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engineering
mechanical engineering

Questions and Answers of Mechanical Engineering

In Problem 14.37, NO2 transport by diffusion in a stagnant atmosphere was considered for steady-state conditions. However, the problem is actually time dependent, and a more realistic approach would
A large sheet of material 40 mm thick contains dissolved hydrogen (H2) having a uniform concentration of 3k mol/m3. The sheet is exposed to a fluid stream that causes the concentration of the
A common procedure for increasing the moisture content of air is to bubble it through a column of water. Assume the air bubbles to be spheres of radius r0 = 1 mm and to be in thermal equilibrium with
Steel is carburized in a high-temperature process that depends on the transfer of carbon by diffusion. The value of the diffusion coefficient is strongly temperature dependent and may be approximated
A thick plate of pure iron at 1000°C is subjected to a carburization process in which the surface of the plate is suddenly exposed to a gas that induces a carbon concentration Cc,s at one surface.
A solar pond operates on the principle that heat losses from a shallow layer of water, which acts as a solar absorber, may be minimized by establishing a stable vertical salinity gradient in the
If an amount of energy Q''o (J/m2) is released instantaneously, as, for example, from a pulsed laser, and it is absorbed by the surface of a semi-infinite medium, with no attendant losses to the
The presence of CO2 in solution is essential to the growth of aquatic plant life, with CO2 used as a reactant in the photosynthesis. Consider a stagnant body of water in which the concentration of
Hydrogen gas is used in a process to manufacture a sheet material of 6-mm thickness. At the end of the process, H2 remains in solution in the material with a uniform concentration of 320 k mol/m3. To
Consider the hydrogen-removal process described in Problem 14.46, but under conditions for which the mass diffusivity of the hydrogen gas (A) in the sheet material (B) is DAB = 1.8 x 10-11 m2 /s
A 1-mm-thick square (100 mm x 100 mm) sheet of polymer is suspended from a precision scale in a chamber characterized by a temperature and relative humidity of T = 300 K and cp = 0, respectively.
Consider a DVD similar to that of Problem 5.86. To protect sensitive information within the storage medium, a very thin film of reactive polymer is embedded within the polycarbonate at a distance d =
Consider the DVD of Problem 14.49, except now the reacting polymer is blended uniformly with the polycarbonate to reduce manufacturing costs. Assume that a first-order homogeneous chemical reaction
A vitreous silica optical fiber of diameter 100μm is used to send optical signals from a sensor placed deep inside a hydrogen chamber. The hydrogen is at a pressure of 20 bars. The mass diffusivity
The surface of glass quickly develops very small micro cracks when exposed to high humidity. Although micro cracks can be safely ignored in most applications, they can significantly decrease the
A person applies an insect repellent onto an exposed area of A = 0.5 m2 of their body. The mass of spray used is M = 10grams and the spray contains 25% (by mass) active ingredient. The inactive
Show that the coefficient of thermal expansion for an ideal gas is 1/T, where T is the absolute temperature.GIVENIdeal gasAbsolute temperature = T
Calculate the coefficient of thermal expansion, β, for saturated water at 403 K from its definition and property values in Appendix 2, Table 13. Then compare your results with the value in the
Calculate the coefficient of thermal expansion, β, from its definition for steam at 450?C and pressures of 0.1 atm and 10 atm from standard steam tables. Then compare your results with the value
A long cylinder of 0.1 m diameter has a surface temperature of 400 K. If it is immersed in a fluid at 350 K, natural convection will occur as a result of the temperature difference. Calculate the
For the conditions given in Problem 5.4, determine the Nusselt Number and the heat transfer coefficient from Figure.From Problem 5.4: A long cylinder of 0.1 m diameter has a surface temperature of
An empirical equation proposed for the heat transfer coefficient in natural convection from long vertical cylinders to air at atmospheric pressure is Where T = the film temperature = 1/2 (Ts + T??)
