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GE Interview Questions and Answers - 05 May 2012

Posted on :17-02-2016
GE Interview Questions and Answers - 05 May 2012:-

Q1. Is the derivation for composite axis symmetric bar subjected to torsion similar to the composite beam derivation? 

ANS:
Yes. Since plane sections remain plane assumption holds good for composite bar subjected to torsion, similar principle as derivation for composite beam can be used to solve for stresses in a composite bar subjected to torsion. 


Q2. Why do they have a tapering change in c.s. for stepped shafts?

ANS:
For stepped shafts, away from the abrupt change in the cross-section, the stresses can be computed using the same formulae derived taking appropriate diameter of the shaft portion in to consideration. At the region of change in geometry, stress concentrations can occur due to abrupt change radius.


Q3. Why is the variation of shear strain with radius linear?
 
ANS:
For non-linear elastic materials again, the elastic moduli are different and are functions of the strains. Therefore, while solving for stresses, appropriate constitutive law should be used before applying the equilibrium conditions. 


Q4. Why do we have to make the assumption that plane sections plane?

ANS:
In order to make calculation of stresses due to torsion easy, we need to make certain simplifying assumptions on the deformation pattern which is realistic. It is has been found from the rigorous solution procedure (elasticity solution) and from the experiments that the circular cross section members subjected to pure torsion in the elastic range satisfy very closely this condition of plane sections remain plane and rigid. 

By making this assumption, the solution procedure becomes simple as shown in the basic concepts section in the derivation for torsional stresses. 


Q5. What if material is non-isotropic? 

ANS:
If the material is non-isotropic (i.e. anisotropic), then the elastic moduli will vary and thus the problem will be completely different with additional stresses appearing since there is a coupling between shear stresses and normal stresses for an anisotropic material. 


Q6. How about bars with non-axis symmetric cross section? 

ANS:
For bars with non-ax symmetric cross section, the assumption of plane section remain plane is not satisfied. Regions of the cross-section undergo deformations in the axial direction leading to warping of the section. There are again certain simplifying assumptions which are relaxation of the ax symmetric bar assumption that can be used to find stresses in a non-ax symmetric bar.


Q7. What if material goes to plastic range? 

ANS:
Even, if the material goes to plastic range, plane sections remain plane assumption is assumed to hold. Therefore, the strains can be found out from which distribution of stresses can be derived from equilibrium principles.


Q8. The formulae derived look very similar to beam and axial deformation formulae? 

ANS:
Yes, that is true. In all the three derivations pertaining to axial, beam and torsional deformations, the assumption of plane section remains plane is used and it leads to very similar formulae for these three types of structural members. 


Q9. What about non-linear elastic materials? 


Q10. Why is the variation of shear strain with radius linear?


Q11. If a surface emits 200 W at a temperature of T, how much energy will it emit at a temperature of 2T?

ANS:
Since E u T4, a 2-fold increase of temperature brings a (24) = 16-fold increase in energy. Thus the surface will emit (16)(200) = 3200 W.


Q12. Explain why the temperature boundary layer grows much more rapidly than the velocity boundary layer in liquid metals.

ANS:
Liquid metals are characterised by very low Prandtl numbers since their thermal conductivity is high, hence the heat diffusion is much faster than momentum diffusion.


Q13. What is the Fourier number?

ANS:
  1. The Fourier number is defined as: Fo = at/L2 where a = thermal diffusivity, t = time L = characteristic length
  2. The Fourier number is a dimensionless measure of time used in transient conduction problems.


Q14. Define a black surface.

ANS:
  1. A black surface is defined by three criteria: it absorbs all radiation that is incident on it emits the maximum energy possible for a given temperature and wavelength of radiation (according to Plancks law)the radiation emitted by a blackbody is not directional (it is a diffuse emitter)
  2. A black surface is the perfect emitter and absorber of radiation. It is an idealized concept (no surface is exactly a black surface), and the characteristics of real surfaces are compared to that of an ideal black surface.


Q15. Your friend asserts that, in a heat exchanger, it is impossible for the exit temperature of the cold fluid to be greater than the exit temperature of the hot fluid when both fluids are single phase fluids. What is your response?

ANS:
The statement is true for a parallel flow heat exchanger. However, in a counter flow heat exchanger the outlet temperature of the cold fluid can in fact exceed the outlet temperature of the hot fluid.


Q16. What is a diffuse surface?

ANS:
A diffuse surface is defined as one for which the emissivity (e) and the absorptivity (a) are independent of direction (q).


Q17. What is the difference between diffusion and radiation heat transfer?

ANS:
Diffusion heat transfer is due to random molecular motion. Neighboring molecules move randomly and transfer energy between one another - however there is no bulk motion. Radiation heat transfer, on the other hand, is the transport of heat energy by electromagnetic waves. All bodies emit thermal radiation. 

In particular, notice that unlike diffusion, radiation heat transfer does not require a medium and is thus the only mode of heat transfer in space. The time scale for radioactive heat transfer is much smaller than diffusive heat transfer.


Q18. What are the conditions to be satisfied for the application of a thermal circuit?

ANS:
The problem must be a steady state, one-dimensional heat transfer problem.


Q19. Define and state the physical interpretation of the Biot number.

ANS:
The Biot number is given by:
Bi = hL/k
where
h = convective heat transfer coefficient,
k = thermal conductivity
L = characteristic length.

