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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In an experiment, the temperature of a hot gas stream is to be measured by a thermocouple with a spherical junction. Due to the nature of this experiment, the response time of the thermocouple to register 69 percent of the initial temperature difference must be within 5 s. The properties of the thermocouple junction are k = 35 W/m•K, ρ = 8500 kg/m3, and cp = 320 J/kg•K. If the heat transfer coefficient between the thermocouple junction and the gas is 250 W/m2•K, determine the diameter of the junction.
A fabricated food, in the form of small spherical pellets, is to be frozen in an air-blast freezer. The air-blast freezer is operating with air at -30°C. The initial product temperature is 25°C. The pellets have a diameter of 1.2 cm, and the density
of the product is 980 kg/m³ . The initial freezing temperature is -2.5°C. The latent heat of fusion for this product is
280 kJ/kg. The thermal conductivity of the frozen product
is 1.9 W/(m°C). The convective heat transfer coefficient is
50 W/(m²
°C). Calculate the freezing time (h)
Humans are able to control their rates of heat production and heat loss to maintain a nearly constant core temperature of Tc = 37°C under a wide range of environmental conditions. This process is called thermoregulation. From the perspective of calculating heat transfer between a human body and its surroundings, we focus on a layer of skin and fat, with its outer surface exposed to the environment and its inner surface at a temperature slightly less than the core temperature, Ti = 35°C = 308 K. Consider a person with a skin/fat layer of thickness L = 2 mm and effective thermal conductivity k = 0.3 Wm ⋅ K. The person has a surface area A = 1.8 m2 and is dressed in a bathing suit. The emissivity of the skin is ε = 0.95.a). When the person is in still air at T∞ = 308 K, what is the skin surface temperature and rate of heat loss to the environment? Convection heat transfer to the air is characterized by a free convection coefficient of h = 2 W?2 ⋅ Kb). When the person is in water at T∞ =…
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- 3.5 In a ball-bearing production facility, steel balls that are each of 15 mm in diameter are annealed by first heating them to 870°C and then slowly cooling in air to 125°C. If the cooling air stream temperature is 60°C, and it has a convective heat transfer coefficient of , determine the time required for the cooling.arrow_forwardDetermine 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 55arrow_forward1.79 Consider the cooling of (a) a personal computer with a separate CPU and (b) a laptop computer. The reliable functioning of these machines depends on their effective cooling. Identify and briefly explain all modes of heat transfer involved in the cooling process.arrow_forward
- The handle of a ladle used for pouring molten lead is 30 cm long. Originally the handle was made of 1.9cm1.25cm mild steel bar stock. To reduce the grip temperature, it is proposed to form the handle of tubing 0.15 cm thick to the same rectangular shape. If the average heat transfer coefficient over the handle surface is 14 W/m K, estimate the reduction of the temperature at the grip in air at 21C.arrow_forward3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the atmosphere from outer space. If its original temperature is 38°C, the effective average temperature of the atmosphere is 1093°C, and the effective heat transfer coefficient is , estimate the temperature of the shell after reentry, assuming the time of reentry is 10 min and the interior of the shell is evacuated.arrow_forward3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.arrow_forward
- A 0.6-cm diameter mild steel rod at 38C is suddenly immersed in a liquid at 93C with hc=110W/m2K. Determine the time required for the rod to warm to 88C.arrow_forwardCitrus fruits are very susceptible to cold weather, and extended exposure to subfreezing temperatures can destroy them. Consider an 8-cm-diameter orange that is initially at 15°C. A cold front moves in one night, and the ambient temperature suddenly drops to –6°C, with a heat transfer coefficient of 15 W/m2·K. Using the properties of water for the orange and assuming the ambient conditions to remain constant for 20400 seconds before the cold front moves out, determine the surface temperature of orange that night. Solve this problem using analytical one-term approximation method. The properties of the orange are approximated by those of water at the average temperature of about 5°C, k = 0.571 W/m·°C and α = 0.136 × 10–6 m2/s. The surface temperature of orange that night isarrow_forwardIn a manufacturing facility, 2-in-diameter brassballs (k = 64.1 Btu/h·ft·°F, r = 532 lbm/ft3, and cp =0.092 Btu/lbm·°F) initially at 250°F are quenched in a waterbath at 120°F for a period of 2 min at a rate of 120 ballsper minute. If the convection heat transfer coefficient is42 Btu/h·ft2·°F, determine (a) the temperature of the balls afterquenching and (b) the rate at which heat needs to be removedfrom the water in order to keep its temperature constant at120°F.arrow_forward
- A small copper wire with a diameter of 0.792 mm and initially at 366.5 K is suddenly immersed in a liquid held constant at 311 K. The convection coefficient h = 85.2 W/m2 · K. The physical properties can be assumed constant and are k = 374 W/m · K, cp = 0.389 kJ/kg · K, and ρ = 8890 kg/m3. Determine the time in seconds for the average temperature of the wire to drop to 338.8 K (one-half the initial temperature difference). Do the same but for h=11.36W/m2·K. Forpart(b),calculate the total amount of heat removed for a wire 1.0 m long.arrow_forwardDuring air cooling of steel balls, the convection heat transfer coefficient is determined experimentally as a function of air velocity to be h = 17.9V 0.54 for 0.5 < V < 2 m/s, where h and V are in W/m2∙K and m/s, respectively. Consider a 24-mm-diameter steel ball initially at 300°C with a thermal conductivity of 15 W/m∙K. The steel balls are cooled in air at 10°C and a velocity of 1.5 m/s. Determine the initial values of the heat flux and the temperature gradient in the steel ball at the surface.arrow_forwardLayers of 23-cm-thick meat slabs (k =0.47 W/m · °C) and α= 0.13x 10-6 m2/s) initially at a uniform temperature of 7°C are to be frozen by refrigerated air at -30°C flowing at a velocity of 1.4 m/s. The average heat transfer coefficient between the meat and the air is 20 W/m2· °C. Assuming the size of the meat slabs to be largerelative to their thickness, determine how long it will take for the center temperature of the slabs to drop to 18°C. Also, determine the surface temperature of the meat slab at that time. Answer: 22.1 h, -26.9 ℃arrow_forward
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