Differentiate between boiling and condensation. Draw a boiling curve with different regimes and explain in brief. Why nucleate boiling is most desirable in boiling regimes?
A hot plate of 30 cm acts as a hot pan of water at 1 atmosphere. The temperature of the pan bottom is at 130°C. Estimate heat transfer rate by the plate. csf=0.013c_{sf} = 0.013, surface tension of water = 58.0 mN/m.
Why is higher heat transfer occurred in drop wise condensation than the film wise condensation?
Define modified latent heat of vaporization. Why is it used?
One hundred tubes of diameter 2.0 cm each are arranged in a squared array and exposed to atmospheric steam, Calculate the mass of steam condenses per unit length of tube for a tube wall temperature of 95 °C.
Define effectiveness of heat exchanger, minimum fluid, fouling resistance, and heat capacity rate.
When is NTU method better than LMTD method? Why is higher heat transfer occurred in counter flow than in parallel flow arrangement? Explain.
A double pipe heat exchanger is used to heat an oil with cp=2.1c_p = 2.1 kJ/kg-K from 50°C to 100°C. The other fluid is water having cp=4.2c_p = 4.2 kJ/kg-K enters the heat exchanger at 160°C and leaves at 90°C. The arrangement is counter flow. UU value is 320 W/m2^2-°C. Calculate the area and effectiveness of the heat exchanger for a total heat transfer rate is 400 kW.
Define mass transfer, mass diffusion coefficient. Explain the mechanism of mass transfer.
Define Lewis number, Sherwood number, and Schemidt number. Explain the physical significance of these.
Draw NTU versus ϵ\epsilon for counter and parallel flow. Is there any limit for effective design of exchanger based on NTU versus ϵ\epsilon? Explain.
What are the parameters that effect the convection heat transfer? What are the effects of turbulence?
What are the significances of Nusselt number (Nu) and Prandtl (Pr) number?
Derive an expression of the mass continuity equation for the Laminar boundary layer on a flat plate.
Define hydrodynamic and thermal boundary layer. Why are they formed? Draw the hydrodynamic boundary layer profile for a flow over the flat plate.
What do you mean by Stanton Number? Derive an expression for Reynolds-Colburn analogy based on fluid friction and heat transfer?
Air is flowing over a flat plate 5m long and 2.5m wide with a velocity of 4 m/sec at 15°C. Calculate:
Helium at 1 atm and 200K is used to cool a 1-m-square plate maintained at 310K. The flow velocity is 75 m/sec. Calculate the total heat-loss from the plate. What is the boundary layer thickness as the flow leaves the plate.
In an Industrial facility, air is to be preheated before entering a furnace by geothermal water at 120°C flowing through the tubes of tube bank located in a duct. Air enters the duct at 20°C and 1 atm with a mean velocity of 4.5 m/sec, and flows over the tubes in normal direction. The outer diameter of the tube is 1.5 cm, and the tubes are arranged in-line with longitudinal and transverse pitches of SL=ST=5m. There are 6 rows in the flow direction with 10 tubes in each row.
What is natural convection? What is the physical mechanism of natural convection heat transfer on a vertical flat plate? Explain in brief.
What are the significances of Grashof number and Rayleigh number? Why heat transfer coefficient is higher in forced convection rather than the free convection?
A 6-m long section of a 8-cm diameter horizontal hot water pipe passes through a large room whose temperature is 20°C. If the outer surface temperature of the pipe is 70°C, determine the rate of heat loss from the pipe by natural convection.
In which case NTU method is preferable than LMTD? Why heat transfer is higher in counter flow heat exchanger than in parallel flow arrangement? Explain.
Define optimum spacing for natural convection with finned surface. A 12m wide and 18cm high vertical hot surface in 25°C air is to be cooled by a heat sink with equally spaced fins of rectangular profile. The fins are 0.1 cm thick, 18 cm long in the vertical direction, and have a height of 2.4 em from the base. Determine the optimum fin spacing and the rate of heat transfer by natural convection from the sink if the base temperature is 80°C.
Hot oil is to be cooled by water in a 1 shell pass and 8 tubes passes heat exchanger. The tubes are thin walled and are made of copper with an internal diameter of 1.4 cm. The length of heat exchanger is 6m where U = 300 W/m²-K. Water flows through the tubes at 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 20°C and 160°C, respectively. Determine the rate of heat transfer in the heat exchanger and the outlet temperature of water and oil.
Differentiate between boiling and evaporation. How can boiling heat transfer be enhanced? Explain in brief.
