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engineering
heat and mass transfer fundamentals and applications

Questions and Answers of Heat And Mass Transfer Fundamentals And Applications

A 0.2 m × 0.2 m street sign surface has an absorptivity of 0.6 and an emissivity of 0.7. Solar radiation is incident on the street sign at a rate of 200 W/m2, and the surrounding quiescent air is at
Consider a 1.2-m-high and 2-m-wide glass window with a thickness of 6 mm, thermal conductivity k = 0.78 W/m · K, and emissivity ε = 0.9. The room and the walls that face the window are maintained
Determine the overall U-factor for a double-door type wood-framed double-pane window with 13-mm air space and metal spacers, and compare your result with the value listed in Table 9–6. The overall
Consider a 2-m-high electric hot-water heater that has a diameter of 40 cm and maintains the hot water at 60°C. The tank is located in a small room at 20°C whose walls and ceiling are at about the
Repeat Prob. 9–129 assuming the plate to be positioned horizontally with(a) Transistors facing up(b) Transistors facing down.Data from problem 129A group of 25 power transistors, dissipating 1.5 W
Skylights or “roof windows” are commonly used in homes and manufacturing facilities since they let natural light in during day time and thus reduce the lighting costs. However, they offer little
Repeat Prob. 9–135E for a circular horizontal duct of diameter 4 in. Evaluate air properties at a film temperature of 115°F and 1 atm pressure. Is this a good assumption?Data from problem 135The
Consider a hot, boiled egg in a spacecraft that is filled with air at atmospheric pressure and temperature at all times. Disregarding any radiation effect, will the egg cool faster or slower when the
A hot object suspended by a string is to be cooled by natural convection in fluids whose volume changes differently with temperature at constant pressure. In which fluid will the rate of cooling be
A spherical block of dry ice at -79°C is exposed to atmospheric air at 30°C. The general direction in which the air moves in this situation is(a) Horizontal(b) Up(c) Down(d) Recirculation around
The primary driving force for natural convection is(a) Shear stress forces(b) Buoyancy forces(c) Pressure forces(d) Surface tension forces(e) None of them
Consider a horizontal 0.7-m-wide and 0.85-m-long plate in a room at 30ºC. Top side of the plate is insulated while the bottom side is maintained at 0ºC. The rate of heat transfer from the room air
Write a computer program to optimize the spacing between the two glasses of a double-pane window. Assume the spacing is filled with dry air at atmospheric pressure. The program should evaluate the
Write a computer program to evaluate the variation of temperature with time of thin square metal plates that are removed from an oven at a specified temperature and placed vertically in a large room.
Two concentric cylinders of diameters Di = 30 cm and Do = 40 cm and length L = 5 m are separated by air at 1 atm pressure. Heat is generated within the inner cylinder uniformly at a rate of 1100
A vertical 0.9-m-high and 1.8-m-wide double-pane window consists of two sheets of glass separated by a 2.2-cm air gap at atmospheric pressure. If the glass surface temperatures across the air gap are
A horizontal 1.5-m-wide, 4.5-m-long double-pane window consists of two sheets of glass separated by a 3.5-cm gap filled with water. If the glass surface temperatures at the bottom and the top are
A vertical double-pane window consists of two sheets of glass separated by a 1.5-cm air gap at atmospheric pressure. The glass surface temperatures across the air gap are measured to be 278 K and 288
A 4-m-diameter spherical tank contains iced water at 0°C. The tank is thin-shelled and thus its outer surface temperature may be assumed to be same as the temperature of the iced water inside. Now
Consider a 0.3-m-diameter and 1.8-m-long horizontal cylinder in a room at 20°C. If the outer surface temperature of the cylinder is 40°C, the natural convection heat transfer coefficient is(a) 3.0
