Chapter 5, Problem 5.22P

The local atmospheric pressure in Denver, Colorado (elevation 1610 m), is 83.4 kPa. Air at this pressure and at 30°C flows with a velocity of 4.8 m/s over a 2.3-m × 8-m flat plate whose temperature is 120°C. Determine the rate of heat transfer from the plate if the air flows parallel to the 2.3-m-side. The properties of air at 0.823 atm and at the film temperature of (120 + 30)/2 = 75°C are k = 0.02971 W/m·°C, ν = 2.486 × 10-5 m2/s, and Pr = 0.7166.

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.

Thick 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

Question 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 doubled

The airflow at 1 atm has a free stream velocity and temperature of 5 cm/s and 25°C, respectively. Determine the heat flux on the plate surface and the convection heat transfer coefficient of the airflow.

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)

Explain and discuss the significance of (i) Rayleigh number and (ii) Nusselt number in natural convection

A 1.2-in-outer-diameter pipe is to span across a river at a 115-ft-wide section while being completely immersed in water. The average flow velocity of the water is 8 ft/s, and its temperature is 70°F. Determine the drag force exerted on the pipe by the river.