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Fundamentals of Heat and Mass Transfer
- 1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical hairdryer is idealized as a thin-walled cylindrical duct with a 6-cm diameter with a fan at the inlet that blows air over an electric heating coil as schematically shown in the figure. The design of this appliance requires two power settings, with which the air blown over the electric heating coil is heated from the ambient temperature of to an outlet temperature of and with exit air velocities of 1.0 m/s and 1.5 m/s. Estimate the electric power required for the heating coil to meet these conditions, assuming that heat loss from the outside of the dryer duct is neglected.arrow_forwardThe furnace wall shown in the figure is made of a material with a thermal conductivity of 5 W / m ° C, the radiant emission coefficient of the outer surface of the wall is 0.95, Stefan Boltzman constant is 5.67x104 W / m ^ 2 (K ^ 4), ambient temperature and air temperature 297 K is. The heat transfer coefficient between the outer surface of the wall and the air is h = 20 W / m (K ^ 2). Wall inner surface temperature 573 K, outer surface temperature 308 K Since the value is kept constant a) Find the layer thickness of the wall? b) Find the layer thickness of the wall if the moving fluid medium on the outer surface of the wall is corroded?arrow_forwardThe front window of an automobile has a thickness of 4 mm and is defrosted by the passage of a stream of hot air through the inner surface. The thermal conductivity of glass is around 0.78 W/(m oC). Assume the ambient air temperature is -10°C and the external convection coefficient is 65W/(m2 K). If the hot air blowing to defog the glass internally is at 40 ºC and the internal convection coefficient corresponds to 30 W/(m2 K), the internal surface temperatures will be _____________. However, if the car accelerates and maintains high speed, the external convection coefficient should _____________ [increase or decrease?] and if the condition is maintained long enough the temperature of the inner surface of the glass should _____________ [increase or decrease?].arrow_forward
- Q1 A. 1. When may one expect radiation heat transfer to be important? 2. Name some good conductors of heat; some poor conductors. B. Derive an expression for the temperature distribution in a sphere of radius r with uniform heat generation q and constant surface temperature Tw.arrow_forwardThe interior of a refrigerator whose dimensions are 0.05 x 0.05 dam base area and 1.25 m high, must be kept at 4 °C. The refrigerator walls are constructed of two steel sheets (k= 35 kcal/h.m.°C) 3 mm thick, with 65 mm of material insulation (k=0.213 kcal/h.m.°C) between them. The film coefficient of the inner surface is 10 kcal/h.m².°C, while on the external surface it varies from 8 to 12.5 kcal/h.m².°C. Calculate: a) The power (in HP) of the refrigerator motor so that the heat flux removed from the inside the refrigerator maintain the specified temperature, in a kitchen whose temperature can vary from 21 to 36 °C; b) The temperatures of the inner and outer surfaces of the wall. Given 1 HP = 641.2 Kcal/harrow_forwardan unsisulated 100 mm diameter steam pipe runs for 25 meters inside a room whose walls and air are at a temperature of 25 C. the superheated steam inside the pipe maintains the temperature at the pipe surface at 150 C. if the natural convection heat transfer coefficient of the air outside the pipe is 10 w/m^2 k and the surface emissivity is 0.8, compute for the convection thermal resistance of the air film surrounding the pipe in k/Warrow_forward
- 3. A food-processing plant requires that two large process areas A and B be kept at constant temperature TA 12°C. The refrigeration load of area A is 175 kW. The heating load of area B is 210 kW. Conditioning of the two spaces is achieved by an (cyclic) electrical air-conditioning and heating unit (CM) that also interacts with the environment at To = 5°C, as shown in the В., 12°С 8°C and TB Work R 5°C figure. А, —8°С a. What is the least amount of electrical power required to operate the air- conditioning and heating unit? b. Determine the rate of energy exchange between the unit (CM) and the environment in case a.arrow_forward1 - A square chip, with side w = 5 mm, operates under isothermal conditions.The chip is positioned on a substrate so that its side and bottom surfaces are thermally insulated, while its top surface is exposed to theflow of a refrigerant at T∞ = 15°C. From reliability considerations, the chip temperature cannot exceed T = 85°C. The refrigerant being air, with a convection heat transfer coefficientcorresponding h = 200 W/(m2K), what is the maximum allowable power for the chip? Since the coolant is a dielectric liquid for which h = 3000 W/(m²K), what is the maximum allowed power?arrow_forwardThe temperature distribution across a wall 0.25 m thick at a certain instant of time is T(x) = a + bx + cx², where T is in degrees Celsius and x is in meters, a = 200 C, b = -200 C/m, and c = 30 C/m². The wall has a thermal conductivity of 2.5 W/m.K. (a) Determine the heat flux into and out of the wall (q"in and q'out). (b) If the cold surface is exposed to a fluid at 100 C, what is the convection coefficient h? - Degree Celsius 200°C q" In- q'in q'out= h = Choose... Choose.... Choose... L₂x K = 2.5 W/m.k T(x)-200-200 x +30x² q" Out 142.7 C 11 L=0.25 m Fluid Too = 100 °C harrow_forward
- A gas filled tube has 2 mm inside diameter and 25 cm length. The gas is heated by an electrical wire of diameter 50 microns (o.05 mm) located along the axis of the tube. Current and voltage drop across the heating element are 0.5A and 4 volts, respectively. If the measured wire and inside tube wall temps are 175C and 150C respectively, find the thermal conductivity of the gas filling the tube.arrow_forward19 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.arrow_forwardA thermometric well is placed in a pipe having diameter of 55 mm. Pipe wall temperature is 100°C and heat transfer coefficient inside the pipe is 300 W/m² K. Thickness of thermowell is 1.2 mm and its length is 50 mm. Thermal conductivity of thermowell material. Is 30 W/mK. If the temperature of the gas flowing through the pipe is recorded by thermometer as 200°C, determine the true temperature of gas. If the error in gas temperature is to be reduced by 80% by increasing the length of thermowell, determine the new length of thermowell. Draw a sketch of this thermowell. Take perimeter to area ratio for thermowell as 1/T (T= thickness).arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning