Questions and Answers of Fundamentals Thermal Fluid

Oxy-fuel combustion power plants use pulverized coal particles as fuel to burn in a pure oxygen environment to generate electricity. Before entering the furnace, pulverized spherical coal particles
Consider a sphere of diameter 5 cm, a cube of side length 5 cm, and a rectangular prism of dimension 4 cm × 5 cm × 6 cm, all initially at 0°C and all made of silver (k = 429 W/m·K, r = 10,500
What is an infinitely long cylinder? When is it proper to treat an actual cylinder as being infinitely long, and when is it not? For example, is it proper to use this model when finding the
What is the physical significance of the Fourier number? Will the Fourier number for a specified heat transfer problem double when the time is doubled?
Why are the transient temperature charts prepared using nondimensionalized quantities such as the Biot and Fourier numbers instead of the actual variables such as thermal conductivity and time?
Can the transient temperature charts in Fig. 18€“14 for a plane wall exposed to convection on both sides be used for a plane wall with one side exposed to convection while the other side is
The Biot number during a heat transfer process between a sphere and its surroundings is determined to be 0.02. Would you use lumped system analysis or the transient temperature charts when
A body at an initial temperature of Ti is brought into a medium at a constant temperature of T∞. How can you determine the maximum possible amount of heat transfer between the body and the
In a meat processing plant, 2-cm-thick steaks (k = 0.45 W/m·K and α = 0.91 × 10-7 m2/s) that are initially at 25°C are to be cooled by passing them through a refrigeration room at -11°C. The
A 10-cm thick aluminum plate (Ï = 2702 kg/m3, cp= 903 J/kg·K, k = 237 W/m·K, and Î± = 97.1 Ã— 10€“6m2/s) is being heated in liquid with
In a production facility, 3-cm-thick large brass plates (k = 110 W/m·K, Ï = 8530 kg/m3, cp= 380 J/kg·K, and Î± = 33.9 Ã— 10-6m2/s) that are initially
Reconsider Prob. 18€“43. Using an appropriate software, investigate the effects of the temperature of the oven and the heating time on the final surface temperature of the plates. Let the
Layers of 23-cm-thick meat slabs (k = 0.47 W/m·K and α = 0.13 × 10-6 m2/s) initially at a uniform temperature of 7°C are to be frozen by refrigerated air at 230°C flowing at a velocity of 1.4
In an annealing process, a 50-mm-thick stainless steel plate (Ï = 8238 kg/m3, cp= 468 J/kg·K, k = 13.4 W/m·K, and Î± = 3.48 Ã— 10€“6m2/s)
Layers of 6-in-thick meat slabs (k = 0.26 Btu/h·ft·°F and α = 1.4 × 10-6 ft2/s) initially at a uniform temperature of 50°F are cooled by refrigerated air at 23°F to a temperature of 36°F at
A long cylindrical wood log (k = 0.17 W/m·K and α = 1.28 × 10-7 m2/s) is 10 cm in diameter and is initially at a uniform temperature of 15°C. It is exposed to hot gases at 550°C in a fireplace
Long cylindrical AISI stainless steel rods (k = 7.74 Btu/h·ft·°F and Î± = 0.135 ft2/h) of 4-in-diameter are heat treated by drawing them at a velocity of 7 ft/min
A long iron rod (Ï = 7870 kg/m3, cp= 447 J/kg·K, k = 80.2 W/m·K, and Î± = 23.1 Ã— 10€“6m2/s) with diameter of 25 mm is initially heated to
A 30-cm-diameter, 4-m-high cylindrical column of a house made of concrete (k = 0.79 W/m·K, α = 5.94 × 10-7 m2/s, ρ = 1600 kg/m3, and cp = 0.84 kJ/kg·K) cooled to 14°C during a cold night is
A long 35-cm-diameter cylindrical shaft made of stainless steel 304 (k = 14.9 W/m·K, ρ = 7900 kg/m3, cp = 477 J/kg·K, and α = 3.95 × 10-6 m2/s) comes out of an oven at a uniform temperature of
Reconsider Prob. 18–52. Using an appropriate software, investigate the effect of the cooling time on the final center temperature of the shaft and the amount of heat transfer. Let the time vary
A 2-cm-diameter plastic rod has a thermocouple inserted to measure temperature at the center of the rod. The plastic rod (Ï = 1190 kg/m3, cp= 1465 J/kg·K, and k = 0.19 W/m·K)
A long Pyroceram rod (Ï = 2600 kg/m3, cp= 808 J/kg·K, k = 3.98 W/m·K, and Î± = 1.89 Ã— 10€“6m2/s) with diameter of 10 mm has an initial
Steel rods, 2 m in length and 60 mm in diameter, are being drawn through an oven that maintains a temperature of 800°C and convection heat transfer coefficient of 128 W/m2·K. The steel
For heat transfer purposes, an egg can be considered to be a 5.5-cm-diameter sphere having the properties of water. An egg that is initially at 8°C is dropped into the boiling water at 100°C. The
Citrus fruits are very susceptible to cold weather, and extended ex posure to subfreezing temperatures can destroy them. Consider an 8-cm- diameter orange that is initially at 158C. A cold front
A person puts a few apples into the freezer at - 15°C to cool them quickly for guests who are about to arrive. Initially, the apples are at a uniform temperature of 20°C, and the heat transfer
Reconsider Prob. 18–61. Using an appropriate software, investigate the effect of the initial temperature of the apples on the final center and surface temperatures and the amount of heat transfer.