Solar One is the first large-scale (10 MW electric) solar-thermal electric-power-generating plant in the U.S. It is located near Barstow, CA. A schematic diagram of the receiver and tower is shown
Compare the rate of heat loss from a human body with the typical energy intake from consumption of food (1033 kcal/day). Model the body as a vertical cylinder 30 cm in diameter and 1.8 m high in
An electric room heater has been designed in the shape of a vertical cylinder 2 m tall and 30 cm in diameter. For safety, the heater surface cannot exceed 35?C. If the room air is 20?C, find the
Consider a design for a nuclear reactor using natural-convection heating of liquid bismuth. The reactor is to be constructed of parallel vertical plates 6 ft tall and 4 ft wide, in which heat is
A mercury bath at 60?C is to be heated by immersing cylindrical electric heating rods, each 20 cm tall and 2 cm in diameter. Calculate the maximum electric power rating of a typical rod if its
An electric heating blanket is subjected to an acceptance test. It is to dissipate 400 W on the high setting when hanging in air at 20?C. If the blanket is 1.3 m wide:(a) What is the length required
An aluminum sheet, 0.4 m tall, 1 m long, and 0.002 m thick is to be cooled from an initial temperature of 150?C to 50?C by immersing it suddenly in water at 20?C. The sheet is suspended from two
A 0.1 cm thick flat copper plate, 2.5 m x 2.5 m square is to be cooled in a vertical position. The initial temperature of the plate is 90?C with the ambient fluid at 30?C. The fluid medium is either
A laboratory apparatus is used to maintain a horizontal slab of ice at 28?F so that specimens can be prepared on the surface of the ice and kept close to 32?F. If the ice is 4 in. by 1.5 in and the
An electronic circuit board is the shape of a flat plate 0.3 m x 0.3 m in plan-form and dissipates 15 W. It may be placed in operation on an insulated surface in a horizontal position or at an angle
Cooled air is flowing through a long sheet metal air conditioning duct, 0.2 m high and 0.3 m wide. If the duct temperature is 10?C and passes through a crawl space under a house at 30?C, estimate(a)
Solar radiation at 600 W/m2 is absorbed by a black roof inclined at 30?C as shown. If the underside of the roof is well insulated, estimate the maximum roof temperature in 20?C air.GIVENInclined
A 1 m square copper plate is placed horizontally on 2 m high legs. The plate has been coated with a material that provides a solar absorptance of 0.9 and an infrared emittance of 0.25. If the air
A 2.5 x 2.5 m steel sheet 1.5 mm thick is removed from an annealing oven at a uniform temperature of 425?C and placed in a large room at 20?C in a horizontal position.(a) Calculate the rate of heat
A thin electronic circuit board, 0.1 m by 0.1 m in size, is to be cooled in air at 25?C. The board is placed in a vertical position and the back side is well insulated. If the heat dissipation is
A plot of coffee has been allowed to cool to 17?C. If the electrical coffee maker is turned back on, the hot plate on which the pot rests is brought up to 70?C immediately and held at that
A laboratory experiment has been performed to determine the natural-convection heat transfer correlation for a horizontal cylinder of elliptical cross section in air. The cylinder is 1 m long, has a
A long, 2-cm-OD horizontal copper pipe carries dry saturated steam at 1.2 atm absolute pressure. The pipe is contained within an environmental testing chamber in which the ambient air pressure can be
Compare the rate of condensate flow from the pipe in Problem 5.24 (air pressure = 2.0 atm) with that for a 3.89-cm-OD pipe and 2.0 atm air pressure. What is the rate of condensate flow if the 2 cm
A thermocouple (0.8 mm OD) is located horizontally in a large enclosure whose walls are at 37?C. The enclosure is filled with a transparent quiescent gas which has the same properties as air. The
Only 10 percent of the energy dissipated by the tungsten filament of an incandescent lamp is in the form of useful visible light. Consider a 100 W lamp with a 10 cm spherical glass bulb. Assuming an
A sphere 20 cm in diameter containing liquid air (??140?C) is covered with 5 cm thick glass wool (50 kg/m3 density) with an emissivity of 0.8. Estimate the rate of heat transfer to the liquid air
A 2-cm-OD bare aluminum electric power transmission line with an emissivity of 0.07 carries 500 amps at 400 kV. The wire has an electrical resistivity of 1.72 micro-ohms cm2/cm at 20?C and is
An 8-in.-OD horizontal steam pipe carries 220 lbm/h dry saturated steam at 250°F. If ambient air temperature is 70°F, determine the rate of condensate flow at the end of 10 ft of pipe. Use an
A long steel rod (2 cm in diameter, 2 m long) has been heat-treated and quenched to a temperature of 100°C in an oil bath. In order to cool the rod further it is necessary to remove it from the bath
In petroleum processing plants, it is often necessary to pump highly viscous liquids such as asphalt through pipes. In order to keep pumping costs within reason, the pipelines are electrically heated
Estimate the rate of convective heat transfer across a 1 m tall double-pane window assembly in which the outside pane is at 0°C and the inside pane is at 20°C. The panes are spaced 2.5 cm apart.
