Heat Transfer Important Questions

 

Unit -I

Short answer questions:

Q.1 Identify the different modes of heat transfer in the following systems/ operations.

(a) Steam raising in a steam boiler.

(b) Air / water cooling of an I.C. engine cylinder.

(c) Heat loss from a thermos flask.

(d) Heating of water in a bucket with an immersion heater.

(e) Heat transfer from a room heater.

(f) Heat transfer in a refrigerator cabin.

Q.2 (a) With relevant examples, explain the mechanisms of conduction, convection and

radiation heat transfer.

Q.3. Explain clearly the basic laws of Heat transfer.

     Long answer questions:

Q.1 State and explain Fourier's law of conduction. What is the significance of negative sign

in the equation? A temperature difference of 8450C is impressed across a fiberglass layer of

13cm thickness. The thermal conductivity of the fiberglass is 0.035 W/m K.Computethe heat

transferred through the material per hour per unit area.

Q.2 Derive general heat conduction equation in Cartesian co-ordinates.

Q.4 Derive general heat conduction equation in Cylindrical co-ordinates.

Q.4 Derive general heat conduction equation in Spherical co-ordinates.

Unit –II

Short answer questions:

Q.1 Explain clearly the analogy between heat and electricity.

Q.2 What do you under- stand by the term over all heat transfer coefficient?

Q.3 What is meant by a lumped capacity?

Q.4 What are the physical assumptions necessary for a lumped- capacity unsteady-state

      analysis to apply?

Q.5 What is critical thickness of insulator on a small diameter wire or pipe.

Long answer questions:

Q.1 Determine the steady heat transfer per unit area through a 3.8 cm thick homogeneous slab with its two faces maintained at uniform temperatures of 350C and 250C. The thermal

conductivity of wall material is 1.9 X 10-4 kW/m-K.

Q.2 Derive an expression for temperature distribution in a slab when T1 and T2 are its surface temperatures. Assume that the thermal conductivity of the slab varies with temperature k = k0(1+αT)

Q.4 Heat is generated at a constant rate of 4X108W/m3 in a copper rod (3.86 W/mK) of

radius 5 mm. The rod is cooled by convection from its cylindrical surface into an ambient at

300C with a heat transfer coefficient of 2000 W/m2K. Determine the surface temperature of

the rod.

Q.5 During a heat treatment process a spherical object of 5 cm diameter is cooled in one

minute in an oil bath from 150C to 60C. If a cube made of the same material with a side of 50 mm is to be cooled between the same temperature limits, calculate the time required. Assume negligible internal thermal resistance.

Unit –III

Short answer questions:

Q.1 What is the significance of dimensional analysis.

Q.2 What is meant by Reynolds’s Analogy in forced convection.

Q.3 Briefly explain the Buckingham's π-Theorem for dimensional analysis.

Q.4 What are repeating variables and how are they selected for dimensional analysis.

Q.5 What do you understand by the hydrodynamics and thermal boundary layers. Illustrate

with reference to flow over a at heated plate.

Long answer questions:

Q.1 Show by dimensional analysis that data for forced convection may be correlated by an

equation of the form Nu=f(Re,Pr).

Q.2(a) Explain the Reynolds’s Analogy in forced convection.

(b) Water flows inside a smooth tube at a mean flow velocity of 3.0 m/s. The tube diameter is 25mm and constant heat flux condition is maintained at the tube wall such that the tube

temperature is always 200C above the water temperature. The water enters the tube at 300C and leaves at 500C. Calculate the tube length necessary to accomplish the indicated heating

Q.3 A pipe with a diameter of 2 cm is kept at a surface temperature of 40 0C. Find the heat

transfer rate per m length of this pipe if it is

i. Placed in an air flow in which the temperature is 50 0C and

ii. placed in a tank of water kept a temperature of 30 0C. The heat transfer coefficient in these two situations, which involve A. forced convection in air and B. free convection in water, are estimated to be 20 W/m2 K and 70 W/m2 K respectively.

Q.4 A thin at plate has been placed longitudinally in a steam of air at 200C and while flows

with undisturbed velocity of 7.5 m/s. The surface of plate is maintained at a uniform

temperature of 1200C.

i. calculate the heat transfer coefficient 0.8m from the leading edge of the plate,

ii. Also calculate the rate of heat transfer from one side of the plate to the air over the first 0.8 m length. Assume unit width of the plate.

Q.5 How are the local and average convection coefficients for a flow past a flat plate are

related? Derive the relationship.

Q.6 A 0.5cm thick and 4cm long fin has its base on a plane plate which is maintained at

1100C. The ambient air temperature is 200C. The conductivity of the fin material is 60 W/m-

K and the heat transfer coefficient h= 150 W/m2K. Determine. Assume that the tip of the fin

is insulated.

i. Temperature at the end of the fin

ii. Temperature at the middle of the fin

iii. Total heat dissipated by the fin.

(b) Derive an expression for the temperature distribution in a short fin with convection taking

place at the tip.

 

Unit –IV

Short answer questions:

Q.1 Why are heat transfer rates high for a phase change process?

Q.2 Differentiate between pool boiling and film boiling.

Q.3 List various regimes of pool boiling and show it with neat sketch.

Q.4 Write short notes on nucleate boiling.

Q.5 What are radiation shape factors? Why are they used?

Q.6 Differentiate between contact and in-direct contact type heat exchanger.

