What is the physical mechanism of heat conduction in a solid, a liquid, and a gas?
> A 3-kg plastic tank that has a volume of 0.2 m3 is filled with liquid water. Assuming the density of water is 1000 kg/m3, determine the weight of the combined system.
> Reconsider Prob. 16–37. Using appropriate software, plot the convection heat transfer coefficient as a function of the wire surface temperature in the range of 100°C to 300°C. Discuss the results. Data from Prob.
> A 1.4-m-long, 0.2-cm-diameter electrical wire extends across a room that is maintained at 20°C. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 240°C in stea
> The 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
> A 200-ft-long section of a steam pipe whose outer diameter is 4 in passes through an open space at 50°F. The average temperature of the outer surface of the pipe is measured to be 280°F, and the average heat transfer coefficient on that surface is determ
> A transistor with a height of 0.4 cm and a diameter of 0.6 cm is mounted on a circuit board. The transistor is cooled by air flowing over it with an average heat transfer coefficient of 30 W/m2⋅K. If the air temperature is 55°
> An electric current of 5 A passing through a resistor has a measured voltage of 6 V across the resistor. The resistor is cylindrical with a diameter of 2.5 cm and length of 15 cm. The resistor has a uniform temperature of 90°C, and the room air temperatu
> In a power plant, pipes transporting superheated vapor are very common. Superheated vapor is flowing at a rate of 0.3 kg/s inside a pipe 5 cm in diameter and 10 m in length. The pipe is located in a power plant at 20°C and has a uniform surfac
> Air at 20°C with a convection heat transfer coefficient of 20 W/m2⋅K blows over a pond. The surface temperature of the pond is at 40°C. Determine the heat flux between the surface of the pond and the air.
> An engineer who is working on the heat transfer analysis of a house in English units needs the convection heat transfer coefficient on the outer surface of the house. But the only value he can find from his handbooks is 14 W/m2⋅K, which is in SI units. T
> How do the thermal conductivity of gases and liquids vary with temperature?
> One way to improve the fuel efficiency of a car is to use tires that have a lower rolling resistance tires that roll with less resistance. Highway tests at 65 mph showed that tires with the lowest rolling resistance can improve fuel efficiency by nearly
> Reconsider Prob. 16–28. Using appropriate software, plot the rate at which ice melts as a function of the container thickness in the range of 0.1 cm to 1.0 cm. Discuss the results. Data from Prob. 16-28: A hollow spherical iron contain
> A hollow spherical iron container with outer diameter 20 cm and thickness 0.2 cm is filled with iced water at 0°C. If the outer surface temperature is 5°C, determine the approximate rate of heat gain by the iced water in kW and the
> A concrete wall with a surface area of 20 m2 and a thickness of 0.30 m separates conditioned room air from ambient air. The temperature of the inner surface of the wall (T1) is maintained at 25°C. (a) Determine the heat loss Q(W) through the concrete wal
> One way of measuring the thermal conductivity of a material is to sandwich an electric thermofoil heater between two identical rectangular samples of the material and to heavily insulate the four outer edges. Thermocouples attached to the inner and outer
> In a certain experiment, cylindrical samples of diameter 4 cm and length 7 cm are used. The two thermocouples in each sample are placed 3 cm apart. After initial transients, the electric heater is observed to draw 0.6 A at 110 V, and both differential th
> The north wall of an electrically heated home is 20 ft long, 10 ft high, and 1 ft thick and is made of brick whose thermal conductivity is k = 0.42 Btu/h⋅ft⋅°F. On a certain winter night, the temperatures of the inner and the outer surfaces of the wall a
> Engine oil at 40°C is flowing over a long flat plate with a velocity of 6 m/s. Determine the distance xcr from the leading edge of the plate where the flow becomes turbulent, and calculate and plot the thickness of the boundary layer over a length of 2xc
> An aluminum pan whose thermal conductivity is 237 W/mâ‹…K has a flat bottom with diameter 15 cm and thickness 0.4 cm. Heat is transferred steadily to boiling water in the pan through its bottom at a rate of 800 W. If the inner surface of
> Repeat Prob. 15–97 by considering the more general form of Stokes’ law expressed as FD = 3πμDV + (9π/16)ρV2D2 where ρ is the fluid density. Data from Prob. 15-97: During an experiment, three aluminum balls (ρs = 2600 kg/m3) having diameters 2, 4, and
> Reconsider Prob. 16–21. Using appropriate software, plot the amount of heat loss through the glass as a function of the window glass thickness in the range of 0.1 cm to 1.0 cm. Discuss the results. Data from Prob. 16-21: The inner and outer surfaces of
> Consider a 2100-kg car cruising at constant speed of 70 km/h. Now the car starts to pass another car by accelerating to 110 km/h in 5 s. Determine the additional power needed to achieve this acceleration. What would your answer be if the total mass of th
> During an experiment, three aluminum balls (ρs = 2600 kg/m3) having diameters 2, 4, and 10 mm, respectively, are dropped into a tank filled with glycerin at 22°C (ρf = 1274 kg/m3 and μ = 1 kg/m·s). The terminal settling velocities of the balls are meas
> The inner and outer surfaces of a 0.5-cm thick 2-m × 2-m window glass in winter are 10°C and 3°C, respectively. If the thermal conductivity of the glass is 0.78 W/m⋅K, determine the amount of heat loss through the glass over a period of 5 h. What would y
> A 17,000-kg tractor-trailer rig has a frontal area of 9.2 m2, a drag coefficient of 0.96, a rolling resistance coefficient of 0.05 (multiplying the weight of a vehicle by the rolling resistance coefficient gives the rolling resistance), a bearing fricti
> The inner and outer glasses of a 4-ft × 4-ft double-pane window are at 60°F and 48°F, respectively. If the 0.25-in space between the two glasses is filled with still air, determine the rate of heat transfer through the window.
