Question

001: Ability to formulate and experimentally discover the energy balance accordingly to the first law of thermodynamics for a system. Numbers of engineering devices are designed to operate mainly under the same conditions for long periods, and it is known as steady flow devices. Given here are the examples of the flow devices. a. Nozzle and diffuser b. Turbine and compressor C. Throttling valve d. Mixing chamber e. Heat exchanger f. Pipe and duct flow 1. Briefly explain the function of each device. 2. Explain how the mentioned devices different from each other in term of energy analysis. In your report, state the reason if any energy components can be neglected in the analysis.

Answer 1

(a) Nozzle and Diffuser Nozzle is a device that is used to increase the velocity of fluid at the expense of fluid's pressure. Diffuser is a device that's designed to reduce the velocity of a fluid (which result in the increase of fluid pressure). (b) Turbines and Compressors Turbine is a device which is designed to produce shaft work (power) at the expense of fluid's pressure. As the working fluid (steam or gas) passes through the turbine it does work against the blades which are attached to a shaft. The shaft rotates and produce work. The purpose of a compressor is to increase the pressure of a fluid. (The term compressor is used when the working fluid is a gas, while the term pump is used when the working fluid is a liquid) (c) Throttling Valve Throttling valves are essentially flow restricting devices (for ex. Capillary tubes), placed in flow passage which cause a significant drop in the fluid pressure. This drop in fluid pressure is generally accompanied by a significant change in fluid's temperature. (d) Mixing Chamber Mixing chamber is a section in the flow passage where two or more streams of same or different fluids get mixed.

(e) Heat Exchanger Heat exchanger is a device, where the heat is exchanged between two fluids without mixing. (1) Pipe and Ducts Pipes and Ducts are used to transport a fluid (gases or liquids) from one location to another location. (2) First law analysis First law of thermodynamics (conservation of energy) for a steady flow device can be written (on rate basis) as follows 2 ** (****+92) - 2 m (n* "* + 92) out Where, I is the rate at which heat is transferred to the system and W is the rate at which work is done by the system. (a) Nozzles and diffusers Since the flow through the Nozzles (or diffusers) generally, take place at very high velocities, there's little time available for the heat transfer between the fluid and surroundings. Therefore, 0 is negligible compared to the kinetic and enthalpy change of the fluid (0 - 0). Also, the changes in elevation head is negligible. Therefore, the potential energy change of the fluid can be neglected. Since there are no moving parts - W = 0. Substituting these assumptions and results, first law equation for a Nozzle (or diffuser) can be approximated as

M. + - Now out 2 Or hoin = ho out Where V2 ho = 1+2 Known as stagnation (or total) enthalpy of the fluid. (b) Turbine and Compressor Heat transfer between the fluid and surrounding is usually neglected in the analysis of flow through the turbine or compressor, as is the kinetic and potential energy changes (since they are negligibly smaller compared to the enthalpy changes of the fluid). Therefore, the first law for a Turbine (or) compressor process can be approximated as W = -m(hout -hin) (for a turbine W will be positive since the work is done by the fluid and for a compressor, W will be negative since the work is done on the fluid.) (e) Throttling Valve Throttling valves are generally small devices. The heat transfer between the fluid and surroundings while flowing through a throttling device can be neglected since there's not large enough area or sufficient time for the heat transfer to take place. There's no external work transfer involved in the throttling process (W = 0). Also the changes in the potential and kinetic energy changes are negligible compared to the enthalpy change of fluid. Therefore, the first law equation for a throttling process reduces to hin = hout

(d) Mixing Chamber Mixing chambers ae usually well insulated. Therefore, external heat transfer, 0 -0. There's no work transfer involved: W = 0. Also, the kinetic and potential energy changes of the fluid in a mixing chamber is negligible. From the conservation of mass principle, the sum of rate at which the fluids enter the mixing chamber should be equal to the rate at which mixture leaves the mixing chamber. Mathematically, m = m in out From the conservation of energy n (1) (n) = m(h) in out (e) Heat Exchangers Heat exchangers do not involve any external work transfer (W = 0). Also the kinetic and potential energy changes for each fluid stream is negligibly small. Heat exchangers are usually well insulated to prevent any heat exchange between the working fluid and the surroundings (that's it allows only the heat exchange between the working fluids). Under this condition, if the whole heat exchanger is considered as the control volume for the analysis then 0 -0. (Remember, if the first law is applied to each of the fluid stream separately, then + 0.) (1) Pipe and Duct flow For liquids flowing through the ducts or pipe, kinetic energy changes are usually small (it's especially true when the pipe or duct has constant cross sectional area). However, for a gas flow in ducts of variable cross sectional area, the kinetic energy changes may not be negligible. Similarly, if the selected control volume involves any work transfer devices such as electric

wire, fan etc., then W70. In some situations, heat transfer between the fluid and external surroundings may not be negligible (0 = 0). Otherwise, the work and heat transfer effects can be neglected. When there's a considerable change in the elevation of the fluid, potential energy changes can be significant should be considered during the analysis.