A 4-m-long section of a 5-cm-diameter horizontal pipe in which a refrigerant flows passes through a room at 20°C. The pipe is not well insulated and the outer surface temperature of the pipe is
An average person generates heat at a rate of 84 W while resting. Assuming one-quarter of this heat is lost from the head and disregarding radiation, determine the average surface temperature
In an experiment, the temperature of a hot air stream is to be measured by a thermocouple with a spherical junction. Due to the nature of this experiment, the response time of the thermocouple to
Repeat Prob. 7–106 by solving it for staggered arrangement with SL = ST = 1.5 cm, and compare the performance of the evaporator for the in-line and staggered arrangements.Data from problem 106Air
Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at 220°C. Air
Exhaust gases at 1 atm and 300°C are used to preheat water in an industrial facility by passing them over a bank of tubes through which water is flowing at a rate of 6 kg/s. The mean tube wall
Repeat Prob. 7–103 for in-line arrangement with SL = ST = 6 cm.Data from problem 103Air is to be heated by passing it over a bank of 3-m-long tubes inside which steam is condensing at 100°C. Air
Air is to be heated by passing it over a bank of 3-m-long tubes inside which steam is condensing at 100°C. Air approaches the tube bank in the normal direction at 20°C and 1 atm with a mean
Water at 15°C is to be heated to 65°C by passing it over a bundle of 4-m-long, 1-cm-diameter resistance heater rods maintained at 90°C. Water approaches the heater rod bundle in normal direction
A thermocouple with a spherical junction diameter of 1 mm is used for measuring the temperature of a hydrogen gas stream. The properties of the thermocouple junction are k = 35 W/m · K, ρ = 8500
A glass (k = 1.1 W/m · K) spherical tank is filled with chemicals undergoing exothermic reaction. The reaction keeps the inner surface temperature of the tank at 80°C. The tank has an inner radius
A 0.2 m × 0.2 m street sign surface has an absorptivity of 0.6 and an emissivity of 0.7, while the street sign is subjected to a cross flow wind at 20°C with a velocity of 1 m/s. Solar radiation is
Reconsider Prob. 7–97. Using the EES (or other) software, evaluate the hot air velocity on the convection heat transfer coefficient. By varying the hot air velocity from 0.15 to 0.35 m/s, plot the
In flow across tube banks, how does the heat transfer coefficient vary with the row number in the flow direction? How does it vary with in the transverse direction for a given row number?
In flow across tube banks, why is the Reynolds number based on the maximum velocity instead of the uniform approach velocity?
Reconsider Prob. 7–90. Using the EES (or other) software, evaluate the effect of the hot air velocity on the thermocouple junction diameter that would satisfy the required response time of 5 s. By
Air (1 atm, 5°C) with free stream velocity of 2 m/s flows in parallel to a stationary thin 1 m × 1 m flat plate over the top and bottom surfaces. The flat plate has a uniform surface temperature of
An array of electrical heating elements is used in an air-duct heater as shown in Fig. P7–130. Each element has a length of 250 mm and a uniform surface temperature of 350°C. Atmospheric air
For laminar flow of a fluid along a flat plate, one would expect the largest local convection heat transfer coefficient for the same Reynolds and Prandl numbers when(a) The same temperature is
Air at 20°C flows over a 4-m long and 3-m wide surface of a plate whose temperature is 80°C with a velocity of 5 m/s. The length of the surface for which the flow remains laminar is(a) 1.5 m(b) 1.8
Air at 20°C flows over a 4-m-long and 3-m-wide surface of a plate whose temperature is 80°C with a velocity of 5 m/s. The rate of heat transfer from the laminar flow region of the surface is(a) 950
Engine oil at 105°C flows over the surface of a flat plate whose temperature is 15°C with a velocity of 1.5 m/s. The local drag force per unit surface area 0.8 m from the leading edge of the plate
Air (k = 0.028 W/m. K, Pr = 0.7) at 50°C flows along a 1-m-long flat plate whose temperature is maintained at 20°C with a velocity such that the Reynolds number at the end of the plate is 10,000.
