Question 1:

A 50 mm diameter, thin wall metal pipe covered by 25 mm thick layer of insulation (ki = 0.085 W/m.K) and carrying a superheated steam at atmospheric pressure is suspended from the ceiling of a large room. The steam temperature entering the pipe is 120 oC, and the air temperature is 20 oC. The overall heat transfer coefficient on the outer surface of the covered pipe is 10 W/m2.K. if the velocity of the steam is 10 m/s, at what point along the pipe will the steam begin condensing and what distance will be required for the steam to reach a mean temperature of 100 oC?

Question 2:

Consider a horizontal, thin walled circular tube of diameter D = 0.025 m submerge in a container of n-octadecane (paraffin), which is used to store thermal energy. As hot water flows through the tube, heat is transferred to the paraffin, converting it from the solid to liquid state at the phase change temperature of T∞ = 27.4 oC. The latent heat of fusion and density of paraffin are hsf = 244 kJ/kg and = 770 kg/m3, respectively and thermophysical properties of the water may be taken as cp = 4.185 kJ/kg.K, k = 0.635 W/m.K, and = 467 x 10-6 kg/s.m. Assuming the tube surface to have a uniform temperature corresponding to that of the phase change, determine the water outlet temperature and total heat transfer rate for a water flow-rate of 0.1 kg/s and inlet temperature of 60 oC. If the height, width and length of the container are 0.25 m, 0.25 m and 3 m, respectively, how long would it take to completely liquefy the paraffin, from an initial state for which all of the paraffin is solid and at 27.4 oC?

Question 3:

Consider the process by which ice is formed for an indoor rink. A parallel array of cooling tube is submerge in a shallow layer of water, and a refrigerant (Freon-12) is passed through the tubes. The layer height is H = 60 mm and the tube pitch, diameter and length are S = 50 mm, D = 12 mm, and L = 5 m, respectively. The temperature and flow rate of refrigerant entering each tube are -33 oC and 0.02 kg/s, respectively. The refrigerant remains at liquid state through out the tube, and average thermo physical property value may be taken to be cp = 900 J/kg.K, k = 0.07 W/m.K, m = 3.5 x 10-4 kg/s.m, and Npr = 4.6. The density of water = 1000 kg/m3, and the latent heat of fusion is hsf = 3.34 x 105 J/kg.

a. consider the process whereby the water is taken from the saturated liquid to solid states to be at the freezing point throughout the process, at what temperature does the refrigerant leave the tube? What is the rate of heat transfer to the refrigerant for a single tube length?

b. For the condition of part (a), how long would it take to completely freeze the water.

Question 4:

Water enters counter flow, double pipe heat exchanger at a rate of 150 lbm/min and is heated from 60 oF to 140 oF by an oil with specific heat of 0.45 Btu/lbm.oF. The oil enters at 240 oF and leaves at 80 oF. The overall heat transfer coefficient is 50 Btu/hr. oF.

a. What heat transfer area is required ?b. What area is required if all condition remain the same except that a shell and

tube heat exchanger is used with the water making one shell pass and the oil making two tube passes?

c. What exit water temperature would result if, for the exchanger part (a), the water flow rate were decreased to 120 lbm/hr?

Question 5:

A cold air chamber is proposed for quenching steel ball bearings of diameter D = 0.2 m and initial temperature Ti = 400 oC. Air in the chamber is maintained at – 15 oC by refrigeration system, and the steel ball pass through the chamber on a conveyor belt. Optimum bearing production requires that the temperature at the center of ball bearing reach 80 oC. Recommended the drive velocity of conveyor belt and estimated the power is required to maintain the chamber condition.

Question 6:

A sphere 30 mm in diameter initially at 800 K is quenched in a large bath having a constant temperature of 320 K with a convection heat transfer coefficient of 75 W/m2.K. The thermophysical properties of the sphere material are: = 400 kg/m3, cp = 600 J/kg.K, and k = 1.7 W/m.K.

a) calculate the time required for the surface of the sphere to reach 415 K.b) determine the heat flux at the outer surface of the sphere at the time determined

in part (a)c) determine the energy has been lost by the sphere during the process of cooling to

the surface temperature of 415 K.

