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.
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
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 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.
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.
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?