A 9-cm-diameter potato (ρ = 1100 kg/m3, cp = 3900 J/kg·K, k = 0.6 W/m·K, and α = 1.4 × 10-7 m2/s) that is initially at a uniform temperature of 25°C is baked in an oven at 170°C until a
In Betty Crocker’s Cookbook, it is stated that it takes 2 h 45 min to roast a 3.2-kg rib initially at 4.5°C “rare” in an oven maintained at 163°C. It is recommended that a meat thermometer be
Repeat Prob. 18–64 for a roast rib that is to be “welldone” instead of “rare.” A rib is considered to be well-done when its center temperature reaches 77°C, and the roasting in this case
Oranges of 2.5-in-diameter (k = 0.26 Btu/h·ft·°F and α = 1.4 3 10-6 ft2/s) initially at a uniform temperature of 78°F are to be cooled by refrigerated air at 25°F flowing at a velocity of 1
Under what conditions can a plane wall be treated as a semi-infinite medium?
What is a semi-infinite medium? Give examples of solid bodies that can be treated as semi-infinite mediums for heat transfer purposes.
Consider a hot semi-infinite solid at an initial temperature of Ti that is exposed to convection to a cooler medium at a constant temperature of T∞, with a heat transfer coefficient of h. Explain
The walls of a furnace are made of 1.2-ft-thick concrete (k = 0.64 Btu/h·ft·°F and α = 0.023 ft2/h). Initially, the furnace and the surrounding air are in thermal equilibrium at 70°F. The
Consider a curing kiln whose walls are made of 30-cm-thick concrete with a thermal diffusivity of Î± = 0.23 Ã— 10-5m2/s. Initially, the kiln and its walls are in equilibrium
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
A highway made of asphalt is initially at a uniform temperature of 55°C. Suddenly the highway surface temperature is reduced to 25°C by rain. Determine the temperature at the depth of 3 cm from the
A thick aluminum block initially at 20°C is subjected to constant heat flux of 4000 W/m2 by an electric resistance heater whose top surface is insulated. Determine how much the surface temperature
Refractory bricks are used as linings for furnaces, and they generally have low thermal conductivity to minimize heat loss through the furnace walls. Consider a thick furnace wall lining with
Thick slabs of stainless steel (k = 14.9 W/m·K and Î± = 3.95 Ã— 10-6m2/s) and copper (k = 401 W/m·K and Î± = 117 Ã— 10-6m2/s) are
A thick wood slab (k = 0.17 W/m·K and α = 1.28 × 10-7 m2/s) that is initially at a uniform temperature of 25°C is exposed to hot gases at 550°C for a period of 5 min. The heat transfer
The soil temperature in the upper layers of the earth varies with the variations in the atmospheric conditions. Before a cold front moves in, the earth at a location is initially at a uniform
We often cut a watermelon in half and put it into the freezer to cool it quickly. But usually we forget to check on it and end up having a watermelon with a frozen layer on the top. To avoid this
What is the product solution method? How is it used to determine the transient temperature distribution in a two-dimensional system?
