In an air standard cycle, heat addition is assumed to occur ________________.

Explanation:

In an air standard cycle, heat addition is assumed to occur from an external constant high-temperature source. This is the correct answer because, in the idealized analysis of thermodynamic cycles, such as the air standard cycle, it is typically assumed that the heat addition process happens at a constant high temperature provided by an external source. This simplifies the analysis and helps in understanding the fundamental thermodynamic principles governing the cycle.

Detailed Solution:

Thermodynamic cycles are used to model the operation of heat engines, refrigerators, and heat pumps. One of the fundamental concepts in analyzing these cycles is the assumption of an air standard cycle. An air standard cycle is an idealized thermodynamic cycle in which air is assumed to be the working fluid, and the cycle is analyzed using the properties of air as an ideal gas. The air standard cycle helps in simplifying the complex processes involved in real cycles by making several assumptions.

Assumptions of Air Standard Cycle:

• The working fluid is air, which behaves as an ideal gas throughout the cycle.
• The heat addition and heat rejection processes are idealized as occurring at constant temperatures.
• The specific heats of air are assumed to be constant.
• All processes are internally reversible.
• There are no changes in kinetic or potential energy of the working fluid.

In the context of the air standard cycle, the heat addition process is particularly important. It is assumed to occur at a constant high temperature provided by an external source. This external source is typically envisioned as a large thermal reservoir that can supply heat at a constant temperature without undergoing any significant temperature change itself.

Why Option 1 is Correct:

Option 1 states that heat addition occurs from an external constant high-temperature source. This aligns with the assumptions of the air standard cycle, where the heat addition process is idealized to occur at a constant temperature. This simplification makes the analysis more tractable and allows for the application of the first and second laws of thermodynamics to determine the efficiency and work output of the cycle.

The heat addition process in an air standard cycle is typically represented as an isothermal process in idealized cycles like the Carnot cycle or as an isobaric or isochoric process in cycles like the Otto or Diesel cycles. In all these cases, the heat addition is assumed to come from an external source at a constant high temperature.