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 and Mass Transfer: Fundamentals and Applications
- Under 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 16°C throughout the year, while the walls of the room are nominally at 30°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. Heat flux due to convection: q'onv W/m? Heat flux due to radiation in the summer: i W/m? Heat flux due to radiation in the winter: qad W/m? i ||arrow_forwardA 3-m-internal-diameter spherical tank made of 1-cm-thick stainless steel is used to store iced water at 0°C. The tank is located outdoors at 25°C. Assuming the entire steel tank to be at 0°C and thus the thermal resistance of the tank to be negligible, determine (a) the rate of heat transfer to the iced water in the tank and (b) the amount of ice at 0°C that melts during a 24- hour period. The heat of fusion of water at atmospheric pressure is h = 333.7 kJ/kg. The emissivity of the outer surface of the tank is 0.75, and the convection heat transfer coefficient on the outer surface can be taken to be 30 W/m2 · °C. Assume the average surrounding surface temperature for radiation exchange to be 15°C.arrow_forwardA bioengineer working at chemical company intends to dry an insoluble wet granular material in a pan 0.6 x 0.6 m and 3 cm deep. The material is at the same dept in the pan, whereas the sides and the bottom can be considered to be insulated. Heat transfer is by convection from an air stream flowing parallel to the surface at a velocity of 10 m/s. The air is at 65.6°C and has a humidity of 0.010 kg H2O/kg dry air. Calculate the rate of drying for constant-rate periodarrow_forward
- A fabricated food, in the form of small spherical pellets, is to be frozen in an air-blast freezer. The air-blast freezer isoperating with air at -40°C. The initial product temperatureis 25°C. The pellets have a diameter of 0.5 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 productis 1.9 W/(m°C). The convective heat transfer coefficient is 50 W/(m2 °C). Calculate the freezing time.arrow_forwardHeat transfer is of critical importance in various industrial applications, including manufacturing. During machining, both the cutting tool and the workpiece will be significantly heated by friction heating. The heating of the cutting tool will reduce the tool hardness and strength, deteriorate the cutting quality, and shorten the tool life. Therefore, it is essential to prevent the overheating of the cutting tool during machining. Coolants are an instrumental part of machining to help cool the tool and the workpiece, provide lubricant, flush away chips, and prevent corrosion. The task of this project is to design the coolant to maintain the maximum machine tool temperature below 100 °C during the side milling process. As shown in the figure below, the machine tool has a diameter of 10 mm (D) and a length of 5 cm. The tool material is M2 high speed tool steel (T11302) and the workpiece is aluminum 6061. The spindle speed (w) is 2000 RPM and the cutting speed (v) is 50 mm/min. The feed…arrow_forwardThe boiling temperature of nitrogen at atmospheric pressure at sea level (1 atm) is -196°C. Therefore, nitrogen is commonly used in low temperature scientific studies since the temperature of liquid nitrogen in a tank open to the atmosphere will remain constant at -196°C until the liquid nitrogen in the tank is depleted. Any heat transfer to the tank will result in the evaporation of some liquid nitrogen, which has a heat of vaporization of 198 kJ/kg and a density of 810 kg/m3 at 1 atm. Consider a 3-m-diameter spherical tank initially filled with liquid nitrogen at 1 atm and 196°C. The tank is exposed to 22°C ambient air with a heat transfer coefficient of 22 W/m2 · °C. The temperature of the thin-shelled spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Disregarding any radiation heat exchange, determine the rate of evaporation of the liquid nitrogen in the tank as a result of the heat transfer from the ambient air in kg/sec. Answer in…arrow_forward
- An electric resistance space heater is designed such that it resembles a rectangular box 50 cm high, 80 cm long, and 15 cm wide filled with 45 kg of oil. The heater is to be placed against a wall, and thus heat transfer from its back surface is negligible. The surface temperature of the heater is not to exceed 75°C in a room at 25°C for safety considerations. Disregarding heat transfer from the bottom and top surfaces of the heater in anticipation that the top surface will be used as a shelf, determine the power rating of the heater in W. Take the emissivity of the outer surface of the heater to be 0.8 and the average temperature of the ceiling and wall surfaces to be the same as the room air temperature. Also, determine how long it will take for the heater to reach steady operation when it is first turned on (i.e., for the oil temperature to rise from 25°C to 75°C). State your assumptions in the calculations.arrow_forwardA heating system is to be designed to keep the wings of an aircraft cruising at a veloeity of 900 km/h above freezing temperatures during flight at 12.200-m altitude where the standard atmospheric conditions are -55.4°C and 188 kPa. Approximating the wing as a cylinder of 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_forward8,000 kilograms of dressed poultry are blast-frozen on hand trucks each day (24 hrs) in a freezing tunnel. The poultry is to precooled to 7°C before entering the freezer where it is frozen and its temperature is lowered to -20°C for storage. The specific heat of poultry above and below freezing are 3.18 and 1.55 kJ/kg-°C, respectively. The latent heat is 246 kJ/kg and the freezing temperature is -2.75°C. Neglecting the heat load from the hand trucks, determine the amount of heat needed to be removed from the dressed poultry in kW.arrow_forward
- A vertical 1.5-m-high, 2.8-m-wide double-pane window consists of two layers of glass separated by a 2.0-cm air gap at atmospheric pressure. The room temperature is 26°C while the inner glass temperature is 18°C. Disregarding radiation heat transfer, determine the temperature of the outer glass layer and the rate of heat loss through the window by natural convection.arrow_forwardA vertical furnace wall is made up of an inner wall of firebrick 20 cm thick followed by insulating brick 15 cm thick and an outer wall of steel 1 cm thick. The surface temperature of the wall adjacent to the combustion chamber is 1200°C while that of the outer surface of steel is 50°C. The thermal conductivities of the wall material in W/m-K are: firebrick, 10; insulating brick, 0.26; and steel, 45. Neglecting the film resistances and contact resistance of joints, determine the heat loss per sq. m. of wall area. Badly needed asap. I will rate helpful. Thank you in advance.arrow_forwardDefine the cooling of a boiled egg by forced and natural convection.arrow_forward
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