A short cylinder initially at a uniform temperature Ti is subjected to convection from all of its surfaces to a medium at temperature T∞. Explain how you can determine the temperature of the
Consider a short cylinder whose top and bottom surfaces are insulated. The cylinder is initially at a uniform temperature Ti and is subjected to convection from its side surface to a medium at
Consider a cubic block whose sides are 5 cm long and a cylindrical block whose height and diameter are also 5 cm. Both blocks are initially at 20°C and are made of granite (k = 2.5 W/m·K
Repeat Prob. 18€“87 with the heat transfer coefficient at the top and the bottom surfaces of each block being doubled to 80 W/m2·K.Repeat Prob.Consider a cubic block whose sides are
A hot dog can be considered to be a cylinder 5 in long and 0.8 in in diameter whose properties are ρ = 61.2 lbm/ft3, cp = 0.93 Btu/lbm·8F, k = 0.44 Btu/h·ft·8F, and α = 0.0077 ft2/h. A hot dog
A 2-cm-high cylindrical ice block (k = 2.22 W/m·K and α = 0.124 × 10-7 m2/s) is placed on a table on its base of diameter 2 cm in a room at 24°C. The heat transfer coefficient on the exposed
A short brass cylinder (Ï = 8530 kg/m3, cp= 0.389 kJ/kg·K, k = 110 W/m·K, and Î± = 3.39 Ã— 10-5m2/s) of diameter 8 cm and height 15 cm is initially
A semi-infinite aluminum cylinder (k = 237 W/m·K, α = 9.71 × 10-5 m2/s) of diameter D = 15 cm is initially at a uniform temperature of Ti = 115°C. The cylinder is now placed in water at 10°C,
A long roll of 2-m-wide and 0.5-cm-thick 1-Mn manganese steel plate coming off a furnace at 820°C is to be quenched in an oil bath (cp= 2.0 kJ/kg·K) at 45°C. The metal sheet is moving
Large steel plates 1.0-cm in thickness are quenched from 600°C to 100°C by submerging them in an oil reservoir held at 30°C. The average heat transfer coefficient for both faces of steel plates is
Aluminum wires, 3 mm in diameter, are produced by extrusion. The wires leave the extruder at an average temperature of 350°C and at a linear rate of 10 m/min. Before leaving the extrusion room, the
During a picnic on a hot summer day, the only available drinks were those at the ambient temperature of 90°F. In an effort to cool a 12-fluid-oz drink in a can, which is 5 in high and has a diameter
Two metal rods are being heated in an oven with uniform ambient temperature of 1000°C and convection heat transfer coefficient of 25 W/m2·K. Rod A is made of aluminum (ρ = 2702 kg/m3, cp = 903
Stainless steel ball bearings (ρ = 8085 kg/m3, k = 15.1 W/m·°C, cp = 0.480 kJ/kg·°C, and α = 3.91 × 10-6 m2/s) having a diameter of 1.2 cm are to be quenched in water. The balls leave the oven
During a fire, the trunks of some dry oak trees (k = 0.17 W/m·K and Î± = 1.28 Ã— 10-7m2/s) that are initially at a uniform temperature of 30°C are exposed to hot
In Betty Crocker’s Cookbook, it is stated that it takes 5 h to roast a 14-lbm stuffed turkey initially at 40°F in an oven maintained at 325°F. It is recommended that a meat thermometer be used to
Spherical glass beads coming out of a kiln are allowed to cool in a room temperature of 30°C. A glass bead with a diameter of 10 mm and an initial temperature of 400°C is allowed to cool for
The water main in the cities must be placed at sufficient depth below the earth’s surface to avoid freezing during extended periods of subfreezing temperatures. Determine the minimum depth at which
A 40-cm-thick brick wall (k = 0.72 W/m·K, and α = 1.6 × 10-6 m2/s) is heated to an average temperature of 18°C by the heating system and the solar radiation incident on it during the day. During
A large heated steel block (ρ = 7832 kg/m3, cp = 434 J/kg·K, k = 63.9 W/m·K, and α = 18.8 × 10–6 m2/s) is allowed to cool in a room at 25°C. The steel block has an initial temperature of
A large iron slab (Ï = 7870 kg/m3, cp= 447 J/ kg·K, and k = 80.2 W/m·K) was initially heated to a uniform temperature of 150°C and then placed on concrete floor
A hot dog can be considered to be a 12-cm-long cylinder whose diameter is 2 cm and whose properties are Ï = 980 kg/m3, cp= 3.9 kJ/kg·K, k = 0.76 W/m·K, and Î± =
Reconsider Prob. 15–35. Using an appropriate software, investigate the effect of car speed on the required power to overcome(a) Rolling resistance,(b) The aerodynamic drag,(c) Their combined
During steady motion of a vehicle on a level road, the power delivered to the wheels is used to overcome aerodynamic drag and rolling resistance (the product of the rolling resistance coefficient and
A 5-ft-diameter spherical tank completely submerged in freshwater is being towed by a ship at 12 ft/s. Assuming turbulent flow, determine the required towing power.