Air at 15°C flows over a flat plate subjected to a uniform heat flux of 300 W/m2 with a velocity of 3.5 m/s. The surface temperature of the plate 6 m from the leading edge is(a) 1648C(b) 68.38C(c)
Air at 20°C flows over a 4-m-long and 3-m-wide surface of a plate whose temperature is 80°C with a velocity of 5 m/s. The rate of heat transfer from the surface is(a) 7383 W(b) 8985 W(c) 11,231
Water at 75°C flows over a 2-m-long, 2-m-wide surface of a plate whose temperature is 5°C with a velocity of 1.5 m/s. The total drag force acting on the plate is(a) 2.8 N(b) 12.3 N(c) 13.7 N(d)
Air at 25°C flows over a 5-cm-diameter, 1.7-m-long smooth pipe with a velocity of 4 m/s. A refrigerant at -15°C flows inside the pipe and the surface temperature of the pipe is essentially the same
Air at 25°C flows over a 5-cm-diameter, 1.7-m-long pipe with a velocity of 4 m/s. A refrigerant at -15°C flows inside the pipe and the surface temperature of the pipe is essentially the same as the
Combustion air in a manufacturing facility is to be preheated before entering a furnace by hot water at 90°C flowing through the tubes of a tube bank located in a duct. Air enters the duct at 15°C
Repeat Prob. 7–108 for staggered arrangement with SL = ST = 6 cm.Data from problem 108Combustion air in a manufacturing facility is to be preheated before entering a furnace by hot water at 90°C
Air at 15°C and 1 atm flows over a 0.3-m-wide plate at 65°C at a velocity of 3.0 m/s. Compute the following quantities at x = xcr:(a) Hydrodynamic boundary layer thickness, m(b) Local friction
Reconsider Prob. 8–89. Using EES (or other) software, investigate the effect of air velocity and the surface emissivity on the exit temperature of air and the rate of heat loss. Let the air
Repeat Prob. 8–91 for a circular horizontal duct of 15-cm diameter.Data from problem 91A concentric annulus tube has inner and outer diameters of 25 mm and 100 mm, respectively. Liquid water flows
Liquid water flows at a mass flow rate of 0.7 kg/s through a concentric annulus tube with the inlet and outlet mean temperatures of 20°C and 80°C, respectively. The concentric annulus tube has
Reconsider Prob. 8–84. Using the EES (or other) software, evaluate the effect of the liquid ammonia mass flow rate on needed insulation thickness for keeping the outer surface temperature at 20°C
Reconsider Prob. 8–87. Using EES (or other) software, investigate the effect of the volume flow rate of air on the exit temperature of air and the rate of heat loss. Let the flow rate vary from
Wind at 30°C flows over a 0.5-m-diameter spherical tank containing iced water at 0°C with a velocity of 25 km/h. If the tank is thin-shelled with a high thermal conductivity material, the rate at
Ambient air at 20°C flows over a 30-cm-diameter hot spherical object with a velocity of 2.5 m/s. If the average surface temperature of the object is 200°C, the average convection heat transfer
Kitchen water at 10°C flows over a 10-cm-diameter pipe with a velocity of 1.1 m/s. Geothermal water enters the pipe at 90°C at a rate of 1.25 kg/s. For calculation purposes, the surface temperature
Jakob (1949) suggests the following correlation be used for square tubes in a liquid cross-flow situation:Water (k = 0.61 W/m · K, Pr = 6) at 50°C flows across a 1-cm square tube with a Reynolds
Air (Pr = 0.7, k = 0.026 W/m . K) at 200°C flows across 2-cm-diameter tubes whose surface temperature is 50°C with a Reynolds number of 8000. The Churchill and Bernstein convective heat transfer
Jakob (1949) suggests the following correlation be used for square tubes in a liquid cross-flow situation:Water (k = 0.61 W/m · K, Pr = 6) flows across a 1-cm square tube with a Reynolds number of
On average, super insulated homes use just 15 percent of the fuel required to heat the same size conventional home built before the energy crisis in the 1970s. Write an essay on super insulated
The decision of whether to invest in an energy-saving measure is made on the basis of the length of time for it to pay for itself in projected energy (and thus cost) savings. The easiest way to reach
What do the average velocity Vavg and the mean temperature Tm represent in flow through circular tubes of constant diameter?
What is the generally accepted value of the Reynolds number above which the flow in smooth pipes is turbulent?
What fluid property is responsible for the development of the velocity boundary layer? For what kinds of fluids will there be no velocity boundary layer in a pipe?
In the fully developed region of flow in a circular tube, will the velocity profile change in the flow direction? How about the temperature profile?
Consider laminar flow in a circular tube. Will the friction factor be higher near the inlet of the tube or near the exit? Why? What would your response be if the flow were turbulent?
How does the friction factor f vary along the flow direction in the fully developed region in(a) Laminar flow and(b) Turbulent flow?
Consider the flow of oil in a tube. How will the hydrodynamic and thermal entry lengths compare if the flow is laminar? How would they compare if the flow were turbulent?
Consider the velocity and temperature profiles for airflow in a tube with diameter of 8 cm can be expressed as with units in m/s and K, respectively. If the convection heat transfer coefficient is
Consider the velocity and temperature profiles for a fluid flow in a tube with diameter of 50 mm can be expressed aswith units in m/s and K, respectively. Determine the average velocity and the mean
Consider the flow of mercury (a liquid metal) in a tube. How will the hydrodynamic and thermal entry lengths compare if the flow is laminar? How would they compare if the flow were turbulent?
