please very urgent i need the right answer please be carfully A 25 cm thick brick wall of 4 m by 3 m andthermal conductivity 0.75 W/m°C has inner andouter surfaces at 35°C and 7°C, respectively.The rate of heat transfer through the wall is:a.1008 W○ b. 707 WC.984 Wd. 1028 WClear my choice A 50 mm*45 mm*20 mm cell phone charger(the shape of the charger is a rectangularparallelepiped) has a constant surfacetemperature when plugged into an electricalwall outlet. The surface of the charger is subjectto a free convection heat transfer coefficient ofh = 4.5 W/m^2.K with the room air having atemperature of 15°C. By assuming steadyconditions and neglecting radiation, if theelectric power of the charger is 0.816 W, thesurface temperature of the charger is:a. 36.85°CO b. 32.14°CО с. 29.16°C○ d. 39.15°CClear my choice

Answered: A 25 cm thick brick wall of 4 m by 3 m…

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.

Chapter2: Steady Heat Conduction

Section: Chapter Questions

Problem 2.54P

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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.
3.17 A 1.4-kg aluminum household iron has a 500-W heating element. The surface area is . The ambient temperature is 21°C, and the surface heat transfer coefficient is . How long after the iron is plugged in does its temperature reach 104°C?
Show that the rate of heat conduction per unit length through a long, hollow cylinder of inner radius ri and outer radius ro, made of a material whose thermal conductivity varies linearly with temperature, is given by qkL=TiTo(rori)/kmA where Ti = temperature at the inner surface To = temperature at the outer surface A=2(rori)/ln(ro/ri)km=ko[1+k(Ti+To)/2]L=lenthofcyclinder
A plane wall, 7.5 cm thick, generates heat internally at the rate of 105 W/m3. One side of the wall is insulated, and the other side is exposed to an environment at 90C. The convection heat transfer coefficient between the wall and the environment is 500 W/m2 K. If the thermal conductivity of the wall is 12 W/m K, calculate the maximum temperature in the wall.
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.
A high-speed computer is located in a temperature-controlled room at 26C. When the machine is operating, its internal heat generation rate is estimated to be 800 W. The external surface temperature of the computer is to be maintained below 85C. The heat transfer coefficient for the surface of the computer is estimated to be 10W/m2K. What surface area would be necessary to assure safe operation of this machine? Comment on ways to reduce this area.
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.
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.
The heat transfer across a 5" wall of firebrick is 500 W/m2. If the surface temperature thermalconductivity of brick is 0.7 Btu/hr-ft-°F, find he on cold side is 30°C and temperature on hot side. O 65.37 deg C 87.25 deg C 73.45 deg C 82.41 deg C
a. The wall of a building has a surface area of 50 m2. The outside layer of the wall is 20 cm thick concrete with thermal conductivity kcon = .8 W/m-K. The inner layer is 10 cm thick balsa wood (kbalsa = .048 W/m-K) as an insulator. Outside temperatures of 47o C are expected, while an inside temperature of 21o C is maintained by the cooling system. Find the rate of heat transfer through the wall.
pls answer:The temperature at the inner and outer surfaces of a boiler wall made of 20 mm thick steel and covered with an insulating material of5 mm thickness are 300 C and 50 C respectively. If the thermal conductivities of steel and insulating material are 58W/m.C and 0.116W/m.C respectively, determine the heat flux through the boiler wall in W/m2.

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