- Reconsider Prob. 13–34. Using the EES (or other) software, evaluate the effect of the distance L between the black coaxial parallel disks (D = 1 m) on the radiation heat transfer coefficient. By
- Pure hydrogen gas, at 25°C, is flowing through an iron pipe with an inner diameter of 25 mm. The iron pipe is 10 m long and has a wall thickness of 5 mm. If the concentration of hydrogen in the iron
- Reconsider Prob. 14–56. Using the EES (or other) software, evaluate the effect of pipe wall thickness on the diffusion rate of hydrogen through the pipe wall. By varying the pipe wall thickness
- Helium gas is stored at 293 K in a 3-m-outer-diameter spherical container made of 5-cm-thick Pyrex. The molar concentration of helium in the Pyrex is 0.00073 kmol/m3 at the inner surface and
- Pure helium gas is stored at 20°C in the cylindrical Pyrex tank of 2 m in length with an inner diameter of 12 cm and a wall thickness of 2.5 mm. A helium detector situated beside the tank has
- Hydrogen can cause fire hazards, and hydrogen gas leaking into surrounding air can lead to spontaneous ignition with extremely hot flames. Even at very low leakage rate, hydrogen can sustain
- Pure N2 gas at 1 atm and 25°C is flowing through a 10-m-long, 3-cm-inner diameter pipe made of 2-mm-thick rubber. Determine the rate at which N2 leaks out of the pipe if the medium surrounding the
- Helium gas is stored at 293 K and 500 kPa in a 1-cmthick, 2-m-inner-diameter spherical tank made of fused silica (SiO2). The area where the container is located is well ventilated. Determine(a) The
- You probably have noticed that balloons inflated with helium gas rise in the air the first day during a party but they fall down the next day and act like ordinary balloons filled with air. This is
- Reconsider the balloon discussed in Prob. 14–63. Assuming the volume to remain constant and disregarding the diffusion of air into the balloon, obtain a relation for the variation of pressure in
- What are the adverse effects of excess moisture on the wood and metal components of a house and the paint on the walls?
- How does the condensation or freezing of water vapor in the wall affect the effectiveness of the insulation in the wall? How does the moisture content affect the effective thermal conductivity of
- Moisture migration in the walls, floors, and ceilings of buildings is controlled by vapor barriers or vapor retarders. Explain the difference between the two, and discuss which is more suitable for
- A glass of milk left on top of a counter in the kitchen at 15°C, 88 kPa, and 50 percent relative humidity is tightly sealed by a sheet of 0.009-mm-thick aluminum foil whose permeance is 2.9 ×
- Why are the insulations on the chilled water lines always wrapped with vapor barrier jackets?
- Express the mass flow rate of water vapor through a wall of thickness L in terms of the partial pressure of water vapor on both sides of the wall and the permeability of the wall to the water vapor.
- Consider a 20-cm-thick brick wall of a house. The indoor conditions are 25°C and 50 percent relative humidity while the outside conditions are 50°C and 50 percent relative humidity. Assuming that
- The roof of a house is 15 m × 8 m and is made of a 20-cm-thick concrete layer. The interior of the house is maintained at 25°C and 50 percent relative humidity and the local atmospheric pressure is
- Reconsider Prob. 14–74. Using EES (or other) software, investigate the effects of temperature and relative humidity of air inside the house on the amount of water vapor that will migrate through
- The diffusion of water vapor through plaster boards and its condensation in the wall insulation in cold weather are of concern since they reduce the effectiveness of insulation. Consider a house that
- Reconsider Prob. 14–74. In order to reduce the migration of water vapor, the inner surface of the wall is painted with vapor retarder latex paint whose permeance is 26 × 10-12 kg/s · m2 · Pa.
- Reconsider Prob. 14–77. In order to reduce the migration of water vapor through the wall, it is proposed to use a 0.051-mm-thick polyethylene film with a permeance of 9.1 × 10-12 kg/s·m2 · Pa.
- In transient mass diffusion analysis, can we treat the diffusion of a solid into another solid of finite thickness (such as the diffusion of carbon into an ordinary steel component) as a diffusion
- When the density of a species A in a semi-infinite medium is known at the beginning and at the surface, explain how you would determine the concentration of the species A at a specified location and
- A thick wall made of natural rubber is exposed to pure oxygen gas on one side of its surface. Both the wall and oxygen gas are isothermal at 25°C, and the oxygen concentration at the wall surface is
- Define the penetration depth for mass transfer, and explain how it can be determined at a specified time when the diffusion coefficient is known.
