O H.W.: The heat transfer coefficient for air flowing over a sphere is to be determined by observing the temperature-time history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm in diameter, is at 66°C before it is inserted into an airstream having a temperature of 27°C. A thermocouple on the outer surface of the sphere indicates 55°C 69 s after the sphere is inserted in the airstream. Assume, and then justify, that the sphere behaves as a spacewise isothermal object and calculate the heat transfer coefficient. 3 p= 8933 kg/m, Cp 389 J/kg-K, k 398 %3D Too =27°C T(0) = 66°C T(69s) =55°C -D=12.7mm 21 Warith Alanbiyaa (Dr. ALI M) Heat Transfer

O H.W.: The heat transfer coefficient for air flowing over a sphere is to be determined by observing the temperature-time history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm in diameter, is at 66°C before it is inserted into an airstream having a temperature of 27°C. A thermocouple on the outer surface of the sphere indicates 55°C 69 s after the sphere is inserted in the airstream. Assume, and then justify, that the sphere behaves as a spacewise isothermal object and calculate the heat transfer coefficient. 3 p= 8933 kg/m, Cp 389 J/kg-K, k 398 %3D Too =27°C T(0) = 66°C T(69s) =55°C -D=12.7mm 21 Warith Alanbiyaa (Dr. ALI M) Heat Transfer

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.

Chapter3: Transient Heat Conduction

Section: Chapter Questions

Problem 3.10P: 3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the...

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O H.W.: The heat transfer coefficient for air flowing over a sphere is to be determined by observing the temperature-time history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm in diameter, is at 66°C before it is inserted into an airstream having a temperature of 27°C. A thermocouple on the outer surface of the sphere indicates 55°C 69 s after the sphere is inserted in the airstream. Assume, and then justify, that the sphere behaves as a spacewise isothermal object and calculate the heat transfer coefficient. p= 8933 kg/m", Cp 3 = 389 J/kg-K, k= 398 %3D -T(0)= 66°C T(69s) =55°C Teo =27°C -D=12.7mm Worith Alenbiveo (Dr ALLMA Hoot Tronofor
Q2/ An aluminum sphere weighting 7kg and initially at a temperature of 533K is suddenly immersed in a fluid at 283K. if heat transfer coefficient between the sphere and fluid is 50W/m?.°C. Take density=2707kg/m3, specific heat=0.9KJ/kg.ºC and thermal conductivity= 204W/m°C. Determine th Bi number and And determine the time required to cool sphere to 263k
A thermocouple is initially at temperature of T; and it is used to measure the temperature of air (Tair). It would take a while for the temperature of the thermocouple junction to approach Tair. The thermocouple junction can be approximated by a copper sphere of 0.5 mm in diameter. Using the lumped system analysis (chapter 5, undergraduate heat transfer textbook), derive an expression for T(t) (it should look like: (T(t)-Tair )/(Ti-Tair) = e, where T; is the initial temperature of the thermocouple, and Tair is the air temperature). Calculate the time constant 7 (assuming the convection heat transfer coefficient h=5 W/m²K). Use the thermal properties of copper from your undergraduate textbook or any other books. Usually t=3t is used as the response time of the thermocouple with 5% measuring error. Explain why? If the thermocouple size is increased to 5 mm in diameter, what is the response time?
A composite plane wall consisting of materials, 1.5-in steel (k = 312 BTU-in/HR.ft2.0F) and 2-in aluminum (k = 1400 BTU-in/HR.ft2.0F), separates a hot gas at Ti = 2000F, hi = 2 BTU/HR.ft2.0F, from cold gas at To = 80 deg F, ho = 5. If the hot fluid is on the aluminum side, find: a) Transmittance, U; b) The heat through 100 sq. ft of the surface under steady state condition and c) The interface temperature at the junction of the metals.
Heat transand P 5.8 The heat transfer coefficient for air flowing over a sphere is to be determined by observing the temperature-time history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm in diameter, is at 66°C before it is inserted into an airstream having a temperature of 27°C. A thermocouple on the outer surface of the sphere indicates 55°C 69 s after the sphere is inserted into the airstream. Assume and then justify that the sphere behaves as a spacewise isothermal object and calculate the heat transfer coefficient. T 2706
Brass cube "p = 8530 kg/m , c_= 380 J/kg.°C, k= 110 W/m.°C, a = 33.9E-6 W/m.°C", side length =9 mm, are annealed by heating them first to 929°C in a furnace and then allowing them to cool slowly to 130°C in ambient air at 28°C. If the average heat transfer coefficient is 17.3 W/m .°C, how long will the annealing process will (seconds)?
Small potato balls of 0.5 cm radius (can be approximated as a sphere) baked in the oven and initially at a uniform temperature of 160°C. Assume, potatoes have a density of 650 kg/m³, specific heat of 3400 J/kgK and conductivity of 1 W/mK. You want to eat potatoes quickly without burning your tongue so that you have decided to cool them down with forced convection. Air h, T. hot potato initially Air temperature is 20'C and the convective heat transfer coefficient is 140 W/m²K. Calculate the time (in seconds) required for its surface temperature to drop 90°C. Round the value you calculated and write it as an integer!! For example, if you find 8.2, write is as 8. Answer:
Small potato balls of 0.5 cm radius (can be approximated as a sphere) baked in the oven and initially at a uniform temperature of 160°C. Assume, potatoes have a density of 650 kg/m, specific heat of 3400 J/kgK and conductivity of 1 W/mK. You want to eat potatoes quickly without burning your tongue so that you have decided to cool them down with forced convection. Air h, T. hot potato initially Air temperature is 20°C and the convective heat transfer coefficient is 140 W/m?K. Calculate the time (in seconds) required for its surface temperature to drop 90°C.
1. A composite furnace wall is made up of a 12-in. lining of magnesite refractory brick, a 5-in.thickness of 85% magnesia, and a steel casing 0.10-in. thick. Flue gas temperature is 2200 F andthe boiler room is at 80 F. Gas side film coefficient is 15 Btu/hr-sq.ft-F and air side is 4.0.Determine:a. The thermal current Q/Ab. Interface temperaturesc. Effect on thermal current and inside refractory wall temperature if the magnesia insulation weredoubled.
VI.2 A coolant is transported in a pipe with external wall temperature of -30 °C and with outer diameter of 10 cm. The tube is thermally isolated by two layers: 1) an internal layer of foamed polypropylene with thermal conductivity of 0.08 W.m.K and thickness of 10 cm, and 2) an external felt layer with thermal conductivity of 0.05 W.m.K and thickness of 5 cm. Temperature of the outer surface is 25 °C. Calculate the heat flow from the surroundings to the tube with length of 100 m. What is the temperature on the boundary between polypropylene and felt layers? Result: The heat flow from the surroundings is approx. 1.77 kW. Temperature between layers of isolation is 8.8 °C.
Carbon steel balls (p=7833 kg/m', k=54 W/m.C, Cp=0.465 kJ/kg.K ) 10 mm in diameter are annealed by heating then first 910 °C in a furnace and then allowing them to cool slowly to 120 °C in ambient air at 36 °C. If the average heat transfer coefficient is 80 W/m?.°C.
A sphere which is 12.7 mm in diameter, is at 66 °C before it is inserted into an airstream having a temperature of 27 °C. The surface temperature of the sphere is 55 °C after 69 seconds measured by a thermocouple. This sphere has density 8933 kg/m³, cp = 389 J/kg∙K, k=398 W/m K Assume this sphere can be treated as a lumped system. Find the convective heat transfer coefficient and then calculate Bi number to justify the assumption that this sphere can be treated as a lumped system.