Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
8th Edition
ISBN: 9781305387102
Author: Kreith, Frank; Manglik, Raj M.
Publisher: Cengage Learning
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Chapter 3, Problem 3.9P
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
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19 mm diameter steel balls are quenched by heating to 989 K followed by slow cooling to 400 K in an environment with air at T∞ = 325 K and h = 39 W/m2.K. Assuming that the steel properties are k = 40 W/m.K, ρ = 7800 kg/m3 and C = 600 J/kg.K, estimate the time (in "minutes") required for the cooling process.
Bolas de aço com 19 mm de diâmetro são temperadas pelo aquecimento a 989 K seguido pelo resfriamento lento até 400 K em um ambiente com ar a T∞ = 325 K e h = 39 W/m2.K. Admitindo que as propriedades do aço sejam k = 40 W/m.K, ρ = 7800 kg/m3 e C = 600 J/kg.K, estime o tempo (em "minutos") necessário para o processo de resfriamento.
A food product with 80% moisture content in a 5 cm diameter can wants to be frozen. The density of the product is 1000 kg / m cubic, the thermal conductivity is 1.0 W / (m K), and the initial freezing temperature is -1.75 ° C. After 10 hours in -25 ° C freezing medium, the product temperature becomes -10 ° C. Estimate the convection heat transfer coefficient of the freezing medium. Assume the can as an infinite cylinder
John is employed by a new food processing plant where eggs are
to be boiled in large quantities to produce egg salad. John has
been assigned the task of determining the minimum cooking
time for the eggs, utilizing his vast knowledge of heat transfer.
As a basis of calculation, he assumed that the eggs are perfect
spheres with a diameter of 5 cm and uniform material properties
(r =1000kg/m'; Cp = 3300 J/(kg K) and k=0.75 W/(m-K)). The
eggs were taken from a cold storage room at 5°C and
immediately put into pots of boiling water at 100°C. The
convective heat transfer coefficient of the water is h = 1500
W/m°K. For health safety reasons, the plant requires that the
entire egg must be at a temperature above 75°C for at least 20
seconds in order for the cooking process to be complete.
a. What is the minimum time necessary to cook the egg to
company specifications?
b. In the testing lab, John forgot to remove a batch of eggs, and
they remained in the cooking pot long enough to reach a…
Chapter 3 Solutions
Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
Ch. 3 - Consider a flat plate or a plane wall with a...Ch. 3 - 3.2 High-strength steel is required for use in...Ch. 3 - Prob. 3.3PCh. 3 - 3.5 In a ball-bearing production facility, steel...Ch. 3 - A 0.6-cm diameter mild steel rod at 38C is...Ch. 3 - Prob. 3.7PCh. 3 - Prob. 3.8PCh. 3 - 3.9 The heat transfer coefficients for the flow of...Ch. 3 - 3.10 A spherical shell satellite (3-m-OD,...Ch. 3 - Prob. 3.11P
Ch. 3 - Prob. 3.12PCh. 3 - Prob. 3.13PCh. 3 - 3.14 A thin-wall cylindrical vessel (1 m in...Ch. 3 - A thin-wall jacketed tank heated by condensing...Ch. 3 - 3.16 A large, 2.54-cm.-thick copper plate is...Ch. 3 - 3.17 A 1.4-kg aluminum household iron has a 500-W...Ch. 3 -
3.28 A long wooden rod at with a 2.5-cm-OD is...Ch. 3 - A mild-steel cylindrical billet 25 cm in diameter...Ch. 3 - Prob. 3.37PCh. 3 -
