11.8 A chromel-alumel thermocouple junction, which can be approximated with a sphere, has an effective diameter of 1 mm. It is used to measure the temperature of a gas flow with an effective heat-transfer coefficient of 500 W/m²-°C. (a) Determine the time constant of this thermocouple. (b) If the gas temperature suddenly increases by 100°C, how long will it take the thermo- couple to attain a temperature rise within 1% of the gas temperature rise? (c) Find the answer to the previous questions if the diameter of the bead is doubled. (Assume that the heat-transfer coefficient remains the same.)

11.8 A chromel-alumel thermocouple junction, which can be approximated with a sphere, has an effective diameter of 1 mm. It is used to measure the temperature of a gas flow with an effective heat-transfer coefficient of 500 W/m²-°C. (a) Determine the time constant of this thermocouple. (b) If the gas temperature suddenly increases by 100°C, how long will it take the thermo- couple to attain a temperature rise within 1% of the gas temperature rise? (c) Find the answer to the previous questions if the diameter of the bead is doubled. (Assume that the heat-transfer coefficient remains the same.)

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 3.9P: 3.9 The heat transfer coefficients for the flow of 26.6°C air over a sphere of 1.25 cm in diameter...

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3.9 The heat transfer coefficients for the flow of 26.6°C air over a sphere of 1.25 cm in diameter are measured by observing the temperature-time history of a copper ball the same dimension. The temperature of the copper ball was measured by two thermocouples, one located in the center and the other near the surface. The two thermocouples registered, within the accuracy of the recording instruments, the same temperature at any given instant. In one test run, the initial temperature of the ball was 66°C, and the temperature decreased by 7°C in 1.15 min. Calculate the heat transfer coefficient for this case.
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.
What duty is required to cool 1 t of Melons in 12 h from 20°C to 4°C? (Use Table 21.1 data
5
Given a plaster of paris (PoP) slab with L = 0.052 cm, W = 0.052 cm, and H = 0.0095 cm, what is the heat transfer coefficient if the biot number is 0.19682? Attached is the Heisler Chart below. Other data (if necessary):Initial Temperature: 328.2 CSurrounding Temperature: 27.9 CPoP Density: 680 kg/m^3Heat Capacity: 1090 J/kgKThermal Conductivity: 0.1185 W/mK
a. An air stream passing through a 2-inch (1/6 ft) diameter, thin-walled tube is to be heated by high- pressure steam condensing on the outer surface of the tube at 320 °F. The overall heat transfer coefficient, h between steam and air can be assumed to be 25 Btu/(ft2.hr °F) with the air entering at 100 ft/sec, 10 psia, 40 °F. The air is to be heated to 150 °F. Determine the tube length required. Assuming Rayleigh Line flow, calculate the static pressure change due to heat addition. Also, for the same inlet conditions, calculate the pressure drop due to friction, assuming Fanno flow in the duct with f = 0.018. b. c. d. To obtain an approximation to the overall pressure drop in this heat exchanger, add the two results. Discuss the accuracy of this calculation.
19.6 In a thermal heat sink the heat flux variation along the axis of a cooling passage is approximated as 9 A TX = a + b sin (77) where x is measured along the passage axis and Lis its total length. A large installation involves a stack of plates with a 3-mm air space between them. The flow passages are 1.22 m long, and the heat flux in the plates varies according to the above equation where a = 900 W/m² and v = 2500 W/m². Air enters at 100°C
What is the rate of rice husk consumption of a 2-ton grain dryer requiring to dry paddy from 22 to 14% in 8 hrs. The dryer overall thermal efficiency is 45%. Assume a heat of vaporization for paddy of 600 kCal/kg of moisture evaporated and heating value for rice husk of 3,000 kcal/kg.
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6.81 An industrial process involves evaporation of a thin water film from a contoured surface by heating it from below and forcing air across it. Laboratory measure- ments for this surface have provided the following heat transfer correlation: 0.58 NuL = 0.43 Re8 P04 The air flowing over the surface has a temperature of 290 K, a velocity of 10 m/s, and is completely dry ( = 0). The surface has a length of 1 m and a surface area of 1 m². Just enough energy is supplied to maintain its steady-state temperature at 310 K. (a) Determine the heat transfer coefficient and the rate at which the surface loses heat by convection. (b) Determine the mass transfer coefficient and the evaporation rate (kg/h) of the water on the surface. (c) Determine the rate at which heat must be supplied to the surface for these conditions.
The TPD method measures temperature elevations in a tissue region during a heating pulse and its later temperature decay after the pulse. It is then using the Pennes bioheat equation to perform a curve fitting to determine the local blood perfusion rate. If the TPD probe is placed in the vicinity of very large blood vessel, will the TPD technique provide an accurate measurement of the local blood perfusion in the vicinity of this large blood vessel? Explain briefly. (Hint: Is the Pennes bioheat equation accurate surrounding a large blood vessel?)
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