In areas where the air temperature remains below 0°C for prolonged periods of time, the freezing of water in underground pipes is a major concern. Fortunately, the soil remains relatively warm during those periods, and it takes weeks for the subfreezing temperatures to reach the water mains in the ground. Thus, the soil effectively serves as an insulation to protect the water from the freezing atmospheric temperatures in winter.
The ground at a particular location is covered with snowpack at -8°C for a continuous period of 60 days, and the average soil properties at that location are
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HEAT+MASS TRANSFER ACCESS CODE
- Consider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his body to air flow. The air temperature is 32°C, and the fan is blowing air at a velocity of 5 m/s. The surrounding surfaces are at 40°C, and the emissivity of the person can be taken to be 0.9. If the person is doing light work and generating sensible heat at a rate of 90 W, determine the average temperature of the outer surface (skin or clothing) of the person. The average human body can be treated as a 30-cmdiameter cylinder with an exposed surface area of 1.7 m2. Evaluate the air properties at film temperature of 35°C and 1 atm.arrow_forwardConsider 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.arrow_forward1. (a) Consider a room with a 1.8-m-high and 2.0-m-wide double-pane window consisting of two 4-mm-thick layers of glass separated by a 10-mm-wide stagnant air space. The convection heat transfer coefficients on the inner and outer surfaces of the window are 12 W/m2 K and 25 W/m2 K, respectively, while the average thermal conductivity of glass is 0.78 W/m K; and the air, 0.026 W/m K. If the room is maintained at 22 oC, the outside temperature is -4 oC and heat transfer due to radiation can be neglected, determine: (i) Draw the sketch and thermal resistance network; (ii) the total thermal resistance; (iii) the steady rate of heat transfer through this double-pane window; (iv) the temperature of the inner surface of the window.…arrow_forward
- When is the heat transfer through a fluid conduction and when is it convection? For which case is the rate of heat transfer higher? How does the convection heat transfer differ from the thermal conductivity of the fluid?arrow_forwardWhat is a semi-infinite medium? Give examples of solid bodies that can be treated as semi-infinite mediums for heat transfer purposes.arrow_forwardUnder conditions for which the same room temperature is maintained by a heating or cooling system, it is not uncommon for a person to feel chilled in the winter but comfortable in the summer.Consider a room whose air temperature is maintained at 18ºC throughout the year, while the walls of the room are nominally at 24°C and 14°C in the summer and winter, respectively. The exposed surface of a person in the room may be assumed to be at a temperature of 32°C throughout the year and to have an emissivity of 0.90. The coefficient associated with heat transfer by natural convection between the person and the room air is approximately 2 W/m2·K. Calculate the following. a. heat flux due to convection in W / m^2 b. Heat flux due to radiation in the summer in W / m^2 c. Heat flux due to radiation in the winter in W / m^2arrow_forward
- If we neglect radiative heat transfer, what is the appropriate heat transfer coefficient value for stagnant ambient air?arrow_forwardA heating system is to be designed to keep the wings of an aircraft cruising at a velocity of 900 km/h above freezing temperatures during flight at 12,200-m altitude where the standard atmospheric conditions are 55.4°C and 18.8 kPa. Approximating the wing as a cylinder the elliptical cross section whose minor axis is 30 cm and disregarding radiation, determine the average convection heat transfer coefficient on the wing surface and the average rate of heat transfer per unit surface area.arrow_forwardIn a food industry, pieces of meat with approximately 30 cm in diameter and 20 mm in thickness are stored in a industrial freezer and thawed by exposure to ambient air at 15°C with convective heat transfer coefficient equal to 10W/(m2.K). Consider a piece of meat that, when removed from the freezer, had a temperature of -12°C. For defrosting, the piece is hung on a "line", so that both larger surfaces are exposed to the ambient air. For frozen meat, the following properties can be assumed ρ=1090 kg/m3, cp=3.54 kJ/(kg.K), k=0.47 W/(m.K). Ask: (1) Indicating the control volume and the simplifications adopted, obtain the differential energy balance equation for the cooling the piece of meat and describe the conditions of outline and initial that apply to the process, justifying them; (2) Use the appropriate analytical solution to determine the time necessary for the complete defrosting of the part. The part is considered to be completely thawed when a minimum temperature of 5oC is reached…arrow_forward
- A long 10-cm-diameter steam pipe whose external surface temperature is 110°C passes through some open area that is not protected against the winds . Determine the rate of heat loss from the pipe per unit of its length when the air is at 1 atm pressure and 10°C and the wind is blowing across the pipe at a velocity of 8 m/s.arrow_forwardIn a production facility, large brass plates of 4-cm thickness that are initially at a uniform temperature of 25°C are heated by passing them through an oven that is maintained at 455°C . The plates remain in the oven for a period of 7 min. Taking the combined convection and radiation heat transfer coefficient to be h = 120 W/m2·K, determine the surface temperature of the plates when they come out of the ovenarrow_forward
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