a) What is forced convection? How does it differ from natural convection? Isconvection caused by winds forced or natural convection?b) What is external forced convection? How does it differ from internal forcedconvection? Can a heat transfer system involve both internal and external convectionat the same time? Give an example.c) In which mode of heat transfer is the convection heat transfer coefficient usuallyhigher, natural convection or forced convection? Why?d) Consider a hot baked potato. Will the potato cool faster or slower when we blow thewarin air coming from our lungs on it instead of letting it cool naturally in the coolerair in the room? Explain.e) What is the physical significance of the Nusselt number? How is it defined?f) When is heat transfer through a fluid conduction and when is it convection? For whatcase is the rate of heat transfer higher? How does the convection heat transfercoefficient differ from the thermal conductivity of a fluid?g) Define incompressible flow and incompressible fluid. Must the flow of acompressible fluid necessarily be treated as compressible?un

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a) What is forced convection? How does it differ from natural convection? Is convection caused by winds forced or natural convection? b) What is external forced conv How does it differ from internal forced stem involve both internal and external convection convection? Can a heat transi at the same time? Give an example. c) In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why? d) Consider a hot baked potato. Will the potato cool faster or slower when we blow the warm air coming from our lungs on it instead of letting it cool naturally in the cooler air in the room? Explain. e) What is the physical significance of the Nusselt number? How is it defined? f) When is heat transfer through a fluid conduction and when is it convection? For what case is the rate of heat transfer higher? How does the convection heat transfer coefficient differ from the thermal conductivity of a fluid? g) Define incompressible flow and incompressible fluid. Must the flow of a compressible fluid necessarily be treated as compressible? un r. Luma

a) What is forced convection? How does it differ from natural convection? Is convection caused by winds forced or natural convection? b) What is external forced conv How does it differ from internal forced stem involve both internal and external convection convection? Can a heat transi at the same time? Give an example. c) In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why? d) Consider a hot baked potato. Will the potato cool faster or slower when we blow the warm air coming from our lungs on it instead of letting it cool naturally in the cooler air in the room? Explain. e) What is the physical significance of the Nusselt number? How is it defined? f) When is heat transfer through a fluid conduction and when is it convection? For what case is the rate of heat transfer higher? How does the convection heat transfer coefficient differ from the thermal conductivity of a fluid? g) Define incompressible flow and incompressible fluid. Must the flow of a compressible fluid necessarily be treated as compressible? un r. Luma

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.

Chapter8: Natural Convection

Section: Chapter Questions

Problem 8.13P

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The heat transfer rate from hot air by convection at 100C flowing over one side of a flat plate with dimensions 0.1m0.5m is determined to be 125 W when the surface of the plate is kept at 30C. What is the average convection heat transfer coefficient between the plate and the air?
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 170°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, Te hot potato initially @ T; Air temperature is 20°C and the convective heat transfer coefficient is 80 W/m?K. Calculate the time (in seconds) required for its surface temperature to drop 130°C. Round the value you calculated and write it as an integer!!! For example, if you find 8.2, write is as 8. Answer:
When a flat plate is subjected to uniform heat flux, how different are the relations compared to the isothermal plate case? Questions above are related to Heat transfer
A flat aluminum plate 8 m long and 1.5 m wide has a surface temperature of 180°C. For its cooling, air at 1 atm and speed of 10 m/s circulates at 20°C parallel to the length of the plate.a) Determine the rate of heat transfer by convection in steady state. b) In order to increase the heat dissipation from the plate, the engineers considered placing rectangular fins of the same material (Kaluminium = 200 W/m.K). The fins are 50 mm long and 0.5 mm thick and the placement density is 75 fins per unit length of the plate (the fins are equally spaced and span the entire width of the plate). The convective coefficient of the air over the plate resulting from the placement of fins is 10 W/m².K. Determine the heat transfer rate and percentage increase over part (a).
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A food product with 80% moisture content in a 10 cm diameter can wants to be frozen. The density of the product is 1000 kg / m³, the thermal conductivity is 1.0 W / (m K), and the initial freezing temperature is -2 ° C. After 14 hours in the freezing medium -30 ° C, the product temperature becomes -10 ° C. Estimate the convection heat transfer coefficient of the freezing medium. Assume the can as an infinite cylinder.
Answer: Calculate the rate of heat transfer by natural convection between a shed roof of area 20m x 20m and ambient air, if the roof surfacetemperature is 27 C, the air temperature is -3 C, and the average convection heat transfer coefficient is 10 W/m K.
A closed room containing hot air is used to cool down slabs of asolid with a conductivity of 160 W/m*K, density of 2950 kg/m3), and a heat capacity of 920 J/kg*K). Each solid slab is 2 cm thick. Air circulates withing the room at 15 °C with a convection heat transfer coefficient of 200 W/m2*K. Do not assume steady heat transfer.(a) What is the final temperature of slabs upon leaving the room, if thetemperature of slabs before entering the room is 70 °C.(b) How much more time is needed for the temperature of the solid slabs to decrease by an additional 20 °C?
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A food product wants to be produced in a small round shape (pellet) by freezing it in a water blast freezer freezer. Air freezer operates at -30 ° C. The initial product temperature is 25 ° C. The pellet has a diameter of 1.2 cm, and a density of 980 kg / m³. The initial freezing temperature is -2.5 ° C. The latent heat of freezing of the product is 280 kJ / kg. The thermal conductivity of the frozen product is 1.9 W / (m ° C). The convective heat transfer coefficient is 40 W / (m² K). Calculate the freeze time. t f = Answer hour.
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3- You have experienced convection cooling if you have ever extended your handout of the window of a moving vehicle or into a flowing water stream. With the surface of your hand at a temperature of 30 oC, determine the convection heat flux for a) a vehicle speed of 35 km/h in air at -5 oC with convection coefficient of 40 W/m2 .K and b) a velocity of 0.2 m/s in a water stream at 10 oC with convection coefficient of 900 W/m2.K. Which condition would feel colder? Contrast theseresults with a heat loss of approximately 30 W/m2 under normal room conditions.