Why is modified latent heat used? Explain also why higher heat transfer occurred in dropwise condensation than in filmwise.
Steam at 105°C at atmospheric pressure condenses on a 4m high and 3m wide vertical plate which is maintained at 80°C by circulating cooling water through one side. Determine
Define heat capacity rate, minimum fluid and heat exchanger effectiveness. Explain the effects of NTU on effectiveness.
A double-pipe heat exchanger is constructed of a Cu inner tube of internal diameter (Dᵢ) = 1.2cm, external diameter (D₀) = 1.6cm and an outer tube of diameter 3 cm. Determine
Why is correction factor needed?
Define Grashof number, Rayleigh number and Buoyancy force. How we detect a system is dominating by force convection or free convection?
Why is lower heat transfer experienced in free convection than in force convection? Mention some uses of free convection.
Consider a 2 × 2 ft² thin square plate in a room of 75°F. One side of the plate is maintained at a temperature of 130°F, while the other side is insulated. Determine the rate of heat transfer from the plate by natural convection if the plate is
Define convective heat transfer coefficient. How does it vary? Why is higher heat transfer occurred in turbulent flow than in laminar flow?
Define Stanton number. Derive an expression for Reynolds-Colburn analogy.
A 2m×3m flat plate is suspended in a room and is subjected to air flow parallel to its surfaces along its 3m long side. The free stream temperature and velocity are 25°C and 6 m/s. The total drag force acting on the plate is measured to be 0.82N. Determine the average convection heat transfer coefficient for the plate.
State 5 types of diffusion process.
Determine the mole fraction of the water vapor at the surface of a lake whose temperature is 15°C. Compare the mole fraction of water in the lake having atmospheric pressure of 92 kPa.
Dry air at 15°C and 92 kPa flows over a 2m long wet surface with a free stream velocity of 4 m/s. Determine the average mass transfer coefficient.
Define Reynolds number, Nusselt number, Boundary layer and Prandtl number. Describe their physical significances. Also define "no slip" condition.
Air at 25°C and atmospheric condition having no moisture is blowing with a velocity of 2.5 m/s over a squared flat plate of 3m × 3m. The plate is maintained at 90°C throughout the length. Calculate :
What is the physical significance of Prandtl number (Pr)? How Pr behaves on the following cases
The convection heat transfer coefficient for a clothed person standing in moving air is expressed as h = 14.8v⁰.⁶⁹ for 0.15 < v < 1.5 m/s where v is the air velocity. For a person with a body surface area of 1.7 m² and average surface temperature of 29°C, determine the rate of heat loss from the person in a windy air at 10°C by convection for air velocity of 1.5 m/s.
For laminar free convection from a heated vertical surface, the local convection coefficient is expressed as hₓ = Cx⁻ⁿ. Obtain an expression for hₓ / h.
Why heat transfer coefficient is important for convection heat transfer? On what factors does the heat transfer coefficient depend? Explain two factors.
Dry air at atmospheric pressure and 20°C is flowing with a velocity of 3 m/s along the length of a long flat plate of 0.3m wide, maintained at 100°C. Calculate:
Why "Colburn analogy" is significant and important? Explain.
How "Reynolds analogy" is different from "Colburn analogy"? When "Reynolds analogy" can be used?
Air at 27°C and 1 atm flows over a flat plate at a speed of 2 m/s. Assuming that the plate is heated over its entire length to a temperature of 60°C. Calculate the heat transfer for the first 0.4 m of the plate. Properties of air at 316.5K: v=17×10⁻⁶ m²/s, Cp=1006 J/kg-K, Pr=0.7, k=0.027 W/m-K and ρ=1.12 kg/m³.
Air at 25°C flows across a circular tube of diameter 6 cm, perpendicular to the tube axis with a velocity of 3 m/s. Tube surface is maintained at 55°C. Determine the value of convection heat transfer coefficient. Air properties at 40°C, v=18×10⁻⁶ m²/s, Pr=0.7, k=0.027 W/m-K.
Water at 20°C flows through a tube, 4 cm diameter, 9 m length, tube surface being maintained at 90°C. Temperature of water increases from 20°C to 60°C. Find the mass flow rate. Use the equation Nu=0.023 Re⁰.⁸ Pr⁰.⁴. Properties of water at 40°C: ρ=993 kg/m³, cp=4170 J/kg-K, k=0.64 W/m-K and v=0.6×10⁻⁶ m²/s.
Why natural convection is important? Mention uses of natural convection heat transfer.
How Grashoff number differs from Reynolds number?