An architect is asked to determine the heat loss through a wall of a building constructed as shown in the sketch. If the wall spacing is 10 cm, the inner surface is at 20°C and the outer surface is
A flat plate solar collector of 3 m x 5 m area has an absorber plate that is to operate at a temperature of 70?C. To reduce heat losses, a glass cover is placed 0.05 m from the absorber and its
Determine the rate of heat loss through a double glazed window, as shown in the sketch, if the inside room temperature is 65?C and the average outside air is 0?C during December. Neglect the effect
Calculate the rate of heat transfer between a pain of concentric horizontal cylinders 20 mm and 126 mm in diameter. The inner cylinder is maintained at 37°C and the outer cylinder is maintained at
Two long concentric horizontal aluminum tubes of 0.2 m and 0.25 m diameter are maintained at 300 K and 400 K respectively. The space between the tubes is filled with nitrogen. If the surfaces of the
A solar collector design consists of several parallel tubes each enclosed concentrically in an outer tube which is transparent to solar radiation. The tubes are thin walled with diameter of the inner
Liquid oxygen at ?? 183?C is stored in a thin walled spherical container with an outside diameter of 2 m. This container is surrounded by another sphere of 2.5 m inside diameter to reduce heat loss.
The surfaces of two concentric spheres having radii of 75 and 100 mm are maintained at 325 K and 275 K, respectively.(a) If the space between the spheres is filled with nitrogen at 5 atm, estimate
Estimate the rate of heat transfer from one side of a 2 m diameter disk rotating at 600 rev/min in 20?C air, if its surface temperature is 50?C.GIVENA disk rotating in airDiameter (D) = 2 mRotational
A sphere 0.1 m diameter is rotating at 20 RPM in a large container of CO2 at atmospheric pressure. If the sphere is at 60?C and the CO2 at 20?C, estimate the rate of heat transfer.GIVENA rotating
A mild steal (1% carbon), 2 cm OD shaft, rotating in 20?C air at 20,000 rev/min, is attached to two bearings 0.7 m apart. If the temperature at the bearings is 90?C, determine the temperature
An electronic device is to be cooled in air at 20?C by an array of equally spaced vertical rectangular fins as shown in the sketch below. The fins are made of aluminum and their average temperature,
Consider a vertical 20 cm tall flat plate at 120?C suspended in a fluid at 100?C. If the fluid is being forced past the plate from above, estimate the fluid velocity for which natural convection
Suppose a thin vertical flat plate. 60 cm high and 40 cm wide, is immersed in a fluid flowing parallel to is surface. If the plate is at 40?C and the fluid at 10?C, estimate the Reynolds number at
A vertical isothermal plate 30 cmhigh is suspended in an atmosphere air streamflowing at 2 m/s in a vertical direction. If the air is at 16?C, estimate the plate temperature for which the
A horizontal disk 1 m in diameter rotates in air at 25?C. If the disk is at 100?C, estimate the RPM at which natural convection for a stationary disk becomes less than 10% of the heat transfer for a
The refrigeration system for an indoor ice rink is to be sized by an HVAC contractor. The refrigeration system has a COP (coefficient of performance) of 0.5. The ice surface is estimated to be ??2?C
A 0.15 m square circuit board is to be cooled in a vertical position as shown. The board is insulated on one side while on the other, 100 closely spaced square chips are mounted, each of which
A gas-fired industrial furnace is used to generate steam. The furnace is a 3 m cubic structure and the interior surfaces are completely covered with boiler tubes transporting pressurized wet steam at
An electronic device is to be cooled by natural convection in atmospheric air at 20?C. The device generates internally 50 W and only one of its external surfaces is suitable for attaching fins. The
To measure the mass flow rate of a fluid in a laminar flow through a circular pipe, a hot wire type velocity meter is placed in the center of the pipe. Assuming that the measuring station is far from
Nitrogen at 30?C and atmospheric pressure enter a triangular duct 0.02 m on each side at a rate of 4 x 10??4 kg/s. If the duct temperature is uniform at 200?C, estimate the bulk temperature of the
Air at 30?C enters a rectangular duct 1 m long and 4 mm by 16 mm in cross-section at a rate of 0.0004 kg/s. If a uniform heat flux of 500 W/m2 is imposed on both of the long sides of the duct,
Engine oil flows at a rate of 0.5 kg/s through a 2.5 cm ID tube. The oil enters 25?C while the tube wall is at 100?C.(a) If the tube is 4 m long. Determine whether the flow is fully developed.(b)
The equation was recommended by H. Hausen (Zeitschr. Ver. Deut. Ing., Belherft No. 4, 1943) for forced-convection heat transfer in fully developed laminar flow through tubes. Compare the values of