> Consider a blimp that can be approximated as a 3-m diameter, 8-m long ellipsoid and is connected to the ground. On a windless day, the rope tension due to the net buoyancy effect is measured to be 120 N. Determine the rope tension when there are 50 km/h
> Which is a better heat conductor, diamond or silver?
> It is proposed to meet the water needs of a recreational vehicle (RV) by installing a 3-m-long, 0.5 m-diameter cylindrical tank on top of the vehicle. Determine the additional power needed at a speed of 80 km/h when the tank is installed such that its ci
> The inner and outer surfaces of a 4-m × 7-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m⋅K are maintained at temperatures of 20°C and 5°C, respectively. Determine the rate of heat tr
> A paratrooper and his 7-m-diameter parachute weigh 1200 N. Taking the average air density to be 1.2 kg/m3, determine the terminal velocity of the paratrooper.
> A wood slab with a thickness of 0.05 m is subjected to a heat flux of 40 W/m2. The left and right surface temperatures of the wood slab are kept at constant temperatures of 40°C and 20°C, respectively. What is the thermal conductivity of the wood slab?
> An escalator in a shopping center is designed to move 50 people, 75 kg each, at a constant speed of 0.6 m/s at 45° slope. Determine the minimum power input needed to drive this escalator. What would your answer be if the escalator velocity were to be dou
> An automotive engine can be approximated as a 0.4-m-high, 0.60-m-wide, and 0.7-m-long rectangular block. The ambient air is at 1 atm and 15°C. Determine the drag force acting on the bottom surface of the engine block as the car travels at a ve
> What is a blackbody? How do real bodies differ from blackbodies?
> A commercial airplane has a total mass of 150,000 lbm and a wing planform area of 1700 ft2. The plane has a cruising speed of 625 mi/h and a cruising altitude of 38,000 ft where the air density is 0.0208 lbm/ft3. The plane has double slotted flaps for us
> Define emissivity and absorptivity. What is Kirchhoff’s law of radiation?
> The passenger compartment of a minivan traveling at 50 mi/h in ambient air at 1 atm and 80°F is modeled as a 4.5-ft high, 6-ft-wide, and 11-ft-long rectangular box. The airflow over the exterior surfaces is assumed to be turbulent because of t
> Consider two walls of a house that are identical except that one is made of 10-cm-thick wood while the other is made of 25-cm-thick brick. Through which wall will the house lose more heat in winter?
> During flow over a given body, the drag force, the upstream velocity, and the fluid density are measured. Explain how you would determine the drag coefficient. What area would you use in the calculations?
> Consider two houses that are identical except that the walls are built using bricks in one house and wood in the other. If the walls of the brick house are twice as thick, which house do you think will be more energy efficient?
> Stokes’ law can be used to determine the viscosity of a fluid by dropping a spherical object in it and measuring the terminal velocity of the object in that fluid. This can be done by plotting the distance traveled against time and obse
> Reconsider Prob. 15–87. Using appropriate software, investigate the effect of boat speed on the drag force acting on the bottom surface of the boat, and the power needed to overcome it. Let the boat speed vary from 0 to 100 km/h in incr
> Consider a room that is initially at the outdoor temperature of 20°C. The room contains a 40-W lightbulb, a 110-W TV set, a 300-W refrigerator, and a 1200-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy c
> Consider heat loss through two walls of a house on a winter night. The walls are identical except that one of them has a tightly fitted glass window. Through which wall will the house lose more heat? Explain.
