Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III HEAT TRANSFER Previous Question Paper (August/September - 2024 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
PART – A
What are the different modes of heat transfer?
Answer
There are three fundamental modes of heat transfer:
- Conduction: Heat transfer through a stationary medium (solid or fluid) due to a temperature gradient, occurring via molecular interaction or lattice vibration.
- Convection: Heat transfer between a surface and a moving fluid due to the combined effects of conduction and bulk fluid motion (advection).
- Radiation: Heat transfer in the form of electromagnetic waves (or photons) emitted by matter as a result of its thermal energy, which does not require a physical medium for propagation.
What do you understand by initial and boundary conditions?
Answer
In heat conduction problems, the temperature distribution within a solid depends on both time and spatial coordinates. To obtain a unique solution for the heat diffusion equation, specific conditions must be defined.
Initial conditions specify the temperature distribution of the system at the starting time, typically denoted as t = 0. For a one-dimensional system, this is expressed as T(x, 0) = f(x).
Boundary conditions describe the physical conditions existing at the surfaces of the solid for all time t > 0. These are essential to account for the interaction between the system and its surroundings. Common types include:
- Dirichlet condition: Prescribed surface temperature, T(0, t) = T_s.
- Neumann condition: Prescribed surface heat flux, -k(∂T/∂x) = q_s''.
- Robin condition: Convection boundary condition, where the surface heat flux equals the convective heat transfer, -k(∂T/∂x) = h(T_s - T_∞).
Write down the momentum equation for the hydrodynamic boundary layer in the Cartesian coordinate system over a flat plate.
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Define Fin efficiency and fin effectiveness.
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Explain how 'black surface', 'gray surface'?
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What are the advantages and limitations of dimensional analysis?
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Define effectiveness of a heat exchanger.
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Define Grashoff Number and Stanton Number.
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Differentiate between film-wise and drop-wise condensation.
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Write the use and importance of Plank’s Law.
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PART – B
Derive general heat conduction equation for isotropic material in cylindrical co-ordinates.
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What do you mean by boundary and initial condition?
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A Stainless steel plate is of 2 cm thick is maintained at a temperature of 550°C at one face and 50°C on the other. The thermal conductivity of stainless steel at 300°C is 19.1 W/m K. Calculate the heat transferred through the material per unit area?
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In what way is the science of heat transfer different from thermodynamics? Explain.
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Explain why the conductivity of metals decreases and conductivity of insulating material increases with increases in temperature?
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A metallic plate, 3cm thick is maintained at 400°C on one side and 100°C on the other side. How much heat is transferred through the plate? Take k for the metallic plate as k = 370 W/m-K?
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List out the assumptions of Fourier's law of heat conduction.
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State the physical interpretation of Biot number.
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Write short notes on: (i) Stefan-Boltzmann's law (ii) Plank's law.
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A system in space which generates a lot of waste heat has to be get rid of by radiation into deep, cold space. The power required to dump is 1 × 10³ W. The external radiator has a surface area of 1m × 2m, useful emission from only one side and emissivity of 0.99. What is the equilibrium temperature of the radiator in degree Celsius?
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Air at 27°C and 1 atm flows over a flat plate at a speed of 2m/s. Calculate the boundary layer thickness at a distance of 20 and 40 cm from the leading edge of the plate. Calculate the mass flow which enters the boundary layer between x= 20 cm and x = 40 cm. The viscosity of the air is at 27°C is 1.85 × 10⁻⁵ kg/m s. Assume the unit depth in the z–direction.
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What is the effect of the presence of non-condensable gases on the condensation process?
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Explain the concept of LMTD for a counter flow heat exchanger.
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A counter flow heat exchanger is used to cool oil at a rate of 0.6 kg/s (Cp = 2.5) from 110°C to 35°C using water at 20°C. The overall heat transfer coefficient is 1500 W/m²K. Assuming cooling water outlet temperature as 80°C and using NTU method calculate: (a) (i) Water flow rate. (ii) Surface area required (b) Effectiveness of the heat exchanger?
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Steam condenses at atmospheric pressure on the external surface of the tubes of a steam condenser. The tubes are 12 in number and each is 30 mm in diameter and 10 m long. The inlet and outlet temperatures of cooling water flowing inside the tubes are 25°C and 60°C respectively. If the flow rate is 1.1kg/s, calculate (a) The rate of condensation of steam (b) The number of transfer units (c) The effectiveness of the condenser.
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Define mass concentration, molar concentration, mass fraction and mole fraction.
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The diffusivity of CCl4 in air is determined by observing the steady state evaporation of CCl4 in a tube of 1 cm diameter exposed to air. The CCl4 liquid level is 10 cm below the top level of the tube. The system is held at 25°C and 1 bar pressure. The saturation pressure of CCl4 at 25°C is 14.76 kPa. If it is observed that the rate of evaporation of CCl4 is 0.1 kg/hour determine the diffusivity of CCl4 into air?
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