Reconsider Prob. 15€“32. Using an appropriate software, investigate the effect of wind speed on the torque applied on the pivot. Let the wind speed vary from 0 to 50 m/s in increments of 5
A wind turbine with two or four hollow hemispherical cups connected to a pivot is commonly used to measure wind speed. Consider a wind turbine with four 8-cm-diameter cups with a center-to-center
A 70-kg bicyclist is riding her 15-kg bicycle downhill on a road with a slope of 8° without pedaling or braking. The bicyclist has a frontal area of 0.45 m2 and a drag coefficient of 1.1 in the
During major windstorms, high vehicles such as RVs and semis may be thrown off the road and boxcars off their tracks, especially when they are empty and in open areas. Consider a 5000-kg semi that is
Wind loading is a primary consideration in the design of the supporting mechanisms of billboards, as evidenced by many billboards being knocked down during high winds. Determine the wind force acting
A submarine can be treated as an ellipsoid with a diameter of 5 m and a length of 25 m. Determine the power required for this submarine to cruise horizontally and steadily at 40 km/h in seawater
At highway speeds, about half of the power generated by the car’s engine is used to overcome aerodynamic drag, and thus the fuel consumption is nearly proportional to the drag force on a level
Advertisement signs are commonly carried by taxicabs for additional income, but they also increase the fuel cost. Consider a sign that consists of a 0.30-m-high, 0.9-m-wide, and 0.9-m-long
Bill gets a job delivering pizzas. The pizza company makes him mount a sign on the roof of his car. The frontal area of the sign is A = 0.612 ft2, and he estimates the drag coefficient to be CD =
A circular sign has a diameter of 50 cm and is subjected to normal winds up to 150 km/h at 10°C and 100 kPa. Determine the drag force acting on the sign. Also determine the bending moment at the
Reconsider Prob. 15–22E. Using an appropriate software, investigate the effect of frontal area on the annual fuel consumption of the car. Let the frontal area vary from 10 to 30 ft2 in increments
To reduce the drag coefficient and thus to improve the fuel efficiency, the frontal area of a car is to be reduced. Determine the amount of fuel and money saved per year as a result of reducing the
A car is moving at a constant velocity of 110 km/h. Determine the upstream velocity to be used in fluid flow analysis if(a) The air is calm,(b) Wind is blowing against the direction of motion of the
During a high Reynolds number experiment, the total drag force acting on a spherical body of diameter D = 12 cm subjected to airflow at 1 atm and 5°C is measured to be 5.2 N. The pressure drag
The resultant of the pressure and wall shear forces acting on a body is measured to be 580 N, making 35° with the direction of flow. Determine the drag and the lift forces acting on the body. FR
The drag coefficient of a car at the design conditions of 1 atm, 25°C, and 90 km/h is to be determined experimentally in a large wind tunnel in a full-scale test. The height and width of the car are
In general, how does the drag coefficient vary with the Reynolds number at(a) Low and moderate Reynolds numbers(b) At high Reynolds numbers (Re > 104)?
What is drafting? How does it affect the drag coefficient of the drafted body?
What is flow separation? What causes it? What is the effect of flow separation on the drag coefficient?
What is the effect of streamlining on (a) friction drag (b) Pressure drag? Does the total drag acting on a body necessarily decrease as a result of streamlining? Explain.
What is the effect of surface roughness on the friction drag coefficient in laminar and turbulent flows?
What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils?
What is terminal velocity? How is it determined?
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
What is lift? What causes it? Does wall shear contribute to the lift?
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?
What is the difference between streamlined and bluff bodies? Is a tennis ball a streamlined or bluff body?
Name some applications in which a large drag is desired.
What is drag? What causes it? Why do we usually try to minimize it?

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