Repeat Prob. 8–22 for a heat transfer coefficient of 40 W/m2?K.Data from problem 22Combustion gases passing through a 3-cm-internaldiameter circular tube are used to vaporize waste water at
Repeat Prob. 8–24 for steam condensing at a rate of 0.60 kg/s.Data from problem 24Cooling water available at 108C is used to condense steam at 30°C in the condenser of a power plant at a rate of
Reconsider Prob. 8–27. Using the EES (or other) software, evaluate the effect of the cooling water mean velocity on the rate of steam condensation in the condenser. By varying the cooling water
Reconsider Prob. 8–24. Using the EES (or other) software, investigate the effect of the cooling water average (mean) velocity on the number of tubes needed to achieve the indicated heat transfer
Reconsider Prob. 8–29. Using the EES (or other) software, evaluate the effect of the tube length on the average convection heat transfer coefficient of air. By varying the tube length from 3 to 18
How is the friction factor for flow in a tube related to the pressure drop? How is the pressure drop related to the pumping power requirement for a given mass flow rate?
Consider fully developed laminar flow in a circular pipe. If the diameter of the pipe is reduced by half while the flow rate and the pipe length are held constant, the pressure drop will(a)
The velocity profile in fully developed laminar flow in a circular pipe of inner radius R = 10 cm, in m/s, is given by u(r) = 4(1 - r2/R2). Determine the mean and maximum velocities in the pipe, and
Consider fully developed laminar flow in a circular pipe. If the viscosity of the fluid is reduced by half by heating while the flow rate is held constant, how will the pressure drop change?
In fully developed laminar flow inside a circular pipe, the velocities at r = 0.5R (midway between the wall surface and the centerline) are measured to be 3, 6, and 9 m/s.(a) Determine the maximum
Water is flowing in fully developed conditions through a 3-cm-diameter smooth tube with a mass flow rate of 0.02 kg/s at 15°C. Determine(a) The maximum velocity of the flow in the tube and(b) The
Water at 15°C is flowing through a 5-cm-diameter smooth tube with a length of 200 m. Determine the Darcy friction factor and pressure loss associated with the tube for(a) Mass flow rate of 0.02
Water at 10°C (ρ = 999.7 kg/m3 and μ = 1.307 × 10-3 kg/m?s) is flowing in a 0.20-cm-diameter 15-m-long pipe steadily at an average velocity of 1.2 m/s. Determine(a) The pressure drop(b) The
Determine the hydrodynamic and thermal entry lengths for water, engine oil, and liquid mercury flowing through a 2.5-cm diameter smooth tube with mass flow rate of 0.01 kg/s and temperature of 1008C.
Determine the average velocity, hydrodynamic and thermal entry lengths for water, engine oil, and liquid mercury flowing through a standard 2-in Schedule 40 pipe with mass flow rate of 0.1 lbm/s and
An engineer is to design an experimental apparatus that consists of a 25-mm-diameter smooth tube, where different fluids at 100°C are to flow through in fully developed laminar flow conditions. For
In a chemical process plant, liquid isobutane at 508F is being transported through a 30-ft-long standard 3/4-in Schedule 40 cast iron pipe with a mass flow rate of 0.4 lbm/s. Accuracy of the results
Water at 15°C is flowing through a 200-m-long standard 1-in Schedule 40 cast iron pipe with a mass flow rate of 0.5 kg/s. If accuracy is an important issue, use the appropriate equation to
Water at 15°C (ρ = 999.1 kg/m3 and m = 1.138 × 10-3 kg/m? s) is flowing in a 4-cm-diameter and 25-m-long horizontal pipe made of stainless steel steadily at a rate of 7 L/s. Determine(a) The
Reconsider Prob. 8–52. Using the EES (or other) software, investigate the effect of the pipe diameter on the pumping power requirement to overcome the pressure loss. By varying the pipe diameter
Consider a fluid with mean inlet temperature Ti flowing through a tube of diameter D and length L, at a mass flow rate ṁ. The tube is subjected to a surface heat flux that can be expressed as
Repeat Prob. 8–58 by replacing air with helium, which has six times the thermal conductivity of air. Evaluate properties of helium from EES at a bulk mean temperature of 25°C. Is this a good
A fluid is flowing in fully developed laminar conditions in a tube with diameter D and length L at a mass flow rate ṁ. The tube is subjected to a surface heat flux that can be expressed as q̇s(x)
Reconsider Prob. 8–58. Using EES (or other) software, investigate the effects of air velocity at the inlet of the channel and the maximum surface temperature on the maximum total power dissipation
Reconsider Prob. 8–62. Using the EES (or other) software, evaluate the effect of water mass flow rate on the free-stream H2 gas velocity and the surface temperature of the parallel plates, and the
Consider the flow of oil at 108C in a 40-cm-diameter pipeline at an average velocity of 0.5 m/s. A 1500-m-long section of the pipeline passes through icy waters of a lake at 0°C. Measurements

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