Q.7 Briefly explain the regenerative heat exchanger.

Q.8 Write short notes on recuperative heat exchanger

Q.9 Write short notes on LMTD and AMTD

Q.10 Differentiate between counter flow and parallel flow heat exchangers.

Long answer questions:

Q.1 (a) How does the log mean temperature difference for a heat exchanger differ from the

arithmetic mean temperature difference? For specified inlet and outlet temperatures, which

one of these two quantities is larger?

(b) A shell-and-tube heat exchanger has condensing steam at 100 0C in the shell side with one shell pass. Two tube passes are used with air in the tubes entering at 10 0C. The total surface area of the exchangers is 30 m2 and the overall heat-transfer coefficient may be taken as 150 W/m2.K. If the effectiveness of the exchanger is 85 percent, what is the total heat transfer

rate?

Q.2 Hot oil is to be cooled by water in a one shell pass and eight tube passes heat exchanger.The tubes are thin walled and made of copper with an internal diameter of 14 mm. The length of each tube pass is 5 m and U0 = 310 W/m2K. Water flows through the tubes at a rate of 0.2 kg/s and the oil through the shell at a rate of 0.3 kg/s. The water and the oil enter at temperatures of 200C and 1500C respectively. Determine the rate of heat transfer and the exit temperatures of the water and the oil

Q.3 (a) In a gas to liquid heat exchanger, why are fins provided on gas side? Explain.

(b) Determine the overall heat transfer coefficient based on the outer area of a 3.81 cm O.D.

and 3.175 cm I.D. brass tube ( k = 103.8 W/m.K) if the heat transfer coefficients for flow

inside and outside the tube are 2270 and 2840 W/m2K respectively and the unit fouling

resistances at inside and outside are Rfi = Rfo = 0.0088m2 K/W

Q.4 (a) Derive an expression for logarithmic mean temperature difference for the case of

counter flow of heat exchanger.

(b) A hot fluid enters a heat exchanger at a temperature of 200 0C at a flow rate of 2.8 kg/s

(sp. heat 2.0 kJ/kg-K) it is cooled by another fluid with a mass flow rate of 0.7kg/sec

(Specific .heat 0.4 kJ/kg-K). The overall heat transfer coefficient based on outside area of

20m2 is 250W/m2-K. Calculate the exit temperature of hot fluid when fluid when fluids are

in parallel flow.

Q.5 . A one shell pass, two tube pass heat exchanger has a total surface area of 5 m2 and its overall heat transfer coefficient based on that area is found to be 1400 W/m2K.

If 4500 kg/h of water enters the shell side at 315oC while 9000 kg/h of water enters the tube

side at 400C, find the outlet temperatures using

(a) The correction factor LMTD method and

(b) Effectiveness-NTU method. Take cp for both fluid streams as 4.187 kJ/kg

Q.6 (a) Deduce average heat transfer co-efficient equation in film condensation on a Verticalat plate using Nusselt's theory.

(b) A heated brass plate at 1600C is submerged horizontally in water at a pressure

corresponding to a saturation temperature of 1200C. What is the heat transfer per unit area?

Calculate also the heat transfer coefficient in boiling.

Q.7 The condenser of a steam power plant operates at a pressure of 7.38 kPa. Steam at this pressure condenses on the outer surfaces of horizontal pipes through which cooling water circulates. The outer diameter of the pipes is 2 cm, and the outer surfaces of the pipes are maintained at 30 0C. Determine

(a) the rate of heat transfer to the cooling water circulating in the pipes and

(b) the rate of condensation of steam per unit length of a horizontal pipe.

Q.8 Explain the conditions under which drop wise condensation can take place. Why does the rate of heat transfer in drop-wise condensation many times larger than in film-wise

condensation?

(b) A steam condenser consists of 100 tubes, each 1.27mm in diameter are arranged in a

square array. If the tubes are exposed to dry steam at atmospheric pressure and the tube

surface temperature is maintained at 98oC, what is the rate at which steam is condensed per unit length of the tubes?

Q.9 Show that the direct heat exchange area between a disc of radius `r' and a sphere of

radius `R', whose center is on the normal through the center of the disc separated by a center to center distance `H' is 2 π R2 [ 1 - {H / √(H2 + r2)g}].

Unit –V

Short answer questions:

Q.6 Define the terms

i. Absorptivity ii. Reflectivity and iii. Transmissivity

Q.7 Differentiate between specular and diffused radiation.

Q.8. State Stefan Boltzman law and Planck;s law of radiation.

Long answer questions:

Q.1 (a) Explain the utility of radiation shields.

(b) Two large parallel planes having emissivities 0.3 and 0.5 are maintained at temperatures

of 900 0C and 4000C respectively. A radiation shield having an emissivity of 0.05 is placed

between the two planes. Estimate: i. Heat exchange per m2 of area if the shield were not

present

ii. Temperature of the shield, and

iii. Heat exchange per m2 area when the shield is present.

Q.2 Fused quartz transmits 90% of the incident thermal radiation between 0.2 and 4 μm.

suppose a certain heat source is viewed through the quartz window, what heat flux in Watts

will be transmitted through the material from black body radiation sources at:

i. 800 0C ii. 550 0C

Q.3 (a) State and prove reciprocity theorem as applied to radiation shape factors.

(b) Two concentric cylinders having diameters of 10cm and 20 cm have a length of 20cm.

Calculate the shape factor between the open ends of the cylinders.