- Reconsider Prob. 14–82. Using the EES (or other) software, evaluate the oxygen concentration with varying time and distance from the wall surface. By varying the distance from the wall surface for
- A thick nickel wall is exposed to pure hydrogen gas at 165°C on one side of its surface. The hydrogen concentration at the wall surface is constant. Determine the hydrogen concentration at the
- Reconsider Prob. 14–85. Using the EES (or other) software, evaluate the hydrogen concentration with varying time and distance from the wall surface. By varying the elapsed time for 0 ≤ t ≤
- Consider a piece of steel undergoing a decarburization process at 925°C. The mass diffusivity of carbon in steel at 925°C is 1 × 10-7 cm2/s. Determine the depth below the surface of the steel at
- A layer of glucose is submerged under a deep layer of water at 25°C. As the layer of glucose dissolves in the water, its density at the glucose-water interface is maintained constant. Determine(a)
- A long nickel bar with a diameter of 5 cm has been stored in a hydrogen-rich environment at 358 K and 300 kPa for a long time, and thus it contains hydrogen gas throughout uniformly. Now the bar is
- When handling corrosive and toxic substances, chemical resistant gloves should be worn. When selecting gloves to handle a substance, the suitability of the gloves should be considered. Depending on
- A steel part whose initial carbon content is 0.12 percent by mass is to be case-hardened in a furnace at 1150 K by exposing it to a carburizing gas. The diffusion coefficient of carbon in steel is
- Repeat Prob. 14–90 for a furnace temperature of 500 K at which the diffusion coefficient of carbon in steel is DAB= 2.1 × 10-20 m2/s.Data from problem 90A steel part whose initial carbon content
- A pond with an initial oxygen content of zero is to be oxygenated by forming a tent over the water surface and filling the tent with oxygen gas at 25°C and 130 kPa. Determine the mole fraction of
- A heated piece of steel, with a uniform initial carbon concentration of 0.20% by mass, was exposed to a carburizing atmosphere for an hour. Throughout the entire process, the carbon concentration on
- A large pond, at a temperature of 25°C, has an initial uniform oxygen density of 2 kg/m3. During an aeration process, the pond surface has its oxygen density suddenly increased to 9 kg/m3. Determine
- What is diffusion velocity? How does it affect the mass-average velocity? Can the velocity of a species in a moving medium relative to a fixed reference point be zero in a moving medium? Explain.
- What is the difference between mass-average velocity and mole-average velocity during mass transfer in a moving medium? If one of these velocities is zero, will the other also necessarily be zero?
- Define the following terms: mass-average velocity, diffusion velocity, stationary medium, and moving medium.
- Consider one-dimensional mass transfer in a moving medium that consists of species A and B with ρ = ρA + ρB = constant. Mark these statements as being True or False._____(a) The rates of mass
- A researcher is using a 5-cm-diameter Stefan tube to measure the mass diffusivity of chloroform in air at 25°C and 1 atm. Initially, the liquid chloroform surface was 7.00 cm from the top of the
- What is Stefan flow? Write the expression for Stefan’s law and indicate what each variable represents.
- A 1-in-diameter Stefan tube is used to measure the binary diffusion coefficient of water vapor in air at 80°F and 13.8 psia. The tube is partially filled with water with a distance from the water
- The mass diffusivity of ethanol (ρ = 789 kg/m3 and M = 46 kg/kmol) through air was determined in a Stefan tube. The tube has a uniform cross-sectional area of 0.8 cm2. Initially, the ethanol surface
- Methanol (ρ = 791 kg/m3 and M = 32 kg/kmol) undergoes evaporation in a vertical tube with a uniform cross-sectional area of 0.8 cm2. At the top of the tube, the methanol concentration is zero, and
- Benzene is classified by the U.S. Department of Health and Human Services (DHHS) as a carcinogen. Exposure to benzene increases the risk of cancer and other illnesses in humans. The Occupational
- An 8-cm-internal-diameter, 30-cm-high pitcher halffilled with water is left in a dry room at 15°C and 87 kPa with its top open. If the water is maintained at 15°C at all times also, determine how
- A tank with a 2-cm-thick shell contains hydrogen gas at the atmospheric conditions of 25°C and 90 kPa. The charging valve of the tank has an internal diameter of 3 cm and extends 8 cm above the
- Reconsider Prob. 14–105. Using EES (or other) software, plot the mass flow rate of hydrogen lost as a function of the diameter of the charging valve as the diameter varies from 1 cm to 5 cm, and
- The pressure in a pipeline that transports helium gas at a rate of 5 lbm/s is maintained at 14.5 psia by venting helium to the atmosphere through a 0.25-in-internal-diameter tube that extends 30 ft
- Repeat Prob. 14–109E for a pipeline that transports carbon dioxide instead of helium.Data from problem 109 The pressure in a pipeline that transports helium gas at a rate of 5 lbm/s is maintained
- A large tank containing ammonia at 1 atm and 25°C is vented to the atmosphere through a 2-m-long tube whose internal diameter is 1.5 cm. Determine the rate of loss of ammonia and the rate of
- What is a concentration boundary layer? How is it defined for flow over a plate?
