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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An average person generates heat at a rate of 84 W while resting. Assuming one-quarter of this heat is lost from the head and disregarding radiation, determine the average surface temperature of the head when it is not covered and is subjected to winds at 10°C and 25 km/h. The head can be approximated as a 30-cm-diameter sphere. Assume a surface temperature of 15°C for evaluation of ms. Is this a good assumption?
Air at 300 degrees Celsius flows over a plate of dimension 0.5 m by 0.25 m. If the convection heat transfer coefficient is 250 Watts per Sq.m Kelvin, determine the heat transfer rate from the air to one side of the plate if the plate is to be maintained at 40 degrees Celsius.
Consider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his entire body to air flow. The air temperature is 85°F and the fan is blowing air at a velocity of 6 ft/s. If the person is doing light work and generating sensible heat at a rate of 300 Btu/h, determine the average temperature of the outer surface (skin or clothing) of the person. The average human body can be treated as a 1-ft-diameter cylinder with an exposed surface area of 18 ft2. Disregard any heat transfer by radiation. What would your answer be if the air velocity were doubled? Evaluate the air properties at 100°F.
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- When the convection heat transfer coefficient is very large (h → ∞), the convection resistance becomes zero and Ts ≈ T. This situation is approached in practice at surfaces where ------------------------ occur.arrow_forwardQuestion No. 3: During a cold winter day, wind at 55 km/h is blowing parallel to a 4-m-high and 10-m-long wall of a house. If the air outside is at 5°C and the surface temperature of the wall is12°C, determine the rate of heat loss from that wall by convection. What would your answer be if the wind velocity was doubledarrow_forwardConsider a refrigeration truck traveling at 55 mph at a location where the air temperature is 80°F. The refrigerated compartment of the truck can be considered to be a 9-ft-wide, 8-ft-high, and 20-ft-long rectangular box. The refrigeration system of the truck can provide 3 tons of refrigeration (i.e., it can remove heat at a rate of 600 Btu/ min). The outer surface of the truck is coated with a lowemissivity material, and thus radiation heat transfer is very small. Determine the average temperature of the outer surface of the refrigeration compartment of the truck if the refrigeration system is observed to be operating at half the capacity. Assume the air flow over the entire outer surface to be turbulent and the heat transfer coefficient at the front and rear surfaces to be equal to that on side surfaces. For air properties evaluations assume a film temperature of 80°F. Is this a good assumption?arrow_forward
- A 6cm diameter shaft rotates at 3000rpm in a 20cm long bearing with a uniform clearance of 0.2mm. At steady operating conditions, both the bearing and shaft in the vicinity of the oil gap are 50°C. The thermal conductivity and the viscosity of the lubricating oil are k=0.17 W/m-K and µ= 0.05 N-s/m2, respectively. By simplifying and solving the differential continuity, momentum and energy equation, determine the maximum temperature of oil. (Hint: Couette flow)arrow_forwardWhat does the Grashof number represent? How does the Grashof number differ from the Reynolds number?arrow_forwardWhat is natural convection? How does it differ from forced convection? What force causes natural convection currents?arrow_forward
- A horizontal pipe having a surface temperature of 67 °C and diameter of 20 cm is buried at a depth of 1 m in the earth at a location where k = 1.8 W/m-°C. The earth surface temperature is 15 °C. Calculate the shape factor if the pipe length is 10 m.arrow_forwardConsider laminar flow of air across a hot circular cylinder. At what point on the cylinder will the heat transfer be highest? What would your answer be if the flow were turbulent?arrow_forwarda 6cm diameter shaft rotates at 3000rpm in a 20cm - long bearing with a uniform clearance of 0.2mm. At steady operating conditions, both the bearing and the shaft in the vicinity of the oil gap are at 50°c, and the viscosity and thermal conductivity of lubricating oil are 0.05N.s/m^2 and 0.17W/m. k. By simplifying and solving the continuity, momentum, and energy equations, determine: a) the maximum temperature of oil. b) the rate of heat transfer to the bearing and the shaft, and the mechanical power wassted by the viscous dissipation in the oil?arrow_forward
- Air is flowing in parallel over the upper surface of a flat plate with a length of 4 m. The first half of the plate length, from the leading edge, has a constant surface temperature of 50°C. The second half of the plate length is subjected to a uniform heat flux of 86 W/m2. The air has a free stream velocity and temperature of 0.5 m/s and 10°C respectively. The properties of air at Tf = 30°C are k = 0.02588 W/m∙K, ν = 1.608 × 10−5 m2/s, and Pr = 0.7282. Determine the local convection heat transfer coefficients at 1 m and 3 m from the leading edge.arrow_forwardAir is flowing in parallel over the upper surface of a flat plate with a length of 4 m. The first half of the plate length, from the leading edge, has a constant surface temperature of 50°C. The second half of the plate length is subjected to a uniform heat flux of 86 W/m2. The air has a free stream velocity and temperature of 0.5 m/s and 10°C respectively. The properties of air at Tf = 30°C are k = 0.02588 W/m∙K, ν = 1.608 × 10−5 m2/s, and Pr = 0.7282.Determine the local convection heat transfer coefficients at 1 m from the leading edge.arrow_forwardAddition of fins may not necessarily increase the heat transfer from a surface; itmay even decrease the heat transfer. Comment on this statemearrow_forward
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