20.A cylindrical resistance heater is placedhorizontally in air. The outer surface temperatureof the resistance wire is to be determined.The environment temperature Tair = 80.0°Cheater is 350.owThe length and the diameter of the resistanceheater is 0.75 m and 0.5 cm.AirResistanceheater, T,[D=0.5 cmL = 0.75 mThe properties of air at 1atm0158502k = 0.05572W - m%3D-1K1v = 7.804 x 10°m - s-1Pr = 0.6986B= 0.001294K安が-1安苏白91915850E10691915850E106Hint: Start your guess from 1 2U C106Pr = 0.698606L= 0.75 mThe properties of air at 1atm and 500°C are:k = 0.05572W - m-1. K-1v = 7.804 x 10°m -s-1|安苏白3 =0.001294K 1%3D91915850E106Hint:苏白o苏白Nu =| 0.6 +0.387RA%3D1+0599/PrIterate at least for 2 times.安苏白91915850E106

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20.A cylindrical resistance heater is placed horizontally in air. The outer surface temperature of the resistance wire is to be determined. The environment temperature Tair = 80.0°C and the power heater is 350.0w The length and the diameter of the resistance heater is 0.75 m and 0.5 cm. 白 Air Resistance heater, T, (D=0.5 cm L= 0.75 m properties of air at 1atm The 500°C are: 015890 || 1 v = 7.804 x 10 °m -s-1 Pr = 0.6986 B = 0.001294K %3D -1 安苏白 50E106 50E106

20.A cylindrical resistance heater is placed horizontally in air. The outer surface temperature of the resistance wire is to be determined. The environment temperature Tair = 80.0°C and the power heater is 350.0w The length and the diameter of the resistance heater is 0.75 m and 0.5 cm. 白 Air Resistance heater, T, (D=0.5 cm L= 0.75 m properties of air at 1atm The 500°C are: 015890 || 1 v = 7.804 x 10 °m -s-1 Pr = 0.6986 B = 0.001294K %3D -1 安苏白 50E106 50E106

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 7.4P

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7.43 Liquid sodium is to be heated from 500 K to 600 K by passing it at a flow rate of 5.0 kg/s through a 5-cmID tube whose surface is maintained at 620 K. What length of tube is required?
Determine the rate of heat transfer per meter length to a light oil flowing through a 2.5-cm-ID, 60-cm-long copper tube at a velocity of 0.03 m/s. The oil enters the tube at 16C, and the tube is heated by steam condensing on its outer surface at atmospheric pressure with a heat transfer coefficient of 11.3 kW/m K. The properties of the oil at various temperatures are listed in the following table: Temperature, T(C) 15 30 40 65 100 (kg/m3) 912 912 896 880 864 c(kJ/kgK) 1.80 1.84 1.925 2.0 2.135 k(W/mK) 0.133 0.133 0.131 0.129 0.128 (kg/ms) 0.089 0.0414 0.023 0.00786 0.0033 Pr 1204 573 338 122 55
What regime favors heat transfer and why?Laminar, Turbulent or Transitional
The temperature of an air stream flowing is measured by a copper – constantanthermocouple which may be approximated as a cylinder of 1.7 mm diameter. Initiallythe junction and air at a temperature of 25 oC. The air temperature suddenly changesto and is maintained at 215 oC. The thermal junction properties are: ρ = 8750 kg/m3; Cp = 380 J/kgoC; k(thermocouple) = 28W/moC; h = 145 W/m2-oC. a. Determine the time required for the thermocouple to indicate temperature of 165oC. Also, determine the thermal time constant and the temperature indicated bythe thermocouple at that instant.b. Discuss the stability of this thermocouple to measure unsteady state temperatureof a fluid when the temperature variation in the fluid has a time period of 3.6 s.
During air cooling of oranges, grapefruit, andtangelos, the heat transfer coefficient for combined convection,radiation, and evaporation for air velocities of0.11 < V < 0.33 m/s is determined experimentally and isexpressed as h = 5.05 kair Re1/3/D, where the diameter Dis the characteristic length. Oranges are cooled by refrigeratedair at 5°C and 1 atm at a velocity of 0.3 m/s. Determine(a) the initial rate of heat transfer from a 7-cm-diameterorange initially at 15°C with a thermal conductivity of0.50 W/m·K, (b) the value of the initial temperature gradientinside the orange at the surface, and (c) the value of theNusselt number.
For air flow at a constant wall temperature of 100 ◦C and average bulk temperature of 40 ◦C through a 4-cm-ID pipe, determine the value of average convection coefficient for an inlet velocity of 0.8 m/s if the pipe length is (i) 1 m, (ii) 3 m, (iii) 10 m.
An average man has abody surface area of 1.8m2 and a skin temperature of 33degrees celcius .The convection heat transfer coefficient for a clothed person walking in still air is expressed as {h,8.6V^0.53}FOR 0.5<v<2m/s,where V is the walking velocityin m/s.Assuming the average surface temperature of the clothed person to be 30degrees celcius, determine the rate of heat loss from an average man walking in still air at 10degrees celcius by convectionat a walking velocity of (a)0.5m/s (b)1.0m/s (c)1.5m/s (d)2.0m/s
Consider the flow of oil (use unused engine oil properties) at 20? in a 30-cm-diameter pipeline at an average velocity of 2 m/s. A 200-m-long section of the pipeline passes through icy waters of a lake at 0?C. Measurements indicate that the surface temperature of the pipe is very nearly 0?C. Disregarding the thermal resistance of the pipe material, determine (a) the temperature of the oil when the pipe leaves the lake, (b) the rate of heat transfer from the oil, and (c) the pumping power required to overcome the pressure losses and to maintain the flow of the oil in the pipe.
Air at 10oC enters a 10-cm-diameter and 4-m-long pipe at a rate of 0.035 kg/s. The inner surface of the pipe has a roughness of 0.22 mm, and the pipe is nearly isothermal at 60o Determine the rate of heat transfer to air using the Nusselt number relation given by (a) Eq. 8–66 and (b) Eq. 8–71. Evaluate air properties at a bulk mean temperature of 20oC. Is this a good assumption?
Dry and saturated steam at 6 MPa abs. enters a 40 m length of 11.5 cm O.D steel pipe at a flow rate of 0.12 kg/s. The pipe is covered with 5 cm thick asbestos insulation (k= 0.022 W/m-K). The pipe is located in a tunnel with stagnant air temperature of 27°C. The unit outside convective coefficient is 10 W/m^2-K. Neglecting steam film and pipe wall resistance, determine the mass of steam.
Example 9: A 2-kW resistance heater wire whose thermal conductivity is k = 15 W/m ·°C has a diameter of D = 4 mm and a length of L = 0.5 m, and is used to boil water. If the outer surface temperature of the resistance wire is Ts = 105°C, determine the temperature at the center of the wire.
Saturated dry steam at 1500 kPa passes through a 100 mm steel pipe (t = 5 mm), with a total length of 100 m. the steam line is insulated with a 50 mm thickness of 85% magnesia which has a thermal conductivity of 0.25 W/m degrees C. For a mass flow rate of 600 kg steam per hour and an ambient air temperature of -10 degrees C, determine the quality of steam after passing through this pipe. Surface film conductance of condensing steam, h = 5500 W/m2 - degrees C Surface film conductance of air, h = 12 W/m2 - degrees C Thermal conductivity of pipe material, k = 153 W/m degrees C Properties of steam, 1500 kPa(tsat= 198.32 degrees C) hg = 2792.2 kJ/kg hf = 844.89 kJ/kg hfg = 1947.3 kJ/kg