3.38 An egg, which for the purposes of this...
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- An electrical transmission line of 1.2-cm diameter carries a current of 200 amps and has a resistance of 310-4 ohm per meter of length. If the air around this line is at v, determine the surface temperature on a windy day, assuming a wind blows across the line at 33 km/h.arrow_forwardHeat Transferarrow_forwardIn presence of an external flow when will the convective heat transfer be dominated by free convection and not by the forced convection due to the external flow? Gr ≫ Re². Gr ≪ Re. Gr ≫ Re. Gr ≪ Re For ideal gases, the specific heat capacity at constant pressure, thermal conductivity, dynamic viscosity, and Prandtl number are independent of pressure. How will the ratio of thicknesses of velocity boundary layer and thermal boundary layer change if pressure is increased from 1 atm to 5 atm for the flow of an ideal gas over a flat plate? It will decrease by a factor of 5. It will remain the same. It will increase by a factor of 5. We need to know the actual fluid properties to tell.arrow_forward
- Heat Transfer Lesson The water will be heated from 10 (Degrees centigrade oC) to 90 (Degrees centigrade oC) while flowing through a smooth pipe with 1.5 cm inner diameter and 7 meters length.The entire surface of the pipe is equipped with an electric heater, which ensures uniform heating throughout.The outer surface of the heater is well insulated and therefore all heat generated in the heater in continuous operation is transferred to the water in the pipe.If the system provides 6 Liter / minute flow rate of hot water. (Thermophysical properties of water at 50 oC:ρ = 988 m3/kg, k= 0.6305 W/m oC, cp=4181 J/kg oC, Pr=3.628, μ= 0.0005471 kg/m.s ) a)Find the power of the resistance heater [W]. b)Calculate the inner surface temperature [oC] of the pipe at the outlet. c)Find the pressure drop [Pa]. d)Find the pump power [W] required to overcome this pressure drop.arrow_forwardHEAT TRANSFER A 0.5 cm diameter copper wire is exposed to environment air in cross flow over the wire with uniform velocity of 0.5 m/s. The copper wire is insulated by a rubberized sheath with thermal conductivity of 0.13 W/m∙K. The outer surface of the wire has a temperature of 60˚C due to the electric current flowing through it. The thermo-physical properties of the environment are υ =15.89 × 10-6 m2 /s, Pr = 0.707, k = 26.3 x 10-3 W/m.K. The outer surface of the insulation must be kept at temperature of 45˚C or below. Design a suitable thickness of the insulation in order toallow the use of such insulated wire in a room temperature of 27˚C.arrow_forwardThe temperature of a gas stream is measured by a thermocouple whose junction can be approximated as a 1-mm-diameter sphere. Take the junction’s properties as: k of 32 W/m K, density of 8.2 kg/m^3, c of 300 J/Kg K. On its surface, the overall heat transfer coefficient is 200 W/m^2 K. Neglect any conduction loss from the sphere to other parts of the thermocouple. Create a plot of measurement error as a function of time for the thermocouple, expressed as a fraction of the initial temperature difference.arrow_forward
- African elephants have a large body with a surface area of the trunk as A= 11 m². Typically, elephants roll into wet mud in summer time. The evaporation of moisture from skin surface enhances the convection heat transfer rate to h= 30 W/m²K (without evaporation the h value is very small ~ h=8 W/m²K). The normal metabolism of elephants (the total heat generation rate in the elephant body) is Qgen = 2500 W. Based on what you learned in ENME321, the surface temperature of the elephant (or any object with heat generate rate) is calculated using the following equation: Ts = Too+ Qgen /(h A). The air temperature Too is 32°C. Elephants are not considered as being overheated if the surface temperatures are lower than 37°C. a. The elephant has not rolled into wet mud. Use h=8 W/m²K, estimate the surface temperature of the elephant. Do you think that the elephant will be overheated? b. The elephant has rolled into wet mud. Use h=30 W/m²K, estimate the surface temperature of the elephant. Do you…arrow_forwardWhat’s the correct answer for this please ?arrow_forwardThis food product is produced in the form of small spheres or pellets by freezing it in the water freezer. Blast freezer air freezers operate at a temperature of -35 degrees Celsius The initial product temperature is 25 degrees Celsius.The pellets have a diameter of 0.5 cm and a density of 980 kg / m³ temperature initial freezing is -2.5 degrees Celsius latent heat of product manufacture is 280 kj / kg the thermal conductivity of frozen products is 1.9 W / (m degrees Celsius). Convection heat transfer coefficient is 50 W / (m² degrees Celsius) calculated freezing time. tf =arrow_forward
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