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Interview Questions Mechanical Engineering Part-4 :: Saudi Aramco Standard

 Mechanical Engineering Interview Questions Part-4 

151. Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon ?
Liquid hydrogen has higher burning velocity.

152. Why axial flow compressor is preferred for gas turbines for aeroplanes ?
Because it has low frontal area.

153. What is the effect of inter cooling in gas turbines ?
It decreases thermal efficiency but increases net output.

154. Why iso-octane is chosen as reference fuel for S.I. engines and allotted 100 value for its octane number ?
Iso-octane permits highest compression without causing knocking.

155. Why thermal efficiency of I.C. engines is more than that of gas turbine plant ?
In I.C. engine maximum temperature attained is higher than in gas turbine.

156. Which are the reference fuels for knock rating of S.I. engines ?
n-heptane and ISO-octane.

157. When effect of variations in specific heats is considered then how do maximum temperature and pressure vary compared to air standard cycle ?
Temperature increases and pressure decreases.

158. Quantities like pressure, temperature, density, viscosity, etc. are independent of mass. What are these called ?
Intensive properties.

Also read :

159. The amount of radiation emitted per scm per sec is called …. ?
Emissive power.

160. In convection heat transfer, if heat flux intensity is doubled then temperature difference between solid surface and fluid will ?
Get doubled.

161. How you can define coal ?
Coal is a naturally occurring hydrocarbon that consists of the fossilised remains of buried plant debris that have undergone progressive physical and chemical alteration, called coalification, in the course of geologic time.

162. Which pollutant is major greenhouse gas and what is its effect ?
CO is major greenhouse gas and it traps the radiation of heat from the sun within earth’s atmosphere.

163. In order to increase efficiency and reduce CO emissions and other emissions, clear coal technologies are receiving major attention. What are these ?

  1. Advanced pulverised and pressurised pulverised fuel combustion.
  2. Atmospheric fluidised bed combustion and pressurised fluidised bed combustion.
  3. Supercritical boilers.
  4. Integrated gasification combined cycle systems.
  5. Advanced integrated gasification, including fuel cell systems.
  6. Magneto hydrodynamic electricity generation.

164. What are the important operational performance parameters in design of fuel firing equipment ?
Fuel flexibility, electrical load following capability, reliability, availability, and maintenance ease.

165. What is the difference between total moisture and inherent moisture in coal ?
The moisture content of the bulk as sampled is referred to as total moisture, and that of the air dried sample is called inherent moisture.

166. Proximity analysis of coal provides data for a first, general assessment of a coal’s quality and type. What elements it reports ?
Moisture, volatile matter, ash and fixed carbon.

167. Ultimate analysis of coal is elementary analysis. What it is concerned with ?
Carbon, hydrogen, nitrogen, and sulphur in coal on a weight percentage basis.

168. Explain the difference between AFBC, BFBC, PFBC and PCFB in regard to fluidised bed technologies.
AFBC (Atmospheric fluidised bed combustion) process consists of forming a bed of inert materials like finely sized ash or ash mixed with sand, limestone (for sulphur removal), and solid fuel particles in a combustor and fluidising it by forcing combustion air up through the bed mixture. The gas flows thorugh bed without disturbing particles significantly but gas velocity is high enough to support the total weight of bed (fluidisation). At slightly higher velocity excess gas passes through the bed as bubbles (fluidised bed) and gives the bed the appearance of a boiling liquid.
Bubbling fluidised bed combustion (BFBC) has a defined height of bed material and operates at or near atmospheric pressure in the furnace.

Also read :

Pressurised fluidised bed combustion (PFBC) system operates the bed at elevated pressure. Exhaust gases have sufficient energy to power a gas turbine, of course, gases need to be cleaned.
In fluidised combustion, as ash is removed some unburned carbon is also removed resulting in lower efficiency. In circulating fluidised bed combustion (CFBC) system, bed is operated at higher pressure leading to high heat transfer, higher combustion efficiency, and better fuel feed. Circulating fluidised beds operate with relatively high gas velocities and fine particle sizes. The maintenance of steady state conditions in a fast fluidised bed requires the continuous recycle of particles removed by the gas stream (circulating bed). The term circulating bed is often used to include fluidised bed systems containing multiple conventional bubbling beds between which bed material is exchanged.

169. What for Schmidt plot for is used in heat transfer problems ?
Schmidt plot is a graphical method for determining the temperature at any point in a body at a specified time during the transient heating or cooling period.

170. In which reactor the coolant and moderator are the same ?
Pressurised water reactor.

171. Which reactor has no moderator ?
Fast breeder reactor.

172. What are thermal neutrons ?
Thermal neutrons are slow neutrons (having energy below 1 eV) which are in thermal equilibrium with their surroundings.

173. What is big advantage of fast breeder reactor ?
It has rapid self breeding of fissile fuel during the operation of the reactor, and thus, it offers about sixty times the output with same natural uranium resources through ordinary non-breeder nuclear reactor.

