GATE TF key terms

Energy cannot be created or destroyed, only converted between forms. For a closed system, the heat added equals the change in internal energy plus the work done by the system.

Spontaneous processes increase total entropy, so heat cannot be fully converted into work in a cyclic device. It sets the direction of energy transfer and bounds thermal efficiency.

A thermodynamic property defined as internal energy plus the product of pressure and volume. It represents the heat exchanged in constant-pressure processes such as combustion and flow through boilers.

A measure of molecular disorder and of how much energy is unavailable for useful work. It increases in irreversible processes and stays constant in ideal reversible adiabatic processes.

The maximum useful work obtainable when a system is brought reversibly into equilibrium with its environment. Unlike energy, exergy is destroyed by irreversibilities such as friction and mixing.

A fixed region in space selected for analysis, across whose boundaries mass and energy may flow. Steady-flow energy balances for turbines, compressors, and heat exchangers are written on this basis.

The highest possible efficiency of a heat engine operating between a hot reservoir and a cold reservoir, equal to one minus the ratio of their absolute temperatures. No real cycle can exceed it.

The ideal vapour power cycle used in steam plants, comprising pump, boiler, turbine, and condenser processes. Practical plants add superheating and reheating to raise efficiency and avoid wet steam in the turbine.

The ideal gas-turbine cycle of two isentropic and two constant-pressure processes. Its efficiency rises with pressure ratio and turbine inlet temperature, and it underlies combined-cycle power plants.

The ratio of actual work output to ideal isentropic work for a turbine, or ideal to actual work for a compressor. It quantifies how far a real machine falls short of a reversible adiabatic process.

A power plant arrangement in which hot exhaust from a gas turbine generates steam for a bottoming steam turbine. It achieves thermal efficiencies well above either cycle operating alone.

The heat released per unit mass or volume when a fuel burns completely. The higher value includes latent heat of water vapour in the products; the lower value excludes it.

A fuel test reporting moisture, volatile matter, fixed carbon, and ash content by mass. It indicates combustion behaviour and is usually paired with ultimate analysis of elemental composition.

Determination of the elemental composition of a fuel, chiefly carbon, hydrogen, nitrogen, sulphur, and oxygen. It supports stoichiometric air calculations and flue-gas estimation.

The minimum mass of air required to burn unit mass of fuel completely, derived from combustion equations. Actual furnaces run lean or rich relative to this value, expressed as the equivalence ratio.

Air supplied beyond the stoichiometric requirement, expressed as a percentage. Enough ensures complete combustion, but too much raises heat losses through the stack and lowers efficiency.

The ratio of heat absorbed by the working fluid to heat released by fuel combustion, often found indirectly by summing stack, radiation, and unburned-carbon losses.

The ratio of actual heat transfer rate to the maximum thermodynamically possible rate for given inlet conditions. It depends on flow arrangement and the number of transfer units.

The driving temperature difference used to size heat exchangers, computed as the logarithmic average of terminal differences for counterflow or parallel flow. It corrects for non-uniform temperature profiles.

A lumped parameter giving heat flux per unit area per unit temperature difference across a wall, combining convective and conductive resistances in series, including fouling layers.

A material property describing the rate of heat conduction per unit area per unit temperature gradient. Metals are high, gases low, and insulation very low.

The ratio of internal conduction resistance to external convective resistance for a solid body. Small values justify a lumped-capacitance transient analysis with uniform internal temperature.

A dimensionless convective heat transfer coefficient, equal to the convective coefficient times a characteristic length divided by fluid thermal conductivity. It expresses convection strength relative to pure conduction.

The ratio of inertial to viscous forces in a flow, equal to density times velocity times length over viscosity. It determines whether flow is laminar, transitional, or turbulent.

A dimensionless measure of pressure drop in pipe flow, related to wall shear stress. The Darcy form appears in the Darcy–Weisbach head-loss equation; the Fanning form is one quarter as large.

The shaft power needed to move a fluid, obtained from volumetric flow rate, density, gravitational acceleration, head, and pump efficiency. It is a key parasitic load in power plants.

The mass of fluid passing a cross-section per unit time, equal to density times area times velocity. It is conserved in steady flow and links energy balances to equipment sizing.

Rotodynamic devices such as turbines, compressors, and pumps that exchange work with a fluid through rotating blades. Velocity triangles and Euler's equation relate blade speeds to energy transfer.

Generation of voltage and current when light is absorbed in a semiconductor junction, separating electron-hole pairs. Output depends on irradiance, cell temperature, and the load operating point.

The ratio of a photovoltaic cell's maximum power to the product of its open-circuit voltage and short-circuit current. It measures the squareness of the current-voltage curve and cell quality.

The terminal voltage of a photovoltaic cell under illumination with no external load. It falls as cell temperature rises and rises logarithmically with irradiance.

The available wind power per unit swept area, proportional to air density times the cube of wind speed. It explains why doubling wind speed multiplies potential output eightfold.

The theoretical maximum fraction of wind kinetic energy a rotor can extract, about 0.593. It follows from momentum theory assuming an ideal, frictionless actuator disc.

The ratio of actual energy generated over a period to the energy the plant would produce running at rated power continuously. It captures resource variability and availability.

The ratio of average load to peak load over a period on a power system or consumer. A high value means demand is steady and generation assets are used efficiently.

Splitting of a heavy nucleus into lighter fragments after neutron absorption, releasing energy and further neutrons. A self-sustaining chain reaction requires the effective multiplication factor to exceed one.

A material such as light water, heavy water, or graphite that slows fast neutrons to thermal energies, increasing the probability of further fission in thermal reactors.

A movable absorber of neutrons, often boron or cadmium, inserted into a reactor core to reduce reactivity and regulate or shut down the chain reaction.