Question 7:

As permanent space stations increase in size, there is an attendant increase in the amount of electrical power they dissipate. To kept station compartment temperature from exceeding prescribed limits , it is necessary to transfer the dissipate heat to space. A novel heat rejection scheme that has been proposed for this purpose in termed of a Liquid Droplet Radiation (LDR). The heat is first transferred to a high vacuum oil, which is than injected into outer space as a stream of small droplets. The stream is allowed to traverse a distance L, over which is cools by radiating energy to outer space at absolute zero temperature. The droplets are then collected an routed back to the space station.

Consider condition for which droplets of emmisivity = 0.95 and diameter D = 0.5 mm are injected at a temperature at a temperature of Ti = 500 K and a velocity of v = 0.1 m/s. properties of oil are = 885 kg/m3, cp = 1900 J/kg.K and k = 0.14 W/m.K. Assuming each drop to radiate to deep space at Tsur = 0 K, determine the distance L required for the droplets to impact the collector a final temperature of Tf = 300 K. What is the amount of thermal energy rejected by each droplet?

QUESTION 8:

A tile iron consists of a massive plate maintained at 150 oC by an imbedded electrical heater. The iron is placed in contact with a tile to soften the adhesive, allowing the tile to be easily lifted from the subflooring. The adhesive will soften sufficiently if heated above 50 oC for at least 2 min, but its temperature should not exceed 120 oC to avoid deterioration of the adhesive. Assume the tile and subfloor to have an initial temperature of 25 oC and have equivalent thermophysical properties of k = 0.15 W/m.K and cp = 1.5 x 106 J/m3.K.

a) how long will it take a worker using the tile iron to lift a tile? Will the adhesive temperature exceed 120 oC?

b) If the tile iron has a square surface area 254 to the side, how much energy has been removed from it during the time it has taken to lift tile?

Question 9:

The coating on a 1 m by 2 m metallic surface is cured by placing it 0.5 m below an electrically heated surface of equivalent dimensions. The heater is well insulated on its top side and is aligned with the coated surface. Both the heater and coated surface may be approximated as black bodies. During the curing procedure, the heated and coated surface are maintained at 700 K and 400 K, respectively. Convection effect may be neglected.

a. If the assembly is exposed to large surrounding at 300 K, what is the electrical power required to operate the heater?

b. If the surface are connected with reradiating sidewall, what is the electric power required to operate the heater?

Question 10:

One way to deliver a timed dosage of a drug within the human body is to ingest a capsulate and allow it to settle in the gastroinstestinal system. Once inside the body, the capsule slowly releases the drug by a diffusion- limited process. The drug capsule consists of a 0.652 cm diameter spherical bead containing a uniform initial concentration of 68.9 mg/cm3 Dramamine.

a. What is the residual concentration of Dramamine at the centre of the spherical bead after 48 hours?

b. The capsule is now a cylindrical tablet of diameter 0.652 cm and thickness 0.3 cm. What is the residual concentration of Dramamine at the centre of the spherical bead after 48 hours?

c. The capsule is now a cube 0.652 cm on a side. What is the residual concentration of Dramamine at the centre of the spherical bead after 48 hours?

The diffusion coefficient of Dramamine (species A) in the gel matrix (species B) is 3 x 10-7 cm2/s at a body temperature of 37 oC.

Question 11:

Two parallel, diffuse-gray surfaces, each 1 m by 2 m, face each other and are separated by 1 m. Each surface has an emissivity of 0.2. One surface is at 27 °C and the other is at 277°C.

a. Calculate the net radiation transfer from the hotter surface, if the surroundings are at 0 K.

b. If well-insulated, diffuse-gray sidewalls are added to this configuration, what will be the net radiation heat transfer rate from the hotter surface?

c. Review the assumptions you made in performing the calculations and briefly explain what effect they could have on the accuracy of your results.

2 m

1 m0.5 m

Coated surface

Heater surface

surroundings

Question 12:

Consider a room that is 4 m long by 3 m wide with a floor-to-ceiling distance of 2.5 m. The four walls of the room are well insulated, while the surface of the floor is maintained at a uniform temperature of 30°C by means of electric resistance heaters. Heat loss occurs through the ceiling, which has a surface temperature of 12 °C. If all surfaces have an emissivity of 0.9, what is the rate of heat loss by radiation from the room?

Question 13:

Two parallel, aligned disks, 0.4 m in diameter and separated by 0.1 m, are located in a large room whose walls are maintained at 300 K. One of the disks is maintained at a uniform temperature of 500 K with an emissivity of 0.6, while the back side of the second disk is well insulated. If the disks are diffuse, gray surfaces, determine the temperature of the insulated disk.