Steady Flow Devices l2c4i

Nozzles and diffs, turbines, compressors and pumps, heat exchangers, and throttling devices.

Control Volume Energy Rate Balance • Including all the energy forms such as Kinetic Energy, Potential Energy, Internal Energy and the Energy associated wit the Control Volume will result in the Universal Energy Balance Equation

Since u, p, and v are all properties, this combination is also a property and therefore can be rewritten as Enthalpy,

Control Volume Energy Rate Balance • The energy equation can be rewritten in the following form

Nozzles and Diffs • A nozzle is a flow age of varying cross-sectional area in which the velocity of a gas or liquid increases in the direction of flow. • In a diff, the gas or liquid decelerates in the direction of flow

Nozzles and Diffs • nozzle and diff are combined in a wind-tunnel test facility.

Nozzles and Diffs • For nozzles and diffs, the only work is flow work at locations where mass enters and exits the control volume, so the term Wcv drops out of the energy rate equation. • Assuming potential energy is negligible.

Nozzles and Diffs example

Turbine • A turbine is a device in which work is developed as a result of a gas or liquid ing through a set of blades attached to a shaft free to rotate. • Turbines are widely used in vapor power plants, gas turbine power plants, and aircraft engines. • In these applications, superheated steam or a gas enters the turbine and expands to a lower exit pressure as work is developed

Turbine Example

Power washer

Sample with potential energy

Solution

1. 2. 3.

4. Evaluate change in specific enthalpy

5. Evaluate change in specific kinetic energy 6. Evaluate change in specific potential energy 7. Calculate Wcv Answer: Wcv = -1.54 kW

Heat Exchangers • Devices that transfer energy between fluids at different temperatures by heat transfer modes.

Vessel Type Heat Exchanger Hot and Cold Water are Mixed in one chamber

Recuperators Heat exchanger in which a gas or liquid is separated from another gas or liquid by a wall through which energy is conducted. (Counter-flow and parallel tube-within-a-tube)

Recuperators (Parallel-flow and parallel tube-within-a-tube)

Recuperators (cross-flow Heat exchanger) Same as in automobile radiators, and multiple- shell-and-tube condensers and evaporators.

Heat Exchanger Example

Solution

1. Energy balance (underline elements are negligible for heat exchangers)

2. Determine enthalpies

3. Calculate mass ratio 4. Energy balance on the steam side of the condenser only,

Answer: mass ratio = 36.6 Qcv = -2276.7 kJ/kg

Throttling Devices • A significant reduction in pressure can be achieved simply by introducing a restriction into a line through which a gas or liquid flows. This is commonly done by means of a partially opened valve or a porous plug,

Throttling Devices

There is usually no significant heat transfer with the surroundings and the change in potential energy from inlet to exit is negligible.

Although velocities may be relatively high in the vicinity of the restriction, measurements made upstream and downstream of the reduced flow area show in most cases that the change in the specific kinetic energy of the gas or liquid between these locations can be neglected

Example Problem

Seat Work