1st Law Flashcards
Can a steady-flow device involve more than one inlet and one outlet.
True because it is designed to handle multiple streams of fluid entering/exiting
Explain the energy analysis of steady-flow systems.
The fluid properties may be different at different inlets and exits during a steady-flow process.
The mass content of control volume during a steady-state process is constant.
The energy content of control volume during a steady-state process is constant.
The fluid properties at an inlet or exit remain constant during a steady-flow process.
Energy can be transferred to or from a system by…
Mass flow, heat transfer and work
What are the quantities which are part of the total energy of a closed system.
Internal energy
Kinetic energy
Facts regarding energy transfer mechanisms.
There cannot be any net heat transfer between two systems that are at the same temperature.
Heat transfer is recognized only as it crosses the boundary of a system.
Heat always flows from high temperature to low temperature.
If a system undergoes an adiabatic process, what happens to the temperature
In an adiabatic process, there is no heat exchange to the surroundings. Therefore if the system expands, the temp decreases.
If the system compresses the temp increases with internal energy increasing due to work being done on the system.
Which device below increases the pressure of gas usually with negligible changes in kinetic and potential energies of fluid streams?
Compressor
Steady flow devices that result in a drop in working fluid pressure from inlet to exit are
Nozzle, turbine, throttling device
What happens to a throttling valve pressure with negligible heat transfer and negligible changes in kinetic and potential energies?
Throttling valves produce a pressure drop without involving any work.
The thermodynamic performance of a device such as a turbine through which mass flows is best analysed by studying what?
The flowing mass alone.
Why is the statement false: For a control volume at steady state, mass can accumulate within the control volume.
The mass within the control volume remains constant. The mass flow rate entering =mass flow rate exiting.
what is a A diffuser?
A diffuser is a flow passage of varying cross-sectional area in which the velocity of a gas or liquid increases in the direction of flow.
For a one-inlet, one-exit control volume at steady state, the mass flow rates at the inlet and exit are equal but the inlet and exit volumetric flow rates may ….
Not be equal because of the changes in fluid density. density=mass/volume.
Facts about regarding heat and work interactions.
The temperature of a well-insulated system can be changed by energy transfer as work.
Both heat and work are boundary phenomena.
The magnitudes of heat and work depend on the path followed during a process.
The magnitudes of heat and work depend on the initial and final states of a system.
Why are Heat and work not defined at a state of a system.
Heat and work are not properties of the system, they are forms of energy transfer. A state of a system is things like pressure,temp and volume.
Mass flow rate for a flow modelled as one-dimensional depends on all except
Total volume of working fluid
As velocity decreases in a diffuser, pressure what?
Decreases
If an energy transfer between a closed system and its surroundings is not heat, it must be
Work
Facts about specific heat
The energy needed to raise the temperature of a unit mass of a substance by 1ºC depends on how the process is executed.
The specific heat units kJ/kg°C and kJ/kg°K are identical and can be used interchangeably.
The specific heat at constant volume is a measure of the variation of internal energy of a substance with temperature.
The specific heat at constant pressure is a measure of the variation of enthalpy of a substance with temperature
Why is the statement incorrect: The specific heat at constant pressure can be less than the specific heat at constant volume.
For an ideal gas, the specific heat at constant pressure is always greater than the specific heat at constant volume. Due to that when a gas is heated at constant pressure, it does work to expand, which requires additional energy compared to heating at constant volume (no work),
Which device handles a liquid and requires work input, usually with negligible heat transfer and negligible changes in kinetic and potential energies of fluid streams?
Pump
For one-dimensional flow, mass flow rate is the product of ….
density, area, and velocity.