A furnace door, 1.5 m high and 1 m wide, is insulated from inside and has an outer surface temperature of 70°C. If the surrounding ambient air is at 30°C, calculate the heat loss from the door. Properties of air at 50°C: ρ=1.09 kg/m³, v=18×10⁻⁶ m²/s, cp=1005 J/kg-K, Pr=0.7 and k=0.028 W/m-K.
Why LMTD is used instead of arithmetic mean of temperature? Between counter and parallel flow which one has higher heat transfer and why?
Define NTU method, effectiveness, minimum fluid. Upon what factors effectiveness depends? Explain.
A counter flow double pipe heat exchanger is to heat water from 25°C to 85°C at the rate of 1.2 kg/s. The heating is accomplished by geothermal water available at 160°C at a mass flow rate of 1 kg/s. The inner tube is thin-walled and has a diameter of 15 cm. If the overall heat transfer coefficient of the heat exchanger is 620 W/m²°C, determine the length of the heat exchanger required to achieve the desired heating.
Differentiate boiling, evaporation and condensation. Enlist the different types of boiling.
Describe the pool boiling mechanism with neat sketches.
How can boiling heat transfer be enhanced?
What is the difference between film- and drop-wise condensations? Which is more effective mechanism of heat transfer? Why? How can film-wise condensation be converted into drop-wise condensation?
What is the modified latent heat of vaporization? For what is it used? How does it differ from ordinary latent heat of vaporization?
Saturated steam at atmospheric pressure condenses on a 3m high and 2m wide vertical plate which is maintained at 75°C by circulating cooling water through one side. Determine:
Define Fick's law of mass transfer. What is the mechanism of mass transfer? Describe in brief.
Write short notes on:
Determine the mole fraction of air dissolved in water at the surface of a lake whose temperature is 17°C. Take the atmospheric pressure at lake level to be 92 kPa.
Describe the mechanism of force convection. Why is boundary layer formed? Define velocity and thermal boundary layer thickness.
For a flat plate 3 m × 2 m having uniform temperature 50 °C. Air flows over the flat plate with 2.5 m/s at atmospheric pressure. Find the critical boundary layer thickness. Also find the boundary layer thickness at 1.5 cm from the leading edge. (Use ν=1.68×10−5\nu = 1.68 \times 10^{-5} m²/s).
What is physical mechanism that causes the friction factor to be higher in turbulent flow?
What is the physical significance of the Reynolds number? How it is defined for external flow over a plate of length L?
Calculate the ratio of thermal boundary layer thickness to hydrodynamic boundary layer thickness of air at 1 atm and 300 K.
A thin walled, steel pipe of 6 cm length and 50 mm internal diameter is heated by condensing steam from the outside, which maintains the internal wall temperature at 100 °C. The tube has 2.5 × 10⁻⁴ m³/s of water flowing through it, that is heated from 20 °C to 60 °C. What is the mean heat transfer coefficient over the pipe length?
Air flows across a 4 cm diameter smooth pipe with 20 °C, 1 atm and velocity 20 m/s (free stream condition). If the tube surface temperature is 150 °C. Calculate the heat transfer from the tube per meter length.
What is natural convection? How does it differ from forced convection? Give examples.
Consider a fluid whose volume does not change with temperature at constant pressure. What can you say about natural convection heat transfer in this medium?
A 10 m long section of a 6 cm diameter horizontal hot-water pipe passes through a large room whose temperature is 27 °C. If the temperature of the outer surface of the pipe is 73 °C, determine the rate of heat loss from the pipe.
Why is higher heat transfer experienced in counter flow than in parallel flow?
Under what conditions is the thermal resistance of the tube in a heat exchanger negligible?
A double pipe heat exchanger is used to heat an oil with cp_p = 2.1 kJ/kg.°C from 50 °C to 100 °C. The other fluid in water having cp_p = 4.2 kJ/kg.°C enters the heat exchanger at 160 °C and leave at 90 °C. The arrangement is counter flow. U = 320 w/m²°C. Calculate the area and effectiveness of the heat exchanger for a total heat transfer rate 500 kW.
What is boiling? What is the difference between pool boiling and flow boiling? Give examples of pool boiling and flow boiling.
Draw the pool boiling curve and identify all the boiling regimes.
A hot plate of 30 cm by 30 cm acts as a hot pan of water at 1 atmosphere. The temperature of the pan bottom in at 120 °C. Estimate heat transfer rate by the plate csf_{sf} = 0.013, surface tension of water = 58.0 mN/m.
