Mechanical Engineering UPSC PYQ 2024

A refrigerator in a laboratory uses R-134a as the working substance. The high pressure is 1200 kPa, the low pressure is 101.3 kPa and the compressor is reversible. It should remove 500 W from a specimen currently at - 20°C (not equal to T_L in the cycle), that is inside the refrigerated space. Find the cycle COP and the electrical power required. The enthalpy of superheated R-134a at 1200 kPa may be taken as 430 kJ/kg at compressor outlet. Use the R-134a property table attached. The refrigerant enters the compressor as saturated vapour.

Explain the principle of electric discharge machining (EDM) using suitable diagram. Describe the effect of process parameters of electric discharge machining on process performance characteristics such as material removal rate, tolerance and surface characteristics.

A rectangular plate with a central circular hole of diameter 250 mm is subjected to two mutually perpendicular direct stresses of magnitudes 20 MN/m^2 and 10 MN/m^2 along the x and y directions, respectively. In addition, a shear stress of magnitude 7.5 MN/m^2 acts on all the four planes on which the direct stresses act (see the figure below). The circular hole is deformed into the shape of an ellipse. If the value of Young's modulus of the material is 210 GN/m^2 and Poisson's ratio is 0.3, find the lengths of the major and minor axes :