Thermal Engineering Archives - Page 22 of 22 - Learn Mechanical Engineering

Thermodynamics | Gate Mechanical Basic Concepts Thermodynamics is another important topic for gate examination .thermodynamics mainly consists of sub subjects are IC engine ,Refrigeration and Air Conditioning. Some important points are mentioned below: In the polytropic process we have pvn = constant, if the value of n is infinitely large, the process is called constant volume process. The molecular volume of any perfect gas at 600 +103 N/m2 and 270c will be 4.17 m3/kg mol. The thermodynamic cycle used in a thermal power plant is Rankine. For the same heat input and same compression ratio efficiency depend mainly on working substance. The change in enthalpy for small temperature change ∆T for an ideal gas is given by the relation ∆H = Cp ∆T. Entropy, for reversible flow, is given by the relation dS = dqh/T. The S.I. unit of pressure is Pascal. Superheated vapour behaves approximately as a gas. A sterling cycle has two isothermal and two constant volume processes. Brayton cycle has two isentropic and two constant pressure processes. Otto cycle consists of two isentropic and two constant volume processes. Diesel cycle consists of two isotropic, one constant volume one constant pressure processes. Ericsson cycle has two isothermals and two constants pressure processes. Brayton cycle cannot be used in reciprocating engines even for same adiabatic compression ratio and work output because large volume of low pressure air cannot be efficiently handled in rotation engines. Brayton cycle is generally used for gas turbine. Antifreeze chemicals are those that lower down the freezing points of liquids. Clog point of an oil refer to the temperature at which paraffin and waxes in oil start precipitating. If value of n is defiantly large in a polytropic pVn = C. Then the process is called constant volume process. A heat exchange process where the product of pressure and volume remains constant is called hyperbolic process. Zeroth law of thermodynamics provides the basis for measuring thermodynamic property of temperature. 1 watt = 1 Nm/s. For a six compression of air set, with the minimum work condition the temperature rise in the cylinders will be the same. In case of steam engine the cut off ratio is the ratio of piston stroke which the piston has travelled when cut off occurs. The reason for insulating the pipe is to minimize the heat loss from the surface. Under ideal conditions isothermal, isobaric, isochoric and adiabatic processes are quasi-static processes. Isentropic flow is reversible adiabatic flow. Exhaust gases from a engine possess kinetic energy The extension and compression of a helical spring is an example of reversible process. At critical point the latent enthalpy of vaporization is zero. Change in temperature = zero relation is not applicable in a free expansion process. An ideal engine absorbs heat at a temperature of 1270c and exhausts heat at a temperature of 770c.Its efficiency will be 13%. The triple point of a substance is the temperature and pressure at which the solid does not melt, the liquid does not boil and the gas does not condense.