Draw the temperature and velocity profile for condensation in a vertical plate. Why is higher heat transfer in drop wise condensation than in film wise condensation?
Why is modified latent heat of condensation used in condensation process?
Vertical plate of 60 cm × 60 cm is maintained at 95 °C (one side, other side is insulated). It is exposed to a superheated stream of temperature 105 °C at 1 atmosphere. Calculate the amount of steam condensed per hour. Also find h value at the end of the plate.
How does mass transfer differ from bulk fluid flow? Can mass transfer occur in a homogeneous medium?
Define Lewis number and Schmidt number. What are the significances of these numbers?
An open pan 20 cm in diameter and 10 cm deep contains water at 25 °C and is exposed to atmospheric air at 25 °C. Calculate the evaporation rate of water. Diffusion coefficient of water vapor in air at 25 °C and 1 atm is 0.256 cm²/s.
What is convection heat transfer? How Nusselt's number and Reynold's number are defined? What are the significances of these numbers?
Consider a flat plate of length 2 m and 1 m wide having temperature of 40 °C. Air flows over the flat plate with 2 m/s at 1 atm. Find where the critical Reynolds number occurred? Determine boundary layer thickness at a distance 20 cm from the leading edge. (v= 1.69 × 10⁻⁵ m²/s)
What do you mean by Prandtl's number? Explain the physical significance.
Engine oil at 60 °C flows over the upper surface of a 6 m long flat plate whose temperature is 20 °C with a velocity of 2 m/s. Determine the total drag force and rate of heat transfer per unit width of the entire plate.
Differentiate between laminar flow and turbulent flow. Why is higher heat transfer in turbulent flow than laminer flow?
What is hydraulic diameter? How is it defined? What is equal to for a circular tube diameter?
What fluid property is responsible for the development of the velocity boundary? For what kind of fluid will there be no velocity boundary layer in a pipe?
Air flows across a 5 cm diameter smooth pipe with free stream conditions of 20 °C, 1 atm and u∞ = 25 m/s. If the tube surface temperature is 120 °C, Calculate the heat loss per unit length.
Describe the mechanism of free convection heat transfer. Why is higher heat transfer in force convection compared to free convection?
A plate of length L is placed in an atmosphere of low temperature. In which condition heat transfer is higher for the followings : i) The plate is vertical ii) The plate is horizontal iii) The plate is 30° inclined as the vertical. Explain why?
A 60 m long section of an 8 cm diameter horizontal hot water pipes shown in figure 4(c) passes through a large room whose temperature is 20 °C. If the outer surface of the pipe is 70 °C, determine the rate of heat loss from the pipe by natural convection. What will happen if the pipe is vertical?
What is the difference between pool and flow boiling? Draw the pool boiling curve and identify the different boiling regimes. Also explain the characteristics of each regime.
A heated 35 cm by 35 cm square plate serves as the bottom for a pan of water at 1 atm pressure. The temperature of the plate is maintained at 119 °C. Estimate the heat transfer rate by the plate Cf = 0.013. Vapor-liquid surface tension for water = 58.8 mN/m.
What is the difference between film and drop-wise condensation?
What is the modified latent heat of vaporization? For what is it used?
A 50 cm by 50 cm square vertical plate is maintained at 95 °C and exposed to saturated steam at 1 atm pressure. Calculate the amount of stream condensed per hour.
When is LMTD method applied? Define minimum fluid and effectiveness of heat exchanger. Between counter and parallel flow which one has higher heat transfer? Why?
Under what conditions will the temperature rise of the cold fluid in a heat exchanger be equal to the temperature drop of the hot fluid?
A double-pipe heat exchanger is used to heat an oil with C = 2.2 KJ/Kg. °C from 50 °C to 100 °C. The other fluid having C = 4.2 KJ/Kg. °C enters the heat exchange at 160 °C and leaves at 90 °C. The overall heat transfer coefficient is 300 W/m².°C. Calculate the area and effectiveness of the heat exchange for a total heat transfer rate of 600 KW.
Write the expression for Fick's law of diffusion. Why does mass transfer occur?
Write short notes on (Any three) : i) Sherwood number ii) Mass transfer coefficient iii) Schemidt number iv) Lewis number v) Stanton number
A mixture of head N₂ gas is contained in a pipe at 298 K and 1 atm, total pressure of which is constant throughout. At one end of the pipe at point 1, the partial pressure PA1 Of He is .6 atm and at the other end 0.2 m distance is PA2 = 0.2 atm. Calculate the flux of He at steady state if DAB Of He-N₂ mixture is 0.687 × 10⁻⁴ m²/s.