- What is the physical significance of the Schmidt number? How is it defined? To what dimensionless number does it correspond in heat transfer? What does a Schmidt number of 1 indicate?
- What is the physical significance of the Lewis number? How is it defined? What does a Lewis number of 1 indicate?
- Under what conditions will the normalized velocity, thermal, and concentration boundary layers coincide during flow over a flat plate?
- Heat convection is expressed by Newton’s law of cooling as Q̇ = hAs (Ts - T∞). Express mass convection in an analogous manner on a mass basis, and identify all the quantities in the expression
- What is the physical significance of the Sherwood number? How is it defined? To what dimensionless number does it correspond in heat transfer? What does a Sherwood number of 1 indicate for a plane
- In natural convection mass transfer, the Grashof number is evaluated using density difference instead of temperature difference. Can the Grashof number evaluated this way be used in heat transfer
- What is the relation (f/2) Re = Nu = Sh known as? Under what conditions is it valid? What is the practical importance of it?
- What is the name of the relation f/2 = St Pr2/3 = StmassSc2/3 and what are the names of the variables in it? Under what conditions is it valid? What is the importance of it in engineering?
- Using the analogy between heat and mass transfer, explain how the mass transfer coefficient can be determined from the relations for the heat transfer coefficient.
- What is the relation hheat = ρcphmass known as? For what kind of mixtures is it valid? What is the practical importance of it?
- What is the low mass flux approximation in mass transfer analysis? Can the evaporation of water from a lake be treated as a low mass flux process?
- Consider a flat plate made of rubber separates carbon dioxide gas at 25°C and air. On the surface exposed to air, there is mass convection. Determine the mass concentration gradient of carbon
- Consider a wet concrete patio is covered with a thin layer of water film. At the surface, mass convection of water to air occurs at an average mass transfer coefficient of 0.03 m/s. If the air is at
- A fluid with a free stream velocity of 2 m/s is flowing over a surface of a body with characteristic length of 2 m. The flow conditions are maintained at 298 K and 1 atm, and the average mass
- The average heat transfer coefficient for air flow over an odd-shaped body is to be determined by mass transfer measurements and using the Chilton–Colburn analogy between heat and mass transfer.