> A plastic boat whose bottom surface can be approximated as a 1.5-m-wide, 2-m-long flat surface is to move through water at 15°C at speeds up to 45 km/h. Determine the friction drag exerted on the boat by the water and the power needed to overc
> Consider heat transfer through a windowless wall of a house on a winter day. Discuss the parameters that affect the rate of heat conduction through the wall.
> A 2-m-high, 4-m-wide rectangular advertisement panel is attached to a 4-m-wide, 0.15-m-high rectangular concrete block (density = 2300 kg/m3) by two 5-cm-diameter, 4-m-high (exposed part) poles, as shown in Fig. P15–86. If the sign is t
> A 7-ft-diameter spherical tank completely submerged in fresh water is being towed by a ship at 8 ft/s. Assuming turbulent flow, determine the required towing power.
> How does forced convection differ from natural convection?
> A 1.2-m-external-diameter spherical tank is located outdoors at 1 atm and 25°C and is subjected to winds at 48 km/h. Determine the drag force exerted on it by the wind.
> Define thermal conductivity, and explain its significance in heat transfer.
> An airplane is consuming fuel at a rate of 7 gal/min when cruising at a constant altitude of 10,000 ft at constant speed. Assuming the drag coefficient and the engine efficiency remain the same, determine the rate of fuel consumption at an altitude of 30
> The lighting needs of a storage room are being met by six fluorescent light fixtures, each fixture containing four lamps rated at 60 W each. All the lamps are on during operating hours of the facility, which are 6 a.m. to 6 p.m. 365 days a year. The stor
> An airplane has a mass of 48,000 kg, a wing area of 300 m2, a maximum lift coefficient of 3.2, and a cruising drag coefficient of 0.03 at an altitude of 12,000 m. Determine (a) the takeoff speed at sea level, assuming it is 20 percent over the stall spee
> A small airplane has a total mass of 1800 kg and a wing area of 42 m2. Determine the lift and drag coefficients of this airplane while cruising at an altitude of 4000 m at a constant speed of 280 km/h and generating 190 kW of power.
> Consider a light plane that has a total weight of 11,000 N and a wing area of 39 m2 and whose wings resemble the NACA 23012 airfoil with no flaps. Using data from Fig. 15–45, determine the takeoff speed at an angle of attack of 5° at sea level. Also dete
> Name some applications in which a large drag is desired.
> A small aircraft has a wing area of 40 m2, a lift coefficient of 0.45 at takeoff settings, and a total mass of 4000 kg. Determine (a) the takeoff speed of this aircraft at sea level at standard atmospheric conditions, (b) the wing loading, and (c) the re
> Reconsider Prob. 15–77. Using appropriate software, investigate the effect of passenger count on the takeoff speed of the aircraft. Let the number of passengers vary from 0 to 500 in increments of 50. Tabulate and plot the results. Data from Prob. 15-77
> A jumbo jet airplane has a mass of about 400,000 kg when fully loaded with over 400 passengers and takes off at a speed of 250 km/h. Determine the takeoff speed when the airplane has 150 empty seats. Assume each passenger with luggage is 140 kg and the w
> Consider an airplane whose takeoff speed is 220 km/h and that takes 15 s to take off at sea level. For an airport at an elevation of 1600 m (such as Denver), determine (a) the takeoff speed, (b) the takeoff time, and (c) the additional runway length requ
> Consider an aircraft that takes off at 260 km/h when it is fully loaded. If the weight of the aircraft is increased by 10 percent as a result of overloading, determine the speed at which the overloaded aircraft will take off.
> A 2.4-in-diameter smooth ball rotating at 500 rpm is dropped in a water stream at 60°F flowing at 4 ft/s. Determine the lift and the drag force acting on the ball when it is first dropped in the water.
> A water pump increases the water pressure from 15 psia to 70 psia. Determine the power input required, in hp, to pump 0.8 ft3/s of water. Does the water temperature at the inlet have any significant effect on the required flow power?
> How do flaps affect the lift and the drag of wings?
> Explain why endplates or winglets are added to some airplane wings.
> What is induced drag on wings? Can induced drag be minimized by using long and narrow wings or short and wide wings?
> What is the effect of wing tip vortices (the air circulation from the lower part of the wings to the upper part) on the drag and the lift?
> What is the difference between streamlined and bluff bodies? Is a tennis ball a streamlined or bluff body?
> Air is flowing past a spherical ball. Is the lift exerted on the ball zero or nonzero? Answer the same question if the ball is spinning.
> Air is flowing past a symmetrical airfoil at an angle of attack of 5°. Is the (a) lift and (b) drag acting on the airfoil zero or nonzero?
> Why is the contribution of viscous effects to lift usually negligible for airfoils?