174. What is the purpose of biological shield in nuclear plants ?
Biological shield of heavy concrete prevents exposure to neutrons, beta rays and gamma rays which kill living things.

175. Which two elements have same percentage in proximate and ultimate analysis of coal?
Moisture and ash.

176. On which analysis is based the Dulong’s formula for the heating value of fuel ?
On ultimate analysis.

177. Which element causes difference in higher and lower heating values of fuel ?
Hydrogen.

Also read :

178. Which heating value is indicated by a calorimeter and why ?
Gross heating value because steam is condensed and heat of vapour formed is recovered.

179. State the difference between ultimate and proximate analysis of coal ?
In ultimate analysis, chemical determination of following elements is made by weight: Fixed and combined carbon, H, O, N, S, water and ash. Heating value is due to C, H and S.
In proximate analysis following constituents are mechanically determined by weight. Moisture, volatile matter, fixed carbon and ash. Heating value is due to fixed carbon and volatile matter.

180. What is fuel ratio ?
Fuel ratio is the ratio of its % age of fixed carbon to volatile matter.

181. How the analyses and calorific values of fuels can be reported ?
It may be reported as
(a) as received or fired (wet) basis
(b) dry or moisture free basis
(c) combustible or ash and moisture free basis

182. What is the difference between nuclear fission and fission chain reaction.
The process of splitting of nucleus into two almost equal fragments accompanied by release of heat is nuclear fission. Self sustained, continuing, sequence of fission reactions in a controlled manner is fission chain reaction.

183. Explain difference between fissile and fertile materials.
The materials which can give nuclear fission e.g. U 35, Pu 39, U 33 are fissile materials. Fertile material itself is not fissionable, but it can be converted to a fissionable material by irradiation of neutrons in a nuclear reactor.

184. What do you understand by fuel cycle in nuclear plants ?
Fuel cycle a series of sequential steps involved in supplying fuel to a nuclear power reactor. The steps include : Mining, refining uranium, fabrication of fuel elements, their use in nuclear reactor, chemical processing to recover remaining fissionable material, re-enrichment of fuel from recovered material, refabrication of new fuel elements, waste storage etc.

185. What is heavy water and what is its use in nuclear plants ?
Water containing heavy isotopes of hydrogen (Deuterium) is known as heavy water. Heavy water is used as a moderator. Heavy water has low cross section for absorption of neutrons than ordinary water. Heavy water slows down the fast neutrons and thus moderates the chain reaction.

186. What is a converter reactor ?
A reactor plant which is designed to produce more fuel than it consumes. The breeding is obtained by converting fertile material to fissile material.

187. Explain nuclear reactor in brief.
A plant which initiates, sustains, controls and maintains nuclear fission chain reaction and provides shielding against radioactive radiation is nuclear reactor.

188. What is the difference between conversion and enrichment ?

  • The process of converting the non fissile U 38 to fissile U-35 is also called “Conversion”. The material like U 38 which can be converted to a fissile material by the neutron flux is called “fertile material”. The conversion is obtained within the nuclear reactor during the chain reaction.
  • Enrichment is the process by which the proportion of fissile uranium isotope (U-35) is increased above 0.7% (original % in natural uranium).
  • The concentration of U-35 in the uranium hexafluoride is increased from the 0.7% in natural uranium to to 4%. This is called enrichment and is accomplished in an enrichment plant.

189. Disposal of radioactive waste materials and spent fuel is a major and important technology. How the waste radioactive material is disposed off ?
Nonusable fission products are radioactive and take short/medium/long time for radioactive decay to reach safe level of radioactivity. Accordingly three methods of disposal are :
(a) Zero or low radioactivity material is dispersed or stored without elaborate shielding.
(b) Medium radioactivity material is stored for short duration of about 5 years to allow decay of radioactivity.
(c) High radioactive material. They are stored in water for several months to permit radioactive decay to an accepetable low level.

190. Which nuclear reactor uses water as a coolant, moderator and reflector ?
Pressurised water reactor.

191. Which reactor produces more fissionable material than it consumes ?
Breeder reactor.

192. Which reactor uses natural uranium as fuel ?
Gas cooled reacator.

193. Which reactor uses heavy water as moderator ?
CANDU.

194. Which reactor requires no moderator ?
Breeder reactor.

195. Which reactor uses primary coolant as fluoride salts of lithium, beryllium, thorium and uranium ?
Molten salt breeder reactor.

196. Why an increase in area is required to produce an increase of velocity in case of supersonic flow ?
Increase in area for increase in velocity for supersonic flow is required because the density decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass, area must increase.

197. Under what circumstances would there be an increase in pressure in a divergent nozzle ?
For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will exist at the exit.

198. Why water can’t be used as refrigerant for small refrigerating equipment ?
The refrigerant should be such that vapour volume is low so that pumping work will be low. Water vapour volume is around 4000 times compared to R- for a given mass.

199. Which parameter remains constant in a throttling process ?
Enthalpy.

200. What is the difference between isentropic process and throttlinglprocess ?
In isentropic process, heat transfer takes place and in throttling process, enthalpy before and after the process is same.

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