- Dry air at 15°C and 85 kPa flows over a 2-m-long wet surface with a free stream velocity of 3 m/s. Determine the average mass transfer coefficient. Dry air 15°C, 85 kPa 3 m/s Wet Evaporation
- Local convection heat transfer coefficient for air flowing parallel over a 1-m-long plate with irregular surface topology is experimentally determined to be hx = 0.5 + 12x – 0.7x3, where hx is in
- Ethyl alcohol is spilt and spread over a flat table into a layer of approximately 1 m in diameter wide. Dry air at 25°C and 1 atm flows over the ethyl alcohol surface with a free stream velocity of
- In a manufacturing facility, 40 cm × 40 cm wet brass plates coming out of a water bath are to be dried by passing them through a section where dry air at 1 atm and 25°C is blown parallel to their
- A thin slab of solid salt (NaCl), with dimension of 0.15 m × 0.15 m, is being dragged through seawater (v = 1.022 × 10–6 m2/s) at an average relative velocity of 0.6 m/s. The seawater at 18°C
- Air at 80°F, 1 atm, and 30 percent relative humidity is blown over the surface of a 15-in × 15-in square pan filled with water at a free stream velocity of 10 ft/s. If the water is maintained at a
- Repeat Prob. 14–132E for temperature of 60°F for both the air and water.Data from problem 132Air at 80°F, 1 atm, and 30 percent relative humidity is blown over the surface of a 15-in × 15-in
- Benzene (M = 78.11 kg/kmol) is a carcinogen, and exposure to benzene increases the risk of cancer and other illnesses in humans. A truck transporting liquid benzene was involved in an accident that
- Consider a 5-m × 5-m wet concrete patio with an average water film thickness of 0.3 mm. Now wind at 50 km/h is blowing over the surface. If the air is at 1 atm, 15°C, and 35 percent relative
- Air at 40°C and 1 atm flows over a 5-m-long wet plate with an average velocity of 2.5 m/s in order to dry the surface. Using the analogy between heat and mass transfer, determine the mass transfer
- A 2-in-diameter spherical naphthalene ball is suspended in a room at 1 atm and 80°F. Determine the average mass transfer coefficient between the naphthalene and the air if air is forced to flow over
- Consider a 3-mm-diameter raindrop that is falling freely in atmospheric air at 25°C. Taking the temperature of the raindrop to be 9°C, determine the terminal velocity of the raindrop at which the
- Consider a circular pipe of inner diameter D = 0.7 in. whose inner surface is covered with a thin layer of liquid water as a result of condensation. In order to dry the pipe, air at 540 R and 1 atm
- Consider a 15-cm-internal-diameter, 10-m-long circular duct whose interior surface is wet. The duct is to be dried by forcing dry air at 1 atm and 15°C through it at an average velocity of 3 m/s.
- Reconsider Prob. 14–140. Using EES (or other) software, plot the mass transfer coefficient as a function of the air velocity as the velocity varies from 1 m/s to 8 m/s, and discuss the results.Data
- Does a mass transfer process have to involve heat transfer? Describe a process that involves both heat and mass transfer.
- During evaporation from a water body to air, under what conditions will the latent heat of vaporization be equal to convection heat transfer from the air?
- Jugs made of porous clay were commonly used to cool water in the past. A small amount of water that leaks out keeps the outer surface of the jug wet at all times, and hot and relatively dry air
- Consider a shallow body of water. Is it possible for this water to freeze during a cold and dry night even when the ambient air and surrounding surface temperatures never drop to 0°C? Explain.
- Reconsider Prob. 14–146. Using EES (or other) software, plot the water temperature as a function of the relative humidity of air as the relative humidity varies from 10 to 100 percent, and discuss
- During a hot summer day, a 2-L bottle drink is to be cooled by wrapping it in a cloth kept wet continually and blowing air to it with a fan. If the environment conditions are 1 atm, 80°F, and 30
- A water-soaked 10-cm × 10-cm-square sponge is experiencing parallel dry air flow over its surface. The average convection heat transfer coefficient of dry air at 20°C flowing over the sponge
- Consider a thin layer of liquid water on a concrete surface. The surrounding air is dry with a convection heat transfer coefficient of 50 W/m2 . K. The liquid water has an emissivity of 0.95, and the
- A soaked sponge is experiencing dry air flowing over its surface. The air is at 1 atm and zero relative humidity. Determine the difference in the air temperature and the surface temperature of the
- One way of increasing heat transfer from the head on a hot summer day is to wet it. This is especially effective in windy weather, as you may have noticed. Approximating the head as a 30-cm-diameter
- A 2-m-deep 20-m × 20-m heated swimming pool is maintained at a constant temperature of 30°C at a location where the atmospheric pressure is 1 atm. If the ambient air is at 20°C and 60 percent
- Repeat Prob. 14–152 for a pool temperature of 25°C.Data from problem 152A 2-m-deep 20-m × 20-m heated swimming pool is maintained at a constant temperature of 30°C at a location where the
- A glass bottle washing facility uses a well agitated hot water bath at 50°C with an open top that is placed on the ground. The bathtub is 1 m high, 2 m wide, and 4 m long and is made of sheet metal
- Determine the mole fraction of the water vapor at the surface of a lake whose temperature is 15°C and compare it to the mole fraction of water in the lake. Take the atmospheric pressure at lake
- Repeat Prob. 14–154 for a water bath temperature of 55°C.Data from problem 154A glass bottle washing facility uses a well agitated hot water bath at 50°C with an open top that is placed on the