> Why are flaps used at the leading and trailing edges of the wings of large aircraft during takeoff and landing? Can an aircraft take off or land without them?
> Both the lift and the drag of an airfoil increase with an increase in the angle of attack. In general, which increases at a higher rate, the lift or the drag?
> At winter design conditions, a house is projected to lose heat at a rate of 60,000 Btu/h. The internal heat gain from people, lights, and appliances is estimated to be 6000 Btu/h. If this house is to be heated by electric resistance heaters, determine th
> Air is flowing past a symmetrical airfoil at zero angle of attack. Is the (a) lift and (b) drag acting on the airfoil zero or nonzero?
> What is stall? What causes an airfoil to stall? Why are commercial aircraft not allowed to fly at conditions near stall?
> Dust particles of diameter 0.06 mm and density 1.6 g/cm3 are unsettled during high winds and rise to a height of 200 m by the time things calm down. Estimate how long it takes for the dust particles to fall back to the ground in still air at 1 atm and
> A person extends his uncovered arms into the windy air outside at 1 atm and 60°F and 25 mi/h in order to feel nature closely. Treating the arm as a 2-ft-long and 4-in-diameter cylinder, determine the combined drag force on both arms.
> A 0.12-mm-diameter dust particle whose density is 2.1 g/cm3 is observed to be suspended in the air at 1 atm and 20°C at a fixed point. Estimate the updraft velocity of air motion at that location. Assume Stokes’ law to be applicable. Is this a valid assu
> What is the difference between the upstream velocity and the free-stream velocity? For what types of flow are these two velocities equal to each other?
> A 2-m-long, 0.2-m-diameter cylindrical pine log (density = 513 kg/m3) is suspended by a crane in the horizontal position. The log is subjected to normal winds of 40 km/h at 5°C and 88 kPa. Disregarding the weight of the cable and its drag, det
> A 1.2-in-outer-diameter pipe is to span 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.
> Consider 0.8-cm-diameter hail that is falling freely in atmospheric air at 1 atm and 5°C. Determine the terminal velocity of the hail. Take the density of hail to be 910 kg/m3.
> A long 5-cm-diameter steam pipe passes through some area open to the wind. Determine the drag force acting on the pipe per unit of its length when the air is at 1 atm and 10°C and the wind is blowing across the pipe at a speed of 50 km/h.
> A vertical piston–cylinder device contains water and is being heated on top of a range. During the process, 65 Btu of heat is transferred to the water, and heat losses from the side walls amount to 8 Btu. The piston rises as a result of evaporation, and
> A 5-mm-diameter electrical transmission line is exposed to windy air. Determine the drag force exerted on a 160-m-long section of the wire during a windy day when the air is at 1 atm and 15°C and the wind is blowing across the transmission line at 50 km/
> In flow over cylinders, why does the drag coefficient suddenly drop when the boundary layer becomes turbulent? Isn’t turbulence supposed to increase the drag coefficient instead of decreasing it?
> Why is flow separation in flow over cylinders delayed when the boundary layer is turbulent?
> In flow over bluff bodies such as a cylinder, how does the pressure drag differ from the friction drag?
> The forming section of a plastics plant puts out a continuous sheet of plastic that is 1.2 m wide and 2 mm thick at a rate of 18 m/min. The sheet is subjected to airflow at a velocity of 4 m/s on both top and bottom surfaces normal to the direction of mo
> The weight of a thin flat plate 50 cm × 50 cm in size is balanced by a counterweight that has a mass of 2 kg, as shown in Fig. P15–50. Now a fan is turned on, and air at 1 atm and 25°C flows downward over both s
> Define the planform area of a body subjected to external flow. When is it appropriate to use the planform area in drag and lift calculations?
> Repeat Prob. 15–48 for water. Data from Prob. 15-48: Air at 25°C and 1 atm is flowing over a long flat plate with a velocity of 8 m/s. Determine the distance from the leading edge of the plate where the flow becomes turbulent, and the thickness of the b
> Air at 25°C and 1 atm is flowing over a long flat plate with a velocity of 8 m/s. Determine the distance from the leading edge of the plate where the flow becomes turbulent, and the thickness of the boundary layer at that location.
> Reconsider Prob. 15–46E. Using appropriate software, investigate the effect of truck speed on the total drag force acting on the top and side surfaces and the power required to overcome it. Let the truck speed vary from 0 to 100 mi/h in
> How are heat, internal energy, and thermal energy related to each other?
> Consider a refrigeration truck traveling at 70 mi/h at a location where the air is at 1 atm and 80°F. The refrigerated compartment of the truck can be considered to be a 9-ft-wide, 8-ft-high, and 20 ft-long rectangular box. Assuming the airflo
