a refrigerator is constructed of fiberglass insulation (k=0.035 W/ m. C) sandwiched between two layers of l-mm-thick sheet metal (k-15.1 W/m. C). The refrigerated space is maintained at 3C, and the average heat transfer coefficients at the inner and outer surfaces of the wall are 4 W/m2.C and 9 W/m2. "C, respectively. The kitchen temperature averages 25°C. It is observed that condensation occurs on the outer surfaces of the refrigerator when the temperature of the outer surface drops to 20°C. Determine the minimum thickness of fiberglass insulation that needs to be used in the wall in order to avoid condensation on the outer surfaces. Sheet metal Kitchen Refrigerated air space 3°C 25°C Insulation 10°C 1 mm mm

Q2/ The wall of a refrigerator is constructed of fiberglass insulation (k=0.035 W/ m. C) sandwiched between two layers of l-mm-thick sheet metal (k-15.1 W/m. C). The refrigerated space is maintained at 3C, and the average heat transfer coefficients at the inner and outer surfaces of the wall are 4 W/m2.C and 9 W/m2. "C, respectively. The kitchen temperature averages 25°C. It is observed that condensation occurs on the outer surfaces of the refrigerator when the temperature of the outer surface drops to 20°C. Determine the minimum thickness of fiberglass insulation that needs to be used in the wall in order to avoid condensation on the outer surfaces. Sheet metal Kitchen Refrigerated air space 3°C 25°C Insulation 10°C 1 mm mm

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.

Chapter7: Forced Convection Inside Tubes And Ducts

Section: Chapter Questions

Problem 7.29P

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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.
An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70C. It is to be cooled in 25C air by attaching aluminum fins with a total surface area of 12cm2. The convection heat transfer coefficient between the fins and the air is 20W/m2K. Estimate the operating temperature when the fins are attached in such a way that (a) there is a contact resistance of approximately 50 K/W between the surface of the device and the fin array and (b) there is no contact resistance (in this case, the construction of the device is more expensive). Comment on the design options.
2.2 A small dam, which is idealized by a large slab 1.2 m thick, is to be completely poured in a short Period of time. The hydration of the concrete results in the equivalent of a distributed source of constant strength of 100 W/m3. If both dam surfaces are at 16°C, determine the maximum temperature to which the concrete will be subjected, assuming steady-state conditions. The thermal conductivity of the wet concrete can be taken as 0.84 W/m K.
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.)
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
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Q) The temperature of a heated plate is 88°C which is cooled by using pin-fins having a length of 5-cm, diameter of 1-cm, and thermal conductivity of (k=201 W/m-°C). The number of pins is (N=1800) and the temperature of the atmosphere is 10°C, while the convection heat transfer coefficient on the surfaces is 37 W/m2.°C. Assuming steady one-dimensional heat transfer along the fin and taking the nodal spacing to be 2.5-cm, determine (a) the temperature at the nodes along the fins, (b) the heat transfer rate from a single fin, (c) the heat transfer rate from all fins, and (d) the overall heat transfer rate from the heated plate which has dimensions of 0.6-m x 0.5-m with the fins.
Q1: Consider a large plane wall of thickness L = 0.4 m, thermal conductivity k=2.3 W/m °C, and surface area A= 20 m2. The left side of the wall at x= 0 is subjected of T1 = 80°C. while the right side losses heated by convection to the surrounding air at T-15 °C with a heat transfer coefficient of h=24 W/m2 C. Assuming constant thermal conductivity and no heat generation in the wall, (a) express the differential equation and the boundary conditions for steady one-dimensional heat conduction through the wall, (b) obtain a relation for the variation of temperature in the wall by solving the differential equation, and (c) evaluate the rate of heat transfer through the wall Ans : (c) 6030 W
Q2. Steam pumped through a long- insulated pipe at a temperature of T= 500 K and provides a convection coefficient of h, = 100 W/m?K at the inner surface of the pipe. The inner and outer radius of the pipe and insulation material are r1 = 10, r2 = 12 and r3 = 17 cm, respectively. The thermal conductivity of the pipe is 100 W/mK. The insulation material is glass fiber and its outer surface is exposed to ambient air at 300 K. If the ambient air provides a convection coefficient of ho = 20 Internal flow Ambient air W/m?K, determine the followings: a. What are the thermal resistance coefficients for convections and conductions b. What is the heat transfer rate per unit length of the pipe c. If the pipe is 30 m long, what will be total heat transfer rate from the pipe. t00 noints)
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Outer surface of a tube-steel oven is at 110 oC. What would be the heat transfer rate tothe surrounding air at 25 oC. if the natural convective heat transfer coefficient ish = C (Ts – Ta)1/4where; C= 4.5 W/(m2.K5/4), Ta and Ts are air temperature and surface temperature,respectively. Surface area of the wall is 125 m2.(heat transfer by radiation can be neglected)