Air at an average temperature of 150?C flows through a short square duct 10 x 10 x 2.25 cm at a rate of 15 kg/h. The duct wall temperature is 430?C. Determine the average heat transfer coefficient,
Water enters a double pipe heat-exchanger at 60?C. The water flows on the inside through a copper tube 2.54 cm (1 in) ID at a velocity of 2 cm/s. Steam flows in the annulus and condenses on the
An electronic device is cooled by passing air at 27?C through six small tubular passages in parallel drilled through the bottom of the device as shown below. The mass flow rate per tube is 7 x 10??5
Unused engine oil with a 100?C inlet temperature flows at a rate of 250 g/sec through a 5.1-cm-ID pipe that is enclosed by a jacket containing condensing steam at 150?C. If the pipe is 9 m long,
Determine the rate of heat transfer per foot length to a light oil flowing through a 1-in.-ID, 2-ft-long copper tube at a velocity of 6 fpm. The oil enters the tube at 60?F and the tube is heated by
Calculate the Nusselt number and the convection heat transfer coefficient by three different methods for water at a bulk temperature of 32?C flowing at a velocity of 1.5 m/s through a 2.54-cm-ID duct
Atmospheric pressure air is heated in a long annulus (25 cm ID, 38 cm OD) by steam condensing at 149?C on the inner surface. If the velocity of the air is 6 m/s and its bulk temperature is 38?C,
If the total resistance between the steam and the air (including the pipe wall and scale on the steam side) in Problem 6.12 is 0.05 m2 K/W, calculate the temperature difference between the outer
Atmospheric air at a velocity of 61 m/s and a temperature of 16°C enters a 0.61-m-long square metal duct of 20
Compute the average heat transfer coefficient hc for 10?C water flowing at 4 m/s in a long, 2.5-cm-ID pipe (surface temperature 40?C) by three different equations and compare your results. Also
Water at 80?C is flowing through a thin copper tube (15.2 cm ID) at a velocity of 7.6 m/s. The duct is located in a room at 15?C and the heat transfer coefficient at the outer surface of the duct is
Mercury at an inlet bulk temperature of 90?C flows through a 1.2-cm-ID tube at a flow rate of 4535 kg/h. This tube is part of a nuclear reactor in which heat can be generated uniformly at any desired
Exhaust gases having properties similar to dry air enter a thin-walled cylindrical exhaust stack at 800 K. The stack is made of steel and is 8 m tall and 0.5 m inside diameter. If the gas flow rate
Water at an average temperature of 27°C is flowing through a smooth 5.08-cm-ID pipe at a velocity of 0.91 m/s. If the temperature at the inner surface of the pipe is 49°C, determine (a) the heat
An aniline-alcohol solution is flowing at a velocity of 10 fps through a long, 1-in.-ID thin wall tube. On the outer surface of the tube, steam is condensing at atmospheric pressure, and the
In a refrigeration system, brine (10 percent NaCl) by weight having a viscosity of 0.0016 (Ns)/m2 and a thermal conductivity of 0.85 W/(m K) is flowing through a long 2.5-cm-ID pipe at 6.1 m/s. Under
Derive an equation of the form hc = f(T, D, V) for turbulent flow of water through a long tube in the temperature range between 20? and 100?C.GIVENTurbulent water flow through a long tubeWater
The intake manifold of an automobile engine can be approximated as a 4 cm ID tube, 30 cm in length. Air at a bulk temperature of 20?C enters the manifold at a flow rate of 0.01 kg/s. The manifold is
High-pressure water at a bulk inlet temperature of 93?C is flowing with a velocity of 1.5 m/s through a 0.015-m-diameter tube, 0.3 m long. If the tube wall temperature is 204?C, determine the average
Suppose an engineer suggests that air is to be used instead of water in the tube of Problem 6.24 and the velocity of the air is to be increased until the heat transfer coefficient with the air equals
Atmospheric air at 10?C enters a 2 m long smooth rectangular duct with a 7.5 cm x 15 cm cross-section. The mass flow rate of the air is 0.1 kg/s. If the sides are at 150?C, estimate(a) The heat
Air at 16?C and atmospheric pressure enters a 1.25-cm-ID tube at 30 m/s. For an average wall temperature of 100?C, determine the discharge temperature of the air and the pressure drop if the pipe is
The equation has been proposed by Hausen for the transition range (2300 GIVENWater flowing through a pipeThe Hausen correlation given aboveWater temperature = 15?CPipe temperature =
Water at 20°C enters a 1.91 cm ID, 57 cm long tube at a flow rater of 3 gm/s. The tube wall is maintained at 30°C. Determine the water outlet temperature. What error in the water temperature
A solar thermal central receiver generates heat by focusing sunlight with a field of mirrors on a bank of tubes through which a coolant flows. Solar energy absorbed by the tubes is transferred to the
Determine the heat transfer coefficient for liquid bismuth flowing through an annulus (5 cm ID, 6.1 cm OD) at a velocity of 4.5 m/s. The wall temperature of the inner surface is 427°C and the

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