ermometer is placed in a stream of air with convection coefficient of 60 W/m2 -K for measuring transient temperature of air. Consider cylindrical thermometer bulb consists of mercury only. For which k = 8.9 W/m.K and
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- A 10 cm * 10 cm plate has a constant surface temperature of 150°C. Determine the Grashof number if the chip is placed in the following fluids: air (1 atm, 30°C), liquid water (30°C), and engine oil (10°C). Discuss how the Grashof number affects the natural convection flow.A 5.0-cm-diameter cylinder is heated to a temperature of 200 oC, and air at 30 oC forced across it at a velocity of 50 m/s (h = 180 W/m2K). If the surface emissivity is 0.7, calculate the total heat loss per unit length if the walls of the enclosing room are at 10 oC. Comment on your calculation. COMPLETE SOLUTIONThick fluids such as asphalt and waxes and the pipes in which they flow are often heated in order to reduce the viscosity of the fluids and thus to reduce the pumping costs. Consider the flow of such a fluid through a 100-m-long pipe of outer diameter 30 cm in calm ambient air at 0oC. The pipe is heated electrically, and a thermostat keeps the outer surface temperature of the pipe constant at 25oC. The emissivity of the outer surface of the pipe is 0.8, and the effective sky temperature is −30oC. Determine the power rating of the electric resistance heater, in kW, that needs to be used. Also, determine the cost of electricity associated with heating the pipe during a 10-h period under the above conditions if the price of electricity is $0.09/kWh. Properties The properties of air at 1 atm and the film temperature of (Ts+T∞)/2 = (25+0)/2 = 12.5°C are: k = 0.02458 W/m.oC, ν = 1.448x10-5 m2/s, Pr = 0.7330
- 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 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?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)Consider a 50-cm-diameter and 95-cm-long hot water tank. The tank is placed on the roof of a house. The water inside the tank is heated to 80°C by a flat-plate solar collector during the day. The tank is then exposed to windy air at 18°C with an average velocity of 40 km/h during the night. Estimate the temperature of the tank after a 45-min period. Assume the tank surface to be at the same temperature as the water inside, and the heat transfer coefficient on the top and bottom surfaces to be the same as that on the side surface. Evaluate the air properties at 50°C.
- Hot water is being transported in a horizontal pipe (k = 0.15 W/m∙K, Di = 2.5 cm, Do = 4.5 cm), where the pipe inner surface temperature is at 100°C. A length of 2 m of the pipe outer surface is exposed to cool air at 12°C. Assuming that the properties of air can be evaluated at 40°C and 1 atm pressure, determine the outer surface temperature of the pipe. Is 40°C an appropriate film temperature for evaluation of the air properties?During a cold winter day, wind at 55 km/h is blowing parallel to a 4m high and 10m long wall of a house. If the air outside is at 5 degrees Celsius and the surface temperature of the wall is 12 degrees Celsius , determine the rate of heat loss from the wall by convection. What would your answer be if the wind velocity has doubled? From Table A-1:thermal conductivity, k = 0.0246 W/m degrees Celsiusviscosity, v = 1.4 x 10^-5 m^2/sPrandtl no. Pr = 0.717A 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.
- The emissivity of galvanized steel sheet, a common roofing material, is ε = 0.13 at temperatures around 300 K, while its absorptivity for solar irradiation is αS = 0.65. Would the neighborhood cat be comfortable walking on a roof constructed of the material on a day when GS = 750 W/m2, T∞ = 16°C, and h = 7 W/m2 · K? Assume the bottom surface of the steel is insulated.If forced convection uses Reynolds number to account for fluid movement during forced convection, what dimensionless number is used for natural convection?A 6-mm-diameter electrical transmission line carries an electric current of 50 A and has a resistance of 0.002 ohm/m length. Determine the surface temperature of the wire during a windy day when the air temperature is 10°C and the wind is blowing across the transmission line at 40 km/h.