It is a ratio of the temperature drop in the solid material and the temperature drop the solid and the fluid. So when the Bi <<1 , most of the temperature drop is in the fluid and the solid may be considered isothermal.


Q20. Define overall heat transfer coefficient.

ANS:
The overall heat transfer coefficient is defined in terms of the total thermal resistance between two fluids. If there are a number of thermal resistances between the two fluids, the overall heat transfer coefficient is given by:
U = 1/SR


Q21. You might have observed early morning frost on a clear day even when the minimum air temperature during the night was above 0 degree C. On a clear day, the effective sky temperature can be as low as -45 degree C. Explain how such frost formulation takes place.

ANS:
The frost is created because of radioactive losses to the sky.


Q22. How is natural convection different from forced convection?

ANS:
In natural convection, the movement of the fluid is due entirely to density gradients within the fluid (e.g. hot air rises over cold air). There is no external device or phenomenon which causes fluid motion. In forced convection, the fluid is forced to flow by an external factor - e.g. wind in the atmosphere, a fan blowing air, water being pumped through a pipe. 

Typically heat transfer under forced convection conditions is higher than natural convection for the same fluid.


Q23. What is the range of values for the emissivity of a surface?

ANS:
The emissivity e ranges between 0 and 1.


Q24. State the condition which must be satisfied to treat the temperature distribution in a fin as one-dimensional.

ANS:
When ht/k <<1 where h is the convective heat transfer coefficient, t is the thickness of the fin and k is the thermal conductivity of the fin, one can consider that the temperature gradient in the thickness direction is very small and the analysis can be considered as one-dimensional.


Q25. What is a lumped system?

ANS:
A lumped system is one in which the dependence of temperature on position (spatial dependence) is disregarded. That is, temperature is modeled as a function of time only.


Q26. A greenhouse has an enclosure that has a high transmissivity at short wavelengths and a very low transmissivity (almost opaque) for high wavelengths. Why does a greenhouse get warmer than the surrounding air during clear days? Will it have a similar effect during clear nights?

ANS:
Solar radiation is skewed towards shorter wavelengths. On a clear day the glass of the greenhouse admits a large proportion of the incident radiation. Inside the greenhouse, the various surfaces (plants etc.) reflect the radiation; but the reflected radiation is spectrally different, having more of a high wavelength contribution. 

Thus the reflected radiation is not transmitted well by the glass, and is reflected back into the greenhouse. The interior heats up due to this trapped radiation. The same effect will not be seen on a clear night, since there is no solar radiation.


Q27. Define a view factor.

ANS:
A view factor is defined in the context of two surfaces A and B. It is defined as the fraction of radiation leaving A which is incident directly on surface B. A view factor must be defined in terms of surface A to surface B (FAB).


Q28. What is the effect of the Prandtl number of a fluid on the relative thicknesses of velocity and temperature boundary layers when the fluid flow is parallel to a flat plate?

ANS:
For laminar flow, the ratio of the boundary layer thickness d to that of the thermal boundary layer, dt, is given by:

d/dt u Prn.

The higher the Prandtl number, the larger is the ratio.


Q29. What is internal energy generation? Give examples where internal energy generation occurs.

ANS:
Internal energy generation is the generation of heat within a body by a chemical, electrical or nuclear process. Examples are the heating of a nuclear fuel rod (due to fission within the rod), the heating of electrical wires (due to the conversion of electrical to heat energy), microwave heating and the generation of heat within the Earth. The heat generated in each case is being converted from some other form of energy.


Q30. What do you understand by the terms fully developed velocity and temperature profile regions in internal flow?

ANS:
In the fully developed region, the cross-sectional velocity/temperature profile is of a constant shape at any axial location. Thus the profile has ceased to change. Also there is no radial component of velocity i.e. every particle of fluid is flowing purely in the axial direction.


Q31. What is a gray surface?

ANS:
A gray surface is defined as one for which the emissivity (e) and the absorptivity (a) are independent of wavelength (l).


Q32. Both the Nusselt number and the Biot number have the same form. What are the differences between them in terms of the variables employed and their physical significance?

ANS:
Both the Biot number and the Nusselt number are of the form (hL/k). However, for the Biot number, the thermal conductivity k used is that for the solid; for calculating Nusselt number the k value as that of the fluid. The Biot number is a measure of the ratio of the temperature drop in the solid material and the temperature drop between the solid and the fluid. The Nusselt number is a dimensionless version of the temperature gradient at the surface between the fluid and the solid, and it thus provides a measure of the convection occurring from the surface.


Q33. What do you understand by stability criterion for the solution of transient problems?

ANS:
When solving transient problems using finite-difference methods, it is possible that the solution undergoes numerically induced oscillations and becomes unstable i.e. the temperature values diverge. The stability criterion is a restriction on the values of Dt and Dx which ensures that the solution remains stable and converges. The criterion is usually expressed as a function of Fouriers number.


Q34. Will the thermal resistance of a rectangular slab be increased or decreased if the thermal conductivity is increased?

ANS:
Thermal resistance will decrease.


Q35. In a particular case of fluid flow over a flat plate the temperature boundary layer thickness is much smaller than the velocity boundary layer thickness. What can you conclude about the nature of the fluid?

ANS:
The fluid is a high Prandtl number fluid. e.g.oil.


  
   






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